EUROPA

Europa's icy crust records active and rapid resurfacing by tectonic and thermal processes, such that the current surface is <10⁸ yrs old, as shown by the paucity of craters. The resurfacing is driven by tides, which cause internal heating, surface stress, rotational torque and orbital evolution. Tidal effects (besides maintaining the subsurface ocean) can explain the formation of observed tectonic crack patterns, ridges, crustal displacement, and chaotic terrain, largely by processes involving connections between the surface and the underlying ocean through cracks, melt sites, and occasional impacts. An older notion that the ice is too thick for such connections (isolating the ocean from surface oxidants) motivated considerable speculation about life associated with imagined undersea volcanic vents, for which there is no observational evidence. However, the permeability of the observed icy crust allows exchange of materials, including radiogenic oxidants and exogenic organics from the surface and endogenic substances from the ocean. Calculations of plausible transport rates now show that oxidants may be plentiful in the ocean, and could support a substantial biomass. Moreover, the calculations show a billion-yr delay time before the oxygen reached the ocean, providing (as on Earth) an anaerobic interval needed for initial formation of biological systems and structures. The crust itself may provide viable ecological niches as well as channels for transport of oxidants for deep ocean life. Continual changes in environmental conditions in the ice crust, such as deactivation of individual cracks after thousands of years (due to non-synchronous rotation), effects of long-term orbital changes, and crustal thawing (releasing any trapped organisms), could provide drivers for biological adaptation, as well as opportunity for evolution. The permeable crust means that a habitable biosphere might extend to within a few centimeters of the surface, making Europan life accessible at the surface, without a need to drill through kms of crust, and also vulnerable to contamination.