[P-PS08] Formation and evolution of planetary materials in the Solar System

[PPS08-01] Chemical evolution of primordial salts and organic sulfur molecules in the asteroid Ryugu

★Invited Papers

*Toshihiro Yoshimura¹, Yoshinori Takano¹, Hiroshi Naraoka², Toshiki Koga¹, Daisuke Araoka³, Nanako O. Ogawa¹, Philippe Schmitt-Kopplin^4,5, Norbert Hertkorn⁴, Yasuhiro Oba⁶, Jason P. Dworkin⁷, José C. Aponte⁷, Takaaki Yoshikawa⁸, Satoru Tanaka⁹, Naohiko Ohkouchi¹, Minako Hashiguchi¹⁰, Hannah McLain⁷, Eric T. Parker⁷, Saburo Sakai¹, Mihoko Yamaguchi¹¹, Takahiro Suzuki¹¹, Tetsuya Yokoyama¹², Hisayoshi Yurimoto⁶, Tomoki Nakamura¹³, Takaaki Noguchi¹⁴, Ryuji Okazaki², Hikaru Yabuta¹⁵, Kanako Sakamoto¹⁶, Toru Yada¹⁶, Masahiro Nishimura¹⁶, Aiko Nakato¹⁶, Akiko Miyazaki¹⁶, Kasumi Yogata¹⁶, Masanao Abe¹⁶, Tatsuaki Okada¹⁶, Tomohiro Usui¹⁶, Makoto Yoshikawa¹⁶, Takanao Saiki¹⁶, Satoshi Tanaka¹⁶, Fuyuto Terui¹⁷, Satoru Nakazawa¹⁶, Sei-ichiro Watanabe¹⁶, Yuichi Tsuda¹⁶, Shogo Tachibana^18,16, Hayabusa2-initial-analysis SOM team (1.Japan Agency for Marine-Earth Science and Technology, 2.Kyushu University, 3.National Institute of Advanced Industrial Science and Technology, 4.Helmholtz Zentrum München, 5.Technische Universität München, 6.Hokkaido University, 7.NASA Goddard Space Flight Center, 8.HORIBA Advanced Techno, 9.HORIBA Techno Service Co., 10.Nagoya University, 11.Thermo Fisher Scientific Inc., 12.Tokyo Institute of Technology, 13.Tohoku University, 14.Kyoto University, 15.Hiroshima University, 16.Japan Aerospace Exploration Agency, 17.Kanagawa Institute of Technology, 18.University of Tokyo)

Keywords:Hayabusa 2, Ryugu, sulfur-bearing soluble organic matters, salt

Samples of the carbonaceous asteroid (162173) Ryugu, collected by the Hayabusa 2 spacecraft, provide information on its similarities to CI chondrites and evidence for extensive aqueous alteration (e.g. Yokoyama et al., 2023; Nakamura et al., 2023). There is also evidence that organic matter has also undergone aqueous alteration and that the organic molecules are present as salts on the Ryugu surface (Naraoka et al., 2023). Based on the results of sequential solvent extraction of components with different solubilities, such that salts, carbonates and phyllosilicates (via hot water, formic acid and hydrochloric acid extracts, respectively), this study reports on the elemental distribution of major cations and anions on the Ryugu matrix and the diversity of sulfur-bearing soluble organic molecules. Sodium was the most abundant element (~90%) in the salt fraction of Ryugu, and was probably the main electrolyte when salts were precipitated. Here we show the element partitioning of the major component ions by sequential extractions of salts, carbonates, and phyllosilicate-bearing fractions to reveal primordial brine composition of the primitive asteroid. Sodium is the dominant electrolyte of the salt fraction extract. Anions and NH₄⁺ are more abundant in the salt fraction than in the carbonate and phyllosilicate fractions, with molar concentrations in the order SO₄²⁻ > Cl⁻ > S₂O₃²⁻ > NO₃⁻ > NH₄⁺. The salt fraction extracts contains anionic soluble sulfur-bearing species such as S_n-polythionic acids (n < 6), C_n-alkylsulfonates, alkylthiosulfonates, hydroxyalkylsulfonates, and hydroxyalkylthiosulfonates (n < 7). The sulfur-bearing soluble compounds may have driven the molecular evolution of prebiotic organic material transforming simple organic molecules into hydrophilic and amphiphilic molecules, suggesting that they may have directly influenced the chemical behaviour of organic matter.