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EXOBIOLOGY IN 
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EXOBIOLOGY 
EARTH ORBIT 




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The Results of Science Workshops Held 
at NASA Ames Research Center 



Edited by 

D. Deuees, D. Brownlee, J. Tarter, 
D.'usher, W. Irvine, and H. Klein 





Prepared at Ames Research Center 



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W/NSA 



National Aeronautics and Space Administration 
Office of Management 
Scientific and Technical Information Division 
Washington, DC 1989 



Library of Congress Cataloging-in-Publication Data 

Exobiology in earth orbit: the results of science 

workshops held at NASA Ames Research Center / edited by D. 

DeFrees ... (et al.) : prepared at Ames Research Center, 
p. 126 cm. - (NASA SP ; 500) 

1. Space biology-Congresses. 2. Life-Origin-Congresses. 
I. DeFrees, D. (Doug) II. Ames Research Center. III. Series. 
QH327.E95 1989 89-3231 

577-dc19 CIP 



Contents 



Page 

FOREWORD v 

PREFACE vii 

WORKSHOP MEMBERS xi 

EXECUTIVE SUMMARY xv 

CHAPTER I - BIOLOGY AND THE SPACE SCIENCES 1 

CHAPTER II -OBSERVATIONAL AND EXPERIMENTAL 

OPPORTUNITIES IN EARTH ORBIT 7 

2.1 Free-Flying Spacecraft and the Space Shuttle 7 

2.2 Space Station 13 

2.3 Lunar Base 15 

CHAPTER III -OBSERVATIONAL EXOBIOLOGY 19 

3.1 Cosmic History of the Biogenic Elements and Compounds 20 

3.2 Remote Observations 24 

3.3 Planetary Atmospheres 29 

3.4 Titan 35 

3.5 Comets 37 

3.6 Asteroids and Meteors 41 

3.7 The Protosolar Nebula and its Analogs 45 

3.8 The Detection of Other Planetary Systems 53 

3.9 Molecules in Space 57 

3.10 Interstellar Dust 64 

3.1 1 Evolution of Elemental Abundances in Galaxies 72 

CHAPTER IV - COSMIC DUST COLLECTION 77 

4.1 State of Knowledge 78 

4.2 Science Questions 79 

4.3 Technical Approach 81 

4.4 Technology Needs 85 

4.5 Opportunities 86 

4.6 Conclusions 87 



mi 



Page 

CHAPTER V - IN SITU INVESTIGATIONS 89 

5.1 Models of Interstellar Grains 89 

5.2 Reactions of Neutral Atomic Oxygen 92 

5.3 An Artificial Comet 93 

5.4 Microbial Survival in Space 96 

CHAPTER VI -SUMMARY OF PROPOSED EXPERIMENTS 101 

6.1 Observational Exobiology 101 

6.2 Cosmic Dust Collection 107 

6.3 In Situ Investigations 109 

APPENDIX A - ACRONYMS AND ABBREVIATIONS 111 

APPENDIX B - CHARACTERISTICS OF ORBITAL OBSERVATORIES .113 

APPENDIX C - CONVERSIONS: WAVELENGTH/FREQUENCY 123 

APPENDIX D- CONVERSIONS: LINEAR SIZE/ANGULAR SIZE 125 



IV 



Foreword 



One might think that the origin of life is strictly a biological question, to be 
attacked by experiments in Earth-based laboratories. That is largely true. But the 
chemical processes that operated over the eons of remote geological time to 
produce the first reproducing organisms occurred in the terrestrial environment 
of that time— and that environment was established by the processes that led to 
the formation and chemical evolution of the Earth itself. The traces of such 
processes to be found on the Earth today are studied by Earth scientists, and 
there is much to be learned of relevance to the origin of life. 

But the Earth did not spring from nothing; it was formed at the same time as 
the Sun, the planets, and small bodies of the Solar System about 4.5 billion 
years ago from primitive material in interstellar space. At least the relative 
abundances of the chemical elements, probably the chemical compounds into 
which they were combined, and perhaps the detailed physical properties of the 
pre-Earth material, were established by processes taking place in space before the 
Earth was formed. At least in principle, then, those who study space have 
insights to contribute. 

Exobiology, the study of processes relevant to biology that are occurring in 
space, is becoming a mature field. One immediately thinks of experiments to 
detect life elsewhere, like the Viking experiments at Mars and searches for radio 
signals from life elsewhere. Indeed, such studies have been undertaken by 
NASA's Life Sciences Program. But they are only the most visible elements of a 
much broader program to uncover phenomena relevant to the origin and evolu- 
tion of life throughout the universe. Astronomical studies reveal that the abun- 
dances of the elements in space seem to be very similar throughout our galaxy. 
Our Sun fits the general pattern because it and the other stars are so massive that 
as they form, they gravitationally draw in all types of material without regard to 
differences in chemical or physical properties. But on-site studies of the planets 
and satellites of the Solar System-including the Earth-show large variations 
that can be interpreted in terms of different chemical and physical processes 
occurring in the solar nebula of gas and dust before the planets formed. The fate 
of carbon at different places in the solar nebula is an example, and one of great 
importance for the origin of life. 

In interstellar space, carbon takes a variety of forms. In the dense "molecular 
clouds" from which stars (and presumably planets) are forming at the present 



epoch, most of the carbon is observed to take the form either of carbon monox- 
ide or of solid material in microscopic dust grains. Some of the grains may be 
made of graphite, while others appear to contain relatively heavy organic mole- 
cules, rather like those forming the organic residue in carbonaceous meteorites 
that have been recovered after they have fallen to the Earth's surface, and 
analysed in the laboratory. How did carbon become part of the Earth? Clearly 
not as a gas like methane or carbon monoxide, as it is known that the noble 
gases of even greater molecular weight are enormously depleted relative to their 
abundances elsewhere. It is more likely that at the time the Earth formed the 
local temperature and pressure favored the retention of carbon in solid form, 
possibly in the form of the heavy organic molecules like those in meteorites. 
Such a hypothesis raises questions open to further study: Were the organics 
simply those astronomers infer to be part of interstellar dust? Did they survive 
the heating believed to have accompanied the formation of the Solar System? If 
so, how? Can such primitive materials be observed at the current epoch, perhaps 
in comets? 

All these questions are interesting, and many of them can be addressed 
using the techniques of space research. Infrared telescopes in Earth orbit, like 
the projected Space Infrared Telescope Facility (SI RTF) and Infrared Space 
Observatory (ISO), can make more definitive studies of the carbon in molecules 
and solids in interstellar molecular clouds. They and the Hubble Space Telescope 
(HST) can provide more accurate information about asteroids thought to be 
representative of the bodies where carbonaceous meteorites originate. High- 
flying aircraft can recover interplanetary particles, some of which could be 
fragments of comets. And a rendezvous mission to a comet, such as the Comet 
Rendezvous Asteroid Flyby (CRAF), could yield far more information about 
comets, of which the Comet Halley flyby missions gave tantalizing hints. 

This volume gives a status report on the scientific investigations which can be 
undertaken in the field of exobiology using instruments in Earth orbit. The 
reader will find that there is much to be done and a whole host of questions that 
can be addressed. Every scientist interested in exobiology should consider how 
his or her work will be affected by the opportunities described here. They 
should also see what they can do to assist the Nation to reestablish a first-class 
space-science program, given the constraints imposed by the aftermath of the 
Challenger accident. 

George Field 
Harvard-Smithsonian Center for Astrophysics 



VI 



Preface 



The Science Workshops on Exobiology in Earth Orbit were held to thor- 
oughly explore all concepts for scientific experiments of exobiological interest 
to be carried out on any type of Earth-orbiting spacecraft over the next few 
decades and to make recommendations on which classes of experiments should 
be carried out. The Workshops grew out of the realization that many new 
opportunities would become available to exobiology before the end of this 
century. Furthermore, three series of workshops redefining the scope of the field 
of exobiology had just concluded. (The results of these workshops are published 
in three NASA Special Publications, SP-476, SP-477, and SP-478, listed at the 
end of Chapter 1.) It was thus an opportune time to connect the basic science 
objectives defined by these three efforts to the spaceflight missions that were 
being contemplated for the remainder of this century and that could be used to 
realize the science objectives. The primary focus was on missions sponsored by 
the National Aeronautics and Space Administration (NASA) and the European 
Space Agency (ESA). Necessarily, only those opportunities which were known 
at the time of the Workshop meetings were considered; subsequent missions 
were omitted not because of lack of interest, but rather because we were not 
aware of them. The January 1986 Challenger accident has led to uncertainty in 
the timetables for the various Earth-orbital missions discussed in this report, and 
in some cases to the vehicle that will be used to launch them. This must be kept 
in mind; while the basic scientific objectives and plans described in this report 
are still valid, the specifics of their implementation are subject to change. 

The Workshops on Exobiology in Earth Orbit should be viewed as the second 
step of a three-part process, each step of which increases the number of people 
who are thinking about the role of Earth-orbital space missions in the field of 
exobiology. The first step was a series of informal meetings held at NASA's 
Ames Research Center during 1982-83 that included primarily Ames investiga- 
tors. The Workshops on Exobiology in Earth Orbit, the second step of the 
process, was composed of about 40 scientists from around the world, including 
astronomers, chemists, biologists, and geologists; the recommendations for 
experiments, summarized in Chapter 6, represent the consensus opinion of this 
particular group of experts. The third step in the process begins with the distri- 
bution of this report. All interested scientists can now consider the classes of 



VII 



experiments recommended by the Workshops, discuss their merits, and formu- 
late their own ideas for experiments to be carried out in Earth orbit. The recom- 
mendations of these Workshops should not be viewed as final, but rather as a 
starting point for further discussions within the general scientific community. If 
exobiologists are inspired to give serious consideration to performing experi- 
ments in Earth orbit, and if nonexobiologists are inspired to look at their work 
from a new point of view, then these Workshops will have succeeded. 

The scientists who provided their time and expertise, attended the meetings, 
discussed the ideas, and who did the thinking, the writing, and the rewriting are 
listed at the end of this Preface. It is they who are responsible for the content of 
this report. The chairpersons of the Workshops were Harold P. Klein from Santa 
Clara University and William M. Irvine from the University of Massachusetts. 
Three science working groups were formed -reflecting the basic nature of exo- 
biological investigation, which includes observation, collection, and simulation. 
These were led by Jill Tarter from the University of California at Berkeley and 
the SETI Institute (Observational Exobiology), Don Brownlee of the University 
of Washington (Cosmic Dust Collection), and David Usher from Cornell Univer- 
sity [In Situ Experiments). John Billingham of NASA Ames provided the prin- 
cipal liaison between the Workshops and the Ames Research Center. The Work- 
shops members benefited from many experts who came to the meetings and gave 
tutorials on specific aspects of Exobiology in Earth Orbit. They are; 

Roger Arno, NASA Ames Research Center (spacecraft opportunities) 

Martin Barmatz, Jet Propulsion Laboratory (containerless processing) 

Peter Banks, Stanford University (Space Station) 

Bill Berry, NASA Ames Research Center (spacecraft opportunities) 

Don Brownlee, University of Washington (cosmic dust) 

Ted Bunch, NASA Ames Research Center (cosmic dust collection) 

Graham Cairnes-Smith, Glasgow University (origin of life) 

Sherwood Chang, NASA Ames Research Center (cosmic history of the 
biogenic elements and compounds) 

Martin Cohen, University of California at Berkeley (proto-planetary systems) 

Robert Davies, University of Pennsylvania (panspermia) 

Don DeVincenzi, NASA Headquarters (exobiology) 

Mike Duke, NASA Johnson Space Center (lunar bases) 

Mayo Greenberg, University of Leiden (bacterial survival in grains) 

Gerda Horneck, Institut fur Flugmedizen, Koln, West Germany (microb- 
ial survival in space; ESA activities) 

William Kinard, NASA Langley Research Center (Long-Duration Expo- 
sure Facility) 

Michael Lampton, University of California at Berkeley (the space 
environment) 



vu I 



Joseph Nuth, NASA Goddard Spaceflight Center (small-particle nuclcation) 
Jeff Scargle, NASA Ames Research Center (planetary detection) 
Jill Tarter, University of California at Berkeley and SETI Institute (observa- 
tional opportunities) 
Chandra Wickramasinghe, University College, Cardiff (panspermia) 

We are indebted to all of these people for their time and effort. Additional 
thanks are due to Bill Berry and Mike Duke who contributed written summaries 
of their tutorials that are included in Chapter 2 of this report. The many logis- 
tical details that go along with running meetings which exceed 50 attendees, all 
of whom need to be housed, fed, reimbursed, and aided in a myriad of ways, 
were efficiently handled by Wanda Davis from the Molecular Research Institute. 
We also owe special thanks to Vera Buescher and Elyse Murray of the SETI 
Institute who provided invaluable assistance. Finally, we thank Guy Fogclman of 
RCA Government Services for reading the full manuscript. 

Doug DeFrees 
Molecular Research Institute 



IX 



Workshop Members 



Harold P. Klein, Chairperson 
Department of Biology 
Santa Clara University 
Santa Clara, CA 95053 

William M. Irvine, Co chairperson 
Department of Physics and Astronomy 
University of Massachusetts 
Amherst, MA 01 003 

Lou Allamandola 

Mail Stop 245-6 

NASA Ames Research Center 

Moffett Field, CA 94035 

Akiva Bar-Nun 

Department of Geophysics and 

Planetary Science 

Tel Aviv University 

Ramat Aviv 

69 978 Tel Aviv, ISRAEL 

Michael J. S. Belton 

Kitt Peak National Observatory 

P.O. Box 26732 

Tucson, AZ 85726 

Albert L. Betz 
Space Science Laboratory 
University of California 
Berkeley, CA 94720 

John Billingham 

Mail Stop 239-11 

NASA Ames Research Center 

Moffett Field, CA 94035 



John Harry Black 
Steward Observatory 
University of Arizona 
Tucson, AZ 85721 

Thomas Brock 
Department of Bacteriology 
University of Wisconsin 
Madison, Wl 53706 

Donald E. Brownlee 
Department of Astronomy 
University of Washington 
Seattle, WA 98195 

Ted Bunch 

Mail Stop 239-4 

NASA Ames Research Center 

Moffett Field, CA 94035 

A. G. Cairns-Smith 
Department of Chemistry 
Glasgow University 
Glasgow G1 2 8QQ 
SCOTLAND 



Glenn Carle 

Mail Stop 239-12 

NASA Ames Research Center 

Moffett Field CA 94035 

Sherwood Chang 

Mail Stop 239-4 

NASA Ames Research Center 

Moffett Field, CA 94035 



Martin Cohen 

Radio Astronomy Laboratory 
University of California 
Berkeley, CA 94720 

Robert E. Davies 

Department of Astronomy and 

Astrophysics 

University of Pennsylvania 

Philadelphia, PA 19104 

Douglas J. DeFrees 
Molecular Research Institute 
701 Welch Road, Suite 213 
Palo Alto, CA 94304 



Eric Herbst 

Department of Physics 
Duke University 
Durham, NC 27706 

Larry Hochstein 

Mail Stop 239-10 

NASA Ames Research Center 

Moffett Field, CA 94035 

Gerda Horneck 

Institut fur Flugmedizin 

DFVLR 

LinderHohe, 5000 Koln 90 

WEST GERMANY 



Donald DeVincenzi 

Mail Stop 239-11 

NASA Ames Research Center 

Moffett Field, CA 94035 



Wes Huntress 
Jet Propulsion Laboratory 
4800 Oak Grove Drive 
Pasadena, CA 91 103 



Therese Encrenaz 
Observetoire de Paris Meudon 
92195-Meudon Principal Cedex 
Meudon, FRANCE 

Neal Evans 

Astronomy Department 
University of Texas 
Austin, TX 78712 

Paul D. Feldman 
Department of Physics 
Johns Hopkins University 
Baltimore, MD 21218 

J. Mayo Greenberg 
Huygens Laboratory 
University of Leiden 
Wassenarseweg 78 
RA Leiden 2300 
NETHERLANDS 



John Kerridge 
Institute of Geophysics and 
Planetary Science 
University of California 
Los Angeles, CA 90024 

Chris McKay 

Mail Stop 239-12 

NASA Ames Research Center 

Moffett Field, CA 94035 

Stanley Miller 

Department of Chemistry, B-01 7 
University of California, San Diego 
La Jolla,CA 92093 

Ken Nealson 

Center for Great Lakes Studies 
600 East Greenfield Ave. 
Milwaukee, Wl 53204 



XII 



Al Nier 

University of Minnesota 

School of Physics and Astronomy 

116 Church Street, S.E. 

Minneapolis, MN 55455 

Joe Nuth 

Chemical Dynamics Inc. 
8251 Parkham Court 
Severn, MD 21144 

John Oro 

Department of Biochemistry and 
Biological Sciences 
University of Houston 
Houston, TX 77004 

Tom Roellig 

Mail Stop 245-6 

NASA Ames Research Center 

Moffett Field, CA 94035 

Tom Scattergood 

Mail Stop 239-4 

NASA Ames Research Center 

Moffett Field, CA 94035 



Jill Tarter 

Mail Stop 229-8 

NASA Ames Research Center 

Moffett Field, CA 94035 

Xander Tielens 
Astronomy Department 
University of California 
Berkeley, CA 94720 

David A. Usher 
Department of Chemistry 
Cornell University 
Ithaca, NY 14853 

Joe Veverka 
Space Sciences Building 
Cornell University 
Ithaca, NY 14853 

John Wolfe 

Mail Stop 239 12 

NASA Ames Research Center 

Moffett Field. CA 94035 



XIII 



Executive Summary 



One of the principal products of these Workshops is the list of experiments 
which the members recommend and which is summarized in Chapter 6. A brief 
list of these recommended experiments is given here. Those listed for the Obser- 
vational Exobiology section are in order of decreasing priority as determined by 
a consensus of the Workshop members. Otherwise, no priorities are implied by 
the order in which the experiments are listed. 

Observational Exobiology 

1 . Search for extrasolar planetary systems 

2. Study star-forming regions in the galaxy— analogs for the solar nebula 

3. Study comets, asteroids, Titan, and the giant planets in our own solar 
system 

4. Study the organic chemistry of interstellar molecular clouds 

Cosmic Dust Collection 

1. Develop and implement capture techniques which preserve biogenic 
material 

2. Develop and implement techniques to determine the orbits of dust 
particles 

3. Refine laboratory methods for the analysis of small particles 

In Situ Experiments 

1. Study the formation, condensation, aggregation, and surface chemistry of 
suspended dust grains 

2. Create, release, and monitor an artificial comet in space 

3. Determine the viability of microorganisms in space 



xv 



Chapter I 
Biology and the Space Sciences 



H. P. Klein 



It became more and more apparent, as data from the two Viking landers on 
Mars began to accumulate during 1976 and 1977, that Mars is an inhospitable 
planet for life. The excitement generated by initial, presumptive indications of 
metabolic activity gave way under subsequent experimentation, both on the 
Martian surface and in ground-based laboratories, to the realization that simple, 
unanticipated, physico-chemical rather than biological processes were probably 
the basis for the observed phenomena. To many this signaled an attenuation, if 
not the end, of interest by biologists in conducting research in the space 
sciences. From time to time "exobiology," i.e., the study of extraterrestrial 
organisms, has been referred to as "an endeavor to study that which is nonexis- 
tent," and the Viking results seemed to end any hope of studying the biota of 
another planet. The conclusion was drawn, by some, that biologists involved 
with such matters would now withdraw from participation in space exploration 
and redirect their efforts to terrestrial biology. 

Proponents of this view were mistaken, however, because they did not under- 
stand the biological context within which the search for life on Mars was carried 
out. For the biologist, the Viking mission was an important test of ideas about 
how life arises from relatively simple nonbiological materials. These ideas have as 
their central theme the concept that living systems arise through a process of 
chemical evolution, a process in which molecules of increasing complexity are 
produced under the influence of natural energy sources until a stable, self- 
reproducing system is established. According to this view, simple compounds 
containing the "biogenic" elements— carbon, hydrogen, nitrogen, oxygen, sulfur, 
and phosphorus— condense under appropriate conditions to form the direct pre- 
cursors of living matter, ultimately resulting, with further chemical modifica- 
tions, in replicating organisms. 

Once it became clear that direct analysis of other objects in the solar system 
would become feasible through the use of spacecraft technology, attention 



The Martian surface, as seen by the Viking lander. 



naturally turned to Mars as the most accessible, "Earth-like" extraterrestrial 
object upon which to test this theory of chemical evolution. Before the Viking 
mission it was assumed that organic materials, produced either photochemically 
from the Martian atmospheric components or derived from meteoritic infall 
from the neighboring asteroid belt, would be present on the Martian surface. 
Indeed, one of the key objectives at that time was to ascertain the level of com- 
plexity of these putative organic compounds. Did they include organics related 
to terrestrial biological matter? Had chemical evolution on Mars advanced to the 
point of producing replicating molecules? To biologists the search for life on 
Mars was characterized by these and related questions, and not solely by a quest 
for evidence of living or fossil "Martians." In retrospect, the Viking results 
underscored how naive our assumptions were about the ease with which repli- 
cating organic systems are formed and evolve on a planetary body. The results 
emphasize how much additional knowledge we need about chemical and bio- 
logical evolution. 

These uncertainties, of course, are applicable when contemplating the origins 
of life on our own planet. The answers to many of the key questions in our 
scenario of chemical evolution still elude us. How and where did the carbon- 
containing precursors of terrestrial biology arise? What contributions to the 
inventory of such precursors were made by comets, asteroids, or cosmic dust 
during or after the accretion of the Earth? Indeed, what is the history of the bio- 
genic elements themselves? Can we trace their origins and subsequent interac- 
tions back through pre-solar system epochs to cosmic dust and interstellar 
gases? During the past three decades, considerable strides have been made by 
chemists and biochemists in demonstrating the relative ease with which the 
biogenic elements can condense to form complex, biologically important organic 
compounds. However, we are on tenuous ground when we try to pin down these 
chemical processes in time and space. How much of the process took place on 
this planet? What relationships exist between the organic compounds found in 
the interstellar medium and those found in meteorites, comets, other solar 

system objects, and the Earth? 

The open issues regarding the accumulation of the organic material that set 

the stage for the appearance of replicating molecules on Earth are further 
obscured because our models for the early Earth preceding and during the period 
when organic chemical evolution "went critical" (i.e., gave rise to reproducing 
chemicals) are relatively imprecise. Not only is it important to understand the 
physical environment within which replicating systems first appeared, but such 
knowledge is equally necessary to gain insight into the earliest stages of biologi- 
cal evolution— that is, how life, once initiated on Earth, was sustained and 
became diversified. Uncertainties exist in specifying the history of the terrestrial 
atmosphere. How, and over what time span, did the Earth's volatiles accumu- 
late? Was the atmosphere strongly reducing or more oxidized at various stages in 



the process? Just what is the detailed history of molecular oxygen as a compo- 
nent of the Earth's atmosphere? For our models of the prebiological era on 
Earth, what are we to assume to be the history of the flux of solar (and other?) 
radiations reaching the surface? What is the possible significance of targe-body 
impacts with the Earth during some or all of this phase of evolution? 

From the foregoing discussion, it should be clear that the intellectual content 
in the field of exobiology goes far beyond attempts to detect life on another 
planet. Thus, while exobiology has historically been narrowly viewed as the 
search for extraterrestrial life, in point of fact, the field today is better described 
as an interdisciplinary science devoted to the study of evolutionary biology. As 
such, it encompasses the origins and history of the major elements required for 
life; their processing in the interstellar medium and in protostellar systems; 
their incorporation into organic compounds on the primitive Earth and on other 
celestial objects; the interactions of an evolving planet with the evolution of 
complex organic compounds; the conditions under which chemical evolution 
resulted in replicating molecules; and the subsequent interactions between an 
evolving biota and further planetary evolution. To implement the objectives of 
this discipline, investigators in the field are studying different aspects of the 
evolutionary process in order to synthesize from these studies a plausible "road 
map" that leads from the origin of the universe to the establishment of a sus- 
tained biota on Earth. It is reasonable to expect that biologists will acquire new 
and important information in the future from ground-based studies of terrestrial 
and extraterrestrial materials, as well as from laboratory demonstrations of 
critical chemical and biochemical pathways involved in chemical evolution. 
Moreover, one can readily assume that telescopic probing of the solar system 
and beyond by physical scientists will provide fresh insights into many of these 
issues. However, successful implementation of the broad program of inquiry that 
constitutes modern exobiology requires that biologists interact directly with 
astronomers, astrophysicists, atmospheric chemists, geochemists, and other 
physical scientists in order to resolve many of the open questions in this field. In 
this regard, the opportunities provided by space technology are especially 
intriguing. Direct measurements of the compositions of the atmospheres and/or 
surfaces of objects such as comets, Jupiter, Titan, Saturn, Neptune, and 
Uranus— all of which are known to contain at least simple organic molecules— 
and investigations of the chemistry of carbonaceous asteroids, can provide 
valuable insights into the nature of organic chemical evolution within the solar 
system. Detailed analysis of solar system objects, particularly the Moon and 
Mars, including careful assessment of their cratering histories, could advance our 
understanding not only of the early history of the solar system during the 
prebiological era on Earth, but also of later epochs throughout biological 
evolution. 



With the various space missions now under development and on the drawing 
boards, both in the United States and abroad, it is clear that much progress is to 
be expected in exobiological science. It thus seemed important to study the 
extent to which Earth-orbiting space missions, such as the Space Station, might 
make useful contributions. A priori, it would appear attractive to utilize such 
permanently orbiting facilities to observe a wide range of objects, from inter- 
stellar clouds to solar system bodies; to collect cosmic materials for subsequent 
analysis; and to conduct in situ experiments, taking advantage of the natural 
environment of space. The subsequent sections of this publication are devoted to 
a critical examination of these possibilities, as well as to a consideration of other 
ways in which they could provide definitive insights for exobiological science. 



4 



Suggestions for Further Reading 

Ezell, E. C; and Ezell, L. N.: On Mars: Exploration of the Red Planet, 
1 958-1 978. NASA SP-421 2, 1 983. 

Klein, H. P.: The Viking Mission and the Search for Life on Mars. Rev. 
Geophys. Space Phys., vol. 17, 1979, p. 1655. 

Billingham, J., ed.: Life in the Universe. NASA CP-2156, 1981. 

Wood, J. A.; and Chang, S., eds.: The Cosmic History of the Biogenic Ele- 
ments and Compounds. NASA SP-476, 1985. 

Hartman, H.; Lawless, J. G.; and Morrison, P., eds.: Search for the Universal 
Ancestors. NASA SP-477, 1985. 

Milne, D. H,; Raup, D. M.; Billingham, J.; Niklas, K.; and Padian, K., eds.: 
The Evolution of Complex and Higher Organisms. NASA SP-478, 1 985. 








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V 




o 



Chapter II 

Observational and Experimental 
Opportunities in Earth Orbit 



W. Berry, M. Duke and J. Tarter 



As a prelude to the discussion of scientific questions and the Earth-orbital 
investigations that can be applied to answer them, it is appropriate to briefly 
describe the Earth-orbital opportunities available to the exobiologist. These have 
been limited to those projects which are already proposed and which have some 
probability of going into operation before the end of this century. An excep- 
tion, the lunar base, which may be operating in the first to second decade of the 
21st century, has been included as it has particularly intriguing possibilities for 
exobiological investigations. There are five classes of Earth-orbital flight oppor- 
tunities: Space Shuttle (Space Transportation System (STS)) laboratories, 
spacecraft deployed from the Shuttle, spacecraft deployed from expendable 
launch vehicles (ELVs), the Space Station, and lunar bases. 

2.1 Free-Flying Spacecraft and the Space Shuttle 

The ELV a program provides access to a variety of orbital inclinations and 
altitudes. (Until the Shuttle is operational out of Vandenburg Air Force Base, 
California, these vehicles offer the only means of reaching orbits of greater than 
57° inclination, such as polar and sun-synchronous orbits.) Current U.S. civil 
ELVs can insert from 200 kg into low Earth orbit with the Scout, to 2000 kg 
into the higher-energy geostationary orbit with the Atlas-Centaur. NASA man- 
ages payload launches with these ELVs and with the intermediate capability 
Delta and Atlas-F systems. Commercial firms are also marketing launch services 
with the expendable vehicles which supplement NASA's ELV capability. The 



A complete list of abbreviations is included in Appendix A. 



An Artist's concept of the Space Station in Earth orbit 



7 



Air-Force-managed Titan 34D (which can insert 5000 kg into geostationary 
orbit), the French-managed Ariane, and the U.S.S.R. Proton ELVs are also 
potential launch vehicles. 

The Space Shuttle can reach an altitude of 600 km with a payload the mass 
of the Space Telescope (11,600 kg) and will be able to reach more than twice 
this altitude if plans go forward to add additional Orbital Maneuvering System 
kits. The Shuttle will be able to carry 30,000 kg to a 400-ktn orbit and half this 
into a polar orbit when the western launch site (Vandenburg) begins supporting 
launches. A variety of payloads can be carried and either retained aboard the 
Shuttle during the mission or deployed into space or into Earth orbit. If 
retained, the payload will be functional only during the 7-day period typical of 
orbital operations. Small payloads may be flown as payloads of opportunity in 
the Shuttle cargo bay as "getaway specials" (GAS) or as part of the Hitchhiker 
program on the orbiter mid-deck, provided that they have minimal resource 
needs. More complex and/or larger payloads fly in the 60- by 15-foot Shuttle 
cargo bay as part of a Spacelab manifest, either inside Spacelab or exposed to 
space on a pallet. Either configuration has access to the extensive services 
provided by Spacelab, such as communications, fluid loops, and control panels. 
More complex, and likely more costly, independent payloads using a portion or 
all of the bay can also be considered; these could be extended beyond the bay 
once orbit is attained. 

Spacecraft deployed from the Shuttle provide long exposures, flexibility in 
orbit selection, and the possibility for retrieval. These payloads can be visited 
periodically by the Shuttle for servicing, retrieval and replacement of experiment 
packages, and recovery for return to Earth. Another option would be to share a 
spacecraft performing another primary task; this option relies on an already 
approved mission which would not be compromised by the addition of an exo- 
biology experiment. Drawbacks to this approach are that the orbit, altitude, 
resource availability, etc., are dictated by the host. 

Greater flexibility is provided by sharing accommodations on a spacecraft 
designed to support multiple tasks, in that the accepted payloads are roughly 
equal in priority; such spacecraft will be available in the near term. In order of 
increasing capability they include NASA's Long Duration Exposure Facility 
(LDEF), Spartan, and the European Retrievable Carrier (EURECA). LDEF 
will be flown in a series of missions providing exposure to space for a number of 
self-contained experiments; i.e., the spacecraft does not provide power, propul- 
sion, communication, or pointing. (LDEF I was deployed in April 1984 and 
because of the Challenger accident has yet to be retrieved.) After exposure to 
the space environment for a period of up to several years, LDEF is retrieved by 
the Shuttle and returned to the ground for experiment evaluation. NASA's 
Spartan is an autonomous package intended for short, dedicated, space astron- 
omy missions, but can be used for other types of experiments. It is deployed 
from the Shuttle and then retrieved after approximately 100 hours in space. 



8 



EURECA, managed by the European Space Agency (ESA), will be launched by 
the Shuttle and is intended to perform a number of experiments within defined 
volume, power, and telemetry capabilities. The first EURECA is manifested for 
deployment in 1990, with retrieval and return to Earth about 6 months later. 
The experiments for this EURECA have already been selected, but a second 
EURECA is expected to fly 18 months later, and there are plans to support 
exobiology experiments with this flight. 

There are thus a wide variety of opportunities in Earth orbit that exobiolo- 
gists can potentially exploit during the next 15 years using either expendable 
launch vehicles or the Space Shuttle. The vehicle chosen to carry out any inves- 
tigation will depend on the services required, the science objectives, and the 
anticipated funds available. 

In addition to the orbital spacecraft providing permanent or retrievable 
platforms for exobiology experimentation, a number of telescopes are expected 
to be launched in the next two decades. Some of these will be free-flyers, and 
some will be attached to the Space Station or one of its platforms. They will 
have lifetimes ranging from a few years to decades— depending on the plans for 
on-orbit servicing. These telescopes will provide facilities for the exobiology 
community to study the origin and evolution of the biogenic elements and 
compounds remotely. 

Appendix B presents a compilation of the currently envisioned instrumental 
capabilities of the various orbital telescopes. The spectral coverage and approxi- 
mate launch dates are summarized in figure 2-1. It is not yet clear how the 
Challenger accident or the construction of the Space Station will affect these 
specific dates, and they should probably be interpreted as giving only the most 
likely order of launch. Appendix C provides a definition of frequency and wave- 
length equivalents and Appendix D gives the apparent angular scales of various 
objects at specified distances; these will be useful for interpreting figure 2-1 and 
subsequent tables. Table 2-1 enumerates the telescopes being planned by NASA; 
Table 2-2 lists the ESA telescope projects that have been given approval or 
strong support in the long-term planning process and which have invited U.S. 
participation. For every telescope these tables contain, a definition of the mis- 
sion name or acronym, a tentative launch date, and a description of the current 
status of the instrumentation is included. Table 2-1 also lists the NASA Center 
or Research Unit with primary responsibility for the telescope instrumentation. 
Appendix B provides more detail and describes each of the proposed observing 
instruments in terms of its frequency or wavelength coverage, the size of the 
instantaneous field of view that can be imaged, the best spatial resolution that 
can be achieved, the spectral resolving power, and the limiting sensitivity that 
can be obtained with modest integration time. Where applicable, the maximum 
sampling rate and polarization characteristics are also provided. In many 
instances the data are incomplete because instrument packages have not yet 
been chosen or outlined. 



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TABLE 2-1 



NASA Telescopes 



ORBITAL 

IUE 



HST 



ASTRO 



COBE 



EUVE 



GRO 



AXAF 



International Ultraviolet Explorer (1977) [Goddard Space Flight 
Center] . This spacecraft has outlived its original planned lifetime, 
but it continues to function and is supported by an active guest 
investigator program. Longevity is unpredictable, no plans exist 
to decommission the spacecraft. 

Hubble Space Telescope (1989) [Space Telescope Science Insti- 
tute] . The first generation of instruments is constructed and 
integrated into the telescope, awaiting launch. These instruments 
are detailed in Appendix B. The next generation of instruments 
(to be installed on orbit ~1994) have been selected for competi- 
tive development during 1986-88. All three selected instruments 
are described in Appendix B. 

Hopkins Ultraviolet Telescope (HUT) [Johns Hopkins Univer- 
sity], Ultraviolet Imaging Telescope (UIT) [Goddard Space Flight 
Center], Wisconsin Ultraviolet Photopolarimetry Experiment 
(WUPPE) [University of Wisconsin] . These instruments have been 
finalized by their respective proposers and will be flown at least 
twice as a package Shuttle payload. Collaboration with the prin- 
cipal investigators as a guest investigator is possible and will be 
solicited by an announcement of opportunity (AO) for flights in 
1989. 

Cosmic Background Explorer (1989) [Goddard Space Flight 
Center]. The instrument complement is finalized and is detailed 
in Appendix B. 

Extreme Ultraviolet Explorer (1991) [University of California 
Berkeley Space Sciences Laboratory]. The instrument comple- 
ment is finalized and is detailed in Appendix B. 

Gamma Ray Observatory (1990) [Marshall Space Flight Center] . 
The instrument complement is finalized and is detailed in Appen- 
dix B. 

Advanced X-ray Astrophysics Facility (1994) [Goddard Space 
Flight Center] . The instrument complement has been selected on 
the basis of a competitive AO, but implementation is not yet 



11 



complete and some modification may still be possible. The 
selected instrumentation is detailed in Appendix B. 

FUSE Lyman Far Ultraviolet Spectroscopy Explorer (LYMAN) (1995). 

Potential Explorer-class mission if collaboration can be secured 
to provide cost-sharing. A strawman instrument complement has 
been defined for planning purposes, no AO has yet been issued. 
The strawman instrument complement is listed in Appendix B. 

Block I Initial Configuration for the Space Station (1995). Phase A 

studies of the Block I Space Station have been completed. The 
final definition of the module structures and positioning and the 
selection of the instrumentation associated with these modules is 
expected to be completed during 1988. 

SI RTF Space Infrared Telescope Facility (1996) [Ames Research Cen- 

ter]. The instrument complement has been selected on the basis 
of a competitive AO, but implementation is not yet complete and 
some modification may still be possible/The selected instrumen- 
tation is detailed in Appendix B. 

LDR Large Deployable Reflector (2000) [Jet Propulsion Laboratory 

and Ames Research Center] . In preproject phase, waiting phase A 
study monies. Strawman instrument complement detailed in 
Appendix B. 

OVLBI Orbiting Very Long Baseline Interferometry (1998) [Jet Propul- 

sion Laboratory]. Possible Explorer-class mission. In preproject 
phase, awaiting phase A study monies. Strawman instrument 
complement detailed in Appendix B. 

AIRBORNE 

KAO Kuiper Airborne Observatory (now) [Ames Research Center]. 

Telescope is a permanent fixture; new backends are supplied by 
each observer. 

3M Balloon Balloon-Borne Three-Meter Telescope for Far-Infrared and Sub- 
millimeter Astronomy (1989) [Smithsonian Institution Astro- 
physical Observatory, University of Arizona, and University of 
Chicago]. One of three balloon programs under consideration. 
The instrument complement is finalized and is detailed in Appen- 
dix B. 



12 



SOFIA 



Stratospheric Observatory for Infrared Astronomy (1992) [Ames 
Research Center]. A 3.5-m telescope proposed to be flown on 
a 747SP aircraft as a replacement for the very successful KAO 
during the interval prior to SI RTF. Backend instrumentation 
supplied by observers. Some performance specifications are given 
in Appendix B. 



ORBITAL 



TABLE 2-2 



ESA Telescopes with Potential for U.S. Collaboration 



ISO 



Infrared Space Observatory (1992). Approved mission. The 
instrument complement has been selected on the basis of a 
competitive AO, but implementation is not yet complete and 
some modification may still be possible. The selected instrumen- 
tation is detailed in Appendix B. 



FIRST Far Infrared Space Telescope (1995). One of the four corner- 

stones of ESA's long-term plans for space missions. Mission not 
yet approved and the telescope may turn into an interferometer. 
Strawman instrument complement described in Appendix B. 

XMM X-ray Multiple Mission (1993-2004). A series of missions that 

form one of the cornerstones of ESA's long-term plans for space 
missions. Intended to complement NASA's AXAF (which excels 
at spatial resolution) by providing excellent spectral resolution 
from soft to hard X-ray regime. No missions yet approved. 

COLUMBUS Space-Station-related modules. This label covers ESA efforts to 

define which of their spacecraft and on-orbit experiments could 
benefit from a human-tended station. 



2.2 Space Station 

The Space Station is now envisioned as an evolving facility, with the first 
phase, called Block I, scheduled to be functioning during the mid- to late-1990s. 
The exact configuration of the station and the experiments that will be included 
in Block I have yet to be determined. The station configuration and its evolution 
from Block I to Block II appear to be well defined, but decisions on science 
issues have not been made; the summary that follows will require updating. 



13 



The Space Station is not just one structure, but is composed of four basic 
components: the manned core, free flyers tended by the station, co-orbiting 
platforms, and polar orbiting platforms. The core is by far the largest and most 
complex element. It must provide the main structure to which the Shuttle will 
dock and in which the astronauts will live. It will be constructed in a low Earth 
orbit at an estimated altitude of 500 km with an orbital inclination of 28.5° to 
the equator. Block I will consist of a 135-m horizontal boom with photovoltaic 
arrays at each end to produce 50 kW of power. A single polar orbiting platform 
for Earth remote sensing will also be part of the Block I configuration. At the 
center of mass will be two interconnected pressurized modules. One of these 
modules will be for the six crew members, providing living quarters, storage 
areas, and communications, and one will be a laboratory. It is anticipated that 
the modules will later be joined by two additional laboratory modules, one from 
Japan and one from Germany. Block II will add two horizontal towers with an 
upper cross boom for mounting attached payloads and telescopes and a lower 
boom for Earth-looking instruments. The power will be increased to 75 kW. 
Solar dynamic arrays and a satellite servicing facility for free flyers and 
co-orbiting platforms will be added. 

The free-flying or attached platforms provide a shared environment for instru- 
mentation modules, which can be periodically serviced and changed out. The 
co-orbiting free-flying platforms will be tended by the station crew, but the 
polar orbit platforms will be serviced by the Shuttle. The free flyers provide 
their own power, and have generally been derived from the scientific satellites 
and telescopes that have been planned in advance of the Space Station concept. 
In addition, platforms can be attached to the Space Station boom, providing 
greater ease in servicing and experiment retrieval, though in a more contami- 
nated environment. These platforms generally provide their own power, but 
experiment control and telemetry is possible through sharing of Space Station 
facilities. The Space Station offers the possibility that the platforms can be 
tended or retrieved by the crew using an orbital maneuvering vehicle (OMV) that 
is being developed in parallel with the Space Station, but will not be functional 
at Block I. Another parallel development is the orbital transfer vehicle (OTV), 
required to place free flyers or platforms in higher orbits, up to geosynchronous 
orbit. 

The scientific community has been quick to review the potential offered by 
the Space Station. With the caveat that sufficient funds be provided to continue 
the orderly development of the well-planned, long-term space missions, some of 
which are already under way, they have endorsed the use of the Space Station 
for many of the other missions and have conceived of new experiments that take 
specific advantage of the Station environment. Competition for space in Block I 
laboratory modules or on platforms is severe. The following list presents the 
facilities of interest to the exobiology community. These items have high 
priority, and should remain in the final version of Block I for 1993 to 1996. 



14 



Tentative Space Station Complement at Block I 

CORE 

Astrometric Telescope Facility 
Life Sciences Research Facility 
Microgravity and Materials-Processing Facility 
Cosmic Dust Collector 

FREE FLYERS 
HST 
GRO 
AXAF 

CO-ORBITING PLATFORMS (NUMBER OF PAYLOADS UNDER REVIEW) 
SIRTF 

POLAR PLATFORM(S) 



2.3 Lunar Base 

A lunar base is a natural extension of NASA's Space Station program, tech- 
nically and programatically, and could be in its initial stages of implacement in 
the first decade of the 21st century. Technically, the lunar base can utilize or 
build directly upon many of the systems developed for Space Station. Pro- 
gramatically, the construction of a lunar base fits logically after the development 
of Space Station and OTV technology. 

The rationale for developing a lunar base is to advance scientific understand- 
ing, to learn to utilize lunar material and the lunar environment beneficially in 
the development of the space infrastructure, and to develop the capability for 
extended, even permanent, habitation in space. Investigations into the origin and 
history of the Moon can uniquely be carried out at a lunar base and these may 
help us to better understand the early history of the Earth and the prevailing 
conditions that lead to the origin of life. Long-baseline radio interferometers 
and very large optical arrays can improve upon currently planned orbital obser- 
vatories in both sensitivity and spatial resolution. These are crucial to the study 
of the origin and evolution of the biogenic elements and compounds; particu- 
larly, attempts to image and spectroscopically examine the atmosphere of a 
distant planet, or to study the formation of a nearby planetary system and 
understand the processes by which complex interstellar organic compounds 
might survive their inclusion into the protoplanetary nebula. New investigations 
into the properties of matter may be possible utilizing the high vacuum and 
extremes of temperature that may be easily maintained on the Moon for very 
long times. 



15 



Among the exobiological studies that can be carried out uniquely at a lunar 
base are the direct search for evidence of life forms in interplanetary space 
(panspermia); determination of the quantity and form of organic and biogenic 
compounds in lunar soil (in a very-low-organic-background laboratory environ- 
ment); study of the survivability of terrestrial organisms in the lunar environ- 
ment; study of the frequency history of cometary impacts on the Moon (and, 
consequently, Earth); gathering of evidence pertinent to the theory of episodic 
extinctions on Earth related to cosmic events; and conduct of astronomical 
investigations of interest to the exobiology community such as those previously 
mentioned. Also, the search for extraterrestrial intelligence (SETI) may require 
the radio-quiet lunar far side in order to conduct a sufficiently sensitive search, 
shielded from terrestrial interference. 

According to current concepts, the initial lunar base will have aspects of the 
Earth-orbit Space Station from which it will evolve. The early base will have geo- 
logical and biological laboratory facilities focusing on understanding how to best 
utilize the lunar materials and environment for scientific investigations and life- 
support functions. As production capability grows at the lunar base, new facili- 
ties may become available for research. This includes very large, very high 
vacuum chambers; very high temperature and very low temperature systems of 
substantial dimensions; chambers of very low natural magnetic field; very low 
natural radiation background facilities (buried facilities, with natural-material 
barriers constructed from uranium-potassium-free lunar materials); facilities 
nearly totally free of light (organic) elements and free or capable of being iso- 
lated from human-introduced biogenic compounds; and laboratories free of 
metallic contaminants found in many terrestrial environments. All of these 
should be rather easily developed on the Moon and can be made available inex- 
pensively once the initial base is able to govern its own growth. 



16 



Suggestions for Further Reading 

Mendell, W. W., ed.: Lunar Bases and Space Activities of the 21st Century. 
Lunar and Planetary Institute, Houston, 1985. 

Clark, L. G.; Kinard, W. H.; Carter, D. J., Jr.; and Jones, J. L., Jr.: The 
Long Duration Exposure Facility (LDEF), Mission 1 Experiments. NASA 
SP-473, 1984. 

Banks, P. M.; Black, D. C; and Hudson, H. S.: Space Station Summer Study 
Report. NASA, 1986. 

Harwit, M.; and Neal, V.: The Great Observatories for Space Astrophysics. 
NASA Astrophysics Division, Washington, 1985. 



17 




-- , . s > . ;». JT*> «_— 



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Chapter III 
Observational Exobiology 



M. Belton, A. Betz, J. Black, M. Cohen, T. Encrenaz, 

N. Evans, P. D. Feldman, E. Herbst, J. Kerridge, 
C. McKayJ. Nuth, T. Roellig, T. Scattergood, J. Tarter, 

A. Tielens, and J. Veverka 



Since life (as we know it) depends upon its environment for development 
and survival, the origin and evolution of life are integral parts of the physical and 
chemical processes that govern the formation and evolution of the planets. The 
formation and evolution of a planet is intimately bound to the evolution of its 
sun, a star. Stars themselves are the manufacturing plants for all of the biogenic 
elements; their birth and death cycle is governed by the laws of physics in this 
universe. Thus it should be no surprise that in attempting to understand the 
origin and evolution of the biogenic elements and compounds, the exobiology 
community has, over the past decade, developed an increasing interest in the 
results of astronomical observations. Much of what is known or conjectured 
about the processes that led to the abiotic chemical evolution of organic matter 
in the vicinity of the planet Earth was derived from the observations conducted 
by astronomers and astrophysicists. The observational results have been com- 
bined with knowledge gained from the collection and analyses of pristine mate- 
rials of terrestrial and extraterrestrial origin, and have been supported by labora- 
tory attempts at chemical synthesis and theoretical models of complex 
chemistries in various environments. There now appears to be a marginally self- 
consistent outline of the pathways leading from the stellar nucleosynthesis of 
carbon, oxygen, nitrogen, phosphorus, sulfur, and other biologically significant 
trace elements to their inclusion into planetesimal sized bodies within the young 
protosolar nebula. While this outline is self-consistent, it is far from complete, 
and many of the most interesting details remain to be supplied by more obser- 
vational, experimental, and theoretical effort. 



Hubble Space Telescope 



19 



3.1 Cosmic History of the Biogenic Elements and Compounds 

The principal matter of the early universe was hydrogen and helium (fig. 3-1). 
Local density concentrations in the overall expansion of the young universe led 
to the gravitational contraction of galactic-mass gas clouds within which further 
fragmentation and further collapse led to the formation of the first generation 
of Milky Way stars. Or, perhaps the stars began to collapse first and aggregations 
of these protostellar clouds eventually formed galactic-mass assemblages of gas 
and stars. Either way, in those collapsing stellar-mass fragments where the cen- 
tral temperature rose to ~10 7 K, hydrogen atoms fused to form helium and 
produced a stable, self-luminous, main-sequence star. When the hydrogen fuel 
was exhausted, further contraction of the stellar core raised the temperature to 
~10 8 K, whereupon helium could fuse to form carbon. Eventual depletion of 
helium resulted in further core contraction and an increased temperature until 
oxygen could be fused from carbon, and so on and so on, until the peak of the 
nuclear binding energy curve was encountered at 56 Fe. 

Further fusion reactions being endothermic, the fate of the now-evolved, 
old giant star depends on its initial mass and how much of its outer layers it shed 
with each phase of core burning and gravitational readjustment. Sufficiently 
small stars enter a stable, nonnuclear-burning, white dwarf configuration in 
which the pressure from a degenerate gas provides the needed support against 
further gravitational collapse. Gradually the central temperature drops as the 
white dwarf cools off, evolving into an increasingly unobservable black dwarf; 
it eventually "goes out." From the point of view of the biogenic elements, 
except for the mass that has been lost from the stellar surface along the way, 
such stars represent a graveyard in which the elements are entombed and are 
never again accessible for chemical evolution. However, if the star runs out of 
nuclear fuel and is still sufficiently massive, no stable white dwarf configuration 
is accessible. These massive stars end their life cycle not with a whimper, like the 
white dwarfs, but with a bang. In a spectacular supernova explosion, much of 
the outer mass of the star is hurled back into the interstellar medium while the 
stellar core implodes to another stable configuration: a neutron star or a black 
hole. During this violent stellar demise, enough energy is available to drive the 
endothermic fusion reactions, thereby producing the full repertoire of stable 
elements as well as many unstable isotopes. Thus, the first generation of stars 
seeded the interstellar medium with heavy elements that became incorporated 
into subsequent generations of stars. These then have the potential for somewhat 
more complex nuclear reactions, particularly in the conversion of hydrogen to 
helium. Like their predecessors, some of these later stars return enriched matter 
to the interstellar medium and lock the rest of it away into stable, degenerate 
configurations. Currently, something like a few solar masses of enriched material 
are added to the Milky Way's interstellar medium each year, and the mean metal- 
licity (i.e., the abundance of chemical elements heavier than helium) of the 
galaxy does not appear to have changed much since the collapse of the protosun. 

20 



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ofc N O ^from diffuse . 

of ariP (p , gases <S^^,, ; ^ 

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^ cloud r material 





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cf bioqenic elements: 

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during col lapse 

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some 'ich/ 
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in recto .• in icu . . . Ejectionv P iar * ie ?r 

planetesunals planetesirnals Dfrom r)o.s comets 

in inner solar in outer sok}r v sofar ^ //'/' 

^system • sqstem sustem Ui///s 

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. >C incorporation of . 

chemistru mQS t planetesirnals 

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Figure 3-1. The cosmic history of the biogenic elements and compounds— from 
the Big Bang to the formation of planets. 



21 



On average, it takes about five billion years for the biogenic elements pro- 
duced within one star to become gravitationally bound within a later-generation 
star. Along the way from one stellar interior to the next, the biogenic elements 
cycle in and out of several types of environments within the interstellar medium. 
Here they undergo chemical and isotopic alterations and suffer a large number of 
near misses in the star-formation game. One important type of interstellar envi- 
ronment is the giant molecular cloud. These are observed to be active nurseries 
for stars being born at this time. Each such cloud may have a mass of up to 
1 6 Mq, densities ranging from 1 4 to 1 6 cm -3 , and coot temperatures, ~1 to 
50 K. By the time it is finally dispersed (in part by the energetics of star forma- 
tion), such a cloud will have converted only about 100 M into stars. During 
their lifetimes the giant molecular clouds serve as very effective molecule and 
dust factories powered by radiation fields, hydrodynamical energies of the 
embedded newly forming stars, and interstellar cosmic ray flux. More than 
70 organic molecules have been detected in interstellar space, mostly within 
giant molecular clouds, and the unidentified spectral lines resulting from radio 
observations indicate that the list is incomplete. 

Within the giant molecular cloud complexes it is clear that H 2 is formed on 
the surface of dust grains, but the composition and origin of the dust are much 
debated. Dust is formed when the enriched elemental gas in the outer layers of 
an evolving star is blown off in a stellar wind. Subsequent cooling of the wind 
leads to condensation of grains far from the stellar surface. Grains are also 
formed in the winds that blow off newly formed stars; in the novae outbursts of 
dying stars; perhaps in the rapidly cooling regions behind shock waves propagat- 
ing throughout the molecular clouds; and perhaps, too, in the cooling ejecta of 
supernovae, whose strong shock waves also destroy preexisting dust grains 
throughout a large volume of the interstellar medium. This dust may be com- 
posed of carbon and other biogenic elements if it condensed in the wind from a 
giant carbon star, or it may perhaps provide a catalytic surface for the transfor- 
mation of the biogenic elements and compounds into ever more complex organic 
molecules within the molecular clouds. The dust in molecular clouds is impor- 
tant as a shield against ultraviolet radiation without doubt, but its composition 
and the exact role it plays in the cloud chemistry beyond the formation of H 2 
is still questioned. Even in the event that the dust plays no active role in the gas- 
phase chemistry, it may still be of particular importance to exobiology if it pro- 
vided a vehicle for inclusion of the biogenic elements into the protosolar nebula 
in a highly processed form. There is a growing body of evidence, based on iso- 
topic anomalies, that some interstellar dust has survived its introduction into the 
nebula. Whether manufactured on the surface of the grains or not, some organic 
molecules do coat solid grains of dust and reveal themselves by the vibrational 
spectra observed in the infrared. 



22 



Not all of the gas and dust bound in the initial fragment that collapsed to 
form the Sun was incorporated into the central star, and some that was may 
subsequently have been liberated via a strong T Tauri wind during the Sun's 
infancy. A certain unknown fraction of the collapsing cloud must have been 
incorporated into a toroidal nebulosity orbiting the developing stellar core. 
This solar nebula provided the material from which the solar system planets 
were formed as well as an environment for chemical processing of interstellar 
biogenic elements and compounds into both more and less complex forms. The 
solar nebula flattened into a viscous accretion disk, bounded above and below by 
shocks. Late-arriving interstellar materials experienced more or less traumatic 
chemical processing, depending on the location at which they crossed these 
shock fronts and the initial velocity of the particles falling onto the shocks. The 
picture is particularly unclear with respect to dust grains; some grains coated 
with organic mantles may have entered the solar nebula with their complement 
of biogenic elements unaltered; some grains without complex organic mantles 
may have grown them as a result of the chemistry induced by the added energies 
during the encounter; or some (perhaps all) grains may have been sputtered away 
and converted into their basic elemental components during the shock wave pas- 
sage, only to condense anew within the early nebula itself. That grains were 
present within the early nebula seems undeniable. Dust is observed associated 
with comets which presumably preserve the primitive nebular material ; whether 
the grains are of nebular or presolar origin is unknown. Processing of and on the 
grains did not cease with their inclusion in the solar nebula. Turbulence may 
have cycled the nebular material between extremely diverse thermal locales on 
a time scale that would have resulted in transformed, possibly enhanced, chem- 
ical complexity. Aggregation would have caused material to settle into the mid- 
plane of the accretion disk, and fragmentation of this plane would have led to 
the incorporation of grains and macromolecules into kilometer or larger size 
planetesimals where internal heating and collisions may have strongly metamor- 
phosed the included biogenic elements and compounds. Aggregates of this size 
would have been resistant to the subsequent dispersal of the solar nebula, and 
would eventually have formed into the planets, satellites, asteroids, and comets. 
Some of the nebular material, initially dispersed, might have condensed and 
reentered the solar system as icy comets at a later date. 

Chemical processing in the early solar system, evidenced by certain elemental 
fractionation patterns in primitive meteorites, could also potentially cause small 
isotopic fractionations for some of the elements in those meteorites. Isotopic 
variations are, in fact, observed in such meteorites, but in a significant number of 
cases the nature and/or magnitude of those variations are incompatible with a 
local, i.e., solar system, process and are therefore attributed to presolar process- 
ing. That processing can be either nucleosynthetic or chemical. An example of 



23 



the former is supplied by certain carbon-bearing compounds found in carbona- 
ceous meteorites, characterized by a 12 C/ 13 C ratio of 42, compared with the 
canonical solar system value of 89. An origin by formation in the atmosphere of 
a red giant star is inferred for such grains. An example of evidence for presolar 
chemical processing is the deuterium content of meteoritic organic matter, with 
D/H values over 20 times the galactic value. This is commonly attributed to ion- 
molecule reactions at the low temperatures of interstellar clouds. The presence 
of such material in meteorites is evidence that some presolar organic matter was 
able to survive entry into the solar nebula. 

Suggestions for Further Reading 

Woods, J. A.; and Chang, S., eds.: The Cosmic History of the Biogenic Ele- 
ments and Compounds. NASA SP-476, 1985. 



3.2 Remote Observations 

Most of our information concerning extraterrestrial objects and environments 
comes from the analysis of electromagnetic radiation. The intensity and polariza- 
tion of the radiation, at a particular frequency, in a certain direction, and at a 
specific time, can be measured to provide information of particular interest to 
exobiology, for example: 

1. The existence of extra-solar planets that might serve as suitable hosts for 
the chemical evolution of life 

2. The abundances and distribution of biogenic substances throughout the 
cosmos 

3. The conditions under which complex organic molecules form or are 
destroyed 

4. The conditions under which solids composed of the biogenic elements 
form from gases 

5. The processes in circumstellar, interstellar, and nebular stages of physical- 
chemical evolution which govern the composition and distribution of biogenic 
matter during its transit from stage to stage 

Exobiologists need access to observing platforms in space. Observing from 
spacecraft has clear advantages over ground-based viewing because of the opacity 
of the atmosphere in many regions of the electromagnetic spectrum (fig. 3-2): 
the gamma-ray, X-ray, ultraviolet, parts of the infrared, most of the submilli- 
meter, and parts of the millimeter. Even in those spectral regions where the 
atmosphere is generally transparent, there are specific frequencies where absorp- 
tion or emission by species in the air precludes ground-based observations. 



24 



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Further benefits accrue due to the possibility for improved spatial resolution 
(fig. 3-3). The resolution of a telescope in orbit is limited by the quality of its 
optics rather than by the stability of the atmosphere. Also, in the near-weightless 
environment of low Earth orbit, larger collecting areas are possible, providing 
higher spatial resolution and greater sensitivity. Spatial resolution can also be 
improved by constructing interferometers with much longer baselines than are 
possible on the ground. Sensitivity at wavelengths for which thermal emission 
is important can be improved in Earth orbit relative to the ground by utilizing 
cryogenically cooled telescopes and receivers. The decreased levels of such 
human-caused "noise" as light pollution and radio frequency interference (RFI) 
also can lead to improved sensitivity in orbit. Note that many of the improve- 
ments gained by placing a telescope in low Earth orbit are magnified by going to 
a lunar base, particularly if it is located on the Moon's far side. 

Although gamma-ray measurements are useful for tracing the broad distri- 
bution of interstellar matter and cosmic rays in the galactic plane, and although 
X-ray measurements are useful for studying the very active phases in the devel- 
opment of recently formed stars, they preferentially sample the hottest and 
most energetic environments, as revealed by electronic transitions within atomic 
species. Since these wavelengths cannot directly investigate the biogenic ele- 
ments in their more complex molecular configurations, their utility for exobiol- 
ogy is less immediate than other wavelengths. Absorption line studies in the 
ultraviolet of regions rich in molecules and dust particles are impossible at large 
distances because of extinction by dust. Even with the high-resolution spectro- 
graph on the Space Telescope, only relatively transparent regions with visual 
extinctions A v < ~4 magnitudes (corresponding to foreground hydrogen column 
densities < ~7X10 21 cm -2 ) will be accessible in the ultraviolet at high resolu- 
tion. Even so, these thin regions can yield valuable information about elemental 
abundances, depletion of elements from the gas onto solids, and the chemistry 
of small molecules. Of greatest value to exobiology will be the exploitation of 
those parts of the spectrum in the infrared, submillimeter, and millimeter regions 
that are inaccessible from Earth. These spectral regions typically bracket the 
condition where the excitation temperature of the molecular transitions can be 
provided by the ambient radiation field (hv = kT ex ) for many chemical species 
of exobiological interest for a wide variety of environments. Thus both absorp- 
tion and emission can be observed, depending on the physical conditions. The 
infrared region from 2 to 20 jum has many diagnostic advantages for studying 
dilute matter. Chemically important, nonpolar molecules such as methane, 
acetylene, and carbon dioxide, which lack strongly allowed radio spectra, have 
strong vibrational transitions in this region of the spectrum. They are among the 
simplest molecular forms of carbon, the central element of exobiology, and thus 
they are missing links in our understanding of the chemical evolution of inter- 
stellar and circumstellar matter. Observation at high spectral resolution can 
provide the rotational structure of a vibrational band. It is thus possible to 



26 



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27 



obtain in a single measurement complete information on energy-level popula- 
tion distributions and hence to infer densities, temperatures, and total abun- 
dances accurately for material that is either sufficiently warm to excite these 
transitions or that lies in front of a strong continuum source. Such information 
is more difficult to obtain from isolated measurements of individual rotational 
transitions in the radio regime, although this approach is necessary for the typi- 
cally cold interstellar clouds. In the infrared, submillimeter, and millimeter spec- 
tral regions, it is possible from space to observe chemically important species like 
oxygen and water that cannot readily be studied from Earth because their spec- 
tra are obliterated by atmospheric absorption. 

Spectrometers of high spectral resolution (with resolving power A/AA~ 10 s ) 
are needed for observations of molecular lines to obtain information of diagnos- 
tic value. High spatial resolution is also desirable to investigate phenomena 
occurring both at distant locations within the Milky Way and in other galaxies. 
The study of protoplanetary systems and accretion shocks surrounding them in 
star-forming regions provides a particularly stringent requirement on spatial 
resolution: ~0.01 arcsec (see Appendix C). For the immediate future, this may 
not be available. To achieve both high spectral and high spatial resolution simul- 
taneously requires extremely large apertures (and/or interferometers) to collect 
enough photons. 

The orbital observatories will permit access to the far-infrared and submilli- 
meter regions of the spectrum that have previously been mostly unexplored. The 
required data base of fundamental frequencies and preferred molecular config- 
urations does not now exist to guide the conduct of observations or the inter- 
pretation of data from this wavelength regime. In order to take full advantage of 
orbital observation opportunities, a strong ground-based laboratory research 
program must be pursued. The infrared and submillimeter spectra of many 
biogenic compounds are extremely rich, and a very high degree of accuracy is 
required to allow chemical specificity. Laboratory studies at low pressures and 
temperatures representative of the interstellar environment can provide definite 
predictions of spectral characteristics. However, theoretical calculations are also 
required to deduce the frequencies of the most probable transitions for mole- 
cules and radicals which are too reactive or otherwise not amenable to ordinary 
laboratory techniques. This becomes both increasingly more difficult and more 
important with increasing molecular weight. Laboratory study is also needed to 
understand the properties of the dust grains that were cycled through many 
phases of the stellar and interstellar media before being incorporated into the 
solid bodies of the solar system. These grains may be a primary source of carbon, 
and in addition the molecular mantles manufactured on the grain surfaces in the 
molecular cloud complexes may provide a significant source of highly processed 
organic compounds that could find their way into other protostellar nebulae and 
strongly influence their future evolution toward life. 



28 



In the subsequent sections of this chapter, specific examples are presented 
of the kinds of data that would be most useful for exobiology. From a study of 
the type of instrumentation that will probably be available in orbit (described in 
Appendix B), the members of the Workshop have concluded that many of these 
data should be obtainable in the near future. Attempts have been made to 
identify those areas in which modification of planned instrumentation would be 
desirable. The individual subsections are organized so that each one discusses 
problems relating to a particular astrophysical environment. They are ordered 
with respect to distance, starting with those closest in our own solar system and 
progressing outward to studies of external galaxies. Chapter 6 provides a priori- 
tized summary of the various observational projects discussed in all of these 
subsections. 

Suggestions for Further Reading 

Harwit, M.; and Neal, V.: The Great Observatories for Space Astrophysics. 
NASA Astrophysics Division, Washington, 1985. 

European Space Science Horizon 2000. ESA SP-1070, 1984. 



3.3 Planetary Atmospheres 

What useful exobiological information can be obtained from the study of 
planetary atmospheres? Although little can be learned about actual local biology 
(should it exist) through comparative studies of the planets and their atmo- 
spheres, much can be learned about the formation and properties of environ- 
ments that may be necessary for life and, by extension, about the development 
of our own environment. Information about environments in which life arose in 
the solar system and those in which it apparently did not will provide con- 
straints on the conditions favorable for the origin of life and, by implication, the 
likelihood of the existence of such environments and life elsewhere. At the most 
general level, we want to understand the chemistry of the biogenic elements and 
compounds in the solar system at the present time as well as in the past. Some 
basic problems are summarized by the following questions: 

1. From what did the planets form; what can the planets and their atmo- 
spheres tell us about the primordial, pre-solar nebula? 

2. How did the planets form? Did they form by simple gravitational collapse 
of parts of the nebula or by accretion of grains? What does this say about the 
possible existence of planets suitable for life in other stellar systems? 

3. What are the conditions necessary for the stable existence of the different 
atmospheres found in the solar system, and how did they form? 

4. How have these atmospheres evolved since their initial formation? This 



29 



question encompasses sources for the observed molecules and aerosols, produc- 
tion of useful biological precursors, effects of various types of energy, processes 
that lead to the conversion of an atmosphere from oxygen-free to oxygen- 
containing, and any other conditions that led to the various planetary 
atmospheres. 

5. What drives atmospheric dynamics and how may these drivers affect con- 
ditions favorable for the origin of life? 

6. Are the other atmospheres found in the solar system useful as models for 
the atmosphere of the early Earth? 

It is probable that the information we can obtain from Earth orbital observa- 
tions of the planets and other solar system bodies will allow us to formulate and 
test models for the formation and subsequent evolution of the solar system. 

Lander and orbital spacecraft studies of Mars and Venus have yielded detailed 
information on the composition of their atmospheres and some indications of 
surface composition. Further investigations concerning the possibility of fossil 
remains of microbial life, the search for subsurface water, and the determination 
of the length of time that liquid water was present on the surface of both planets 
are very relevant to exobiology. Detailed data are needed for these studies and 
will require in situ experiments or sample returns. Several such studies are in 
the conceptual and planning stages and one, Mars Observer, which will study the 
atmosphere and surface from local orbit, is being developed for launch. Further 
observations from the ground or Earth orbit will be useful in the selection of 
sites for in situ studies or collection of samples. 

The outer planets, however, present an area where extremely useful remote 
observations could be made because less is known about their atmospheres. 
Moreover, in some ways they are models of the early Earth, particularly of the 
period when there was no free oxygen in the atmosphere. One interesting study 
has demonstrated that it is energetically feasible for organisms to live in a liquid 
ocean under the crustal ice on Europa. Even if life itself has not developed on 
the outer planets, or their satellites, study of their atmospheres will most cer- 
tainly be useful for placing the origin of life in the context of the origin of the 
solar system. From an exobiological perspective, three basic areas of study are 
appropriate for the outer planets: origin of the atmospheres, evolution of the 
atmospheres, and searches for other examples of satellites possessing atmo- 
spheres. Each of these will be discussed in the following paragraphs. 

There are currently two basic models, with some variations, to explain the 
origin of the outer planets, and they make different predictions about the nature 
of the planets' atmospheres. In the first model, a gravitational instability in the 
primordial nebula leads to the formation of giant gaseous protoplanets that col- 
lapse and later segregate out a core. The second model begins with the nuclea- 
tion of grains and their subsequent aggregation to form the cores of the giant 
planets followed by a runaway accretion to collect the atmospheres. If the first 
model was operative the chemical elements should have been retained in solar 



30 



proportion and consequently the atmospheres should also be in solar propor- 
tions. The situation for the second model is not so simple because various frac- 
tionation processes can occur during nucleation, aggregation, and accretion. 
Thus the initial elemental composition would not have been preserved. Calcula- 
tions suggest that, as compared to solar composition, the heavier elements 
(carbon, oxygen, nitrogen, etc.) should be enriched relative to the lighter hydro- 
gen and helium. This model would favor the development of an atmosphere 
richer in the biogenic elements and more conducive to the abiotic synthesis of 
biogenic compounds and even, possibly, some form of life. 

The data that are available for Jupiter and Saturn show factors of 2 to 3 
enrichment in carbon (primarily in the form of methane, CH 4 ) relative to solar 
abundances and apparently a similar enrichment in nitrogen, although the latter 
is complicated. Nitrogen occurs mostly as ammonia, NH 3 , that is not uniformly 
distributed in the atmosphere because it freezes out and also reacts with hydro- 
gen sulfide, H 2 S, another atmospheric gas. These data support the nucleation 
model. However, oxygen (as water, H 2 0) is depleted in the Jovian atmosphere 
by about a factor of 30. This could be because water condenses to the liquid or 
solid state at the higher elevations: water vapor exists only lower in the atmo- 
sphere where observation is difficult. Sulfur has not yet been detected, but 
H 2 S is very photolabile (to form elemental sulfur) and readily reacts with NH 3 
to form solid compounds. The abundance of phosphorus varies too much to be a 
useful discriminator, perhaps because of the reaction with H 2 in the lower 
atmosphere. 

Isotopic data are of poor quality and provide conflicting evidence: Low- 
temperature equilibration of hydrogen and water in icy materials leads to an 
enrichment of deuterium of up to five times solar, but observations of hydrogen 
deuteride (HD) and deutero-methane (CH 3 D) suggest a nearly solar ratio. This 
result implies that low-temperature H/D equilibration has not contributed to the 
isotopic composition and therefore that aggregation, fractionation, and accretion 
of an atmosphere characterized the formation of Jupiter. However, data on the 
carbon isotopes suggest that 12 C/ 13 C is about 1.8 times solar, opposite what 
should be observed if fractionation processes did take place. One hypothesis is 
that the Jovian 12 C/ 13 C ratio is representative of that in the primordial nebula, 
but if this is true why is the ratio different from that in the Sun? It may be that 
the isotopic composition of the solid carbonaceous condensates in the outer 
solar system differed from that of the bulk of the carbon that prevailed as gas. 

Obviously, higher-quality observations than those currently available are 
necessary to more accurately define these isotopic ratios on Jupiter and Saturn 
and to determine them on Uranus and Neptune. The relative isotopic composi- 
tions of these four planets may be required to disentangle clues to the formation 
processes from signatures of early solar nebula conditions. For H 2 , HD, CH 4 , 
CH 3 D, and NH 3 , most of the observations have been made on molecular lines 
that occur in the visible and near-infrared regions of the spectrum. Measurements 



31 



from Earth orbit will open the far-infrared and submillimeter regions where 
these small molecules have detectable transitions. Very detailed spatial studies 
are needed to determine the distributions of the various species over the Jovian 
and Saturnian disks. Velocity can also serve as a useful discriminant against the 
influence of local chemical and meteorological effects. Adequate determination 
of H 2 and HD profiles around 7000 A requires a spectral resolving power of 
>10 4 . Determination of isotopic and elemental ratios can also be made from 
observations of NH 3 and phosphine, PH 3 , in the infrared region. High-altitude 
or Earth-orbiting facilities are necessary to minimize the effects of atmospheric 
water and carbon dioxide (see fig. 3.2). Typical resolving powers of 1000 or 
better are required for adequate measurements of isotope ratios, e.g., 
15 NH 3 / 14 NH 3 , although resolving powers of 10 5 are necessary to observe indi- 
vidual rotation lines. If sufficient spatial resolution (<1 arcsec) can be achieved, 
then local effects such as atmospheric turbulence and chemistry may be studied 
(see Appendix C). 

Questions about the evolution of atmospheres with time may be easier to 
answer than questions about origins. We know from the detection of molecules 
that are not predicted by simple thermodynamic condensation models of the pri- 
mordial nebula that outer-planet atmospheres are not in chemical equilibrium. 
We also know from the presence of these molecules, and from the existence of 
unidentified colored species that cannot be formed by freezing any of the 
known atmospheric constituents, that active chemistry is occurring, or has 
occurred, in the outer solar system. We do not know the details of such chemis- 
try or what drives it. For example, in the upper atmosphere of Jupiter, photo- 
chemistry of methane is occurring, but are there other types of chemistry going 
on? What are the identities and sources of the (colored) compounds observed in 
the lower atmosphere where direct photochemistry of methane is unlikely? Are 
any of these organic compounds, and is the photochemistry there in any way 
related to the photochemistry likely to have been driven in the early atmosphere 
of the Earth? Might there indeed be sufficient chemical gradients and energy 
sources in the Jovian atmosphere to support the simple life forms once postu- 
lated in a paper entitled "Floaters, Bobbers, and Sinkers"? 

Further information on the nature and distribution of compounds in plane- 
tary atmospheres must be obtained. For this the infrared region of the spectrum 
is potentially the most useful, and many observations (e.g., Voyager Infrared 
Interferometric Spectrometer (IRIS)) have been made. However, increased spec- 
tral and spatial resolution are required to determine how various chemical 
species are distributed in the atmosphere and to sort out the region from 10 to 
15 jLtm where many molecules have absorptions. Study of the nature and distri- 
bution of compounds in the atmosphere will also yield information about the 
effects of various energy sources (solar ultraviolet, energetic particles, lightning) 
at different levels within the atmosphere and hence their relative importance to 
atmospheric chemistry and evolution. 



32 



Remote observations can also help determine the physical properties of 
atmospheres, in addition to the chemical properties. Information regarding tem- 
perature and pressure profiles and atmospheric dynamics can be obtained by 
measurements in the infrared and near-infrared regions and to some extent in the 
visible region. Both high spectral resolution and signal sensitivity are important, 
as they are necessary for accurate determination of line profiles and continuum 
absorption. Information in the far-infrared region of the spectrum will help 
extend temperature profiles to regions deeper in the atmospheres than is cur- 
rently possible. Observations over long periods of time will help clarify effects of 
atmospheric dynamics on chemical abundances and, by inference, on local 
production of chemical species. 

Another problem that may be clarified by observations from Earth-orbiting 
facilities is the difference between the atmospheres of Uranus and Neptune and 
those of Jupiter and Saturn. If all of these planets were formed by the same pro- 
cesses from a nebula of homogeneous composition, then the atmospheres should 
be similar, except for the effects of decreasing temperature with distance from 
the Sun. Present data suggest that although the two pairs of atmospheres are 
qualitatively similar, the elemental ratios (C/H, N/H, etc.) are quite different, 
implying different sources or formation pathways. Because of their much greater 
distance from us and the Sun, Uranus and Neptune appear much smaller in the 
sky and are dimmer than Jupiter and Saturn. Useful data are much more diffi- 
cult to obtain and instruments with greater sensitivity and less atmospheric 
interference are needed for studies of Uranus and Neptune. 

One last field of study is the search for and characterization of tenuous atmo- 
spheres in the outer solar system. Both Pluto and Triton (Neptune's satel- 
lite) have atmospheres, but very little is known about them. More information 
about such atmospheres would assist in understanding the overall distribution of 
the biogenic elements within the solar system and the associated occurrences of 
planetary atmospheres and hence their likelihood around planets elsewhere in 
the universe. Observations using instruments with extremely high sensitivity are 
necessary for these studies. 

The study of planetary atmospheres can best be accomplished by in situ mea- 
surements from remote probes such as Galileo or Cassini. However, the number 
of such missions planned for the next two decades is extremely limited and the 
information obtainable relates only to a specific location and time, so that these 
observations must be complemented and extended by global and synoptic obser- 
vations from Earth or Earth-orbit. Of the orbital spacecraft listed in Appendix B, 
those that will be most useful in making planetary observations are those capable 
of detecting the ultraviolet, infrared, and submillimeter regions of the spectrum. 
HST, ASTRO, 1UE, FUSE, SOFIA, SIRTF, ISO, FIRST, and LDR satisfy these 
requirements. For continuing efforts to determine the identities, abundances, 
and distributions of molecular species, the greatest information to date has come 
from the 5-jum window and the 10- to 15-jum region in the infrared. From the 



33 



point of view of sensitivity, SI RTF would be the facility of choice, providing the 
capability to observe weak lines (e.g., those of 15 NH 3 and H 2 S). However, the 
spectral and spatial resolution as presently defined may not be adequate to make 
the required measurements. ISO will have a similar capability. For the stronger 
lines, and especially at wavelengths longer than 10 jum, SOFIA could provide 
superior spatial and spectral resolution, and in the very near future. For faint 
sources and high resolution, we will have to wait for LDR. For Jupiter at least, 
Galileo may obtain the desired information first. 

Measurements of isotope ratios require very high spectral resolution (resolving 
power, X/AX > 10 4 ), and, if local variations of the ratio are to be determined, 
high spatial resolution as well (see Appendix C). For H 2 , HD, CH 4 , CH 3 D, and 
NH 3 , these observations have used the visible and near-infrared regions of the 
spectrum, but determinations of 13 C/ 12 C and 15 N/ 14 N for Jupiter were made 
using bands in the infrared. If resolving powers around 10 5 can be achieved with 
sufficient sensitivity, then the instruments aboard HST, SOFIA, SI RTF, and 
LDR should be able to make useful observations. Resolving power X/AX > 1 3 is 
not planned for the faint-object spectrograph (FOS) on HST. IUE and then 
FUSE will be of use if bands of HD (and of C 2 H 2 , etc.) below 3000 A can be 
detected. For work requiring high spatial resolution (e.g., 0.8 arcsec— about 
one-tenth the size of the great red spot of Jupiter) we will have to wait for LDR. 

Physical properties of planetary atmospheres can also be determined if accu- 
rate line shapes can be obtained. High spectral resolution and detector sensitivity 
are necessary to adequately characterize line profiles and to determine con- 
tinuum absorption. High spatial resolution is necessary for the study of local 
conditions. For this work, observations must be made in the infrared or far- 
infrared regions. Combined data acquired with SOFIA, the SI RTF wide-field 
camera (WFC) or the LDR medium-resolving-power spectrometer (MRPS) 
should be useful to meet the proposed resolution and sensitivity requirements at 
different wavelengths. 

The study of tenuous atmospheres surrounding Pluto and moons in the outer 
solar system essentially requires high detector sensitivity while maintaining 
sufficient spectral resolution (X/AX > 10 2 ) to detect molecular absorption or 
emissions. Spatial resolution is not of great importance provided that individual 
objects can be isolated. For these studies the HST faint-object spectrograph 
(FOS) and the LDR MRPS should be useful. 

Since the observations of planetary atmospheres described here primarily 
involve obtaining and interpreting spectra, laboratory spectra of individual 
molecules under proper conditions are necessary for comparison. Not only is 
information about line positions and strengths needed for the determination of 
the identities and abundances of atmospheric constituents, but the effects of 
pressure and temperature on spectral line shapes (and positions) need to be mea- 
sured to determine the atmospheric conditions, hence the locations, where 



34 



molecular absorptions and emissions originate. A strong laboratory effort must 
therefore be carried out in conjunction with the observational program. 

Suggestions for Further Reading 

Encrenaz, T.: Primordial Matter in the Outer Solar System: A Study of 
its Chemical Composition from Remote Spectroscopic Analysis. Space Science 
Reviews, vol. 38, 1984, p. 35. 

Trafton, L: The Atmospheres of the Outer Planets and Satellites. Reviews 
of Geophysics and Space Physics, vol. 1 9, 1 981 , p. 43. 



3.4 Titan 

Some of the most interesting discoveries of the Voyager mission concern 
Saturn's planet-sized moon, Titan. Titan, like the Earth, has an atmosphere 
dominated by nitrogen with a surface pressure comparable to the terrestrial 
value; its surface is strongly suspected to be (at least partly) liquid, possibly 
composed of a mixture of methane and ethane. A number of organic molecules 
have been identified on Titan: hydrogen cyanide (HCN), cyanoacetylene 
(HC 3 N), and cyanogen (CN) 2 . These are postulated to be products of methane 
(CH 4 ) and nitrogen (N 2 ) photodissociation reactions. Thus, Titan may serve as a 
Miller-Urey-type model of a highly reduced early Earth atmosphere in which the 
first stages of organic chemical evolution could take place in the atmosphere. 
Indeed, laboratory simulations show that a Titan-like atmosphere, primarily a 
N 2 /CH 4 mixture, under various energetic excitations forms not only HCN, 
(CN) 2 , and HC 3 N (observed on Titan), but also CH 3 CN, (HCN) 4 , and finally, 
adenine (HCN) S , a component of DNA. Titan thus appears as an object of 
exceptional interest for exobiology. 

It is likely that Titan will be extensively explored by a space mission at the 
end of the century: a joint ESA-NASA project, Cassini, is now under study. 
Before this exploration, many observations need to be performed from Earth 
orbit. Observing Titan is difficult since it is small and faint. Sensitive Earth-orbit 
observations of Titan in the ultraviolet, visible, and infrared spectral ranges could 
significantly improve our knowledge in the 10 to 15 forthcoming years and 
provide valuable data complementing that which Cassini may provide. 

The ultraviolet and visible spectral ranges are suitable for studying Titan's 
photochemistry. Two types of observations can be identified. 

The first is imaging Titan in the ultraviolet and visible regions. This imaging 
can be accomplished in various bandpasses to within 0.1-arcsec resolution using 
the Space Telescope wide-field/planetary camera (WF/PC) or the faint-object 
camera (FOC). This camera would give about eight resolution elements across 



35 



the Titan disk (Appendix C), which might be sufficient to detect limb darkening 
or brightening, but not much in the way of detailed structure. In the far- 
ultraviolet region, both cameras are hampered by the red sensitivity of the 
detectors, particularly the charge-coupled devices (CCD) used by the WF/PC. 
The ultraviolet filters have relatively poor transmission and appreciable out-of- 
band transmission, particularly in the red. Moreover, the far-ultraviolet emissions 
(e.g., of H, N, C, N 2 , and H 2 ) originate mainly in the exosphere of Titan, excited 
by interaction with charged particles trapped on magnetic field lines of Saturn, 

Second is spectroscopy of Titan between 1000 and 3000 A. A study in the 
ultraviolet range at medium and high resolution would be the best way to search 
for the spectral signatures of the photodissociation products of organic mole- 
cules that may be present in the atmosphere. The advantage of the HST, relative 
to previous ultraviolet satellites such as the IUE, will be a higher spectral resolu- 
tion and a higher sensitivity (Titan is too faint to be observed with IUE below 
2000 A). The instruments to be used would be the faint-object spectrograph, for 
a medium-resolution spectrum of the whole ultraviolet range, and the high- 
resolution spectrograph, for investigating specific spectral signatures at high 
spectral resolution using different modes (depending upon the wavelength and 
the lines to be searched for). However, at this stage it is not clear whether either 
of these two spectrographs has sufficient sensitivity to detect any species other 
than the dominant atoms and diatomic molecules of the outer atmosphere 
of Titan. 

Many complex organic molecules exhibit strong vibration-rotation lines in 
the infrared and rotational transitions in the far-infrared, submillimeter, and 
millimeter ranges. These lines cannot be observed from the ground. Moreover, in 
the case of a weak source such as Titan, the background noise is very strong. A 
cooled instrument in space, like SI RTF or ISO, is needed to perform the obser- 
vations. An alternative solution is to observe Titan in the millimeter range with 
a heterodyne system and long-baseline interferometer. This is certainly a ground- 
based program to be developed in the near future. However, in the millimeter 
range, rotational transitions, under the excitation conditions of planetary atmo- 
spheres, are usually much weaker than transitions corresponding to higher J 
values, i.e., in the submillimeter and far infrared. The search for complex mole- 
cules on Titan with ISO, SI RTF, and LDR thus appears to be a promising 
method, complementing ground-based millimeter programs. Molecules to be 
searched for include the following nitriles: CH 3 CN (acetonitrile), C 2 H 5 CN (pro- 
pionitrile), C 2 H 3 CN (acrylonitrile), and possibly adenine. The instruments to be 
used would be high-resolution spectrometers with a spectral resolving power as 
high as possible; however, at long wavelengths (>50 jumjthe observations will 
have to separate Titan from Saturn— i.e., will have to be performed with the best 
possible angular resolution when Titan is near elongation. Initially, SOFIA may 
offer the superior resolution needed and have the sensitivity to detect at least 
the strongest features. 



36 



A quantitative inventory of organic compounds and their distribution with 
altitude within the atmosphere may illuminate the specific production mecha- 
nisms responsible for their existence. At the moment, such mechanisms are 
largely matters of theoretical speculation. Detailed atmospheric observations will 
also contribute to an understanding of the aerosols and surface materials that 
have accumulated over the age of Titan. 

Suggestions for Further Reading 

Hunten, D. M.; Tomasko, M. G.; Flasar, F. M.; Samuelson, E. E.; Strobel, 
D. F.; and Stevenson, D. J.: Titan. In Saturn, T. Gehrels and M. S. Mathews, 
eds., University of Arizona Press, 1984. 



3.5 Comets 

Comets are especially interesting as frozen remnants of the primordial solar 
system. As such, they might provide clues needed to understand the subsequent 
evolution and differentiation of the planets and their satellites following the 
initial condensation of the solar nebula. Was there something special about the 
conditions in the early evolution of the solar nebula that created the terrestrial 
planets at the right distance from the Sun so that at least one of them was able 
to chemically evolve life? In addition, the comets themselves may have contrib- 
uted substantially to the inventory of volatiles and organics available to the 
primitive Earth. We can estimate the rate of cometary impacts early on from the 
cratering histories of the Moon and Mars, but we need to know the abundance 
and probable structure of the biogenic elements within the comets to know how 
much they might have affected the earliest phases of abiotic chemical evolution 
on Earth. Three of the basic questions to be answered are 

1 . What are the compositions and structures of comets? 

2. Where and how were they formed? 

3. Did they contribute volatiles and/or organics to Earth? 

While specialized missions that approach or penetrate particular comets may 
yield the most precise chemical information, such missions sample only one 
comet at a time. To understand the global characteristics of the early solar 
nebula will require a large statistical sample of cometary properties. Such data 
can be collected only by remote observational surveys. What is learned from 
these observations may well influence the detailed design of individual future 
cometary missions such as CRAF. There will necessarily be much indirect infer- 
ence associated with remote cometary observations, but that does not reduce 
their importance. 



37 



Following the extensive studies of comet Halley, and in the interim period 
before another major comet mission, the first problem can now be approached 
only with models based on phenomena observable by optical and radio tech- 
niques. In particular it is possible to differentiate and study the coma and tail 
when the comet is at small heliocentric distances, while an unresolved asteroid- 
like inactive nucleus is all that is accessible for comets at several astronomical 
units from the Sun. 

The generally accepted cometary model of an icy conglomerate a few kilo- 
meters in diameter containing both volatile and refractory components was first 
proposed on the basis of an analysis of the "nongravitational" force perturba- 
tions of the orbit of periodic comet Encke. These forces arise from the jet action 
produced by the nonuniform vaporization of matter from the surface of a 
rapidly rotating comet nucleus. Subsequent investigations have concluded that 
the major volatile component controlling the vaporization must be water ice, 
and recent ultraviolet spectra of a number of comets provide strong confirming 
evidence for this idea. Not only is water ice confirmed as the dominant volatile 
in the nucleus, but an initial composition of volatiles similar to what is found in 
interstellar molecular clouds appears to be needed to account for the observed 
abundances of visible radicals. In a different approach, which ignores the details 
of the chemistry and simply counts the end-product atoms that are then assem- 
bled into a hypothetical nucleus, water ice again emerges as the dominant 
constituent. 

Unlike planetary atmospheres, which are gravitationally bound and exhibit 
only relatively mild temporal variations, the atmosphere of a comet is a highly 
transitory and variable phenomenon. The most rudimentary coma models 
assume that the parent molecules sublimate at the surface of the nucleus and 
flow radially outward, subject only to the solar particle and ultraviolet radiation 
fields that progressively decompose them into their constituents atoms (or ions), 
which continue to flow radially outward. Although rudimentary, this one- 
dimensional picture serves as a basis for derived abundances of H, O, and OH in 
the coma that appear to be consistent with an H 2 source that is at least an 
order of magnitude more abundant than any other hydrogen-bearing molecule. 
For comet Halley the spacecraft measurements verified this predominance of 
water. 

The fundamental compositional differences between comets, as deduced from 
ground-based observations, is the dust-to-gas ratio. This is related to the amount 
of observed continuum radiation (sunlight reflected from solid grains) relative to 
gas fluorescence produced by C 2t C 3 , CN, etc., which is taken as a measure of 
the total gas production rate. Among the species detectable in the visible (which 
represent less than 1% of the total volatile component), there appears little 
variation from comet to comet, provided that observations are compared at simi- 
lar heliocentric distances. The abundance of these species relative to H 2 (or at 
least to OH presumably derived from H 2 0) is also relatively constant from 



38 



comet to comet. One exception does exist, and that is CO^, whose fluorescence 
in the comet-tail band system in the blue gives the ion tail its visibility. This 
species is often absent from the coma, and no ion tail is seen, particularly in 
old, short-period comets. On the basis that spectacular ion tails are most often 
associated with bright, new comets, and that both CO and C0 2 are considerably 
more volatile than H 2 0, it has been suggested that new comets contain a signifi- 
cant fraction of either of these species and that, for at least a part of their orbit, 
the sublimation of gases from the nucleus is controlled by this more volatile 
fraction. 

Spectroscopy in the far-ultraviolet provides a convenient means of studying 
compositional variations of the dominant volatile species. H 2 0, C0 2 , and CO 
all have signatures in this spectral region, as do the dissociation products of 
H 2 (H, O, and OH), as well as C, C , CO , and C0 2 . Several sulfur com- 
pounds also fluoresce in the far-ultraviolet, including S, CS, and the recently 
discovered diatomic sulfur, S 2 . Since the terrestrial atmosphere is opaque to 
wavelengths below 3000 A, observations in the far-ultraviolet have to be made 
from above the absorbing 2 and 3 . 

In May 1983, comet IRAS-Araki-Alcock (1983d) passed to within 0.032 AU 
of the Earth, providing a unique opportunity to study the structure of a come- 
tary coma on a scale of tens of kilometers from the ground and from Earth 
orbit. A major contribution of the IUE satellite observatory was the discovery of 
S 2 in a region ~500 km around this comet. The spatial distribution and the 
expected photochemical lifetime of S 2 imply that it is produced directly from 
the nucleus at ~10~ 3 the rate of water molecule production. Furthermore, 
the short lifetime makes this species an ideal tracer of short-term cometary activ- 
ity. IUE data on S 2 obtained over a 32-hour period have shown a marked tem- 
poral variation that can be associated with the visual Sunward "fan" of this 
comet. The presence of S 2 in cometary ice is also potentially an indicator of the 
physical conditions prevalent at the time of the formation of the comets (or 
"cometesimals") in the solar or pre-solar nebula. S 2 is generally much reduced 
in abundance relative to other sulfur polymers in the gas phase at low tempera- 
tures. The significance of S 2 will not be fully appreciated until its presence in 
other comets is confirmed in future ultraviolet observations. 

The successful encounter missions with comet Halley in March 1986 (Giotto, 
Vega, and Suisei) and the plethora of Earth-based observations carried out 
during this apparition have served to largely confirm the aforementioned models 
of the cometary coma. It is still too early to draw any significant conclusions 
from the images of the nuclear region of comet Halley regarding the mechanism 
of dust and gas ejection (or sublimation) from the dark nucleus, but the in situ 
measured chemical composition of both the gaseous and solid components of 
the coma was found to be consistent with what was expected on the basis of 
remote observations of comets performed over the past decades. Water, the 
dominant constituent, was also detected remotely using very-high-resolution 



39 



interferometric spectroscopy at ~3 /jm from the KAO, illustrating the potential 
of this technique in the future for the detection of other parent molecular 
species. Surprisingly, neither the in situ measurements nor the remote ultraviolet 
observations have reported the presence of any gaseous chemical species pre- 
viously unknown in comets. However, there has been a report, from low- 
resolution infrared spectroscopy, of the detection of spectral features due to 
organic compounds in the cometary grains, but this interpretation remains 
highly controversial. Unfortunately, the in situ dust analyses cannot resolve this 
question since they could measure only the atomic constituents of the dust 
particles; molecular bonds were broken upon impact of the dust on the instru- 
ment because of the large velocity of the spacecraft relative to the comet. None- 
theless, a significant fraction of the small particles consisted almost entirely of 
the biogenic elements, C, H, O, and N, in various combinations. These results are 
very preliminary, and much detailed information about the gas and dust in the 
coma is expected after further analysis of the wealth of comet Halley data. How- 
ever, it seems likely that new information will emerge to encourage further 
remote study of biogenic elements in future comets, particularly "new" comets 
which are of most interest in this regard. 

The infrared range is the best spectral region for analyzing micrometer-sized 
particles, typical of cometary dust, wherein may be found the majority of the 
biogenic elements and compounds that could be incorporated into the early 
Earth by cometary impact. Several observations can be considered: 

1. Mapping of the dust between 1 and 18 jum at moderate spectral resolution 
(X/AX ~ 100 - 1000). The instruments to be used are infrared cameras with 
medium-resolution filters. The scientific objectives in mapping the dust are 

a. Searches for the silicate signature at 10 and 18 jum. 

b. Searches for the signatures of O-H, C-H, and C-C bonds in the 1- to 
7-jum region in order to recognize organic and carbonaceous dust grains as well as 
the hydrated phases. Data from the Giotto mission to Halley point strongly to 
the existence of carbon-rich or entirely carbonaceous grains. 

c. Search for an icy halo at 3 /am (in the very center of the coma). 

d. Constraints upon the size and the composition of the dust, by analyzing 
the general shape of the thermal flux between 4 and 1 8 (Jim. 

2. Observations of cometary dust at longer infrared wavelengths 
(20-200 jum). The shape of the spectrum beyond 20 jum is very sensitive to the 
size of dust particles. An accurate measurement of the slope of the spectrum 
between 20 and 200 fjtm would provide new constraints, especially for the large 
particles. 

3. Observations of the coma with the high spatial resolution from LDR. 
Eventually, LDR and/or FIRST will provide the possibility of directly observing 
the primary constituents of the coma after they have been sublimated from the 
nucleus. H 2 could be easily detected and mapped at 557 GHz. 



40 



All these observations will be complemented by continuing ground-based 
observations at microwave frequencies. The potential for extremely high spatial 
resolution using interferometers may prove to be valuable in interpreting the 
lower-spatial-resolution infrared data, and distinguishing between several possible 
parent species and production mechanisms. 

J ust as the infrared is best suited to studies of cometary grains, the ultraviolet 
is the spectral region in which the most abundant atoms and small molecules 
fluoresce. The primary objectives, therefore, of Space Telescope observations of 
comets would concentrate on the photochemical evolution of the gaseous 
component of the inner coma. There are few spectral signatures of simple 
organic molecules in this part of the spectrum so that the presence of prebiotic 
molecules must be inferred from observations of the spatial distribution of 
dissociation products resulting from photochemical decomposition of the 
heavier molecules. Observations of comets made over the past 6 years using the 
IUE satellite have provided a wealth of new information about the chemistry 
and evolution of the inner coma. These studies will be strongly enhanced with 
the use of ultraviolet spectrographs on both HST and ASTRO. 

Suggestions for Further Reading 

Nature, vol. 321, 15 May 1986, p. 6067. 

Wilkening, L. L.; and Matthews, M. S., eds.: Comets. University of Arizona 
Press, Tucson, 1982. 



3.6 Asteroids and Meteors 

Most of the 3500 asteroids for which orbits are known circle the Sun between 
Mars and Jupiter and are found between 2.1 and 3.2 AU. In addition to this 
main belt, there are important classes such as the Trojans at the L 4 and L 5 
points of the Sun-Jupiter system and the near-Earth asteroids such as the Amors, 
Apollos, and Atens. The largest of the asteroids (1 -Ceres) is about 1000 km 
across while the smallest for which physical data are available have diameters of 
only 500 to 1000 m. 

Because of their small size it has been common to impute dull geological 
histories to the asteroids as a group, but considerable evidence against such a 
view can be marshalled. In particular, it has been demonstrated that at some 
time during its 4.5-billion-year history, the asteroid 4-Vesta was hot enough for 
lavas to have erupted onto its surface. 

On the basis of reflectances, colors, and other optical properties it has been 
possible to divide asteroids into several groups. The most abundant of these are 
the S-types (reflectances near 0.15 and colors similar to those of ordinary chon- 
drites) and the C-types (very low reflectances, often less than 0.05, and colors 



41 



similar to carbonaceous chondrites). By the mid-1980s more than half a dozen 
"types" had been identified and their distributions charted as a function of dis- 
tance from the Sun. In part, the observed pattern has been interpreted to reflect 
the temperature gradient believed to have controlled the condensation of solids 
in the early solar nebula. For instance, in accordance with such a model the 
probably more volatile-rich and carbonaceous objects are relatively more abun- 
dant at distances farther from the Sun than the asteroid belt. 

A major breakthrough during the past decade has been the development of 
two techniques for the routine determination of diameters (and therefore, 
reflectances). These radiometric and polarimetric techniques have been verified 
in several instances in which asteroid diameters could be obtained directly from 
observations of stellar occultations. As a result, the average diameters of hun- 
dreds of asteroids are known reasonably well, as are the average absolute reflec- 
tances of their surfaces. A complication arises in that many asteroids are known 
to have irregular shapes (as deduced from observed periodic fluctuations in their 
brightness). 

Asteroids are seen as distant point objects from Earth, and any compositional 
information is derived from measurements of how their surfaces scatter sunlight. 
Such spectral-reflectance techniques have been developed extensively over the 
past two decades. Characteristic absorption features make it possible to identify 
some key minerals, and although matching is not always exact, a striking corre- 
spondence between the spectra of certain asteroids and some meteorites has 
been established. The lack of precise matching must have several causes. First, 
there are measurement uncertainties in both the asteroid and the meteorite data. 
Second, and more importantly, it is very unlikely that meteorites provide a full 
sample of surface materials present in the asteroid belt. Finally, it must be 
realized that asteroid surfaces have undergone long histories of space weathering, 
whereas space-weathered samples of meteoroid surfaces are not available for 
laboratory measurements. In spite of such difficulties, it has proved possible to 
characterize the likely compositions of many asteroid surfaces and to examine 
the distribution of various compositions as a function of distance from the Sun. 
In most cases spectral-reflectance measurements are restricted to the global 
properties of asteroid surfaces, although careful observations made at different 
phases of an asteroid's rotation period have recently documented evidence of 
variations in composition on the surfaces of a few asteroids such as 4-Vesta and 
8-Flora. 

Spectral-reflectance measurements provide data on surface composition only; 
they contain no direct information on the internal makeup of the body. Such 
information can sometimes be inferred from an accurate measurement of the 
mean density. Unfortunately, approximated masses have been determined from 
orbital perturbations for only four asteroids so far, and the determination of 
additional masses must await close spacecraft flybys. Such flybys will also pro- 
vide accurate measurements of volumes, and will represent a future source of 
very precise density determinations for asteroids. 



42 



One known difficulty with spectral-reflectance measurements is that it can 
be difficult to derive uniquely and accurately the relative abundances of key 
constituents if one does not know a priori the whole suite of materials present 
and the texture of the regolith. This problem is especially acute when dealing 
with the abundances of optically opaque materials (metallic nickel/iron, magne- 
tite, carbonaceous material). This is a major contributing factor to disagreements 
in the literature over the metallic content of S-asteroid surfaces and the rela- 
tionship of dark asteroids such as Pallas to carbonaceous chondrites. 

One major question concerns the types and distributions of organic-rich and 
volatile-rich materials that formed in the solar system as a result of direct con- 
densation from the solar nebula or aggregation from primitive presolar dust, or 
subsequent processing on or within parent bodies. Several types of parent bodies 
are involved— main-belt asteroids, Trojan asteroids, small satellites of the outer 
planets, and comets. Each population of parent body has experienced its own 
long history of evolution and it is difficult to infer the original characteristics 
from what we observe today. Is any interrelationship that we observe representa- 
tive of that which obtained in the early stages of the solar system? A particular 
difficulty is that our only compositional information on distant objects comes 
from remote-sensing data that refer only to the surfaces of these bodies. How do 
we relate the surface composition to internal bulk characteristics or to the 
detailed properties of meteorite samples measured in the laboratory? 

One important investigation would involve a concerted effort to define the 
inventory of carbonaceous materials in the solar system, as a prelude to deter- 
mining its precise composition and ultimately its provenance and history. From 
the study of meteorites it is clear that several types of carbonaceous materials 
are present. Some of this material has undergone evident processing on parent 
bodies and it is suspected that most of these parent bodies were asteroidal rather 
than cometary in origin. Whether or not any carbonaceous meteorites could be 
derived from comets remains an issue of current debate. 

Spectral-reflectance measurements in the near infrared have made it possible 
to identify certain asteroid surface materials with carbonaceous meteorite 
analogs: for example, 1 -Ceres has been associated with a CI or CM carbonaceous 
chondrite. It is evident from such observations that the surfaces of C-asteroids in 
the main belt vary in the strength of the 3-^im absorption feature attributed to 
water-of-h yd ration in the clay minerals that make up these materials. In general, 
such measurements are difficult to make and require high sensitivity and high 
resolution. Since it appears possible to relate the strength of the 3-jum feature to 
carbonaceous meteorite mineralogy, an important goal of asteroid research 
should be to establish how widespread such material is and to determine the 
strength of this feature for as many C-asteroids as possible. 

The carbonaceous material associated with asteroids in the outer solar system 
is different spectrally from that in many of the bodies in the asteroid belt. A 
distinction has been made between the C-material in the main belt and the 
D-material (very dark, but redder) that is found in the outer fringes of the 

43 



main belt and at the orbit of Jupiter. Evidence has been presented that the car- 
bonaceous materials in comets might be similar to this D-material. The spectral 
characteristics of the D-material have been defined poorly, and high-resolution 
spectrometry in the infrared would help constrain its composition and test avail- 
able speculations concerning its nature. There is a tendency to regard D-material 
as a less "processed" version of what becomes C-material, but such vague sugges- 
tions need to be tested. 

A fundamental question in dealing with carbonaceous material in the solar 
system concerns the possible interrelationship of comets and some types of 
asteroids. It has long been suspected that some comets must evolve into objects 
resembling Earth-crossing asteroids. Two recent observations have added circum- 
stantial support to this general view. The first was the discovery by IRAS of a 
small, asteroid-like body (1983 TB) whose orbit is closely similar to the Geminid 
meteor stream. The second was the discovery of disturbances in the solar wind 
that might be attributed to the release of volatile material from the Apollo 
asteroid 2201-Oljato. Several Earth-crossers have been identified as likely dead 
comets on the basis of their orbital characteristics. Thus, an important problem 
is how to identify the dead comets among the population of Earth-crossers 
(from orbital or remote-sensing observations) and then to obtain high-resolution 
spectra of their surfaces for comparison with meteorite samples and other solar 
system small bodies. Another opportunity for understanding the interrelation- 
ships may arise from a study of meteor showers. 

Current measurements of the properties of comets constrain the abundances 
of volatile materials; the refractory component abundances must be inferred by 
other means. This may be possible from measurements of the ultraviolet spectra 
of meteor showers associated with active and extinct comets. Because many 
meteors are too fragile to survive passage through the Earth's atmosphere, such 
spectroscopic measurements are the only method by which elemental abundance 
ratios for a large number of known cometary samples can be determined. 
Ground-based optical efforts have not been sufficiently quantitative because of 
the large number of iron lines that mask other interesting features. There are 
several "iron-free" windows in the ultraviolet (1150-1250 A, 1300-1350 A, 
1370-1550 A, 1800-1900 A, 1970-2080 A) through which lines of H, C, SiO, 
P, S, N, O, Al, Mg, Mn, Ca, Na, and Zn may be strong enough to be visible from 
low-Earth-orbit observations of meteor streams. If practical, such observations 
might be conducted from a dedicated instrument launched from the Shuttle or 
an ELV, or from one of the polar-orbiting platforms associated with the Space 
Station composed primarily of Earth-observing instruments. This may provide 
additional data to combine with inventories of meteoritic abundances and help 
identify the dead cometary component among the Earth-crossing asteroids. 

A summary of important measurements to be taken and questions to be 
answered follows: 

1. High-spectral-resolution infrared (1 to 5 /im) spectra of C-asteroids and 



44 



related objects such as D-asteroids and comets should be obtained. 

2. A comprehensive survey of the strength of the 3-jum water-of-hydration 
feature in the spectra of dark asteroids should be carried out. 

3. The 3.3- to 3.5-jum region indicative of C-H bonds in organic matter should 
be searched. 

4. The relationship of meteorite samples to asteroid parent bodies and to 
contemporary asteroids (especially for the more volatile-rich types) should be 
determined. 

5. The provenance of Earth-approaching asteroids should be determined. 
(How can we distinguish between the asteroid-derived population and the come- 
tary one?) 

6. What clues about the makeup of parent bodies can be inferred from 
studies of cosmic dust collected in the Earth's upper atmosphere and from 
meteor investigations? 

These measurements will not of themselves be sufficient to decipher the his- 
torical connection between asteroids and comets, but they will be helpful 
adjuncts to the necessarily limited number of flybys that are now being planned 
by NASA, ESA, and the Soviet Union. 

Suggestions for Further Reading 

Gehrels, T.; and Matthews, M. S., eds.: Asteroids. University of Arizona 
Press, 1979. 



3.7 The Protosolar Nebula and its Analogs 

The protosolar nebula represents a critical boundary between the local 
chemistry and processing of the biogenic elements and compounds and the 
rather ubiquitous organic chemistry that is observed in a large number of inter- 
stellar and circumstellar regions. The formation of the protosolar nebula was an 
energetic process, and the fundamental question is how the energetics affected 
the preexisting complex chemistry. From our studies of primitive solar system 
bodies we may be able to infer the distribution of the biogenic elements and 
compounds early in the history of the solar nebula. It is doubtful whether we 
will discover a timekeeper of sufficient precision to be able to distinguish 
between the possibility that the deduced distribution of chemical complexity 
arose ab initio from an elemental mixture within the protosolar nebula, or was 
imposed upon it by the processing history within the cloud from which it 
formed. For this reason it is extremely important to search for analogs of the 
protosolar nebula evolving today within regions of active star formation. The 
organic chemistry within and without the nebula must be compared in detail to 
decide whether the material from whence planets might eventually form retains 



45 



a memory of the gas phase and grain surface chemistry that had been slowly 
accumulating within the parent cloud. Whether complex organic chemistry origi- 
nated early in the history of the solar system or arrived from elsewhere will 
profoundly influence the likelihood of life arising elsewhere. Analogs of the 
protosolar nebula are critical to our understanding. 

Stars form through the gravitational collapse of interstellar molecular clouds. 
This process starts with the fragmentation of the parent cloud into a number of 
clumps, many of which may collapse to form stars. According to present models, 
the initial collapse phase is isothermal, but when the cloud becomes opaque to 
visible radiation it heats up. Since the initial cloud is rotating, the cloudlet will 
flatten during the collapse. The core collapses faster than the outside, resulting 
in a core-envelope structure, with the envelope falling freely onto the core. The 
core itself consists of a central object (the protostar) in hydrodynamical equilib- 
rium surrounded by an accretion disk (the protoplanetary disk). The core is 
separated from the envelope by an accretion shock. In this accretion shock, 



Dusty 
Envelope 



Cloud 
Boundary 



Dust Destruction 
Region 



Dust Free 
Envelope 




"- / 

Accretion 
Shock 



Dust Free 
Disk 



Hydrostatic 
Core 



3x10 11 cm 
i r 



Dusty 
Disk 



3x10 10 cm 



+ 



+ 



2x10 14 cm 2x10 17 cm 
Size scale for 1M© protostar 



Figure 3-4. The different regions in a collapsing, rotating interstellar cloud. 



46 



nearly all of the kinetic energy of the falling envelope is transformed into inter- 
nal energy of the gas and radiated away. For a central object of approximately 
one solar mass, the shock velocity on its surface is about 300 km/sec, while the 
shock luminosity is about 40 L^. There will also be an accretion shock in the 
disk, with a range of velocities depending on the position within the disk. How- 
ever, the main heating of the disk is thought to be viscous heating driven by tur- 
bulence. Close to the central object the dust initially present in the collapsing 
envelope will have been vaporized by the strong radiation field generated by the 
shock, creating a dust-free zone. The different zones in a collapsing, rotating 
interstellar cloud are illustrated in figure 3-4, which shows the typical sizes for 
these zones. 

This global picture of star formation has recently been modified with the 
discovery of strong stellar winds from protostars. The effect of the protostellar 
wind on the molecular cloud is thought to cause two shocks. First, the wind will 
shock against the shell of swept-up material (i.e., the wind shock, <400 km/sec). 
Second, this shell of swept-up material will be driven supersonically into the 
surrounding molecular cloud material (v ~ 15 km/sec). The size of the wind- 
acceleration zone is about protostellar size or perhaps the size of the circum- 
stellar disk. The size of the wind shock and the molecular cloud shock is much 
larger, possibly even larger than the size of the collapsing envelope. This is shown 
in figure 3-5. 

Carbon monoxide observations in the vicinity of pre-main-sequence stars 
reveal blue-shifted and red-shifted emission in two opposing lobes. The observed 
outflow velocities around T Tauri stars (pre-main-sequence stars with M ~ 1M@) 
are about 15 km/sec or less. Emission from shocked, vibrational^ excited H 2 
has also been detected in these regions. The most spectacular sign of outflow 
from protostars is, however, the phenomenon of Herbig-Haro (HH) objects. 
These nebulae show a strong emission-line spectrum due to cooling behind 
the shocks. Proper motion studies and optical line studies reveal space velocities 
of up to 400 km/sec. The proper motion vectors of the HH objects in a family 
point back toward the exciting star. These shocked clumps of gas are either 
ejected directly from the circumstellar disk of the protostars, or they may 
represent the interaction between the directed flows from these objects and their 
molecular cloud environment. It is believed that the responsible stars are the 
precursors of even the T Tauri stars, which themselves are pre-main-sequence 
stars. The evolution of the protosolar nebula is thus envisaged to incorporate 
phases of both infall and outflow, possibly simultaneously. The importance of 
the HH-exciting stars is twofold: first, they indicate hitherto unsuspected violent 
activity around the protostar, and second, their study reveals clues to the physi- 
cal and chemical nature of protostellar disks and circumstellar nebulae. 

Both the nebular morphology of the HH objects themselves and their proper 
motions, where these can be determined, indicate that the outflows from their 
exciting stars are highly anisotropic. The flows are either unipolar or bipolar 



47 



OUTFLOW FROM PROTOSTARS 




'. Molecular Cloud 






Collapsing Envelope 



•>■ ■ ■■*■- ». -i ■* >. 












Protostar 



-L 









Proto Planetary Disc 



Molecular Cloud Shock 
(-15 Mms) 




Jet and Associated 
Herbig-Harbo Objects 



Figure 3-5. Schematic of a stellar wind-driven shock model of a protostar. 



with oppositely directed velocity fields. Some even show evidence for precession 
(i.e., the gradual rotation of the axis of outflow with time). In a few cases the 
HH objects can be traced back, essentially to the ejecting stars, in the form of 
well-collimated "jets." In two cases, a radio jet continues to the star from the 
nearest HH object. Stellar binarity is reasonably well established for two systems 
and may be important in others, but the mass ratios of these binaries are totally 
unknown. The inferred (projected) orbital planes of these binaries are orthog- 
onal to the flow direction from the system. Airborne far-infrared measurements 
for a number of HH-exciting stars have resolved the emitting regions at 50 and 
100 jum orthogonal to the flow direction. But even the smallest far-infrared 
beam sizes obtainable with the present airborne observatories are unable to 
resolve these regions in the flow direction. The emitting regions have radii of 
about 10 3 to 10 4 AU and presumably they represent the flattened, collapsing 



48 



envelope of the protostar. The mass in these regions is difficult to estimate from 
the far-infrared observations because the dust properties are essentially 
unknown. Moreover, these observations probe only the dust and not the gas, 
which makes up most of the mass. Ground-based aperture-synthesis observations 
of the gas through molecular line transitions in the millimeter range will be 
forthcoming. Although such observations can yield much insight into the 
dynamics of these systems, mass estimates will still be difficult, because the 
dominant component of the gas, molecular hydrogen (H 2 ), cannot be observed 
this way. 

T Tauri stars are newly formed stars as evidenced by their high lithium abun- 
dance, their high rotation velocities, and their general association with dark 
clouds. Their ages, as estimated from their position in the Hertzsprung-Russell 
(HR) diagram, range from about 10 5 to 10 7 years. T Tauri stars are often sur- 
rounded by a circumstellar disk. As inferred from statistics, these disks are 
presumed to have a large radius-to-thickness ratio and they give the impression 
of a continuous transition in the morphology of protostellar nebulae from the 
very earliest protostellar stages (the HH-exciting star), through the pre-main- 
sequence stars (the T Tauri stars), to the main sequence, where protoplanetary 
"disks" may be detected. Recent studies indicate that T Tauri stars are very 
active and exert a significant influence on their environment. Besides the strong 
stellar wind mentioned before, X-ray and chromospheric emission have been 
detected, indicating strong flare activity on the protostellar surface. All T Tauri 
stars may undergo the FU Orionis phenomenon (a major stellar eruption) during 
their evolution. 

Among the T Tauri stars, the object HL Tau is a keystone. It is currently the 
only T Tauri star to show ice absorption in its circumstellar shell. It also exhibits 
the strongest silicate absorption feature at 10 fJtm and the greatest degree of 
visible linear polarization. Polarimetric, near-infrared images indicate a flattened, 
extended object on a scale of TO 2 AU. The star shows HH object nebulosity as 
well as an extended streamer or jet. All of these characteristics point toward the 
presence of a flattened dust distribution close to the exciting star, possibly a 
dusty disk. In the HR diagram, HL Tau is located at the top of the convective 
track for a 1-M @ star, with a stellar age of about 10 5 years. HL Tau may there- 
fore provide an ideal analog for the protosun at age 10 5 years. 

Estimates of the column density in small particles provide a lower limit to 
the mass of dust that is consistent with the far-infrared estimates of 
1-6X10" 5 Mq. Much more material might be hidden at the much smaller scale 
size than presently resolvable, or the gas in the immediate vicinity of HL Tau 
may have already condensed into solid grains and accreted into undetectable 
large (centimeter-sized) particles. 

Understanding the formation of stars and planets and the distribution and 
composition of matter (particularly the biogenic compounds) in protostellar 
nebulae is a topic for exobiology. This includes studies of the physical and 



49 



chemical conditions in the collapsing envelope, the formation and evolution of 
circumstellar disks and protostars, and the interaction of the protostellar wind 
with the collapsing envelope and the parental molecular cloud. A study of these 
questions as a function of age of the protostar, coupled with information on the 
effects on biogenic materials from these environmental factors and the corre- 
sponding role of biogenic compounds in the collapse process (e.g., as cooling 
agents), could provide general constraints on the origin of life in other solar 
systems. In particular, an important question to answer is that of the relative 
time scales for planet formation, the origin of life, and the decay of the eruptive 
activity of star formation. It should be repeated here that objects such as HL 
Tau may provide excellent analogs for studying the early stages of formation of 
the solar system and the study of astrophysical boundary conditions on the 
origin of life in planetary systems. 

Although there is secure theoretical support for this global picture of star 
formation, the observational evidence is rather limited. Astronomers lack an 
adequate observing tool. In particular, as evidenced by Appendix C, a resolution 
of about 0.01 arcsec is required to resolve planet formation in the dusty disk 
around protostars in the Taurus dark cloud, the nearest site of star formation 
(see fig. 3-3). Such resolution will not be available in the near future. However, a 
permanent manned presence in Earth orbit, along with construction capabilities, 
raises the possibility of large arrays of orbiting telescopes. These could easily 
have baselines capable of resolving planets at 1 AU from their stars. Over the 
longer term, observing facilities could be placed at a lunar base. 

Detailed studies of collapsing protostars with sufficient spatial and spectral 
resolution to decipher the chemical and kinetic variations inflicted on the bio- 
genic elements and compounds as a function of time is essential to understand- 
ing whether the formation of a protoplanetary system similar to our own proto- 
solar nebula is a common occurrence in the universe. Further, having formed a 
protoplanetary system, it is also critically important to understand whether the 
final stages of the star-formation processes in the center of the nebula have any 
significant influence over the planet-formation process, and/or the early history 
of the newly formed planetary surfaces and atmospheres. Observationally, we 
have recently understood that the star-formation process is far from quiescent, 
even for the low-mass stars similar to our Sun. What is yet unknown is when the 
precise epoch of planetary system formation occurs in relation to the energetic 
outflows and HH object formation and acceleration and the existence of a very 
large rotating disk system of molecular gas and dust. And how do these various 
phenomena affect the newly formed planets? What are the signposts of systems 
that are about to form planets versus those that have already formed planets 
versus those that never will? Will these planets be like the assemblage in our own 
solar system? What is the distribution of raw materials from which they can 
form? In time it may be possible to detect and spectroscopically examine planets 
in orbit about other stars, as well as any debris left over from the formation of 



50 



the planets, but until that time, studying the precursors of the planetary systems 
and using them as analogs for understanding our own solar nebula may help us 
to assess the potential for life sites elsewhere in the universe. 

During the collapse phase, the opacity in the collapsing envelope is very high 
in the visible and near-infrared. Essentially, one can hope to see the central 
object and the dusty disk only in the far-infrared and submillimeter. Fortu- 
nately, many molecules, atoms, and ions have transitions in these wavelength 
regions. Poor atmospheric transmission in these wavelength regions mandates 
observations from Earth orbit. The LDR will be a good instrument for the study 
of the collapse phase of star formation. The LDR, with a 20-m aperture, can 
achieve a resolution at 100 micrometers of about 100 AU for protostars in 
the Taurus cloud, sufficient to resolve the collapsing envelope. Studies of molec- 
ular rotational transitions can then yield important chemical, spatial, and kine- 
matic information on the collapsing envelope and the interaction of the outflow 
with the circumstellar envelope. High spectral resolution (X/AX = 10 5 ) is 
required for such studies. Such high resolution is also required to provide veloc- 
ity discrimination across the accretion shock on different parts of the proto- 
planetary disk and on the central object, as well as to study the interaction of 
the outflow with the circumstellar disk and with the parent molecular cloud. 
Important lines for these studies are the far-infrared, fine-structure transitions of 
Cll, Ol, and Ol II, the high-lying rotational transitions of CO, and the rotational 
transitions of HD and H 2 . 

Important information on the kinematical structure of the collapsing enve- 
lope and the outflow, and their interaction, can also be obtained from near- 
infrared absorption-line studies. Molecular vibrational transitions are particularly 
useful, because the detailed rotational structure can be used to derive excitation 
temperatures and total column densities. This may aid in relating potential 
multiple components spatially. Such studies have already been performed for the 
luminous, massive protostar in Orion. With a cooled telescope from Earth orbit 
(e.g., SI RTF and ISO) this also will become possible for less-luminous, less- 
massive protostars, such asTTauri stars. High spectral resolution (X/AX = 10 5 ) is 
imperative to resolve the rotational structure. 

The HST may provide an excellent platform to observe protostars during the 
latter phases of the collapse, when the envelope has been dispersed by the strong 
stellar wind. Its- high spatial resolution and good ultraviolet sensitivity will allow 
studies of HH objects and jets much closer to the photospheric surface than 
presently possible. The ultraviolet lines of high ionization stages of C and O are 
useful diagnostics for the shock conditions (e.g., pre-shock density, shock veloc- 
ity). The 0.1-arcsec resolution of the WF/PC on the HST corresponds to about 
10 AU at the distance of the Taurus cloud. This is probably still insufficient for 
spatial resolution of the actual acceleration zone of the stellar wind, yet the 
question of precisely where HH objects originate may be amenable to study 
from the HST. By imaging through narrow-band filters that isolate specific 



51 



shock-cooling emission lines and using the HST's high ultraviolet spatial resolu- 
tion, one might hope to trace details of the HH flows into the immediate stellar 
vicinity using their characteristic shock lines. These observations might bear at 
least indirectly on the issue of confinement of HH flows by circumstellar disks. 
Examination of the "base" of an HH flow might serve to determine whether a 
mechanism similar to a solar coronal hole is operating near the protostellar pole 
or whether the confinement and orientation of these flows arises some distance 
above the stellar surface. 

A crucial question, of course, relates to the nature of those stars that are 
known to drive HH flows or jets. It is believed that they are the precursors of 
even T Tauri stars, but they are often so deeply embedded in dark clouds that 
they are not directly visible, optically. One technique for studying their photo- 
spheres is to investigate the spectrum of scattered starlight from circumstellar 
clouds and nebulae. This method has begun to bear fruit, optically, and it could 
certainly be extended into the ultraviolet with the HST. 

Polarization studies on this scale may also yield important information on 
the structure of the circumstellar disk, through the asymmetrical scattering of 
starlight by grains in the disk. 

Recent work with the IUE has demonstrated that variable ultraviolet absorp- 
tion lines occur in the spectrum of Beta Pictoris. These lines are thought to arise 
in the circumstellar disk detected in the infrared around this star. Such spectra 
could be obtained with the HST for a much larger sample of candidate stars 
around which IRAS observations indicate the potential presence of dusty disks. 

SI RTF and ISO will be able to detect the infrared continuum emission from 
low-mass protostars throughout the galaxy, although their relatively large beam 
size in the far-infrared (~30 arcsec), where most of the luminosity is emitted, 
will present some confusion problems for distances of about 10 kpc or larger. 
LDR will be able to detect low-mass protostars to a distance of 100 kpc. This 
includes, besides our own galaxy, the Large and Small Magellanic Clouds. 

SI RTF and ISO could provide diffraction-limited, two-dimensional imaging of 
near-infrared starlight scattered by disk particles. Because of its larger aperture 
size, SOFIA could improve upon these telescopes for far-infrared studies of the 
collapsing envelope. In particular, because of the increased spatial resolution, 
one might hope to resolve all dimensions of the collapsing envelope. Such studies 
will also be helped by the advent of new detector technology (e.g., the develop- 
ment of linear and fully two-dimensional detector arrays designed to oversample 
the Airy disk, thereby producing the spatial resolution close to the theoretical 
limit). 

Suggestions for Further Reading 

Black, D. C; and Matthews, M. S., eds.: Protostars and Protoplanets II. 
University of Arizona Press, Tucson, 1985. 
Rev. Mexicana Astron. Astrofis., vol. 7, 1 983. 

52 



3.8 The Detection of Other Planetary Systems 

One major goal for exobiology is to determine the distribution of potential 
sites for the emergence of life in the universe. Since life as we know it is a plane- 
tary phenomenon, this means detecting and surveying extrasolar planetary sys- 
tems as well as systems where planets are about to form or where planets have 
failed to form. It is of particular interest to discover whether the general charac- 
teristics of our solar system, with its trio of early water-bearing planets, bom- 
barded by organic-rich meteorites and comets, are repeated frequently in other 
systems. We may be able to make great progress in understanding why Venus 
and Mars apparently contain no extant life by comparative planetology studies 
within our own solar system alone. However, to the extent that the limitations 
on life involved the cosmic environment of these two planets, other planetary 
systems must be studied to assess how probable the sets of conditions favorable 
to the formation of life really are in the universe. 

There is currently no unambiguous evidence for the existence of any plane- 
tary system outside the solar system. Recent advances in ground-based instru- 
mentation have resulted in the detection of large disks of dust and gas surround- 
ing nearby stars. Nonetheless, evidence for extrasolar planetary systems is still 
lacking. Star-formation theory predicts that planets will abound, but none has 
been found, only the intriguing clues. This is perhaps not too surprising; until 
recently, the measuring tools available were woefully inadequate for the discov- 
ery of all but the most massive planets around the nearest stars. On more than 
one occasion, the classical ground-based observational programs have resulted in 
exciting discovery claims of one or more planets orbiting familiar stars, such as 
Barnard's star. However, more recent measurements with modern photoelectric 
detectors negate these earlier discoveries and illustrate how difficult it is to 
fully account for all the systematic biases that may develop as equipment ages in 
a dynamic environment under the influence of 1 Earth gravity. The opportuni- 
ties for stably orbiting long-lived instrumentation above the Earth's atmosphere 
in the near future promise to revolutionize this situation. 

There are four different ways to search for extrasolar planetary systems; all 
are complementary and all will probably be necessary to detect and study all 
the members of another system. Of these four methods, three will derive great 
benefit from orbiting instrumentation which will remove the performance limita- 
tions imposed by the atmosphere. Definitive results can be expected within the 
decade, and spectacular successes are possible if dedicated systems can be 
afforded. 

How do you detect another planetary system? Given sufficient angular reso- 
lution and the ability to detect a very faint object in contrast to a very bright 
object, one might hope to directly image extrasolar planets at optical or infrared 
wavelengths. The task is extremely difficult. To detect Jupiter in orbit about 
the Sun, if the system were at a distance of 10 parsecs (33 light-years), requires 
the ability to distinguish two objects whose relative brightness is 2X1 Q~* and 

53 



whose apparent separation is 0.5 arcsec. For the Earth/Sun system, at 10 par- 
sees, the contrast ratio is 2X10 -10 and the separation on the sky is only 
0.1 arcsec. Contrast ratios in the infrared may be less severe for the giant planets, 
and in general smaller, fainter stars make the contrasts more favorable. The 
primary limitation for ground-based systems is that the atmosphere spreads out 
the image of a star over a circle that is at best 0.5 arcsec in diameter, inhibiting 
the discovery of any close-in orbiting planets. Planets in larger orbits will reflect 
less starlight and thus be intrinsically fainter and more difficult to detect with 
any imaging device. 

There are also three indirect methods for deducing the presence of planetary 
mass companions in orbit about nearby stars. The first measures the reflex 
motion of the star resulting from the fact that both stars and planets orbit about 
a common center of mass. This orbital motion introduces a tiny "wobble" into 
the space motion of a nearby star when measured against the more distant fixed 
stars. Astrometry is the attempt to accurately measure the relative position of a 
star over decades in order to uncover this small periodic displacement. To infer 
the presence of J upiter in orbit about a solar mass star at a distance of 1 parsecs 
requires the position of the star to be measured with an accuracy of 0.3 milliarc- 
sec, while an accuracy of 1 microarcsec is required to detect the influence of 
Earth. With extreme care and modern instrumentation, ground-based observator- 
ies can achieve an accuracy of 1 milliarcsec by combining many observations 
over a period of months. The atmosphere imposes an absolute limit of 0.1 milli- 
arcsec, but long-term systematic changes in the telescope itself may prevent this 
limit from being achieved. Unlike direct imaging, astrometric measurements 
must be continued over time scales comparable to the orbital periods of the 
planets in order to verify that the detected wobble has the expected periodic 
shape. 

If the orbital plane of the planets is nearly perpendicular to the plane of the 
sky, two other indirect detection techniques are possible. The orbital motion 
about a common center of mass can be detected because of the periodic Doppler 
shift it introduces into the spectrum of the star. An emission or absorption line 
from the atmosphere of the star will be blue-shifted as the star approaches the 
observer and red-shifted as the star recedes along its orbit. Searches based on 
these shifts in the wavelength of the stellar lines are called spectroscopic, and 
this is the method of detection that is least disturbed by atmospheric attenua- 
tion and turbulence. Great precision in measuring the change in the wavelength 
is required over periods comparable to planetary orbit time scales. Great preci- 
sion is currently being achieved from the ground. This method is nearly indepen- 
dent of the distance to the star, provided that the star is sufficiently bright. The 
effects of a Jupiter-sized planet in orbit about a large number of relatively 
nearby stars should be discernible from the ground, but the searches have been 
under way for only a few years and there are not yet enough data. Terrestrial 
atmospheric effects will preclude the detection of the smaller wavelength shifts 



54 



introduced by an Earth-sized planet and some, as yet unknown, but quasi- 
periodic fluctuation in stellar atmospheres themselves may impose a more 
stringent limitation on this technique. 

The last detection method, called "photometric," is statistical. If the orbital 
plane is favorably inclined, then the luminosity of the star may be observed to 
decrease slightly every time the star is occulted by one of the orbiting planets. 
The event is short-lived and unpredictable because of the unknown inclination of 
the orbital plane and the phases of the planets when the measurements are 
begun. Thus many stars must be monitored continuously with precision suffi- 
cient to detect a change in stellar luminosity of 10~ 6 in each of two broad-band 
colors, so that fluctuations in the stellar emission can be distinguished from the 
real eclipse events. Ground-based studies of new instrumentation for this tech- 
nique are just beginning, and it is not yet clear whether the required technology 
can be achieved. 

The HST is unlikely to achieve the earlier advertised contrast ratio of 
2X1 CF at a separation of 1 arcsec at visible wavelengths, and its usefulness for 
direct imaging of planetary systems is dubious. It is more likely to be of greater 
use in the study of the larger and more extended precursors to planetary systems 
(see section 3.7). However, if time can be obtained, this instrument may be 
able to search a handful of nearby stars or follow up on other indications of 
planetary systems; a systematic survey is unlikely given the enormous demand 
for observing time. The second generation of HST instrumentation will include 
one of two infrared instruments currently being developed competitively, either 
a near-infrared camera (NICMOS) or an imaging Michelson spectrometer (IMS). 
Neither is ideal for planetary detection, but they do open up the wavelength 
region with the most favorable contrast ratios for the giant planets. Evaluation 
studies of the HST optics and the imaging performance of these two instruments 
should be closely watched by the exobiology community to attempt to ensure 
that the superior planet detector is selected. ISO and SI RTF will operate in the 
infrared, and have been discussed as possible opportunities for imaging nearby 
planetary systems. Although their sensitivity will be extremely good, both tele- 
scopes lack the required spatial resolution. The proposed "superresolution" 
capability requires signal-to-noise ratios that are probably greater than those pro- 
vided by a faint planet. The prospects for direct imaging of a large number of 
planets in the near term do not look hopeful. 

The HST will also have two instrument systems that can make astrometric 
measurements. Neither will improve on the current ground-based accuracy and 
neither will be available for the extensive observing time needed for multiple 
observations over the decades required to discern the systematic stellar motion. 
The single best near-term candidate instrumentation for a systematic search for 
extrasolar planetary systems is a dedicated imaging astrometric telescope 
mounted on the upper boom of the Space Station. NASA's Task Force on the 
Scientific Uses of the Space Station has recently concluded that it should be 



55 



possible to build an orbiting astrometric telescope, associated with the manned 
Space Station, which would be capable of positional accuracy of 10 micro- 
arcsec on stars in limited areas of the sky for periods as long as 10 to 20 years. 
This accuracy exceeds current ground-based performance by two orders of mag- 
nitude. With such a capability it would be possible to detect, with some 
assurance, planets like Uranus around the nearest few hundred stars, and Jupiter- 
like systems would, if they exist, be easily found. A reasonable statistical sample 
of other planetary systems could be developed with an astrometric telescope in 
orbit, or very meaningful limits could be placed on their existence. 

As already stated, spectroscopic searches for extrasolar planetary systems are 
little affected by the atmosphere. As long as the apparent brightness of the star 
is great enough, this technique can provide exactly the same capability to find 
planets orbiting distant stars as it can for nearby ones. Systems with the required 
stability to recognize the periodic wavelength shifts induced by a Jupiter-mass 
planet already exist; detection of the smaller shifts induced by terrestrial-type 
planets will ultimately probably be limited by the atmosphere. Current ground- 
based efforts are now limited by the lack of detailed information about the pres- 
ence of any long-term periodic or quasi-periodic changes within the stellar 
photospheres that could mimic or mask the planetary velocity signature. It 
seems quite likely that this technique will begin to pay off consistently in the 
near future. 

Photometric searches capable of monitoring 4000 stars continuously in two 
colors with an absolute precision in the luminosity determination of 10~ 6 might 
detect about one planet per month. Most of the detections will be the short- 
period planets, and planets as small as Earth may not be detectable during the 
active phases of the stellar cycles. A number of technological obstacles must be 
overcome before this technique can be demonstrated. This may eventually be 
possible with instrumentation proposed for the Block II Space Station; work is 
beginning on the required technology. Photometry could prove to be an espe- 
cially critical technique for studying the distribution of mass and orbits in other 
planetary systems, since it works best for the short-period inner planets, unlike 
the other approaches. 

In contrast to photometry, the opportunity for making an early start on an 
orbiting astrometric telescope has never been clearer or more promising. An 
orbiting manned Space Station would ensure that the necessary orbiting super- 
structures will be available to allow the development and operation of a dedi- 
cated astrometric telescope at very low cost in a reasonable time. The manned 
Space Station is a key element for a number of reasons. First is an ability to per- 
form consistent on-orbit maintenance and service, which will ensure that the 
system can operate consistently over the required time. Second, although the 
telescope system should be designed for largely routine, automatic operation, 
there would no longer be a requirement for it to be completely autonomous, 
thus reducing the cost substantially. Third, on-orbit integration with the Space 



56 



Station would relieve any requirement that it be transported to orbit in a fully 
integrated way. Finally, the Space Station will provide a stable platform from 
which to conduct observations for the decades that are required to accomplish 
a systematic astrometric search of a large number of stars. Because the stellar 
position is measured only relative to the reference stars in the surrounding field, 
and not absolutely, an astrometric telescope is uniquely immune to nearly all of 
the mechanical perturbations introduced by manned use of the Station. It is 
therefore one of the few astronomical tools that can be considered for an 
attached payload— the most cost-efficient way to interact with the Space 
Station. 

In the hopeful event that planets begin to be identified around nearby stars, 
the next task will be to attempt to discern whether some form of life might 
exist there. New technologies and a new generation of dedicated orbital instru- 
mentation would be required to tackle this problem. Some very preliminary 
suggestions have been made; they require either an interferometer or a very large 
single aperture operating at visible wavelengths. If the separation of the 
~1-m elements of the interferometer can be maintained to within 10~ 6 cm 
for periods of ~100 hours, then it should be possible to image the atmosphere 
surrounding the distant planet and spectroscopically detect the ozone band near 
6800 A, assuming the concentration of ozone is similar to the very large non- 
equilibrium (biologically generated) value of the terrestrial atmosphere. Require- 
ments on the surface accuracy of a large monolithic mirror greatly exceed those 
achieved for the HST. If such a mirror quality can be achieved and combined 
with occulting masks that suppress the bright stellar image, similar searches for 
2 and ozone could be conducted. Near-term ground-based studies should begin 
on these or any other approaches to explore the potential for developing the 
technologies required to meet what today appear to be impossible specifications. 

Suggestions for Further Reading 

Black, D. C; and Brunk, W. C, eds.: An Assessment of Ground-Based Tech- 
niques for Detecting Other Planetary Systems. NASA CP-21 24, 1 986. 

Tarter, J. C; Black, D. C; and Billingham, J.: Review of Methodology Avail- 
able for the Detection of Extrasolar Planetary Systems. J. British Interplanetary 
Soc, vol. 39, 1986, p. 41 8. 



3.9 Molecules in Space 

There are two reasons for the relevance of interstellar chemistry to exobiol- 
ogy. First, the presence of large amounts of fairly complex organic molecules 
(up to at least 13 atoms in size) in interstellar clouds raises the question of what 
the limit is to the complexity attainable by interstellar molecules. Second, if 



57 



complex molecules are synthesized in the interstellar medium, the question of 
the possible connections between the existence of these species and the exis- 
tence of biological molecules on Earth is of interest. The first of these questions 
can be addressed by both theoretical and observational approaches. It is feasible 
to discuss whether the theoretical models predict large abundances of yet-to-be- 
detected complex molecules, whether observational techniques can locate such 
complex molecules if they exist in appreciable abundance, and what the best 
frequencies and observational tools might be. The second of these questions is 
very difficult to answer. The processes of star and planet formation from collaps- 
ing interstellar clouds are not yet understood in any detail. An understanding of 
the chemical relationship between organic interstellar molecules and those on 
newly formed planets such as the primeval Earth requires an understanding of 
the physical conditions experienced by the molecules during the transitions from 
interstellar cloud to protosolar nebula and comets, from protosoiar nebula to 
planetesimals, and from planetesimals and comets to planets. (Some of these 
issues are addressed in other sections of this report.) We will discuss answers to 
the first of the two questions only. In addition, we will briefly discuss small 
molecules of exobiological interest— chief among these is water. 

Interstellar clouds are giant accumulations of gas and dust particles that exist 
primarily in the planes of the Milky Way and other spiral galaxies. Typical tem- 
peratures in these clouds range from 10 to 100 K except in localized star- 
forming regions, which are considerably warmer. The matter is mainly gaseous, 
with perhaps only 1% (by mass) in the form of small dust particles, or grains, 
roughly 0.1 jum in radius. The density of the gas in a typical, dense, interstellar 
cloud is in the range 10 4 to 10 6 cm" 3 ; that in the so-called diffuse interstellar 
clouds is typically in the range 10" 1 to 10 2 cm -3 . The major constituents of the 
gas are molecular hydrogen (H 2 ) and helium. Molecules involving heavier atoms, 
of which close to 70 are known, possess lower abundances by significant factors. 
With respect to H 2 , the (fractional) abundance of CO in dense clouds is typically 
10" 4 ; that of water, as large as 10" 5 ; that of methanol, 10~ 7 ; and that of even 
more complex species 10~ 8 to 10~ 10 . The molecules range in complexity from 
diatomics such as H 2 and CO up to a 13-atom, linear, unsaturated nitrile 
(HC n N) and include many of the better-known small organic molecules. Note 
that even though the detected molecules involving heavy atoms are trace con- 
stituents of a rarified gas in the clouds, the clouds are so vast that the amount of 
organic matter in them surpasses that on Earth. 

The mechanism by which gaseous molecules in dense interstellar clouds are 
excited into emission in the microwave region of the spectrum is predominantly 
inelastic collisions. These collisions convert translational energy into rotational 
energy, promoting one of the collision partners into an excited rotational state 
that then emits as it relaxes to a lower rotational level. In the bulk of interstellar 
clouds, there is insufficient thermal energy to populate excited vibrational 
states via inelastic collisions. Consequently, vibrational emission, which would be 



58 



detected in the infrared, can be seen only in selected hot regions such as star- 
forming areas. Vibrational spectra can sometimes be seen in absorption if there 
exists a suitable continuum infrared source such as hot dust surrounding a star 
embedded deep in the cloud. Nevertheless, practical difficulties in the use of 
ground-based infrared astronomy, such as atmospheric interference, have hereto- 
fore relegated this technique to a distant second place behind radio astronomy in 
the observation of gaseous interstellar molecules. 

The two important In situ mechanisms for the production of complex gas- 
phase molecules in dense interstellar clouds are synthesis from atoms and smaller 
molecules by reactions on the surface of dust grains followed by desorption into 
the gas or by reactions in the gas phase. Both of these mechanisms have been 
studied in some detail. In general, it has been difficult to determine whether gas- 
phase reactions or grain-surface reactions are more important, although the 
former are currently favored. Quantitative, time-dependent treatments that con- 
tain calculated abundances of large molecules have been undertaken for both the 
grain-surface mechanism and for the gas-phase mechanism. These calculations 
involve sizable numbers of molecules and reactions, but terminate at species of 
rather small size in the context of biology. It is possible to simplify the compu- 
tational problems involved in these models and still obtain agreement with 
observation. In particular, a "semidetailed" steady-state treatment of the gas- 
phase chemistry in the dense interstellar cloud TMC-1 can account for the abun- 
dances of organic hydrocarbons as complex as methyl acetylene (C 3 H 4 ) or the 
radical C 4 H. Unfortunately, extension of even this treatment to larger molecules 
is hindered by a lack of rate-coefficient data for important reactions. As these 
data are obtained in the laboratory, the semidetailed treatment can be extended 
to include larger molecules. A still more approximate solution of the kinetic 
equations has been utilized to estimate how efficient gas-phase reactions are in 
producing complex interstellar hydrocarbons. In this approach, hydrocarbons 
through 12 carbon atoms in size have been considered. 

A severe upper limit to the abundances of complex hydrocarbons in the gas 
phase can be obtained from cosmic-abundance arguments. Assume that the total 
fractional hydrocarbon abundance (with respect to the gas density) in the gas 
phase of dense interstellar clouds is about 10" 4 . This is approximately the total 
fractional carbon abundance and is itself probably an upper limit. Also assume 
that the total abundance of all hydrocarbons with n carbon atoms is a factor of 
a times the abundance of all hydrocarbons with /?-1 carbon atoms where 
< a < 1. This assumption is clearly an idealization, but it is hard to imagine 
the abundance of all hydrocarbons with n carbon atoms increasing as /? increases 
for a significant range of n. According to a variety of theoretical treatments, the 
fractional abundance of 1 -carbon hydrocarbons, fQ 1 (mainly methane), is 10~ 6 . 
Then a is equal to 0.99, and complex hydrocarbons possibly can have high 
abundances. For example, the fractional abundance of 12-carbon hydrocarbons 
would have an upper limit of 9.0X10" 7 . Thus, the use of cosmic-abundance 



59 



arguments and a constant multiplicative factor a do not lead to stringent limits 
on the abundances of complex hydrocarbons unless the total cosmic abundance 
resides in the smallest hydrocarbons (i.e., fQ approaches 10~ 4 ). 

The abundance of hydrocarbons produced by gas-phase reactions has been 
estimated using the approximate kinetic model mentioned previously. With this 
model, the total fractional abundance of all 12-carbon-atom hydrocarbons is 
approximately 10~ 10 , far below the upper limit we have developed here. The 
semidetailed treatment extends only to 4-carbon-atom hydrocarbons. A crude 
extrapolation of these data leads to predictions that fQ l2 is somewhere in the 
range 10~ 9 to 10 _1 1 , in good agreement with the former hydrocarbon estimate. 
So it would appear that despite great uncertainties, approximate and semide- 
tailed gas-phase theoretical treatments "predict" abundances far below the 
cosmic-abundance-derived upper limits. The detailed grain-surface treatment is 
a time-dependent model, as is the detailed gas-phase model. The use of these 
models to extrapolate complex hydrocarbon abundances will lead to answers 
that depend on the age of the interstellar cloud. In the gas-phase model, abun- 
dances of hydrocarbons are maximized at intermediate cloud lifetimes (10 5 to 
10 6 years) well before steady state is achieved. Extrapolation of calculated 
abundances at such lifetimes leads to higher estimates than extrapolations based 
on the semidetailed model. However, an estimate of the minimum time neces- 
sary to synthesize such complex hydrocarbons via gas-phase reactions yields an 
answer of approximately 10 7 years, far longer than the time at which the smaller 
hydrocarbons exist at maximum abundance, thus throwing doubt on the extrap- 
olations. The grain-surface treatment, on the other hand, shows smaller hydro- 
carbons with high abundances at a time as large as 10 7 years. Extrapolation of 
these results yields a value of fQ below 1 _1 2 . 

It is difficult to conclude much at all from these extrapolations other than 
that they yield complex hydrocarbon abundances significantly smaller than 
upper limits based on cosmic-abundance arguments. Increased measurements of 
important rate coefficients will enable the heretofore successful gas-phase semi- 
detailed treatment discussed above to be extended to much larger species. 

What do current observations tell us about the abundances of very complex 
molecules in dense interstellar clouds? A broad infrared feature at 3.4 jtim has 
been interpreted as indicative of complex organic matter on the surfaces of 
grains. Likewise, other broad infrared features have been interpreted as indica- 
tive of a wide variety of unresolved polycyclic aromatic hydrocarbons in the gas 
phase. However, our best extrapolations based on distinctive, sharp, gas-phase 
microwave spectra involve the linear cyanopolyynes, HC /7 N, where n is an odd 
integer. In the well-studied source TMC-1, HC 3 N, HC 5 N, HC 7 N, HC 9 N, and 
HCjjN have been observed. Their abundance drops by a factor of approxi- 
mately four each time two more carbons are added from n = 5 through n - 9. 
With a fractional abundance of 10~ 8 for HC 5 N, simple extrapolation using 
this factor yields a fractional abundance of approximately 6X10 -13 for the 



60 



hypothetical interstellar molecules HC 19 N. This rather large result is in reason- 
able agreement with the extrapolation of the successful gas-phase semi detailed 
calculation to fQ (10 -11 to 1CT 15 ) only if we assume a synthetic bias toward 
linear, unsaturated species. The number of possible 20-carbon hydrocarbons is 
quite large and encompasses many different types of skeletal branching. If many 
of these species have abundances similar to the estimated abundance of HCj 9 N, 
then our large cosmic-abundance-determined limit may indeed be appropriate. 
However, the gas-phase mechanism favors highly unsaturated species. If this 
mechanism is operative, according to current opinion, then the large abundance 
of HC 19 N is in line with our extrapolation of the semidetailed calculation. 
Despite this agreement, we are forced to conclude that there is indeed great 
uncertainty in trying to estimate the abundances of complex hydrocarbons and, 
by extension, all other complex organic species based on the limited, present- 
day observations. It seems reasonable to suggest that our knowledge of the abun- 
dances of these species in dense interstellar clouds based on microwave observa- 
tions will increase gradually and incrementally as species slightly larger than 
those previously seen are detected. 

What are the prospects for observing species more complex than HCjjN? 
Consider the spectral region currently utilized by radio astronomers; this extends 
roughly from 1 to 300 GHz, with the lower frequencies labeled microwaves 
and the higher frequencies labeled millimeter waves. Studies of the radiation 
physics of emission lines in this spectral region show that for each molecule 
there is an optimum emission frequency, or frequency of maximum emission 
intensity, which is a function of both temperature and the size of the molecule. 
As temperature increases, the optimum emission frequency increases as the 
square root of temperature. As the size of the molecule increases, the optimum 
emission frequency decreases dramatically. As an example, for a linear hydrocar- 
bon with 20 carbon atoms in a cool interstellar cloud (T = 10 K) the optimum 
emission frequency is about 5 GHz, whereas in a warm cloud (T = 50 K) this 
frequency rises to about 1 1 GHz. If the calculations are performed to maximize 
the observed signal-to-noise ratio, this latter frequency falls to 8 GHz, because 
the noise level of the receiver increases with frequency. This example shows that 
the detection of complex molecules via rotational transition frequencies is best 
attempted at microwave and millimeter rather than submillimeter wavelengths. 

However, there is a fundamental problem in observing complex molecules via 
rotational transitions at any frequency. As molecules become more complex, 
their density of rotational states increases, typically (for linear molecules) as the 
cube of the number of heavy atoms. This means that the number of molecules in 
any given rotational level decreases dramatically as the number of atoms 
increases. The intensity of an emission spectral line is proportional to the 
number of molecules in a given energy level. Hence, the intensity of any spe- 
cific emission line decreases strongly as the number of atoms increases, because 
the emission is spread out into many more emission lines. In particular, a linear 



61 



hydrocarbon with 19 carbon atoms will have its emission line of optimal fre- 
quency reduced in intensity by a factor 300 from a linear hydrocarbon with 
3 carbon atoms, assuming the species have the same dipole moment and the 
same abundance. 

Although these considerations suggest that observing more complex mole- 
cules in the interstellar medium via microwave and millimeter studies will be 
difficult, it is possible that other wavelength regions may provide a better 
method of detecting these molecules. Studies in the infrared region can be con- 
veniently divided into low-resolution and high-resolution investigations. High- 
resolution studies can be used to study gas-phase interstellar molecules in both 
absorption and emission. Low-resolution studies can be used for both the 
gaseous and condensed phases. Emission studies in the infrared require a warm 
or nonthermal (e.g., shocked) portion of an interstellar cloud, whereas absorp- 
tion studies require a suitable background continuum source. At first glance it 
would appear that infrared observations avoid the severe problems encountered 
in the microwave and millimeter ranges caused by the dilution of rotational 
states of complex molecules since the density of vibrational levels of complex 
molecules is quite low even for very complex species, at least at low excitation 
energies. This apparent advantage versus rotation cannot be realized in high- 
resolution studies because gas-phase molecules rotate as well as vibrate and a 
high-resolution infrared transition involves a change in the rotational, as well as 
the vibrational, quantum state. Low-resolution infrared studies avoid this prob- 
lem of rotational fine structure, but at the expense of facile identification of the 
emitting or absorbing species. To minimize the rotational fine structure, it would 
be desirable to study molecules in absorption at the lowest possible excitation 
temperatures and densities in order to minimize the excitation of rotational 
levels. Unfortunately, most theories of interstellar chemistry do not predict 
significant abundances of complex molecules in low-density, diffuse clouds. In 
addition, the Doppler broadening caused by large-scale rotation within the 
clouds may blend the closely spaced lines together and make them unresolvable 
even with ultrahigh-resolution infrared spectroscopy. However, high-resolution 
infrared studies will be beneficial for smaller emitting or absorbing species in the 
gas phase once the practical difficulties of this type of astronomy are overcome 
by space observatories such as SI RTF. 

It would thus seem that infrared studies at lower resolution offer our best 
hope for complex molecule observation. However, the difficulty of identifying 
the carrier of a low-resolution spectrum can be great. Consider the broad, 
3.4-jum feature which has been explained by varieties of organic matter on dust 
surfaces, but has also been claimed to be due to bacteria. Or consider the tenta- 
tive, but hardly unambiguous, identification of gaseous polycyclic aromatic 
hydrocarbons via broad features at 6.2 and 7.7 jum. Perhaps the best that can be 
hoped for is the functional group analysis used by organic chemists who observe 



62 



a low- or medium-resolution infrared spectrum of a complex molecule and 
deduce what functional groups and/or bonds the molecule possesses. This type 
of analysis utilized for interstellar spectra would not yield specific molecules, 
but would indicate the types of complex molecules present. Any organic mole- 
cule would be expected to show a C-H stretch at 3.38 to 3.51 jum, an olefinic 
hydrocarbon could show a C=C stretch at 5.95 to 6.17 jum, an alcohol could 
show an O-H stretch at 2.75 to 2.79 jum, etc. A special area of investigation that 
may hold unusual promise is the spectra of complex molecules in the far infrared 
(over 50 jum or less than 6 THz); far-infrared spectra correspond to low- 
frequency vibrational transitions, which are often more precise indicators of the 
specific species than are the higher-frequency vibrations. Whether this specificity 
would remain under low resolution is unclear at present in large part because of 
a lack of spectroscopic data in this region of the spectrum. The best-studied 
far-infrared spectrum is probably that of methanol, in which it arises from a 
so-called torsional motion. Torsional spectra of many other simple species still 
remain to be studied in detail in the laboratory. 

It has been suggested that the well-known diffuse bands, seen in the visible 
and ultraviolet as starlight passes through diffuse interstellar clouds, are due to 
complex gas-phase organic hydrocarbons, specifically C^ species with n > 5. 
Although it is unclear how complex molecules can be efficiently synthesized 
under the low-density conditions of diffuse clouds, it is worthwhile to pursue 
this line of reasoning. The claim is that long-chain C /7 molecules might be able 
to withstand photodissociation via the interstellar radiation field that penetrates 
into diffuse, but not dense, clouds. Instead, the molecules manage to utilize the 
energy they receive from an absorbed photon to internally convert from one 
state to another until they re-emit the excess energy and avoid dissociation. The 
internal conversion process leads to a finite width for the excited states of the 
molecules, which is supposed to cause the diffuseness of the visible bands. 
Also, as discussed earlier, a new variant of this claim has been made that links 
the diffuse bands with polycyclic aromatic hydrocarbons (PAHs) and suggests 
the width of the features to be due to the unresolved rotational fine structure. 
These claims should be investigated in some detail because if they are at all valid, 
they represent a manner of specifically detecting complex species via line width. 
What must be understood are the precise spectra of these species, their photo- 
dissociation rates in the diffuse interstellar medium, and the source or sources 
of the large spectral widths. 

Theoretical models of interstellar chemistry in the future should include 
significantly more complex molecules than have heretofore been the case. This 
is especially true of gas-phase models because many ion-molecule laboratories 
are currently measuring relevant rate coefficients. The semidetailed approach to 
chemical modeling offers the best opportunity for making predictions of the gas- 
phase abundances of complex species. 



63 



Observations from above the atmosphere are needed for a number of small 
molecules in both diffuse and dense interstellar clouds. Not only are observa- 
tions of molecules of direct exobiological interest such as H 2 and 2 needed, 
but many other molecules whose study is important to obtain an understanding 
of the chemical and physical conditions in interstellar space are unobservable 
from the ground. 

The unambiguous observation of increasingly complex molecules in the 
interstellar medium will not be achieved easily. Difficulties exist in every region 
of the electromagnetic spectrum considered. In the realm of radio astronomy, 
the most likely wavelength region appears to be at low frequencies. These fre- 
quencies do not require telescopes on satellite platforms, but are quite accessible 
from the Earth. Certainly, as current receiver technology improves, it should be 
possible to extend the size of the molecules currently detected in the microwave 
range. In addition, it might be possible to detect functional groups of complex 
species or even the species themselves via low-resolution infrared spectroscopy, 
and even visible and ultraviolet studies will be significantly enhanced via space- 
based observatories such as HST, SI RTF, and LDR. The more complex the 
organic chemistry detected in interstellar clouds, the more interesting the inter- 
stellar medium becomes to exobiologists. 

Suggestions for Further Reading 

Irvine, W. M.; Schloerb, F. P.; Hjalmarson, A.; and Herbst, E.: In Protostars 
and Protoplanets II, D. C. Black and M. S. Matthews, eds., University of Arizona 
Press, Tucson, 1985. 

Winnewisser, G.; and Herbst, E.: Organic Molecules in Space. Topics in 
Current Chemistry, vol. 139, 1987, p. 119. 

Herbst, E.: On the Formation and Observation of Complex Interstellar 
Molecules. Origins of Life, vol. 16, 1985, p. 3. 



3.10 Interstellar Dust 

Small, solid particles (dust) must have played an important role in determin- 
ing which biogenic compounds were incorporated into the early solar nebula and 
at which stage of chemical processing. The exact role dust played cannot be 
understood until the nature of interstellar and circumstellar dust has been deter- 
mined. Was dust the bulk source of carbon? Was the dust siliceous but coated 
with an icy organic mantle? What volatiles were trapped within grains? How big 
were the grains before and after passage through the accretion shock? Collected 
specimens of interstellar dust particles and remote observations of many physi- 
cally different environments may answer these questions. Interstellar dust can 
be studied through its interaction with electromagnetic radiation by measuring 



64 



the extinction, the linear and circular polarization, and the scattering of nebular 
and star light. Interstellar extinction and polarization curves have been deter- 
mined for numerous stars with diffuse interstellar material along the line of 
sight. The observed extinction and polarization curves can be used to determine 
grain sizes if their composition (i.e., optical constants) is known. However, very 
little information of this sort is available for dust in molecular clouds. 

The dust can also be studied through its interaction with the gas, although it 
is difficult to extract information on the dust from these data. For example, the 
dust may significantly influence the molecular composition of the gas, but 
because of the difficulty in disentangling gas-phase and grain-surface reactions 
only H 2 is unambiguously attributed to grain-surface reactions. 

Finally, the dust can be studied indirectly by determining the depletion of 
the elements in the gas phase with respect to solar abundances. In such an 
approach it is generally assumed that the missing elements are locked in solid 
dust grains. Correlations between the observed depletions and physical param- 
eters, such as condensation temperature, may then lead to insight on the specific 
composition and condensation history of the dust grains. 

Despite 50 years of active research, the composition of interstellar grains is 
still highly uncertain. Most researchers in this field agree that silicates and some 
form of carbon are present. The detection of the 10- and 20-jum features (pre- 
sumably the Si-0 stretch and O-Si-0 bend in silicate materials) in a large variety 
of objects points toward the ubiquitous presence of silicates in the interstellar 
medium. The composition of the carbonaceous component is, however, more 
controversial. Some researchers think that the carbon is mainly in one or another 
highly condensed form, based on the strong 2200-A peak in the interstellar 
extinction curve, which can be attributed to ~200-A nearly spherical carbona- 
ceous grains. In such a model it is assumed that larger (~1000-A) carbon grains 
are also present, which then produce a large fraction of the visible extinction. A 
small fraction of the visible and the far-ultraviolet extinction, and the visible 
polarization, is produced by silicate grains with sizes in the range of 100 to 
2500 A. Inside molecular clouds these grains can accrete molecular mantles 
consisting of simple molecules such as water, ammonia, and carbon monoxide, 
plus all the other possible condensable interstellar molecules. But in the diffuse 
interstellar medium these grain mantles are presumably quickly removed by 
photodesorption and sputtering in low-velocity shocks. Good observational 
evidence exists for the presence of icy grain mantles inside molecular clouds and 
for the absence of water ice grain mantles in the diffuse interstellar medium. 

In an alternative model, the carbon is mainly in the form of large molecules 
in grain mantles on silicate cores. Some carbon (~25%) is still in some highly 
condensed form, in order to explain the 2200-A peak. The bulk of the visible 
extinction and polarization in the diffuse interstellar medium, however, is due to 
these grain mantles. The molecular mantles are presumed to be formed by ultra- 
violet photolysis of the simple molecular mixtures accreted on the silicate grains 



in molecular clouds. The more complex molecules produced by the ultraviolet 
photolysis of the simple molecules in the mantles are more refractory than the 
simple ices and therefore can better survive in the harsh environment of the 
diffuse interstellar medium. These processes can be studied in the laboratory, 
and preliminary results of such studies indicate the ultraviolet photolysis can be 
important in the evolution of interstellar dust. Some observational support for 
the presence of complex molecular-grain mantles in the diffuse interstellar 
medium exists. 

Models for the formation of the dust are likewise controversial. The refrac- 
tory grain materials, such as silicates and the carbonaceous grains, are presum- 
ably formed at high temperatures (100-2000 K) in the carbon-rich or oxygen- 
rich outflow from late-type red giants, supergiants, planetary nebulae, novae, 
supernovae, and protostellar nebulae. The detection of large infrared excesses 
supports the picture of grain condensation around late-type red giants, super- 
giants, and planetary nebulae. No unambiguous observational evidence exists yet 
for the condensation of dust around supernovae. The primary locations for the 
condensation of grains composed of the biogenic elements is one of the ques- 
tions that the exobiology community would like to answer. As discussed earlier, 
the complex grain mantles are presumed to be formed after the condensation 
process, when the grains have been cycled within molecular clouds in the inter- 
stellar medium. 

Little is known about the actual dust condensation process in the outflow 
from stellar objects. Observations typically lack the spatial resolution required to 
resolve the condensation zone, while experiments and theory are hampered by 
the lack of knowledge of the relevant physical conditions in these regions. 
Condensation possibly occurs in thermodynamic equilibrium with the most 
refractory material condensing into small nuclei onto which somewhat less 
refractory materials condense out sequentially when cooling nuclei reach the 
condensation temperature of those materials in the outflow. Alternatively, 
condensation might take place under highly supersaturated conditions and all 
materials might then condense out more or less simultaneously into an 
amorphous material. Since grains offered the best opportunity for biogenic 
molecules to become incorporated into the protosolar nebula, it is most impor- 
tant to understand the composition and elemental inventory of grains in distinct 
locales. Of particular interest is the incorporation of trace constituents, which 
have been detected in meteorites (e.g., 2e Al and S-process Xe) into this Stardust. 
These tracers may allow the nucleosynthetic history of the parent grains to be 
reconstructed. 

Finally, another important characteristic of grains, their distribution, is also 
highly dependent on the condensation process. In particular, is coagulation 
important for the formation of large grains? Obviously, large fluffy conglomer- 
ates of small particles may have quite different physical properties (surface area, 
strength, extinction properties) than large homogeneous particles. 



66 



It is appropriate to single out infrared spectroscopy because of its great 
potential for remote sensing of dust material. Infrared spectroscopy is a power- 
ful tool for the study of the composition of interstellar dust. Broad absorption 
features appear superimposed on the infrared continuum of many interstellar 
sources. Because of their spectral width, these features are generally attributed 
to molecular vibrational transitions in solid materials. 

A molecular group (such as the methyl group, CH 3 ) absorbs at a few charac- 
teristic vibrational frequencies. The peak frequency and absorption strength of 
such a group does not vary much between different molecules within a given 
class of molecules (such as saturated hydrocarbons). This makes the identifica- 
tion of molecular groups (and classes of molecules) from observed infrared 
spectra relatively easy. It does, however, somewhat hamper the precise identifi- 
cation of the specific molecule involved in the absorption or emission process. 
Precise identification can be rendered even more difficult by the presence of a 
collection of molecules and the possible overlap of their absorption bands. The 
detection of rather subtle spectral variations and thus relatively high-spectral- 
resolution studies (X/AX> 10 3 ) are needed to identify specific molecules. Fortu- 
nately, identification can be assisted by studying the spectra of a large sample of 
sources and correlating the spectral variations with the physical conditions in 
these sources. 

The possibilities and pitfalls of infrared spectroscopy of interstellar grains will 
be illustrated with two examples: interstellar grain mantles and large molecules. 

Ground-based observations around 3 jum have confirmed the presence of 
water ice in grain mantles inside molecular clouds through the detection of the 
3.08-jLim OH-stretching vibration. Evidence for the C-H bond stretch has also 
been found in the 3.2- to 3.6-pm band. The detection of other features, in 
particular the carbon-bearing molecules, is hampered by atmospheric absorption 
in the 5- to 8-/jm region and the presence of the strong ice and silicate bands, 
which dominate the 3- and lO-jum regions, respectively. Airborne or spaceborne 
observations of the 5- to 8-jLim region of the spectrum are therefore extremely 
important to determine the composition of interstellar grain mantles. 

Figure 3-6 shows the 5- to 8-jum spectrum of the bright, protostellar object 
W33A obtained with the KAO. Deep absorption features at 6.0 and 6.8 jum 
are apparent. The 6.0-jum band can be identified with the bending mode in water 
ices, reinforcing the identification of the 3.08-^m band with the OH-stretching 
mode in water ice. A good agreement is obtained with the spectrum of pure 
water or mixtures of water and other molecules as long as the concentration of 
water is greater than 50%. The 6.8-jum band is at the correct wavelength to be 
the C-H deformation mode in saturated hydrocarbons. A comparison of the 
observed spectrum with that of methanol (CH 3 OH) shows reasonable agreement 
(fig. 3-6). Higher-resolution observations are needed to confirm this identifica- 
tion by detecting the subtle variations in the 6.8-jum band, as present in the 
methanol spectrum. The position and shape of the 6.8-;um band show that 



67 



unsaturated hydrocarbons— such as aromatics, alkynes, and simple aliphatic 
ketones— are not dominant in interstellar grain mantles along the line of sight 
toward this object. The 5- to 8-jum spectra of other objects do suggest the 
presence of such molecules in the grain mantles in those regions. 



t/t 



max 




X, ^m 



Figure 3-6. The 5- to 8-micrometer spectrum of the bright, protostellar object 
W33A obtained with the KAO. 



Many celestial objects show infrared emission features at 3.3, 3.4, 6.2, 7.7, 
8.6, and 11.3 jum. Some researchers have attributed these features to ultraviolet- 
pumped infrared fluorescence from a collection of PAHs. In particular, the 3.3- 
and 6.2-jum features point strongly toward fundamental vibrations in aromatic 
ring molecules. A particularly striking Raman spectrum from a collection of such 
molecules (auto exhaust) is compared in figure 3-7 with the 5- to 10-Mm spec- 
trum of the emission features in the Orion bar. The close agreement between the 
Raman spectrum and the interstellar spectrum is strong circumstantial evidence 
that they arise from similar groups of species, since Raman-active and infrared- 
active modes are similar in frequency and number in these molecules. The 
number of carbon atoms in the emitting molecules, derived from the observa- 
tions, is about 50. The abundance of these species is estimated to be about 
2X10" of that of hydrogen. If this interpretation of the infrared emission 



68 



features is correct, then about 1% of the elemental carbon is locked up in these 
species. It is noteworthy that the carbonaceous grains in carbonaceous meteor- 
ites also exhibit Raman spectroscopic features similar to those of figure 3-7. 
These grains are also the bearers of isotopic ratios of D/H that approach those 
ratios observed in interstellar molecules. 



CM 



E 
- 20 



E 
o 



o 10 



X 

LL 







10 

T 



8.3 



Microns, |jm 
7.14 6.25 

— I 



T 



(a) Auto Soot-Raman 



A. 
• \ 







/ w i 



1 



(b) Orion Bar-Emission 



nl 



W 



1 



1 



1000 



1200 1400 

Wavenumbers, cm 



1600 

-1 



5.55 
~l~ 





1800 



Figure 3-7. Comparison of the Raman spectrum of auto soot, a collection of 

polycyclic aromatic molecules, with the 5- to 1 0-micrometer emission 
features in the Orion bar. 



69 



The composition of interstellar grains is a most important question for exo- 
biology. Infrared telescopes in Earth orbit can make an important contribution 
to answering this question. It is important to obtain spectra covering the full 
wavelength scale from 2 to about 20 jum to identify as many characteristic 
groups as possible. Because the 5- to 8-£tm region of the spectrum, containing, 
for example, the important C-C stretches and C-H bending modes, is blocked 
from the ground, infrared observations from Earth orbit (ISO and SI RTF) are 
required. These satellites have the added advantage of the reduced thermal 
background of a cooled telescope. Thus, even in the 10-Mm atmospheric window, 
they can make an important contribution because much fainter sources can be 
observed than from the ground. 

Among the questions that have to be addressed are, "What is the composition 
of molecular grain mantles accreted inside molecular clouds?" and "Are photo- 
lyzed grain mantles an important component of the interstellar dust?" Carbon 
dioxide is one of many molecules that cannot be observed from the ground. 
Theoretical calculations have suggested that C0 2 may be the dominant constitu- 
ent of accreted grain mantles under certain physical conditions. Absorption by 
atmospheric C0 2 makes detection of interstellar C0 2 impossible, even from air- 
plane altitudes. Ultraviolet photolysis will produce complex molecules that 
cannot be formed by grain-surface reactions. Detection of such molecules will 
help unravel the photochemistry taking place. Specifically, the determination of 
the relative abundance of aromatic to aliphatic species is very important. The 
former may be related to dust formed in stars (e.g., small condensed carbona- 
ceous grains), while the latter may be produced by photolysis. High-resolution 
spectra in the 3.2- to 3.5-/jm region could answer this question. The C-H stretch- 
ing modes are weak compared to the continuum dust extinction. Because there 
is significant atmospheric CH 4 absorption even at a good site (such as Mauna 
Kea), cooled space infrared telescopes are needed. Faint sources with long path 
lengths through the diffuse interstellar medium can then be studied. 

Infrared observation can also be used to study the formation sites of inter- 
stellar grains. One particularly important question is, of course, "Do carbona- 
ceous or other biogenic-element-bearing grains condense in the outflow from 
novae and supernovae?" and if so, "What is their precise composition?" The 
tentative identification of infrared fluorescence from PAHs around planetary 
nebulae is particularly interesting. These species, which are presumably the 
molecular precursors of carbon grains, have obviously been formed only recently 
in these objects. 

High-resolution studies of the 7.7-/im band in a variety of sources could reveal 
profile variations related to differences in the collection of PAHs. Correlation 
studies among the different infrared emission bands are also important in this 
respect and could produce insight into the actual synthetic route. As yet undis- 
covered infrared emission features may be present in wavelength regions that are 
not observable from airplane altitudes. Searches for the emission features in 



70 



absorption against background stars with diffuse interstellar medium material 
along the line of sight is important to assess the possibility of the ubiquitous 
presence of PAHs in the diffuse interstellar medium. Such absorption studies of 
the outflow from carbon-rich, late-type giants could elucidate their formation 
process. 

Important progress in the study of interstellar dust can also be expected from 
visible and ultraviolet studies. In particular, ultraviolet extinction and polariza- 
tion studies of stars behind dark clouds represent a largely unexplored area, and 
with both HST and ASTRO it will be possible to make such studies and derive 
average grain sizes for such regions. This could answer the important question, 
"Do interstellar grains coagulate into fluffy aggregates inside molecular clouds or 
does grain growth mainly take place through grain mantle formation?" It may 
also be possible to study how the composition of the grain surface influences the 
rate of grain growth. The HST could also search for structure in the ultraviolet 
extinction curve, similar to the diffuse interstellar bands in the visible. Optical 
and ultraviolet spectroscopy with HST of high-velocity shocks (100-200 km/sec) 
can yield the elemental abundances in the post-shock gas and provide insight 
into the destruction of interstellar grains. Finally, with the ASTRO satellite, it 
will perhaps be possible to study the dust extinction curve toward nearby stars 
for wavelengths shorter than the Lyman limit. Presently no information of this 
sort is available. Many materials have electronic transitions in this wavelength 
region that may show up as prominent extinction bumps for small particles. 

The observational projects outlined here require much laboratory and 
theoretical effort. Studies of the condensation process in the outflow from late- 
type giants, planetary nebulae, novae, and supernovae have to be undertaken. In 
particular, the role of large aromatic molecules in the formation of interstellar 
carbon grains should be investigated and their physical and chemical properties 
determined in the laboratory. Laboratory spectra of candidate materials have to 
be measured for a successful interpretation of interstellar spectra. Laboratory 
studies are a prerequisite for our understanding of the role of grain-mantle pho- 
tolysis in the interstellar medium. Such studies may also provide insights into the 
interchange of molecules between the gas phase and the solid phase in molecular 
clouds. 

Suggestions for Further Reading 

Nuth, J. A., IV; and Stencel, R. A., eds.: Interrelationships among Circum- 
stellar, Interstellar, and Interplanetary Dust. NASA CP-2403, 1986. 

Tielens, A. G. G. M.; and Allamondola, L. J.: The Composition, Structure, 
and Chemistry of Interstellar Dust: In Physical Processes in the Interstellar 
Medium, D. Hollenbach and H. Thronson, eds., 1986. 



71 



3.1 1 Evolution of Elemental Abundances in Galaxies 

The biogenic elements available for the formation of planets, and perhaps 
life, in external galaxies must be measured in order to estimate the probability 
for planetary and life formation. To have an exobiological impact, these ele- 
ments must be present in the interstellar medium rather than locked up in stars. 
There, they can be in either gaseous or solid form; measurements are needed of 
both states in order to estimate the total abundances available for planetary 
formation. Furthermore, elemental abundance gradients within a galaxy will 
determine where planets having the "necessary" inventory of biogenic elements 
are most likely to form. Thus, measurements should be made of galaxies of dif- 
ferent ages and morphological types so that the production rate of biogenic 
elements in the universe as a whole can be accurately estimated. Is it possible 
that there are galaxies that have not yet attained the stage of nucleosynthetic 
evolution necessary to provide the raw materials from which geologically active 
planets can be produced? 

Observations of the biogenic elements in the gas phase can be made only 
spectroscopically, primarily in the ultraviolet, visible, and infrared spectral 
regions for atoms and in the infrared, submillimeter, and microwave regions for 
molecules formed from the biogenic elements. Spectral resolutions of 
X/AX ~ 1000 are needed to clearly separate the spectral lines from the back- 
ground continuum. Even higher spectral resolutions allow studies of the velocity 
distributions within the lines, reducing the spatial resolution penalty incurred by 
studying objects as distant as external galaxies. For evaluating Earth-orbiting 
observatories, we split the observations into separate spectral regimes. 

Ultraviolet or visible lines must be seen either by emission from, or in absorp- 
tion against, a high-temperature gaseous environment such as a stellar photo- 
sphere or an HI I region. Studies of heavy-element (carbon, nitrogen, oxygen, 
etc.) abundances inside stellar atmospheres are important, since the elements can 
be returned to the interstellar medium in the course of stellar evolution, making 
them available for incorporation into subsequent generations of stars and 
planets. Outside stellar atmospheres the material in interstellar space will gener- 
ate absorption features in the ultraviolet and visible regions. These features can 
be strong in comparison to features at longer wavelengths because the oscillator 
strengths are greater at the higher frequencies. Thus ultraviolet/visible spectro- 
scopic observations are very sensitive to trace amounts of biogenic elements in 
the diffuse interstellar medium. However, as will be noted later, these observa- 
tions must be complemented by longer-wavelength measurements of the dense 
molecular cloud regions. Studies of emission lines from extragalactic HII regions 
will also be useful in surveying heavy-element abundances, although the infrared 
fine-structure line observations (see below) require less correction for extinction. 

Infrared observations of biogenic elements in the gas phase are necessary to 
complement the shorter-wavelength observations. Abundances derived solely 



72 



from the shorter-wavelength studies will be too low, in general, since the dense 
clouds, where many of the interstellar heavy elements reside, are impenetrable to 
visible or ultraviolet light. However, these regions are transparent to longer- 
wavelength radiation; thus smaller extinction corrections are necessary in the 
infrared. Infrared line radiation can be used to measure the abundances in both 
neutral and ionized regions. The lines of interest have wavelengths ranging from 
4 to 200 jLtm, with the different lines being sensitive to different excitation 
temperatures and densities. Some of the more important lines include the 
157-Mm C + line in neutral regions as well as the Olll lines at 88 and 52 ^m and 
the Nelll line at 16 jum in the ionized regions. 

The rotational transitions of many of the molecules containing biogenic ele- 
ments lie in the microwave region of the electromagnetic spectrum. Observations 
at these wavelengths will complete the biogenic element surveys by probing the 
very densest and coldest molecular clouds. 

Measurements of the biogenic elements residing in the dust component of 
external galaxies require different instruments and observing approaches than do 
gas-phase studies. The dust itself can be seen either in absorption in the ultravio- 
let through near-infrared or in emission at longer wavelengths. High-resolution 
spectrometers are not advantageous in determining the dust composition, since 
the solid-phase absorption features are broad in comparison with the gas-phase 
features. Resolving powers of X/AX ~ 100 or less are best since they are sensi- 
tive. The total quantity of dust in galaxies is best measured through broad-band 
infrared photometry; telescopes with cooled optics are desirable because of their 
high inherent sensitivity. 

The planned Earth-orbiting space observatories (Appendix B) will be of tre- 
mendous utility in measuring biogenic element abundances in other galaxies. 
The elimination of atmospheric effects will increase sensitivity so that objects, 
which from the ground can be studied only in the Milky Way, will be routinely 
observed in nearby galaxies. The sensitivity gain will also allow detailed studies 
of even more distant objects for the first time, so that cosmological evolution 
effects on the availability of biogenic elements can be studied. 

The three major observatory-class orbiting telescopes— HST, SI RTF, and 
LDR— will be the most useful of all the planned astronomical spacecraft since 
they alone have the fine angular resolution and pointing ability needed to work 
on extragalactic objects. 

For ultraviolet, visible, and near-infrared studies, the HST will be the most 
useful observing platform. The advertised angular resolution of better than 
0.1 arcsec is sufficient to study individual stars in selected regions of nearby 
galaxies such as M31. In addition, the first generation of science instruments 
includes two spectrometers operating between 1150 and 8000 A with resolving 
powers ranging from X/AX = 10 2 to 10 s . This is more than adequate for visible 
gas-phase elemental abundance studies in external galaxies. 



73 



For wavelengths from 2.5 to 100 jum SI RTF is the most sensitive telescope 
now planned for Earth orbit. At very high spectral resolutions (XI AX » 10 3 ), 
HST (with a possible second-generation infrared spectrometer) and LDR will 
have an advantage at wavelengths less than 2.5 jum and longer than 100 jum, 
respectively, because of their larger collecting areas. At very high resolving 
powers, this advantage overtakes the cooled-optics advantage of SI RTF. There is 
an infrared spectrometer proposed for SI RTF that is planned to operate between 
2.5 and 200 jum with two resolution modes of X/AX = 50 and 1000. The high- 
resolution mode is adequate for extragalactic studies of the strongest infrared 
fine structure lines; for example, it will be capable of measuring the very impor- 
tant 157-jum Cll line out to a redshift of z = 0.3. The sensitivity of theSIRTF 
infrared spectrometer is expected to be high enough that observations can easily 
be made of other infrared lines required to trace the biogenic elements encased 
within dense molecular clouds in either neutral or ionized states in normal 
galaxies out to distances of 500 Mpc. This distance encompasses a very large 
number of galaxies, enough that biogenic elemental abundance studies can be 
correlated with other factors, such as galactic morphological type. 

There are two infrared spectrometers of high enough resolution for infrared 
fine-structure line observations listed in the strawman instrument complement 
of LDR. The large aperture of this telescope will allow good angular resolution 
for abundance gradient observations of the biogenic elements and the large col- 
lecting area will make it unequalled for submillimeter line measurements. 

Of all the planned Earth-orbiting telescopes, SI RTF is the best suited for 
carrying out dust studies in other galaxies. The cold optics of this telescope will 
allow for very high sensitivities. For example, the planned infrared photometer 
will be capable of measuring the integrated mass of dust in normal spiral galaxies 
out to red shifts of z = 2. The composition of the dust can be determined using 
the planned SI RTF infrared spectrometer, which has a low-resolution mode that 
was developed for detecting dust features in galaxies with large red shifts. In the 
low resolving power mode of X/AX = 50, the SI RTF spectrometer sensitivity is 
limited only by the background emission from the zodiacal dust. This high sen- 
sitivity would allow observations of the 10-jum silicate dust feature in spiral 
galaxies out to a red shift of z = 1 .5. While the warm optics of the LDR make it 
less sensitive for observations of the most distant and faint extragalactic objects, 
its larger aperture will allow studies of the distribution of dust in nearby galaxies 
with 20 times the angular resolution of SI RTF. 



74 



Suggestions for Further Reading 

Morris, M.; and Rikard, L. J.: Molecular Clouds in Galaxies. Ann. Rev. 
Astron. Astrophys., vol. 20, 1982, p. 517. 

Stein, W. A.; and Soifer, B. T.: Dust in Galaxies. Ann. Rev. Astron. 
Astrophys., vol. 21 , 1 983, p. 1 77. 



75 




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Chapter IV 



Cosmic Dust Collection 



D. Brownlee, T. Bunch, S. Chang, J. Kerridge, 

and J. Wolfe 



Primitive solar system objects such as asteroids and comets occupy an impor- 
tant place in exobiology because their contents include water, organic matter, 
and minerals that, together, are considered prerequisites for the prebiotic evolu- 
tion of biological materials from chemical systems. In addition, these materials 
are samples of planetesimals that contributed some fraction of the volatile con- 
tents of the terrestrial planets. They also contain abiotic organic compounds and 
carbonaceous materials that may be attributed to origins in the parent bodies, 
the so!ar nebula, and galactic environments that preceded the nebula. Therefore, 
their origins and the origins of their components have a bearing on understand- 
ing the cosmic evolution of biogenic compounds and the development of models 
for prebiotic evolution on planets. To asteroids and comets, however, should be 
added a third category of materials of exobiological interest, "cosmic dust," 
the information about the content of which shows promise of supplementing 
and complementing that of the first two types of material. 

The designation cosmic dust is applied to extraterrestrial particles less than 
1 millimeter in diameter. Some of these particles that have entered Earth's 
atmosphere have been collected by high-flying aircraft. It is believed that most 
are debris from comets and asteroids that predate our planetary system and may 
have thus originated outside the solar system. Isotopic anomalies in primitive 
carbonaceous meteorites suggest that dust originating from their cometary or 
asteroidal parent bodies contains components formed in the presolar epoch. In 
any case, cosmic dust should be viewed as possible samples of primordial 
material that have been preserved since the early history of the solar system. 
In the exobiological context, laboratory studies of the biogenic elements and 
compounds in these minute objects will open windows through which physical 
and chemical processes operating in the galaxy or early solar system may be 



Scanning Electron Micrograph of a porous chondritic micrometeorite collected in 
the stratosphere by a NASA Ames U-2 aircraft. 



77 



discerned, as has been the case with meteorites. Through such windows it may 
be possible, in ways not available with meteorites, to elucidate the pathways 
taken by biogenic materials from their origins in stars to their incorporation in 
some of the earliest objects formed in the solar system. 

Meteorites found on the surface of the Earth have been the traditional source 
of primordial extraterrestrial materials, and they have been, and will continue to 
be, valuable windows into the past. However, there are two major limitations 
with this source of samples. One is that the stress of atmospheric entry prevents 
structurally weak materials from surviving in pieces as large as conventional 
meteorites. This exclusion effect prevents typical cometary matter from surviv- 
ing as recoverable macroscopic samples. Studies of the fragmentation of meteors 
in cometary meteor showers have shown that typical cometary material is more 
than an order of magnitude weaker than the most friable of the recovered 
meteorites. The second limitation with meteorites is that, with the exception of 
the lunar meteorites, it has not been possible to associate collected samples with 
their sources. It is widely believed that conventional meteorites are fragments of 
asteroids, but it is not possible to prove this or to identify the specific parent 
bodies of recovered samples. There are many distinct families of meteorites, and 
it is unfortunate that their measured chronologies and detailed physical and 
chemical properties cannot be associated with specific bodies or locations within 
the solar system. 

An approach for overcoming the problems of the atmospheric exclusion 
effect and the unknown origin of meteoritic samples is to collect meteoroids 
directly in space. From Earth orbit this is not practical for meteoroids as large as 
conventional meteorites because the flux is too low, but it is possible to collect 
the much more abundant submillimeter particles, cosmic dust. The flux of 
0.1 -mm particles is 1 m _t yr -1 and that of 0.01 -mm particles is 1 m~ 2 day -1 . 
With large areas and long exposure, it should be possible to collect a few par- 
ticles of millimeter size and large numbers of smaller samples. Dust-collection 
experiments in space began during the Mercury program, but progress was slow, 
partly because of limitations on the size and exposure times of collectors that 
could be exposed and then returned to the laboratory. This situation changed 
dramatically in April 1 984 with the launch of the LDEF. Opportunities for long 
exposures with Earth return will be common in the future, and it is likely that 
considerable progress can be made in the meteoroid collection field. 

4.1 State of Knowledge 

The most detailed information on the properties of individual interplanetary 
dust grains has come from laboratory studies of 5- to 50-jum particles that have 
been collected in the stratosphere with U2 aircraft. The extraterrestrial nature of 
the samples has been proven by detection of trapped solar wind particles and 
tracks of solar cosmic rays, unique indicators of exposure to space. 



78 



A variety of particle types have been identified, but the most common are 
particles that have chondritic (solar) abundances for all the major and minor ele- 
ments found in meteorites. They are black, contain 2- to 5-wt% carbon, and have 
elemental compositions identical to CI and CM carbonaceous chondrites. Among 
the chondritic composition particles there are two clear subdivisions: those that 
contain hydrated minerals and those that are anhydrous. The hydrated particles 
are often rather compact and the bulk of their mass is contained in clay miner- 
als. In some cases the abundant hydrated phases are serpentine minerals that 
closely resemble those in CI/CM chondrites. However, in other particles the 
hydrated minerals are distinct from those common in carbonaceous meteorites 
and they also contain minor anhydrous phases, such as low Ni-pentlandite, that 
have not been identified in meteorites. Some of the hydrated cosmic dust par- 
ticles may be samples of the same parent bodies that produce the CI and CM 
meteorites, but others in this class are mineralogically distinct and probably did 
not come from the CI/CM parent bodies. The anhydrous particles are unique and 
are unlike any established meteorite type. They are the only known case of a 
carbon-rich chondritic composition material that is composed entirely of anhy- 
drous phases. These particles are often exceedingly porous and are similar in 
strength to the fragile materials that are observed as cometary meteors. The pore 
spaces in the particles may have originally been filled with ice. The anhydrous 
particles are aggregates of grains ranging in size from ~50 A to micrometers. 
Some grains are single minerals (enstatite, olivine, iron sulfide, or carbides), but 
others are themselves microaggregates of very tiny crystals embedded in carbon- 
aceous material. Carbon occurs as binding material for the grains, as discrete 
amorphous clumps, and as coatings, lsotopic analysis of hydrogen associated 
with carbon in the particles has shown D/H enhancements as high as a factor of 
10 relative to solar. The bulk D/H enhancement is higher than that found in 
most meteorites. High D/H fractionation in interstellar environments is usually 
attributed to ion-molecule reactions. 

4.2 Science Questions 

The primary goals of future dust-collection experiments in Earth orbit are to 
collect materials that cannot be recovered as conventional meteorites and to 
directly associate this material with specific comets, with specific parent bodies 
(particular asteroids), or with interstellar origins. The samples should then be 
returned to Earth for detailed elemental, isotopic, molecular, mineralogic, and 
structural study, with exobiological interest focusing on the nature and abun- 
dances of the biogenic elements (carbon, hydrogen, nitrogen, oxygen, sulfur, 
phosphorus) and their compounds. The particles obtained would always be 
small, and depending on the collection technique, they would be at least some- 
what altered during acquisition. Even if the particles could not be collected in 
pristine condition from Earth orbit, they could at least be collected in a form 
that would give accurate elemental and isotopic compositions for grains over a 
large size range. 

79 



The collected dust samples can play a unique and important role in the future 
study of primitive solar system materials. Probably the most significant aspect 
of this effort would be the collection of materials from a variety of specific 
comets and the capture of interstellar dust in transit through the solar system. 
The laboratory analysis of both cometary and interstellar particles would 
provide the first direct information on cometary and interstellar grains. It is par- 
ticularly intriguing that it might be possible to examine the abundances and pos- 
sible forms of the biogenic elements and compounds in materials that existed 
before and just after the formation of the solar system. In addition, elements 
heavier than oxygen in the interstellar medium occur primarily in the form of 
interstellar grains, and contemporary grains are probably very similar to those 
that contained most of the condensable elements in the solar nebula during its 
collapse stage. Dust particles in comets are believed to be well preserved grains 
that may have been typical of the materials that existed at the outer fringes of 
the solar nebula. The collection of dust may then provide direct information on 
the composition of some of the original material from which the solar system 
formed, and the composition of collected grains can then provide useful data for 
modeling solar nebula processes and environments. 

Among the important scientific questions pertinent to exobiology that 
could be addressed by detailed laboratory studies of cosmic dust are the 
following: 

1. What similarities exist in the relative abundances of the biogenic elements 
and compounds in interstellar, cometary, and meteoritic samples? Can these 
similarities be traced to common sources and histories in the interstellar 
medium, the solar nebula, or the parent objects? 

2. What connections exist between the gas-phase chemistry observed in inter- 
stellar clouds and the organic chemistry of cometary and interstellar dust? 

3. What evidence of grain-mediated organic chemistry in interstellar clouds 
can be found in interstellar or cometary dust? 

4. What role has water played in the chemistry and mineralogy of interstellar 
dust particles? Do they contain hydrous silicates? 

5. What evidence is there for the influence of liquid water on the chemistry 
and mineralogy of cometary samples? 

6. What materials in interstellar and cometary dust can be attributed to dis- 
crete nucleosynthetic sources? Are they the same as can be inferred from some 
components of meteorites? 

7. What correlations can be drawn between astronomical observations of 
interstellar clouds and the structure and compositions of collected cometary and 

interstellar dust? 

8. Did radiation-induced polymerization of organics occur in presolar or early 

solar system environments? 

A special attraction of the dust-collection approach is the abundance of dust 
particles, providing the capability to collect samples from a variety of comets 
and thus address the possible diversity among comets. The properties of come- 

80 



tary bodies may differ because of conditions and processes that existed at the 
time and location of their origin, or they may differ because of endogenic alter- 
ation processes such as might occur from heating caused by 26 AI decay. The 
capability of evaluating diversity among comets is a unique attribute of the 
dust-collection approach. 

A major goal of solar system research is the direct return of a pristine come- 
tary sample collected from a dedicated mission to a comet nucleus. However, 
even when such advanced missions occur, they will be limited by cost to a small 
number of bodies and will not adequately study the diversity question which can 
be addressed by cosmic dust collection. The dust collection may also provide 
new insights into existing meteorites by providing criteria for identifying pos- 
sible cometary or presolar materials in existing collections of meteorites and 
interplanetary dust. For example, the recent identification of meteorites from 
the Moon was possible only because analyses of lunar samples had defined sev- 
eral characteristic properties of lunar material. It is assumed that appropriate 
analysis of meteorites and interplanetary dust particles collected via U2 aircraft 
will continue and that collections can be made in low Earth orbit, but in addi- 
tion it would be highly desirable to sample directly the source regions of the 
interplanetary dust population. Those sources are comets and asteroids, with the 
former believed to be by far the dominant source. Sample-return missions to 
comets are currently under consideration and, although that topic is strictly 
outside the scope of this report, its relevance and potential value to the Earth 
orbital investigations justify reiteration of the importance of such a mission. 

Finally, a conceivable but remote possibility concerns the issue of pan- 
spermia. If extraterrestrial microbes have been dispersed in space and make a real 
contribution to the cosmic-dust population, they may be included in any col- 
lected samples. Routine microscopic surveys of the collected particles could be 
followed up by detailed nondestructive chemical analyses. In the event that a 
number of putative microbes were detected, attempts at culturing them for 
viability and reproduction could be considered. Because of contamination and 
collection problems in space, it is probably best to conduct panspermia experi- 
ments in the stratosphere where large numbers of particles can be obtained with 
minimal contamination. Samples could be cultured directly on the collection 
substrate and correlation of growth with an actual extraterrestrial particle would 
be evidence for capture of an organism. 

4.3 Technical Approach 

The proposed new dust-collection experiments will require both particle 
collection and measurement of orbital parameters. The trajectory and speed of 
an incoming particle must be measured with sufficient precision so that a good 
match can be made between the collected particle and the source body. The 
orbits of dust particles evolve because of Poynting-Robertson drag, which 
decreases the eccentricity and semimajor axis with time, but does not alter the 

81 



inclination. Most particles probably will not have uniquely determinable parents 
and will be sporadic. A small fraction of the detected particles will have not yet 
had their orbits significantly modified and it will be possible to match them with 
their source body. Because it may be possible to detect large numbers of parti- 
cles, the total number of particles with identified sources could, in principle, be 
large. Orbital evolution caused by Poynting-Robertson drag is slowest for the 
larger particles and so the best collection experiment is one that collects large 
numbers of relatively large dust particles. The science return from laboratory 
analysis is also greater for large particles. 

The most straightforward technique for determining the orbital elements of 
an impacting dust particle is to measure the time of flight (TOF) and path direc- 
tion of a particle that first penetrates a thin, front film, passes through an open 
space, and then enters a rear-collection substrate or device. This basic technique 
for orbit measurement was actually first used on Pioneers 8 and 9 over 15 years 
ago. The detection of the front-film penetration can be made from the light, 
shock wave, or ion pulse generated during the impact. The velocity (TOF) and 
direction measurements can be made solely by electronic methods or by a com- 
bination of real-time electronic measurement followed by later measurements of 
the impact sites in the laboratory. The TOF between the front film and substrate 
must be established electronically at the time of collection and must have a 
precision of a few percent. The path direction can be determined by position- 
sensitive detectors similar to those used for two-dimensional ionizing radiation 
detectors. The acoustic, light, or ion pulse can be detected by a sensor-array grid 
square. The path direction requires the measurement of the location of the 
front-film penetration and entry into the rear substrate. A modification of the 
purely electronic approach is to use coarse position sensors to identify the 
general region where the impact occurred and to precisely measure the TOF. 
After return to the laboratory, the penetration hole and rear impact can be 
located and measured to determine the particle path to great precision. 

Dust collection from Earth orbit is complicated by the high velocity of the 
incoming particles. Impact velocities range from 4 to over 70 km/sec and the 
typical impact velocity for 10-jum particles is about 15 km/sec. Even at the 
minimum velocity, the kinetic energy exceeds the binding energy, and non- 
destructive collection is by no means a trivial process. The most developed col- 
lection concepts involve direct impact and collection by totally mechanical 
processes. Material was collected with some of these techniques on Gemini, 
Skyiab, and Solar Max, and there is considerable expectation that the large 
number of similar collection experiments on LDEF will also return important 
data. Although the direct-impact techniques can recover some unmelted mate- 
rial, in general the collection is destructive and most structural information is 
lost. True nondestructive collection would require a device that could decelerate 
incoming particles without causing excessive heating or mechanical stress. Low- 
pressure gas cells, foams, and electrostatic or electromagnetic devices have been 



82 



suggested as possible approaches for this type of collection, but they are only in 
early stages of development. 

An example of an impact collection technique is the capture cell in which a 
particle penetrates a thin film and enters an enclosed volume where vapor and 
debris are trapped. For capture cells the information that is obtained for a par- 
ticle is bulk elemental and isotopic composition, as well as some limited data on 
shape and density from the shape and size of the penetration hole. The compart- 
mentalization of capture cells allows the collection of discrete particles over a 
wide size range. A major advantage of capture cells is that all condensable mate- 
rials are trapped in the cell for later analysis. Other destructive collection 
schemes involve direct cratering into either a solid or a porous material. For 
impacts at moderate velocity into some metals, the efficiency of retention of 
meteoroid residue as material lining the crater bottom can be appreciable. This 
technique has the advantage that the sample is highly concentrated and is not 
diluted with collection material, as is the case with capture cells. Disadvantages 
are that volatile materials are lost preferentially and at the higher velocities all 
the projectile is vaporized. The shock-loading of the impacting particle can be 
considerably decreased by using a low-density substrate material. This can be a 
foam, a stack of thin foils, or a suspension of particles or fibers. If the velocity 
is not extreme, particles can be collected intact with this approach in the sense 
that some original phase and structural information is preserved. It is unlikely 
that fragile particles can be captured in pristine condition, but it is likely that 
particles or components of particles that are strong mineral grains can be decel- 
erated without melting or severe heating. Recent work with impacts recovered 
from Solar Max have demonstrated that some fragments of fragile particles can 
be captured in unmelted form. 

There are two generic approaches to "nondestructive capture" of hyper- 
velocity particles: passive capture and active capture. The passive approach uses 
input into suitable low-density, inert capture media to absorb kinetic energy in a 
manner that minimizes alteration of the particle. Active collection utilizes force 
fields to decelerate particles. 

The success of passive intact capture of hypervelocity particles rests on the 
ability to absorb the maximum amount of particle kinetic energy by a passive 
capturing medium while maintaining the energy removal rate below the 
threshold that will cause particle damage. Examples of possible passive capture 
media are low-density polymeric foams, suspended micrograms, gas, void-metal 
composites, felts, and aerogels. Considerable laboratory success has been realized 
from the use of low-density polymeric foams to capture comet analog silicate 
grains in an organic binder at speeds up to 6 km/sec. Laboratory tests with pure 
aluminum projectiles have demonstrated recovery of 60% of the projectile mass 
at speeds of 7.9 km/sec. 

A considerable body of laboratory data suggests that the passive capture tech- 
niques can be used to collect meteoroids in space. It is likely that both mineral 



83 



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grains and organic materials can be collected in relatively unmodified form. 
Strong, low-velocity particles can be captured intact. The fragile particles that 
may be characteristic of comets will probably fragment during collection, but 
even for the most fragile materials it is expected that individual solid compo- 
nents will be recovered without major modification. The passive technique is 
relatively well developed and could be used at the present time. 

Totally nondestructive collection is theoretically possible using electrostatic 
or magnetic fields to decelerate micrometer-sized meteoroids from cosmic 
velocities. Figure 4-1 shows a conceptual design of an electrostatic collector. 
Particles are charged with an electron beam and then decelerated by electro- 
static fields that retard the particle motion. This is essentially the inverse of elec- 
trostatic dust accelerators used in the laboratory to calibrate micrometeoroid 
impact sensors such as those used on the flybys of comet Halley. In the labora- 
tory, micrometer and submicrometer particles are highly charged and then accel- 
erated with a potential of a few million volts to attain velocities as high as 
50 km/sec. 

Feasibility studies are currently under way to determine whether the electro- 
static technique can be used for practical and efficient collection in space. 
Although there have been conceptual studies, there is no laboratory experience 
with actual capture of hypervelocity particles. The active capture approach has 
many attractions, but there are formidable problems that must be solved before 
a significant number of particles in the nanogram to microgram range can be 
collected. 

4.4 Technology Needs 

Only a few meteoroids have been collected in space and, although orbital 
measurements have been made for particles, they were not made for collected 
material. Presently there is a considerable amount of activity in the collection 
and detection fields. An impressive set of particle impact detectors flew on the 
comet Halley probes, Giotto and Vega, and are planned for the Jupiter probe, 
Galileo. Particle collection experiments involving capture cells and cratering into 
solids are carried on LDEF and the Soviet space station. The technology of 
hypervelocity capture of small meteoroids is also under development for possible 
use on a comet-sample-return mission using a flyby spacecraft on an Earth-return 
trajectory. This mission is part of the Solar System Exploration Committee 
(SSEC) core program. There is progress being made in the collection and the 
detection of particles, and it is timely to combine the technologies to produce a 
device that collects particles and measures their orbital parameters. The major 
technology needs are to refine the collection techniques and to adapt detection 
methods so that precise speeds and impact directions can be measured. 

The capture techniques need to be refined so that some intact particles can 
be collected, and volatile elements such as carbon, nitrogen, and hydrogen will 



85 



not be lost to analyses. For example, capture cells made of "getter" materials 
have been discussed wherein the meteoroid vaporizes enough getter material to 
trap reactive elements. The specific needs in the passive collection area are to 
develop efficient materials or devices that trap meteoroids or their debris with- 
out causing significant chemical or physical alteration or contamination with 
collector or spacecraft materials. Some of this development can be done on the 
ground using particles accelerated to high velocity with light gas guns, plasma 
drag accelerators, or electrostatic accelerators. The ultimate tests must be made 
in Earth orbit with real meteoroids. Extensive development will require frequent 
exposure opportunities. 

The detection technology needs to be adapted to the particular problem of 
accurately measuring orbital parameters of micrometer- to millimeter-sized par- 
ticles. The required accuracy of the velocity vector is about a few percent. 
Plasma pulse detectors were used for TOF and impact-angle measurements on 
Pioneers 8 and 9 and the LEAM (lunar ejecta and meteorites) instrument placed 
on the lunar surface by Apollo 17 astronauts. Improvements in the plasma area 
would be to simplify the technique so that large areas could be instrumented at 
minimal cost. Another position-sensitive technique that is being actively studied 
involves piezoelectric sensors that use arrival times in a detector array to pin- 
point the exact impact location. A final technique uses a polarized PVDF plastic 
film that generates electric pulses when it is perforated. This material needs no 
electrical bias. The impact point is measured by signal-delay times, it appears 
that there are several detector technologies that are suitable for orbital param- 
eter measurement and the major task is probably to develop a workable system 
combined with the collection capability. 

4.5 Opportunities 

In the near term, collection experiments can use collection areas approxi- 
mately a meter squared and exposure times of months. Much larger areas and 
exposure times would be desirable to collect very large or very rare particles. 

Near-term exposure and recovery opportunities include recoverable satellites 
such as LDEF and EURECA. A first-rate collection and trajectory measurement 
experiment of several square meters' area could be included on an Explorer-class 
mission, with subsequent sample recovery and return to Earth. This could be a 
dedicated mission or an add-on to a multiinstrumented spacecraft. A timely 
completion of such a mission would be of considerable value in planning larger 
and more complex collectors for the Space Station. The Shuttle itself does not 
have sufficient exposure times to be useful except for simple tests involving a 
few impacts. A 2-m 2 collector exposed for a year would provide an excellent 
opportunity to collect a significant number of 0.01- to 0.1 -mm particles. A 
100-m 2 collector on Space Station would provide the unique opportunity to 
collect millimeter-sized particles. Because the reason for having a large area is 



86 



the collection of rare particles, only cells that were impacted would have to be 
recovered. Once set into operation, the collector would require very little atten- 
tion. Perhaps twice a year cells that received major hits would be recovered for 
Earth return. The scientific reward for operating a large area collector would be 
the collection of the actual particles that produce the annual showers of come- 
tary meteors. The larger particles collected would have become radar or faint 
optical meteors had they been allowed to reach the atmosphere. 

4.6 Conclusions 

The analysis of recovered samples of primitive solar system material provides 
fundamental insights into the materials and processes that existed in the early 
history of the solar system and thus into the cosmic evolution of the biogenic 
elements and compounds. With the improving capabilities to conduct ambitious 
experiments in Earth orbit, it now seems possible to collect small meteoroid 
samples (extraterrestrial dust) that can be associated with their parent sources on 
the basis of common orbital parameters. The major scientific advance that could 
come from such an endeavor would be the collection of samples that could be 
confidently identified as originating from the interstellar medium or from 
specific comets. Even though, for some experiments, particles may be signifi- 
cantly altered during capture, it is believed that at least their elemental and iso- 
topic compositions can be accurately measured in the laboratory. The ultimate 
challenge will involve the development of a device for use on the Space Station 
that will collect cosmic dust without significant concomitant alteration of its 
chemical and physical properties. 



Suggestions for Further Reading 

McDonnell, J. A. M.: Cosmic Dust. Wiley and Sons, New York, 1978. 

Sandford, S. A.; and Walker, R. M.: Laboratory Infrared Transmission Spec- 
tra Measurements of Individual Interplanetary Dust Particles from 2.5 to 
25 Microns. Ap. J., vol. 291, 1985, p. 838. 

McKeegan, K. D.; Walker, R. M.; and Zinner, E.: Ion Microprobe Isotopic 
Measurements of Individual Interplanetary Dust Particles. Geochim. Cosmochim. 
Acta, vol. 49, 1985, p. 1971. 

Brownlee, D. E.: Cosmic Dust: Collection and Research. Ann. Rev. Earth 
Planet. Sci., vol. 13, 1985, p. 147. 

Tsou, P.; Brownlee, D. E.; and Albee, A. L.: Comet Coma Sample Return 
Via Giotto II. J. British Interplanetary Soc, vol. 38, 1985, p. 232. 



87 



1 k 

fij 










Chapter V 
In Situ Investigations 



L. Allamandola, A. Bar-Nun, T. Brock, S. Chang, 

R. E. Davies, J. M. Greenberg, L. Hochstein, 
G. Horneck, W. Huntress, S. Miller, K. Nealson, 

and D. Usher 



A spacecraft in Earth orbit provides a uniquely useful environment for studies 
that are of central importance to modern exobiology. Such a space platform 
makes available special conditions including microgravity, a reasonably good 
vacuum with a very high pumping speed, a continuous source of high-energy 
neutral atomic oxygen, and a solar flux that is less attenuated than that at the 
Earth's surface. 

In this Chapter we discuss four different types of experiments that can be 
performed on a space platform. These are the study of suspended grains, reac- 
tions of neutral atomic oxygen, studies of an artificial comet, and possible 
experimental tests of the viability of microorganisms in space. 

5.1 Models of Interstellar Grains 

Interactions between a gas phase and a solid phase are well known and 
include sorption phenomena, heterogeneous catalysis, and many other familiar 
terrestrial physical-chemical processes. Such interactions are also important in 
the space environment, particularly for the roles they play in the cosmic history 
of the biogenic elements and compounds. Elucidation of this history involves 
tracing the physical and chemical pathways taken by the biogenic elements 
and compounds from their origin in stars to their incorporation into preplane- 
tary bodies, measuring the biogenic elements and compounds in the galaxy 
and solar system to develop theories about the formation of the solar system, 
and determining how the physical and chemical properties of the biogenic 
elements and compounds influenced the formation of the solar system and the 
bodies within it. I n this context, interactions among gases and grains in space are 



Artist's concept of a laboratory module for the Space Station. 



89 



fundamental to theories of the origins of the constituents of interstellar clouds, 
comets, meteorites, interplanetary dust, and all of the bodies in the solar system. 
Experiments capable of yielding insight into the nature of these processes are 
thus of great value in confirming or modifying various aspects of these theories. 

Nucleation, condensation, and growth of carbonaceous particles must occur 
in the envelopes of carbon stars to yield the observed circumstellar dust and 
molecules. Similar processes are thought to occur under conditions as diverse as 
those in interstellar clouds and in the atmospheres of some of the outer planets 
and their satellites; observational evidence points to the presence of fine-grained 
dust (from less than 0.1 {dm to about 1 jum in diameter, presumably containing 
varying proportions of hydrogen, carbon, nitrogen, and oxygen) in both types of 
environments. Although there are some theoretical discussions of the properties 
of dust based on remote spectrophotometric observations, the physical and 
chemical character of the materials remains poorly understood, as does the 
nature of the processes that produced them. 

Although theories of grain nucleation, condensation, and dust growth are 
being developed, the complexities of the natural processes make them difficult 
to model. The few experimental studies that have been conducted were per- 
formed under conditions that do not permit scaling to relevant astrophysical 
environments. A common feature of the processes in all the environments men- 
tioned above is that grains form and evolve over substantial lengths of time while 
suspended in a thin gas phase, largely, if not entirely, independent of other 
grains. This condition should influence the rate of formation, the chemistry, 
structure, morphology, and other characteristics of the dust. While this condi- 
tion is difficult, if not impossible, to achieve in a terrestrial laboratory, it may 
be effectively simulated under microgravity conditions. Experiments conducted 
in Earth orbit would provide "space truth" for analogous experiments carried 
out in terrestrial laboratories or on computers. Furthermore, they would yield 
samples formed under well-defined conditions, the properties of which could be 
readily determined and compared with those of natural material either remotely 
sensed or obtained from meteorites, interplanetary dust, and dust returned from 
a comet. 

Once grains are formed in the solar nebula, they must accrete to form the 
larger planetesimal-sized objects thought to have been the building blocks of 
planets. The rate and mechanism for planetesimal formation are believed to 
depend on the size distribution, composition, and structure of the original 
nebular dust. In theory, the ability of colliding grains to stick together depends 
largely on short-range, Van der Waals interactions, although electrostatic and 
ferromagnetic forces may come into play. It has been suggested that grains 
endowed with mantles containing organic matter and/or icy components should 
accrete and grow faster than others. Despite its implications for early solar 
system history, this suggestion has never been tested experimentally. Micro- 
gravity facilities would provide excellent opportunities for model studies of grain 



90 



accretion in the space environment. Rates of accretion couid be determined as a 
function of the chemical composition, physical structure, and relative velocities 
of the grains. 

For example, the effects of grain rotation on accretion are unknown. Large 
grains in interstellar space may be spun up nonthermally to angular speeds of 
10 5 -10 8 rev/sec. What happens when submicrometer core-mantle particles that 
are so rapidly spinning collide? Do they melt and stick or do they immediately 
tear apart? Up to what limiting rotational frequencies will they predominantly 
coagulate? One approach to answering this question would be to insert ice- 
coated iron needles about 0.1 micrometer thick into a cooled microgravity 
chamber, and spin them up with a high-frequency magnetic field. The chamber 
walls would have to be maintained at low temperatures-preferably ~20 K. Light 
scattering is an excellent method to then follow the process of aggregation by 
measuring both the scattered intensity at several angles and the polarization of 
the transmitted and the scattered light. 

In addition to growing by the passive accretion of gaseous species to its sur- 
face, a dust grain can provide an active surface to catalyze reactions of species 
sorbed to it or it can itself be changed by chemical reactions with sorbed gases. 
Chemical reactions between gas and dust have been hypothesized to occur in 
interstellar clouds and in the solar nebula to account for the organic matter 
observed by radio astronomers in the clouds and by chemists in meteorites, 
comets, and interplanetary dust. These ideas are often expressed in terms of a 
Fischer-Tropsch-Type (FTT) synthesis in which the surfaces of silicate, metal, 
or metal oxide grains suspended in interstellar clouds or the solar nebula provide 
active sites for catalysis. Molecules of hydrogen, carbon monoxide, carbon 
dioxide, and ammonia sorbed on the sites at temperatures from 300 to 600 K 
may have been converted spontaneously to organic compounds and other car- 
bonaceous phases. These products, in the case of the solar nebula, were subse- 
quently retained on the grains and accreted into primitive planetesimals. Accord- 
ing to the FTT synthesis scenario, interstellar molecules represent products of 
nebular synthesis that were ejected into the surrounding medium during dissi- 
pation of prestellar nebulae. 

Recent data from analyses of organic matter in meteorites and from labora- 
tory FTT syntheses suggest, however, that the FTT processes cannot explain all 
the observed molecular and isotopic characteristics of the natural products. 
What they can account for remains to be clearly established, and additional 
artificial syntheses may provide the clues, provided they are conducted under 
conditions that may be related to the natural processes. 

All laboratory FTT reactions have been conducted at or near a total pressure 
of 1 atm with a bed of catalysts. Under these conditions, grains contact each 
other, chemical intermediates can migrate from catalyst sites on one grain to 
those on others, and opportunities exist for a diverse chemistry. In the nebular 
environment, the total pressure is 10~ 3 to 10~ 6 atm and dust is expected to com- 



91 



prise only about 1% of the mass. Under these conditions, sorbed gases and 
reactive intermediates produced on a grain would remain on that grain (or 
desorb into the gas phase where other processes would govern their fate) until 
the grain is accreted with others into larger objects; consequently, the composi- 
tion and abundance of the products and the rates at which they could form may 
well be strongly constrained and different from those observed in terrestrial 
laboratories. Gas-grain interactions that are independent of a bulk solid phase 
should be amenable to study under microgravity conditions. 

Other hypothetical gas-grain processes of nebular or interstellar relevance that 
merit study include the hydration of silicate grains to phyllosilicates by gaseous 
water, the photoirradiation of icy mantles of grains by starlight, and the thermal 
evolution of interstellar condensates in the solar nebula. 

Technology is being developed on Spacelab to levitate individual small 
neutral particles (<1 -cm-diameter) in acoustic levitation chambers. (Charged 
particles can be levitated electrostatically.) These levitation devices allow full 
control of the dynamics of the particle, including translation about the levitation 
chamber, spin angular momentum and orientation, and shape for liquid droplets. 
This technology could be used to suspend grains in gas chambers to study the 
physical and chemical interactions between gases and grains. The goal of such 
experiments would be to understand gas-grain processes in interstellar clouds and 
in the primordial solar nebula. An additional experiment might be to examine 
the collective response of a large number of small suspended particles with 
various initial conditions, including a central force, to simulated nebula conden- 
sation processes. 

5.2 Reactions of Neutral Atomic Oxygen 

The continuous source of oxygen atoms flowing with a narrow range of high 
velocities in low Earth orbit provides a unique opportunity to study oxygen 
atom chemistry as it relates to biological and biogenic molecules. The glow 
observed on the windward surfaces of the Space Shuttle shows that the differ- 
ential velocity of about 8 km/sec between the Shuttle and ambient oxygen 
atoms is sufficient to overcome the activation energy for reaction between 
ground-state oxygen atoms and certain, as yet unspecified, large molecules at 
rates sufficient to permit detection. Oxygen atom reaction rates and activation 
energies could be studied by injecting various reactants into a cell through which 
the oxygen atoms flow and monitoring the products downstream by either 
absorption or emission spectroscopy in the ultraviolet, visible, and perhaps 
infrared. Impact velocity can be varied from an upper limit of 8 km/sec to a few 
meters per second by having the incoming stream of oxygen bounce off several 
deflection plates or by injecting the reactants with a velocity vector either paral- 
lel, antiparallel, or perpendicular to the incoming beam. Interesting reaction 
partners to choose from are plentiful with, for example, bacteria and biologically 



92 



relevant molecules forming one class, and simple molecules relevant to planetary 
atmospheres and interstellar chemistry forming another. 

The reactions of energetic neutral oxygen atoms with large molecules is a 
completely unexplored field, and it is quite likely that a new area of chemistry 
exists. That the source of the Shuttle oxygen glow has not been determined by 
an extensive literature search underscores the uniqueness of a continuous source 
of energetic neutral oxygen atoms. 

5.3 An Artificial Comet 

Organic components of the gas and dust play a prominent part in many of 
the new and exciting results from the study of comet Halley. Foremost among 
these results were the discoveries of abundant, fine-grained dust composed of 
dark, apparently carbonaceous, matter that mantles inactive regions and is 
ejected from active regions of the comet surface; evidence for CO and C0 2 as 
parent molecules, in addition to H 2 and HCN; and CN jets associated with the 
emission of dust from the nucleus. Yet, unanswered questions remain regarding 
parent-daughter molecular relationships and the structure and composition of 
the dust rich in the biogenic elements. For instance: What parental species in 
addition to HCN are needed to explain the abundance of the CN radical? What 
is the role of dust in contributing to gaseous species in the coma? What proper- 
ties predispose the grains to be retained as mantling material? What are parent 
molecules for CS, C 2 , and C 3 ? What species are responsible for the 3.2- to 
3.6-jum emission features suggestive of C-H bonds? 

Although these questions specifically address unknowns in the composition 
and chemical evolution of comet Halley, the answers would apply to virtually 
ail comets. They reflect continuing uncertainties about the interstellar versus 
nebular origin of these objects and their constituents, their relationships to other 
primitive bodies in the solar system, and the contributions they may have made 
to prebiotic evolution and the inventories of the biogenic elements and com- 
pounds on Earth and the other terrestrial planets. The pertinence of all these 
issues to exobiology continue to make the study of comets an area of high 
priority. 

In 1984 the injection of barium particles into the solar wind on the Active 
Magnetosphere Particle Tracer Explorers (AMPTE) mission was carried out as 
an "artificial comet" experiment to elucidate aspects of the plasma interaction 
with the comet. In the future, the placement and operation of platforms in space 
will provide opportunities to conduct analogous simulation studies of other 
facets of cometary phenomena involving more realistic materials. These studies 
should be conducted to provide assessments of two interesting questions: the 
origin of daughter molecules and ions from putative parent species in either the 
gas or the dust emitted from comets, and the dynamical evolution of cometary 
dust mantles, aggregates, and grains as a function of their physical properties and 
chemical composition. 

93 



Two types of experiments are envisioned: releasing water vapor and selected 
gases, singly or as mixtures, from canisters and releasing a meter-sized ball of 
water ice laden with either gas or dust or both. The measurement objective in 
both types of experiments would be to follow spectroscopically in the ultravio- 
let, visible, or infrared (or some combination of wavelength regions) the changes 
in composition in the coma and tail as functions of time and distance from 
release. An additional objective in the case of the ice ball would be to determine 
the physical evolution of the ice and dust with respect to chemical reactions at 
the surface and to formation of dust mantle, ejection of grains or grain aggre- 
gates from the surface or interior, and their disaggregation and dissipation over 
time. Embedded thermal sensors could be used to follow the thermal evolution 
of the interior. 

The interaction of the unattenuated solar flux with the volatiles and dust 
would be examined, as would the processes of sublimation and photolysis. The 
interaction of the Earth's magnetic field and ionosphere moving past the sub- 
limed and ionized gases at about 7 km/sec could be studied as analogs to pro- 
cesses that occur at a real comet because of solar wind interaction with the 
cometary ionosphere. 

For both types of experiments, selecting the mode(s) of observation (ground- 
based, airborne, spaceborne, or combinations thereof) to follow the resulting 
course of chemical evolution of nucleus, coma, and tail would have to take into 
account the time scales available for observations of each feature; the require- 
ments for spatial, spectral, and temporal resolution; and the need to synchronize 
the release with the onset and continuation of observations. 

One major problem in either type of experiment is the interference of the 
Earth's atmosphere. The coma of an artificial comet cannot be studied at the 
orbit of the Space Station (~300 km) because of the reactivity of a large flux of 
oxygen atoms (~4X10 14 cm -2 sec -1 ) and a somewhat smaller flux of nitrogen 
molecules. By comparison, the flux of photons of wavelength less than 2000 A 
is only 1 .1X 1 1 3 cm' 2 sec -1 . Therefore, the gas or ice ball should be released at 
an altitude of ~1000 km. At this altitude, the fluxes of oxygen and hydrogen 
atoms are only ~6X10 9 and ~3X10 10 cm* 2 sec" 1 , respectively; the flux of 
thermal ions is ~6X10 9 and of 100 MeV photons only 10 3 cm" 2 sec" 1 . All of 
these are much smaller than the flux of photons with wavelength less than 
2000 A; and, therefore, the coma chemistry is not likely to be much affected by 
them. 

For the gas-release experiments, several canisters, each containing a selected 
pressurized gas, would be placed at the appropriate orbit. The water canister 
should be heated to a high temperature in order to supply water vapor at the 
required rate. The valve of each canister should be operated separately, enabling 
the formation of various gas mixtures and also imitating to some extent the 
gradual release of gases from a gas-laden ice ball. 



94 



The gases selected for release would depend on the theoretical parent- 
daughter relationships that are being tested and could include carbon monoxide, 
carbon dioxide, methane, acetylene, ammonia, hydrogen cyanide, and acetoni- 
trile, among others. While lacking a full complement of gases, ices, or other 
grains, these experiments would necessarily be incomplete simulations. On the 
other hand, ground-based experiments would be similarly limited; moreover, the- 
simulations of the vacuum, volume, and time scales of the inner coma would be 
impossible in the laboratory. 

Formation of a gas-laden ice ball may be achieved by expanding a mixture of 
water vapor with various gases, through a nozzle, into a vacuum chamber to 
produce a snow, which may then be consolidated by compaction to the desired 
extent. Dust particles could be injected simultaneously and mixed with the 
snow. Alternatively, quantities of water vapor and gases could be frozen on a 
cold surface and continuously scraped from it, until a large enough quantity is 
accumulated to make into the ~1-m (or larger) ice ball. An ice ball of this 
size is calculated to last for about 10 5 sec (about 28 hours) under full solar 
heating. 

Ices could be prepared at various densities and at different temperatures 
corresponding to amorphous or crystalline states. Recent laboratory experiments 
indicate that, depending on the temperature of formation and thermal history of 
the ice, gases may be occluded in amorphous or crystalline forms of ice; differ- 
ent gas-release characteristics may prevail, depending on the ice phase, in some 
cases accompanied by ice grain ejection. Preservation of such sites would be 
essential for study. If formed at temperatures lower than 80 K, the ball should 
be kept in a dewar cooled by liquid helium, itself enclosed in a dewar of liquid 
nitrogen, until the time of release. 

In the cases where dusts of either silicate or carbonaceous composition or 
both were embedded in ice, the choice of material to simulate the dust would 
necessarily be model-dependent. Candidate materials for carbonaceous grains, 
for instance, could be amorphous carbon, polymers of HCN, coal dust, and ter- 
restrial kerogens, among others. 

Experiments with an ice ball could not simulate the heterogeneity of physical 
and chemical composition or the irregularity in the distribution of surface-active 
sites that were actually observed in comet Halley. Determining how simpler 
model systems behave, however, would be a prerequisite for gaining understand- 
ing of the more complex systems. 

Precursor laboratory simulations should be conducted in a large, ground- 
based, high-vacuum chamber where an ~1-m ice ball could be formed and sub- 
jected to heating by infrared or xenon arc lamps. 

When compared with the large body of observational data now available as a 
result of studies of comet Halley, it is expected that observations made on an 
artificial coma and tail produced by a gas-laden ice ball in Earth orbit, together 



95 



with observations obtained on a coma produced by gas-release experiments and 
in large-scale laboratory experiments, will contribute substantially to our knowl- 
edge of the structure and composition of cometary nuclei. Among the questions 
that will be addressed are: How are gases occluded in the nucleus and how are 
they released? What gases or grains and what sequence of their release could 
produce the species distribution observed in real cometary comae? What is the 
contribution of gas-laden ice grains in the coma to its chemistry? What proper- 
ties of grains are conducive to the formation of dusty mantles? 

5.4 Microbial Survival in Space 

Panspermia is the concept that life on Earth arose not de novo but from an 
inoculum that reached the Earth from space. If panspermia were true, the prob- 
lem of the origin of life would be transferred from the relatively known environ- 
ment of Earth to a relatively unknown environment elsewhere. If panspermia has 
occurred it is required that 

1. Life arose somewhere else in the universe and flourished. 

2. Some of the living organisms in this location left their origin and entered 
outer space. 

3. Some of these latter organisms survived space travel and reached Earth. 

4. Some of these surviving organisms penetrated the Earth's atmosphere in 
viable form. 

5. Some (or at least one) of these viable organisms found conditions favora- 
ble on Earth and flourished. 

6. The descendants of this xenobiont led to the vast array of organisms on 
Earth today. 

It would, of course, be difficult to prove unambiguously that panspermia 
occurred. However, it might be possible to determine by measurement, calcula- 
tion, and experiment how likely it is that panspermia could occur, that it is 
impossible to disprove panspermia experimentally, or that the probability of 
panspermia is vanishingly small. (In this discussion, directed panspermia is not 
being considered as there is yet no evidence that intelligent life exists in the 
universe outside Earth.) 

In light of the above scenario, there are several approaches for investigating 
certain aspects of the concept of panspermia: 

1. Measurement or calculation of the rate at which organisms could escape 
from a planet 

2. Measurement or calculation of the rate at which organisms would be 
killed during space travel 

3. Calculation of the time it would take for xenobionts to reach Earth from 
various sources in outer space 

4. Calculation of the concentrations of xenobionts that might be expected in 
the Earth's orbit 



96 



5. Calculation, using results from 3 and 4, of a flux rate for xenobionts 
reaching the Earth's surface, permitting determination of the experimental 
requirements that would be needed for detecting by direct measurement any 
putative xenobionts 

One critical problem in carrying out the above scenario is that material col- 
lected in Earth orbit (Chapter 4) may also contain viable organisms derived from 
Earth (perhaps ejecta attached to volcanic debris). Thus, it would be extremely 
important to determine the flux rate of organisms from Earth into space. The 
final result of such studies might be that viable organisms found in Earth orbit 
are most likely to have been derived from the Earth's biosphere, A direct micro- 
biological test of panspermia with existing tools is difficult but possible: for 
example, one could search for the presence of novel amino acids or nucleosides. 

Microbial survival is one of several parameters that can be used to put proba- 
bility boundaries on the hypothesis of panspermia. While one can hypothesize 
that the evolution of life on other planets might have led to organisms more 
tolerant to the conditions of interstellar space than the terrestrial creatures with 
which we are familiar, such a hypothesis cannot be tested at present. This dis- 
cussion is thus organized around the notion that extraterrestrial organisms will 
be fundamentally similar to those found on Earth. The proposed experiments 
are concerned with determining the survival potential of terrestrial organisms 
subjected to conditions characteristic of space. Since the major environmental 
variables in space are radiation flux (particularly ultraviolet), vacuum condi- 
tions, and low temperatures, these parameters should be critically examined. The 
studies can be organized around three general areas or questions: 

1. To what extent does an organism's physiological state affect its survival 
potential? 

2. What is the range of resistance to ultraviolet and other injurious factors 
among microbes isolated from terrestrial and aqueous environments? 

3. What are the critical factors that affect any resistance to the conditions 
found in space? 

Some experimental work has been done in each of these areas, including carrying 
microbes into space aboard Spacelab, but additional ground-based studies are 
needed to enable proper planning for further in situ space-based experiments. 
Ground-based studies have the advantage that they are relatively inexpensive and 
the experimental design is flexible. Thus, use of space simulations and atmo- 
spheric controls should allow various hypotheses dealing with space survival to 
be tested. These hypotheses can be expected to evolve to a point at which they 
could be developed as experiments or experimental programs suitable for further 
testing in space. However, since a satisfactory, ground-based simulation of the 
complex interplay of all of the environmental factors of outer space is difficult, 
if not impossible, to attain, the performance of experiments on living micro- 
organisms in Earth orbit is required to properly study the viability of microbes 
in space. 



97 



To determine the relation between physiological state and viability, the fol- 
lowing parameters should be examined: 

1. The effect of the particular portion of the growth cycle (i.e., exponential 
vs. stationary phase cells, including cells maintained for extended periods in the 
stationary phase) 

2. The effect of the growth medium (i.e., complex vs. simple) 

3. The effect of the type of energy-generating processes (i.e., fermentative 
vs. respiratory) 

4. Resistance to oxidation (i.e., the presence of superoxide dismutase and/or 
catalase) 

5. The effect of lyophilization on resistance 

Since exposure to the low temperatures and vacuum of outer space would effec- 
tively lyophilize microbial cells, an examination of the role of the state of hydra- 
tion on resistance and survival is of paramount importance. Experiments should 
be planned so as to consider synergistic effects. The current literature contains 
much information on the effect of various physical and chemical factors on the 
survival of microorganisms. However, many of these studies were carried out 
with "hydrated" cells and are probably irrelevant to the lyophilized state. The 
degree to which hydrated and dehydrated cells exhibit different sensitivities to 
various agents should be thoroughly tested. Another particularly important 
parameter is temperature. The effects of very low temperatures on resistance and 
survival should be studied. 

The selection of appropriate test organisms should be made with care. Ideally, 
one should employ organisms that are easy to handle and which are well known 
with respect to their genetics, nutrition, and physiology. In addition, organisms 
exhibiting greater than usual resistance should be considered. The following two 
organisms are possible prototypes: 

1. Escherichia coii, a gram-negative heterotroph with about average powers 
of resistance and capable of reasonable survival in nature 

2. Bacillus subtilis, a gram-positive heterotroph, the vegetative cells of which 
are no more resistant than those of E. coli, but which produces a structure, the 
endospore, capable of surviving extremes of temperature, desiccation, and 
nutritional deprivation 

Throughout the 3.5 to 4 billion years of life history on Earth, adaptation to 
conditions comparable to outer space seems never to have been required. In view 
of the lack of perfectly suitable test systems, one could turn to the use of organ- 
isms adapted to growth or to survival, at least in extreme regions of the bio- 
sphere such as in soil or rock from deserts or Antarctica, or in the upper layers 
of the atmosphere. Airborne microbes commonly are in a resting or temporarily 
inactive state, either as a spore with built-in resistance mechanisms against envi- 
ronmental extremes or modified by desiccation and starvation. They are repre- 
sented by endospores of bacteria and spores of actinomycetes, fungi, ferns, 
mosses, pollen of flowering plants, and cysts of protozoa. Some of them are 



98 



especially adapted for dissemination over the biosphere. Endolithic life forms, 
such as cyanobacteria, algae, fungi, and lichens of unusual organization, have 
been detected inside rocks from the Antarctic dry valleys as well as inside desert 
sandstone. They represent an example of a simple ecosystem with a favorable 
microclimate surrounded by an extreme environment of low humidity, tempera- 
ture extremes, and a high influx of solar radiation. Resistant organisms could be 
sought by the wholesale exposure to extreme conditions of samples taken from a 
variety of such inhospitable localities on Earth. 

The study of any given organism (or small group of organisms) has the disad- 
vantage that the results obtained may not be representative of the total picture. 
Misleading "dogma" can be established through the rigorous and in-depth study 
of a few easy-to-handle organisms. Therefore, the variability in levels of resis- 
tance for a variety of microorganisms should be systematically examined. 



Suggestions for Further Reading 

Wood, J. A.; and Chang, S., eds.: The Cosmic History of the Biogenic Ele- 
ments and Compounds. NASA SP-476, 1985. 

Davies, R. E.: Panspermia: Unlikely, Unsupported, But Just Possible. Acta 
Astronautica, in press. 

Bar-Nun, A.; Herman, G.; Laufer, D.; and Rappaport, M. L.: Trapping and 
Release of Gases by Water Ice and Implications for ley Bodies. Icarus, vol. 63, 
1985, p. 317. 

Whipple, F. L.; and Huebner, W. F.: Physical Processes in Comets. Annual 
Review of Astronomy and Astrophysics, vol. 14, 1976, p. 143. 



99 



Chapter VI 
Summary of Proposed Experiments 



The next several decades offer many exciting opportunities to conduct exo- 
biological observations and experiments in Earth orbit, and to attack a wide 
variety of questions that are not directly amenable to ground-based studies. This 
summary highlights the most important aspects of the exobiology research 
identified in this report, prioritizes the various projects, and indicates which can 
be pursued with facilities available now and which must wait for capabilities that 
are planned for the future. The order in which the different research topics are 
discussed in the Observational Exobiology section indicates, by the consensus of 
the members of this Workshop, their relative importance to exobiology. The 
ordering within the other two sections, Cosmic Dust Collection and In Situ 
Experiments, does not imply any prioritization, nor is the order in which the 
three major sections appear meant to imply anything about their relative impor- 
tance. These selected experiments are for Earth-orbital activities only, and they 
augment the very strong interests of the exobiology community in ground-based 
research and solar system missions. 

6.1 Observational Exobiology 

Since life, as we know it, is a planetary phenomenon, it is of utmost impor- 
tance to know whether planets exist outside our own solar system. The orbital 
observatories of the next few decades should allow us to answer this fundamen- 
tal question. In addition, we anticipate that the greatest increase in our under- 
standing of the origin and evolution of the biogenic elements will come from the 
use of telescopes above the Earth's atmosphere, opening up the wavelength 
region from the far infrared to the submillimeter. This portion of the spectrum 
provides unique information about many molecules and particles in diverse 



A famous early 20th century engraving (1911) erroneously thought to be a 17th 
century woodcut of a Medieval astronomer passing through the sphere of the stars 
to see the mechanisms of the Ptolemaic universe beyond. (Courtesy of Science 
Graphics, Tucson, AZ.) 



101 



cosmic environments and to date is almost completely unexplored. High sensi- 
tivity, in addition to good spatial and spectral resolution, will be extremely 
important, as will the laboratory and theoretical studies needed to correctly 
interpret the new observational data. 

6.1.1 Detection of Extrasolar Planetary Systems 

The best opportunity for conducting a systematic survey of the nearest stars 
for massive, long-period planets is a dedicated astrometric telescope to be 
mounted on the upper boom of the Space Station. Alternative approaches such 
as searches for short-period planets will require technical breakthroughs to 
develop a double differential photometer, or a long-lived platform to monitor 
thousands of stars for periodic occultations may be accommodated by Space 
Station. The detection of extrasolar planetary systems is best pursued at optical 
and near-infrared wavelengths. Earth-orbital observations are crucial as the 
Earth's atmosphere imposes severe limitations on the ability to either directly 
detect a planetary companion to a star or to indirectly deduce the presence of a 
planet from its influence on the stellar motion or apparent brightness. While the 
HST will be able to measure, with observations taking minutes, the relative 
position of a nearby star that is as accurate as can be obtained with modern 
detectors and hours of ground-based telescope time, and it will have the orbital 
longevity and mechanical stability needed for such an astrometric search pro- 
gram, it is unlikely that sufficient time will be allocated for a systematic search 
for extrasolar planetary systems. 

The logical followup to an astrometric detection of an extrasolar planet is its 
direct detection. Once again, this is best done from observatories in Earth orbit. 
The HST will provide the best opportunity in the near term to attempt imaging 
any extrasolar planets detected astrometrically or tentatively identified from 
ground-based observations. Imaging is easier at near-infrared wavelengths where 
the contrast ratio of planet and star is larger and either the NICMOS infrared 
spectrometer or the imaging Michelson interferometer selected for the second 
generation of HST instrumentation may provide a capability to do just that. If 
either SI RTF or ISO prove to be capable of producing a "superresolution," 
they may be able to image the outer planets around nearby stars. FIRST or LDR 
should eventually provide the necessary better resolution, but a free-flying infra- 
red interferometer will be required to provide imaging capabilities for more 
distant stars. 

6.1 .2 The Solar Nebula and its Analogs 

Observations of regions of star formation are important to exobiology in two 
ways. First, they will determine how stars hospitable to the origin of life are 



102 



formed, and second, they are necessary to make estimates of the likelihood of 
planet formation. Because of the clouds of dust obscuring star-forming regions, 
protostellar observations must be made principally at far-infrared and longer 
wavelengths. Studies of the far-infrared continuum and low-resolution spec- 
troscopy will help determine the quantity and composition of material available 
for the formation of planets. High-spectral- and high-spatial-resolution observa- 
tions at submillimeter and far-infrared wavelengths are needed to identify the 
molecules present in the gaseous state; these can be used to determine the nature 
of the physical environment and to investigate the link between organic chemis- 
try in the dense molecular protostellar clouds and the protostellar nebula. In 
order for the far-infrared-continuum observations to be sufficiently sensitive, a 
cryogenically cooled telescope must be employed. Of lesser importance, ultra- 
violet studies of objects in the late stages of star formation, when the obscuring 
dust clouds have been blown away, can yield information on the radiation envi- 
ronment in planetary systems in the era of the origin of life. All of the observa- 
tions listed here are strongly affected by atmospheric absorption, and thus are 
best made with orbiting observatories. 

The contribution of biogenic and other heavy elements from stars to the 
interstellar medium is also of exobiological importance. This happens late in the 
life of a star, either as grain formation and ejection in the atmospheres of giant 
stars, or in supernovae ejecta. To a lesser extent, it also occurs during the main- 
sequence stage of stellar evolution through the action of stellar winds (for exam- 
ple, the solar wind). Observations of these evolved objects are best made in the 
visible and near infrared. 

The first-generation instruments on the HST are capable of performing only 
the ultraviolet and visible observations discussed here. SI RTF is necessary for the 
other studies mentioned. More detailed work, especially in the high-resolution 
studies of molecular and atomic lines, requires LDR for its high sensitivity and 
spatial resolution, which is necessary to determine the composition and structure 
of matter within these energetic systems. 

6.1 .3 Solar System Observations 

While we recognize the importance of measurements by direct missions to 
solar system objects, three broad areas of solar system observations have been 
identified as holding promise for exobiological studies from Earth orbit. These 
are primarily related to the identification of biogenic elements and complex 
molecules in primitive bodies and in the atmospheres of the giant planets and 
satellites. Significant progress is expected, particularly from the use of the HST. 
Nonetheless, many species of interest will remain undetectable until the develop- 
ment of sensitive orbiting telescopes and spectrographs for the spectral range 
from the far-infrared through millimeter wavelengths. 



103 



Comets and asteroids present two challenges for study from Earth-orbiting 
observatories. Studies of cometary comae are similar to the atmospheric obser- 
vations of the giant planets described later, although the lack of pressure broad- 
ening makes very high spectral resolution both extremely useful and desirable; 
this can be obtained with LDR and/or a higher-resolution SI RTF spectrometer 
than is currently planned. Determining the composition of the solid (ice or 
mineral) surfaces can be performed by means of reflectance spectroscopy, at 
reasonably high resolution, in the infrared. Some of this work can be done in 
the near infrared when HST acquires this capability, but most likely an infrared 
orbiting observatory such asSIRTF or ISO will be required. The WUPPE that is 
part of the ASTRO Shuttle package will allow the size and composition of the 
cometary dust to be inferred from the percentage of polarization in scattered 
light from 1300 to 3300 A. 

The objectives for Titan, the only known planetary satellite with a significant 
atmosphere, are similar to those below for the giant planets. Because of the very 
small angular size of the satellite, the measurable flux is much smaller than that 
for the planets, and initial observations may have to await the development of 
more sensitive, second-generation HST instruments. 

For the giant planets the need is to increase knowledge of the inventory of 
trace molecular species in the predominantly hydrogen atmospheres and to 
determine, using line-shape measurements at high spectral resolution, the vertical 
and meridional distributions of these species in the atmosphere. Isotope ratios 
should also be determined for the most abundant molecules. When HST becomes 
operational it will permit high-resolution studies of molecules detectable in the 
ultraviolet and visible and, in addition, will allow these studies to be extended to 
Uranus and Neptune. ASTRO will provide limited observations of solar system 
targets, but should add significant information about their atmospheric composi- 
tion. Further progress will require high-resolution infrared, submillimeter, and 
millimeter observations from space using platforms such as SI RTF, ISO, or LDR. 
Determination of abundances from such observations will need extensive labora- 
tory data currently not available. 

6.1.4 Molecules in the Interstellar Medium 

The study of molecules in interstellar space is an important component of 
exobiology. Three important questions to be answered in this field are the 
extent of chemical evolution throughout the universe, the availability of bio- 
genic elements, and the existence of molecules such as water and organic com- 
pounds. The extent of chemical evolution can be determined by spectroscopic 
studies of complex molecules in the gas phase of interstellar clouds, especially 
in the infrared and the millimeter, and by studies of interstellar dust particles in 
the infrared, visible, and ultraviolet. In the near future, most detailed observa- 
tions of molecules will be undertaken via ground-based millimeter- and 



104 



submillimeter-wave telescopes. However, infrared spectra are especially impor- 
tant in determining complex molecular signatures; because of atmospheric inter- 
ference, such space-based observatories as SI RTF and LDR will be crucial in 
this regard. A high-resolution spectrometer on SI RTF would be especially useful 
for precise molecular identification. A vigorous program of laboratory spectro- 
scopic studies is necessary to complement the observational program because the 
laboratory spectra of many important interstellar molecules have not yet been 
fully studied. 

The availability and relative abundances of biogenic elements can be deter- 
mined by observational studies directed at either atoms or molecules. The deter- 
mination of the relative abundances of carbon and oxygen in various sources is 
important because it is thought by some investigators that molecular complex- 
ity can occur only in carbon-rich regions. An inventory of the biogenic elements 
in external galaxies would be especially interesting and would be aided by 
space-based observatories in all wavelength regions. 

Finally, the existence of water and organic compounds can be investigated 
via spectroscopic studies in a variety of wavelength regions, especially the sub- 
millimeter and infrared, depending upon the excitation conditions in the source. 
It is obvious that space-based observatories such as HST, LDR, and SI RTF will 
aid immeasurably in the search for these species. Whether molecules such as 
these are widespread in our own and other galaxies is intimately related to the 
probability that life exists elsewhere. 

6.1 .5 Time Scales 

The order in which observations of interest to exobiologists are performed 
and the pace at which our understanding increases will be determined by the 
timetables established within NASA and ESA for the development of the orbital 
facilities identified earlier. The following list represents the current best estimate 
of the order of implementation of the telescopes judged most useful to observa- 
tional exobiology. The observations listed under each spacecraft reflect the 
priorities outlined above. 

Hubble Space Telescope (HST): NASA 1989 

1. Astrometric and coronographic searches for extrasolar-system planets to 
be augmented by second-generation infrared instrumentation in 1994 

2. Ultraviolet observations of young, stellar objects— determination of the 
radiation environment in early stages of planetary systems 

3. Visible and near-infrared observations of grains in stellar atmospheres 

4. High-resolution spectroscopy (ultraviolet/visible) of planetary atmo- 
spheres, comets, and asteroids; imaging of planets at wavelengths of specific 
molecular bands 



105 



ASTRO 1: NASA 1989 

1 . Spectroscopy (500-3200 A) of planetary atmospheres 

2. Ultraviolet imaging of planets and comets in selected bands 

3. Ultraviolet polarimetry of dust in comets and in interstellar space 

4. Extinction-curve measurements to study size and distribution of interstel- 
lar grains 

Space Station-Block I: NASA 1995 

1. Dedicated astrometric telescope 

2. Service and instrument changeout on free-flying orbiting telescopes 

3. Construction and servicing of later generations of orbiting telescopes 

Space Infrared Telescope Facility (SI RTF): NASA 1996 
Infrared Space Observatory (ISO):ESA 1992 

1 . Possible direct imaging of extrasolar planets using superresolution 

2. High-spectral-resolution observations of atoms and molecules in star- 
forming regions, e.g., water 

3. Continuum and low-resolution observations of dust in star-forming regions 

4. Observations of complex organic molecules 

5. Continuum and low-spectral-resolution studies of grain formation and 
ejection from stellar atmospheres in the late stages of stellar evolution 

6. Continuum and low-resolution studies of grains in supernovae ejecta 

7. Molecular spectroscopy of solar system planets, satellites, comets, and 
asteroids 

8. Thermal emission from dust in comets 

9. Reflectance spectroscopy of solid surfaces (asteroids) and ices (comets) 

Far Ultraviolet Spectroscopy Explorer (FUSE): NASA 1996 (Joint with ESA 
under the name Lyman) 

1. High-resolution, extreme-ultraviolet spectroscopy of the interstellar 
medium and the intergalactic medium 

Large Deployable Reflector (LDR): NASA 2000 
Far Infrared Space Telescope (FIRST): ESA 1995 

1 . Direct imaging of extrasolar planets 

2. High-resolution studies of accretion shocks surrounding protostellar nebula 

3. High-spectral-resolution observations of molecules in star-forming regions 

4. Studies of molecular oxygen and water in many transitions in regions of 
star formation and planets 

In closing, we note that while the next few decades offer exciting opportuni- 
ties for observational exobiology, even more exciting prospects lie just a little 
farther in the future. For example, having detected examples of extrasolar 



106 



planets, exobiologists would no doubt wish to study them in detail. This will 
require a whole new generation of telescopes or arrays of telescopes in high orbit 
or on the lunar far side with capabilities in excess of the near-term orbiting 
systems. There is much to be learned. 

6.2 Cosmic Dust Collection 

Collection of cosmic dust is important to exobiology because collectable dust 
particles are samples of comets and asteroids, primitive bodies that are likely to 
preserve compounds that formed during or before the origin of the solar system. 
Cometary particles are of particular interest because comets probably formed in 
the coldest, most-remote, regions of the solar nebula. Collection from Earth 
orbit can provide an unbiased sample of nanogram to microgram particles 
that complement meteorites and samples of extraterrestrial dust in the strato- 
sphere. The most exciting aspect of meteoroid collection from orbital platforms 
is the active measurement of impact velocity and directions to identify the 
sources of individual collected particles. 

6.2.1 Capture Techniques 

The unintentionally captured meteoroids collected from returned Solar Max 
surfaces have shown that analyzable material can be collected even with simple 
materials such as aluminum sheet metal and multilayer plastic thermal blankets. 
Capture experiments were recently flown on Salyut and currently are being 
exposed on LDEF. They were specifically designed to collect meteoroids, and 
their results will play a role in developing future experiments. The development 
of special capture materials or cells for the collection of the biogenic material in 
analyzable form is needed for exobiology. For example, special metal capture 
cells where low-atomic-weight elements could be recondensed on cell walls fol- 
lowing hypervelocity impact could be used— retention of some elements may 
require a "getter." The development of clean cells made of appropriate materials 
should lead at least to the capture of enough material to determine elemental 
hydrogen, carbon, and nitrogen abundances and isotope ratios. The hydrogen 
isotopic composition is of particular importance because large isotope effects 
have been seen for this element in interplanetary dust samples. No previous 
experiments exist for the capture of light elements, and the development of 
capture cell techniques will involve laboratory simulation experiments (using 
dust accelerators and lasers) and actual orbital collection experiments. 

The ultimate particle collection experiment is the so-called "intact capture" 
in which the particle, or at least parts of the particle, are collected without heat- 
ing or significant modification. One approach is to use "soft" collection sub- 
strates where the particle is decelerated slowly. Soft materials include porous 
targets and gas cells. Experiments under way are promising; porous targets can 



107 



definitely be used for intact collection of particles impacting at less than 
6 km/sec. The lowest impact velocity on an orbiting collector is 3 km/sec. 
Collectors of this type that do not contain hydrogen, carbon, nitrogen, or 
oxygen and hence will not contaminate samples, will have to be developed. The 
development of soft collection substrates involves dust accelerators and actual 
flight exposures. In the long term, it is hoped that electromagnetic decelerators 
can be developed so that meteoroids can be collected, ideally, with no heating 
at all. Near-term work in this area will be to develop a prototype design and to 
conduct a feasibility study. 

6.2.2 Orbital Parameter Measurement 

An electronic technique to measure the impact velocity and direction of par- 
ticles was used on Pioneers 8 and 9 and on the LEAM experiment placed on the 
Moon during Apollo 17. Other techniques have been suggested and investigated 
in at least a casual way. It is critical for the future of meteoroid collection in 
Earth orbit to fully develop at least one approach that can be used with a prac- 
tical system for the collection of 10- to 250-jum particles. The speed and impact 
angle should be measured to an accuracy of a few percent so that the orbits of 
collected samples can be determined with sufficient accuracy. There appear to 
be no technological hurdles to achieving this accuracy, and the development will 
probably be more of an adaptation and refinement of existing techniques. 

6.2.3 Laboratory Analyses 

The collectable cosmic dust samples will be small and techniques for their 
analysis must be refined. As examples, isotopic, mass spectrometric, gas chro- 
matographic, and spectroscopic analyses are possible for samples this small, 
but techniques to do so have not been adequately developed. Laboratory tech- 
niques that could be refined for this study include mass spectrometry, gas chro- 
matography, Auger spectroscopy, microESCA (Electron Spectroscopy for Chem- 
ical Analysis), Secondary Ion Mass Spectrometry (SIMS) microprobe, and a 
variety of other techniques, including energy-loss spectroscopy. Developments of 
such microanalytical techniques would also be of considerable value for the 
analysis of meteorites, interplanetary dust, and samples returned from comets 
and Mars. 

6.2.4 Flight Exposures 

Development and implementation of the new generation of orbital dust col- 
lectors will require several types of long-exposure flight opportunities. For the 
development of techniques, a square meter exposed for several months to a year 



108 



would be adequate in many cases. Ultimately, the collection of large or other- 
wise rare particle types will require several square meters exposed for at least a 
year. On an operational Space Station one can envision a 10- to 100-m 2 area 
exposed continuously with astronaut recovery of selected modules that are hit. 
Development of a contamination monitoring system for the collection environ- 
ment should take place in parallel with any collection program. 

6.3 In Situ Investigations 

Spacecraft in Earth orbit, in particular the Space Shuttle and Space Station, 
allow experiments to be done that are not possible on the ground. Space labora- 
tories provide a unique combination of conditions that cannot be duplicated in 
terrestrial laboratories. Four different types of experiments have been identi- 
fied as being able to take advantage of the special opportunities provided in 
Earth orbit; three of these are highlighted here. 

6.3.1 Studies of Suspended Dust Grains 

Detailed studies of the fundamental physical and chemical processes leading 
to the formation, condensation, and aggregation of dust grains, and the processes 
occurring on the surfaces of grains in circumstellar shells, in interstellar dust 
clouds, and in presteliar nebula, can be best done on a space platform in Earth 
orbit. Such experiments are directly relevant to the growth of planetesimals and, 
ultimately at least, the terrestrial planets. Increasing interest is being shown in 
the constituents of these environments, and the mechanisms of molecular and 
grain synthesis that may occur there. Whether organic molecules or carbona- 
ceous grains that originated in these environments could have survived intact 
during the accretion of the Earth is not known. However, the analysis of meteo- 
rites and cosmic dust clearly shows that molecules and carbonaceous grains of 
extraterrestrial origin can survive entry into the atmosphere of the contempo- 
rary Earth. The proposed experiments may therefore also be relevant to Earth- 
based exobiological research: what organic compounds should one use in labora- 
tory simulations of reactions that may have been important for the origin of 
life? 

6.3.2 Artificial Comets 

Interest in comets is presently intense as the many exciting results from the 
study of comet Halley are being analyzed. As is normal in scientific investiga- 
tions, many new questions are arising from these studies. A deeper understand- 
ing of the chemical and physical processes that occur at the interface between 
gas-and-dust-laden ice and space, under the influence of solar heating, is not only 



109 



of interest in itself, but will help in planning for future comet sample return 
missions. An incremental increase in the depth of understanding of cometary 
processes could be achieved by the study of artificial comets in space. Two types 
of experiments are envisioned, one in which gas is released and monitored to 
simulate the coma and tail, and one in which a dust-and-gas-laden ice ball is 
released and monitored to simulate the nucleus and its immediate vicinity. Such 
experiments in space are the logical next step in the study of comets, bodies that 
contain much information of interest to exobiologists concerning the early 
history of the solar system and the environment of the early Earth. 

6.3.3 The Survival of Microorganisms in Space 

While most scientists are convinced that life evolved on Earth from nonliving 
matter, the idea that life evolved elsewhere, traveled across space, and inoculated 
our own world is an intriguing one. One of the requirements of this panspermia 
hypothesis is that microorganisms can survive in the space environment. As this 
environment cannot be simulated on the ground, the viability of microorganisms 
in space must be tested in space. Therefore, a variety of microorganisms should 
be exposed to the space environment and their viability determined under differ- 
ing, controlled conditions. Such studies are relevant to exobiology in the strict 
sense of the word, and can provide an experimental test of the panspermia 
hypothesis; they are important to biology in providing new information on the 
adaptability of life to extreme environments. 



110 



Appendix A 



ACRONYMS AND ABBREVIATIONS 



AMPTE 

AO 

AU 

AXAF 

CCD 

COBE 

CRAF 

ELV 

ESA 

ESCA 

EURECA 

EUVE 

FIRST 

FOC 

FOS 

FTT 

FUSE 

GAS 

GRO 

GSFC 

HH 

HR 

HST 

HUT 

IMS 

IR 

IRAS 

IRIS 

ISO 

IUE 

KAO 

LDEF 

LDR 

LEAM 



Active Magnetosphere Particle Tracer Experiment 

Announcement of Opportunity 

Astronomical Unit 

Advanced X-ray Astronomy Facility 

Charge-Coupled Device 

Cosmic Background Explorer 

Comet Rendezvous Asteroid Flyby 

Expendable Launch Vehicle 

European Space Agency 

Electron Spectroscopy for Chemical Analysis 

European Retrievable Carrier 

Extreme Ultraviolet Explorer 

Far-Infrared Space Telescope 

Faint Object Camera (on HST) 

Faint Object Spectrograph (on HST) 

Fischer-Tropsch type (synthesis) 

Far Ultraviolet Spectroscopy Explorer 

Getaway Special (on Space Shuttle) 

Gamma Ray Observatory 

Goddard Space Flight Center 

Herbig-Haro (object) 

Hertsprung-Russell diagram of stellar luminosities plotted against 

effective stellar temperature 
Hubble Space Telescope 
Hopkins Ultraviolet Telescope (on ASTRO) 
Imaging Michelson Spectrometer 
Infrared region of the electromagnetic spectrum 
Infrared Astronomical Satellite 
Infrared I nterferometric Spectrometer (on Voyager) 
Infrared Space Observatory 
International Ultraviolet Explorer 
Kuiper Airborne Observatory 
Long Duration Exposure Facility 
Large Deployable Reflector 
Lunar Ejecta and Meteorites 



111 



MRPS 

MSFC 

NASA 

NICMOS 

OMV 

OTV 

OVLBI 

PAH 

PVDF 

SETI 

SIMS 

SIRTF 

SOFIA 

SSEC 

STS 

TOF 

uv 

WFC 

WF/PC 

WUPPE 

XMM 



Medium-Resolving-Power Spectrometer (on LDR) 

Marshall Space Flight Center 

National Aeronautics and Space Administration 

Near Infrared Camera and Multi-Object Spectrometer 

Orbital Maneuvering Vehicle 

Orbital Transfer Vehicle 

Orbiting Very Long Baseline Interferometer 

Polycyclic Aromatic Hydrocarbon 

Polyvinylidene Fluoride 

Search for Extraterrestrial Intelligence 

Secondary Ion Mass Spectrometry 

Space Infrared Telescope Facility 

Stratospheric Observatory for Infrared Astronomy 

Solar System Exploration Committee 

Space Transportation System 

Time of Flight 

Ultraviolet region of the electromagnetic spectrum 

Wide Field Camera (on SI RTF) 

Wide-Field/Planetary Camera (on HST) 

Wisconsin Ultraviolet Photopolarimetry Experiment (on ASTRO) 

X-ray Multiple Mission 



112 



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Appendix C 



CONVERSIONS: WAVELENGTH/FREQUENCY 



Domain Wavelength Frequency (GHz) 

Radio >1 cm <30 

Millimeter 1-10 mm 300-30 

Submillimeter 600-1 000 jum 500-300 

Far-infrared 2-600 jum 1.5X10 5 -500 

Near-infrared 0.7-2 jum 4-1.5X10 5 

Visible 4000-7000 A 7.5-4X 1 5 

Ultraviolet 2000-4000 A 1.5-0.75X10 6 

Far-ultraviolet 1 00-2000 A 3-0.1 5X 1 7 

X-ray 1-1 00 A 3-0.03X1 9 

Gamma ray <1 A >3X10 9 

1 jum = 10" 6 m 

1 A=10~ 8 m 

1 Hz = 1 cycle/sec 

1 GHz= 10 9 Hz 

1 Jy=10~ 26 W/m 2 /Hz 

log [f v (in Jy)] = -0.4 m v + 3.57 

m v = 20 is equivalent to f v = 4X1 0~ 5 Jy 



123 



Appendix D 



CONVERSIONS: LINEAR SIZE/ANGULAR SIZE 



Domain 


Linear size 


Distance 


Angular size 


Moon, diameter 


3476 km 


0.00257 AU 


31.09' 


Mars 


6786 km 


0.524 AU 


17.9" 


10 km at Mars 


10 km 


0.524 AU 


0.0263" 


Sun 


1 391 980 km 


1 AU 


31.99' 


Ceres 


772 km 


1.892 AU 


0.563" 


Jupiter 


142 796 km 


4.203 AU 


46.9" 


lo 


3630 km 


4.203 AU 


1.19" 


Europa 


3138 km 


4.203 AU 


1.03" 


Great Red Spot 


24 000 km 


4.203 AU 


7.88" 




X 15 000 km 




X 4.92" 


10 km at Jupiter 


10 km 


4.203 AU 


0.0033" 


Saturn 


120 660 km 


8.555 AU 


19.5" 


Titan (visible cloud layer) 


5550 km 


8.555 AU 


0.895" 


lapetus 


1460 km 


8.555 AU 


0.118" 


Phoebe 


220 km 


8.555 AU 


0.036" 


Uranus 


51 200 km 


18.218AU 


3.9" 


Neptune 


50 460 km 


29.110AU 


2.4" 


Pluto 


3400 km 


38.44 AU 


0.12" 


Earth-Sun orbit, radius 


1 AU 


1 pc 
(nearest star) 


1" 






160pc 


0.0062" 






(nearest star 








formation) 




Jupiter-Sun orbit 


5.20 AU 


1 pc 


5.2" 


Solar System diameter 


-100AU 


1 pc 


1.67' 






160 pc 


0.625" 


1 solar mass protostar 


6X10 5 km 


160 pc 


0.025 mas 


core 








Accretion Disk around 


2X10 9 km 


160pc 


0.08" 


1 solar mass protostar 




10 kpc 
(to galactic 
center) 


1.28 mas 



125 



Domain 


Linear size 


Distance 


Angular size 


Dusty Envelope around 


2X10 12 km 


160pc 


1.33' 


1 solar mass protostar 




10 kpc 


1 .28" 


Diffuse Molecular Cloud 


1 pc 


100pc 


34.38' 






1 kpc 


3.44' 






10 kpc 


20.6" 






30 kpc 


6.8" 






(across Milky 








Way) 








1 Mpc 


0.206" 






(nearest 








galaxy) 




Giant Molecular Cloud 


5 pc 


1 kpc 


17.19' 






10 kpc 


1.719' 






30 kpc 


34.38" 






1 Mpc 


1 .03" 


Giant Molecular Cloud 


0.1 pc 


1 kpc 


20.62" 


Core 




10 kpc 


2.06" 






30 kpc 


0.688" 






1 Mpc 


20.62 mas 


Compact HI I Region 


0.01 pc 


100pc 


20.62" 






1 kpc 


2.06" 






10 kpc 


0.206" 






30 kpc 


68.67 mas 






1 Mpc 


2.06 mas 


HII Region Complex 


2pc 


1 kpc 


6.87' 






10 kpc 


41.25" 






30 kpc 


13.75" 






1 Mpc 


0.41 2" 



For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 



126 



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