Abstract
The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard
the Cassini spacecraft obtained its first spectral map of the satellite
Iapetus in which new absorption bands are seen in the spectra of
both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon
dioxide is identified in the low-albedo material, probably as a photochemically
produced molecule that is trapped in H2O ice or in some mineral or
complex organic solid. Other absorption bands are unidentified. The
spectrum of the low-albedo hemisphere is satisfactorily modeled with
a combination of organic tholin, poly-HCN, and small amounts of H2O
ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice
slightly darkened with tholin. The detection of CO2 in the low-albedo
material on the leading hemisphere supports the contention that it
is carbon-bearing material from an external source that has been
swept up by the satellite's orbital motion.
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