Abstract
Galileo spacecraft data obtained from Jupiter's ice-covered satellite
Europa suggest the existence of a subsurface water ocean. The formation
of this ocean is a direct consequence of early igneous and hydrothermal
processes that may have facilitated oxidation through H-2 and CH4
escape. These processes could have led to a moderately alkaline sulfate-carbonate
ocean and a moderately oxidized magnetite-bearing silicate mantle.
When the ocean formed, low-temperature chemical disequilibria involving
oceanic sulfate, bicarbonate, and ferrous minerals in underlying
rocks could have provided multiple energy sources for chemotrophic
organisms. Potential metabolic processes include oxidation of ferrous
iron, sulfides, native sulfur, methane, hydrogen, and organic compounds,
as well as reduction of water, sulfate, bicarbonate and carbonate
ions (methanogenesis and acetogenesis), native sulfur, and ferric
iron. If they occurred, these reactions would have provided close
coupling for biogeochemical cycles of S, C, and Fe in the ocean.
Periodic supplies of fresh rocks and/or aqueous fluids at the oceanic
floor could drive these cycles throughout the satellite's history,
including the present epoch. Signs of chemical disequilibria in the
ocean would indicate chemical sources of energy for metabolism but
should not be considered as indicators of life. In turn, observational
signs of low-temperature redox equilibration among sulfur and carbon
species would be suggestive of ancient life.
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