Abiotic synthesis as a new source of organic matter on Enceladus

The Cosmic Dust Analyzer (CDA) instrument onboard the Cassini mission found complex macromolecular organic material in ice grains erupting from the plume of Enceladus [1]. While [1] suggested that these organics could be primordial material processed in Enceladus' core and delivered to the ocean by hydrothermal activity, it has been demonstrated that complex organics can also be synthesized abiotically from the reduction of inorganic carbon species by molecular hydrogen (H₂) during hydrothermal reactions on Earth [2-4]. Here, we use the kerogens described in [5] as analogs for the high-mass organics detected on Enceladus to explore this hypothesis. We generate stability diagrams for kerogens relative to oceanic carbonate species at temperatures from 50°C to 300°C as functions of oxidation state (represented by hydrogen activity, aH₂) and pH. We apply constraints from [6] and [7] on the temperature, oxidation state and pH of the hydrothermal fluid to test whether the kerogens could be formed under these conditions. We find that, within our lower allowed temperature range (T ~ 100°C) , kerogen formation from ∑CO₂ and H₂ is thermodynamically favorable within the entire constrained pH and aH₂ boundaries. As temperature increases, the aH₂ required to form kerogens increases more rapidly than our predicted aH₂ range for Enceladus' interior. Thus, we find that kerogens could only form within a small region of the constrained parameter space at higher temperatures, and would be unlikely to form at all at the high end of our temperature range (T ~ 300°C). We present the implications of our findings for the plausibility of abiotic synthesis of complex organic material in the Enceladus interior, and motivate the need to measure carbon isotopes on Enceladus to further constrain the origin of this material.

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