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Oberon: Color photometry from voyager and its geological implications

1991; Elsevier BV; Volume: 90; Issue: 1 Linguagem: Inglês

10.1016/0019-1035(91)90065-2

1090-2643

P. Helfenstein, J. Hillier, C. M. Weitz, Joseph Veverka,

Impact of Light on Environment and Health

Three primary terrains are exposed at the surface of Oberon: average cratered terrain, bright crater rays, and prominent dark deposits often seen on (but not restricted to) crater floors. We have applied Hapke's (1986, Icarus 67, 264–280) photometric model to Voyager imaging and Earth-based observations to determine photometric properties and colors of Oberon's terrains and compare the results with those for the other Uranian satellites. All terrains on Oberon exhibit photometric roughnesses (θ = 21°) that are lunar-like and similar to average global photometric roughnesses of Titania and Umbriel. The asymmetry factors are backscattering for all terrains (−0.35≤g≤−0.20) and fall within the observed range for other Uranian satellites. At visual wavelengths, g becomes more backscattering with decreasing single-scattering albedo (0.25 ≤ ω0 ≤ 0.61). The brightest materials on Oberon are relatively neutral in color, typical of materials covering other Uranian satellites, but the violet/clear and green/clear albedo ratios for the darkest terrains on Oberon are atypically large for Uranian satellites. The variation can be modeled in terms of a binary mixture of low-albedo and high-albedo materials having slightly different colors. Oberon's leading hemisphere is dominantly covered with dark surface materials identical in color to dark deposits on crater floors, while materials on the trailing hemisphere are richer in a spectrally neutral icy component, with dark materials occurring in small irregular patches. Bright ray craters appear to have excavated into a relatively cleaner, spectrally neutral, icy crust. Dark materials at the surface may be effusive flood deposits, a dark layer below the icy crust which is more exposed on the leading hemisphere due to preferential erosion by impacts or possibly exogenic in origin.