10:45 〜 12:15
[MIS20-P06] Investigations of phosphate behavior within subsurface ocean of Enceladus through hydrothermal experiments and geochemical modeling
キーワード：エンセラダス、内部海、リン酸、熱力学平衡計算、熱水実験
Enceladus, Saturnian icy satellite, has CO2-rich alkaline subsurface ocean (e.g., Postberg et al., 2009). Recent geochemical modeling predicts the presence of abundant phosphor in the ocean based on solubilities of phosphates under CO2-rich aqueous conditions (Hao et al., 2022). In addition, re-analysis of the observational data also suggests ~1–10 mmol/kg of phosphate concentrations in the ocean. Such highly abundant phosphate could support the potential activity of life. However, the previous modeling just assumed mineral assemblages controlling the system, rather than considering elemental compositions of rocky components within Enceladus.
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.
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.