Chemical composition of carbonaceous asteroid Ryugu from synchrotron spectroscopy in the mid- to far-infrared of Hayabusa2-returned samples

¹ Institut des Sciences Moléculaires d’Orsay, CNRS, Univ. Paris-Saclay, 91405 Orsay, France
e-mail: emmanuel.dartois@universite-paris-saclay.fr
² Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa, 240-8501, Japan
³ Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁴ Institut Chimie Physique (ICP), UMR 8000, Université Paris-Saclay, CNRS, Orsay, 91405, France
⁵ IJCLab, UMR 9012, Université Paris-Saclay, CNRS, 91405 Orsay, France
⁶ Institut de Minéralogie, Physique des Matériaux et de Cosmochimie, Museum National d’Histoire Naturelle, CNRS, Sorbonne Université, Paris 75231, France
⁷ Institut de Planétologie et d’Astrophysique, Université Grenoble Alpes, Grenoble 38000, France
⁸ Synchrotron SOLEIL, CNRS, CEA, Paris-Saclay, France
⁹ Earth and Planets Laboratory, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015, USA
¹⁰ Materials Science and Technology Division, US Naval Research Laboratory, Washington, DC 20375, USA
¹¹ Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
¹² Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8229, USA
¹³ The Graduate University for Advanced Studies, SOKENDAI, Hayama, Kanagawa 240-0193, Japan
¹⁴ Department of Earth Sciences, Waseda University, Shinjuku-ku, Tokyo 169-8050, Japan
¹⁵ Centro de Química Estrutural, Institute of Molecular Sciences and Department of Chemical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
¹⁶ Department of Earth Sciences, Tohoku University, Sendai 980-8578, Japan
¹⁷ École normale supérieure de Lyon, Université de Lyon, 69342 Lyon, France
¹⁸ Institute for Molecular Science, UVSOR Synchrotron Facility, Myodaiji, Okazaki 444-8585, Japan
¹⁹ Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
²⁰ NASA Ames Research Center, Moffett Field, CA 94035-1000, USA
²¹ Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²² Materials Science and Technology Division, US Naval Research Laboratory, Washington, DC 20375, USA
²³ Spectroscopy Division, Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo 679-5198, Japan
²⁴ Institute of Materials Structure Science, High Energy Accelerator Research Organization, KEK, Tsukuba, Ibaraki, 305-0801, Japan
²⁵ Research and Utilization Division, Japan Synchrotron Radiation Research Institute (JASRI), Sayo-gun, Hyogo 679-5198, Japan
²⁶ Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan
²⁷ Department of Earth and Planetary Sciences, Kyushu University, Fukuoka 819-0395, Japan
²⁸ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
²⁹ Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
³⁰ Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
³¹ Kanagawa Institute of Technology, Atsugi 243-0292, Japan
³² Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8601, Japan

Received: 5 August 2022
Accepted: 19 October 2022

Abstract

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/CH₃ in Ryugu than generally measured in C1 chondrites collected on Earth, and possibly a higher carbonate content.