DPS 34th Meeting, October 2002
Session 23. Other (than Io and Europa) Planetary Satellites
Oral, Chair(s): C.A. Hibbitts and W.M. Grundy, Wednesday, October 9, 2002, 9:20-10:30 and 11:00-11:20am, Room M

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[23.01] CO2/Water-ice/Non-ice relationships on the Galilean satellites Ganymede and Callisto

C. A. Hibbitts, G. B. Hansen, T. B. McCord (The Planetary Science Institute)

Most of the carbon dioxide detected on the surfaces of the Galilean satellites Ganymede and Callisto is contained in the non-ice materials. Reflectance spectra returned by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft show that the water-bearing non-ice material on Ganymede [McCord et al., 2001] hosts some of the CO2, but on Callisto an anhydrous OH-bearing material hosts the CO2 [McCord et al., 1998; Hibbitts et al., 2001]. The CO2 absorption band in reflectance spectra of both satellties has a minimum at ~ 4.26 \mum implying the CO2 is trapped by similar physical mechanisms onto or into dissimilar non-ice host materials. The detected CO2 is in the top ~ 1 mm of the surface, and its abundance for both moons is less than 0.3%, by weight. On Ganymede, spectra from areas of relatively fine-grained ice are sufficiently bright at 4 \mum for the CO2 absorption to be detected if it were present. CO2 is absent from the fine-grained icy poles and, in non-polar regions containing mixtures of ice and non-ice materials, CO2 is less in fine-grained icy areas compared to adjacent larger-grained icy areas. Similarly on Callisto, CO2 abundance is less at the poles where the ice is finer-grained than in non-polar regions. Thus, it appears that CO2-poor fine-grained water-ice, perhaps a layer, can cover CO2-bearing non-ice material on both moons. The lack of CO2 in fine-grained ice, though, does not imply that there is no CO2 in larger-grained ice on these moons. CO2 in larger-grained ice will simply not be detected by NIMS because of large-grained ice's very low reflectance at 4 \mum. Subsurface ice or clathrate may in fact act as a reservoir for supplying CO2 to the tenous exosphere on Callisto and to the CO2-rich impact craters on both moons.

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