Reconstruction of aqueous environment of the Ryugu's parent boby using synchrotron radiation X-ray analysis

Abstract

We analyzed the chemical species of each element in the Hayabusa2 return sample by X-ray absorption near edge structure (XANES) analysis. In addition, the major minerals of Ryugu were analyzed by EPMA elemental analysis, and the composition of phyllosilicate was analyzed. Based on these results, chemical equilibrium calculations of the cation selectivity coefficients and solubilities between the phyllosilicate layers were performed to estimate the aqueous environment of Ryugu's parent body, especially the redox potential (Eh) and pH. As a result, Ryugu's water is estimated to be highly reductive (Eh = -0.5~-0.6V) and basic (pH = 10~11). The valence of trace elements such as vanadium and chromium were also measured and found to be consistent with these results. The estimated Eh values are reductive to the extent that water is reduced and produces hydrogen gas at each pH, suggesting that Ryugu's water may have been lost not only through ice sublimation and mineral hydration, but also through water-reduction reactions.