The impact of a weak south pole on thermal convection in Enceladus' ice shell

Enceladus exhibits a strong hemispheric dichotomy of tectonism and heat flux, with geologically young, heavily tectonized terrains and a high heat flux in the South Polar Terrain (SPT) and relatively ancient terrains with presumably lower heat fluxes over the rest of the satellite. To understand the convective pattern and its relationship with surface tectonics, we present three-dimensional numerical models of convection in Enceladus' ice shell including basal heating and tidal heating. Our thermal boundary conditions exhibit no north-south asymmetries, but because the tectonism at the SPT may weaken the ice there, we impose a mechanically weak lithosphere within the SPT. The weakening is parameterized by adopting a reduced viscosity contrast within the SPT. Without such a weak zone, convection (if any) resides in stagnant-lid mode and exhibits no hemispheric dichotomy. In the presence of such an SPT weak zone, however, we find vigorous convection in the ice underneath the SPT, with convective plumes rising close to the surface. In contrast, only stagnant lid convection, or no convection at all, occurs elsewhere over the satellite. Away from the SPT, the heat flux in our models is small (5-10mWm ^-2) and the surface strains are small enough to imply surface ages >10 ⁹years. Within the SPT, however, our models yield peak heat fluxes of ~70-200mWm ^-2, implying heat flows integrated across the SPT of up to 5GW, similar to that inferred from Cassini thermal observations. The surface strains in our models are high enough near the south pole to cause intense tectonism and imply surface ages of ~10 ⁶-10 ⁷years, consistent with age estimates of the SPT.