Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa

Geochemical models are used to explore the possibility that lithoautotrophic methanogenesis (the conversion of CO₂ plus H₂ to methane) could be a source of metabolically useful chemical energy for the production of biomass at putative European hydrothermal systems. Two cases are explored: a relatively reduced methane-rich ocean and a relatively oxidized sulfate-and bicarbonate-rich ocean. In the case of a methane-rich ocean, a source of CO₂ for methanogenesis is provided by conversion of dissolved methane to CO₂ during reaction of ocean water with igneous rocks at high temperatures in the subsurface. Fluid-rock reactions also provide a source of dissolved H₂ in the hydrothermal fluid. When this fluid circulates back to the ocean floor and mixes with seawater, conversion of the dissolved CO₂ and H₂ to methane provides a potential source of chemical energy that can be used to drive metabolic processes. For the case of a sulfate- and carbonate-rich ocean, reaction with reduced igneous rocks at high temperatures will also produce hydrothermal fluids with high H₂ concentrations (as occurs in hydrothermal systems on Earth). Mixing of the resulting hydrothermal fluid with seawater in a relatively oxidized ocean could supply energy from either methanogenesis or sulfate reduction. For plausible compositions of a European ocean, methanogenesis can supply similar amounts of energy to that which supports the prolific ecosystems surrounding submarine hydrothermal vents on Earth. Even in the most optimistic case, however, the total amount of biomass that could be supported globally by lithoautotrophic microbes on Europa is extremely small compared to the biomass produced photosynthetically on Earth. Nevertheless, sufficient metabolic energy could apparently be available at hydrothermal systems on Europa to support an origin of life and localized ecosystems.