A subsurface "ocean" of liquid water on Europa was suggested in the early 1970s (26), and further considered subsequent to the Voyager spacecraft flybys (27). The ground-based spectroscopic signature of Europa is dominated by water ice (28). The paucity of craters on Europa's surface, combined with estimates of the impact flux, suggest a geological resurfacing timescale 10 million years (29, 30). Galileo spacecraft gravity measurements indicate that Europa has a combined ice/liquid water shell 80-170 km thick overlying a metallic and rocky core and mantle (31, 32). Models indicate sufficient geothermal and tidal heating to maintain much of the ice shell as liquid water beneath an outer ice layer 10 km thick (26, 27, 33, 34).
High-resolution images of Europa seem consistent with this picture (35). The orientation and relative age relationships of lineaments is consistent with nonsynchronous rotation of an ice shell decoupled from a synchronously rotating interior by liquid water or ductile ice (36). There are regions of chaotic terrain, where broken pieces of the surface seem to have "rafted" into new positions (35, 37, 38), cracks and extensional bands, which likely were filled in with new, fluid material (39), and cycloidal cracking explicable in terms of changing diurnal stress (40). Such features could have been formed in a thin (1 km thick) frozen crustal layer overlaying liquid water (41), but solid-state formation mechanisms also have been suggested. The latter typically involve diapirism within a thick (tens of kilometers thick) ice shell, possibly including bodies of melt or partial melt, overlying a liquid water ocean (35, 42-44).
Perhaps the most compelling evidence for a subsurface liquid water layer on Europa comes from magnetic field results (45) that show the signal of an induced field. This field requires a near-surface global conducting layer, for which the most probable explanation is a salty ocean. All of this evidence, however, remains indirect in nature (46). A definitive answer must await the arrival of the Europa Orbiter spacecraft.
The abundance of most biogenic elements on Europa is not known. It is common to assume Europa's composition to be that of a carbonaceous chondrite meteorite (47), in which case biogenic elements would be abundant. Little is known observationally. Spectral evidence reveals certain organic functional groups (C---H, CN) on Jupiter's moons Ganymede and Callisto, and hints at their presence on Europa (48). Comet impacts over solar system history should have provided Europa with a supply of biogenic elements irrespective of its initial inventory. If comets have typical densities of 1 g·cm3, the quantity of biogenic elements accreted by Europa over 4 Gyr is quite substantial (49). However, more material would be lost in impact ejecta if comets are highly porous objects, and cometary porosity is poorly constrained.