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 CO₂ and SO₂ on the surface of Callisto have been analyzed and mapped in detail. CO₂ of varying concentrations appears to exist everywhere on Callisto, except at higher latitudes, where it may be masked by frost. The CO₂ 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 CO₂ concentrations within or near them. The CO₂ 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 CO₂ on the surface of Callisto. The center of the absorption band has been refined to be at 4.258+/-0.004μm. The presence of a single band shape and band minimum wavelength position in all data sets for the CO₂ absorption implies the physical state of CO₂ is similar 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 correlated with ice-rich impact craters. Large-scale patterns include the depletion 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 depletion. The center of the SO₂ band is determined to be between 4.01 and 4.02 μm.