小惑星リュウグウと初期太陽系：はやぶさ2探査の成果と未解決の謎

抄録

Remote sensing observations and returned sample analysis in Hayabusa2 mission revealed the origin and evolution of the asteroid 162173 Ryugu and its parent body and provided insights into the early solar system history. Here first we review findings of the Hayabusa2 mission, mostly focusing on Ryugu’s physical, chemical, and mineralogical properties. Spectral properties, elemental composition, and mineralogy of the asteroid Ryugu informed from remote-sensing observations of the asteroid and analysis of its returned samples show that Ryugu closely resembles CI chondrite meteorites, while its properties more primitive than CI chondrite meteorites are also identified. The presence of CO₂ fluid inclusions and nucleosynthetic isotopic characteristics point to a distant formation location distinct from those of other carbonaceous chondrite parent bodies for Ryugu’s and CI chondrite parent bodies, possibly beyond Jupiter’s formation location. The homogeneity of Ryugu’s surface suggests that the parent body may be an undifferentiated body. Next, we place those findings in broader context with the solar system explorations, astronomical observations, and theoretical studies to highlight unsolved issues on the origin and evolution of the asteroid Ryugu and the early solar system. While Ryugu’s origin beyond Jupiter’s orbit has been widely discussed in previous studies, combined evidence including Jupiter’s envelope composition, asteroid taxonomy, and theoretical studies do not rule out other possibilities. Observational evidence and theoretical studies suggest water-rock differentiation of large asteroids (parent bodies) and its role in chemical evolution. We conclude with discussion on future studies to better understand the origin of the asteroid Ryugu and the evolution of the early solar system.