小惑星リュウグウの岩石は氷を十億年も持っていた！～地球の材料天体に従来見積もりの2〜3倍の水があった可能性～

2025-09-11 東京大学

図1：炭素質小惑星リュウグウおよびその母天体の歴史

＜関連情報＞

• https://www.s.u-tokyo.ac.jp/ja/press/10908/
• https://www.nature.com/articles/s41586-025-09483-0

リュウグウにおけるルテチウム–ハフニウム同位体による原始的な太陽系小惑星における後期流体の流れの解明 Late fluid flow in a primitive asteroid revealed by Lu–Hf isotopes in Ryugu

Tsuyoshi Iizuka,Takazo Shibuya,Takehito Hayakawa,Tetsuya Yokoyama,Ikshu Gautam,Makiko K. Haba,Kengo T. M. Ito,Yuki Hibiya,Akira Yamaguchi,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,… Hisayoshi Yurimoto
Nature  Published:10 September 2025
DOI:https://doi.org/10.1038/s41586-025-09483-0

Abstract

Carbonaceous asteroids are the source of the most primitive meteorites¹ and represent leftover planetesimals that formed from ice and dust in the outer Solar System and may have delivered volatiles to the terrestrial planets^2,3,4,5. 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, in particular pristine samples returned from asteroid Ryugu, have provided detailed information on fluid–rock interactions within a few million years after parent-body formation^{6,7,8,9,10,11}. However, the long-term fate of asteroidal water remains poorly understood. Here we present evidence for fluid flow in a carbonaceous asteroid more than 1 billion years after formation, based on the ¹⁷⁶Lu–¹⁷⁶Hf decay systematics of Ryugu samples, which reflect late lutetium mobilization. Such late fluid flow was probably triggered by an impact that generated heat for ice melting and opened rock fractures for fluid migration. This contrasts the early aqueous activity powered by short-lived radioactive decay, with limited fluid flow and little elemental fractionation¹². 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 upwards revision of the inventory of their water delivery by a factor of two to three.