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Introduction: We analyzed more than 800 small Ryugu grains as a part of the initial analysis team [1]. Their average sizes of grains collected from the first and the second touchdown sites are ~70 and ~50 μm, respectively. In addition, we also investigated one and three large grains from the "chemistry" and the "stone" teams, respectively. Our "sand" team investigated these grains using FE-SEM, (S)TEM, STXM-XANES, nanobeam XANES, nano-CT, and APT. Several updated results investigated by our team and some results as collaborative work between teams will be presented at this meeting [e.g. 2-8]. The main scientific goals of our team are to clarify the mineralogy of the finegrained samples and to reveal the nature of the space weathering of the asteroid Ryugu, which belongs to the most abundant C-type asteroids. The surfaces of Solar System bodies that lack atmospheres are exposed to the space environment and experience gradual alteration, called space weathering [e.g. 9]. Its effects on C-type asteroids are poorly understood [9]. Samples returned from the C-type asteroid Ryugu by Hayabusa2 enable resolution of this issue. Results and discussion: The mineralogy of most Ryugu grains investigated by (S)TEM is similar to that of CI chondrites, which are chemically the most primitive materials in the Solar System. This result is consistent with other recent studies [10-12]. The major modes of space weathering on Ryugu grains are amorphization and partial melting of phyllosilicate surfaces. Reduction from Fe3+ to Fe2+ in phyllosilicates and dehydration decomposition of the phyllosilicates are associated with the space weathering. Although the physical processes that cause space weathering on Ryugu (solar wind irradiation and (micro)meteoroid impact) are identical to those on the Moon and the S-type asteroid Itokawa [13], the results are substantially different from space weathering of these anhydrous airless bodies. Space weathering of Ryugu promotes dehydration of abundant phyllosilicates via decomposition of serpentine and saponite that has already lost interlayer water molecules (H2O) and may, therefore, weaken the 2.7-μm band assigned to hydroxyl (-OH) in reflectance spectra [14]. Therefore, the weak 2.7-μm band of at least a part of C-type asteroids might be caused by space weathering. [ABSTRACT FROM AUTHOR]

Introduction: Solar wind irradiation and micrometeorite bombardments alter the physical, chemical, and optical properties of materials on airless bodies. These modifications are so called space weathering [1]. C-type carbonaceous Ryugue shows variations in reflectance spectra which are likely related to the geologic activity and surface exposure age [2]. Space weathering features in the returned samples will provide key information to interpret the surface evolution of Ryugu. One of the major causes to alter the reflectance spectra of airless bodies is considered to be nanometersized particles of opaque mineral phases (nanophase opaque minerals) produced by space weathering [1]. In this study, we examined the appearance of nanophase opaque minerals in Ryugu grains. Methods: We analyzed two fine Ryugu grains, A104-021002 and A104-028098. These grains were fixed on gold plates with epoxy resin. After the grains were observed by scanning electron microscopy (SEM), electron-transparent sections were extracted from the grain surface using the focused ion beam system. The extracted sections were analyzed by scanning transmission electron microscopy (TEM/STEM). 4D-STEM imaging (electron diffraction mapping using nano-beam) was applied to identify nanometer-sized inclusions. Results: The two Ryugu grains consist mainly of phyllosilicate-rich matrix, iron sulfides, magnetite, and carbonates. SEM observation shows that the phyllosilicate of the two grains have smooth surfaces with tiny bubbles, corresponding to the modified morphologies related to space weathering [3]. The magnetite and iron sulfides have porous surfaces indicative of space weathering [4]. TEM/STEM analysis shows that the phyllosilicate surfaces are coated by modified layers up to ~60 nm in depth (Fig. 1a). 4D-STEM imaging reveals that the surface modified layers are amorphous. Nanophase inclusions are rarely observed in the amorphous layers. STEM-EDS mapping of the phyllosilicate surfaces shows that outer 7-13 nm-wide zone is different in chemical composition from the phyllosilicate substrate (Fig. 1). The outermost rim of A104-021002 includes nanophase particles with high Z-contrast in HAADFSTEM image (Fig. 1b), and is rich in sulfur, iron, and nickel (Fig. 1c). 4D-STEM imaging of the outermost rim shows reflections corresponding to pentlandite and troilite with random orientations. The outermost rim of A104-028098 is rich in iron and has high Z-contrast. 4D-STEM data obtained from the rim shows reflections of troilite. Discussion: The amorphous layers on phyllosilicate are probably produced by solar wind irradiation [3]. Studies of lunar soils and grains from S-type asteroid Itokawa has indicated that the outermost rims with distinct chemical composition correspond to materials deposited from vapors produced by micrometeorite bombardments and solar wind sputtering [1]. Thin deposits including nanophase iron sulfides, pentlandite, and magnetite were produced by the pulsed-laser experiment for carbonaceous chondrite simulating micrometeorite bombardments [5]. Therefore, the outermost rims on Ryugu grains are probably vapor-deposited materials. The nanometer-sized iron sulfides in the vapor deposits can be produced when the sulfur fugacity in the vapor cloud was high enough to condense sulfide minerals [5]. Plausible sulfur source for the high sulfur fugacity may be iron sulfide minerals, because iron sulfides are the second major minerals in Ryugu samples and the space-weathered iron sulfides show the evidence of sulfur loss [4]. The experimentally produced vapor-deposits showed a strongly reddened reflectance slope across the visible to near-infrared wavelengths, because of the opaque nano-particles [5]. The nanophase iron sulfides on Ryugu grains may contribute to the color variations of Ryugu, where surface materials are expected to be reddened with time [2]. The apparent scarcity of nanophase inclusions in the irradiation-damaged rim suggests that vapor deposition processes play an important role for the production of nanophase opaque minerals on Ryugu rather than solar wind irradiation. [ABSTRACT FROM AUTHOR]