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A 'dark secret' of Ganymede revealed

Wednesday, June 16, 2004
Hibbitts (PSI)

Ganymede, with the iciest surface of the three icy Galilean satellites, is the only one to exhibit two spectrally distinct nonice materials on its surface. One of these materials appears to be a heavily hydrated material [McCord et al., 2001], somewhat similar to that found on Europa [McCord et al., 1998], and another less hydrated material spectrally quite similar to that found on Callisto [Hibbitts et al., 2003]. Whereas the nonice materials on Europa and Callisto appear to be of uniform (but different) compositions and present over most of their surfaces [McCord et al., 1998, 2001;Carlson et al., 1999], the nonice materials on Ganymede are not as ubiquitous. On Ganymede, the Europa-like heavily hydrated nonice material is associated with the older regions morphologically similar to the palimpsests on Callisto, not the more geologically-active sulci whereas on Europa the distribution of the nonice material is strongly correlated with geologically 'recent' terrain. This association of this heavily hydrated nonice material with the oldest terrain on Ganymede suggests a stability of billions of years. The less hydrated Callisto-like nonice material on Ganymede has been found to be associated with the ejecta of dark-ray craters. The correlation of a compositionally unique nonice material with dark ray craters is strong evidence for impactor contamination, an idea brought up repeatedly for over 20 years [e.g. Conca, 1981; Schenk and McKinnon, 1991].

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