記述言語：英語   掲載種別：研究論文（学術雑誌）  

Meteoritic evidence suggests that oxygen isotopic exchange between 16O-rich amorphous silicate dust and 16O-poor water vapor occurred in the early solar system. In this study, we experimentally investigated the kinetics of oxygen isotopic exchange between submicron-sized amorphous forsterite grains and water vapor at protoplanetary disk-like low pressures of water vapor. The isotopic exchange reaction rate is controlled either by diffusive isotopic exchange in the amorphous structure or by the supply of water molecules from the vapor phase. The diffusive oxygen isotopic exchange occurred with a rate constant D (m2 s−1) = (1.5 ± 1.0) × 10−19 exp[−(161.5 ± 14.1 (kJ mol−1))R−1(1/T−1/1200)] at temperatures below ∼800–900 K, and the supply of water molecules from the vapor phase could determine the rate of oxygen isotopic exchange at higher temperatures in the protosolar disk. On the other hand, the oxygen isotopic exchange rate dramatically decreases if the crystallization of amorphous forsterite precedes the oxygen isotopic exchange reaction with amorphous forsterite. According to the kinetics for oxygen isotopic exchange in protoplanetary disks, original isotopic compositions of amorphous forsterite dust could be preserved only if the dust was kept at temperatures below 500–600 K in the early solar system. The 16O-poor signatures for the most pristine silicate dust observed in cometary materials implies that the cometary silicate dust experienced oxygen isotopic exchange with 16O-poor water vapor through thermal annealing at temperatures higher than 500–600 K prior to their accretion into comets in the solar system.

DOI： 10.3847/1538-4357/aadcee

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Amorphous silicate dust grains, dominant solid components in the interstellar medium, are converted into crystalline silicate dust through thermal annealing in protoplanetary disks. Water vapor is a major reactive gas species in the protoplanetary disk, and it may affect the crystallization behavior of amorphous silicates. In this study, the water vapor pressure dependence of the crystallization kinetics of amorphous silicate with forsterite composition was investigated under controlled water vapor pressures ranging from ∼1 × 10–9 to 5 × 10–3 bar at 923–1023 K. We found that the crystallization rate depends on the water vapor pressure and becomes faster at higher water vapor pressures. We also found that the activation energy and the pre-exponential factor for crystallization rate decreases with increasing water vapor pressure. Water molecules dissolving into amorphous forsterite cut atomic bonds such as Si–O–Si and Mg–O–Mg through a hydroxyl (−OH) formation reaction. Rearrangement of structural units cut by hydroxyls occurs with a smaller energetic barrier, and thus water vapor can act as a catalyst to promote crystallization of amorphous forsterite. Based on the experimental data, we conclude that the temperature required for crystallization of amorphous forsterite within the lifetime of protoplanetary disks is ∼620–700 K irrespective of the water vapor pressure in the disk and that the observed crystalline forsterite dust in protoplanetary disks indicates the presence of dust annealed at temperatures above ∼620–700 K. Extraterrestrial materials record various thermal events in the early Solar System (e.g., chondrule formation). Considering that meteoritic evidence indicates that the H2O/H2 ratio was enhanced over the canonical ratio in the early Solar System, the thermal evolution of amorphous forsterite dust during various thermal events in the early Solar System should be discussed taking the effect of water vapor pressure into account.

DOI： 10.1021/acsearthspacechem.8b00047

出版者・発行元：Icarus  

The evolution associated with coagulation/fragmentation processes of dust to planetesimals in the protosolar disk is the critical phase of planet formation in the Solar System. The meteoritic components, such as calcium‑aluminum-rich inclusions (CAIs), will provide essential constraints on the coagulation/fragmentation process in the early stage of the disk. We applied the bright-field grinding tomography method to an Allende meteorite (CV3) slab to visualize the coarse-grained CAIs (CG-CAIs) in a colorized 3D model with high spatial resolution. We found four mm-scale CG-CAIs that experienced deformation and/or fragmentation processes within ∼1.8 × 10³ mm³ Allende slab. An angular-shaped CG-CAI's surface showed the anisotropy of red-gray and white sides, which suggests that the fragmentation results in the loss of the primitive Wark-Lovering rim. We also found a vesicular-shaped CG-CAI, which indicates that the fracturing and complex formation process of this CG-CAI likely proceeded prior to the accretion of the Wark-Lovering rim. Our observations reveal that the fragmentation of Allende CAIs occurred during the parent body accretion stage and also in the protosolar disk.

DOI： 10.1016/j.icarus.2025.116648

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DOI： https://doi.org/10.3847/1538-4357/ad7795

出版者・発行元：Astrophysical Journal  

Meteorites and their components exhibit a diverse range of oxygen isotope compositions, and the isotopic exchange timescale between dust grains and ambient gas is a key parameter for understanding the spatiotemporal evolution of the solar nebula. As dust grains existed as macroscopic aggregates in the solar nebula, it is necessary to consider the isotopic exchange timescales for these aggregates. Here, we theoretically estimate the isotope exchange timescales between dust aggregates and ambient vapor. The isotope exchange process between aggregates and ambient vapor is divided into four processes: (i) supply of gas molecules to the aggregate surface, (ii) diffusion of molecules within the aggregate, (iii) isotope exchange on the surface of constituent particles, and (iv) isotope diffusion within the particles. We evaluate these timescales and assess which one becomes the rate-determining step. We reveal that the isotope exchange timescale is approximately the same as that of the constituent particles when the aggregate radius is smaller than the critical value, which is a few centimeters when considering the exchange reaction between amorphous forsterite aggregates and water vapor.

DOI： 10.3847/1538-4357/ad7795

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記述言語：英語   出版者・発行元：Elsevier  

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (P<inf>CO</inf>) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and P<inf>CO</inf> of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H<inf>2</inf>O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and P<inf>CO</inf> of 0.3 Pa, and the reaction rate of D (m²/s) = (3.1 ± 2.3) × 10⁻²³ exp[−41.7 ± 9.6 (kJ mol⁻¹) R⁻¹ (1/T − 1/1200)] was obtained. We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H<inf>2</inf>O gases are larger than those of gaseous isotope exchange between CO and H<inf>2</inf>O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H<inf>2</inf>O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H<inf>2</inf>O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H<inf>2</inf>O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

DOI： 10.1016/j.gca.2024.04.014

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CiNii Research

記述言語：日本語   出版者・発行元：一般社団法人日本地球化学会  

<p>Ca-Al-rich Inclusions (CAIs) に含まれるAlやTiに富む輝石 (Fassaite) は、原始太陽系円盤ガスの環境変化を反映することが期待される。希土類元素(REE)は起源や分別プロセスだけでなく、酸化還元指標として活用できる可能性があるものの、fassaite中の結晶成長に伴った特徴は未だに報告されていない。本研究ではEPMA分析によるTi濃度を用いた結晶成長を考慮して、LA-ICP-MSを用いた微量元素分析を行った。REEパターンやSc/Luはメルトからの分別結晶によるトレンドを示す。Y/HoやTm/Tm*からはCAI前駆物質が分別したガスからの凝縮物でない可能性が示唆された。一方で、Ce/Ce*やYb/Yb*は酸化還元指標としては活用できないものの、Eu/Eu*はfassaite結晶成長中のCAIメルトの酸化還元指標として使用できる可能性を示唆する。</p>

DOI： 10.14862/geochemproc.71.0_161

CiNii Research

記述言語：日本語   出版者・発行元：一般社団法人日本地球化学会  

<p>太陽系の始原的固体物質における酸素同位体組成の非質量依存型の多様性は、原始太陽系円盤での始原的物質と円盤ガスとの酸素同位体交換反応に起因する。本講演では、酸素同位体組成分布を解釈するための基礎データ取得を目的としておこなった、フォルステライト組成の非晶質ケイ酸塩と低圧H2O、COガスとの酸素同位体交換実験の結果と、それから見えてきた原始太陽系円盤でのケイ酸塩ダスト酸素同位体進化の描像に関して紹介する。講演者のおこなった上記速度論的研究から、非晶質ケイ酸塩とCO、H2Oガスとの酸素同位体交換は、CO-H2Oガス間の同位体交換速度よりも圧倒的に速く進行することがわかった。このことは、非晶質ケイ酸塩ダストの両ガスとの効果的な同位体交換反応により、従来考えられてきたよりも低温でケイ酸塩ダスト-CO-H2O間の同位体平衡が達成される可能性を示す。上記3体間の酸素同位体平衡が達成される温度は、~600-700 Kと推定される。太陽系の多くのケイ酸塩の16Oに乏しい同位体組成は、大部分のケイ酸塩が、円盤で~600-700 Kよりも高温を経験した可能性を示唆する</p>

DOI： 10.14862/geochemproc.71.0_143

CiNii Research

出版者・発行元：Icarus  

Oxygen isotope compositions of chondrules reflect the environment of chondrule formation and its spatial and temporal variations. Here, we present a theoretical model of oxygen isotope exchange reaction between molten silicate spherules and ambient water vapor with finite relative velocity. We found a new phenomenon, that is, mass-dependent fractionation caused by isotope exchange with ambient vapor moving with nonzero relative velocity. We also discussed the plausible condition for chondrule formation from the point of view of oxygen isotope compositions. Our findings indicate that the relative velocity between chondrules and ambient vapor would be lower than several 100ms⁻¹ when chondrules crystallized.

DOI： 10.1016/j.icarus.2023.115690

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記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Dust particles in protoplanetary disks experience various chemical reactions under different physicochemical conditions through their accretion and diffusion, which results in the radial chemical gradient of dust. We performed three-dimensional Monte Carlo simulations to evaluate the dust trajectories and the progress of fictitious irreversible reactions, of which kinetics is expressed by the Johnson-Mehl-Avrami equation. The distribution of the highest temperature that each particle experiences before the degree of reaction exceeds a certain level shows the log-normal distribution, and its mode temperature was used as the effective reaction temperature. Semi-analytical prediction formulas of the effective reaction temperature and its dispersion were derived by comparing a reaction timescale with a diffusive transport timescale of dust as a function of the reaction parameters and the disk parameters. The formulas reproduce the numerical results of the effective reaction temperatures and their dispersions within 5.5 and 24 %, respectively, in a wide temperature range (200-1400 K). We applied the formulas for the crystallization of amorphous silicate dust and its oxygen isotope exchange with the H2O vapor based on the experimentally determined kinetics. For sub-micron sized amorphous forsterite dust, the predicted effective reaction temperature for the oxygen isotope exchange was lower than that of crystallization without overlap even considering their dispersions. This suggests that the amorphous silicate dust in the protosolar disk could exchange their oxygen isotopes efficiently with the 16O-poor H2O vapor, resulting in the distinct oxygen isotope compositions from the Sun.

DOI： 10.3847/1538-4357/acf310

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/acf310

Web of Science

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Oxygen isotope compositions of chondrules reflect the environment of chondrule formation and its spatial and temporal variations. Here, we present a theoretical model of oxygen isotope exchange reaction between molten silicate spherules and ambient water vapor with finite relative velocity. We found a new phenomenon, that is, mass-dependent fractionation caused by isotope exchange with ambient vapor moving with nonzero relative velocity. We also discussed the plausible condition for chondrule formation from the point of view of oxygen isotope compositions. Our findings indicate that the relative velocity between chondrules and ambient vapor would be lower than several when chondrules crystallized.

DOI： https://doi.org/10.1016/j.icarus.2023.115690

その他リンク： https://pdf.sciencedirectassets.com/272593/1-s2.0-S0019103523X0013X/1-s2.0-S0019103523002671/main.pdf?X-Amz-Security-Token=IQoJb3JpZ2luX2VjEMv%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaCXVzLWVhc3QtMSJIMEYCIQDwA1Sz95wWGYtpNkGTrhOjqRmQH%2FMnMwWEoLWW4UxqAwIhAPiqUw0%2F

記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Meteoritics and Planetary Science  

Al–Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) have revealed the initial 26Al/27Al ratios, (26Al/27Al)0, for individual Ca-Al-rich inclusions (CAIs) in meteorites. We find that the relative sensitivity factors of 27Al/24Mg ratio for SIMS analysis of hibonite, one of the major constituent minerals of CAIs, exhibit variations based on their chemical compositions. This underscores the critical need for using appropriate hibonite standards to obtain accurate AlMg data. We measured the AlMg mineral isochron for hibonite in a fine-grained CAI (FGI) from the Northwest Africa 8613 reduced CV chondrite by SIMS using synthesized hibonite standards with 27Al/24Mg of ~30, ~100, and ~400. The obtained mineral isochron of hibonite in the FGI yields (26Al/27Al)0 of (4.73 0.09) 9 105, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI (Kawasaki et al., 2020). The uncertainties of (26Al/27Al)0 indicate that the constituent minerals in the FGI formed within ~0.02 Myr in the earliest solar system. The disequilibrium O-isotope distributions of the minerals in the FGI suggest that the O-isotope compositions of the nebular gas from which they condensed underwent a transitional change from 16O-rich to 16O-poor within ~0.02 Myr in the earliest solar system. Once formed, the FGI may have been removed from the forming region within ~0.02 Myr and transported to the accretion region of the parent body.

DOI： 10.1111/maps.13989

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記述言語：英語   出版者・発行元：Science  

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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PubMed

記述言語：英語   出版者・発行元：Science  

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

DOI： 10.1126/science.abn9033

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PubMed

記述言語：英語   出版者・発行元：Science  

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37° ± 10°C, about 5:2^+0.8<inf>-0.7</inf> million (statistical) or 5:2^+1.6-<inf>2.1</inf> million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles that of the Sun's photosphere than other natural samples do.

DOI： 10.1126/science.abn7850

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PubMed

記述言語：英語   出版者・発行元：Science  

The near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth’s volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.

DOI： 10.1126/science.abo0431

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PubMed

記述言語：英語   出版者・発行元：Science  

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

DOI： 10.1126/science.abn9057

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PubMed

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.

DOI： DOI: 10.1126/science.abn9057

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

DOI： DOI: 10.1126/science.abn9033

記述言語：英語   出版者・発行元：Springer Nature  

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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CiNii Research

記述言語：英語   出版者・発行元：Science Advances  

The extraterrestrial materials returned from asteroid (162173) Ryugu consist predominantly of low-temperature aqueously formed secondary minerals and are chemically and mineralogically similar to CI (Ivuna-type) carbonaceous chondrites. Here, we show that high-temperature anhydrous primary minerals in Ryugu and CI chondrites exhibit a bimodal distribution of oxygen isotopic compositions: ¹⁶O-rich (associated with refractory inclusions) and ¹⁶O-poor (associated with chondrules). Both the ¹⁶O-rich and ¹⁶O-poor minerals probably formed in the inner solar protoplanetary disk and were subsequently transported outward. The abundance ratios of the ¹⁶O-rich to ¹⁶O-poor minerals in Ryugu and CI chondrites are higher than in other carbonaceous chondrite groups but are similar to that of comet 81P/Wild2, suggesting that Ryugu and CI chondrites accreted in the outer Solar System closer to the accretion region of comets.

DOI： 10.1126/sciadv.ade2067

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PubMed

出版者・発行元：Geochimica Et Cosmochimica Acta  

Coarse-grained igneous calcium-aluminum-rich inclusions (CAIs) are suggested to have experienced gas–melt isotope exchange of oxygen during the melting events of their precursors. Therefore, their oxygen isotope variation would preserve information about the high-temperature processes in the earliest Solar System. We experimentally determined oxygen isotope exchange kinetics between CAI analog melt and carbon monoxide (CO) gas at 1420 °C and 1460 °C under CO gas partial pressures of 0.1, 0.5, and 1 Pa to understand the role of CO gas on the oxygen isotope exchange. We observed oxygen isotope zoning profiles inside the reacted samples that formed through the oxygen isotope exchange reaction at the melt surface and oxygen diffusion in the melt. The zoning profiles were fitted using a three-dimensional spherical diffusion model with time-dependent surface concentration. The oxygen isotope exchange efficiency for colliding CO molecules is estimated to be ∼3.3 × 10^–4, which is much smaller than that for H<inf>2</inf>O (0.28). The oxygen diffusion coefficient obtained in this study is similar to that obtained in the oxygen isotope exchange experiments between the CAI melt and H<inf>2</inf>O, suggesting that the diffusion species in the melt is O^2–, despite the surrounding atmospheres. A comparison of the isotope exchange reaction kinetics between (1) CAI melt and CO gas, (2) CAI melt and H<inf>2</inf>O gas, and (3) CO and H<inf>2</inf>O gases shows that the reaction rate decreases in the order of (3), (2), and (1). The rapid isotope exchange of the reaction (1) indicates that the oxygen isotopic compositions of H<inf>2</inf>O and CO should have been equilibrated during the melting and crystallization processes of igneous CAIs. Both H<inf>2</inf>O and CO change the oxygen isotope compositions of molten CAI in the same direction, although reaction (2) controls the isotope exchange timescale between the CAI melt and surrounding gas. Our dataset demonstrates that type B CAIs having melilite with homogeneous oxygen isotope composition should have been heated for 2–3 days at P<inf>H2</inf> > 100 Pa above the melilite liquidus (∼1400 °C) in the solar protoplanetary disk.

DOI： 10.1016/j.gca.2022.09.006

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記述言語：英語   出版者・発行元：American Association for the Advancement of Science (AAAS)  

The Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial ³He/⁴He and ²⁰Ne/²²Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth’s atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.

DOI： 10.1126/sciadv.abo7239

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CiNii Research

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

The near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth’s volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.

DOI： DOI: 10.1126/science.abo043

出版者・発行元：Nature Astronomy  

Volatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.

DOI： 10.1038/s41550-022-01745-5

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）  

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： DOI: 10.1126/science.abn867

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37° ± 10°C, about 5.2+0.8−0.7 million (statistical) or 5.2+1.6−2.1 million (systematic) years after the formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles that of the Sun’s photosphere than other natural samples do.

DOI： DOI: 10.1126/science.abn785

記述言語：日本語   出版者・発行元：日本惑星科学会  

<p>2020年12月6日に小惑星探査機「はやぶさ2」はC型小惑星リュウグウ表層物質を収めた再突入カ プセルを地球に帰還させた．回収された再突入カプセルに収められた試料コンテナは，オーストラリア現地でのガス採取を実施した後，JAXA相模原キャンパスの惑星物質試料受入設備に搬入され，チェンバー導入前の部品取り外し・洗浄等のプロセスを経てクリーンチェンバー内で真空中での開封・高純度窒素環境下での帰還試料の取り出し・初期記載が行われた．これらのリュウグウ帰還試料の初期記載の結果，これまでに回収されたどの隕石よりも反射率が低く，全体密度が小さい事が判明した．また，赤外反射スペクトルの吸収特性から水酸基を含む含水鉱物と炭酸塩鉱物，及びCH結合に富む有機物が試料中に含まれることが明らかになった．これらの情報を既知の隕石と比較すると，CIコンドライト隕石に最も似ていると言える．また探査機搭載機器によって得られた可視・近赤外スペクトルと比較した結果，帰還試料はリュウグウ表層全体を代表している事が分かった．取り出された試料の一部は既に初期分析チーム，2次キュレーションチーム，NASAへ配分され，更に国際公募研究による配布が予定されている．本稿では一連の試料取り扱いプロセス・初期記載内容について述べる．</p>

DOI： 10.14909/yuseijin.31.2_153

CiNii Research

出版者・発行元：Nature Astronomy  

C-type asteroids¹ are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing^2–7 and on-asteroid measurements^8,9 with Hayabusa2 (ref. ¹⁰). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu¹¹, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m⁻³, which is lower than that of meteorites¹², suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements^5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu^3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

DOI： 10.1038/s41550-021-01550-6

Web of Science

Scopus

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

C-type asteroids are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing2,3,4,5,6,7 and on-asteroid measurements8,9 with Hayabusa2 (ref. 10). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu11, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m−3, which is lower than that of meteorites12, suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements5,9. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu3,4 and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.

DOI： https://doi.org/10.1038/s41550-021-01550-6

その他リンク： https://www.nature.com/articles/s41550-021-01550-6

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Calcium–aluminum-rich inclusions (CAIs) are the oldest materials that formed in the protosolar disk. Igneous CAIs experienced melting and subsequent crystallization in the disk during which the evaporation of relatively volatile elements such as Mg and Si occurred. Evaporation from the melt would have played a significant role in the variation of chemical, mineralogical, and petrologic characteristics of the igneous CAIs. In this study, we investigated crystallization of CAI analog melt under disk-like low-pressure hydrogen (PH2) conditions of 0.1, 1, and 10 Pa to constrain the pressure condition of the early solar system in which type B CAIs were formed. At PH2 = 10 Pa, the samples were mantled by melilite crystals, as observed for type B1 CAIs. However, the samples heated at PH2 = 0.1 Pa exhibited random distribution of melilite, as in type B2 CAIs. At the intermediate PH2 of 1 Pa, type-B1-like structure formed when the cooling rate was 5°C hr−1, whereas the formation of type-B2-like structure required a cooling rate faster than 20°C hr−1. The compositional characteristics of melilite in type B1 and B2 CAIs could also be reproduced by experiments. The results of the present study suggest that PH2 required for type-B1-like textural and chemical characteristics is greater than 1 Pa. The hydrogen pressure estimated in this study would impose an important constraint on the physical condition of the protosolar disk where type B CAIs were formed.

DOI： 10.3847/2041-8213/ac3c41

その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/ac3c41

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Coarse-grained, igneous Ca-Al-rich inclusions (CAIs) in CV chondrites formed through multiple melting events. We conducted in situ O-isotope analysis and Al-Mg systematics by secondary ion mass spectrometry of relict and overgrown minerals from a partial melting event in an Allende Type B CAI, Golfball. Golfball has a Type B CAI bulk composition and a unique structure: a fassaite-rich mantle enclosing a melilite-rich core. Many of the blocky melilite crystals in the core have irregularly shaped, Al-rich (Åk5–15) cores enclosed in strongly zoned (Åk30–70) overgrowths. Since the Al-rich melilite grains could not have formed from a melt of Golfball, they are interpreted as relict grains that survived later melting events. The O-isotopic compositions of the blocky melilite crystals plot along the carbonaceous chondrite anhydrous mineral line, ranging between Δ17O ~ −14‰ and −5‰. The Al-rich relict melilite grains and their overgrowths exhibit the same O-isotopic compositions, while the O-isotopic compositions are varied spatially among melilites. We found that the O-isotopic compositions steeply change across several melilite crystals within few tens of micrometers, indicating the O-isotopic compositions of the melt could not have been homogenized during the partial melting in that scale. According to the time scale of O self-diffusivity in the melt, the cooling rate of the partial melting event is calculated to be >6 × 104 K h−1. Al-Mg isotope data for core minerals plot on a straight line on an Al-Mg evolution diagram. A mineral isochron for Golfball gives initial 26Al/27Al of (4.42 ± 0.20) × 10–5 and initial δ26Mg* of −0.035 ± 0.050‰. The chemical and O-isotopic compositions of melilite and those initial values imply that its precursor consisted of fluffy Type A and/or fine-grained CAIs. The partial melting event for Golfball may have occurred in very short order after the precursor formation.

DOI： https://doi.org/10.1111/maps.13701

開催年月日： 2024年9月

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://confit.atlas.jp/guide/event/jams2024/session/3Poster14-18/detail?lang=ja

開催年月日： 2024年5月

記述言語：日本語  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS08-16/detail

開催年月日： 2024年5月

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PCG22-03/detail

開催年月日： 2024年5月

記述言語：日本語   会議種別：ポスター発表  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS08-P15/detail

開催年月日： 2024年5月

記述言語：日本語   会議種別：ポスター発表  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS03-P05/detail

開催年月日： 2024年5月

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS07-12/detail

開催年月日： 2024年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京大学宇宙線研究所(   国名：日本国  

開催年月日： 2023年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：大阪公立大学　杉本キャンパス　全学共通教育棟   国名：日本国  

プレソーラーSiC 粒子は、原始太陽系円盤の酸化的環境下では蒸発反応を起こすため (Larimer and Bartholomay, 1979)、円盤の円盤物理化学条件 (温度、圧力等) の指標となり得る。円盤でSiCは、酸化還元状態に応じてSiO2残渣層を形成しないアクティブ酸化またはSiO2残渣層を形成しSiO2が蒸発するパッシブ酸化を経験する (e.g., Narushima et al., 1997)。Mendybaev et al. (2002) では、1気圧の酸素分圧をコントロールした様々な酸化還元環境化でSiC蒸発実験をおこなった。その結果、酸化的な環境下 (IW-03) では、SiO2残渣層の形成が確認され、一方還元的な環境下では (IW-06) ではSiO2残渣層が形成されない (または極めて薄い) ことが確認された。SiO2残渣層は円盤ガス・SiC間の直接の接触を妨げることでSiC蒸発速度を左右する。更に始原的隕石中のプレソーラーSiC粒子表面に見られる10–30 nm程度の酸化リム (Bernatowicz et al., 2006) の起源においても重要である。SiC蒸発速度やメカニズムは、ガス分圧やガス組成に関連している可能性があり、1気圧条件での反応速度を円盤環境での反応速度に直接応用できない可能性がある。そのため、本研究では、原始太陽系円盤でのプレソーラーSiC粒子の残存可能性の理解のため、円盤を模した低圧H2-H2O混合ガス環境下でのSiCの蒸発実験をおこなった。 実験出発物質には、主要なプレソーラーSiC粒子の多形であるβ-SiCのチップを用いた。加熱実験には、ガス供給系を備えた真空高温加熱縦型炉を用いて、温度1250–1450°C、H2-H2O混合ガス圧力0.5, 2.5 Paで4–110.3 hおこなった。ガス供給系では、室温でのH2Oからの蒸発ガスとガスボンベから供給されるH2ガスとが混合され、H2Oの消費量から推定したPH2Oは、全圧0.5, 2.5 Paの実験でそれぞれPH2O = 2.2 × 10–3 Pa, 6.0 × 10–3 Paであった。また全圧2.5 Paの一部の実験では、H2Oを50, 75°Cに加熱し室温での飽和水蒸気圧を作り出すことで、PH2O = 3.7 × 10–2 Paの条件での実験もおこなった。サンプル質量変化から蒸発量の推定をおこなった。いくつかのサンプルに関してFIB切片を作成し、透過型電子顕微鏡 (STEM-EDS; JEOL JEM-2800) で観察をおこなった。 STEM-EDS分析から表面に酸素を含む極めて薄い層 (~5–10 nm) の存在が確認され、パッシブ酸化を通した蒸発の可能性が示唆された。これは、プレソーラーSiC粒子表面の酸化リムが円盤でも形成可能であることを示唆する。蒸発速度k (cm s–1) は質量損失と初期サンプルサイズにより推定し (Takigawa et al., 2009)、蒸発フラックスJ (g cm–2 s–1) に変換した。Jは1350–1450°Cの高温領域では温度にほとんど依存しない一方、ガス圧・ガス組成に依存した。1300°C以下の低温領域では、Jは大きく温度に依存し、ばらつきの範囲内で異なるガス条件の結果は一致し、さらに先行研究 (Mendybaev et al., 2002) のIW-03, IW-06条件でのJと近い値を示した。高温領域においてJの小さな温度依存性は、ガス供給 (ガス供給速度に律速されたSiO2形成速度) が律速段階である可能性を示す。高温領域でのJは、PH2Oの平方根に依存したことから、表面でのH2O分子の乖離が蒸発速度に関係している可能性がある。一方低温領域では、Jがガス種に非依存、また非常に薄いSiO2残渣層であったことを考慮すると、SiC-SiO2界面反応が反応速度を律速している可能性があり、本研究で得られた活性化エネルギー (~531 kJ mol–1) が先行研究で報告しているSiC-SiO2界面反応における活性化エネルギー (~548–563 kJ mol–1; (Pultz & Hertl, 1966; Mendybaev et al., 2002)) と整合的であることも説明可能である。 本研究で得られた蒸発速度に基づくと、プレソーラーSiC粒子は溶融CAI形成をもたらした熱的イベント (~1400°C, PH2O > 0.1 Pa, 2–3日; Yamamoto et al., 2021) において完全に蒸発する可能性がある。また、プレソーラーSiC粒子は、酸素同位体交換により同位体的特徴を失うプレソーラーケイ酸塩粒子に対して高温で残存する可能性があるが、プレソーラーコランダムが円盤ガスと酸素同位体交換をする温度領域においては、プレソーラーコランダムより先に蒸発し消失する可能性があることがわかった。

開催年月日： 2023年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：東京海洋大学・品川キャンパス   国名：日本国  

始原的太陽系物質に見られる質量非依存型の酸素同位体分別の定量的解釈のため、フォルステライト組成の非晶質ケイ酸塩と低圧C18Oガスとの酸素同位体交換実験をおこなった。酸素同位体交換率の時間変化から、粒子表面へのCO分子供給量や粒子内部への拡散が律速する時の反応速度パラメータを所得した。CO分子供給量が律速するときの非晶質フォルステライトのCOとH2O分子間の反応性の違いは、難揮発性包有物CAIメルトにおけるそれと同程度であった。得られた速度論データから、非晶質ケイ酸塩とCO、H2Oガスとの酸素同位体交換は、CO-H2Oガス間の同位体交換速度よりも圧倒的に速く進行するがわかった。このことは、これまで速度論的に起こらないとされてきたCO-H2Oガス間の同位体平衡が、非晶質ケイ酸塩ダストがCO、H2O両ガスと同位体交換を効率的にすることによって、低温でも達成される可能性を示す。

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉　幕張メッセ   国名：日本国  

コンドルールの酸素同位体組成は形成環境およびその時空間的変動を反映する。我々は、ケイ酸塩メルトと水蒸気ガスとの間の酸素同位体交換反応について、メルトとガスが有限の相対速度を持つ場合にも適用可能な理論モデルを導出し、酸素同位体組成の観点から原始太陽系星雲におけるコンドルール形成環境が満たすべき条件を議論した。その結果、メルトとガスの相対速度が零でない場合には同位体交換によって質量依存の同位体分別が生じることを発見した。我々の結果は、コンドルールは周囲のガスとの相対速度が数100 m/s以下の環境下で結晶化したことを示唆する。

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉　幕張メッセ   国名：日本国  

Forsterite (Mg2SiO4) in glassy state is a major component of interstellar silicate dust grains [1]. The Interstellar dust undergoes phase transitions through various reactions with adsorbed water molecules which called mineral evolution [2]. Various studies have been performed to investigate the reactions between forsterite and water. Yamamoto and Tachibana [3] investigated the effects of water vapor on crystallization of glassy forsterite, and found that the activation energy of the crystallization decreases as the water vapor pressure increases. Furthermore, Yamamoto et al. [4] showed that the serpentine (Mg3Si2O5(OH)4) and brucite (Mg(OH)2) are formed with the serpentinization reaction of glassy forsterite and water vapor in high pressure conditions by the following reaction: 2 Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2 Kubo et al. [5] performed molecular dynamics (MD) calculations of forsterite-water systems and found that brucite is formed from dissolved MgOxHy in liquid water. The MD result suggests that the interstitial water can be a liquid state in glassy forsterite at temperatures above the boiling point without pressurization, because the density of the interstitial water is higher than that of ordinary water. To understand the effects of liquid water on phase transition of glassy forsterite, we investigated the structural changes of forsterite-water systems with hydrothermal reaction. The structural analyses were mainly performed using the X-ray diffraction (XRD) method. Experiments The nanoparticles of glassy forsterite (~100 nm) were synthesized by an induced thermal plasma method from Mg(OH)2 and SiO2 [3,6]. The hydrothermal reactions were performed using an autoclave. The glassy forsterite and distilled water were introduced in a titanium cell. The starting samples were heated from the room temperature to a reaction temperature (473-573K) at 1.67K・min-1 in heating rate. Then, the temperature was kept at the reaction temperature for 0-48 hours. The XRD measurements were performed after cooling and drying the samples. The pressures in the cell were the saturated vapor pressures of each temperature (e.g., 8.7 MPa at 573 K). Run products from crystalline forsterite (~2 µm) with water were used for comparison with the results of glassy forsterite with water. Results and discussion The result shows that brucite and serpentine are formed from glassy forsterite at 573 K, whereas no diffraction peaks are observed for the XRD pattern of the sample from crystalline forsterite after the hydrothermal reaction at 573 K for 20 hours. The results suggest that the serpentinization reaction are promoted in the interface between the glassy forsterite and water in liquid state. For the samples from glassy forsterite, the intensities of the diffraction peaks assigned to brucite and serpentine depend on reacted time and temperature. From the changing rate of the intensities of the diffraction peaks, it was found that the formation of brucite begins at lower temperature and shorter reaction time in comparison with that of serpentine. For the reaction of glassy forsterite with water in liquid state, brucite is formed from dissolved Mg components in liquid water [5], although the formation of serpentine proceeds in the forsterite-water interface. This suggests that the serpentine formed in the early stages at the interface is a glassy state, while the crystalline brucite grows in the liquid water phase. We discuss the mechanisms of the phase transition in the water-forsterite system.

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉　幕張メッセ   国名：日本国  

太陽系天体は、原始太陽系円盤においてダストが集積して形成された。そのため、進化する原始惑星系円盤でダストが経験する化学反応は、化学的性質の動径分布を形成するにあたって重要な役割を果たしたと考えられる。蒸発、凝縮、固相-気相反応、結晶化といった様々な反応が、円盤の広い温度領域、すなわち円盤のあらゆる領域で進行する。異なる反応を異なる程度だけ経験したダストの集積は、天体の化学的多様性を引き起こす。従って、原始太陽系円盤におけるダストの反応進行を理解することは、太陽系の歴史を理解する上で重要である。 Ciesla (2011) は、定常降着円盤を運動する粒子の軌跡を追跡するモンテカルロシミュレーションを開発し、非晶質ケイ酸塩ダストの結晶化について反応速度論を取り入れて計算することで、ダストがある温度を経験するか否かで結晶質ないし非晶質に二極化されることを示した。またStanford & Ciesla (2012) は、同様のモンテカルロシミュレーションによって原始惑星系円盤において氷ダストが曝される環境を追跡し、複雑な有機分子を生産するのに十分なUV照射および加熱を経験することを示した。このように、移流・拡散によって運動するダスト粒子の化学反応を個別に扱った研究はいくつか存在するが、原始惑星系円盤内の広い温度範囲で起こる反応を一般的に扱う研究は、現時点ではなされていない。本研究では、定常降着円盤におけるダスト粒子の運動を考慮して、様々な不可逆化学反応に対する「反応ライン温度」予測式を開発することを目的とした。 原始惑星系円盤において、ダスト粒子の運動を追跡する3Dモンテカルロシミュレーションをおこなった (Ciesla 2010; 2011; Okamoto & Ida 2022)。円盤モデルとしては、ガス密度に比例する粘性加熱 (α粘性モデル; Shakura & Sunyaev 1973) を熱源とする鉛直・動径温度構造を持つ定常降着円盤を採用した。ダストの吸光度は2.5 cm2 g-1、主星質量は太陽質量Msunで一定とした。乱流粘性係数αは10-2、10-3、質量降着率は10-6、10-7、10-8 Msun yr-1とし、計6通りの円盤についてシミュレーションをおこなった。各パラメータセットに対し、10000個のダスト粒子をスノーライン中心面から放出した。ダスト粒子は、移流・拡散によって円盤ガスとよく馴染んで運動する。 各ダストの経験する温度履歴に基づき、Johnson-Mehl-Avrami 方程式 (JMA方程式; Johnson & Mehl 1973; Avrami 1973) で表される仮想的な不可逆反応の進行を調べた。スノーラインにおいて反応進行度Xが0の状態でシミュレーションを開始し、JMA式を微分することで得られた微小反応進行度δXを積算することで、反応進行を計算した。広範な化学反応を調べるため、活性化エネルギーは20-1000 kJ/mol、前指数因子 (s-1) の自然対数は10-60、アブラミ指数は0.5-4の範囲とし、~200–1500 Kの温度範囲で進行する反応を計算した。各粒子に対し全ての反応が完了した (Xが0.99を超えた) 時点で計算を打ち切り、最大106年にわたって計算をおこなった。 シミュレーションをおこなった結果、反応進行度Xがある特定の値Xrec (0.8、0.9、0.99) を超える以前に経験した最高経験温度 (Tmax,Xrec) のヒストグラムが、対数正規分布によってよくフィッティングされることがわかった。異なる反応が異なるTmax,Xrec分布を示すことから、この分布は反応が効率的に進行する特有の温度範囲を示していると考えられる。全てのヒストグラムに対し対数正規分布によりフィッティングをおこない、その最頻値 (Tline) と分散 (σline) を記録した。様々な反応に対するTline、σlineは、反応パラメータ及び円盤パラメータに依存して変化する。以降、これらを反応ラインの温度および分散として議論をおこなう。 私たちは、これら2つの「反応ライン」パラメータが、反応タイムスケールと局所的な拡散移動タイムスケールを比較することで半解析的に予測できることを明らかにした。これは、ダスト粒子がある温度領域に留まることで、反応が効率的に進行できることによると考えられる。私たちは、あらゆる反応に対するTline、σlineを、4つの反応パラメータ (アブラミ指数、前指数因子、活性化エネルギー、反応進行度) と2つの円盤パラメータ (乱流粘性係数、質量降着率) から予測することができる、反応ライン予測式を開発した。この式は、数値シミュレーションによって求められたTlineと5.5%、σlineと24%以内の範囲でよく一致することが確認された。 本研究で求められた「反応ライン」予測式は、初期太陽系進化において円盤の様々な位置で様々な反応を引き起こした化学進化を議論する上で、強力なツールになり得る。この式により、実験室で決定された反応速度論を直ちに降着円盤に適用し、円盤における物質の分布や複数の反応の競合を議論することができるようになる。

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉　幕張メッセ   国名：日本国  

始原的隕石などに見つかるプレソーラーSiC粒子は、原始太陽系円盤の酸化的環境下では熱力学的に不安定であるため (e.g., Larimer and Bartholomay, 1979)、原始太陽系円盤の熱的プロセスにおける円盤物理化学条件の指標となり得る。Mendybaev et al. (2002) では、1気圧の酸素分圧をコントロールした様々な酸化還元環境化でSiC蒸発実験をおこない、原始太陽系円盤でのプレソーラーSiC粒子の残存可能性を議論している。しかしながら、原始太陽系円盤の低圧環境下では反応ガスのフラックスが全体の反応速度を決定するなど　(e.g., Yamamoto et al., 2018, 2021)、1気圧条件で得られた反応速度を円盤環境での反応速度に直接応用できない可能性がある。そのため本研究では、原始太陽系円盤でのプレソーラーSiC粒子の残存可能性を理解するため、原始太陽系円盤を模した低圧H2-H2O混合ガス環境下でのSiCの蒸発実験をおこなった。 蒸発実験の出発物質には、プレソーラーSiC粒子の中で最も主要な多形であるβ (3C)-SiC (マーチソン隕石中でプレソーラー粒子の~80%を占める (Daulton et al., 2003)) のチップ (横縦4 × (4–5) mm, 厚み0.6 mm. 化学気相成長法により合成) を用いた。加熱実験には、新たに構築したガス供給系を備えた真空高温加熱縦型炉を用いて、温度1250–1450°C、 H2-H2O混合ガス圧力0.5, 2.5 Paで4–110.3 hおこなった。ガス供給系では、室温での脱イオン水からの蒸発ガスとガスボンベから供給されるH2ガスとが混合され、脱イオン水の消費量と全ガス流量から推定したH2/H2O比は全圧0.5, 2.5 Paの実験でそれぞれ ~225, 420 (i.e., PH2O of 2.2 × 10–3 and 6.0 × 10–3 Pa) であった。 サンプル質量とサイズの変化は高精度電子天秤及びミクロメーターで推定し、蒸発量の推定をおこなった。いくつかのサンプルに関してFIB切片を作成し (FIB-SEM; Hitachi NX2000)、透過型電子顕微鏡 (STEM-EDS; JEOL JEM-2800) で観察をおこなった。 FactSageを用いた熱力学計算から、実験における温度・圧力・酸化還元条件ではSiO2は安定に存在せず、STEM-EDS分析からも表面付近における酸素原子の過剰は観察されなかったことから、SiCの蒸発はアクティブ酸化 (i.e., SiC (s) + 2H2O (g) = SiO (g) + CO (g) + 2H2 (g)) (e.g., Narushima et al., 1997)) を通して起こる可能性が示唆された。蒸発速度k (cm s–1) はTakigawa et al. (2009)と同様の手法を用いて質量損失と初期サンプルサイズにより推定し、蒸発フラックスJ (g cm–2 s–1) に変換した。Jは1350–1450°Cの高温領域では温度にほとんど依存しない一方、1300°C以下の低温領域では大きく温度に依存した。また高温域において、全圧2.5 Paの場合における蒸発フラックスJが0.5 Paに対して2–3倍大きく、一方低温域においてJ は、ばらつきの範囲内で両圧力条件の結果は一致した。このことは~1300–1350°Cで反応律速段階の変化があることを示しており、このことは1気圧でのH2-H2O混合ガス中での蒸発実験においても観察されている (Kim and Readey, 1989)。高温での温度に依存しない蒸発速度は、SiC表面へのH2O気体フラックスが反応速度を律速している可能性を示し (e.g., Yamamoto et al., 2018, 2021)、気体フラックスは圧力に比例するため、全圧0.5, 2.5 Paの実験におけるJの違いを説明可能である。表面における衝突水分子の反応頻度は0.05–0.1と推定された。低温での温度に依存したJは、Mendybaev et al. (2002) におけるfO2 = IW-3, IW-6におけるJと整合的であった 本研究で得られた蒸発速度に基づくと、プレソーラーSiC粒子は溶融CAI形成をもたらした熱的イベント (~1400°C, PH2O > 0.1 Pa, 2–3日; Yamamoto et al., 2021, 2022) において完全に破壊される可能性がある。また、酸素同位体交換により破壊されるプレソーラーケイ酸塩粒子に対して高温で残存する可能性があるが、プレソーラーコランダムが円盤ガスと酸素同位体交換をする温度領域においてはプレソーラーコランダムより先に破壊される可能性があることがわかった。

開催年月日： 2023年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉　幕張メッセ   国名：日本国  

コンドライト隕石中の細粒CAI（Ca-Al-rich inclusion）は，高温凝縮鉱物の集合体である（Krot et al., 2004）。二次イオン質量分析法（SIMS）を用いたAl−Mg鉱物アイソクロンデータにより，CVコンドライト中の個々の細粒CAIの初生26Al/27Al比は，~5.2 × 10–5から~3.4 × 10–5の間で分布することが明らかになった（MacPherson et al., 2010, 2020; Kawasaki et al., 2020）。形成領域において26Al量が均一であったことを仮定すると，この分布は約50万年の形成年代の広がりに相当し，細粒CAIは太陽系最初期の約50万年間形成し続けていたことを示す。 我々の先行研究において，Northwest Africa 8613 reduced CVコンドライト中のヒボナイトに富む細粒CAI，HKD01を構成するヒボナイト，スピネル，メリライトのAl−Mg同位体分析を行った（Kawasaki et al., 2020; Wada et al., 2020）。HKD01のスピネルとメリライトの鉱物アイソクロンは，初生26Al/27Al比 = (4.81 ± 0.09) × 10–5を示したが，ヒボナイトデータを用いた回帰直線は，初生26Al/27Al比 = (4.55 ± 0.05) × 10–5を示した。HKD01の岩石鉱物組織（Wada et al., 2020）と平衡凝縮計算（Yoneda and Grossman, 1995）から，ヒボナイトはスピネル・メリライトよりも先に形成したと考えられる。そのため，両者の形成順序と逆転した初生26Al/27Al比の関係が正しいとすると，HKD01形成領域において26Al量が不均一であったことになる。一方で，Wada et al. (2020)は，SIMS分析におけるヒボナイトの27Al/24Mgの相対感度係数（RSF）が，化学組成により変動することにより，両者の初生26Al/27Al比が逆転した可能性を提唱した。 本研究ではこの問題を解決し，細粒CAIの正確な初生26Al/27Al比を決定するために，異なる化学組成をもつ3つのヒボナイト標準試料を合成した。そしてそれぞれの27Al/24Mg のRSFを測定し，同分析セッションにおいて細粒CAI HKD01に含まれるヒボナイトのAl−Mg同位体の再分析を行い，スピネル・メリライトの鉱物アイソクロンデータとの直接比較を行った。SIMSによる分析条件は，Kawasaki et al. (2019, 2020, 2021)とWada et al. (2020)とほぼ同様である。 3つの合成ヒボナイト標準試料は，それぞれが数マイクロメートル以下の多数の結晶から成り，均一な27Al/24Mg比をもつ。それぞれの27Al/24Mg比は，31.71 ± 0.05 (Hib30と名付ける), 100.71 ± 0.12 (Hib100), 386.8 ± 1.0 (Hib400)である。SIMSで測定した27Al/24MgのRSFは，化学組成により変動することがわかった。Hib100とHib400は，分析誤差0.3–0.6%内で等しいRSFを示したが，Hib30はそれらとは2–3%ほど異なるRSFを示した。つまり，未知ヒボナイト試料の正確なAl−Mg同位体データを得るためには適切な標準試料が必要である。 合成ヒボナイト試料を用いてRSF補正を行い新しく取得した，Al−Mg同位体データより得られたHKD01のヒボナイトの鉱物アイソクロンは，初生26Al/27Al比 = (4.73 ± 0.09) × 10–5を示した（Fig. 1）。この初生比は，同FGIのスピネルとメリライトの鉱物アイソクロンが示す初生26Al/27Al比 = (4.81 ± 0.09) × 10–5 (Kawasaki et al., 2020)と誤差内で一致する。ヒボナイトはスピネル・メリライトよりも先に形成したものであるため（Wada et al. 2020），両者の最大の年代差は2万年と見積もられる。これは，HKD01の構成鉱物が，最初期太陽系において，2万年以内の間に形成していたことを示す。この期間は，細粒CAIの形成期間である約50万年 (Kawasaki et al., 2020)よりもかなり短い。HKD01は，そのような短期間での形成後に，気相との著しい化学反応，および再溶融を受けることなく速やかにCAI形成領域から取り去られ，母天体集積領域にまで運ばれたものであると考えられる。

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：the Woodland, Texas   国名：アメリカ合衆国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2023/pdf/2011.pdf

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：the Woodland, Texas   国名：アメリカ合衆国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2023/pdf/1394.pdf

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：アメリカ合衆国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2023/pdf/1129.pdf

開催年月日： 2023年3月

記述言語：日本語   会議種別：口頭発表（一般）  

国名：日本国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2023/pdf/1565.pdf

開催年月日： 2023年3月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：アメリカ合衆国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2023/pdf/1128.pdf

開催年月日： 2022年3月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：the Woodland, Texas   国名：アメリカ合衆国  

開催年月日： 2022年2月

記述言語：英語   会議種別：シンポジウム・ワークショップ パネル（公募）  

開催地：Rusutsu, Hokkaido   国名：日本国  

開催年月日： 2021年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2021年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン発表   国名：日本国  

開催年月日： 2020年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：オンライン   国名：日本国  

開催年月日： 2020年3月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：the Woodlands, Texas   国名：アメリカ合衆国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2020/pdf/1809.pdf

開催年月日： 2019年7月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Sapporo   国名：日本国  

開催年月日： 2019年7月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Sapporo, Hokkaido   国名：日本国  

開催年月日： 2019年7月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Sapporo, Hokkaido   国名：日本国  

開催年月日： 2019年7月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Sapporo, Hokkaido   国名：日本国  

開催年月日： 2019年5月

記述言語：日本語   会議種別：口頭発表（一般）  

国名：日本国  

開催年月日： 2019年5月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：千葉 幕張メッセ   国名：日本国  

Mass-independent oxygen isotope variations of extraterrestrial materials is considered to be a result of oxygen isotope exchange between 16O-rich primitive materials and 16O-poor gaseous reservoirs (Yurimoto & Kuramoto, 2004; Lyons & Young, 2005). Because the isotope exchange rate is controlled by the physicochemical condition of protoplanetary disks, oxygen isotopic compositions of extraterrestrial materials could be a good tracer for the disk conditions. However, little has been known about the kinetics of isotope exchange reaction for oxygen in the disks. We focused on oxygen isotope evolution of amorphous silicate dust, which is recognized as primitive material in protoplanetary disks, and investigated oxygen isotope exchange kinetics between amorphous silicates and water vapor. A series of oxygen isotope exchange experiments between submicron-sized grains of amorphous silicate with forsterite and enstatite stoichiometry and 18O-enriched water vapor (97% 18O) were carried out at 803−1123 K and water vapor pressure (PH2O) of 0.01–1 Pa using a gold-mirror vacuum furnace equipped with a gas flow system (Yamamoto & Tachibana, 2018). The run products were analyzed with FT-IR (JASCO FT-IR 4200), XRD (Rigaku SmartLab), and FE-SEM (JEOL JSM-7000F). The bulk oxygen isotope composition was determined as an average of multiple measurements for a pelletized sample by SIMS (Cameca ims-6f and -1280HR). An infrared absorption peak of amorphous silicate at ~10 micron shifted to longer wavelengths with time by annealing with H 18O vapor of 0.3 Pa at 803−883 K for amorphous forsterite and at 923–1003 K for 2 amorphous enstatite, whereas such a shift was not observed for samples annealed in the presence of isotopically normal water vapor of 0.3 Pa. The relative peak shifts of the 10 micron feature of amorphous forsterite and enstatite correlate with their oxygen isotopic compositions, suggesting that the peak shift was caused by the isotope exchange of oxygen between amorphous silicates and H 18O vapor. Temporal change of the degree of isotope exchange under these conditions was governed by diffusive isotope exchange with the isotope exchange rate of D (m2 s–1) = (1.5 ±1.0) ×10–19exp[–161.5 ±14.1 (kJ mol–1) R–1 (1/T–1/1200)] for amorphous forsterite and D (m2 s–1) = (5.0 ±0.2) ×10–21exp[–161.3 ±1.7 (kJ mol–1) R–1 (1/T–1/1200)] for amorphous enstatite. At PH2O= 0.01 Pa, the isotope exchange reaction for amorphous forsterite is controlled by a supply of water vapor at 853 and 883 K, and the supply-controlled isotope exchange rate was evaluated. Crystallization of amorphous forsterite dominated over oxygen isotope exchange at a higher temperature of 1073 K because crystallization precedes the oxygen isotope exchange reaction. The oxygen isotope exchange timescales for amorphous forsterite and enstatite dust under protoplanetary disk conditions suggest that the primary oxygen isotope signatures of amorphous silicate dust would be erased by thermal annealing at temperatures above 500–650 K within the lifetime of protoplanetary disks. Cometary silicate dust collected by the Stardust mission has 16O-poor isotopic compositions close to the terrestrial value (McKeegan et al., 2006; Nakamura et al., 2008). If primitive silicate dust in the protosolar disk has the same 16O-rich composition as the Sun, this observation implies that comets accreted silicate dust that was thermally processed at temperature above ~500–650 K in the inner Solar System and transported to the comet formation region (e.g., Nuth, 1999; Ciesla, 2007). This could also explain that presolar silicate grains are not a major component in cometary silicates (Floss et al., 2013). The effective radial transport of inner disk materials to the outer part of protoplanetary disks may provide some constrains on the dynamics of protoplanetary disks prior to planet formation.

開催年月日： 2019年2月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：Rusutsu, Hokkaido,   国名：日本国  

開催年月日： 2018年3月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：the Woodlands, Texas   国名：日本国  

その他リンク： https://www.hou.usra.edu/meetings/lpsc2018/pdf/1995.pdf

開催年月日： 2018年2月

記述言語：英語   会議種別：シンポジウム・ワークショップ パネル（公募）  

開催地：Rusutsu, Hokkaido   国名：日本国  

開催年月日： 2018年2月

記述言語：英語   会議種別：シンポジウム・ワークショップ パネル（公募）  

開催地：Rusutsu, Hokkaido   国名：日本国  

開催年月日： 2018年2月

記述言語：日本語   会議種別：シンポジウム・ワークショップ パネル（公募）  

開催地：Rusutsu, Hokkaido   国名：日本国  

開催年月日： 2017年8月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Paris   国名：フランス共和国  

開催年月日： 2017年3月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：the Woodlands, Texas   国名：日本国  

開催年月日： 2017年2月

記述言語：英語   会議種別：口頭発表（一般）  

開催地：Rusutsu, Hokkaido   国名：日本国  

開催年月日： 2017年2月

記述言語：英語  

開催地：Rusutsu, Hokkaido   国名：日本国  

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://sites.google.com/view/douitaihi-bukai2024/

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS08-17/detail

記述言語：日本語   会議種別：口頭発表（一般）  

オープンデータURL： https://confit.atlas.jp/guide/event/jpgu2024/subject/PPS08-16/detail