Thermal And Dynamical Histories Of Saturn's Satellites: Evidence For The Presence Of Short Lived Radioactive Isotopes

Abstract

Recent observations of Saturn's satellite system from the Cassini/Huygens mission present serious challenges to understanding the current dynamical states and thermal histories of these icy bodies using conventional thermal models that use long lived radioactive isotopes (LLRI) as the primary heat sources. In particular, the most distant of the regular satellites, Iapetus, is in synchronous rotation about Saturn, implying relatively high levels of dissipation of tidal energy. However, it retains a highly non‐equilibrium, oblate spheroid, shape, implying a thick, cold, mechanically rigid outer layer or lithosphere. Thermal history models of Iapetus that successfully explain these apparently contradictory characteristics require significant heating early in the satellite's history from short lived radioactive isotopes (SRLI), particularly Al, implying a formation time for Iapetus of between 2.5 and 5 Myr after the formation of Calcium26 Aluminum Inclusions (CAIs) [l]. The characteristics of the other icy satellites in the system are consistent with this formation time, and the current thermal geyser activity on the more silicate‐rich satellite Enceladus may be related to such an early heating event. A consequence of these early formation time models is that the early crust of Iapetus is too thin and weak to retain large impact basin topography until about 100 Myr after formation, and despinning to synchronous rotation might have occurred from 200‐900 Myr after formation. This chronology is consistent with the formation of the large impact basins observed on Iapetus' surface by the ‘late heavy bombardment’ or ‘lunar cataclysm’ event recorded in the dating of samples from the Moon at 3900 Ma.