Solar System Remote Sensing Symposium (2002) 4019.pdf THE DIVERSE SURFACE COMPOSITIONS OF THE GALILEAN SATELLITES. R. W. Carlson,, Jet Pro- pulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 ([email protected]). The galilean satellites represent a diverse collection, ranging SO2 are indeed found to concentrated. Thermal processes from the volcanic moon Io, with a surface that is changing such as diapiric heating, accompanied by H2O sublimation, yearly, to Callisto, with a dark, ancient surface overlying ice. can further concentrate the sulfur compounds and produce The composition of these surfaces are also quite different due the variegated surface patterns, while gardening and asyn- to a variety of processes and influences, including tidal heat- ing, radiolysis, gardening, a magnetic field (Ganymede), and chronous rotation modify the global patterns. An alternative meteoritic infall. suggestion for Europa’s hydrate is hydrated sulfate salts from an internal ocean, but the radiolytic lifetime of these com- Io’s surface contains large quantities of sulfur dioxide (SO2) pounds at the surface is < 4000 years so they would be rap- and colorful sulfur allotropes, both originating in plumes and idly assimilated into the H2SO4,SO2,andSx cycle. NIMS µ flows from the tidally driven volcanoes. A broad, 1- mband spectra are best matched with sulfuric acid hydrate. Surficial is found at high latitudes and may be due to absorption by carbon dioxide is also observed on the leading hemisphere of long-chain sulfur polymers produced by SO radiolysis, al- 2 Europa. This CO may arise from the radiolysis of carbona- though iron and iron sulfide compounds are candidates. An 2 unidentified 3.15 µm absorber is equatorially distributed ceous meteoritic material that is deposited preferentially on whilea4.62µm band, perhaps due to a sulfate compound, the leading hemisphere or it may be an outgassing product. exhibits a non-uniform distribution. Hot spots are generally dark, and some exhibit negative reflectance slopes (i.e., Ganymede and Callisto are both covered by a dark de- blue). The composition of these lavas has not been estab- posit overlying ice, the latter exposed on ridges, slopes, and lished spectroscopically, but the high temperatures of some within craters. Both satellites show similar spectroscopic volcanoes suggest ultramafic silicates or perhaps more re- features, generally more pronounced in Callisto’s spectra. fractory material such as oxides. CO2 is observed to be concentrated on Callisto’s trailing hemisphere, suggestive of magnetospheric influence, in par- Europa’s icy surface contains the radiolytic product hydro- ticular radiolysis. A corresponding CO atmosphere is found gen peroxide (H O ). This molecule is rapidly destroyed by 2 2 2 on Callisto. The atmosphere is rapidly depleted by ionization solar ultraviolet radiation, so replenishment of H O by high- 2 2 and sweeping, so there must be a continuous source of CO energy charged particle impact must be occurring continu- 2 such as radiolysis or outgassing. A band at 3.88 µm has been ously. The same process may produce atmospheric and surfi- suggested to be an SH stretch band, but radiolytically- cial O and the hydrated compound that was found on Eu- 2 produced H CO provides an equally good fit. A 4.57-µm ropa, suggested to be hydrated sulfuric acid. The hydrate is 2 3 band has been suggested to be from a CN stretch transition observed to be associated with visually dark material, whose but the shape and position is better fit by carbon suboxide spectra resemble sulfur polymers, and with surficial SO .A 2 (C O ), a radiolysis product of CO . Sulfur dioxide has been radiolytic sulfur cycle continuously creates and destroys 3 2 2 suggested as a surface constituent on both Ganymede and these compounds, and their observed relative abundance Callisto based on a 4-µm infrared feature. However, the cor- agrees with predictions based on laboratory radiolysis ex- responding feature is not observed on Europa even though an periments. The equilibration time for this cycle is ~ 100 SO ultraviolet feature is present in Europa’s trailing side years. Sources of sulfurous material include Iogenic plasma 2 spectra. Ganymede and Callisto’s 4-µm feature may repre- implantation, preferentially deposited on the trailing hemi- sent something other than SO . sphere where the hydrated acid, visually dark material, and 2.