TOF-SIMS analysis of macromolecular organic matter in Ryugu samples
M. Hashiguchi1, D. Aoki1, K. Fukushima1, H. Yabuta2, H. Yurimoto3, T. Nakamura4, T. Noguchi5, R. Okazaki6, H. Naraoka6,
K. Sakamoto7, S. Tachibana7,8 , S. Watanabe1, Y. Tsuda7 and the Hayabusa2-initial-analysis IOM team1

Nagoya University, Nagoya, 464-8601, Japan, 2Hiroshima University, Higashi-Hiroshima,739-8526, Japan, 3Hokkaido
University, Sapporo,001-0021, Japan, 4Tohoku University, Sendai, 980-8577 Japan, 5Kyoto Univerisity, Kyoto,606-8501
6Kyushu University, 819-0395, Japan, 7ISAS/JAXA, Sagamihara, 252-5210, Japan, 8The University of Tokyo, Bunkyo-ku, 1138654, Japan.
Introduction: The Hayabusa2 spacecraft returned samples with total mass of ~5 g from surface of asteroid Ryugu inDecember 2020 [1]. Chamber A aggregates collected upon the first touchdown and chamber C aggregates collected upon thesecond touchdown were investigated by the Hayabusa2 initial analysis team from June 2021 to May 2022.
The initial analysis organic macromolecule team has performed chemical, molecular, and isotopic analysis on the intact Ryugu
grains and insoluble organic matter (IOM) isolated from Ryugu samples by μ-FTIR, micro-Raman spectroscopy, STXM-XANES,STEM-EELS, AFM-IR, and NanoSIMS [e.g., 4–8]. The result showed that chemical and isotopic compositions are broadly
similar between Ryugu IOM and CI chondritic IOM, although some differences between the two IOM were reported by FTIR.
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is useful to analyze the molecular species present on thesample surface. This technique has been applied for a variety of organic matter from macromolecular materials [e.g., 9] andextraterrestrial samples [e.g., 10–11]. In this study, we conducted in-situ analysis on bulk Ryugu samples and the isolated IOM
using TOF-SIMS to investigate the chemical structure of macromolecular organic matter in asteroid Ryugu.
Experimental: IOM were extracted from two Ryugu aggregate samples (A0106 and C0107) with 6M HCl and 1M
HCl/9M HF at Hiroshima University [4]. Those Ryugu IOM samples and intact Ryugu grains A0106 and C0057 were used inthis study. For comparison, Tagish Lake IOM and bulk samples of Tagish Lake (C-ungrouped) and Orgueil (CI) were used. All
those samples were pressed onto clean copper disks (~ 2.1 mmφ) and mounted in in-house stainless steel holder for TOF-SIMS
analysis. Roughness of the sample surface and copper disks were < 10 µm for fragment samples, < 5 µ m for IOM samples,respectively. The TOF-SIMS measurement was performed using a TRIFT III spectrometer (ULVAC-PHI, Inc., Chigasaki,Kanagawa, Japan) at Nagoya University. Positive and negative ion spectra were obtained using Au+ gold primary ion (22 keV,1.2 nA). The measured surface areas were 200 × 200 μm and acquisition time of about 5 min for each sample. The ion intensityof each species was obtained from region of interest (ROI) of the sample of all samples, and were normalized by using total ion
intensity of signals at m/z 0 to 150. After the TOF-SIMS analysis, mineralogical observation was performed on the sample using
SEM/EDS.
Results and Discussions: Various ions derived from hydrocarbon moieties (CnHx+), which containing 1-10 carbons,
were detected from both of extracted IOM and bulk samples from asteroid Ryugu samples and Tagish Lake and Orgueil
meteorites at m/z 10–150 of TOF-SIMS spectra.
Normalized intensities of these fragment ions and the fragment patterns were almost similar between A0106 and C0107
IOM. Most abundant fragment ion was C2H3+ or C2H5+ for both extracted IOM and bulk samples from Ryugu and Tagish Lake.
Lower mass fragment ions (C1~C4) were more abundant than higher mass fragment ions for both Ryugu and the carbonaceouschondrite samples. Furthermore, intensity of CnH2n+1+ (n =1~4) and CnH2n-1+ (n =3~5) ions from Ryugu IOM were higher thanfrom Tagish Lake IOM. This trend implies that Ryugu IOM contains more abundant hydrogen than Tagish Lake IOM. On theother hand, Tagish Lake IOM showed higher intensity of CnH2n+1+ (n>5) and/or CnH2n-1+ (n>6) ions than Ryugu IOM. Although
it is not clear whether these ions are derived from aromatic carbon or aliphatic carbon, the high mass fragment ions would
indicate the presence of aromatic rings [12]. A previous TOF-SIMS study suggests that CnH2+ radical cations and Cn– radicalanions are the fragmentation ions derived from aromatic moieties [12]. In this study, these radical cations or anions were detectedfrom both Ryugu and Tagish Lake IOM, while their abundances from Tagish Lake IOM were higher than those from Ryugu
IOM, except C5H2+. The result suggests that Ryugu IOM is less molecularly evolved relative to Tagish Lake IOM, which isconsistent with the solid-state 1H-NMR results for organic matter in Ryugu samples [13]. Also, the abundances of intensity ofCn– radical anions from the bulk Ryugu samples were lower than those from Tagish Lake, which also supports our observation
for the extracted IOM.

Compared to bulk samples of Tagish Lake and Orgueil, it was hard to see clear difference on hydrocarbon fragment ionssuch as their intensity and H/C ratio between Ryugu samples like result of IOM samples, except C5Hx+. This may probably be
due to sample heterogeneity. However, Tagish Lake and Orgueil bulk samples showed clearly developed C5Hx+ peaks relative
to the bulk Ryugu samples. On the other hand, the extracted IOM showed the opposite trend: the peak intensities of C5Hx+ ionsfrom Ryugu IOM, especially C5H7+, were very high relative to those from Tagish Lake IOM. Such a large difference between
bulk sample and IOM from the same sample has not been observed from meteorites, and therefore the observation implies thedifferent chemical structures of Ryugu IOM and organic matter from bulk samples, which could be result from the acidsusceptible nature of the Ryugu samples. Different chemical structure of organic matter between Ryugu and Tagish Lake may
result from freshness of Ryugu organic matter with no or little terrestrial alteration and/or different aqueous alteration process.
Further TOF-SIMS analysis on CI chondrite IOM will provide more detailed firm interpretation for chemical evolution of Ryugu
OM.

Acknowledgements
We thank to Mr. R. Nishimura at Nagoya University for making in-house holder for TOF-SIMS analysis in this study.
References[1] Yada T. et al. (2021) Nat. Astron. 6, 214–220. [2] Arakawa M. et al. (2020) Science, 368, 67–71. [3] Tsuda, Y. et al. (2020),
Acta Astronautica 171, 42–54, 2020. [4] Yabuta H. et al. (2022) Science, in revision. [5] Kebukawa Y. et al. (2022) 53rd LPSC.
#1271. [6] Stroud R. M. et al. (2022) 53rd LPSC. #2052. [7] Remusat L. et al. (2022) in 53rd LPSC. #1448. [8] Quirico E. etal. (2022) this meeting. [9] Stephan T. et al. (2003), Meteorit. & Planet.Sci. 38, 109–116. [10] Noun M.et al. (2020) Life, 9,p.44. [11] Hashiguchi M. et al. (2022) 53rd LPSC. #1867. [12] Sjövall P et al. (2021) Fuel, 286, 119373. [13] Cody G. D. et al.
(2022) this meeting.