石崎 拓也

Abstract The successful sample return from asteroid (162173) Ryugu by Hayabusa2 has contributed to our understanding of the solar system evolution. Over the course of the initial sample analysis, various measurements were conducted, such as mineralogical observation, chemical analysis, and mechanical property measurement. These pieces of information allow us to give constraints on the essential conditions of Ryugu's formation and evolution processes (e.g., thermal environment, aqueous alteration, formation of a rubble‐pile body), leading to a clearer view of the early solar system. Here, we report the initial results of the elastic properties of Ryugu particles (e.g., P‐ and S‐wave velocities and Young's modulus) obtained via ultrasonic pulse transmission measurement. Our measurement results showed 2.15  0.05 km/s and 1.25  0.05 km/s for the compressional and shear waves, respectively. Regarding Young's modulus, we obtained 7.1  0.6 GPa, consistent with the previously measured value via a nanoindentation test. Compared with the elastic properties of other carbonaceous chondrites (Tagish Lake, Tarda, Ivuna, and Murchison meteorites), we found that Ryugu had distinctly lower rigidity than Ivuna—the most similar material to Ryugu with respect to chemical and mineralogical features. Instead, Tagish Lake showed the closest elastic properties to Ryugu samples. The affinities in chemical and mineralogical features indicate the genetic relationship between Ryugu and Ivuna. On the other hand, the difference in elastic properties might indicate their formation and evolution processes proceeded differently (e.g., formation depth, degree of alteration).