The Topographies of Rhea and Iapetus in Comparison

Abstract

Rhea and Iapetus are similar-sized satellites of Saturn but their densities are different. Rhea has a higher rock mass ratio and, associated with it, more radiogenic heating. This implies a thinner lithosphere on Rhea than on Iapetus during their thermal evolution and is expected to have a topographic expression at the surface of these bodies: the thicker the lithosphere the more topography can be supported. We derived the topography of Rhea from Cassini stereo images and compared it with the topography of Iapetus derived earlier (1). We found that large impact basins on Rhea have relief of up to 7 km whereas those on Iapetus reach relief of up to 14 km. This observation is consistent with a thinner lithosphere on Rhea compared to Iapetus during the early period of heavy bombardment. In particular, while Iapetus has retained impact basins with diameters of up to 800 km, the largest basin on Rhea found so far has a diameter of 440 km only. Larger basins, which are expected to have been formed on Rhea too, have obviously been relaxed rapidly or alternatively, could possibly not develop in a thin lithosphere at all. Depth-to-diameter ratios for craters smaller than 100 km on Rhea fit those determined on Iapetus but the basins on Rhea show a higher degree of relaxation than those on Iapetus. (1) B. Giese at al., The topography of Iapetus' leading side, Icarus (2007), doi:10.1016/j.icarus.2007.06.005, in press.