Distributions of CO2 and SO2 on the surface of Callisto
Journal of Geophysical Research-Planets, 105 (E9):
22541--22557 (2000)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 degrees 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 mu m. 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 mu m.