Convective instability in Europa's floating ice shell

Models of the tidally heated, floating ice shell proposed for the jovian sa tellite Europa generally find shell thicknesses less than 30 km. Past param eterized convection models indicated that such shells are stable against co nvective overturn, which otherwise ostensibly leads to freezing of the ocea n underneath. Here I apply the temperature-dependent viscosity convection s caling developed by Solomatov and coworkers to the Europan ice shell. The t emperature-dependent properties of ice are linearized about 260 K, as any c onvective interior should be close to this temperature, with the colder ice forming an essentially passive, stagnant lid. Ice shells greater than or s imilar to 10 km thick are found to be unstable to convection at their base for melting-point viscosities of 10(13) Pa-s (as linearized by tidal stress es), if the ice deforms by superplastic creep, but such low viscosities req uire small grain sizes (<1 mm). This requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain-rate scaled to Europa. Regardless, convection at the base of the ice shell, if initiated, may not freeze the ocean. Because of tidal heating, a stagnant-lid regime ice shel l is much more dissipative than a conductive shell of the same thickness. S uch a shell should thin, not thicken, and the potential exists for further thermal instabilities and runaways.