Chemical heterogeneity of insoluble organic matter in Ryugu grains
E. Quirico1, L. Bonal1 , Y. Kebukawa2, K. Amano3, H. Yabuta4, E. Dartois5, C. Engrand6, J. Mathurin7, J. Duprat8,
H. Yurimoto9, T. Nakamura3, T. Noguchi6, R. Okazaki10, H. Naraoka10, K. Sakamoto11, S. Tachibana12, S. Watanabe13, Y.
Tsuda3 and the Hayabusa2-initial-analysis IOM team1
Université Grenoble Alpes, CNRS, Institut de Planétologie et Astrophysique de Grenoble (IPAG), UMR 5274, Grenoble F38041, France. 2Yokohama National Univ., Yokohama 240-8501, Japan. 3Department of Earth Sciences. Tohoku University,Senda 980-8578, Japan. 4Hiroshima University (Japan), 5ISMO, Université Paris-Saclay (France). 6IJCLab Université ParisSaclay/IN2P3 (France). 7Institut de Chimie Physique, Université Paris-Saclay (France). 8Museum National d’Histoire Naturelle,Paris (France). 9Hokkaido University (Japan), 10Kyushu University (Japan), 11JAXA Sagamihara (Japan), 12Univ. of Tokyo(Japan), 13Nagoya University (Japan).
Introduction: The Hayabusa2 mission has returned to Earth 5.42 g of samples collected at the surface of asteroid Ryugu(Yada et al., 2022). Two touchdowns were operated: the first on February 21 2019 to collect material from the top surface, andthe second on July 11 2019, ~20 m north from the artificial crater generated by the impact experiment a few months ago, forprobing ejecta coming from deeper in the subsurface (Tsuda et al., 2020). The bulk and multi-instrumental mineralogical andisotopic characterization of Ryugu samples show that they are related to CI chondrites, but experienced a lower stage ofhydrothermalism resulting in a more reduced mineralogy (e.g. Nakamura et al., 2022). Insoluble organic matter (hereafter IOM)
has been extracted from Ryugu’s samples, and infrared spectroscopic analyses have revealed differences with respect to IOMs
extracted from historical falls as Alais, Ivuna and Orgueil (Yabuta et al., 2022; Kebukawa et al., 2022). Here, we investigate thechemical heterogeneity of IOM in a series of Ryugu grains selected from the A and C chambers, corresponding to the first andsecond touchdowns, respectively. Small grains (100-500 µm) were processed with an HF/HCl extraction protocol andcharacterized by micro-FTIR. Some of these grains were also analyzed by Raman micro-spectroscopy (Bonal et al. 2022).
Experimental: IOM samples were extracted with the HF/HCl protocol (Durand and Nicaise 1980), adapted to microsized grains. Infrared transmission spectra were collected with a Bruker HYPERION 3000 micro-FTIR, equipped with a MCTdetector. The spot size at the sample was ~50 µm x 50 µm and the spectral resolution was 4 cm-1. Measurements were run withan environmental cell maintained under secondary vacuum (~ 10-6 mbar) and the sample was heated at 80°C to remove adsorbedterrestrial water.
Results and discussion: The samples are presented in Tab. 1 and some infrared spectra in Fig. 1. The spectra collectedfrom the series of samples show significant chemical variations, which can have different origins : (1) Terrestrial weathering
versus sample freshness; (2) Analytical artefacts; (3) Accretion of distinct organic precursors and (4) Post-accretional processes.
Ryugu samples are extremely fresh and have experienced little interactions with the terrestrial atmosphere. In contrast, thehistorical falls Alais, Orgueil and Ivuna fell in 1804, 1864 and 1938, respectively, and storage conditions in old times were notoptimized. However, systematic measurements on finds and falls covering a long period of time, including recent falls (e.g.
Tagish Lake, Mukundupura), do not support an IOM evolution controlled by terrestrial weathering (Alexander et al. 2007; Codyet al. 2005; Kebukawa et al., 2011; Orthous-Daunay et al., 2013; Quirico et al. 2018). The HF/HCl digestion protocol could besubjected to analytical artifacts, such as oxidation and contamination (e.g. viton particles from the wires). However, we crosschecked the extraction quality by extracting IOM from well known chondrites (e.g. Orgueil), and in some cases in parallel with
two reactors filled from the same solvents and reagents. In the past, comparison with IOM samples obtained independently with
the CsF-technique developed at Carnegie Institute showed little difference with HF/HCl IOMs (Cody and Alexander 2005;Quirico et al., 2018). Artifact measurements are known to arise with infrared micro-FTIR because IOM samples contain insoluble
minerals with varying sizes that generate scattering, which can blur the absorption signal. We have suspicions for C109-12, as
the baseline correction was uncertain in the region 1700-1600 cm-1. The accretion of different organic precursors is difficult toaddress here as no insights into petrography was available for our samples, and we cannot infer the presence of xenolithic clasts.
Last, post-accretional processes are a potential source of heterogeneity, as observed in recent falls like Tagish Lake and Sutter’sMills (e.g. Herd et a., 2011; Quirico et al., 2018). Mineralogy, bulk and isotopic compositions of Ryugu samples are consistent
with a CI chondrite, more reduced than the least altered CI Alais, and which experienced a hydrothermal event of lower intensity.
Therefore, we do not expect a significant impact of fluid circulation, regarding the similarities of IOM composition across CM
and CI chondrites when covering a broad range of petrologic subtypes. Last, a possible source of heterogeneity is short-duration
thermal metamorphism, rated as stages from I to IV (Nakamura, 2005). According to systematics on a broad array of C1 and C2
chondrites, ICH2/ICH3 appears as a suitable tracer of this process, while the aliphatic and carbonyl abundances are less accurate.
In this respect, C0109-12, A0108-6, A0108-10 and A0106-23,24,25 appear inconsistent with the CI chondrites Alais, Ivuna andOrgueil. C0109-12 displays a high ICH2/ICH3, a low intensity aliphatic band and a low intensity carbonyl band (Fig. 2). However,short-duration metamorphism is not confirmed by Raman spectroscopy and the baseline correction of the spectra of this sample
are questionable. A0108-6 and A0108-10 display a high ICH2/ICH3, but they were very thin and translucent samples, optically very
different than other IOMs. In addition, they display a strong OH band, which does not appear in other IOMs as well. Regarding
A0106-23,24,25, it looks similar to IOM from heated stage II C1 and C2 chondrites, but once again Raman spectroscopy does
not report short-duration heating for A0108-10, A0106-23,24,25. Accordingly, the spectral heterogeneity cannot be firmly
interpreted as the result of short duration heating. Last, the sample C0109-9 seems to depart from other samples in terms ofRaman characteristics. However, its infrared spectrum is similar to that of unheated C2s, and the Raman data likely suffer froma statistical bias.
Conclusion: Our data reveal chemical heterogeneity across the IOM samples extracted from a series of Ryugu grains
and aggregates. No clear differences are observed between the two chambers A and C. 6 samples are consistent with the IOMsof the three historical CI falls Alais, Ivuna and Orgueil, and strengthens the link with this chondrite class. 4 samples show
substantial differences, which cannot be firmly interpreted as the evidence of short duration heating.

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