Images of Europa’s surface show regions where the ice shell appears to be expanding in a manner similar to mid-ocean spreading ridges on Earth. While this phenomenon has been observed for some years now, the other side of the tectonic equation – subduction – has not been found.  On Earth, subduction is driven by temperature differences (and resultant differences in density). Europa, however, is too cold for such temperature differences to develop.  A new model developed by Brandon Johnson of Brown University and his colleagues show that subduction could occur on Europa.  Instead of temperature, varying amounts of salt and porosity in the ice shell could provide the necessary density differences for a slab to subduct. READ MORE »