Heat flows through the ice lithosphere of Europa

Some of the gec,logical features on the surface of Europa's ice shell sugge st the existence of a brittle lithosphere that in some regions is no more t han 2 km thick. The surface heat flow needed to put the brittle-ductile tra nsition in ice at a depth of 2 km is at least similar to 100 mW m(-2), much higher than predicted by tidal heating models for a purely conductive ice shell. A possible explanation lies in the probable existence of a convectiv e layer beneath the brittle lithosphere, which, owing to tidal dissipation in warm ice, could contribute to higher heat flows. If a convective subsurf ace layer and grain size dependent flow behavior of ice are admitted, the g rain size at brittle-ductile transition depth cannot be >1 mm, which is con sistent with the required value for the onset of convection in an ice shell thinner than 20 km.