Abiotic and Biotic Formation of Amino Acids in the Enceladus Ocean.

The active plume at Enceladus' south pole makes the indirect sampling of its global ocean possible. The partially resolved chemistry of the plume, which points to conditions that are seemingly compatible with life, has made orbital sampling missions a priority. We present a conceptual model of energy flux, hydrothermal H2 production, and both abiotic and biotic production of amino acids. Based on the energy flux observed at the south pole and the inferred internal hydrothermal activity, we estimate an H2 production of 0.6-34 mol/s from serpentinization, sufficient to sustain abiotic and biotic amino acid synthesis of 1.6-87 and 1-44 g/s, respectively. Two-dimensional (2D) numerical simulations of the hydrothermal vent suggest that the vent fluids could reach the ice-water boundary in less than 11-55 days for a 50 km deep ocean diluted by ambient ocean water 10 to 1. Concentrations of glycine, alanine, α-amino isobutyric acid, and glutamic acid in the plume and in the ambient ocean could all be above 0.01 μM just due to abiotic production. Biological synthesis, if occurring, could produce a maximum of 90 μM concentrations of amino acids based on a methanogenic ecosystem consuming H2 and CO2. Racemization timescales in the ocean are short compared with production timescales. Thus, no enantiomeric excess is expected in the ambient ocean, and if biology is present, enantiomeric excess at the vent fluids is expected to be less than 10% in the plume. From vent H2 concentrations of 7.8 mM (e.g., Lost City) and assuming complete H2 use and conversion to chemical energy by methanogens, cell production is estimated. Annual biomass production in the methanogenic-based biology model is 4 × 104-2 × 106 kg/year. This corresponds to cell concentrations ∼109 cells/cm3 in the vents and ∼108 cells/cm3 in the plume, and when diluted into the ambient ocean, we predict cell concentrations of 80-4250 cells/cm3. Key Words: Abiotic organic synthesis-Enceladus-Extraterrestrial life. Astrobiology 17, 862-875.