have spectra that are characterized by strong infrared bands of water ice.^2,3 Models indicate that the water ice fraction varies from 20 percent in some dark regions to nearly 100 percent in others. The remaining material, which sometimes makes up the majority of the surface, is a good match with a variety of hydrated silicates similar to the hydrated silicates found in carbonaceous chondrites.⁴ In addition, the surfaces have a number of minor constituents that are variable in observability and abundance.

At least three different carbon-bearing molecules have been identified in spectra of the surfaces of these objects. Spectra obtained by Galileo’s NIMS show absorption bands indicative of carbon-rich materials. Absorptions are interpreted to indicate C-H and C≡N present in organic compounds. Spectra also indicate CO₂, and its spectral characteristics suggest that it is trapped within or bound to dark surface materials. A broad absorption in the ultraviolet is suggestive of trapped O₃ and another unidentified band that may be the –CH₂O functional group of an organic compound.⁵

Carbon dioxide shows interesting and suggestive correlations with both albedo and topography in the three icy Galileans. Callisto, the outermost of the four satellites, has an ancient, heavily cratered surface with widespread dark material that is thought to be carbon-rich. Galileo images show that Callisto’s dark material covers the underlying topography, filling in craters and other topographic lows between bright, ice-rich high-standing knobs and crater rims. CO₂ is correlated with fresh material such as bright impact craters, suggesting that CO₂ is a possible component of Callisto’s icy subsurface. CO₂ is strongest on the trailing side of Callisto, suggesting a possible link to irradiation by jovian magnetospheric plasma. Dark heavily cratered terrain constitutes about one-third of Ganymede’s surface. Geological investigations using Galileo’s high-resolution images suggest that the dark material is a relatively thin layer above brighter icy material and has been affected by processes of sublimation, mass wasting, ejecta blanketing, and tectonism. Dark deposits within topographic lows, such as craters,