[P-PS03] Small Solar System Bodies: Latest results and new perspectives on the Solar System evolution

[PPS03-13] Unique organic matter in the asteroid Ryugu samples revealed by infrared transmission spectroscopy

*Yoko Kebukawa¹, Eric Quirico², Emmanuel Dartois³, Lydie Bonal², Cécile Engrand⁴, Jean Duprat⁵, Jérémie Mathurin⁶, Alexandre Dazzi⁶, Ariane Deniset-Besseau⁶, Hikaru Yabuta⁷, Hisayoshi Yurimoto⁸, Tomoki Nakamura⁹, Takaaki Noguchi¹⁰, Ryuji Okazaki¹¹, Hiroshi Naraoka¹¹, Kanako Sakamoto¹², Shogo Tachibana^12,13, Sei-ichiro WATANABE¹⁴, Yuichi Tsuda¹², The Hayabusa2-initial-analysis organic macromolecule team (1.Faculty of Engineering, Yokohama National University, 2.IPAG, Université Grenoble Alpes, 3.Institut des Sciences Moléculaires d'Orsay, UMR8214, Université Paris-Saclay/CNRS, 4.IJCLab, UMR 9012 Université Paris-Saclay/CNRS, 5.Muséum National d’Histoire Naturelle, UMR CNRS, 6.Institut Chimie Physique, UMR 8000, Université Paris-Saclay/CNRS, 7.Hiroshima University, 8.Hokkaido University, 9.Tohoku University, 10.Kyoto University, 11.Kyushu University, 12.Japan Aerospace Exploration Agency (JAXA), 13.The University of Tokyo, 14.Nagoya University)

Keywords:Ryugu, Hayabusa2, Initial analysis, Infrared spectroscopy, Organic matter

JAXA’s Hayabusa2 mission returned the surface samples from the carbonaceous (C-type) asteroid Ryugu. The first touchdown samples and second touchdown samples were separately stored in the sample container chamber A and chamber C, respectively. The organic macromolecule subteam started initial analysis of the Ryugu particles from June 2021, to decipher the nature of the organic matter and its origin, parent body processing, and interaction with water and minerals [1].

As a part of the organic macromolecule subteam, infrared absorption spectra from the intact Ryugu particles and extracted insoluble organic matter (IOM) were obtained using Fourier transform infrared (FTIR) microspectroscopy, to understand the nature of organic matter in Ryugu. Several samples were analyzed in parallel in the team, in Japan (Yokohama National Univ., YNU) and in France (IPAG, Grenoble and Orsay-lab teams) to increase the robustness of the analysis. The aggregates in chamber A (A0108) and chamber C (C0109) were analyzed as intact Ryugu particles. IOM was obtained after solvent extraction and HF/HCl demineralization from the aggregates A0106 and C0107 [1]. The samples were pressed between two diamond windows and then FTIR measurements were performed on the diamond windows in transmission mode.

Both in the case of intact particles and extracted IOM, the IR absorption spectra of chamber A and chamber C were almost identical, but some local heterogeneity exists. The organic features shown in the FTIR spectra of the Ryugu intact particles were aliphatic C-H stretching at 2960 cm⁻¹ (CH₃ asymmetric), 2930-2925 cm⁻¹ (CH₂ asymmetric), 2855-2850 cm⁻¹ (CH₃ and CH₂ symmetric) and aromatic C=C stretching (~1600 cm⁻¹). Carbonyl C=O stretching modes (~1700 cm⁻¹) were not always visible. Extracted IOM displayed these peaks more clearly. In addition, C=O at 1660 cm⁻¹ newly appeared, which may be assigned to unsaturated ketones/aldehydes or amides. The Ryugu IOM had the highest CH₂/CH₃ with the highest aliphatic CH/aromatic C=C ratios compared to chondritic IOM [2,3], indicating that it is rich in long chain aliphatic moieties. We also observed possible N-H features in the Ryugu IOM. The N-H absorption was previously detected from reflectance IR measurements of intact Ryugu particles by JAXA curation [4,5]. The high CH₂/CH₃ ratios, aliphatic-rich, and N-rich nature of Ryugu organic matter might indicate primitiveness and freshness of the Ryugu particles, which is not preserved in carbonaceous chondrites.

References: [1] Yabuta H. et al. (2022) This meeting. [2] Kebukawa Y. et al. (2011) Geochimica et Cosmochimica Acta, 75, 3530–3541. [3] Alexander C. M. O’D. (2014) Meteoritics & Planetary Science 49, 503-525. [4] Yada T. et al. (2021) Nature Astronomy. [5] Pilorget C. et al. (2021) Nature Astronomy.