The search for current geologic activity on Europa

Observational evidence and theoretical arguments suggest that Jupiter's sat ellite Europa could be geologically active and possess an "ocean" of liquid water beneath its surface at the present time. We have searched for eviden ce of current geologic activity on Europa in the form of active plumes vent ing material above the surface and by comparison of Voyager and Galileo ima ges to look for any changes on the surface. So far, we have observed no plu mes and have detected no definitive changes. The lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 k m(2) y(-1) in the regions analyzed, assuming alterations will cover contigu ous areas of at least 4 km(2) over a period of 20 years. Assuming this as a constant, globally uniform resurfacing rate leads to a minimum average sur face age of 30 million years. We also suggest that the lack of obvious circ ular albedo patterns on the surface due to plumes, coupled with the presenc e of bright-rayed craters such as Pwyll and the predicted sputtering erosio n rate, implies that no large-scale plume activity has taken place over at least the last few thousand years. We thus conclude that if Europa's surfac e is currently active, any changes must be relatively small in spatial scal e or episodic in nature rather than continuous. To detect potential small-s cale surface changes, we need high-resolution comparisons between the Galil eo data and future Europa Orbiter images.