Elemental and isotopic compositions of organic grains from asteroid Ryugu
Laurent Remusat1, Maximilien Verdier-Paoletti1, Smail Mostefaoui1, Lydie Bonal2, Hikaru Yabuta3 and The Hayabusa2initial-analysis IOM team, Hisayoshi Yurimoto4, Tomoki Nakamura5, Takaaki Noguchi6, Ryuji Okazaki7, Hiroshi Naraoka7,
Kanako Sakamoto8, Shogo Tachibana8, 9, Sei-ichiro Watanabe10 and Yuichi Tsuda8
1Museum National d’Histoire Naturelle, CNRS, Sorbonne Université, Paris, France. 2Institut de Planétologie etd’Astrophysique de Grenoble, Université Grenoble Alpes, CNRS CNES, Grenoble, France 3Hiroshima Univ., 4Hokkaido
Univ.,5Tohoku Univ., 6Kyoto Univ.,7Kyushu Univ., 8JAXA, 9Univ. of Tokyo, 10Nagoya Univ.

Introduction: Isotope composition of organic material found in extraterrestrial samples is a powerful proxy for tracking its
origin and evolution during the solar system events [1,2]. In situ investigation of isotope and elemental compositions unravel
the heterogeneity and diversity of organic particles embedded within the fine-grained minerals of chondrites and IDPs [3,4].
Understanding the origin of organic matter on carbonaceous asteroids and its subsequent evolution due to secondary processes
as well as space weathering is one of the prime goals of the Hayabusa2 sample-return mission [5]. To document Ryugu’s
inventory of organic material, we have employed NanoSIMS imaging of intact grains, without any chemical treatments, fromboth Chamber A and Chamber C. We present here the first set of data acquired on the NanoSIMS installed at the National
Muséum of Natural History in Paris, and compare the composition of organic particles from the two sampling sites to evaluate
the influence of space weathering and aqueous alteration.
Samples and methods: Several fragments of 3 particles from Chamber A aggregates (A0108) and 3 particles from Chamber Caggregates (C0109) were manually selected under a binocular and pressed on diamond windows. After their analysis by
Raman and transmission FTIR, samples were gold coated (about 25 nm thick) before NanoSIMS imaging. In the first step,secondary ions of 16O, 12C2, 12C14N, 12C15N and 32S were imaged in multicollection mode, to investigate N-isotope distributions
as well as N/C, O/C and S/C elemental ratios. A 2-3 pA primary Cs+ beam with a spatial resolution around 200 nm wasrastered over 20 by 20 m2, divided into 256 by 256 pixels, in association with an electron flooding gun for charge
compensation. Dwell time was set at 2 ms/pixel and about 60 frames were stacked.
Results and discussion: About 11,600 and 9,600 m2 surface area were imaged, of Chamber A and Chamber C samples,respectively. Individual organic particles were identified using the L’image software developed by Larry Nittler (CarnegieInstitution of Washington). The 15N/14N ratios of individual particles are comparable to the values for isotope anomalies in CIand CM insoluble organic matter [3,6,7] whilst samples from Chamber C tend to exhibit a slightly larger 15N enrichment. Inaddition, bulk organic 15N is in the range of IOM in CI chondrites. These values are consistent with independent NanoSIMS
measurements of other Ryugu aggregates [8]. The corresponding elemental ratios point to a larger content in N in the organic
particles from Chamber A, while the particles from Chamber C are more O-rich. Overall, S content is similar in organicparticles from both chambers. It must be noted that O/C and S/C may be biased by the occurrence of fine scale association ofoxides/silicates and sulfides with the organic particles, as observed by TEM [9]. The elemental composition of particulateorganic matter will be further investigated on insoluble organic matter isolated by acid treatment of Ryugu samples. Our datasuggest differences between aggregates from the two sampling sites in terms of elemental and isotope compositions ofindividual organic particles.
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