Hydrothermal Geochemistry as the Source of Plume Gases on Enceladus: a Thermodynamic Evaluation

Introduction: The Cassini spacecraft detected a gas plume emanating from Saturn’s icy satellite Enceladus [13]. Its composition (91% H2O, 3.2% CO2, 4% N2, and 1.6% CH4) was inferred based on measurements by the Ion and Neutral Mass Spectrometer [2]. Jets from warm (110-160 K) fissures (“sulci”) supply the plume [3], and boiling water in a subsurface reservoir may power the jets [1]. Liquid water is possible because radiogenic and tidal heating might be sufficient to prevent freezing [4]. Terrains near sulci are composed of water and CO2 ices mixed with light organic compounds [5]. Here, we consider aqueous geochemistry in an attempt to interpret the plume’s composition. Conceptual model: Enceladus accreted from ices and rocky materials. After differentiation [6], Enceladus began cooling as it radiated heat to space. Presumably, a water ocean was present above a silicate core, and hydrothermal systems developed at the water-rock interface due to high temperatures generated by short-lived radionuclides [7,8]. Reactions between iron-bearing minerals and water governed redox conditions by controlling the fugacity of hydrogen (fH2), influencing abundances of oxidized and reduced species. High-temperature reactions favored oxidation, while low gravity allowed H2 escape [9], irreversibly oxidizing rocks in hydrothermal environments. It seems likely that Saturn’s moons acquired carbon and possibly nitrogen from organic compounds [10,11], thus hydrothermal systems were sites where carbonaceous matter was broken down [12]. Outside hydrothermal systems, cold oceanic water mixed with hot fluids, quenching redox reactions. The plume’s composition could represent ocean chemistry, which may reflect hydrothermal geochemistry. That is, subsurface waters may be envisioned as quenched hydrothermal fluids from the past. Thermodynamic model: We constrained the oxidation state inside Enceladus by performing equilibrium calculations using the SUPCRT92 code [13]. Temperature-redox conditions that match the plume’s CO2/CH4 ratio were evaluated for environments near the water-rock boundary (P = 100 bar). Equations for CH4-CO2 equilibrium are: