Thermal history of Ryugu based on Raman characterization of Hayabusa2 samples
Lydie Bonal1, Eric Quirico1, Mutsumi Komatsu2, Gilles Montagnac3, Hikaru Yabuta4 and The Hayabusa2-initial-analysis IOM
team, Hisayoshi Yurimoto5, Tomoki Nakamura6, Takaaki Noguchi7, Ryuji Okazaki8, Hiroshi Naraoka8, Kanako Sakamoto9,
Shogo Tachibana9, 10, Sei-ichiro Watanabe11 and Yuichi Tsuda91Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, CNRS CNES, Grenoble, France2
SOKENDAI, The Graduate Univ. for Advanced Studies
3
Laboratoire de géologie de Lyon, CNRS/INSU - ENS de Lyon, Lyon, France
4
Hiroshima Univ., 5Hokkaido Univ.,6Tohoku Univ., 7Kyoto Univ.,8Kyushu Univ., 9JAXA, 10Univ. of Tokyo, 11Nagoya Univ.

Introduction: The degree of structural order of the polyaromatic carbonaceous matter present in extraterrestrial samples is atracer of the thermal history they experienced (e.g., primitive chondrites: [1-4]; micrometeorites: [5-6]). To characterize
Ryugu’s thermal history (long vs. short thermal heating and extent of thermal heating), we thus perform Ramancharacterization of several Ryugu particles returned by the Hayabusa2 mission. In order to be fully confident in the obtained
data and interpretation, Raman characterization was led independently by two groups of persons in Japan and in France on
distinct Ryugu particles. The results were subsequently compared.
Samples and methods: Raman point analyses were performed on several fragments of six particles from Chamber A
aggregates (A0108) and six particles from Chamber C aggregates (C0109). To be able to combine in situ IR and NanoSIMS
measurements on the same samples, fragments of particles were manually selected under a binocular and pressed onto
diamond windows. The Raman spectra were acquired with a 532 nm laser in both Japan and France. Because some Raman
bands related to carbonaceous matter are dispersive, data for Ryugu particles and comparison samples have been acquired andanalyzed consistently in both Japan and in France. In particular, a Renishaw InVia Reflex equipped with a 1800 l/mm grating
at Materials Characterization Central Laboratory was used in Waseda University (Japan). The laser was focused at the sample
surface through a 50× objective (spot size around 3-4 μm) and its power was set at 0.24 mW and 1 mW. Each acquisition
comprised five integrations of 10 s that were averaged to make the final spectrum. In France, Raman measurements were
performed at the Ecole Normale Supérieure de Lyon (Laboratoire de Géologie de Lyon—Terre, Planètes, Environnement)using a LabRam Raman spectrometer (Horiba Jobin-Yvon) equipped with a 600 g/mm grating. The laser was focused througha 100× objective to obtain a <2 μm spot size. The power on the sample was 0.3 mW. Each acquisition comprised six
integrations of 15 s that were averaged to make the final spectrum.
Results and discussion: More than 200 spectra were acquired on 12 different particles. The Raman data acquired in Japan andin France are fully consistent. Each acquired spectrum is characterized by a high fluorescence background and by the presenceof the Raman D- (~ 1350 cm-1) and G-bands (~ 1580 cm-1), related to the presence of polyaromatic carbonaceous matter. The
spectral parameters derived from the mathematical fitting of the individual spectra, that is, the band widths (FWHM-D,FWHM-G), the band positions (ωD, ωG), and the band intensity ratio (ID/IG) are comparable between particles. No systematicdifferences have been observed to date between the spectral parameters of Chamber A and Chamber C particles. This point
will be further investigated on insoluble organic matter isolated by acid treatment of Ryugu samples.
The Ryugu particles contain polyaromatic carbonaceous matter that is poorly structured, as reflected by the high fluorescencebackground superimposed to wide D- and G-Raman bands, present at relatively low Raman shifts. The comparison of thesespectral parameters with similarly characterized chondrites shows that the structural order of the polyaromatic carbonaceous
matter present in Ryugu particles is comparable to that in primitive (type 1, type 2) carbonaceous chondrites. Ryugu thus
escaped significant degree of long duration radiogenic thermal metamorphism (as typically experienced by type 3 chondrites),
as well as short-duration heating as experienced by some type 2 chondrites (e.g., [4]).
References[1] Bonal et al. 2006. Geochimica Cosmochimica Acta 70 :1849. [2] Bonal et al. 2016. Geochimica Cosmochimica Acta 189:
312. [3] Busemann et al. 2007. Meteoritics & Planetary Science 42: 1387. [4] Quirico et al. 2018. Geochimica Cosmochimica
Acta 241 : 17 [5] Dobrica et al. 2011. Meteoritics & Planetary Science 46 : 1363. [6] Battandier et al. 2018. Icarus 306 : 74.