[P-PS03] 太陽系小天体：太陽系の形成と進化における最新成果と今後の展望

[PPS03-08] The fate of water in carbonaceous asteroids from excess ¹⁷⁶Hf in Ryugu

*飯塚 毅¹、渋谷 岳造²、早川 岳人³、横山 哲也⁴、ガウタム イクシュ⁴、羽場 麻希子⁴、伊藤 健吾¹、日比谷 由紀¹、山口 亮⁵、ハヤブサ2初期分析 化学チーム、ハヤブサ2初期分析 コア (1.東京大学、2.海洋研究開発機構、3.量子科学技術研究開発機構、4.東京科学大学、5.極地研究所)

キーワード：炭素質天体、ハフニウム同位体、ハヤブサ2、水の起源

Carbonaceous asteroids are the source of the most primitive meteorites and represent leftover planetesimals that formed from ice and dust in the outer Solar System [1]. These planetesimals may have delivered volatiles to the terrestrial planets [e.g., 2-3]. Understanding the aqueous activity of asteroids is key to deciphering their thermal, chemical, and orbital evolution, with implications for the origin of water on the terrestrial planets. Analyses of the objects, particularly pristine samples returned from asteroid Ryugu, have provided detailed information on fluid–rock interactions within a few million years after parent body formation [e.g., 4-5]. However, the long-term fate of asteroidal water remains poorly understood. Here we show that Ryugu samples contain excess radiogenic hafnium-176 unsupported by the decay of co-existing lutetium-176 (Fig. 1). This excess is attributed to lutetium transport by alkali–phosphorous-rich fluids more than one billion years after formation. Such late fluid flow was likely triggered by an impact that generated heat for ice melting and opened rock fractures for fluid migration. This is in contrast to the early aqueous activity powered by short-lived radioactive decay, with limited fluid flow and little elemental fractionation [6]. Our results imply that carbonaceous planetesimals accreted by the terrestrial planets could have retained not only hydrous minerals but also aqueous water, leading to an upward revision of the water inventory by a factor of 2 to 3.

References: [1] DeMeo F. E. et al. Icarus 380, 114971 (2022). [2] Albarède F. Nature 461, 1227–1233 (2009). [3] Alexander C. M. O'D et al. Science 337, 721–723 (2012). [4] Nakamura T. et al. Science 379, eabn8671 (2023). [5] Yokoyama T. et al. Science 379, eabn7850 (2023). [6] Tang, H. et al. Planet. Sci. J. 4, 144 (2023). [7] Iizuka T. et al. PNAS 112, 5331–5336 (2015).

The Hayabusa2-initial-analysis chemistry team: Yoshinari Abe, Jérôme Aléon, Conel M. O'D. Alexander, Sachiko Amari, Yuri Amelin, Ken-ichi Bajo, Martin Bizzarro, Audrey Bouvier, Richard W. Carlson, Marc Chaussidon, Byeon-Gak Choi, Nicolas Dauphas, Andrew M. Davis, Tommaso Di Rocco, Wataru Fujiya, Ryota Fukai, Hiroshi Hidaka, Hisashi Homma, Gary R. Huss, Kiyohiro Ichida, Trevor R. Ireland, Akira Ishikawa, Shoichi Itoh, Noriyuki Kawasaki, Noriko T. Kita, Koki Kitajima, Thorsten Kleine, Shintaro Komatani, Alexander N. Krot, Ming-Chang Liu, Yuki Masuda, Kazuko Motomura, Frédéric Moynier, Izumi Nakai, Kazuhide Nagashima, Ann Nguyen, Larry Nittler, Andreas Pack, Changkun Park, Laurette Piani, Liping Qin, Sara Russell, Naoya Sakamoto, Maria Schönbächler, Lauren Tafla, Haolan Tang, Kentaro Terada, Yasuko Terada, Tomohiro Usui, Sohei Wada, Meenakshi Wadhwa, Richard J. Walker, Katsuyuki Yamashita, Qing-Zhu Yin, Shigekazu Yoneda, Hiroharu Yui, Ai-Cheng Zhang, Hisayoshi Yurimoto
The Hayabusa2-initial-analysis core: Tomoki Nakamura, Hiroshi Naraoka, Takaaki Noguchi, Ryuji Okazaki, Kanako Sakamoto, Hikaru Yabuta, Masanao Abe, Akiko Miyazaki, Aiko Nakato, Masahiro Nishimura, Tatsuaki Okada, Toru Yada, Kasumi Yogata, Satoru Nakazawa, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Yuichi Tsuda, Sei-ichiro Watanabe, Makoto Yoshikawa, Shogo Tachibana, Hisayoshi Yurimoto