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C. M. Dalle Ore (SETI Inst.), D. P. Cruikshank (NASA Ames), T. C. Owen (U. Hawaii), T. R. Geballe (NOAO Gemini), T. L. Roush (NASA Ames), B. N. Khare (SETI Inst.), C. de Bergh (Obs. Paris)
We have extended the recorded spectrum of Saturn's satellite Rhea from the previous limit of 2.5 \mum to 3.6 \mum, using the UKIRT telescope on Mauna Kea and the CGS4 spectrometer. The extended region of the spectrum defines the profile of the strong H2O ice band centered at 3.0 \mum; the long wavelength portion of this profile is diagnostic of the presence of other surface components because the ice itself is strongly absorbing at wavelengths as long as about 3.4 \mum, even for very small grain sizes (e.g., W. M. Calvin and R. N. Clark, Icarus 89, 305, 1991). The shape of this infrared band lends support to the presence of a small quantity of an involatile organic solid (tholin) in Rhea's surface material, as proposed by K. S. Noll et al. ( Nature 338, 45, 1997) to match a portion of the ultraviolet spectrum. An organic solid provides a superior fit to hydrous silicates. We investigate the merits of the various organic solids for which optical constants are presently available.