Compositional Mapping of Saturn's Satellite Dione with Cassini VIMS and Implications of Dark Material in the Saturn System
Cassini VIMS has obtained spatially resolved imaging spectroscopy data on numerous satellites of Saturn. A close fly-by of Dione provided key information for solving the riddle of the origin of the dark material in the Saturn system. The VIMS data show a pattern of bombardment of fine, sub-0.5-micron diameter particles impacting Dione from the trailing side direction. Multiple lines of evidence point to an external origin for the dark material, including the global spatial pattern, local patterns including crater and cliff walls shielding implantation on slopes facing away from the trailing side, exposing clean ice, and slopes facing the trailing direction that show higher abundances of dark material. Multiple spectral features match those seen on Phoebe, Iapetus, Hyperion, Epimetheus and the F-ring, implying the dark material has a common composition throughout the Saturn system. However, the exact composition of the dark material remains a mystery, except that bound water, carbon dioxide and, tentatively, ammonia are detected, and there is evidence both for and against cyanide compounds. A blue peak due to Rayleigh scattering, and a strong UV-Visible absorption, is observed in spectra of all satellites which contain dark material, again pointing to a common origin. The Rayleigh scattering effect is confirmed with laboratory experiments using ice and 0.2-micron diameter carbon grains when the carbon abundance is <3% by weight. Rayleigh scattering in solids is confirmed in naturally occurring terrestrial rocks. The spatial pattern, Rayleigh scattering effect, and spectral properties argue that the dark material is only a thin coating on Dione's surface, and also on Phoebe, Hyperion, Epimetheus, and Iapetus, although the dark material abundance appears higher on Iapetus, and may be locally thick. As previously concluded for Phoebe, the dark material appears to be external to the Saturn system and may be cometary in origin.
AAS/Division for Planetary Sciences Meeting Abstracts #39
- Pub Date:
- October 2007