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C. Sotin, J. Castillo, F. Deschamps, O. Grasset, A. Mocquet (Universite de Nantes)
This study investigates the efficiency of heat transfer by subsolidus convection within the outer ice shell of Europa. Models are set up based on recent numerical models of convection for fluids with strongly temperature dependent viscosity. It is shown that the temperature across the lower thermal boundary layer is fixed by the characteristics of the ice viscous law. With common values of the activation energy and pre-exponential constant, the temperature difference across the lower boundary is around 10 K. It is also shown that the rate of freezing of the internal ocean depends on its composition since the presence of salts may decrease a lot the freezing temperature. The amount of heat which can be transferred through the convective ice I shell is determined using recent scaling laws and it is compared to the radiogenic heating in the core. Models describing the thermal history of the planet allows us to calculate models of the internal structure which are used to determine the gravitational and tidal parameters. Tidal heating due to the eccentricity of Europa may play an important role since it is mainly produced in the convective ice I layer if the viscosity is lower than 1014 Pa.s, a value which seems larger than the value predicted by scaling laws for temperature dependent viscous fluids and laboratory experiments on the creep behavior of ice. The efficiency of heat transfer is also compared to the amount of tidal heating. Predictions concerning the accuracy of gravity measurements for future Europa mission to assess the presence of a deep ocean and to estimate the thickness of the overlying icy crust are also reported and it is shown that the combination of gravity coefficients and shape of the planet are very complementary to assess the presence of a deep ocean.