Reflections on the Origin of Iapetus' Odd shape

Iapetus is a puzzling body in many respects: orbital characteristics, surface composition variations, shape and geology. This satellite presents unusual topography variations at different scales, as presented by Denk et al. (LPSC 2005 #2268). It can be described as a triaxial ellipsoid with radii 732x726x722 km (Denk et al. LPSC 2005 #2262) and large-scale topography variations of up to 10 km. Moreover, a narrow ridge up to 20-km high and 1300 km long makes Iapetus unique among the icy satellites family. We explore scenarios combining thermal, geodynamic and dynamic conditions suitable for Iapetus to develop these topographic features. We study the hypothesis that in its early history, this body went through a stage of high spin rate during which it acquired a hydrostatic ellipsoidal shape. We explore the scenario that while it was in high rotation speed, the body went through a phase of partial melting due to radiogenic decay and tidal heating. We model the differential buoyancy of diapirs originating in different places. The equator appears to be a preferential place for diapirs to rise to the surface. We investigate the potential connection between the latter result and the formation of Iapetus' ridge. We discuss the evolution of these topographic features with time, resulting in a present non-hydrostatic state of the satellite. This allows us to infer some information on its interior and composition. The above scenarios are also considered in the perspective of models of formation and orbital evolution available from the literature, scenarios proposed for the origin of the dark leading side, and comparison with other Saturnian medium-sized satellites.