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G. B. Hansen, C. A. Hibbitts (Planetary Science Institute)
Our study of the lattice order of water ice on the Jovian satellites Ganymede and Callisto has provided us a view of the global distribution of ice grain-size and of non-ice materials. This study uses the near-infrared spectra returned by the Galileo orbiter Near-Infrared Mapping Spectrometer. Average spectra were generated for various regions, and models were fit to the spectra. The models use scaled albedos of water ice linearly mixed with derived and/or scaled non-ice spectra. Using scalings with simple spectral variation, the average spectra could be fit to almost arbitraty precision. For Callisto, three global-scale observations (60-100 km/pixel), one regional-scale (18 km/pixel), and one local-scale (6 km/pixel) observation were analyzed. In each, a group of icy pixels was averaged and a group of ice-poor pixels was avearaged. A small amount of ice spectrum was removed from the ice-poor average, and the resulting non-ice estimate was scaled for use with the icy average. From this are derived non-ice spectral estimates for the two regions and ice grain sizes for those regions. On Ganymede, eight 4-10 pixel averages at a ~100 km scale are analyzed, in this case using a non-ice spectrum scaled from McCord et al. (Science 292:1523, 2001). The Callisto non-ice spectra all have a slightly red slope and a significant 2.7-\mum O-H absorption band (see Hibbitts et al., Eos, 82, Abst. P12B-0495, 2001). The derived Ganymede non-ice spectra have bands of hydration at 1.4 and 1.9 \mum as seen in isolated spectra from the trailing hemisphere (McCord et al., 2001), implying that this material is quite widespread. The icy regions on Callisto have generally fine grained ice (r = 20-100 \mum), although there is a 25% component of coarse (r = 0.2-1 mm ) ice in two areas (much like the ice in ice-poor regions). The ice on Ganymede has grain sizes of r = 10-100 \mum, except on the trailing equatorial latitudes, where r ~ 400 \mum.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.