Numerical simulations of convection in Europa's ice shell: Implications for surface features

Europa's icy surface displays numerous small (5- to 30-km-diameter) pits, spots, and uplifts that have been suggested to result from convection in the ice shell. To test this hypothesis, we present numerical simulations of convection in Europa's ice shell, including temperature-dependent viscosity and tidal heating. Ice shells 15 and 50 km thick are considered, consistent with several estimates of the shell thickness on Europa. The convection produces deep pits (consistent with some of the observed features) when the lithospheric viscosity is 10³-10⁵ times greater than that of the underlying ice, but greater viscosity contrasts lead to topography insufficient to explain the observed pits. If ductile creep is the only deformation mechanism, these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 10¹⁰. However, the strength of Europa's surface may be low enough for plastic deformation to play a role in the convection, opening the possibility that convection could produce some of Europa's pits. For plausible viscosities (10¹³ Pa s at the melting temperature), the pits are 100-300 m deep and 10-20 km in diameter; greater or lesser viscosities lead to wider or narrower pits, respectively. None of our simulations produced isolated uplifts of any diameter, however, so these probably formed by another mechanism. The convection can induce surface stresses >1 bar, which exceeds the inferred strength of Europa's crust and indicates the likelihood of surface disruption. The maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0.05-0.07 W m^-2.