Chemical composition of carbonaceous asteroid Ryugu from synchrotron spectroscopy in the mid- to far-infrared of Hayabusa2-returned samples
Authors: Dartois Emmanuel , Kebukawa Yoko , Yabuta Hikaru , Mathurin Jérémie , Engrand Cécile , Duprat Jean , Bejach Laure , Dazzi Alexandre , Deniset-Besseau Ariane , Bonal Lydie , Quirico Eric , Sandt Christophe , Borondics Ferenc , Barosch Jens , Cody George D. , De Gregorio Brad T. , Hashiguchi Minako , Kilcoyne David A. L. , Komatsu Mutsumi , Martins Zita , Matsumoto Megumi , Montagnac Gilles , Mostefaoui Smail , Nittler Larry R. , Ohigashi Takuji , Okumura Taiga , Remusat Laurent , Sandford Scott , Shigenaka Miho , Stroud Rhonda , Suga Hiroki , Takahashi Yoshio , Takeichi Yasuo , Tamenori Yusuke , Verdier-Paoletti Maximilien , Yamashita Shohei , Nakamura Tomoki , Morita Tomoyo , Kikuiri Mizuha , Amano Kana , Kagawa Eiichi , Noguchi Takaaki , Naraoka Hiroshi , Okazaki Ryuji , Sakamoto Kanako , Yurimoto Hisayoshi , Abe Masanao , Kamide Kanami , Miyazaki Akiko , Nakato Aiko , Nakazawa Satoru , Nishimura Masahiro , Okada Tatsuaki , Saiki Takanao , Tachibana Shogo , Tanaka Satoshi , Terui Fuyuto , Tsuda Yuichi , Usui Tomohiro , Watanabe Sei-ichiro , Yada Toru , Yogata Kasumi , Yoshikawa Makoto 
Published: February 2023

Context.The current period is conducive to exploring our Solar System's origins with recent and future space sample return missions, which provide invaluable information from known Solar System asteroids and comets The Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) recently brought back samples from the surface of the Ryugu carbonaceous asteroid.

Aims.We aim to identify the different forms of chemical composition of organic matter and minerals that constitute these Solar System primitive objects, to shed light on the Solar System's origins.

Methods.In this work, we recorded infrared (IR) hyper-spectral maps of whole-rock Ryugu asteroid samples at the highest achievable spatial resolution with a synchrotron in the mid-IR (MIR). Additional global far-IR (FIR) spectra of each sample were also acquired.

Results.The hyper-spectral maps reveal the variability of the functional groups at small scales and the intimate association of phyl-losilicates with the aliphatic components of the organic matter present in Ryugu. The relative proportion of column densities of the identified IR functional groups (aliphatics, hydroxyl + interlayer and/or physisorbed water, carbonyl, carbonates, and silicates) giving access to the composition of the Ryugu samples is estimated from these IR hyper-spectral maps. Phyllosilicate spectra reveal the presence of mixtures of serpentine and saponite. We do not detect anhydrous silicates in the samples analysed, at the scales probed. The carbonates are dominated by dolomite. Aliphatics organics are distributed over the whole samples at the micron scale probed with the synchrotron, and intimately mixed with the phyllosilicates. The aromatic C=C contribution could not be safely deconvolved from OH in most spectra, due to the ubiquitous presence of hydrated minerals. The peak intensity ratios of the organics methylene to methyl (CH2/CH3) of the Ryugu samples vary between about 1.5 and 2.5, and are compared to the ratios in chondrites from types 1 to 3. Overall, the mineralogical and organic characteristics of the Ryugu samples show similarities with those of CI chondrites, although with a noticeably higher CH2/CH3in Ryugu than generally measured in C1 chondrites collected on Earth, and possibly a higher carbonate content.