Europa
Chris Chyba (Princeton U.)
There is very strong evidence from measurements of Europa's gravity field that the outer ~100 km of Europa's interior is water. There is strong evidence (theoretical and observational, but especially from magnetometer results) that most of this layer is in the liquid phase. Combining magnetometer results with simple spherical interior models implies that the ice is thin (less than 10 km in thickness) and that the underlying ocean is very saline. Europa's likely bulk composition suggests that so-called biogenic elements should be abundant. Important remaining questions include the availability and cycling of these elements, the availability of energy to power life, and the presence of conditions that might be conducive to the origin of life. Models of radiation-driven chemistry in the surface ice suggest that biologically useful energy should be available. As is the case for Mars, preventing the forward contamination of Europa by terrestrial microorganisms carried on spacecraft should be an important priority in future exploration.