[M-IS20] 水惑星学

[MIS20-P07] セレス初期海洋のアンモニウム濃度復元

徳門 弘都²、*福士 圭介¹ (1.金沢大学環日本海域環境研究センター、2.金沢大学自然科学研究科)

キーワード：セレス、アンモニウム濃度、スメクタイト

Space exploration by the Dawn mission has revealed the presence of NH₄⁺-type smectite on the surface of Ceres. The widespread occurrence of smectites suggests the existence of a global subsurface ocean in early Ceres. The Grand Tack model suggests that large-scale mass transfer may have occurred during the planet formation, when the orbits of giant gas giants such as Jupiter and Saturn were evolved. Ceres, a protoplanetary survivor, is also likely to experienced past large-scale mass transfer, being transported from a distant, extremely cold region to its present location (2.8 A.U.). If we can reconstruct the lower limit level of the ammonium concentration in the early ocean of Ceres, the concentration would be an indicator of whether early Ceres was outside the ammonium snow line (8AU). A geochemical model has been developed for the quantitative reconstruction of water chemistry in ancient natural liquid water on Mars (Fukushi et al., 2019 and 2022). According to this model, it is possible to estimate the cation concentrations in ancient liquid water from the cation composition in smectites via the selectivity coefficient of the relevant cations. However, the smectite Na⁺-NH₄⁺ selectivity coefficient, which is essential for reconstructing water quality in the early ocean on Ceres, has not yet been estimated. In this study, the Na⁺-NH₄⁺selectivity coefficient of smectites was measured experimentally. Using the newly estimated selectivity coefficient, we revisit the NH₄⁺ concentrations in the early ocean on Ceres and discuss the early Ceres formation region.