NON-WATER-ICE CONSTITUENTS IN THE SURFACE MATERIAL OF THE ICY GALILEAN SATELLITES FROM THE GALILEO NEAR-INFRARED MAPPING SPECTROMETER INVESTIGATION

We present evidence for several non-ice constituents in the surface ma terial of the icy Galilean satellites, using the reflectance spectra r eturned by the Galileo near infrared mapping spectrometer (NLMS) exper iment. 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 mapp ed on Callisto and Ganymede are each spatially distributed in differen t ways, indicating different materials are responsible for each absorp tion. The spatial distributions are correlated at the local level in c omplex ways with surface features and in some cases show global patter ns. Suggested candidate spectrally active groups, perhaps within large r 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, th at 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, a nd perhaps cyanogen (4.57 mu m) may be present as very small collectio ns of molecules within the crystal structure, perhaps following models for radiation damage and/or for comet and interstellar grain formatio n at low temperatures. Some of the dark material on these surfaces may be created by radiation damage of the CO2 and other carbon-bearing sp ecies and the formation of graphite. These spectra suggest a complex c hemistry within the surface materials and an important role for non-ic e materials in the evolution of the satellite surfaces.