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[3.02] Saturn's Icy Satellites: Infrared Reflectance Spectra of Rhea, Tethys, Dione, and Iapetus at \lambda\geq 2.5\mum

D. P. Cruikshank, T. L. Roush (NASA Ames), T. C. Owen (U. Hawaii), T. R. Geballe (NOAO Gemini), C. M. Dalle Ore, B. N. Khare (SETI Inst.), C. de Bergh (Obs. Paris)

We present near-IR spectra of the icy satellites of Saturn obtained over the past five years with the UKIRT telescope and the CGS4 spectrometer. We have obtained observations for some of these bodies from 1.4 to 4.2 \mum. As has been long known, the spectra of these objects are dominated by strong absorption bands due to solid H₂O, but most of the satellites show reflectance decreases in violet and ultraviolet wavelengths indicative of the presence of other components. The longer wavelength region, particularly around 3.5 \mum, is helpful in discerning the presence of even small amounts of minerals and organic solids because H₂O has very low reflectance in that region. Here we report our study of the spectra of Tethys, Dione, Rhea, and Iapetus, extended beyond 2.5 \mum. In our parallel investigation of the low-albedo hemisphere of Iapetus, we found that the shape of the strong absorption band at 3.0 \mum, including the rise in reflectance beyond 3.2 \mum, is well matched by a surface mixture that includes "Triton" tholin, an involatile organic substance produced by the irradiation of a gaseous mixture of N₂ : CH₄ (99.9 : 0.1) [W. R. Thompson et al. \bf{Icarus 90}, 57, 1991; G. D. McDonald et al. \bf{Icarus 108}, 137, 1994]. Inclusion of small quantities of this N-rich tholin in models of the other, higher albedo satellite surfaces also produces good fits to the new spectral data.

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