Albedo and color maps of the Saturnian satellites

Abstract

Maps of the normal reflectances and color ratios ( 0.58 0.41 μm ) have been prepared from Voyager images for the following Saturian satellites: Mimas, Enceladus, Tethys, Dione, and Rhea. Albedo and color variegations on these objects are in general much less than those of the Galilean satellites. Enceladus and Mimas have no albedo or color variegations greater than 10% in the observed regions of their surfaces, with the exception of a small high albedo area in the southern polar region of Enceladus, which may be similar to the bright wispy streaks (“linea”) on Rhea and Dione. We find an albedo dichotomy of ∼20% between the leading (bright) and trailing sides of Dione, and confirm a similar dichotomy found by Verbiscer and Veverka ( Icarus, 1989 , 82, 336) for Rhea. This dichotomy is not as marked in the color maps: differences of 5–10% were found for the hemispheres of these two satellites. Unlike the Galilean satellites, there is little general correlation between color and albedo; the only clear example we could find of the expected result that higher albedo surfaces are bluer is for the bright wispy streaks (“linea”) on the trailing sides of Rhea and Dione. On Tethys, we see two lower albedo features: the one on the trailing side is redder than the ambient terrain, whereas the one on the leading side is bluer.There is little correlation between mapped geologic units and features and the color/albedo patterns: Rhea and the trailing side of Dione are the only examples where correlations exist. The higher albedo, photometrically bland leading sides of Tethys, Dione, and Rhea suggest enhanced erosion of this hemisphere by meteoritic and cometary impact. In addition, the optical properties of these satellites may be affected by two exogenic processes not considered extensively in previous studies: alteration of the surfaces by ultraviolet radiation, and accretion of particles from Saturn's E-ring.