Interpreting the thermal alteration history from organic matter in Ryugu samples
Queenie Hoi Shan Chan1*, Tomoki Nakamura2, Michael Zolensky3, Jangmi Han4, Mizuha Kikuiri2, Kana Amano2, Ashley
King5, Charlotte Bays1,5, Peter Krizan1, Natasha Almeida5, Megumi Matsumoto2, Akira Tsuchiyama6,7,8, Akira Miyake9, Junya
Matsuno9, Luis Carlos Colocho Hurtarte10, Rosario Brunetto11, Mark Burchell12, Vassilia Spathis12, Ming Chang Liu13 andJunko Isa14, and the Hayabusa2 initial analysis Core team.
1

Centre for Dynamic Earth and the Solar System (CeDESS), Department of Earth Sciences, Royal Holloway University ofLondon, Egham TW20 0EX, UK. 2Department of Earth Sciences, Tohoku University, Tohoku University, Sendai 980-8578,Japan. 3NASA Johnson Space Center; Houston TX 77058, USA. 4Department of Earth and Atmospheric Sciences, University ofHouston, Houston TX 77204, USA. 5Planetary Materials Group, Natural History Museum, London SW7 5BD, UK. 6Research
Organization of Science and Technology, Ritsumeikan University, Kusatsu 525-8577, Japan. 7Chinese Academy of Sciences(CAS) Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics andMaterials, Guangzhou Institute of Geochemistry, CAS, Guangzhou 510640, China. 8CAS Center for Excellence in Deep EarthScience, Guangzhou 510640, China. 9Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan.
10Diamond Light Source, Didcot OX11 0DE, UK. 11Institut d’Astrophysique Spatiale (IAS), Université Paris-Saclay, Orsay
91405, France. 12Centre for Astrophysics and Planetary Science, School of Physics and Astronomy, University of Kent,
Canterbury CT2 7NZ, UK. 13Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, USA.
14Earth-Life Science Institute, Tokyo 152-8550, Japan.
Introduction: The Hayabusa2 mission visited the near-Earth Cb-type asteroid (162173) Ryugu and has returned to Earthwith the surface material of Ryugu. Ryugu is an airless rubble pile asteroid exposed to space weathering [1]. Near infrared
remote observations recorded an absorption feature at 2.72 μm indicating the presence of OH–bearing hydrous components.
However, the 2.72 μm feature is weak, suggesting that Ryugu was once organic- and water-rich and subsequently dehydrated
by a brief period of surficial heating. This feature has also been observed in “thermally metamorphosed carbonaceouschondrites”, “CI-/CM-like”, or “CY” chondrites [2, 3]. Initial examination of the returned samples did not identify any
significant heating at >150 °C [4, 5]; however, amino acid analyses suggested similarities to thermally-altered meteorites on
the grounds of the predominant straight-chain n-ω-amino acid abundances [6]. It remains unclear as to what caused thenonconformal interpretations of the thermal alteration history of Ryugu.
Here we investigate the alteration history of Ryugu by comparing the organic composition of fresh Ryugu samples to thatof naturally and experimentally heated chondritic meteorites. This study describes a coordinated effort in analysing organic
matter in Ryugu samples using micro–x-ray computed tomography (μ-CT), micro-Raman spectroscopy, focused ion beam(FIB), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), micro-X-ray diffraction (μXRD), scanning transmission X-ray microscopy (STXM), X-ray absorption near edge structure (XANES), and synchrotron
radiation-based X-ray computed tomography (SR-CT).
Samples and Analytical Techniques: We studied Ryugu samples allocated during the Hayabusa2 preliminary
examination (A0063-FC016, A0063-FC017, A0064-FO012, A0067-FC008, C0002-FC014, C0002-FC015, C0025-FC003,C0033-FC003, C0046-FO003) and AO1 (A0009 and C0011), plus raw fragments of naturally long-term (CIs: Orgueil, Ivuna;
CMs: Murchison, Murray; CV: Allende; CRs: Grosvenor Mountains 95577; OCs: Yamato (Y)-74191, Queen Alexandra
Range 93010) and short-term heated meteorites (Belgica-7904, Jbilet Winselwan, Y-793321, Y-86720, Y-982086) [7, 8]. We
also analysed samples of Murchison, Murray and Orgueil experimentally heated to 300, 400, 500, 600, 700 and 900 °C for 1,10, 50, 100, 500 and 1000 hours. Raman analyses were conducted using a Jobin-Yvon Horiba LabRAM HR Raman
microprobe with a 514 nm laser for which the power was maintained at <100 μW at the sample surface. The mineralogy of theraw Ryugu samples was characterised using low-voltage SEM-EDS (FEI Quanta 650) and μ-XRD. We identified regions ofinterest (ROIs) within A0009 and C0011 by conducting μ-CT analysis using a cabinet-based industrial Zeiss Xradia Versa 520
CT system. Targeted ROIs were picked using sterile tools, pressed into indium, and prepared into electron-transparent sections(~100 nm thick) mounted on TEM Cu grids using a FEI Quanta 3D 600 dual beam FIB-SEM, which were studied by STXMXANES to determine the organic composition. Euhedral sulphide crystals were lifted and mounted on W-needles by the FIBSEM, and potential fluid inclusions within them were then located by SR-CT conducted at SPring-8 BL47XU at the X-ray
energy of 7 keV.
Results and Discussion: Increased heating of carbonaceous materials results in a transition from amorphous to graphitic
carbon, and such ordering is traceable using Raman spectroscopy as a cosmothermometer [9]. Raman spectroscopic analysis of

the Ryugu samples indicates significant heterogeneity (Figure 1), evident by Raman spectral features that are comparable tothe organic matter in both relatively unheated primitive chondrites and those that experienced short duration heating like Y793321, a Stage II heated chondrite [10].
The Raman spectral features of the organic matter inmost of the Ryugu samples are similar to those observed
from CIs, consistent with previous studies [11]. However,
localised areas within several Ryugu samples show awider G band, comparable to organic matter processed byshort duration heating. The Raman features of the organic
matter in broad areas within A0009, A0064 and C0011
clearly deviate from that observed from the other Ryugu
samples and Orgueil, with significantly lower D band
positions and D band widths that are comparable toFigure 1. Comparison of the Raman D and G band parameters of Ryugu
chondritic organics that had been heated at elevated samples (yellow symbols), and naturally short-term heated samples
temperatures [9], and larger G band widths that are not (black symbols; Stage II CCs peak heating temperature ~250–500°C;
observed in naturally metamorphosed chondritic Stage IV CCs >750°C). Γ=Full width half maximum; ω = Peak location.
meteorites (Figure 1). These trends are similar to thoseexhibited by astromaterials heated by short-term heating, like interplanetary dust particles [12] and some meteorites [13]. The
Raman spectra of A0009, A0064 and C0011 are similar to those of Orgueil experimentally heated to 300 °C for 500 h.
Analysis of A0064 by SEM-EDS and μ-XRD shows
the presence of hydrated phyllosilicates, suggesting that theparticle did not experience significant heating above ~300–
400 °C. Moreover, the SR-CT data validated the μ-CT scans,and indicate that C0011 contains a sulphide grain hosting aninclusion with X-ray attenuation that is clearly distinct fromair (Figure 2). Previous studies have revealed H2O and CO2
within such inclusions [14], suggesting that they have notbeen metamorphosed after aqueous alteration. Analysing Figure 2. SR-CT images of a sulphide grain picked from C0011.
these more “evolved” particles with STXM-XANES helped
us understand the organic structure corresponding to the Raman features. C-XANES spectra of A0009 and C0011 indicate thepresence of aromatic or olefinic groups (C=C) at ~285 eV and carboxylic groups (COOH) at ~288.5 eV within the matrix
associated with carbonate grains (CO3 absorption at 290.3 eV). While thermally stable oxygenated functional groups wereobserved in short term (impact induced) heated meteorites [15], the association of the organic matter with carbonates iscomparable to the carboxylic-rich diffuse organic matter in CM and CI chondrites [16]. Based on our coordinated investigation,
we have drawn two possible scenarios that can explain our observations: (1) some carbonaceous matter in Ryugu had
experienced post aqueous alteration short duration heating (impact induced and/or solar irradiation), resulting in the presenceof thermally stable oxygenated moieties like furan and phenol that were formed via dehydration and cyclisation ofpolyalcohols. These “evolved” materials were subsequently disrupted and redistributed by impacts, thermal fatigue, and/ormass wasting processes [17], intermixed with fresh, undisturbed and pristine Ryugu materials. Alternatively, (2) the formationof the macromolecular organic matter occurred at the final stages of aqueous alteration in low-temperature, highly oxidizing
fluids, e.g., fluid with OH radicals contributing through H2O2 dissociation [18], co-precipitating carboxylic enrichedcomponents in the immediate vicinity of carbonate.
Acknowledgements: We are extremely grateful to the Astromaterials Science Research Group (ASRG) of ISAS/JAXA
for providing the Ryugu samples.
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