Exploring Oxidation Chemistry and Energy Availability in Enceladus' Ocean
Abstract
The presence of molecular hydrogen (H2) in the plume of Enceladus implies that there is free energy available for methanogenesis, the metabolic reaction of H2 with CO2 to form methane and water [1]. While other metabolic pathways could also serve as sources of chemical energy in Enceladus' ocean, they require the availability of oxidants other than CO2. Such oxidants (e.g., O2, H2O2, SO42-, ferric iron as goethite or FeOOH) were not detected in the plume, which motivates modeling to constrain their abundances in the ocean. Here, we present a geochemical model of the ocean based on measurements made by Cassini INMS [1,2], and equilibrium mineralogies of Enceladus' core. We use a model of water ice radiolysis on the surface of Enceladus to estimate the amounts of O2 and H2O2 contained in the ice, and calculate the delivery rate of these oxidants from the surface ice to the ocean using previous estimates of the rate of plume particle deposition on the south polar region [3]. Depending on Enceladus' age and the history of plume activity, we find that over 1016 moles of both O2 and H2O2 could be delivered to the ocean from the ice shell over 4.5 Ga, the upper limit on Enceladus' age. We also consider O2 and H2O2 produced directly in the ocean from radiation chemistry resulting from the decay of dissolved 40K. We find that another 1016 moles of O2 and 1015 moles of H2O2 could be produced through this process. These oxidants could react with sulfides and ferrous iron dissolved in the ocean to produce SO42- and FeOOH, respectively. We calculate upper limits on these constituents based on the equilibrium solubilities of different precursor ocean floor minerals, and find that SO42- and FeOOH abundances greater than 1 mmol/kg H2O and 10 mmol/kg H2O, respectively, could be generated. From the computed oxidant concentrations and production rates, we calculate the amount of chemical energy that could be available from a wide range of alternate metabolic pathways over time, compare them to empirical and theoretical energy requirements for life, and discuss the implications for the potential ecosystems diversity inside Enceladus. [1] Waite et al. (2017) Science, 356, 155-159 [2] Magee and Waite (2017), LPS XLVIII, 2974 [3] Kempf et al. (2010) Icarus., 206, 446-457
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.U14B..15R
- Keywords:
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- 0810 Post-secondary education;
- EDUCATION