Tokyo Institute of Technology

Graduate School of Science, The University of Tokyo

Hokkaido University

Summary of Presentations

A research group led by Professor Tetsuya Yokoyama, School of Science Department of Earth and Planetary Sciences of Tokyo Institute of Technology, Associate Professor Tsuyoshi Iizuka and Professor Shogo Tachibana of the Graduate School of Science,The University of Tokyo, and Professor Hisayoshi Yurimoto of Hokkaido University has measured the isotopic composition of the Cb-type asteroid Ryugu, and found that local heterogeneities in chromium isotopic composition were caused by the severe water metamorphism and water circulation that occurred on Ryugu. The initial analysis of the Ryugu samples revealed the following.

Initial analysis of the Ryugu samples revealed that the chemical and mineral compositions of the Cb-type asteroid Ryugu are similar to those of Ivena-type carbonaceous meteorites, but there is a slight discrepancy between Ryugu and Ivena-type carbonaceous meteorites in terms of chromium nucleosynthesis origin isotope anomalies, the cause of which has been awaited. The research group has identified a total of five Ryugu meteorites.

The research group measured nucleosynthetic origin isotope anomalies of chromium (⁵⁴Cr) and titanium (⁵⁰Ti) in a total of five Ryugu samples. The results showed that the ⁵⁴Cr isotopic anomaly varied significantly from higher to lower than the average value of the Ivna-type meteorite. This variation is inversely correlated with that of chromium isotopes (⁵³Cr) derived from radiolytic decay of manganese-53 (⁵³Mn), a short-lived nuclide,

Suggesting that water metamorphism and water circulation in the Ryugu source produced ⁵⁴Cr-poor and Mn-rich (⁵³Cr-rich) carbonates. The ⁵⁴Cr isotopic anomaly in the combined material (about 90 mg) of the Ryugu samples (7-24 mg) is consistent with the average value of the Ivna-type carbonaceous meteorite, i.e., the original isotopic composition is consistent with the average value of the Ivna-type carbonaceous meteorite. In other words, a certain amount of homogeneous samples should be analyzed in the initial analysis of the Bennu samples brought back by OSIRIS-REx in order to avoid the influence of heterogeneity.

Figure: ε⁵⁰Ti and ε⁵⁴Cr values for all Ryugu samples analyzed in this study combined (corresponding to about 90mg). The larger sample size of the Ryugu analyzed is consistent with the values for the Ivna-type carbonaceous meteorite in the previous study. (© Yokoyama et al., 2023 with some modifications)

The results of this study were published online in Science Advances on November 9 (JST).

For more information, please visit the Tokyo Institute of Technology website（in Japanese）.

Journal

Journal name	Science Advances
Title of paper	Water circulation in Ryugu asteroid affected the distribution of nucleosynthetic isotope anomalies in returned sample