Carbon dioxide and sulfur dioxide on the surfaces of the Galilean satellites Ganymede and Callisto

The surfaces of Ganymede and Callisto are made mostly of water-ice and a spectrally bland non-ice material. Small amounts of other materials, including CO₂ and SO₂, have recently been discovered on the surfaces of the them [Carlson et al., 1996; McCord et al., 1998] as well as a very thin exosphere of CO₂ on Callisto [Carlson, 1999]. These discoveries were made by the detection of absorption bands in infrared spectra obtained from the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft over the last 5 years. Both the CO₂ and SO₂ on the surface appear to be contained in a less-volatile host material that keeps them stable when they would otherwise quickly escape into space. McCord et al. also showed that the distribution of CO₂ and SO₂ over the surfaces of Ganymede and Callisto are affected by the local geology and space environment.; One goal of this dissertation is to pin down the relationships between the surface geology and the distribution of these volatiles on Ganymede and Callisto. The other goals are to determine if all the CO₂ and SO ₂ are trapped in similar host materials, identify this host material, determine the origins of the CO₂ and SO₂, and determine what implications can be inferred from the distribution of CO₂ and SO₂. These are grand objectives to fulfill in a single dissertation and there have been many hurdles along the way, including a very complex and still not completely resolved calibration of the NIMS data. Additionally, the distribution of SO₂ could only be mapped on Callisto because its band is too shallow to be observed in most places on Ganymede. Despite these limitations, I have been able to accomplish most of the goals. The CO ₂ and SO₂ on their surfaces, are indigenous to the satellites, and are contained in the darker, non-ice materials that are likely either oxyhydroxides or clay-like serpentines. It also appears that the subsurface of Ganymede may have lost much of its internal CO₂ compared to Callisto.