臼井 寛裕

Abstract Sample return missions play a significant role in planetary science by providing pristine extraterrestrial materials. JAXA's Hayabusa2 and NASA's OSIRIS‐REx missions have returned samples from the C‐type asteroids Ryugu and Bennu, respectively. The chemical and mineralogical compositions of these samples closely resemble those of CI chondrites, the traditional reference material for solar system abundances. Based on the findings of the Hayabusa2 mission, JAXA launched the Ryugu Reference Project (RRP) to maximize the scientific value of the returned samples and formed the RRP Measurement Definition Team (RRP‐MDT) to elucidate the RRP's scientific goal and objectives. The RRP‐MDT defined the goal of RRP to reassess the elemental abundances and isotopic compositions of the solar system through comprehensive analyses of the returned asteroid samples and CI chondrites. To this end, the team recommended preparing homogeneously powdered Ryugu reference materials (RRM) using approximately 750 and 400 mg of samples from Chambers A and C, respectively, to address observed compositional heterogeneities. The team proposed to measure the elemental abundances and isotopic compositions of the RRM by analytical techniques involving seven specific measurement groups. Through comprehensive analytical methodologies, interlaboratory calibration, and statistical evaluation, the RRP aims to refine our understanding of solar system formation and evolution.

Abstract Analyzing primitive extraterrestrial samples from asteroids is key to understanding the evolution of the early solar system. The OSIRIS‐REx mission returned samples from the B‐type asteroid Bennu, providing a valuable opportunity to compare them with the Ryugu samples collected by the Hayabusa2 mission. This study examines the representativeness of a fraction of the Bennu samples, which was allocated from NASA to JAXA, by nondestructive characterization of their physical and spectral properties without atmospheric exposure. The reflectance and observed spectral features in the visible‐to‐infrared range of the Bennu sample resemble those from the spectroscopic analysis of different fractions. Additionally, we found differences in the slope of the visible range and band‐center of ~2.7 μm band between the samples and the asteroid surface, which could be explained by the degree of space weathering. A comparative analysis of the Bennu and Ryugu samples revealed spectral similarities, including absorption features indicative of Mg‐rich phyllosilicates, organics, and carbonates, without any evidence of sampling bias or terrestrial alteration. This finding can be used as a benchmark for subsequent Ryugu–Bennu comparative studies.

Abstract Silica polymorphs in meteorites provide critical constraints on crystallization processes associated with thermal activity in the early solar system. A detailed investigation of silica polymorphs in eucrites (the largest group of achondrites) using cathodoluminescence imaging and laser‐Raman spectroscopy revealed significant variations in the relative abundance of silica polymorphs. Based on these variations, the eucrites were divided into four “Si‐groups” according to their dominant silica phase: Si‐0 (cristobalite‐dominant eucrites), Si‐I (quartz‐dominant eucrites), Si‐II (quartz and tridymite‐dominant eucrites), and Si‐III (tridymite‐dominant eucrites). In studied eucrites, tridymite and cristobalite form lathy euhedral shapes, while quartz is anhedral, coexistent with opaques and phosphates, suggesting that silica polymorphs were crystallized from different stages and formation processes. We propose a new model that explains the formation pathways of silica minerals in eucrites and accounts for the distinct formation histories represented by each Si‐group: tridymite crystallizes from alkali‐rich immiscible melts (starting at ≥ ~1060°C), cristobalite crystallizes from quenched melts (~1060°C), and quartz crystallizes from extremely differentiated melts and/or by solid‐state transformation from tridymite and cristobalite through interactions with sulfur‐rich vapor below ~1025°C. This model explains the occurrences of silica polymorphs in eucrites without requiring secondary heating or shock processes.

Abstract Subsurface ice in the mid-latitudes of Mars represents one of the largest present-day water ice reservoirs. While atmospheric models predict Late Amazonian (during the past hundreds of millions of years) obliquity-driven ice accumulation, its long-term variations, and the factors influencing accumulation remain unclear. Using geomorphological evidence and numerical modeling, we reveal a southwestern depositional trend within northern mid-latitudinal crater walls and floors. Detailed crater-fill deposit analyses indicate multiple glaciation stages, including an earlier, high-intensity stage followed by a later, lower-intensity stage, both exhibiting this southwestern trend (ca. 640–98 Ma). We conclude that persistent multiple-stage Amazonian glaciations were governed by atmospheric water availability and obliquity-driven climate cycles.