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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.

If the author provided an email address or URL for general inquiries, it is as follows:

hibbitts@rad.ess.washington.edu

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