Convective Instabilities in Europa's Floating Ice Shell

W.B. McKinnon (Dept EPSc, Wash Univ)

Models of the tidally heated ice shell proposed by Ojakangas and Stevenson for Europa generally find shell thicknesses less than 30 km. Past parameterized convection models indicate that these shell thicknesses are stable to convective overturn and should not lead to freezing of the ocean underneath. Here I apply the temperature- dependent viscosity convection scaling developed by Solomatov to the Europan ice shell. This scaling applies to basally heated square boxes with free-slip boundaries, which should be a good match to the Europan situation. The temperature-dependent properties of ice are linearized about 250 K, as any convective interior should be close to this temperature, with the colder ice forming an ostensibly passive, stagnant lid. Ice shells greater than tex2html_wrap_inline12 20 km (at the equator) are found to be unstable to convection at their base, for melting point viscosities of 10 tex2html_wrap_inline14 Pa-s. The critical shell thickness for convective onset is greater at the poles; it also depends on viscosity near the melting point, and so by the latest rheological laws, on grain size. Convection at the base of the ice shell does not freeze the ocean, however. Because of tidal heating, a "stagnant lid regime" ice shell is much more dissipative than a conductive shell of the same thickness. This causes a shell in which convection occurs to be thinner than a conductive shell undergoing the same tidal strain. The overall effect is to moderate the second-degree shell thickness variations found by Ojakangas and Stevenson (unless the shell is sufficiently thin to begin with), and impose an effective upper limit on the shell thickness. Tidal heating in the convecting base of the shell is non-uniform, which may lead to thermal instabilities, and the maximum stresses in the viscously creeping lid may be several 0.1 MPa, which could resolve as drag on the cold, elastic lithosphere. This research supported by NASA Planetary Geology and Geophysics grant NAG5- 3657.