Europa’s ice shell thickness : estimates from thermal evolution models

Europa’s radial structure consists of a thin ice I_h shell overlying a subsurface ocean and a large solid core. The thickness of the outer ice shell is strongly related to Europa’s thermal and geological histories. Estimates of this thickness range from a few kilometers to several tens of kilometers, with values deduced from the analysis of surface geological features being on the lower end (a few kilometers and less), and values predicted by modelling thermal evolution being on the upper end (up to a few tens of kilometers). Here, we model the thermal evolution of Europa's ice shell using a parameterized convection approach that explicitly accounts for the release of heat (assumed to have a tidal origin) within this shell. We explore changes in the thickness of this ice shell depending on several parameters, including the bulk ice viscosity, amount of tidal heating, and ocean composition. We further consider possible cyclical variations in the amount of tidal heating in response to changes in the eccentricity of Europa’s orbit. Our calculations show that ice shell thickness is mostly influenced by both the ice bulk viscosity and tidal heating. While significant in absence of tidal heating, the ocean composition has no or few influence when tidal heating is accounted for. Interestingly, for dissipated tidal heat and viscosity around 1 TW and 10¹⁴ Pa·s, respectively, which are within the expected range of values for these parameters, our calculations predict an ice shell thickness in the range 15-45 km and, at the top of this shell, and a stagnant lid around 10 km in thickness, in agreement with recent estimates from impact basin morphology. Our calculations further indicate that a 10% change in orbital eccentricity may trigger variations in the ice shell thickness of approximately 15 km, which further helps to reconcile estimates based on geological features and modelled thermal history.