安部 正真

Abstract Although CI chondrites are susceptible to terrestrial weathering on Earth, the specific processes are unknown. To elucidate the weathering mechanism, we conduct a laboratory experiment using pristine particles from asteroid Ryugu. Air‐exposed particles predominantly develop small‐sized euhedral Ca‐S‐rich grains (0.5–1 μm) on the particle surface and along open cracks. Both transmission electron microscopy and synchrotron‐based computed tomography combined with XRD reveal that the grains are hydrous Ca‐sulfate. Notably, this phase does not form in vacuum‐ or nitrogen‐stored particles, suggesting this result is due to laboratory weathering. We also compare the Orgueil CI chondrite with the altered Ryugu particles. Due to the weathering of pyrrhotite and dolomite, Orgueil contains a significant amount of gypsum and ferrihydrite. We suggest that mineralogical changes due to terrestrial weathering of particles returned directly from asteroid occur even after a short‐time air exposure. Consequently, conducting prompt analyses and ensuring proper storage conditions are crucial, especially to preserve the primordial features of organics and volatiles.

Abstract The nitrogen isotope compositions of two samples returned from the asteroid Ryugu were determined using a stepwise combustion method, along with Ivuna (CI) and Y‐980115, a CI‐like Antarctic meteorite, as references. The two Ryugu samples A0105‐07 and C0106‐07 showed bulk δ¹⁵N values of +1.7 ± 0.5‰ and +0.2 ± 0.6‰, respectively, significantly lower than Ivuna with +36.4 ± 0.4‰, but close to Y‐980115 with +4.0 ± 0.3‰. The Ryugu samples are further characterized by C/N and ³⁶Ar/N ratios up to 3.4× and 4.9× the value of Ivuna, respectively. Among all Ryugu samples and CI chondrites, a positive correlation was observed between nitrogen concentrations and δ¹⁵N values, with samples with lower nitrogen concentrations exhibiting lower δ¹⁵N. This trend is explained by a two‐component mixing model. One component is present at a constant abundance among all CI‐related samples, with a δ¹⁵N value around 0‰ or lower. The other varies in abundance between different samples, and exhibits a δ¹⁵N value of +56 ± 4‰. The first ¹⁵N‐poor endmember is seemingly tightly incorporated into a carbonaceous host phase, whereas the ¹⁵N‐rich endmember can be mobilized and decoupled from carbon, potentially because it is in the form of ammonia. Asteroid materials with volatile compositions that are similar to those reported here for the Ryugu samples are attractive candidates for the volatile sources among Earth's building blocks.

Abstract The analysis of samples returned from the C-type asteroid Ryugu has drastically advanced our knowledge of the evolution of early solar system materials. However, no consensus has been obtained on the chronological data, which is important for understanding the evolution of the asteroid Ryugu. Here, the aqueous alteration age of Ryugu particles was determined by the Mn–Cr method using bulk samples, yielding an age of 4.13 + 0.62/−0.55 Myr after the formation of Ca–Al-rich inclusions (CAI). The age corresponds to 4563.17 + 0.60/−0.67 Myr ago. The higher ⁵⁵Mn/⁵²Cr, ε ⁵⁴Cr, and initial ε ⁵³Cr values of the Ryugu samples relative to any carbonaceous chondrite samples implies that its progenitor body formed from the least thermally processed precursors in the outermost region of the protoplanetary disk. Despite accreting at different distances from the Sun, the hydrous asteroids (Ryugu and the parent bodies of CI, CM, CR, and ungrouped C2 meteorites) underwent aqueous alteration during a period of limited duration (3.8 ± 1.8 Myr after CAI). These ages are identical to the crystallization age of the carbonaceous achondirtes NWA 6704/6693 within the error. The ε ⁵⁴Cr and initial ε ⁵³Cr values of Ryugu and NWA 6704/6693 are also identical, while they show distinct Δ'¹⁷O values. This suggests that the precursors that formed the progenitor bodies of Ryugu and NWA 6703/6693 were formed in close proximity and experienced a similar degree of thermal processing in the protosolar nebula. However, the progenitor body of Ryugu was formed by a higher ice/dust ratio, than NWA6703/6693, in the outer region of the protoplanetary disk.