Some of the geological features on the surface of Europa's ice shell suggest the existence of a brittle lithosphere that in some regions is no more than 2 km thick. The surface heat flow needed to put the brittle-ductile transition in ice at a depth of 2 km is at least ~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 convective layer beneath the brittle lithosphere, which, owing to tidal dissipation in warm ice, could contribute to higher heat flows. If a convective subsurface layer and grain size dependent flow behavior of ice are admitted, the grain size at brittle-ductile transition depth cannot be >1 mm, which is consistent with the required value for the onset of convection in an ice shell thinner than 20 km. ¿ 2000 American Geophysical Union