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W.B. McKinnon (Washington Univ., St. Louis)
It has been recently been argued by Ruiz (Nature 412, 409-411) that the dominant non-Newtonian creep mechanisms of water ice make the ice shell above Callisto's ocean stable against solid-state convective overturn, based on the convective scaling relationships of V.S. Solomatov. The argument is actually broader, and applies to all radiogenically heated ice I shells in the outer solar system, and by extrapolation to midsized icy satellites as well. Conductive heat transport and internal melting are predicted to be, or have been, widespread in the outer solar system. Convection, and the tectonics that result, would then only occur in ice I shells whose viscosities are lowered by tidal flexing, e.g., Europa (McKinnon, GRL 26, 951-954), Ganymede, or Triton. This analysis is, while correct, incomplete. At low stresses, where non-Newtonian viscosities can be arbitrarily large, convective instabilities may arise in the diffusional creep regime. Lattice diffusion creep has not been directly observed in water ice, but measurements of diffusion coefficients imply Newtonian viscosities low enough (for warm enough ice) that convection is expected above Callisto's internal liquid layer for plausible ice grain sizes (less than 5 mm). Convective heat flows will exceed steady-state radiogenic values unless the convective adiabat is cooler than the minimum melting temperature (251 K). In convective equilibrium, Callisto's stagnant lid is quite thick (about 100 km), and compatible with the lack of active geology, but the existence of the ocean requires an antifreeze.