Distributions of CO2 and SO2 on the surface of Callisto

Abstract

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±0.004 μm. 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.