Carbon Dioxide on Callisto and Ganymede

Details Carbon Dioxide on Callisto and Ganymede Carbon Dioxide on Callisto and Ganymede

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999dps....31.7004h&link_type=abstract

American Astronomical Society, DPS meeting #31, #70.04

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Scientific paper

The surface compositions of Callisto and Ganymede, the most outer two of the Galilean satellites, are dominated by water-ice and darker, non-ice material(s). Recent discoveries of near-infrared absorptions in spectra returned by NIMS aboard the Galileo spacecraft indicate the presence of CO_2, SO_2, and other materials in small amounts [McCord et al, JGR, 1997, 1998]. The CO_2 and SO_2 are believed to exist as 'trapped' molecules in either the dark material and/or the water-ice. The possibility that the surface distributions of CO_2 on Ganymede and Callisto could provide information on the relative effects of impact cratering, ion implantation, and surface sputtering on volatile abundance have been independently addressed [Hibbitts et al., JGR, 1999 (submitted); Hibbitts et al, AGU, 1999]. Here we contrast the similarities and differences in the CO_2 distributions on the two moons as mapped during the Galileo nominal and GEM missions. Many bright, more recent, impact craters on Callisto show relatively elevated concentrations of CO_2. A similar relationship has not been found for Ganymede. CO_2 concentrated about the center of the trailing hemisphere of Callisto in a pattern longitudinally consistent with a sinusoid is interpreted as showing affects of interactions with ions trapped in Jupiter's co-rotating magnetosphere. There is no evidence for a similar pattern on Ganymede. However the highest latitudes of both Callisto and Ganymede appear covered by water-ice frost and are relatively depleted in CO_2, possibly due to masking. Deposition of frost at high latitudes on Ganymede could be augmented with locally-sputtered water molecules [Johnson, Icarus, 1985] explaining the previously reported equatorial distribution of CO_2 on Ganymede, which we now believe is preferentially associated with less icy material and/or specific regiones. The CO_2 in the less icy material of Ganymede appears more variable than on Callisto where the non-ice material appears to always contain some CO_2.

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