Size of particles ejected from an artificial impact crater on asteroid 162173 Ryugu

¹ Planetary Exploration Research Center, Chiba Institute of Technology, Narashino 275-0016, Japan
e-mail: wada@perc.it-chiba.ac.jp
² Department of Planetology, Kobe University, Kobe 657-8501, Japan
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
⁴ JAXA Space Exploration Center, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
⁵ Department of Science and Technology, Kochi University, Kochi 780-8520, Japan
⁶ Department of Basic Sciences, University of Occupational and Environmental Health, Kitakyusyu 807-8555, Japan
⁷ Department of Physics, Rikkyo University, Tokyo 171-8501, Japan
⁸ Aichi Toho University, Nagoya 465-8515, Japan
⁹ School of Computer Science and Engineering, The University of Aizu, Aizu-Wakamatsu 965-8580, Japan

Received: 28 October 2020
Accepted: 26 January 2021

Abstract

A projectile accelerated by the Hayabusa2 Small Carry-on Impactor successfully produced an artificial impact crater with a final apparent diameter of 14.5 ± 0.8 m on the surface of the near-Earth asteroid 162173 Ryugu on April 5, 2019. At the time of cratering, Deployable Camera 3 took clear time-lapse images of the ejecta curtain, an assemblage of ejected particles forming a curtain-like structure emerging from the crater. Focusing on the optical depth of the ejecta curtain and comparing it with a theoretical model, we infer the size of the ejecta particles. As a result, the typical size of the ejecta particles is estimated to be several centimeters to decimeters, although it slightly depends on the assumed size distribution. Since the ejecta particles are expected to come from a depth down to ~1 m, our result suggests that the subsurface layer of Ryugu is composed of relatively small particles compared to the uppermost layer on which we observe many meter-sized boulders. Our result also suggests a deficit of particles of less than ~1 mm in the subsurface layer. These findings will play a key role in revealing the formation and surface evolution process of Ryugu and other small Solar System bodies.