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 of CO 2 and SO 2 on the surface of Callisto have been analyzed and mapped in detail. CO 2 of varying concentrations appears to exist everywhere on Callisto, except at higher latitudes, where it may be masked by frost. The CO 2 concentration on the trailing hemisphere has a longitudinal distribution largely consistent with a sinusoid centered on the equator near 270° longitude. The approximately sinusoidal pattern suggests that exogenic effects related to Jupiter's corotating magnetic field are involved. Closer inspection of both hemispheres reveals that in many cases, visibly bright and ice‐rich impact craters have high CO 2 concentrations within or near them. The CO 2 sometimes appears to be associated more with dark material near the craters than with the water ice. These correlations suggest impact processes may also affect the distribution of CO 2 on the surface of Callisto. The center of the absorption band has been refined to be at 4.258±0.004 μm. The presence of a single band shape and band minimum wavelength position in all data sets for the CO 2 absorption implies the physical state of CO 2 is similar over the surface of Callisto. The distribution of SO 2 on the surface is less well defined owing to characteristically shallower band depths, but it appears generally mottled, with some areas of high concentrations correlated with ice‐rich impact craters. Large‐scale patterns include the depletion of SO 2 in the polar regions and a depletion of SO 2 on the trailing side relative to the leading side. There is no sinusoidal pattern to this depletion. The center of the SO 2 band is determined to be between 4.01 and 4.02 μm.