小惑星リュウグウ中の可溶性有機分子

抄録

The study of extraterrestrial organic molecules has entered a new era, facilitated by the capability to retrieve primordial materials from carbonaceous asteroids and perform comprehensive laboratory analyses on Earth. On December 6, 2020, the asteroid explorer "Hayabusa2" successfully completed its mission, returning 5.4 g of samples from the near-Earth asteroid "Ryugu (162173)". For astroorganic chemists, the initial analysis of Ryugu samples provided a golden opportunity to conduct comprehensive organic chemical analyses on the soluble organic matter (SOM) in the more chemically pristine sample compared to other Solar System materials previously analyzed in laboratories. This review outlines the distribution and characteristics of the organic molecules identified in the Ryugu samples, including amino acids and nucleobases. The diversity of Ryugu's SOM is not only comparable to that of previously analyzed carbonaceous chondrites, but is also characterized by sulfur-rich compounds such as polythionic acids. The discovery of more than ten types of amino acids and the nucleobase 'uracil' in the Ryugu samples marks a significant milestone in both astrochemistry and astrobiology. The molecular distribution of amino acids, amines, and carboxylic acids suggests these molecules were influenced by low-temperature hydrothermal reactions on the Ryugu’s parent body. The prominent distribution of tetracyclic PAHs suggests that it might not only be influenced by hydrothermal reactions but also indicates the possibility that interstellar origins have been partially preserved. The initial SOM analysis showed that despite the harsh conditions of solar heat, ultraviolet light, cosmic ray irradiation, and high vacuum, Ryugu’s surface and subsurface materials have retained a wide variety of organic molecules. Therefore, SOM on C-type asteroids, like Ryugu, could have been supplied to early Earth and other celestial bodies through releases caused by interstellar collisions.