Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Samples collected from the surface/subsurface of C-type asteroid 162173 Ryugu by the Hayabusa2 mission were nondestructively analyzed in three dimensions (3D). Seventy-three small particles (approximately 10–180 µm in size) were observed using X-ray nanotomography, with an effective spatial resolution of approximately 200 nm. Detailed descriptions of these samples in terms of mineralogy, petrology, and variations among particles were reported. The 57 most common particles consisted of a phyllosilicate matrix containing mineral grains, mainly magnetite, pyrrhotite, dolomite and apatite. The remaining particles were mostly monomineralic particles (pyrrhotite, dolomite, breunnerite, apatite, and Mg-Na phosphate) with two unique particles (calcite in a Al2Si2O5(OH)4 matrix, and CaCO3, phyllosilicate, and tochilinite-chronstedtite inclusions in a carbonaceous material matrix). The results confirmed that the samples correspond to Ivuna-type carbonaceous chondrites (CI chondrites) or related materials. Many small inclusions of voids and carbonaceous materials were detected in pyrrhotite, dolomite, breunnerite, and apatite. However, no fluid inclusions were observed, except for those in pyrrhotite that have already been reported. Magnetite exhibited a wide variety of morphologies, from irregular shapes (spherulites, framboids, plaquettes, and whiskers) to euhedral shapes (equants, rods, and cubes), along with transitional shapes. In contrast, the other minerals exhibit predominantly euhedral shapes (pyrrhotite: pseudo-hexagonal plates, dolomite: flattened rhombohedrons, breunnerite: largely flattened rhombohedrons, and apatite: hexagonal prisms) or aggregates of faceted crystals, except for Mg-Na phosphate. The matrices were heterogeneous with variable phyllosilicate particle sizes, Mg/Fe ratios, density (1.7 ± 0.2 g/cm3), nanoporosities (36 ± 9 %), and abundances of nanograins of Fe(-Ni) sulfides. The macroporosity of the particles was estimated as 12 ± 4 %. The observed textural relationships among the minerals suggest a precipitation sequence of: magnetite (spherulite → plaquette/framboid → rod/equant) → pyrrhotite (pentlandite → pyrrhotite) → apatite → dolomite → breunnerite → coarse phyllosilicates. Fe-bearing olivine (or low-Ca pyroxene) might have precipitated later than dolomite, indicating a high Mg activity in the aqueous solution. This precipitation sequence corresponds to a transition from irregular crystal forms (as seen in some magnetite) to regular forms of euhedral crystals (observed in some magnetite and other minerals). Based on the precipitation sequence and mineral morphologies, together with previously reported observations, a model for aqueous alteration in the Ryugu parent body was proposed as follows: CO2-H2O ice, amorphous silicates (GEMS-like material), and some minerals (mostly metal, sulfides, and anhydrous silicates) accumulated to form the parent body of Ryugu. Amorphous silicates and Fe-Ni metal quickly dissolved into the melted ice to form a highly supersaturated aqueous solution. Poorly-crystalized phyllosilicate and spherulitic magnetite precipitated first, followed by plaquette/framboidal magnetites with decreasing degree of supersaturation due to precipitation. Pseudo-hexagonal pyrrhotite plates were formed by dissolution and reprecipitation under relatively low supersaturation. Subsequently, apatite, dolomite, and breunnerite precipitated in this order in response to decreasing supersaturation.

DOI： 10.1016/j.gca.2024.03.032

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.icarus.2024.116068

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Publisher：Meteoritics and Planetary Science  

Regolith samples returned from asteroid 162173 Ryugu by the Hayabusa2 mission provide direct means to study how space weathering operates on the surfaces of hydrous asteroids. The mechanisms of space weathering, its effects on mineral surfaces, and the characteristic time scales on which alteration occurs are central to understanding the spectroscopic properties and the taxonomy of asteroids in the solar system. Here, we investigate the behavior of the iron monosulfides mineral pyrrhotite (Fe1−xS) at the earliest stages of space weathering. Using electron microscopy methods, we identified a partially exposed pyrrhotite crystal that morphologically shows evidence for mass loss due to exposure to solar wind ion irradiation. We find that crystallographic changes to the pyrrhotite can be related to sulfur loss from its space-exposed surface and the diffusive redistribution of resulting excess iron into the interior of the crystal. Diffusion profiles allow us to estimate an order of magnitude of the exposure time of a few thousand years consistent with previous estimates of space exposure. During this interval, the adjacent phyllosilicates did not acquire discernable damage, suggesting that they are less susceptible to alteration by ion irradiation than pyrrhotite.

DOI： 10.1111/maps.14176

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Publishing type：Research paper (scientific journal)   Publisher：Nature Astronomy  

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

DOI： 10.1038/s41550-023-02137-z

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Language：English   Publisher：Springer Nature  

Large amounts of nitrogen compounds, such as ammonium salts, may be stored in icy bodies and comets, but the transport of these nitrogen-bearing solids into the near-Earth region is not well understood. Here, we report the discovery of iron nitride on magnetite grains from the surface of the near-Earth C-type carbonaceous asteroid Ryugu, suggesting inorganic nitrogen fixation. Micrometeoroid impacts and solar wind irradiation may have caused the selective loss of volatile species from major iron-bearing minerals to form the metallic iron. Iron nitride is a product of nitridation of the iron metal by impacts of micrometeoroids that have higher nitrogen contents than the CI chondrites. The impactors are probably primitive materials with origins in the nitrogen-rich reservoirs in the outer Solar System. Our observation implies that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is greater than previously recognized.

CiNii Research

Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Transmission electron microscopy analyses of Hayabusa2 samples show that Ryugu organic matter exhibits a range of morphologies, elemental compositions, and carbon functional chemistries consistent with those of carbonaceous chondrites that have experienced low‐temperature aqueous alteration. Both nanoglobules and diffuse organic matter are abundant. Non‐globular organic particles are also present, and including some that contain nanodiamond clusters. Diffuse organic matter is finely distributed in and around phyllosilicates, forms coatings on other minerals, and is also preserved in vesicles in secondary minerals such as carbonate and pyrrhotite. The average elemental compositions determined by energy‐dispersive spectroscopy of extracted, demineralized insoluble organic matter samples A0107 and C0106 are C₁₀₀N₃O₉S₁ and C₁₀₀N₃O₇S₁, respectively, with the difference in O/C slightly outside the difference in the standard error of the mean. The functional chemistry of the nanoglobules varies from mostly aromatic C=C to mixtures of aromatic C=C, ketone C=O, aliphatic (CH_n), and carboxyl (COOH) groups. Diffuse organic matter associated with phyllosilicates has variable aromatic C, ketone and carboxyl groups, and some localized aliphatics, but is dominated by molecular carbonate (CO₃) absorption, comparable to prior observations of clay‐bound organic matter in CI meteorites.

DOI： 10.1111/maps.14128

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Science Advances  

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.

DOI： 10.1126/sciadv.adi7203

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

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The Hayabusa2 mission from the Japan Aerospace Exploration Agency (JAXA) returned to the Earth samples of carbonaceous asteroid (162173) Ryugu. This mission offers a unique opportunity to investigate in the laboratory samples from a C‐type asteroid, without physical or chemical alteration by the terrestrial atmosphere. Here, we report on an investigation of the mineralogy and the organo‐chemistry of Hayabusa2 samples using a combination of micro‐ and nano‐infrared spectroscopy. Particles investigated with conventional FTIR spectroscopy have spectra dominated by phyllosilicate‐related absorption, as observed for samples of CI‐chondrites, selected ungrouped carbonaceous chondrites, and selected hydrated micrometeorites. Ryugu samples show smaller sulfate‐related absorption than CI‐chondrites. Our samples that were only briefly exposed to the Earth atmosphere show absorptions related to molecular water, revealing fast terrestrial contamination of the spectral signature at 3 μm. Overall, our FTIR data are in agreement with other work done on Ryugu samples, revealing a low degree of mineralogical variability across Ryugu samples. AFM‐IR mapping of the grains shows the presence of a micrometer‐sized organic globule in one of our analyzed grains. The AFM‐IR spectra obtained on this globule are similar to IR spectra obtained on IOM suggesting that it is constituted of refractory organic matter. This globule may host silicate in its interior, with a different mineralogy than bulk Ryugu phyllosilicate. The shape, presence of peculiar silicate, and the nature of organic constituting the globule point toward a pre‐accretionary origin of this globule and that at least part of Ryugu organics were inherited from the protosolar nebulae or the interstellar media. Altogether, our results show the similarities between Ryugu samples and CI chondrites.

DOI： 10.1111/maps.14122

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

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Ryugu asteroid grains brought back to the Earth by the Hayabusa2 space mission are pristine samples containing hydrated minerals and organic compounds. Here, we investigate the mineralogy of their phyllosilicate‐rich matrix with four‐dimensional scanning transmission electron microscopy (4D‐STEM). We have identified and mapped the mineral phases at the nanometer scale (serpentine, smectite, pyrrhotite), observed the presence of Ni‐bearing pyrrhotite, and identified the serpentine polymorph as lizardite, in agreement with the reported aqueous alteration history of Ryugu. Furthermore, we have mapped the d‐spacings of smectite and observed a broad distribution of values, ranging from 1 to 2 nm, with an average d‐spacing of 1.24 nm, indicating significant heterogeneity within the sample. Such d‐spacing variability could be the result of either the presence of organic matter trapped in the interlayers or the influence of various geochemical conditions at the submicrometer scale, suggestive of a range of organic compounds and/or changes in smectite crystal chemistry.

DOI： 10.1111/maps.14124

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Publisher：IEEE International Conference on Plasma Science  

Tantalum nitrides attract much attention due to brilliant thermal stability, high dielectric constant, and unique optoelectronic properties. Therefore, tantalum nitrides have been widely applied on many fields, like water splitting catalyst, capacitor, and conductive barrier layer in integrated circuit manufacturing1. Abundant types of stable phases exist in Ta-N system, like TaN0.5, TaN0.8, TaN, Ta3N5, and Ta5N62. Moreover, TaN exhibits two different structures of δ-phase (high temperature phase) and ε-phase (low temperature phase) under different quenching conditions.

DOI： 10.1109/ICOPS58192.2024.10625940

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Publisher：IEEE International Conference on Plasma Science  

Transition metal nitride nanoparticles are promising material in electronics field because of their many attractive features such as high electrical conductivity, corrosion resistance, and so on1. The development of mass synthesis methods for them with high purity is required, and thermal plasma is one of the candidates with their several advantages such as high enthalpy and high chemical reactivity. DC arc is a practical and versatile method to generate thermal plasma because of its high thermal efficiency and stable discharge. The purpose of this study is to synthesize transition metal nitride nanoparticles by DC arc and clarify the nitridation mechanism of metal nitride nanoparticles.

DOI： 10.1109/ICOPS58192.2024.10626277

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Publisher：IEEE International Conference on Plasma Science  

Refractory metal-based alloy nanoparticles are attractive material due to high hardness, high thermal conductivity, and great thermal resistance1. Synthesis methods of refractory metal-based alloys with a high processing rate are limited because of their high melting point, and other synthesis methods need to be established. Induction thermal plasma is considered as one of effective synthesis methods of refractory metal nanoparticles, due to high enthalpy, flexible atmosphere selection, and strong quenching effect. The purpose of this study is to synthesize tungsten-based alloy nanoparticles by induction thermal plasma and investigate the effects of physical properties on the composition and crystal structure of the products.

DOI： 10.1109/ICOPS58192.2024.10626598

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

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Samples were recently collected from the carbonaceous asteroid (162173) Ryugu, by the Japan Aerospace Exploration Agency (JAXA) Hayabusa2 mission. They resemble CI chondrites material, thus showing clear evidence of extensive aqueous alteration attested by the widespread presence of a mixture of serpentine and saponite. We present here a scanning transmission electron microscopy study of the Ryugu dominant lithology of the phyllosilicate matrix at the nanometer scale, which we compare with that of the Orgueil CI chondrite. In both objects, the phyllosilicates are of comparable nature and texture, consisting of a mixture of small‐sized crystallites of serpentine and saponite. At the micrometer scale or less, the texture is an alternation of fine and coarse domains. The fine‐grained regions are dominated by saponite. In Ryugu, they enclose numerous Fe,Ni nanosulfides, whereas in Orgueil, S‐ and Ni‐rich ferrihydrite is abundant. The coarse‐grained regions contain more serpentine and no or little Fe,Ni sulfides or ferrihydrite. Scanning transmission x‐ray microscopy at the Fe‐L₃ edge also reveals that iron valency of phyllosilicates is higher and more homogeneous in Orgueil (~70% Fe³⁺) than in Ryugu (<50% Fe³⁺). We interpret the observed textures as being mostly a consequence of aqueous alteration, likely resulting from the replacement by phyllosilicates of submicrometric components, initially agglomerated by a primary accretion. The fine‐grained domains may result from the replacement of GEMS (GEMS—glass with embedded metal and sulfides) objects or from other types of nanometric assemblages of silicate and Fe‐based nanophases. On the other hand, the coarse‐grained regions may correspond to the replacement of anhydrous crystalline silicates of the olivine and pyroxene type. The major difference is the presence of Fe,Ni sulfides in Ryugu and of ferrihydrite and higher iron valency of phyllosilicates in Orgueil. This might be due to long‐term terrestrial weathering that would have destabilized the nanosulfides. We also explore an alternative scenario involving more oxidizing hydrothermal conditions on the Orgueil parent body.

DOI： 10.1111/maps.14101

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

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Samples returned from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 mission revealed that Ryugu is composed of materials consistent with CI chondrites and some types of space weathering. We report detailed mineralogy of the fine‐grained Ryugu samples allocated to our “Sand” team and report additional space weathering features found on the grains. The dominant mineralogy is composed of a fine‐grained mixture of Mg‐rich saponite and serpentine, magnetite, pyrrhotite, pentlandite, dolomite, and Fe‐bearing magnesite. These grains have mineralogy comparable to that of CI chondrites, showing severe aqueous alteration but lacking ferrihydrite and sulfate. These results are similar to previous works on large Ryugu grains. In addition to the major minerals, we also find many minerals that are rare or have not been reported among CI chondrites. Accessory minerals identified are hydroxyapatite, Mg‐Na phosphate, olivine, low‐Ca pyroxene, Mg‐Al spinel, chromite, manganochromite, eskolaite, ilmenite, cubanite, polydymite, transjordanite, schreibersite, calcite, moissanite, and poorly crystalline phyllosilicate. We also show scanning transmission electron microscope and scanning electron microscope compositional maps and images of some space‐weathered grains and severely heated and melted grains. Although our mineralogical results are consistent with that of millimeter‐sized grains, the fine‐grained fraction is best suited to investigate impact‐induced space weathering.

DOI： 10.1111/maps.14093

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Geochimica et Cosmochimica Acta  

We have investigated several particles collected during each of two touchdowns of the Hayabusa2 spacecraft at the surface of the C-type asteroid 162173 Ryugu using various electron microscope techniques. Our detailed transmission electron microscopy study shows the presence of magnetite with various morphologies coexisting in close proximity. This is characteristic of CI chondrite-like materials and consistent with the mineral assemblages and compositions in the Ryugu parent body. We describe the microstructural characteristics of magnetite with different morphologies, which could have resulted from the chemical conditions (growth vs. diffusion rate) during their formation. Furthermore, we describe the presence of magnetites with a spherulitic structure composed of individual radiating fibers that are characterized by pervasive, homogeneously distributed euhedral to subhedral pores that have not been described in previous chondrite studies. This particular spherulitic structure is consistent with crystallization under nonequilibrium conditions. Additionally, the presence of a high density of defects within the magnetite fibers, the high surface/volume ratio of this morphology, and the presence of amorphous materials in several pores and at the edges of the acicular fibers further support their formation under nonequilibrium conditions. We suggest that the growth processes that lead to this structure result from the solution reaching a supersaturated state, resulting in an adjustment to a lower free energy condition via nucleation and rapid growth.

DOI： 10.1016/j.gca.2023.02.003

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DOI： 10.21203/rs.3.rs-2389559/v1

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：American Association for the Advancement of Science (AAAS)  

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu’s parent body formed ~2 million years after the beginning of Solar System formation.

DOI： 10.1126/science.abn8671

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

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 Fe³⁺ to Fe²⁺ 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.

DOI： 10.1038/s41550-022-01841-6

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Other Link： https://www.nature.com/articles/s41550-022-01841-6

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

Glass with embedded metal and sulfides (GEMS), the major components of chondritic-porous interplanetary dust particles (CP-IDPs), is one of the most primitive materials in the Solar System and may be analogous to the amorphous silicate dust observed in various astronomical environments. Mineralogical characteristics of GEMS should reflect their formation process and condition. In this study, synthetic experiments in the sulfur-bearing system of Fe–Mg–Si–O–S were performed with a systematic change in redox conditions using thermal plasma systems to reproduce the mineralogy and textures of GEMS. The resulting condensates were composed of amorphous silicates with Fe-bearing nano-inclusions. The Fe content and texture in the amorphous silicates as well as the mineral phases of the nanoparticles correlate with redox conditions. Fe dissolved in the amorphous silicate as FeO in oxidizing conditions formed Fe-metal nanoparticles in intermediate redox conditions, and gupeiite (Fe₃ Si) nanoparticles in reducing conditions. In intermediate to reducing redox conditions, Fe-poor amorphous silicate formed a biphasic texture with Mg- and Si-rich regions, indicating liquid immiscibility during the melt phase. Most Fe-metal particles were surrounded by FeS and formed on the surface of amorphous silicate grains. Condensates produced in intermediate to slightly reducing redox conditions resemble GEMS in that they have similar mineral assemblages and chemical compositions to amorphous silicate, except that the Fe-metal grains are absent from the interior of the amorphous silicate grains. This textural difference can be explained by the sulfidation at high temperatures in this study, in contrast to sulfidation occurring at low temperatures in the presence of H₂ in natural GEMS formation. Based on the two-liquid structures observed in the experimental products and in GEMS, also recognized in infrared spectra, we propose that GEMS condensed as silicate melt under limited redox conditions followed by incorporation of multiple metal grains into the silicate melt or by aggregation of coreshell structured grains before sulfidation of the metallic iron. Condensates produced in oxidizing conditions are similar to GEMS-like material in the matrices of primitive carbonaceous chondrite meteorites, indicating the possibility that they form by direct condensation from nebula gas in relatively oxidizing conditions compared to GEMS.

DOI： 10.1051/0004-6361/202142620

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.gca.2022.02.024

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

DOI： 10.1016/j.gca.2021.12.031

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Glass with embedded metal and sulfides (GEMS) is a major component of chondritic porous interplanetary dust particles. Although GEMS is one of the most primitive components in the Solar System, its formation process and conditions have not been constrained. We performed condensation experiments of gases in the system of Mg–Si–O (MgSiO₃ composition) and of the S-free CI chondritic composition (Si–Mg–Fe–Na–Al–Ca–Ni–O system) in induction thermal plasma equipment. Amorphous Mg-silicate particles condensed in the experiments of the Mg–Si–O system, and their grain size distribution depended on the experimental conditions (mainly partial pressure of SiO). In the CI chondritic composition experiments, irregularly shaped amorphous silicate particles of less than a few hundred nanometers embedded with multiple Fe–Ni nanoparticles of ≤20 nm were successfully synthesized. These characteristics are very similar to those of GEMS, except for the presence of FeSi instead of sulfide grains. We propose that the condensation of amorphous silicate grains smaller than a few tens of nanometers and with metallic cores, followed by coagulation, could be the precursor material that forms GEMS prior to sulfidation.

DOI： 10.1051/0004-6361/202141216

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DOI： 10.3847/1538-4357/abe5a0

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Carbonaceous chondrites are meteorites believed to preserve our planet fs source materials, but the precise nature of these materials still remains uncertain. To uncover pristine planetary materials, we performed synchrotron radiation.based x-ray computed nanotomography of a primitive carbonaceous chondrite, Acfer 094, and found ultraporous lithology (UPL) widely distributed in a fine-grained matrix. UPLs are porous aggregates of amorphous and crystalline silicates, Fe., Ni sulfides, and organics. The porous texture must have been formed by removal of ice previously filling pore spaces, suggesting that UPLs represent fossils of primordial ice. The ice-bearing UPLs formed through sintering of fluffy icy dust aggregates around the H2O snow line in the solar nebula and were incorporated into the Acfer 094 parent body, providing new insight into asteroid formation by dust agglomeration.

DOI： 10.1126/sciadv.aax5078

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Mid-infrared spectroscopic observations of oxygen-rich asymptotic giant branch (AGB) stars show the common presence of dust species that have a broad feature at ∼11-12 μm. Chemically synthesized amorphous alumina (Al2O3) is widely accepted as the source of this emission, although it is not obvious that amorphous alumina can condense in circumstellar conditions. We performed condensation experiments of Al-Si-Mg-O and Mg-Al-O gases using induction thermal plasma systems, in which small particles condense from vapors with a steep temperature gradient. The condensates were analyzed using X-ray diffraction and Fourier transform infrared spectroscopy, and observed with a transmission electron microscope. The condensed nanoparticles from the Al and O gases were transition aluminas based on face-centered cubic (fcc) packed oxygen (δ- and λ-alumina, and an unknown phase). The fcc oxygen frameworks were maintained in the condensed alumina containing small amounts of Mg and Si. Condensates from the gases of Al:Mg = 99:1 and 95:5 had δ- and γ-alumina structures. Particles with λ- and γ-alumina structures formed from starting materials of Al:Si = 9:1 and Al:Si:Mg = 8:1:1, respectively. Amorphous silica-rich particles condensed from gases of Al/(Si+Al) < 0.75. The condensed transition alumina containing ∼10% Si showed similar spectral shapes to the observed dust emission from the alumina-rich AGB star T Cep. Based on the present results, it is reasonable that the source of 11-12 μm broad emission of alumina-rich stars is not amorphous alumina, but is transition alumina containing ∼10% Si.

DOI： 10.3847/2041-8213/ab1f80

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The electronic structures of forsterite and its amorphous state were investigated using soft X-ray absorption near-edge structure (XANES) analysis at the Mg, Si, and O K-edges. The experimental spectra were reproduced theoretically using density functional theory calculations. Surface-sensitive XANES spectra for the (001), (010), and (100) surfaces were also observed. This study reports the first observation at the O K-edge using synchrotron radiation, and, in addition, reveals that the spectra is surface-sensitive at the Mg and Si K-edges. These findings demonstrate that XANES is a powerful technique to study silicate compounds.

DOI： 10.1063/1.5017245

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The presence of FeO particles in circumstellar space has been suggested based on the observation of a mysterious 21 μm emission band. However, the complete infrared spectra of FeO have not been obtained so far; hence, data of the infrared (IR) spectra of FeO need to be investigated. We prepared synthetic and commercial samples of FeO, which were obtained by crushing bulk samples, annealing iron oxalate dihydrate (${mathrm{FeC } }_{2}{ {
m{O } } }_{4}cdot 2{ {
m{H } } }_{2}{
m{O } }$), and mechanical milling of a powder mixture comprising (Fe and ${mathrm{Fe } }_{2}{ {
m{O } } }_{3}$) particles with different milling times. We present a new study on the IR spectra of these samples, and show that these spectra changed according to defects and disorders. Furthermore, FeO particles are very sensitive to oxygen fugacity and temperature. The spectra of FeO particles were compared with the unidentified observed feature. It may be difficult for FeO particles to exist alone in the ISM and circumstellar space. This may be connected to the problem of missing iron in the ISM.

DOI： 10.3847/1538-4357/aa7deb

Language：English   Publishing type：Research paper (scientific journal)  

Refractory organic compounds formed in molecular clouds are among the building blocks of the solar system objects and could be the precursors of organic matter found in primitive meteorites and cometary materials. However, little is known about the evolutionary pathways of molecular cloud organics from dense molecular clouds to planetary systems. In this study, we focus on the evolution of the morphological and viscoelastic properties of molecular cloud refractory organic matter. We found that the organic residue, experimentally synthesized at similar to 10 K from UV-irradiated H2O-CH3OH-NH3 ice, changed significantly in terms of its nanometer to micrometer-scale morphology and viscoelastic properties after UV irradiation at room temperature. The dose of this irradiation was equivalent to that experienced after short residence in diffuse clouds (<= 10(4) years) or irradiation in outer protoplanetary disks. The irradiated organic residues became highly porous and more rigid and formed amorphous nanospherules. These nanospherules are morphologically similar to organic nanoglobules observed in the least-altered chondrites, chondritic porous interplanetary dust particles, and cometary samples, suggesting that irradiation of refractory organics could be a possible formation pathway for such nanoglobules. The storage modulus (elasticity) of photo-irradiated organic residues is similar to 100 MPa irrespective of vibrational frequency, a value that is lower than the storage moduli of minerals and ice. Dust grains coated with such irradiated organics would therefore stick together efficiently, but growth to larger grains might be suppressed due to an increase in aggregate brittleness caused by the strong connections between grains.

DOI： 10.3847/1538-4357/aa5ca6

Language：English   Publishing type：Research paper (scientific journal)  

The morphological properties of 26 regolith particles from asteroid Itokawa were observed using scanning electron microscopes in combination with an investigation of their three-dimensional shapes obtained through X-ray microtomography. Surface observations of a cross section of the LL5 chondrite, and of crystals of olivine and pyroxene, were also performed for comparison. Some Itokawa particles have surfaces corresponding to walls of microdruses in the LL chondrite, where concentric polygonal steps develop and euhedral or subhedral grains exist. These formed through vapor growth owing to thermal annealing, which might have been caused by thermal metamorphism or shock-induced heating in Itokawa's parent body. Most of the Itokawa particles have more or less fractured surfaces, indicating that they were formed by disaggregation, probably caused by impacts. Itokawa particles with angular and rounded edges observed in computed tomography images are associated with surfaces exhibiting clear and faint structures, respectively. These surfaces can be interpreted by invoking different degrees of abrasion after regolith formation. A possible mechanism for the abrasion process is grain migration caused by impact-driven seismic waves. Space-weathered rims with blisters are distributed heterogeneously across the Itokawa regolith particles. This heterogeneous distribution can be explained by particle motion and fracturing, combined with solar-wind irradiation of the particle surfaces. The regolith activity-including grain motion, fracturing, and abrasion-might effectively act as refreshing process of Itokawa particles against space-weathered rim formation. The space-weathering processes affecting Itokawa would have developed simultaneously with space-weathered rim formation and regolith particle refreshment. (C) 2016 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.gca.2016.05.011

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.14909/yuseijin.24.2_86

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1111/maps.12418

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1093/jmicro/dfu055

Language：English   Publishing type：Research paper (scientific journal)  

REDUCTION EXPERIMENT OF FeO-BEARING AMORPHOUS SILICATE: APPLICATION TO ORIGIN OF METALLIC IRON IN GEMS

DOI： 10.1088/0004-637x/792/2/136

Language：English   Publishing type：Research paper (scientific journal)  

We report the investigation of cutting methods for Hayabusa samples. The purpose of our study is to explore the possibility of applying multiple analyses to a single particle effectively. We investigated the cutting performance of a blade dicing saw, laser, focused ion beam (FIB), and physical breaking by microindenter. Cutting performance was examined by estimating the aspect ratio of the cut slit, i.e., depth over width of the slit. We also investigated the possible contamination and sample damage by cutting. The result of the investigation shows that we can cut the samples from <50 μm to 500 μm using those methods with aspect ratios from 10 to 20, although they would introduce some contamination or damage to the samples. Our investigations also provide an important basis for the analysis of samples obtained by future sample return missions. © The Meteoritical Society, 2014.

DOI： 10.1111/maps.12322

Language：Others   Publishing type：Research paper (scientific journal)  

We observed cross sectional ultra-thin sections near the surface of 12 particles recovered from the S-type asteroid Itokawa by the Hayabusa spacecraft in 2010, using spherical aberration-corrected STEM and conventional TEM. Although their mineralogy is almost identical to the equilibrated LL chondrites and therefore basically anhydrous, micrometer-to-submicrometer-sized sylvite was identified on the surface of Itokawa particle RA-QD02-0034. Separately, micrometer-sized halite was also identified on the surface of Itokawa particle RA-QD02-0129. Detailed inspection of the sample processing procedures at the JAXA's Planetary Materials Sample Curation Facility and textural observation of the sylvite and halite indicate that they were clearly present on two Itokawa particles before they were removed from Clean Chamber #2 at JAXA. However, there is no direct evidence for their extraterrestrial origin at present. If the sylvite and halite are extraterrestrial, their presence suggests that they may be more abundant on the surface of S-type asteroids than previously thought. © The Meteoritical Society, 2014.

DOI： 10.1111/maps.12333

Language：English   Publishing type：Research paper (scientific journal)  

On the basis of observations using Cs-corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2-3nm), (2) composite rims (30-60nm), and (3) composite vesicular rims (60-80nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two-layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation-induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa.

DOI： 10.1111/maps.12111

Language：English   Publishing type：Research paper (scientific journal)  

In this study, the three-dimensional (3-D) microstructure of 48 Itokawa regolith particles was examined by synchrotron microtomography at SPring-8 during the preliminary examination of Hayabusa samples. Moreover, the 3-D microstructure of particles collected from two LL6 chondrites (Ensisheim and Kilabo meteorites) and an LL5 chondrite (Tuxtuac meteorite) was investigated by the same method for comparison. The modal abundances of minerals, especially olivine, bulk density, porosity, and grain size are similar in all samples, including voids and cracks. These results show that the Itokawa particles, which are surface materials from the S-type asteroid Itokawa, are consistent with the LL chondrite materials in terms of not only elemental and isotopic composition of the minerals but also 3-D microstructure. However, we could not determine whether the Itokawa particles are purely LL5, LL6, or a mixture of the two. No difference between the particles collected from Rooms A and B of the sample chamber, corresponding to the sampling sequence of the spacecraft's second and first touchdowns, respectively, was detected because of the statistically small amount of particles from Room B.

DOI： 10.1111/maps.12177

Language：English   Publishing type：Research paper (scientific journal)  

We developed a novel technique called "analytical dual-energy microtomography" that uses the linear attenuation coefficients (LACs) of minerals at two different X-ray energies to nondestructively obtain three-dimensional (3D) images of mineral distribution in materials such as rock specimens. The two energies are above and below the absorption edge energy of an abundant element, which we call the "index element". The chemical compositions of minerals forming solid solution series can also be measured. The optimal size of a sample is of the order of the inverse of the LAC values at the X-ray energies used. We used synchrotron-based microtomography with an effective spatial resolution of &gt
200. nm to apply this method to small particles (30-180. μm) collected from the surface of asteroid 25143 Itokawa by the Hayabusa mission of the Japan Aerospace Exploration Agency (JAXA). A 3D distribution of the minerals was successively obtained by imaging the samples at X-ray energies of 7 and 8. keV, using Fe as the index element (the K-absorption edge of Fe is 7.11. keV). The optimal sample size in this case is of the order of 50. μm. The chemical compositions of the minerals, including the Fe/Mg ratios of ferromagnesian minerals and the Na/Ca ratios of plagioclase, were measured. This new method is potentially applicable to other small samples such as cosmic dust, lunar regolith, cometary dust (recovered by the Stardust mission of the National Aeronautics and Space Administration [NASA]), and samples from extraterrestrial bodies (those from future sample return missions such as the JAXA Hayabusa2 mission and the NASA OSIRIS-REx mission), although limitations exist for unequilibrated samples. Further, this technique is generally suited for studying materials in multicomponent systems with multiple phases across several research fields. © 2012.

DOI： 10.1016/j.gca.2012.11.036

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0004-637x/753/2/141

Language：English   Publishing type：Research paper (scientific journal)  

Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles' size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily formmuch of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.

DOI： 10.1126/science.1207807