Ocean underpressure, subsurface boiling, and the upward transport of water on icy moons

Variations in orbital parameters can change the total amount and spatial distribution of tidal heating within icy satellites, leading to changes in ice shell thickness. These thickness changes are accommodated by the melting and solidification of ice at the ocean/ice interface. During the thickening phase of ice shell evolution, the volumetric change as water freezes into ice, in combination with the volumetric shrinkage of a cooling ice shell, generates overpressure within the subsurface ocean and extensional stresses at the satellite’s surface. During the thinning phase of ice shell evolution, the opposite process may occur, with large compressional stresses generated within the cooling ice shell and underpressure within the subsurface ocean. Fracture penetration, ocean pressurization, and eruptions associated with thickening ice shells have been explored for Europa, Enceladus, and Mimas. However, much less work has been done to understand the behavior of the ice-ocean system when the ice shell thins. Here, we use analytic and numerical models of ice shell evolution to compute the conditions within ice shells and subsurface oceans during the thinning phase of ice shell evolution. We map the conditions under which subsurface oceans may develop underpressure sufficient to initiate decompression boiling and we discuss possible upward transport mechanisms for the vapor generated by this process. We also discuss the implications of our model for the interpretation of compressional tectonic features associated with the stresses generated within thinning ice shells.