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Introduction: The mineralogy, petrology and chemical properties of the samples collected from the asteroid 162173 Ryugu by the Hayabusa2 spacecraft show that the samples correspond to CI meteorites not affected by terrestrial weathering (e.g., [1,2]). Our analysis using SR-based X-ray nanotomography (XnCT) is also consistent CI meteorites [3]. Here we report the morphologies of major mineral phases in 3D to discuss their crystallization conditions in aqueous alteration together with their crystallization sequence based on their textures. Methods: We examined 75 particles (10 ~ 180 μm in apparent size) by XnCT at BL47XU of SPring-8 [3]. Most of them were imaged by DET-SIXM method combined with absorption and phase-shift tomography to discriminate mineral phases with the voxel size of 50 ~ 110 nm, their Mg# and densities. Some samples were imaged by absorption tomography with higher resolution (voxel size: 15 ~ 18 nm). Results and Discussion: The matrix of analyzed particles is mainly composed of Mg-rich phyllosilicates (serpentine and saponite, 82 vol.%) with different features (fine to coarse aggregates, Mg# = 0.75 ~ 0.92 (mean: 0.83± 0.04), density = 1.2 ~ 2.1 (mean: 1.7±0.26) g/cm³), IOMs (0.7 vol.%) and voids (12 vol.%). The major minerals in the matrix are magnetite (2.5 vol.%), Fe(-Ni)S (1.4 vol.%; pyrrhotite >> pentlandite), dolomite (1.2 vol.%), breunnerite and apatite (0.2 vol.%). The data here should show the representative feature of Ryugu samples through the 3D analysis of abundant particles without selection bias but may have large uncertainties for the modal abundance in particular IOMs due to insufficient spatial resolution and thresholding of the XnCT analysis. 3D morphologies. Magnetite has a variety of morphology (typically a few ~ 10 μm); spherulite and its aggregate, framboid composed of mono-dispersed grains of a few 100s nm ~ a few μm, plaquette, large equant grains composed of {311} and elongated grains. Transitional morphologies were also observed. Whiskers (~0.5 μm thick and 20 μm long) and cube (~1 μm) are rarely present as well. In contrast, pyrrhotite is mostly separated hexagonal plates ({100} and {001} in the hexagonal setting) with different size (longest length: <1 ~ 60 μm). The width/length ratio ranges from 0.06 to 0.98 (mean: 0.43±0.20) and is independent of the size. Pentlandite (<~5 μm) is embedded in pyrrhotite or present as fragmented grains in the matrix. Nano-sized pyrrhotite and pentlandite grains were also observed in some matrix by TEM [4] although their morphologies were not clearly known. Dolomite is usually present as aggregates of subhedral ~ euhedral grains (<1 ~ 50 μm). Some are euhedral grains of flattened rhombohedron probably composed of {101}. Separated breunnerite grains (30 ~ 100 μm) are thin rhombohedron of possibly {012}. Apatite is usually present as aggregates of subhedral ~ euhedral grains (<1 ~ 50 μm) and one hexagonal prism of {100} (or {110}) and {001} was observed. Crystallization sequence. The crystallization sequence of the major minerals estimated based on inclusions and their relative enclosing relations, which can be easily recognized in 3D, is follows; magnetite → Fe(-Ni)S (pentlandite → pyrrhotite) → apatite → dolomite/breunnerite. For magnetite, the sequence is likely spherulitic → plaquette/framboidal (from fine to coarse) → equant/elongated. This indicates that magnetite crystallization condition changed from high to low supersaturations. The overall sequence can be explained by aqueous alteration of primitive solar materials such as GEMS-like materials. The high supersaturation for magnetite was probably caused by abrupt dissolution of relatively Fe0 or FeO-rich amorphous silicates followed by precipitation of magnetite and Mg-rich phyllosilicates. In contrast, pyrrhotite hexagonal plates with different size might be formed by dissolution and reprecipitation from original Fe(-Ni)S nanograins at relatively low supersaturation. Dolomite and apatite having equilibrium shapes [5,6] indicate that they precipitated at relatively low supersaturations in the late stage. [ABSTRACT FROM AUTHOR]

Introduction: Imaging using X-ray absorption contrast by synchrotron nano-computed tomography of a Ryugu pyrrhotite crystal (C0002-FC012 - from the second Hayabusa2 spacecraft touch-down site) revealed probable fluid inclusions in the center of a crystal of pyrrhotite, suggesting the parent fluids were trapped in the early stages of crystal growth on Ryugu's parent asteroid. We previously described the performance of Time-of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS) depth profiling and high-resolution mapping at -120°C to expose and measure the composition of the trapped fluids in a frozen state in two Ryugu inclusions [1,2]. Here we report results from follow-on measurements of additional inclusions in the same pyrrhotite crystal, and the initial results of measurement of standards. The ultimate goal is to reveal the quantitative composition of the fluids that caused aqueous alteration, in particular the H2O:CO2 ratio, which will also facilitate cosmochemical modeling of the alteration process. Measurements of Ryugu Fluids: TOF-SIMS measurements of four Ryugu fluid inclusions revealed that the ancient, trapped fluids on Ryugu's parent asteroid were saline aqueous solutions containing H2O, CO2, sulfur species, and nitrogen- and chlorine-bearing organic compounds identified by representative secondary ion species including O-, OH-, CO-, S-, Cl-, C2-, C2H-, and CN-. The inferred presence of CO2 indicates formation of the sulfides, and by implication the Ryugu parent body, beyond the H2O and CO2 snow lines of the early solar system, i.e. > 3-4 au from the Sun [3]. In this work we made measurements of only a few planes exposed by sputtering. We later discovered that we should have measured the inclusions from top to bottom, as shown by our subsequent measureents of standards. Standards: The next step in these analyses was the measurement of appropriate fluid standards, reported here. Artificial aqueous fluid inclusions in quartz were made at Virginia Tech (see [4] for technical details), with a known ratio of H2O to CO2 of 1:1. The surface of a quartz grain was carefully polished to bring promising artificial fluid inclusions to within 2 µm of the surface. Before freezing, the boundaries between water and CO2 ice were apparent, as was a separate H2O-CO2 clathrate phase at that boundary. Thus, even before freezing the inclusions were compositionally heteorgeneous, although the bulk compositions should all have been equal and constant. H2O:CO2 Measurements: For the previous smeasurements in the pyrrhotite crystal we used CO-as the molecular fragment to indicate the presence and apparent concentration of CO2 [1]. However, host quartz in the standards contained major oxygen, which precluded reliance on the CO- molecule. Therefore, we used the molecular fragment Cto measure CO2. Measurement of five standard fluid inclusions (entire inclusions rather than measurements of a few "slices") produced the following results. In four of the five standard inclusions the ratio H2O:CO2 varied by less than 10% relative, which was considered a success. The fifth inclusion produced a ratio 2x greater than the others, possibly due to leakage of CO2. The next step is to measure stardards inclusions with at least one additional, different H2O:CO2 ratio, which will permit construction of a calibration curve for determining actual H2O:CO2 ratios in fluid inclusions. In the course of this work we also verified that different species in the fluid inclusions freeze at different temperatures, resulting in frozen inclusions whose composition varies unpredictably as they are sequentially sputtered and measured by TOF-SIMS. This behavior necessitates that fluid inclusion contents must be measured in-toto in order to produce accurate bulk compositions. [ABSTRACT FROM AUTHOR]

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe 3+ to Fe 2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss., Competing Interests: Competing interestsThe authors declare no competing interests., (© The Author(s) 2022.)