  The Cassini mission to the Saturn system discovered a plume of ice grains and
water vapor erupting from cracks on the icy surface of the satellite Enceladus.
This moon has a global ocean in contact with a rocky core beneath its icy
exterior, making it a promising location to search for evidence of
extraterrestrial life in the solar system. The previous detection of H$_2$ in
the plume indicates that there is free energy available for methanogenesis, the
metabolic reaction of H$_2$ with CO$_2$ to form methane and water. Additional
metabolic pathways could provide sources of energy in Enceladus' ocean, but
require the use of other oxidants that have not been detected in the plume.
Here, we perform chemical modeling to determine how the production of
radiolytic O$_2$ and H$_2$O$_2$, and abiotic redox chemistry in the ocean and
rocky core, contribute to chemical disequilibria that could support metabolic
processes in Enceladus' ocean. We consider three possible cases for ocean redox
chemistry: Case I in which reductants are not present in appreciable amounts
and oxidants accumulate over time, and Cases II and III in which aqueous
reductants or seafloor minerals, respectively, convert O$_2$ and H$_2$O$_2$ to
SO$_4^{2-}$ and ferric oxyhydroxides. We calculate the upper limits on the
concentrations of oxidants and chemical energy available for metabolic
reactions in all three cases, neglecting additional abiotic reactions. For all
three cases, we find that many aerobic and anaerobic metabolic reactions used
by microbes on Earth could meet the minimum free energy threshold required for
terrestrial life to convert ADP to ATP, as well as sustain positive cell
density values within the Enceladus seafloor and/or ocean. These findings
indicate that oxidant production and oxidation chemistry could contribute to
supporting possible life and a metabolically diverse microbial community on
Enceladus.
