[M-IS14] Aqua planetology

[MIS14-P19] Selectivity coefficient of Na⁺-NH₄⁺ exchange in saponite interlayer: Reconstruction of ammonium concentration in early ocean on Ceres

*Hiroto Tokumon¹, Keisuke Fukushi², Yasuhito Sekine³, Kosuke Inoue¹, Koki Morida¹ (1.Kanazawa University , 2.Institute of Nature and Environmental Technology Kanazawa University, 3.Tokyo institute of technology )

Keywords:Saponite, Selectivity coefficient, Ceres

Recent space explorations have revealed the existence of a subsurface ocean in early Ceres due to the melting of ice components by the decay heat of short-lived radionuclides. NASA's Dawn survey has also revealed the existence of ammonium-type saponite containing ammonium ions in the interlayer on Ceres. Recent theories of solar system formation as known as the Grand Tuck Model (Walsh et al., 2011) suggest that during the planet-forming period when protoplanets existed, large-scale mass transfer may have occurred as the orbits of gas giant planets such as Jupiter and Saturn evolved. Ceres may have been transported from a distant location to its present position by the migration of these gas giants. If we can estimate the ammonium concentration in the early ocean on Ceres, we can determine whether the location of early Ceres was outside of the ammonia snow line or not. Currently, a geochemical model has been developed that can reconstruct the chemical properties of water with the clay mineral smectite. The model can estimate the cation concentrations in ancient liquid water from the cation composition in smectite via selectivity coefficients of relevant cation. However, the Na⁺-NH₄⁺ selection coefficient of saponite, which is essential to reconstruct the water quality of the early Ceres’ ocean, has not yet been estimated. In this study, we aim to quantitatively estimate the Na⁺-NH₄⁺ selection coefficient of saponite. Furthermore, from the obtained selection coefficients, we re-examine the water quality of the early Ceres’ ocean as reported by De Sanctis et al. (2020).