Does Europa have a subsurface ocean? Evaluation of the geological evidence

It has been proposed that Jupiter's satellite Europa currently possesses a global subsurface ocean of liquid water. Galileo gravity data verify that t he satellite is differentiated into an outer H2O layer about 100 km thick b ut cannot determine the current physical state of this layer (liquid or sol id). Here we summarize the geological evidence regarding an extant subsurfa ce ocean, concentrating on Galileo imaging data. We describe and assess nin e pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, surface frosts, top ography, and global tectonics. An internal ocean would be a simple and comp rehensive explanation for a broad range of observations; however, we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm, soft ice with only localized or partial melting. Two different models of impact flux imply very different surface ages for Euro pa; the model favored here indicates an average age of similar to 50 Myr. S earches for evidence of current geological activity on Europa, such as plum es or surface changes, have yielded negative results to date. The current e xistence of a global subsurface ocean, while attractive in explaining the o bservations, remains inconclusive. Future geophysical measurements are esse ntial to determine conclusively whether or not there is a liquid water ocea n within Europa today.