The icy moons of Saturn boast a wide variety of topographic features, including the deep Herschel crater on Mimas, the prominent equatorial ridge on Iapetus and the famous “tiger stripes” of Enceladus. The lesser known moons, Rhea and Dione also display complex topography, including impact basins, normal faults and ridges. The rich diversity of surface features on these moons is in part due to differences in their thermal histories. High resolution digital elevation models of such features can be used to gain insight into these thermal histories, as well as their internal structures and orbital histories.
For instance, the depths of craters reduce overtime through a process called viscous relaxation, which increases in efficiency with the surface heat flux. Thus by measuring the current depth and estimating the initial depth, the amount of viscous relaxation a crater has undergone can be calculated and the heat flux at the surface can be inferred. Additionally, ridges and normal faults impose loads on the lithosphere, causing it to bend in response. The width and the amplitude of this bending, called flexure, is in part controlled by the elastic thickness, the portion of the ice shell that behaves elastically.
We use digital elevation models of the moons Rhea and Dione to measure crater depth and the topography of ridges and normal faults. With this data we calculate elastic thicknesses and infer heat fluxes that are much greater than those expected for a body heated solely from radioactive decay. Our results suggest that both Rhea and Dione underwent significant tidal heating and that both moons may have had sub-surface oceans early in their history.