2025-07-17 北海道大学,海洋研究開発機構
小惑星「リュウグウ」から見つかった太陽系最古の岩石「CAI」。Al-Mg放射年代測定法により、太陽系誕生直後の約45億6,730万年前に形成されたことが判明。データは電子顕微鏡により取得したMg（赤）―Ca（緑）―Al（青）の合成X線元素マップ。（©Kawasaki et al. 2025）
＜関連情報＞
- https://www.hokudai.ac.jp/news/2025/07/post-1973.html
- https://www.hokudai.ac.jp/news/pdf/250717_pr.pdf
- https://www.nature.com/articles/s43247-025-02511-x
太陽系最古の固体物質から制約するリュウグウとイヴナ型炭素質隕石の集積領域 Solar System’s earliest solids as tracers of the accretion region of Ryugu and Ivuna-type carbonaceous chondrites
Noriyuki Kawasaki,Sota Arakawa,Yushi Miyamoto,Naoya Sakamoto,Daiki Yamamoto,Sara S. Russell & Hisayoshi Yurimoto
Communications Earth & Environment Published:16 July 2025
DOI:https://doi.org/10.1038/s43247-025-02511-x
Abstract
Samples from the carbonaceous asteroid Ryugu and CI (Ivuna-type) chondrites are dominated by low-temperature, aqueously formed secondary minerals, with rare occurrences of anhydrous primary minerals that formed in the high-temperature region of the solar protoplanetary disk. Here, we show that small (<a few tens of micrometers) Ca-Al-rich inclusions (CAIs) in Ryugu and Ivuna formed within ~0.2 Ma of the Solar System’s birth. These CAIs exhibit mineralogical, O-isotopic, and chronological similarities to CAIs in ordinary and other carbonaceous chondrites, indicating their ubiquitous presence across chondrites. In contrast, larger (>submillimeter) CAIs, commonly found in other carbonaceous chondrites and thought to have been retained via pressure bump(s) in the disk, are absent in Ryugu and CI chondrites. This absence implies that their parent planetesimals formed at a greater heliocentric distance, beyond the influence of a pressure bump created by proto-Jupiter, accreting only small CAIs that evaded radial drift toward the Sun.
