THE UNITED KINGDOM INFRARED TELESCOPE
ANNUAL REPORT
1998

2.3. Solar System

In October 1998, a five year CGS4 study of the icy satellites of Saturn was essentially completed by Cruikshank (NASA Ames) and collaborators. Spectra of Tethys, Dione, Rhea, and Iapetus' trailing hemisphere were obtained. As has long been known, the spectra of these objects are dominated by strong absorption bands due to water ice, but most of the satellites show reflectance decreasing in the violet and ultraviolet, which are indicative of the presence of other surface components. The longer-wavelength region, particularly around 3.5 µm, is helpful in discerning the presence of even small amounts of minerals and organic solids, because water ice has a low reflectance in that region. Inclusion of small amounts of N-rich ``Triton'' tholin (an involatile organic substance produced by irradiation of gaseous mixture of nitrogen and methane) produces good fits to the new IR spectral data in combination with the violet/UV data for these satellites. In a parallel investigation of the low-albedo leading hemisphere of Iapetus, the shape of the strong absorption band at 3.0 µm, including the rise in reflectance beyond 3.2 µm, has been shown to be well matched by a surface mixture containing a larger amount of Triton tholin.