This dissertation has been more in the nature of a survey than a focused
attack on a particular well-defined problem. The conclusions are therefore
varied. I present them in this chapter, arranged into categories corresponding
to the various lines of investigation. I finish with an anticipation of
future lines of research, especially the exciting potential of the Galileo
Jupiter orbiter mission, currently expected to reach Jupiter in the mid-90's.
Shape of Thermal Emission Spectra
The emission spectra are smooth and featureless (Chapter 4). This may constrain
surface composition and texture but not without more laboratory and theoretical
work. However the smoothness of the Callisto and Ganymede spectra, compared
to those from the Earth's moon, shows that, if these objects have segregated
surfaces (in which case most of the emission is from the dark component),
the dark material does not have lunar-like silicate composition and texture.
Perhaps the most important conclusions of the analysis of the IRIS
spectra concern the variations in brightness temperature with wavelength
that are seen in the local thermal emission from the Galilean satellites.
This dissertation demonstrates for the first time that the shape of the
thermal emission spectrum of an airless body, and the variation in shape
across the surface, is a useful probe of surface physical conditions.
The spectrum slopes observed by IRIS are real, and are probably due
to local temperature contrasts, not spatially resolved, with amplitudes
of tens of degrees. The different patterns of variation of spectrum slope
on Europa, Ganymede and Callisto imply that the slopes have more than one
cause, different causes being dominant on different bodies (Chapter 4).
On Callisto, the slopes of the daytime spectra appear to be due largely
to topography, and become more pronounced towards the terminator as shadows
lengthen and topographic temperature contrasts increase. A roughness comparable
to the Moon's may be sufficient to produce the observed slopes, though
better topographic temperature models are required to confirm this (Chapter
6). Callisto near-subsolar spectrum slopes are smaller than on Ganymede,
but a little steeper than predicted by current topographic models. This
may indicate the presence of separated dark and bright materials on the
surface, similar to the situation inferred on Ganymede (see below), but
with one component (the dark, low thermal inertia one) dominant, occupying
80%--90% of the surface (Chapter 7). Near-subsolar spectrum slopes are
steeper in the Voyager 2 data (late morning, trailing/farside quadrant)
than Voyager 1 (early afternoon, leading/nearside quadrant). The slopes
on the trailing hemisphere are thus closer to those on Ganymede, which
is interesting as this hemisphere is the more Ganymede-like in its photometry
and polarimetry also (Chapter 4).
The nighttime Callisto spectra are also quite steep, steeper than those
on Ganymede, and require temperature contrasts with amplitudes of around
30oK. Whether this is due to `remnant' topographic effects cannot
be checked without much more complex thermophysical topographic models.
Alternatively, if the possible minor bright component on the surface has
high thermal inertia, this might also explain the steep nighttime slopes
on Callisto (Chapter 7).
The Ganymede spectrum slopes are probably not due to topography, because
they do not increase towards the terminator, and are in fact remarkably
uniform across the satellite. The shallowness of the near-terminator daytime
spectra indicates a smoother surface than Callisto at an undetermined lengthscale:
the higher albedo of Ganymede cannot explain the apparently smaller topographic
temperature contrasts than on Callisto (Chapter 6). The slopes are most
easily explained by a surface with similar coverage of horizontally separated
dark, low thermal inertia and bright, high thermal inertia components (Chapter
7). The very bright ejecta blanket of the crater Osiris shows unusually
small slopes, which is consistent with the 2-component interpretation as
one component (the bright one) would be expected to be dominant in this
region, resulting in reduced temperature contrasts. By far the steepest
spectrum slopes seen on Ganymede occur near the south pole, for reasons
that are not yet clear.
The spectrum slopes on Europa are smaller than on either Ganymede or
Callisto, and require local temperature contrasts of 20oK or
less (Chapter 4). The available coverage is too limited to constrain the
origin of the slopes. However their small amplitude is consistent with
the impression, obtained from imaging and reflectance spectroscopy, that
Europa's surface is topographically smooth and compositionally homogeneous.
Anisotropy of Thermal Emission
As far as can be determined from the Voyager IRIS data, the falloff in
thermal emission from a given point on Ganymede or Callisto with increasing
solar phase angle is comparable to that on the Earth's Moon (Chapter 5
and Appendix A).
Global Temperature Distributions
Equatorial surface temperatures are significantly below equilibrium (zero
thermal inertia) values. The maximum daytime temperatures on Ganymede and
Callisto are about 158 and 147oK respectively, approximately
10oK and 5oK respectively below equilibrium. Nighttime
temperatures are warmest on Ganymede, which is 15oK warmer in
the early part of the night than Europa or Callisto (Chapter 5). This is
due to Ganymede's higher thermal inertia and faster rotation than Callisto,
and its much lower albedo that Europa (Chapter 7). Anomalously cold regions,
not associated with high surface albedos, were seen by Voyager 1 on Ganymede
20o N, 10o W (in the early afternoon), and at
0o N 250o W (just before midnight). Large anomalies
in thermal properties are required to explain these (Chapter 7). Similarly,
temperatures around 20o N, 330o W on Callisto (in
the late afternoon) are also anomalously cold.
The 2-layer surface models of Morrison and Cruikshank (1973) and Hansen
(1973) explain both the eclipse and diurnal thermal behaviors of Ganymede
and Callisto, but only when the top-layer thermal inertias that they used
are increased by about 50%. The diurnal profiles constrain the thermal
inertia of the lower layer of this model to be a factor of several lower
than solid ice values. The eclipse and diurnal behavior of Ganymede,
however, are also consistent with a 2-component surface with equal surface
coverage of a bright high thermal inertia and a dark low thermal inertia
component (Chapter 7).
Thermal segregation of water ice on the icy Galilean satellites is a potentially
powerful process that may dominate the appearance of the surfaces at sub-kilometer
length scales. Ganymede and Callisto, geologically dead at the scale of
the Voyager images, may show features at small scales that reflect equilibrium
with a process that operates on a timescale of decades or less (Chapter
8). The competing ice redistribution processes of ion sputtering and micrometeorite
bombardment are unlikely to prevent segregation except possibly in polar
regions and perhaps on Europa (Chapter 9), especially the trailing hemisphere
where ion sputtering is intense. Evidence for segregation includes direct
imaging by Voyager at Callistoan high latitudes, the apparent lack of the
kilometers of poleward ice migration that would be expected on Ganymede
and especially Callisto if the ice were unsegregated and warm, and the
preservation of local albedo contrasts at low latitudes on Ganymede and
Calliso. The steep subsolar IRIS spectrum slopes on Ganymede may also require
a segregated surface (Chapter 7). The reflectance spectra of Ganymede and
Callisto may be consistent with segregated surfaces, though Europa's spectrum
probably indicates a homogeneous surface (Chapter 10).
If the surfaces of Ganymede and Callisto are influenced by thermal
segregation, the dark material on their surfaces may be mostly in the form
of thin lag deposits, and the substrate composition is likely to be more
ice-rich than the visible surface. Segregated surfaces will have lower
ice mobility and possibly much lower overlying atmospheric pressure (Chapter
Potential for Future Work
The Galileo Jupiter orbiter mission will enormously improve our knowledge
of the surfaces of the Galilean satellites.
Thermal observations, comparable to those made by the Voyager IRIS
instrument, will be obtained by the Galileo Photopolarimeter-Radiometer
(PPR) (see e.g. Hunten et al. 1987). Though designed primarily for Jovian
atmospheric observations, (like IRIS), it will also be very suitable for
satellite work. Below I compare its characteristics to those of IRIS.
Spectral Resolution. Five broadband flux measurements centered
on 17, 21, 27.5, 37, and above 42 microns, compared to the equivalent of
about 250 narrowband measurements in the 8--50 microns range for Voyager
Angular Resolution. 0.14o diameter circular field
(IRIS field is 0.25o diameter).
Spatial Resolution. For typical Galileo satellite flyby altitudes
of about 1000 km, much closer than the Voyager flybys, the smallest regions
measured will be 2.5 km across, compared to 100s of km for Voyager. Coverage
with such high resolution will be very limited but there will be good coverage
at much higher spatial resolutions than possible with Voyager.
Acquisition Rate. One set of fluxes every 18 seconds in `cycle
mode' (photopolarimetry plus radiometry) compared to a spectrum every 48
seconds from Voyager.
Spatial Coverage. Much more comprehensive than from Voyager,
owing to the larger number of flybys. There will be perhaps 11 targeted
encounters of the icy Galilean satellites, plus many untargeted encounters
which will often provide spatial resolution equal to or better than the
So in all respects but spectral resolution Galileo will substantially
exceed Voyager's capabilities for measuring thermal radiation from the
icy Galilean satellites. The low spectral resolution is not a great handicap,
given the smoothness of the thermal spectra revealed by Voyager. If the
data are well calibrated, spectrum shapes directly comparable to those
obtained here (Chapter 4) will be extractable from the radiometer fluxes.
The Galileo data will allow confirmation of the trends of spectrum
shape seen by Voyager, and better characterize the trends on Europa and
on the night sides of all satellites. Differences in thermal emission with
terrain type will be much more easily visible than with Voyager. Improved
absolute pointing accuracy (if only because of the close flybys) and more
comprehensive phase coverage should allow characterization of the beaming
of thermal emission and correction for it. Complete diurnal temperature
profiles should be obtainable for all three satellites. It will be possible
to compare different longitudes at similar illumination and viewing geometries
on successive flybys, so that the global variations in thermal properties
can be better characterized.
There is also the possibility of thermal observations of the Earth's
Moon and several asteroids by Galileo on its way to Jupiter. These observations,
though limited, will be invaluable for comparison with the Galilean satellite
The superior Galileo data set, by allowing better separation and understanding
of geographical (global and terrain), diurnal, and geometrical (beaming)
effects on the observed thermal emission from the satellites, should provide
much tighter constraints on thermal models like those described here in
Chapters 6 and 7. It should therefore be possible to determine much more
clearly the distribution of surface roughnesses, albedos and thermal inertias
on the satellite surfaces.
The other Galileo instruments will also provide a great deal of information
that is relevant to the science discussed in this dissertation, especially
the possibility of widespread ice segregation. Imaging of the satellite
surfaces at very high resolution may detect and characterize ice segregation
if it exists, and determine its dependence on latitude and terrain type.
The NIMS (near-infrared mapping spectrometer) instrument will also be able
to study latitudinal variations in ice distribution on each icy satellite,
and will have sufficient resolution, for instance, to obtain separate spectra
of the bright and dark segregated patches visible in Fig. 32 at high latitudes
on Callisto, thus possibly obtaining a `pure' spectrum of the dark, non-icy
surface component. Characterization of the latitudinal variations in ice
distribution should allow identification of the dominant redistribution
processes and their effects.
The UV stellar occultation technique, used by Voyager to rule out a
\mubar oxygen atmosphere on Ganymede, is sensitive enough for direct detection
of the water vapor in equilibrium with warm ice on the Galilean satellites
(Spencer, 1982). Stellar occultations observed with the Galileo UVS instrument
will thus be useful indirect probes of surface ice temperature and mobility.
In addition, the plasma instruments and dust detector on board Galileo
will greatly improve our understanding of the rates and nature of ice transport
processes competing with thermal sublimation.
Improved topographic temperature models (Chapter 6) are fairly easy to
create, and should allow much better understanding of the effects of topography
on thermal spectrum shape and on beaming. With the help of such models
should come a better understanding of the spectrum shapes on Callisto and
better constraints on the surface roughness on Ganymede and Europa.
Improved eclipse models are possible and should be studied (Chapter
7 and Appendix C). In particular, subsurface
penetration of sunlight and oblique eclipse geometry should be investigated.
Europa's lack of post-eclipse recovery is a major outstanding problem to
be explained. The use of least squares fitting techniques to determine
simultaneous best-fit thermal models to the eclipse, diurnal, and spectrum
slope data may allow a definitive explanation of Ganymede's thermal emission
behavior, which will show whether a segregated or laterally-homogeneous
surface exists there.
Improved groundbased measurements of the thermal emission of Europa,
Ganymede, and Callisto as a function of orbital phase, solar phase, and
wavelength would be complementary to the IRIS data and provide an additional
constraint on comprehensive thermal models. Published values, such as those
tabulated by Morrison (1977), are barely of adequate quality to provide
good constraints, especially when comparing fluxes in the 10- and 20\dmic\
Photometry of Voyager images might provide an independent check on
the conclusion, derived from the IRIS data, that Callisto has a rougher
surface than Ganymede (Chapter 6). By using the Hapke (1984) equations,
particle albedo, surface texture, and larger-scale surface topography can
be determined independently from a complete enough photometric data set,
and this has been done for Ganymede by Helfenstein (1986). A similar exercise
for Callisto might allow quantitative comparison of its surface morphology
with Ganymede's in a way that could be compared to the thermal emission
data. However, because of possible effects such as the lateral conduction
of heat, the scale of the topography that affects the thermal emission
may be different from that that affects the scattering of sunlight, and
the two data sets may not be directly comparable.
The present study of the thermal emission from the Galilean satellites
has been hampered by the lack of comparable disk-resolved data from other
airless bodies. The thermal emission from the Earth's Moon is still poorly
described or understood, and a more complete survey, especially of the
beaming of thermal emission and the wavelength dependence of the emission,
is badly needed. I am not aware of any published data on the emission from
the moon in the 20\dmic atmospheric window, for instance, that would show
whether the Moon's emission spectrum steepens towards the terminator as
does Callisto's. A complete survey may have to wait for the revival of
interest in Lunar studies, which have been dormant since the early 70's.