[P-PS03] Regolith Science

[PPS03-P03] Particle size of Ryugu's subsurface layer inferred from observation of SCI crater’s ejecta curtain.

*Koji Wada¹, Ko Ishibashi¹, Hiroshi Kimura¹, Masahiko Arakawa², Hirotaka Sawada³, Kazunori Ogawa^4,2, Kei Shirai², Rie Honda⁵, Yuichi Iijima³, Toshihiko Kadono⁶, Naoya Sakatani⁷, Yuya Mimasu³, Tomoaki Toda³, Yuri Shimaki³, Satoru Nakazawa³, Hajime Hayakawa³, Takanao Saiki³, Yasuhiko Takagi⁸, Hiroshi Imamura³, Chisato Okamoto², Masahiko Hayakawa³, Naru Hirata⁹, Hajime Yano³ (1.Planetary Exploration Research Center, Chiba Institute of Technology, 2.Department of Planetology, Kobe University, 3.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 4.JAXA Space Exploration Center, Japan Aerospace Exploration Agency, 5.Department of Science and Technology, Kochi University, 6.Department of Basic Sciences, University of Occupational and Environmental Health, 7.Department of Physics, Rikkyo University, 8.Aichi Toho University, 9.School of Computer Science and Engineering, The University of Aizu)

Keywords:Hayabusa2, Small Carry-on Impctor, Impact crater, Subsurface particle size, Ryugu, Deployable Camera 3

The particle size of subsurface layers of asteroids would result from the formation and the surface evolution processes of the asteroids. It is difficult, however, to reveal the properties of subsurface particles with remote sensing observations from spacecraft and telescopes. Hayabusa2, the second asteroid explorer mission led by Japan Aerospace Exploration Agency (JAXA), succeeded in forming an artificial crater on the asteroid Ryugu with Small Carry-on Impactor (SCI). In the course of the crater formation process, an ejecta curtain was observed by Deployable Camera 3 (DCAM3) that was separated from the spacecraft to take time-lapse optical images of the ejecta curtain. Since particles that compose the ejecta curtain emerged from a certain depth of the subsurface layer (~ 1 m deep), we are able to extract physical properties of the subsurface particles from the analysis of the ejecta curtain images.

We constructed a theoretical model of the ejecta curtain based on the crater scaling laws and compared it with the images in terms of optical depth. We found that the typical size of the ejecta particles lies in the range from several centimeters to decimeters, indicating a deficit of particles smaller than ~ 1 mm in the subsurface layer. The lack of minute particles suggests that Ryugu might have experienced some kind of surface flow process such as mass wasting to sink small particles into a deeper region or that small particles would have been eliminated by solar radiation pressure during the formation of a rubble pile asteroid Ryugu. We expect laboratory analysis of the size distribution and the interparticle adhesion of Ryugu returned samples to help elucidate the possibility of these processes.