Enceladus: Implications of its unusual photometric properties

Abstract

We have investigated the photometric properties of Enceladus up to phase angles of 43°, using Voyager 1 and 2 images. Among the satellite's unusual photometric characteristics are its very high geometric albedo (1.0 ± 0.1 near 0.47 μ), which extends down to at least 0.34 μ, and the uniformity of albedos, colors, and scattering properties over the geologically varied surface. Although some albedo variations of up to 15% occur in low resolution Voyager 1 images, the albedos of the four major geological units imaged in the Voyager 2 near-encounter sequence differ by 1–2% or less, even though the ages of these units probably differ by at least a factor of 10 (3.8 billion years to a few hundred million years). The lack of correlation of spectrophotometric properties with terrain type suggests that the optical characteristics of Enceladus are determined by a recently deposited ubiquitous surface layer, possibly originating in Saturn's E-ring. The high geometric albedo implies that the surface layer is remarkably free of opaque material and is much more backscattering than is common for natural or laboratory frost layers on Earth. The unique phase and photometric functions of Enceladus can be explained by the high degree of multiple scattering in its surface. The existing observations suggest that the textural characteristics of Enceladus' regolith are similar to those of other icy Saturnian satellites.