Infrared transmission spectra of Ryugu particles and their unique adsorption behavior
Yoko Kebukawa1, Lydie Bonal2, Eric Quirico2, Emmanuel Dartois3, Cécile Engrand4, Jean Duprat5, Jérémie Mathurin6,Alexandre Dazzi6, Ariane Deniset-Besseau6, Hikaru Yabuta7 and The Hayabusa2-initial-analysis IOM team,
Hisayoshi Yurimoto8, Tomoki Nakamura9, Takaaki Noguchi10, Ryuji Okazaki11, Hiroshi Naraoka11, Kanako Sakamoto12,
Shogo Tachibana12,13, Sei-ichiro Watanabe14 and Yuichi Tsuda12
1Yokohama National Uni., 2IPAG, Univ. Grenoble Alpes, CNRS CNES 3ISOM, 4IJCLab, 5IMPMC/MNHN, 6ICP, 7Hiroshima
Univ., 8Hokkaido Univ., 9Tohoku Univ., 10Kyoto Univ., 11Kyushu Univ., 12JAXA, 13Univ. of Tokyo, 14Nagoya Univ.
The Hayabusa2 spacecraft successfully obtained samples from two different locations from Ryugu, a C-type asteroid, andreturned them to the Earth on December 6, 2020. The first touchdown samples and second touchdown samples were separately
stored in sample container chamber A and chamber C, respectively. The initial analyses began in June 2021, after curation atISAS/JAXA. The insoluble organic matter (IOM) subteam has so far conducted in-situ analyses of the Ryugu particles, inorder to decipher the nature of the organic matter and its origin, parent body processing, and interaction with water andminerals. Fourier transform infrared (FTIR) spectroscopy is a nondestructive technique for functional group chemistry andstructures which is suitable for both organic and inorganic compounds. To date, IR absorption (transmission) spectra have beenobtained from various chondrites and other astromaterials such as interplanetary dust particles (IDPs), micrometeorites andcometary dust particles [e.g., 1-7]. IR spectroscopy is often employed for astronomical observations and remote sensing. The
reflectance spectra of solar system bodies provide information of the compositions of their surfaces. To compare the laboratory
samples and the surfaces of asteroids, IR reflectance spectra have been commonly used [e.g., 8-10]. Both IR transmission andreflectance spectra give intrinsically similar information as both reflect the absorptions of IR frequencies by the target samples,
although some differences do exist. As a part of the initial analysis in the IOM team, IR absorption spectra were obtained using
FTIR microspectroscopy, to provide initial characterization of the Ryugu particles and search for organic matter, as well as forcomparison with the surface reflectance spectra of 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 theanalysis.
Each particle (sub-millimeter in diameter) from the aggregates in chamber A (A0108) or chamber C (C0109), was further
decomposed by gently crushing between two glass slides, or by separation of a fragment with a scalpel. The small fragments
were then further crushed between two diamond windows (YNU and IPAG) and in a diamond compression cell under an
optical microscope to optimize the thickness for further atomic force microscope based infrared spectroscopy (AFM-IR)
measurements (Orsay-lab). In YNU, IR absorption spectra were collected from each diamond window with a micro-FTIR(JASCO FT/IR-6100+IRT-5200), equipped with a ceramic IR light source, a germanium-coated KBr beam splitter, a mercurycadmium-telluride (MCT) detector, and ×16 Cassegrain mirrors, with typically 20 × 20 μm aperture. The microscope and theFTIR were continuously purged with dry N2. To remove adsorbed water further from the sample, a heating stage (Linkam10036L) was employed. The IR absorptions from water were satisfactory removed typically by 60 °C under N2 flow. MicroFTIR measurements were also conducted at IPAG with a Hyperion 3000 Bruker FTIR microscope, operating with a ~50 × 50µm spot and an environmental cell placing the sample under secondary vacuum (< 10 -6 mbar) and gentle heating (< 80° C) toremove adsorbed water. IR spectra by the Orsay-lab team were taken during a week of beamtime at the SOLEIL synchrotron(SMIS beam line). The synchrotron beam was coupled to a Nicolet Continuum 2 IR microscope equipped with a MCT detector
and a ×32 Cassegrain optics, under dry air. For these measurements, the IR spot size was optimized with an aperture of 6 × 6µm, close to the diffraction limit, and hyperspectral maps of Ryugu samples were acquired over several hundreds of µm in size.
The IR absorption spectra of both A0108 and C0109 particles were almost identical, and typically consistent with CI
chondrites. However, some local heterogeneity exists, e.g., IR spectra from some areas were dominated by carbonate features,and opaque minerals are seen in high resolution hyperspectral maps. They display Mg-rich phyllosilicate bands at ~1000 cm-1
(Si-O stretching) and ~3700 cm-1 (OH group in phyllosilicates), organic bands (aliphatic C-H) at 3000-2800 cm-1. The positionand narrowness of the OH band fit those observed in spectra collected in-situ by the spacecraft [11]. Compared to thereflectance spectra obtained by JAXA curation [12], the OH band and aliphatic C-H bands features in the absorption spectra
were roughly consistent, but the abundance of aliphatic C-H peaks were lower than the reflectance spectra. Such differences
could be due to the difference between reflectance spectra and transmission spectra of sample particles. Also, interesting
behavior was observed during FTIR analysis, in that the aliphatic C-H peak increased during analysis for a few hours, likely
due to adsorption of environmental volatile organic compounds (VOCs). It is known that porous OH bearing silicates adsorb

VOCs [e.g., 13-15], particularly when fresh surfaces are exposed by sample crushing [16], however the Ryugu samples appear
unusually reactive and absorb VOC very rapidly, considering that the Ryugu sample analysis was conducted in a clean
environment without abundant contamination sources. This behavior indicates that Ryugu samples are fresh and highly porous.
Measurements conducted under vacuum and gentle heating reveal a rapid and dramatic disappearance of adsorbed water, alongwith an increase of the aliphatic band intensity. This has been previously observed for CI chondrites [6,17,18], and confirms
the high adsorption properties of Ryugu samples. Tight interactions between organics and -OH in phyllosilicates may accountfor this spectral evolution.
Overall, the FTIR organic signatures of the Ryugu samples do not point to a significant post-accretional heating as
observed in heated CM/CI chondrites [19]. Acid extraction of Ryugu samples very soon will provide a fresh IOM isolate, andthereby enable quantitative characterization of chemical group abundances.
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