Rotating convection with a melting boundary: An application to the icy moons

Abstract

A better understanding of the ice-ocean couplings is required to better characterise the hydrosphere of the icy moons. Using global numerical simulations in spherical geometry, we have investigated here the interplay between rotating convection and a melting boundary. To do so, we have implemented and validated a phase field formulation in the open-source code MagIC. We have conducted a parameter study varying the influence of rotation, the vigour of the convective forcing and the melting temperature. We have evidenced different regimes akin to those already found in previous monophasic models in which the mean axisymmetric ice crust transits from pole-ward thinning to equator-ward thinning with the increase of the rotational constraint on the flow. The derivation of a perturbative model of heat conduction in the ice layer enabled us to relate those mean topographic changes to the underlying latitudinal heat flux variations at the top of the ocean. The phase change has also been found to yield the formation of sizeable non-axisymmetric topography at the solid–liquid interface with a typical size close to that of the convective columns. We have shown that the typical evolution timescale of the interface increases linearly with the crest-to-trough amplitude and quadratically with the mean melt radius. In the case of the largest topographic changes, the convective flows become quasi locked in the topography due to the constructive coupling between convection and ice melting. The tentative extrapolation to the planetary regimes yields $O(10^2-10^3)$ meters for the amplitude of non-axisymmetric topography at the base of the ice layer of Enceladus and $O(10^3-10^4)$ meters for Titan.