Along with liquid water and suitable chemical elements, life requires a source of free energy. Photosynthesis would be extremely constrained by Europa's ice cover (50). Gaidos et al. (51) argue that because of this, most metabolic pathways operating on Earth would be denied to putative europan organisms. Methanogenesis at hydrothermal vents at the bottom of Europa's 100 km-deep ocean could supply similar amounts of energy to that which supports ecosystems at terrestrial vents, although the potential annual biomass production would be 108-109 times below terrestrial primary production based on photosynthesis (52). It is also possible that niches might exist within Europa's ice shell where transient near-surface liquid water environments could permit photosynthesis or other metabolic processes (41, 53).