Near-infrared (0.8-4.0 μm) spectroscopy of Mimas, Enceladus, Tethys, and Rhea

Spectral measurements from the ground in the time leading up to the Cassini mission at Saturn provide important context for the interpretation of the forthcoming spacecraft data. Whereas ground-based observations cannot begin to approach the spatial scales Cassini will achieve, they do possess the benefits of better spectral resolution, a broader possible time baseline, and unique veiewing geometries not obtained by spacecraft (i.e., opposition). In this spirit, we present recent NIR reflectance spectra of four icy satellites of Saturn measured with the SpeX instrument at the IRTF. These measurements cover the range 0.8 - 4.0 mu m of both the leading and trailing sides of Tethys and the leading side of Rhea. The L-band region ( 2.8 - 4.0 mu m) offers new opportunities for searches of minor components on these objects. Additionally, these data include 0.8 - 2.5 mu m spectra of both the leading and trailing sides of Mimas and of the ( mostly) trailing side of Enceladus. The spectrum of Enceladus shows activity near 2.25 mu m that we interpret as a possible signature of NH3 ice. The presence of ammonia in the Saturn system is not unexpected, and may help explain the apparent recent geologic activity of Enceladus. Analysis of leading/trailing differences in H2O band depths, spectral slopes, and albedo imply a separate regime of surface modification for Mimas and Enceladus than for the more distant icy satellites ( Tethys, Dione, Rhea). Aside from the potential NH3 on Enceladus, no other minor constituents are detected in these icy surfaces.