Mimas: Photometric roughness and albedo map

Abstract

Voyager imaging observations of Mimas between 6° and 132° in phase angle provide sufficient data to model such surface characteristics as large-scale roughness and directional scattering properties. We have analyzed disk-integrated and disk-resolved Voyager clear filter (0.48 μm) photometric data using a modified version of Hapke's (1986, Icarus 67, 264–280) equation which accommodates anisotropic multiple scattering. Since near-opposition observations are limited, we assume Mimas has a Rhea-like opposition surge, with angular width h = 0.066 and amplitude B o = 0.55. We find a single scattering albedo ω o = 0.951 ± 0.002 , a Henyey-Greenstein asymmetry parameter g = −0.213 ± 0.005, and a macroscopic roughness parameter θ = 30° ± 1° . The photometric roughness θ is greater than those derived in a similar manner from other bright icy satellite surfaces, though comparable to the 30° determined for Mimas by Buratti (1985, Icarus 61, 208–217) who applied the Hapke theory and the 30° ± 25° found by McEwen (1987, personal communication) who used fits to the Minnaert k parameter to evaluate θ . We find a geometric albedo p V = 0.69 ±0.05 and phase integral q = 0.78 ± 0.05, corresponding to a spherical albedo A = p V q = 0.54 ± 0.1 and a mean opposition magnitude V o = +12.62 ± 0.07. Our photometric analysis is summarized in terms of an albedo map which reproduces the satellite's observed lightcurve and indicates that normal reflectance range from 0.64 on the leading hemisphere to 0.74 on the trailing one. Reflectances at extreme southern latitudes are a few percent higher than those at corresponding northern latitudes, though in general the equatorial latitudes are brighter than the polar regions. The photometric properties show no variation across the surface or relation to any particular terrain type.