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G. K. Benedix (San Francicsco State Univ.), T. L. Roush (NASA Ames), T. C. Owen (Univ. of Hawaii), D. P. Cruikshank (NASA Ames), T. R. Geballe (JACH/UKIRT), C. M. Dalle Ore (SETI Institute), B. N. Khare (NRC/NASA Ames), C. de Bergh (Obs. Paris)
High-resolution spectra have recently been obtained for Rhea and Dione covering most of the UV/VIS/NIR (0.2 - 2.5 Ám) wavelengths. The new data were acquired by HST (0.22 - 0.48 Ám; Noll et al., 1997) and at UKIRT (1.42 - 2.09 Ám, 1995; 0.98 - 1.32 Ám, 1997). We rely on earlier data (Clark et al., 1984) for the region 0.48 - 1.0 Ám. Clark et al. (1984) showed that the surfaces of Rhea and Dione are composed primarily of water ice, with the possibility of small amounts of ammonium hydroxide (NH4OH) ice. Noll et al. (1997) detected ozone in small quantities on the surfaces of these moons represented by the absorption band centered at 0.26Ám. For the Hapke scattering models, we have created a set of optical constants for ozone by digitizing absorption coefficient data in the UV (Inn and Tanaka, 1953) and MIR (around 9Ám; Harvey and Bass, 1958). In addition, we are using optical constants of 100K haxagonal water ice (Grundy and Schmitt, 1998) and optical constants of tholin produced by plasma irradiation of an ice mixture of H2O and C2H6. The spectrum of Dioneĺs leading surface is characterized by a blue slope at the near-ir wavelengths, a strong UV absorption band attributed to O3 ice, and deep absorption bands at ~1.5Ám and ~2.05Ám and shallow absorption bands at ~1.05Ám and ~1.3Ám, all attributed to H2O ice. Water ice alone, however, does not appear to account for the spectrum of Dione; some other component is needed to achieve the blue slope at the longer wavelengths. The model for the leading hemisphere of Rhea is characterized by three grain sizes of 100K water ice plus an ice tholin. These modeling efforts will continue to better constrain the composition of the surfaces of these moons.