Abstract
Within the parent body of the asteroid Ryugu, it is estimated that heating caused the melting of water ice, which then reacted with reducing materials to create an alkaline and reductive aqueous environment. Abiotically synthesized organic matter can be the origin of the organic matter on Earth. To estimate the water environment during aqueous alteration in parent body of Ryugu, we measured the X-ray Absorption Near Edge Structure (XANES) of chromium (Cr), titanium (Ti), vanadium (V), arsenic (As), iron (Fe), and sulfur (S) in samples from Ryugu and the CI chondrite Orgueil meteorite. We compared these results with thermodynamically calculated Eh-pH diagrams. In addition, we examined valence changes of these elements due to atmospheric oxidation by comparing them with the Orgueil meteorite results. The findings indicated that Cr was predominantly in the Cr(III) state across the Ryugu samples, with some Cr(II) also observed. Ti was mainly present as Ti(IV), distributed between ilmenite and phyllosilicates. As was primarily in the As(V) state in phyllosilicates and in more reduced species in phosphate-rich areas. V was uniformly distributed as V(III) regardless of mineral structure. These valence states suggest that Ryugu's parent body may have experienced a highly reducing environment where water decomposed to produce hydrogen.
