Distributions of CO2 and SO2 on the surface of Callisto

Absorption bands in the infrared reflectance spectra from the Galileo Near- Infrared Mapping Spectrometer (NIMS) which are attributed to the presence o f CO2 and SO2 on the surface of Callisto have been analyzed and mapped in d etail. CO2 of varying concentrations appears to exist everywhere on Callist o, except at higher latitudes, where it may be masked by frost. The CO2 con centration on the trailing hemisphere has a longitudinal distribution large ly consistent with a sinusoid centered on the equator near 270 degrees long itude. The approximately sinusoidal pattern suggests that exogenic effects related to Jupiter's corotating magnetic field are involved. Closer inspect ion of both hemispheres reveals that in many cases, visibly bright and ice- rich impact craters have high CO2 concentrations within or near them. The C O2 sometimes appears to be associated more with dark material near the crat ers than with the water ice. These correlations suggest impact processes ma y also affect the distribution of CO2 on the surface of Callisto. The cente r of the absorption band has been refined to be at 4.258 +/- 0.004 mu m. Th e presence of a single band shape and band minimum wavelength position in a ll data sets for the CO2 absorption implies the physical state of CO2 is si milar over the surface of Callisto. The distribution of SO2 on the surface is less well defined owing to characteristically shallower band depths, but it appears generally mottled, with some areas of high concentrations corre lated with ice-rich impact craters. Large-scale patterns include the deplet ion of SO2 in the polar regions and a depletion of SO2 on the trailing side relative to the leading side. There is no sinusoidal pattern to this deple tion. The center of the SO2 band is determined to be between 4.01 and 4.02 mu m.