Thermal stress tectonics on the satellites of Saturn and Uranus

Abstract

Most of the major Saturnian and Uranian satellites have undergone some form of extensional tectonism and/or resurfacing early in their histories. We examine the stresses created by the early thermal evolution of these satellites and consider the geologic implications of these stresses. Starting from an initial accretional temperature profile, we calculate the satellites' temperature profiles as they evolve by conduction. Solid state convection, when it occurs, is handled using a simple parameterized convection scheme. The effects of radiogenic heating and release of heat of phase change are included. The thermal stresses built up in the satellites are then calculated, assuming the satellites to have a Maxwell viscoelastic rheology. The stress calculation takes into account both the effects of thermal stresses and of shifts in the position of the ice I/ice II phase boundary in some satellites. If accretion takes place in a cold, gas‐free environment, the stresses that build up in the smaller satellites (Mimas, Miranda, and Enceladus) are minor. In the larger satellites, however, stresses are large enough to cause fracturing and extensional tectonism under these circumstances. The stresses result largely from rapid cooling contraction of a thin outer shell; the ice II → ice I phase transition also contributes significantly for the largest satellites. If accretion occurs in a warm gaseous nebula that dissipates rapidly, stresses large enough to cause failure are attained in all of the satellites. In all cases, the material immediately underlying the zone of failure is the warmest in the satellite and hence may be buoyant and mobile enough to be extruded to the surface through fractures created by the extension. We conclude that thermal stresses following accretion led to a substantial early episode of extensional tectonism and perhaps also associated resurfacing on many of the Saturnian and Uranian satellites. If evidence of this early episode of extensional tectonism has survived subsequent impacts, then the process may have been responsible for much of the early tectonism and resurfacing observed on these satellites. If not, then it may only be responsible for the very early global resurfacing that some of the satellites are inferred to have undergone.