Non-Water-Ice Constituents in the Surface Material of the Icy Galilean Satellites from the Galileo NIMS Investigation

T.B. McCord, F.P. Fanale (PGD/HIGP/SOEST, Univ. of Hawaii), J.C. Granahan (SETS Technology, Hawaii), G.B. Hansen, C.A. Hibbitts (PGD/HIGP/SOEST, Univ. of Hawaii), R.N. Clark (USGS, Denver), D.L. Matson, T.V. Johnson, M. Segura, R.W. Carlson, W.D. Smythe (Jet Propulsion Lab.), and the NIMS Team

The Galileo NIMS experiment has produced a wealth of spectral data for the icy Galilean Satellites in the range 0.7-5.2 microns. We present our preliminary analysis, focused on identifying non-ice components of the surface material. Water-ice features dominate the spectrum of Europa and to a lesser extent of Ganymede but are weak or non-existent for Callisto. Radiative transfer modeling indicates water-ice on the surface usually in the range 0-10% for Callisto and 50% for Ganymede, with higher levels in small areas. The strongest non-ice absorption is the OH stretch at 2.8 microns found in all hydrated materials. More diagnostic are five weak absorptions for Callisto, not reported before NIMS, centered at 3.88, 4.05, 4.25, 4.57, and possibly at 3.4 microns, with the 4.25-micron band the strongest (0-40%). All these bands exist also on Ganymede but they are weaker. The spatial distributions of these band absorption strengths are overall uncorrelated band to band, indicating that the materials responsible are different for each absorption, but they are often correlated in complex ways with surface features and global locations. Candidate materials include, for each of the five new absorptions, CH (3.4), CO2 (4.25), CN perhaps as cyanogen (4.57), SO2 (4.05), and an SH-radical-containing material (3.88). Sulfur-bearing minerals may also be candidates for some of these absorptions, since some minerals with gas inclusions have unidentified bands in this region. We suggest, based on spectroscopic evidence, that these materials may be present as non-free molecules in nano deposits in the surface material.