リュウグウのナトリウムと塩素は、塩ではなく吸着態で存在 ―アミノ酸などの生体物質の合成や地球への水供給への示唆―

2026-10-16 東京大学

リュウグウ試料研磨片の反射電子像と透過電子像
（Po: ピロータイト; Do: ドロマイト; Mag: マグネタイト; Phys: 層状ケイ酸塩）

＜関連情報＞

• https://www.s.u-tokyo.ac.jp/ja/press/11258/
• https://www.nature.com/articles/s41467-026-78348-5

複数の元素の化学形態分析によるリュウグウ母天体の水環境の再構築 Reconstructing the aqueous environment of the Ryugu parent body through the speciation of multiple elements

Takahiro Kawai, Keisuke Fukushi, Tomohiro Ohno, Hiroki Suga, Masayuki Uesugi, Ayu Takemoto, Akiko Yamaguchi, Takaaki Itai, Yuta Ijichi, Shohei Yamashita, Yasuhiro Niwa, Ryoichi Nakada, Mizuho Koike, Hideto Yoshida, Masaki Oura, Megumi Matsumoto, Tomohiro Usui, Yoko Kebukawa, Takazo Shibuya, Shogo Tachibana, Yasuhito Sekine, Tomoki Nakamura & Yoshio Takahashi
Nature Communications  Published:16 October 2026
DOI:https://doi.org/10.1038/s41467-026-78348-5

Abstract

The parent body of the C-type asteroid Ryugu experienced aqueous alteration. The past aqueous environments have been reconstructed using mineralogies and geochemical modeling, often assuming Na-Cl-rich fluid compositions. Here, we reconstruct Ryugu’s aqueous environment using chemical equilibrium modeling constrained by Electron Probe Microanalyzer (EPMA) and X-ray absorption near-edge structure spectroscopy. Our results suggest that Ryugu water possessed lower salinity than previously estimated and was highly reducing (Eh: −0.5 to −0.8 V) and strongly alkaline (pH: 10.3 to 11.5) conditions conducive to prebiotic organic reactions. The inferred high hydrogen fugacity, lack of NaCl, and low salinity recorded in Ryugu sample suggest that aqueous alteration occurred beneath a surface ice layer, with subsequent ice sublimation driving water loss. These findings refine our understanding of C-type asteroids evolution and the quantity and nature of water delivered to the early Earth.