Surface morphologies and space weathering features of Ryugu samples
T. Matsumoto1, T. Noguchi1, A. Miyake1, Y. Igami1, M. Haruta1, H. Saito2, S. Hata2, Y. Seto3, M. Miyahara4, N. Tomioka5, H.
A. Ishii6, J. P. Bradley6, K. Ohtaki6, E. Dobrică6, T. Nakamura7, M. Matsumoto7, A. Tsuchiyama8, 9; M. Yasutake10, J.
Matsuno8, S. Okumura1, K. Uesugi10, M. Uesugi10, A. Takeuchi10, M. Sun9, S. Enju11, A. Takigawa12, H. Leroux13, C. LeGuillou13, D. Jacob13, M. Marinova13, F. de la Peña13, F. Langenhorst14, D. Harries14, P. Beck15, T. H. Van Phan15, R. Rebois15,N. M. Abreu16, T. Zega17, P.-M. Zanetta17, M. Thompson18, M. Lee19, L. Daly19, P. Bland20, R. Stroud21, K. Burgess21, J. C.
Bridges22, L. Hicks22, M. E. Zolensky23, D. R. Frank6, J. Martinez24, H. Yurimoto25, K. Nagashima6, N. Kawasaki25, R.
Okazaki2, H. Yabuta4, H. Naraoka2, K. Sakamoto26, S. Tachibana12, S. Watanabe27, Y. Tsuda26, and the Hayabusa2 InitialAnalysis Team
123Kyoto Univ., Japan; Kyushu Univ., Japan; Kobe Univ., Japan; 4Hiroshima Univ., Japan; 5Kochi Core Center, JAMSTEC,Japan; 6Univ. of Hawaii, USA; 7Tohoku Univ., Japan; 8Ritsumeikan Univ., Japan; 9Guangzhou Institute of Geochemistry,China; 10JASRI, Japan; 11Ehime Univ., Japan; 12Univ. of Tokyo, Japan; 13Univ. de Lille, France; 14Univ. of Jena, Germany;
15Univ. Grenoble Alpes, France; 16Penn State, USA; 17Univ. of Arizona, USA; 18Purdue Univ., USA; 19Univ. of Glasgow, UK;
20Curtin Univ., Australia; 21US Naval Research Laboratory, USA; 22Univ. of Leicester, UK; 23JSC/NASA, USA; 24Jacobs
Engineering, 25USA, Hokkaido Univ., Japan; 26JAXA/ISAS, Japan; 27Nagoya Univ., Japan
Introduction: Asteroids are leftover remnants of planet formation and provide clues as to the origin and evolution of the earlysolar system. C-type asteroids have been expected to be parent bodies of carbonaceous chondrites, containing hydrated silicatesand volatile compounds that could be the origin of water and life on the Earth. JAXA’s Hayabusa 2 spacecraft explored C-type
asteroid Ryugu and collected surface materials at two landing sites on Ryugu [1]. The spacecraft delivered its re-entry capsuleto the Earth, and subsequent initial investigation of the Hayabusa 2 sample container found that millimeter pebbles and finegrains were successfully recovered from the surface of Ryugu [2]. Geologic maps and variations in the reflectance spectra of
Ryugu’s surface suggest that geologic activities and alteration of regolith occur over time [3]. Ryugu samples will provide an
opportunity to understand the dynamic evolution of surface materials on C-type asteroids. Materials exposed to the spaceenvironment are expected to have altered optical, physical, or chemical properties. This process is defined as space weathering,which includes alteration by micrometeoroid bombardments, solar wind implantation, and solar radiation heating [4]. Analysesof lunar soils and regolith particles from S-type asteroid Itokawa have revealed that the uppermost surfaces of regolith grains
record the space weathering features, such as amorphization, melting, and vapor-deposition [5,6]. Thus far, the space weatheringof carbonaceous asteroids is not well understood. The surface microstructures of Ryugu samples will offer insight into theongoing alteration of the regolith on Ryugu. In this study, we report the surface morphologies and mineral structures of Ryugu
samples investigated in the initial analysis by the Mineralogy-Petrology Fine (Sand) sub-team.
Samples and methods: Ryugu samples from the two sampling sites were preserved in chambers A and C of the sample catcher
inside the sample container [2]. We have mainly investigated the fine grains (< 300 µm) picked up from both chambers at theExtraterrestrial Sample Curation Center of JAXA. After the samples were allocated from JAXA, we handled them in a dry glove
box filled with nitrogen at Kyoto University. For surface observation, the fine grains were fixed on gold plates using an Aralditeadhesive. We examined the surface features of the Ryugu samples using a field emission scanning electron microscope (FESEM: JSM-7001F). We then extracted electron-transparent sections of regions of interest on the Ryugu grains fortransmission/scanning transmission electron microscopy (TEM/STEM) studies, using a focused ion beam (FIB) system (HeliosNanoLab G3 CX at Kyoto University, Thermo Scios at Kyushu University). We are now performing TEM/STEM analysis,
synchrotron radiation X-ray absorption fine structure analysis, nano-tomography, and atom probe analysis.
Results: Surface features of Ryugu grains: Fine Ryugu grains have massive, platy, and granular shapes. The majority of thegrain surfaces consist of phyllosilicates with rough surfaces. Coarse and fine surface textures are identified on the phyllosilicates.
Cracks/gaps exist between phyllosilicates and other mineral phases. TEM analysis shows that the phyllosilicates are composed
mainly of serpentine and saponite (detailed TEM observations are described in [7]). The second major mineral phases on thegrain surfaces are sulfides, magnetite, and carbonates. These minerals are ubiquitous, although their abundance varies from grain
to grain. Most of the sulfides are pyrrhotite crystals. They appear as hexagonal plates, cuboids, and irregular shapes with sizes
up to a few tens of micrometers. The pyrrhotite plates exhibit sharp growth steps on their surfaces. Some pyrrhotite crystals withirregular surfaces have numerous voids. Pentlandite often coexists with pyrrhotite. Tiny sulfides (< 1µm), including zinc,chromium, and/or copper, occur as minor phases. Magnetite appears as framboidal aggregates, plaquettes, spherulites, andirregular shapes. The framboidal magnetite on Ryugu grains have distinct sharp edges, when compared to framboidal magnetites

with rounded morphologies in thermally metamorphosed CI chondrites (e.g., Y980115) in which thermal sintering has likelyoccurred. The carbonate phases found on the Ryugu grains are mainly dolomite and minor magnesite-breunnerite. They haveeuhedral, fractured, and irregular surfaces. Calcium phosphate, oxides (chromite, ilmenite), and iron phosphides are found as
minor phases. Na-Mg bearing phosphate with an irregular shape is present as a rare phase and is commonly attached loosely tothe grain surfaces. Anhydrous minerals found on Ryugu grains are forsteritic olivine, low-calcium pyroxene and pure spinel withsizes up to a few micrometers. These phases are rare. Most of them have fractured surfaces.
Space weathering features: We found impact craters, melted drops, splashes, melt sheets, glassy spherules on Ryugu grains
by SEM observation (Fig. 1). Studies of lunar soils and Itokawa particles have shown that these objects are likely products ofmicro-impacts on airless bodies [8, 9]; hence, the grain surfaces with the impact products may have been exposed to the spaceenvironment. The abundance of the impact products varies from grain to grain. Phyllosilicates on the grain surfaces are altered
to have smooth surface textures with tiny voids (Fig. 1 in [7]). TEM analysis shows that the uppermost surface is surrounded byan amorphous rim with a thickness from 50 nm to 2-3 µm. Tiny vesicles and iron compounds appear in the rim. EDX analysis
shows a change in chemical composition near the surface. The altered surfaces of pyrrhotite and pentlandite have shallow
depressions (200 nm – 300 nm in depth) with rugged textures (Fig. 1). Iron metals protrude from the iron sulfide surfaces, andsome are in the form of curved whiskers (Fig. 1). Dark-field TEM imaging suggests disordering of the lattice near the sulfidesurface. Carbonates and magnetite have altered surfaces with rough textures. The breunnerite grain we examined by TEM has adistinct rim with disordered lattice and a thickness of approximately 90 nm. Selected area diffraction patterns obtained from therim imply the appearance of periclase ((Mg,Fe)O) particles in the rim.
Discussion: The dominance of phyllosilicates and the scarcity of anhydrous minerals indicate that Ryugu samples haveexperienced a high degree of aqueous alteration. The morphologies of the major mineral phases, such as the unique shapes ofmagnetite, suggest a close similarity between Ryugu samples and CI chondrite [10].
Previous ion irradiation experiments simulating space weathering suggest that the uppermost surface of phyllosilicates canbe modified by space weathering [11]. The altered phyllosilicate rim found in our analysis may have been formed by solar windimplantation and impact events [7]. Iron metallic whiskers on iron sulfides have recently been identified as space weathering
products in lunar soils and Itokawa particles [12,13]. The iron metals are likely to have formed via selective sulfur loss,accumulation of excess iron atoms, and subsequent growth of iron metals. These alteration processes may be caused by various
phenomena including solar wind implantation, thermal effects produced by micrometeoroid bombardments and solar heating[12]. The appearance of iron whiskers on Ryugu samples implies that the space weathering of iron sulfides on Ryugu is similar
to that on the Moon and Itokawa. The thickness of the altered rim on breunnerite is roughly consistent with the maximum depth
at which extensive atomic displacement is produced by implanted solar wind ions (~ 1 keV/nucleon) [6]. Hence, solar windimplantation may have contributed to the rim formation of the carbonate. Based on our initial analysis, we tentatively conclude
that the Ryugu samples record the combined processes of space weathering on the aqueously altered asteroid, implying that
remote sensing data should be reassessed considering the space weathering effect.

Fig. 1 Space weathered Ryugu grain. (a) Backscattered electron image of a fine Ryugu grain. (b-d) Secondary electron images of a melted
drop (b), pyrrhotite with an iron whisker (c), and an impact crater with a residue on phyllosilicates (d). The image (d) was obtained fromanother fine Ryugu grain. M: melted drop, Po: pyrrhotite, W: iron whisker. Triangles in (d) indicate the crater rim.

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