Earth, Planets and Space
Ogawa, K.; Sakatani, N.; Kadono, T.; Arakawa, M.; Honda, R.; Wada, K.; Shirai, K.; Shimaki, Y.; Ishibashi, K.; Yokota, Y.; Saiki, T.; Imamura, H.; Tsuda, Y.; Nakazawa, S.; Takagi, Y.; Hayakawa, M.; Yano, H.; Okamoto, C.; Iijima, Y.; Morota, T.; Kameda, S.; Tatsumi, E.; Cho, Y.; Yoshioka, K.; Sawada, H.; Matsuoka, M.; Yamada, M.; Kouyama, T.; Suzuki, H.; Honda, C.; Sugita, S.
Japanese Hayabusa2 spacecraft has successfully carried out an impact experiment using a small carry-on impactor (SCI) on an asteroid (162173) Ryugu. We examine the size distribution of particles inside and outside an artificial impact crater (the SCI crater) based on the images taken by the optical navigation camera onboard the Hayabusa2 spacecraft. The circumferential variation in particle size distribution inside the SCI crater is recognized and we interpret that major circumferential variation is caused by the large boulders inside the SCI crater that existed prior to the impact. The size distribution inside the SCI crater also shows that the subsurface layer beneath the SCI impact site had a large number of particles with a characteristic size of - 9 cm, which is consistent with the previous evaluations. On the other hand, the size distribution outside the SCI crater exhibits the radial variation, implying that the deposition of ejecta from the SCI crater is involved. The slope of the size distribution outside the crater at small sizes differs from the slope of the size distribution on the surface of Ryugu by approximately 1 or slightly less. This is consistent with the claim that some particles are buried in fine particles of the subsurface origin included in ejecta from the SCI crater. Thus, the particle size distributions inside and outside the SCI crater reveal that the subsurface layer beneath the SCI impact site is rich in fine particles with - 9 cm in size while the particles on the surface have a size distribution of a power-law form with shallower slopes at small sizes due to the deposition of fine ejecta from the subsurface layer. Finally, we discuss a process responsible for this difference in particle size distribution between the surface and the subsurface layers. The occurrence of segregation in the gravitational flow of particles on the surface of Ryugu is plausible.