Compositional Mapping of Saturn's Satellite Iapetus with Cassini VIMS and Implications of Dark Material in the Saturn System

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) has obtained spatially resolved imaging spectroscopy data on Iapetus and numerous other satellites of Saturn. Multiple spectral features seen in spectra of dark material on Iapetus match those seen on Phoebe, Hyperion, Dione, Epimetheus, and the F-ring, implying the dark material has a common composition throughout the Saturn system. Water, carbon dioxide and, tentatively, ammonia have been detected in the dark material on Iapetus and other satellites. A blue peak due to Rayleigh scattering, and a strong UV-Visible absorption, is observed in spectra on all satellites which contain dark material, again pointing to a common origin. The Rayleigh scattering effect has been confirmed with laboratory experiments using ice and 0.2-micron- diameter carbon grains when the carbon abundance is less than about 3% by weight. Cassini will fly by Saturn's moon Iapetus on September 10, 2007, at a closest approach altitude of about 1644 km. The low fly-by velocity will allow imaging spectroscopy data to be obtained with VIMS over dark terrain, the transition region into bright terrain, and bright (icy) regions of Iapetus, including good views of the equatorial bulge. Using VIMS data, which covers the spectral range from 0.35 to 5.1 microns in 352 wavelengths at spatial resolutions as high as 0.5 x 1 km, we will map materials on Iapetus' surface and study their geologic placement. Materials under study include water ice, carbon dioxide, ammonia ice, and materials contributing to absorptions seen at 1.94, 2.05, 2.16, and 2.42 microns. Other weaker absorptions observed in spectra of dark material on Iapetus in previously obtained VIMS data will be examined.