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[44.04] Titania and Oberon: Surface Composition from New Near-infrared Observations and Reflectance Models

T. L. Roush, D. P. Cruikshank (NASA Ames), T. C. Owen (U. Hawaii), T. R. Geballe (JACH/UKIRT), G. K. Benedix (San Fran. St. U.), C. de Bergh (Obs. Paris), K. S. Noll (STScI), B. N. Khare (NRC/NASA Ames)

Here we report the combination of new near-ir spectra (1.45-2.48 \mum), of Titania and Oberon obtained in September 1995 at a resolving power of ~800, with older near-ir observations (0.5-1.44 \mum), and recent UV (0.22-0.48 \mum) observations obtained with HST. Previous interpretations suggest these surfaces are chiefly composed of water ice and varying amounts of spectrally neutral material. The new near-ir data provide the opportunity to search for absorption bands that could be attributable to surface materials other than water ice and because the combined spectra include such a broad wavelength region, to undertake improved models of water and neutral components on the surface. The calculated near-ir geometric albedos clearly exhibit three broad spectral features. Two (1.52- & 2.05 \mum) have previously been used to demonstrate the presence of water ice on these satellites. The third (~1.65 \mum), suggests the presence of hexagonal water ice at low temperatures, and may provide a mechanism of estimating the surface temperature. There is no spectral evidence for ices of CO₂, CO, NH₃ or CH₄. At UV wavelengths there is a broad absorption near 0.27-0.28 \mum previously attributed to OH formed by magnetospheric-surface interactions and retained at the low surface temperatures of these satellites. Surface components used in a Hapke scattering models include values for a combination of irradiated water ice in the UV and hexagonal water ice at 100K in the near-ir (IW), amorphous carbon (AC), and tholins (T) (produced from gas and solid). Results of these models suggest the surfaces of Titania/Oberon are composed of IW (~77/52 and finally T (~4/7%).