10:45 AM - 12:15 PM
[MIS20-P06] Investigations of phosphate behavior within subsurface ocean of Enceladus through hydrothermal experiments and geochemical modeling
Keywords:Enceladus, Subsurface ocean, Phosphate, Thermodynamic equilibrium calculation, Hydrothermal experiments
Here we conducted hydrothermal experiments using CO2-rich solutions and CM-type carbonaceous chondrite as simulants of Enceladus’ seafloor rock. Our results show ~10-2–1 mmol/kg of phosphate concentrations in solutions. Rock samples after experiments show not only precipitation of calcite but also dissolutions of Ca-phosphates and Ca-silicates. High phosphate concentrations would be explained by instabilities of Ca-phosphates due to calcite precipitation under CO2-rich conditions.
In addition, we performed thermodynamic equilibrium calculations to simulate water-rock interactions in Enceladus, including phosphate behavior. We used CI-type carbonaceous chondrite as the rock composition. Our calculations show that several 10% of Ca-phosphates contained in the rock are necessary to dissolve for reproducing phosphate concentrations suggested by the recent re-analysis of observational data. In our calculations, observed phosphate concentrations are achieved in the cases of moderate or low-temperature conditions with ~0.2 mol/kg of ΣCO2 and ~11 of pH. These chemical parameters are partly consistent with those suggested in previous studies for Enceladus (e.g., Fukushi et al., 2020). In especially, all parameters estimated for Enceladus are achieved with ~2.3 mol/kg of total CO2 abundance in the system. This would be lower than the typical abundance in cometary ice, suggesting that CO2 has been removed from the initial abundance. If the CO2 loss has been caused by water plume activities, the durations of plume activities are calculated as ~2–6 Gyrs using observed CO2 flux in water plumes. This suggests that plume activities of Enceladus could have continued through the Solar system history.