TEMPERATURE AND VELOCITY STRUCTURE OF A CONVECTING ICE SHELL ON JUPITER'S SATELLITE, EUROPA

Jupiter's moon Europa has a layer of H₂O-rich material at it surface approximately 100 km thick which may be largely liquid. The thickness of the solid ice shell at Europa’s surface is uncertain, but if it is thicker than ~15 to 20 km, it is expected to convect , due to thermal buoyancy caused by the temperature difference between the ~100 K surface ice and ~260 K basal ice. We are in the process of applying numerical convection models developed for studying the Earth's mantle (Citcom) to convection within Europa's ice shell.

The temperature structure, velocity structure, and stagnant lid thickness within the ice shell are controlled by the rheology of ice. The two creep mechanisms most favored within Europa’s ice shell are basal slip accommodated by grain-boundary sliding (GBS), which is non-Newtonian and has a stress exponent n=1.8, and diffusional flow, which is Newtonian . We approximate the effects of non-Newtonian rheology by using an effective activation energy, Q^*_eff=Q^*/n. We simulate convection within the ice shell using a range of Q^*_eff between the Newtonian approximation to GBS (Q^*_eff> = 25 kJ/mol) and diffusional flow (Q^*_eff = 60 kJ/mol). Our convection modeling confirms that the stagnant lid thickness is proportional to Q^*_eff and varies between 2 and 6 km for the range of Q^*_eff, melting point viscosity (h_o), and grain sizes examined. The average convective velocities range from ~10 to ~0.1 cm yr^-, and the convective interior temperatures range from 180 to 230 K. We find a 20-km-thick ice shell will convect for Q^*_eff < 30 kJ mol^-1 for h_o=10¹⁵ Pa s, and Q^*_eff < 50 kJ mol^-1 for h_o=10¹⁴ Pa s.

Accurate models of convection within the ice shell in the future should include the effects of tidal dissipation and non-ice materials on the rheology of ice (e.g. salts and/or sulfur compounds).