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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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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.