Abstract
We present evidence for several non-ice constituents in the surface
material of the icy Galilean satellites, using the reflectance spectra
returned by the Galileo near infrared mapping spectrometer (NLMS)
experiment. Five new absorption features are described at 3.4, 3.88,
4.05, 4.25, and 4.57 mu m for Callisto and Ganymede, and some seem
to exist for Europa as well. The four absorption bands strong enough
to be mapped on Callisto and Ganymede are each spatially distributed
in different ways, indicating different materials are responsible
for each absorption. The spatial distributions are correlated at
the local level in complex ways with surface features and in some
cases show global patterns. Suggested candidate spectrally active
groups, perhaps within larger molecules, producing the five absorptions
include C-H, S-H, SO2, CO2, and C=N. Organic material like tholins
are candidates for the 4.57- and 3.4-mu m features. We suggest, based
on spectroscopic evidence, that CO2 is present as a form which does
not allow rotational modes and that SO2 is present neither as a frost
nor a free gas. The CO2, SO2, and perhaps cyanogen (4.57 mu m) may
be present as very small collections of molecules within the crystal
structure, perhaps following models for radiation damage and/or for
comet and interstellar grain formation at low temperatures. Some
of the dark material on these surfaces may be created by radiation
damage of the CO2 and other carbon-bearing species and the formation
of graphite. These spectra suggest a complex chemistry within the
surface materials and an important role for non-ice materials in
the evolution of the satellite surfaces.
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