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R. Greenberg, B. R. Tufts, G. V. Hoppa, P. E. Geissler (Lunar and Planetary Lab, U. Arizona)
Physical characterization of Europa's crust shows it to be rich in potentially habitable niches, with several timescales for change that would allow stability for organisms to prosper and still require and drive evolution and adaptation. Studies of tectonics on Europa indicate that tidal stress causes much of the surface cracking, that cracks penetrate through to liquid water (so the ice must be thin), and that cracks continue to be worked by tidal stress. Thus a global ocean is (or was until recently) well linked to the surface. Daily tidal flow (period ~10-2 yr) transports substances up and down through the active cracks, mixing surface oxidants and fuels (cometary material) with the oceanic reservoir of endogenic and exogenic substances. Organisms moving with the flow or anchored to the walls could exploit the disequilibrium chemistry, and those within a few meters of the surface could photosynthesize. Cracks remain active for at least 104 yr, but deactivate as nonsynchronous rotation moves them to different stress regimes in <106 yr. Thus, to survive, organisms squeezed into the ocean must migrate to new cracks, and those frozen in place must hibernate. Most sites will remelt and release captive critters within ~106 yr based on the prevalence of chaotic terrain, which covers nearly half of Europa. Linkage of the ocean to the surface also could help sustain life in the ocean by delivering oxidants and fuels. Suboceanic volcanism (if any) could provide additional sites and support for life, but is not necessary.
Thus Europa's biosphere (habitable if not inhabited) likely extends from within the ocean up to the surface, with important implications for exploration strategies: The problem becomes not how to drill down to the ocean, but rather how to choose an active (or recently active) landing site where the ocean comes to the surface. Exploration resources need to go into high resolution reconnaissance. Also, with its biosphere reaching the surface, Europa may be highly susceptible to biological contamination.
Moreover, independent of the model of Europa described above, a number of shortcomings in the recent NRC report on contamination of Europa need further consideration before it is used as the basis for planetary protection standards. An objective criterion might be based on comparison with the probability of natural interplanetary contamination.