SURFACE-PROPERTIES AND PHOTOMETRY OF THE URANIAN SATELLITES

Approximately 230 Voyager images in six filters have been reduced and combined with observations from the Voyager Photopolarimeter Science (PPS) experiment to derive accurate measurements of the following photometric parameters of the five largest Uranian satellites (Miranda, Ariel, Umbriel, Titania, and Oberon): spectral geometric and Bond albedos, phase integrals, the color-dependent phase coefficient, and integrated solar phase curves. Among the new results we report are that Titania is slightly redder than the other satellites and that the phase coefficients have a color dependence, with the UV phase coefficient being larger. This "phase reddening" is not completely expected for a spectrally flat object: we suggest that the decreased importance of multiple scattering cannot be the entire explanation for more sharply peaked blue phase curves. The observations have been fit to the radiative transfer model of B. Hapke (Icarus 67, 264-280) to derive single-scattering albedos ranging from 0.25 ± 0.02 for Umbriel to 0.58 ± 0.05 for Ariel. These satellites have similar, backscattering single particle phase functions (Henyey-Greenstein g ∼ -0.25). Ariel, Titania, and Oberon all have very fluffy regoliths with nearly 90% void space similar to the regoliths of Io and Callisto. We confirm the result of Helfenstein et al. (Icarus 74, 231-239) that Umbriel has a more compact surface: we find that its compaction state is similar to Ganymede's. Our values for the mean slope angle representing macroscopic roughness range from 18 ± 3° for Umbriel to 28 ± 3° for Ariel. Our observations are inconsistent with the mean slope angle of 42° for Ariel reported by Helfenstein et al. (Icarus 79, 231-239) and we find no strong evidence that this parameter is correlated with albedo. © 1990.