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C. A. Hibbitts, T. B. McCord, G. B. Hansen (Univ. of Hawaii), J. Klemaszewski (Arizona State Univ.), R. T. Pappalardo, G. C. Collins, L. M. Prockter (Brown Univ.)
The surface compositions of Callisto and Ganymede, the most outer two of the Galilean satellites, are dominated by water-ice and darker, non-ice material(s). Recent discoveries of near-infrared absorptions in spectra returned by NIMS aboard the Galileo spacecraft indicate the presence of CO2, SO2, and other materials in small amounts [McCord et al, JGR, 1997, 1998]. The CO2 and SO2 are believed to exist as 'trapped' molecules in either the dark material and/or the water-ice. The possibility that the surface distributions of CO2 on Ganymede and Callisto could provide information on the relative effects of impact cratering, ion implantation, and surface sputtering on volatile abundance have been independently addressed [Hibbitts et al., JGR, 1999 (submitted); Hibbitts et al, AGU, 1999]. Here we contrast the similarities and differences in the CO2 distributions on the two moons as mapped during the Galileo nominal and GEM missions. Many bright, more recent, impact craters on Callisto show relatively elevated concentrations of CO2. A similar relationship has not been found for Ganymede. CO2 concentrated about the center of the trailing hemisphere of Callisto in a pattern longitudinally consistent with a sinusoid is interpreted as showing affects of interactions with ions trapped in Jupiter's co-rotating magnetosphere. There is no evidence for a similar pattern on Ganymede. However the highest latitudes of both Callisto and Ganymede appear covered by water-ice frost and are relatively depleted in CO2, possibly due to masking. Deposition of frost at high latitudes on Ganymede could be augmented with locally-sputtered water molecules [Johnson, Icarus, 1985] explaining the previously reported equatorial distribution of CO2 on Ganymede, which we now believe is preferentially associated with less icy material and/or specific regiones. The CO2 in the less icy material of Ganymede appears more variable than on Callisto where the non-ice material appears to always contain some CO2.