The Uranian satellites and hyperion: New spectrophotometry and compositional implications

New reflectance spectra at 3.5% resolution have been obtained for Ariel, Titania, Oberon, and Hyperion in the 0.8- to 1.6-μm spectral region. The new spectra show no absoptions other than the 1.5-μm water-ice feature (within the precision of the data), and demonstrate extension into the 0.8- to 1.6-μm region of the 1.5- to 2.5-μm spectral similarity of Ariel to Hyperion (R. H. Brown and D. P. Cruikshank (1983). Icarus 55,93-92). The new data confirm the presence of dark, spectrally bland component on/in the water-ice surfaces of the Uranian satellites, which, with some reservations, has spectral similarities to the dark substance on the leading side of Iapetus and the dark material on/in the surface of Hyperion, as well as other dark, spectrally neutral substances such as charcoal. Attempts were made to match the spectra of Ariel, Titania, and Oberon with additive reflectance mixes (areal coverage) of fine-grained water frost and various dark components such as charcoal, lampblack, and charcoal-water-ice mixtures. The results were broad limits on the amounts of possible areal coverage of a charcoal-like spectral component on the surfaces of the Uranian satellites, but the data are not of sufficient precision to conclusively determine whether the dominant mode of contaminant dispersal is areal or voluminal. The effect of highly variegated albedos on the diameters derived by R. H. Brown, D. P. Cruikshank, and D. Morrison (1982a) (Nature 300, 423-425) is found to be small.