GEOLOGICAL EVIDENCE FOR SOLID-STATE CONVECTION IN EUROPAS ICE SHELL

The ice-rich surface of the jovian satellite Europa is sparsely crater ed, suggesting that this moon might be geologically active today(1). M oreover, models of the satellite's interior indicate that tidal intera ctions with Jupiter might produce enough heat to maintain a subsurface liquid water layer(2-5). But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Euro pa has (or had at one time) a liquid water ocean(6,7). Here we report on the morphology and geological interpretation of distinct surface fe atures-pits, domes and spots-discovered in high-resolution images of E uropa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ic e masses that have risen buoyantly through the subsurface. We find tha t the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liqui d water layer. Our results are consistent with the possibility that Eu ropa has a liquid water ocean beneath a surface layer of ice, but furt her tests and observations are needed to demonstrate this conclusively .