Dust and Snow Cover on Saturn's Icy Moons

摘要

The final analysis of the Cassini radar observations of Saturn's icy moons presented here shows that the exchange of material between the planet's dust rings and moons, which is specific to the Saturnian system, plays a key role in the current state of the airless satellite regolith. Far from Saturn, the vast debris ring from Phoebe progressively coats the leading side of Iapetus with optically dark material reducing its radar brightness. On the contrary, close to the planet, the extreme radar brightness of the innermost moons Mimas, Enceladus, and Tethys (that exceeds that of the Galilean satellites) is most likely related to Enceladus's geysers and the E-ring which brings ultraclean water ice to their surfaces. The measured radar albedos and observed hemispheric dichotomies require at least a few decimeters thick "snow" cover and that the near surfaces of Saturn's innermost moons contain especially efficient backscattering structures whose nature remains an outstanding problem.