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C.F. Chyba (SETI Institute and Stanford University)
Prospects for life in the possible subsurface ocean of Jupiter’s moon Europa depend on sources of free energy. But these may be hard to find on a perpetually ice-covered world (Gaidos, Nealson, and Kirschvink, Science 284, 1631-1632, 1999). However, both simple organics and oxidants should be synthesized in the uppermost layer of Europa’s ice crust via disequilibrium radiation chemistry. If melt-through events then occur, such as those that may have created Conamara Chaos and other similar (though in many cases much smaller) regions on Europa, these molecules would be mixed into the ocean where they could serve as the redox couples to drive a europan biosphere. Quantitative estimates based on terrestrial analogues suggest that over one billion grams of microbial biomass could bloom in a single Conamara-scale event. Life on Europa would migrate from melt-through event to melt-through event, somewhat analogous to the way life moves among hydrothermal vents on Earth as old vents shut down and new ones arise. In the europan case, however, sources of disequilibrium redox reaction couples would lie at the top of the ocean, rather than (or perhaps in addition to) its bottom. Candidate energy sources such as these can be no more than plausibility arguments, but they emphasize the importance of in situ exploration to investigate the question of life on Europa.