Polarization curves for four Galilean satellites of Jupite

Veverka, J.

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POLARIZATION MEASUREMENTS OF THE
GALILEAN SATELLITES OF JUPITER
J. Veverka
Laboratory for Planetary Studies
rAbstract
Polarization curves for the four Galilean Satellites
are presented. They indicate that the surfaces of Io,
4anymede and Europa are covered mostly by a bright, trans-
parent material, possibly Frost. The surface of Callisto
is different. It is more similar to that of the Moon, but
some frost patches are also present.
g
4
P
i
1.) Introduction
Recently there has been renewed interest in the physical
study of the Galilean satellites. Reflectivity measurements
(Harris, 1961,; Johnson, 1970) indicate that the surfaces
are covered with some material which is very bright in the
t
red, but which is much more absorbing in the blue and
ultraviolet than ordinary water snow. The reflectivity
curve of lo, for example, is very unusual; the geometric
albedo drops from 1.15 at 0.82 u to 0.19 at 0.35 1j (Harris,
*	 1961). One suggestion (Veverka, 1970; Sagan, 1970) is that
the surfaces are covered with a bright frost in which multiple
scattering is important, but that in addition this frost
contains very effective ultraviolet absorbers (such as organic
compound, polymers, or free radicals). Photometric evidence
(Harris, 1961; Johnson, 1970) and visual observations	 A.
(Dollfus, 1962) indicate that all satellites are covered with
patches of bright material, interspersed with darker regions.
On Io, Europa and Ganymede the bright regions predominate,
while the reverse seems to be true for Callisto.
AlthoLgh the phase angle of Jupiter as seen from Earth
F	 never exceeds ll°, polarization measurements of the satellites
should be very helpful in studying their surfaces. Since at
small phase angles the degree of polarization, and hence its
variation with color, are likely to be small, it is useful
to begin such a survey with measurements in integrated,white
light.
22.) The Observations
The polarization curves of the satellites were determined
using the Harvard two channel. Wollaston prism polarimeter with
the Harvard 16 1' and 61" telescopes. No filter was used. Since
the polarimeter, uses S20 photomultipliers all light between
0.3 and 0.8 u is registered, thus appreciably increasing the
signal to noise ratio. As the wavelength deperdence of the
Y	 polarization is probably small at small phase angles, little
information is lost by not using a filter in a preliminary
1
survey .
Observations were made during both the 1968 and 1969
oppositions. The major difficulty encountered was to allow
correctly for the scattered Eight from Jupiter. In this
respect the 1969 observations were definitely superior and
only these are considered in this paper. The polarization
curves of the four satellites are shown in Figure 1. The
accuracy of the points is, with a few exceptions, ± 0.1%.
Since T will argue that these curves indicate the
presence of frost or snow on at least the first three satel-
lites, it will be well to review first what is known about
the polarization properties of snow and frost,
3.) The Polarization Curve of Snow
The only published measurements of the polarization of
water snow or frost are those of Lyot (1929) and Dollfus
(1955). No polarization measurements of frosts of other
compositions have been reported. Lyot studied a`variety
3of natural snow surfaces in integrated white light. Between
a = 0 0 and 10 0 the observed polarization was always small,
increasing from values between -0.1 to 0.0% at a = 2 0 to
,0.0 to +0.1% at a = 10 0 (here a is the phase angle).
Dollfus' measurements, also in white light, refer to
laboratory frost layers viewed at 65 0 . Although for dark
lunar-like surfaces the observed polarization depends on the
phase angle only and is largely independent of the angle of
incidence i or the angle of observation e individually, this
is not true for surfaces made up of bright, transparent
particles in which multiple scattering is important (Dollfus,
1961). For example, according to graphs given by Dollfus
(1955) for one particular frost layer at a phase angle of
70 0 , the observed polarization at e = 40 0 is over twice
that at e = 10 1 for an azimuth angle of 90 0 between i and e.
However, for very small phase angles, the polarization of
the frost is roughly independent of e, except for very large
values of E. Thus we can probably use Dollfus' measurements
here for guidance. His results indicate that between 0 0 and
10 0 , the polarization curve of frost will be concave upward
and will lie between -0.4% and +0.4%, depending on the
texture of the layer.
The weak polarization of a frost layer is evidently a
result of the importance of multiple scattering within the
surface. The addition of a strong absorber will hinder mul-
tiple scattering and hence enhance both the negative branches
and the positive branches of the polarization curve. The
I
rr
polarization curves of snows or frosts cannot be diagnostic
of composition, since any bright transparent substance will
give very similar results. But it should be possible to detect
the presence of even small admixtures of a strong absorber by
the development and enhancement of the negative branch.
However it is difficult to compare laboratory measure-
ments accurately with the disk-integrated polari zation of a
snow or frost hovered planet, since the polarization on such
a planet will vary significantly with position on the disk.
This is because for such a surface the polarization depends
riot only on a but also on i and e separately. This effect will
be most important at large phase angles.
4.) Discussion
In interpreting the polarization curves, I will assume
that atmospheric contributions are negligible. There is no
spectroscopic evdonce that any of the Galilean satellites
has a significant atmosphere, and photometric observations
of post-eclipse brightness anomalies are still uncertain
(Binder and Cruiksh ank (1964); Franz and Millis (1970))•
From the absence of a pronounced negative branch in
the polarization curves of To, Europa and Ganymede, it can
be inferred that the surfaces of these satellites consist mostly
of a bright multiply scattering material, such as snow. It is
true that this is also a property of solid (that is, non-par-
ticulate) surfaces but these tend to have Larger polarizations
near a = 10 0 . To keep the polarization at a = 10 0 low, multiple
scattering within the layer is required. Furthermore, the
r"k.
Iphase curves of the satellites measured by Stebbins (1927) and
Stebbins and Jacobsen (1928) reveal the presence of opposition
effects (Veverka, 1970) which are incompatible with solid
smooth surfaces, but indicate rather surfaces which are
microscopically rough and probably particulate.
The polarization curve of Callisto has a definite negative
branch. The presence of a well developed negative branch can
be interpreted as indicating that the surface is particulate
and sufficiently dark for multiple scattering to be unimportant
(Lyot, 1929). This agrees with photometric evidence: The
reflectivity of Callisto is low (Harris, 1961) and quite com-
patible with that of a rocky surface; and Callisto shows a
very strong opposition effect (Stebbins and Jacobsen, 1928),
indicating that the surface is particulate.
The minimum polarization is -0.8%, compared with -1.2%
for the Moon, but the cross-over angle is probably muchly
smaller than the lunar 23 0 . Certainly, the phase angle at
which the minimum polarization is observed (ti 6 0 ) is much
smaller than the lunar 11 0 . The unusually small values of
the cross-over angle and of the angle of minimum polariz-
ation require an explanation since Callisto has a well developed
branch. A possible explanation is that part of the surface is
covered with frost. In this case the negative branch of the dark
soil would be in fact even more pronounced than indicated in
Figure 1. It is also possible that the entt re surface is
covered with frost contaminated with a strong admixture of a
F..
4dark absorbing substance which hinders multiple scattering and
leads to the production of a negative branch. The amount of
this contamination would vary from place to place since visual
observations (Dollfus 1961) and photometric measurements
(Harris, 1961; Johnson, 1970) indicate patches on
Callisto.
Either this model, or the one above in which a rocky
surface of a fairly dark particulate material is covered in
places by patches of uncontaminated frost could explain
the observed negative branch, the small cross--over angle,
the pronounced opposition effect, and the shape of the
reflectivity curve.
The two sides of Callisto differ by 0.2 mag in V (Harris,
1961). Since the leading (darker) side has fewer bright
regions than the trailing (brighter) side, the negative branch
of the leading ,amide ,should be more pronounced than that of
the trailing side. Unfortunately only two measurements of
the trailing side are available (Fig. l) and it is as yet
impossible to determine whether this indeed is true.
Vapor pressure arguments suggest that water frost J>s
likely to be the most common frost on the Galilean satellites
(Moroz, 1966). However it is difficult to explain the observed
reflectivity curves in terms of patchy surfaces of snow and
silicate rock (Johnson, 1970) . The problem is that the
satellite surfaces tend to be very bright in the red,,b
 but quite
L10%.
dark in the UV. This is not a behaviour of snow. By changing
7the particle size a layer of snow can be made bluish, but not
red ('Veverka, 1970)• For this reason, and since the drop in
reflectivity from the TR to the UV increases systematically
from Ganymede to Europa to to Veverka (19'70) has suggested that the
surfaces originally consisted of water frost with small amounts
of impurities such as C' and NH , and have been modified by
the action of the charged particles in the Jovian radiation
belts. A similar suggestion has been made independently by
Sagan (1970, 1971). This has led to the formation in the
surface layer of organic compounds, polymers and free radicals,
which strongly absorb in the UV. Suggested materials include
NH and CH (Ramsey, 1970); CH 3 (Pap azian, 1958); (NH 2NH) n and
CH3CS (Rice, 1956); polymers of HCN, C 2N 2 and HCN.NH 3
 (Woeller
and Ponnamperuma, 1969); and complex organic molecules such as
azobenzene (Sagan et al., 1967). (Callisto can be left out of
this discussion since its reflectivity changes only from 0.31
at 0.82 U to 0.14 at 0.35 p, and is therefore not unlike that
of a silicate material) .
The effect of the charged particle radiation should be
largest on lo, and appropriately lo is not only the reddest
of the four Galilean satellites, but the reddest known object
in the solar system.
Unfortunately polarization measurements can tell us
little about the composition of the surface frosts. Here
near IR spectrophotometry seems more promising. However
polarimetry remains a powerful way of studying the texture
and general nature of the surfaces.
i
IThe next stop in the polarimetry of the Galilean
satellites is a series of color measurements between 0.3
and 1.01j, with special emphasis on to and Callisto.
Measuring the polarization of Callisto at various points
in its orbit is another important project.
Laboratory measurements of the polarimetric (and
photometric) properties of various snows and frosts
(water, carbon dioxide, methane, ammonia, etc.) are
urgently needed. In addition, frosts contaminated with
various chemicals and mineral powders should be carefully
studied.
I wish to thank W. Liller, C. Sagan, F.L. Whipple
and Nancy Morrison for their help with various aspects
of this work. This work was supported in part by NASA
Grant, NGR-33-010-082.
At Harvard the Polarimeter Project is supported by
Grant AFOAR-F19-628-68-C-0228. 1 wish to thank the
Smithsonian Foundatl,on for support during the phase of
this work carried out at Harvardi
4
References
Binder, A.B. and Cruikshank, D.P. (1964). ""Evidence
for an atmosphere on lo," Icarus 3, 299.i
Dollfus, A. (1955) . "Study-of the planets by means of
the polarization of their light." NASA. TTY'--188.
Dollfus, A. (1961). "Polarization studies of the planets"
Chapter 9 in Planets and Satellites. (Kuiper and
;,.	 Middlehurst, eds.) Chicago.
Dollfus, A. (1962). "The surfaces of the Moon and planets."
Handbook of Physics, 54, 180.
Franz, Q.G. and Millis, L. (1970). "A search for an
anomalous brightening of lo after eclipse." Icarus,
in press.
Harris, D.L. (1961). "Photometry and colorimetry of planets
and satellites." Chapter 8 in Planets and Satellites.
(Kuiper and Middlehurst, eds.) Chicago
Johnson, T.V. (1970). "Albedo and spectral reflectivities
of the Galilean satellites of Jupiter." California
Institute of Technology Ph.D. Thesis.
Lyot, B. (1929) . "Research on the polarization of the light
from planets and from terrestrial substances."
NASA TTF-187..
Moroz, VT. (1966). "Infrared spectrophotometry of the
Moon and the Galilean satellites of Jupiter." Soviet
Astronomy A. J. 9, 999.
Papazian, H.A. (1958). "Free radical formation in solids by
ion bombardment." J. Chem. Phys. 29, 4248.
Ramsey, D.A. (1960) . "Optical spectroscopy of trapped radicals"
Chapter ,6 in Formation of Free Radicals. (Bass and
Broida, eds.) Academic Press
Rice, F.O. (1956). 41 ol.ors on Jupiter" J. Chem. Phys. 24, 1259.
Sagan, C. (1970). "The star that failed" in Science Year:
1971 (Field Enterprises Educational Corporation).
Sagan, C. (1971). "The solar system beyond Mars: an exobiological
survey." Sp. Sci. Rev., in press.
Sagan, C., Lippincott, E.R., Dayhoff, M.O., and R.V. Eck (1967)
"Organic molecules and the coloration of Jupiter." Nature213, 273.
b.
10
Stebbins, J. (1927). "The light variations of the satellites
of Jupiter and their application to the measures of the
solar constant-" Lick Obs. Bull. 1.3, 1.
Stebbins, J. and Jacobsen, T.S. (1928), "Further photometric
measures of Jupiter's satellites and Uranus, with tests
for the solar constant." Lick Obs, Bull. ji-3, 180.
Veverka, J. (1970). "Photomet,"ic and polarimetric studies
of minor planets and satellites - " Harvard Ph. D. Thesis.
Woeller,, F. and Ponnamperuma, C. (1969). 11 0rgani6 synthesis
in a simulated Jovian atmosphere." Icarus 10, 386.
I ^
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P_ igure Caption
Polarization curves of the four Galilean satellites
of Jupiter.
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Polarization measurements of the Galilean satellites of Jupiter

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