A Photometric Study of Enceladus

Abstract

We have supplemented Voyager imaging data from Enceladus (limited to phase angles of 13°-43°) with recent Earth-based CCD observations to obtain an improved determination of the Bond albedo, to construct an albedo map of the satellite, and to constrain parameters in Hapke's (1986, Icarus 67, 264-280) photometric equation. A major result is evidence of regional variations in the physical properties of Enceladus' surface. The average global photometric properties are described by single scattering albedo ω̄ o = 0.998 ± 0.001, macroscopic roughness parameter θ̄ = 6 ± 1°, and Henyey-Greenstein asymmetry parameter g = -0.399 ± 0.005. The value of θ̄ is smaller than the 14° found by fitting whole-disk data, which include all terrains on Enceladus. The opposition surge amplitude B o = 0.21 ± 0.07 and regolith compaction parameter h = 0.014 ± 0.02 are loosely constrained by the scarcity of and uncertainty in near-opposition observations. From the solar phase curve we determine the geometric albedo of Enceladus pv = 0.99 ± 0.06 and phase integral q = 0.92 ± 0.05, which correspond to a spherical albedo A = p v q = 0.91 ± 0.1. Since the spectrum of Enceladus is fairly flat (D. P. Cruikshank, 1980, Icarus 41, 246-258; B. J. Buratti, 1984, Icarus 59, 392-405), we can approximate the Bond albedo A B with the spherical albedo. Our photometric analysis is summarized in terms of an albedo map which generally reproduces the satellite's observed lightcurve and indicates that normal reflectances range from 0.9 on the leading hemisphere to 1.4 on the trailing one. The albedo map also reveals an albedo variation of 15% from longitudes 170° to 200°, corresponding to the boundary between the leading and trailing hemispheres.