Spectrophotometric peculiarities of some satellites of planets

SPECTROPHOTOMETRIC PECULIARITIES OF SOME SATELLITES

OF PLANETS

Yu. D. Davudov, I. K. Koval •

ABSTRACT: Stebbin's and Jacobsen's photoelectric measurements
of the solar phase -brightness relationship for Jupiter's
Galilean satellites are used to show that the surface layers
of the satellites are very porous. The parameter g that
characterized relative density of a surface layer is equal to
0.2 for Callisto. Results of spectrophotometric observations
of satellites are discussed briefly from the point of view of
the possible nature of their surfaces.

Photometric observations of .satellites of planets, as a class of objects ^49
devoid of atmosphere (with certain exceptions), can be quite adequately used
to arrive at a judgment as to the microrelief of the upper layer of their sur-
faces, and in part, as to the nature of the surface layer itself. In any. case,
as the long years of experience gained from photometric study of the Moon have
shown, data from photometric observations, such as the brightness -phase angle
relationship, the spectral variation in the albedo, disk brightness distribu-
tion, and other features, had led to rather definite conclusion as to the
microrelief of the lunar surface and had made it possible to arrive at certain
judgments as to the nature of the material of which the lunar surface layer
was comprised.

Unfortunately, as distinguished from the Moon, photometric data on satel-
lites of planets is quite limited because of the small angular sizes of these
bodies and the limitations on the phase angles at which they can be observed
from the Earth. Moreover, the accuracy of photometric observations is greatly
reduced in many cases because the satellites are relatively dim, and are quite
close to the central body.

Still, photometric (using light filters) and spectrophotometric observa-
tions are a quite effective means for studying this group of objects.

The Brightness-Phase Angle Relationship for
the Galilean Satellites of Jupiter

In 1926, Stebbins [1], and in 192?, Stebbins arid Jacobsen [2], collected
numerous photoelectric measurements of the brightness of the bright satellites
of Jupiter for a whole set of observed phase angles (up to 11°). The principal

61

goal of these observations was to study variations in the brightness of the
satellites with phase rotation around a central body. The data obtained are
close to B in the color system and unquestionably indicate the variation in
the brightness of all four satellites with phase rotation.

The data compiled by Stebbins and Jacobsen have been discussed over and
over again, and when these data were supplemented by those obtained in the 1950 's
at the McDonald Observatory it became conclusive there is a longitudinal effect
in the brightness, and sometimes in the color, of the Galilean satellites. The
latter evidently is associated with the mottled structure of their surfaces.
Callisto is distinguished by maximum changes in brightness with phase rotation
when the solar phase angles are large.

Let us pause briefly to consider the results of the observations obtained
by Stebbins and Jacobsen as they pertain to the relationship between the bright-
ness of the satellites and the solar phase angle.

Figure 1 shows the brightness of the satellites in terms of phase angle
from 20-30 measurements that more or less uniformly cover the phase angles
between 0.3° and 11°. The curves approximate well a square-law function of the •
following type in the case of linear extrapolation of the measurements for the
zero phase shown in Figure 1

when constants a and b have the following values ^50

Jupiter I do) a = 0.046; b = 0.001;

II (Europa) 0.0312; 0.00125;

III (Ganymede) 0.323; 0.00066;
I (Callisto) 0.090; O.O36.

Callisto 's phase function is equated to the wing side of the satellite (the
darker side near the opposition). For the leading side

m=:mo+0.112 + 0.060a— O.OOlOa^.
The curve for the lead side in Figure 1 is designated IV.

As will be seen from Figure 1, Callisto 's lead side is distinguished by
maximum increase in brightness toward the opposition. The brightness differen-
tial for the extreme values of a (upon extrapolation of the measurements for
a = according to Stebbins and Jacobsen) is O .55. A more careful consideration

62

Figure 1. Phase -brightness relation-
ship. I- lo, II- Europa, III-
Ganymede, IV- lead and V- wing sides
of Callisto. X- asteroid 20 Massalia.
X-axis- phase angles 5 y-axis- bright-
ness, in stellar magnitudes.

reference [ 3 ] •

of the Stebbins and Jacobsen measure-
ments shows a tendency toward a sharp
rise in brightness for all four satel-
lites between 1° and 0° (the broken
lines in Figure l). This effect is
particularly well defined in the case
of Callisto (lead side) where there are
four measurements in the interval of
phase angles noted.

So the greater prot)ability is that
the actual brightness differential for
the satellites should exceed the value
taken directly from the Stebbins and
Jacobsen phase curves for phase angles
between 0° and 11° (Figure 1, solid
curves ) .

The magnitude of the effect of
opposition between 1° and 0° can be
evaluated for Callisto by using the
results obtained by Stebbins and
Jacobsen and the procedure detailed in

This satellite's geometric albedo is comparatively low (P(B)=0.2). We can,
therefore, as in the case of the moon, disregard the effects of higher order
scattering and make full use of the calculations given in reference [3]. We
can, in particular, determine the parameter g, characterizing the relative
density of Callisto 's surface layer. To do so, let us use the data listed in
Table 1 for the planet's phase -brightness relationship for different g values,
according to reference [3]» By using the phase -brightness relationship for
Callisto between 1° and 11° we obtain g ~ 0.2 for the satellite's lead side
(see Figure 1, IV). According to reference [3]i 9 = O.25 for the lunar surface
layer (see Figure 1, V). The increase in Callisto's brightness (lead side) is
O .28 between 0° and 1° for the g value found, and the brightness differential
for the interval between 0° and 11° is about O .75.

63

This conclusion as to the structure of the surface layer on Callisto's
lead side is valid only if this layer satisfies the Hapke model, of course [4],

T. Gehrels, in 1955, took photoelectric measurements of the phase-bright-
ness relationship for the asteroid 20 Massalia [5]. Table 2 lists the color
characteristics of this asteroid, and Figure 1 shows its phase function in the
V system (crosses), which is virtually no different from the lunar phase function
or that of Callisto's lead side.

Z.51

TABLE 1. PHASE-BRIGHTNESS RELATIONSHIP FOR A PLANET NEAR
OPPOSITION FOR DIFFERENT VALUES OF THE PARAMETER g

0°
1

2
3
4
5
10
15

O^OO
0.62
0.68
0.71
0.72
0.7'3
0.76
0.80

.0"'00
0.47
0.58
0.63
0.66
0.68
0.74
0.78

0"'00
0.28
0.41
0.49

,0.54
0.57
0.67
0.73

0"'00
0.18
0.30
■0.38

■ 0.40
0.48
0.61
0.68

O^OO
0.14
0.23
0.30^
0.35
0.40
0.59
0.63

O^OO
0.11
0.17
0.24
0.29
0.34
0.48
0.58

TABLE 2. SOME CHARACTERISTICS OF PLANETARY SATELLITES

et and

ts

llite

in

3

Mean
density,

Absolute

stellar

magnitudd[v;

sat ^

{V)

U

B

V R I

Jupiter

I - lo

1660

3.9

—1.90

1.70

0.54

■0.00

—0.21

—0.24

0.92

II- Europa

1440

4.0

—1.53

0.62

0.24

0.00

—0.12

—0.14

0.83

III- Ganymede

2470

2.5

—2.16

0.56

0.20

0.00

—0.14

—0.16

0.49

IV- Callisto

2340

1.7

—1.20

0.64

0.23

0.00

—0.16

—0.19

0.26

Saturn

Enceladus

■ 300

0.6

. 2.22

■

0.01

0.00

.

—

0.54

Tethys

600

.0.7

0.72

0.35

0.15

0.00

—

—

0.84

Dione

650

0.85

0.89

0.24

0.08

0.00'

—0.03

—0.06

0.94

Rhea

900

0.75

0.21

0.34

0.13

0.00

-0.16

—0.13

0.82

Titan

250O

2.7

— r.i6

1.28

0.67

0.00

—0.43

—0.25

0.21

Hyperion

20O

3.3

4.61

0.34

0.06

0.00

—

—

—

lapetus

600

5.5

1.48

0.49

0.08

0.00.

—

—

—

Uranus

Titania

500

1.30

0.10 '

-0.01

0.00

—0.07

—0.19

—

Oberon

400

—

1.49

0.12

0.02

0.00

—0.04

—0.08

—

Neptune

Triton

2000

4.6

-1.16

0.40

0.14

0.00

-0.13

—0.28

0.36

Nereid

150

3.6

4.00

—

0.17

0.00

—

—

. —

Luna

1738

3.34

0.21

0.61

0.29

0.00

-0.35

—0.52

0.11

30

— ■

6.77

0.47

0.19

0.00

—

—

—

64

The wing side of Callisto is less porous, so the effect of opposition is
not as sharply defined. We obtain g = 0.6 directly from curve IV in Figure 1,
and by using Table 1. If it is taken that the albedo of the satellite's surface
on the wing side is approximately 10 percent less than on the lead side,
g = 0.5, roughly, for the wing side, and this is approximately the porosity of
the "average Mars" [6].

The high albedo of the first three Galilean satellites is such that it is /52
not as simple to determine g (as in the manner of the foregoing) and this is so
because higher order scatterings must be included. But given the same surface
layer porosity, there should be a decrease in the magnitude of the observed
effect of opposition with increase in surface albedo [7]. If , for example, we
take the case of the very high albedo of lo (the geometric albedo in the B
system is about 0.6) we find that the satellite's phase curve still shows a
brightness differential of about O .4 between 0° and 11°, and this means its
surface porosity also is quite high. This is true to the same degree for the
satellites Europa and Ganymede. Overall, this result is in good concordance
with Ganymede's low thermal inertia, found by measuring the thermal radiation
when the satellite enters Jupiter's shadow [8].

The conclusion as to the high degree of porosity of the surface layers of
the Galilean satellites of Jupiter must be considered when analyzing spectro-
photometric observations in order to study the nature of the upper covers of
satellites.

Albedo and Spectral Energy Distribution
for Satellites

Today we put the relationship between the albedo of the Galilean satellites
of Jupiter and the wavelength at 0.35-2.5 microns. Photoelectric measurements
with light filters [9] are quire reliable in the 0.35-0.82 region. Spectro-
photometric data in the visible [10] and IR regions [11] of the spectrum are
fragmentary and are in need of refinement. The geometric albedo of the first
two satellites in the red region of the spectrum is puzzlingly high (Table 2).

The geometric albedo, p, the apparent albedo in the center of the full disk,
p , and the smoothness factor for the surface , q , for a planet devoid of atmo-
sphere are linked by the relationship [12]

65

2

9+2'

Customarily p > p, but q ci O and p '^ p for Callisto, at least in the short-
wavelength region of the spectrum (where the satellite's geometric albedo is
approximately that of the Moon's), q increases with increase in the albedo [7,
13], so the differential in the apparent albedo between 0.35 and 0.82 microns
should be even greater than that in Table 2 for the geometric albedo differential,
and this would apply to all the Galilean satellites, particularly lo.

Attention is drawn to the fact that the high value of the satellite's albedo
corresponds to its high mean density (Table 2). The conclusion 'that suggests
itself is that the diameters of lo and Europa are greatly in error as determined
(understated) and this is particularly so if it is considered that the angular
diameters of these satellites are comparatively small and they are closer to
Jupiter than are Ganymede and Callisto, making precise micrometric measurements
against Jupiter's background difficult.

Camichel [l4] and Dollfus [15] have obtained lo and Europa diameters that
are somewhat larger than those accepted earlier, but the corresponding correc-
tions do not change the main point; the albedoes of lo and Europa still are very
high.

A high albedo has been determined for Saturn's satellites Tethys , Dione , and
Rhea, but these data cannot be trusted because the diameters of these distant
satellites cannot be measured directly from the earth. Perhaps the only reliable
photometric characteristics of all planetary satellites, with the exception of
the moon and the Galilean satellites of Jupiter, are their brightness and spectral
energy distribution. /53

V. I. Moroz [16] measured the IR spectrum of the Galilean satellites of
Jupiter at 0.7-2.5 microns and has suggested that at the very least much of the
surface of Europa and Ganymede can be covered by ice. However, according to
reference [173 5 ice has an inverse albedo behavior with respect to the spectrum
as compared to that found by observation. If one attempts to explain the com-
paratively small drop in energy toward the violet end of the spectrum for Europa
by the presence of hoarfrost through which the satellite's own surface is
translucent, there is still no way to explain the data for lo. The question of
the presence of an atmosphere for the Galilean satellites still is an open one.

66

The 2-meter reflector at the Shemakha Astrophysical Observatory was used
during I968-I969 by Yu. D. Davudov to obtain some 60 spectrograms of the
Galilean satellites of Jupiter and of Titan in the 0.40-0.65 micron band. The
observations were made in the Cassegrain focus using a diffraction spectrograph
(dispersions 30 and 75 A./ mm). The comparison stars were selected from the
Kharitonov catalog [I8], and G. F. Sitnik's data [I9] were used to effect the
photometric tie between the satellite spectrograms and the sun. Mean curves of
spectral energy distribution in the spectra of lo (16 spectrograms), Europa (9),
Ganymede (7), Callisto (7), and Titan (17), are shown in Figure 2. They are in
satisfactory agreement with data by other authors if consideration is given to
the fact that the color index for the satellites undergoes change with phase
rotation.

The satellite spectrograms were com-
pared with the lunar spectrum in the region
of the 6190 A CH, band. This band is well
separated in the Titan spectrum. Its
equivalent width is about 20 A [20].
Careful measurements of the spectrograms
for Jupiter's satellites showed no traces
of the 6190 A CH, band noted earlier by
other observers. We can add to this the
fact that the spectral behavior of the
lo albedo is , in general , quite difficult
to explain by the influence of the atmo-
sphere. If one assumes the satellite to
have a neutral surface with a high albedo
in this region of the spectrum, it would
be necessary to have an atmosphere with
a quite complex optical modulus in order
*o have satisfactory observations. This
atmosphere would have to have a relatively
great optical thickness and significant
true absorption over the whole of the
0.35-0.82 micron band, and this is hard

Figure 2. Relative spectral energy
distribution. 1- lo; 2- Europa;
3- Ganymede; 4- Callisto; 5- Titan.
x- Harris ' measurements ;
o- Priboyeva's measurements;
y-axis- logarithms of intensity.

to square with the high albedo of the satellite in the red rays.

67

lo's color changes greatly with phase rotation, reaching a value of about
0^.65 in the U-V (~^'".2 in B-V and '=^'".45 in U-B). This means that the magnitude
of the longitudinal effect on lo's brightness increases from V to U. In other
words, the contrast between the dark formations on the satellite's surface and
their surroundings increases toward the violet end of the spectrum. So far as ^k
the other three Galilean satellites are concerned, the contrast in details in
the U, B, and V is almost the same [93- This effect, which can be observed for
lo, could be imparted by an atmosphere in which the true absorption dominates
the scattering , and the optical thickness increases toward the violet end of
the spectrum. Then it would have to be assumed that there are areas of relief
with reduced levels in the longitude region near 300° (corresponding to the
minimum brightness of lo near opposition). But the lack of observed atmospheric
criteria for lo detracts from the validity of this hypothesis.

Evidently the entire surface is primarily responsible for the observed
behavior of spectral values for lo, and this would apply to the overwhelming
majority of other planetary satellites as well, although neither asteroids nor
meteorites have albedo spectral values similar to those for this satellite [17].
On the other hand, there still is not sufficient grounds for the final rejection
of the atmosphere hypothesis. In any case, the comparatively small mass of the
Galilean satellites, and the absence in their spectra of CH, molecular bands,
still is not the deciding argument in favor of this rejection.

REFERENCES

1. Stebbins, J., Lick Obs. Bull. , 1927, p. 385.

2. Stebbins, J., Jacobsen, T. S. , Lick Obs. Bull. , 1928, p. 401.

3. Morozhenko, A. V., Yanovitskiy, E. G. , Astron. zhurn. , 4? , 1970.

4. Hapke, B. W. , J. Geophys. Res. , 68, I963 , P- 4571.

5. Gehrels, T. , Ap. J. , 123 , 1956, p. 2.

6. Morozhenko, A. V., Yanovitskiy, E. G. ', Astron. zhurn. , 48 , I97I.

7. Bugayenko, O. I. et al.. Article in this collection.

8. Murray, B. C. , Westphal , J. A., Wildey, R. L. , Ap. J. , l4l , I965, p. 1590.

9. Harris, D. , Planets and Satellites , Chapter 8, editors D. Kuiper,

B. Middlehurst , Foreign Literature F*ress , I963.

68

10. Priboyeva, N. B. , Trudy A I AN Kaz. SSR , IX , I967.

11. Moroz, V. I., Astron. zhurn. , 42 , I965, p. 128?.

12. Koval • , I. K. , Doctoral Dissertation, Kiev, I968.

13. Akimov, L. A., Barabashov, N. P., ATs AN SSSR , I969, p. 540.

14. Camichel, H. , Ann. d'Astrophys. , I6 , 1953 1 P- 4l.

15. Dollfus, A., C. R. , 238 , 1954, p. 1475-

16. Moroz, V. I., Fizika planet [Physics of Planets], "Naiika" Press, Moscow,

1967, p. 475.

17. Krinov, Ye. L. , Astron. zhurn. , 17 , 1940, p. 4.

18. Kharitonov, A. V., Astron. zhurn. , 40, I963 , p- 2.

19. Sitnik, G. F. , ATs AN SSSR, 1964, p. 292.

20. Davudov, Yu. D. , ATs AN SSSR, 1970, p. 563.

69