An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime M. Arakawa, T. Saiki, K. Wada, K. Ogawa, T. Kadono, K. Shirai, H. Sawada, K. Ishibashi, R. Honda, N. Sakatani, et al. To cite this version: M. Arakawa, T. Saiki, K. Wada, K. Ogawa, T. Kadono, et al.. An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime. Science, American Association for the Advancement of Science, 2020, 368 (6486), pp.67-71. 10.1126/science.aaz1701. hal-02986191 HAL Id: hal-02986191 https://hal.archives-ouvertes.fr/hal-02986191 Submitted on 7 Jan 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Submitted Manuscript Title: An artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime Authors: M. Arakawa1*, T. Saiki2, K. Wada3, K. Ogawa21,1, T. Kadono4, K. Shirai2,1, H. Sawada2, K. Ishibashi3, R. Honda5, N. Sakatani2, Y. Iijima2§, C. Okamoto1§, H. Yano2, Y. 5 Takagi6, M. Hayakawa2, P. Michel7, M. Jutzi8, Y. Shimaki2, S. Kimura9, Y. Mimasu2, T. Toda2, H. Imamura2, S. Nakazawa2, H. Hayakawa2, S. Sugita10,3, T. Morota10, S. Kameda11, E. Tatsumi20,12,10, Y. Cho10, K. Yoshioka10, Y. Yokota2,5, M. Matsuoka2, M. Yamada3, T. Kouyama13, C. Honda14, Y. Tsuda2, S. Watanabe15,2, M. Yoshikawa2,19, S. Tanaka2,19, F. Terui2, S. Kikuchi2, T. Yamaguchi2†, N. Ogawa2, G. Ono16, K. Yoshikawa16, T. Takahashi2‡, Y. 10 Takei16,2, A. Fujii2, H. Takeuchi2,19, Y. Yamamoto2,19, T. Okada2,10, C. Hirose16, S. Hosoda2, O. Mori2, T. Shimada2, S. Soldini17, R. Tsukizaki2, T. Iwata2, M. Ozaki2,19, M. Abe2,19, N. Namiki18,19, K. Kitazato14, S. Tachibana10, H. Ikeda16, N. Hirata14, N. Hirata1, R. Noguchi2, A. Miura2. Affiliations: 15 1. Kobe University, Kobe 657-8501, Japan. 2. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan. 3. Planetary Exploration Research Center, Chiba Institute of Technology, Narashino 275-0016, Japan. 20 4. University of Occupational and Environmental Health, Kitakyusyu 807-8555, Japan. 5. Kochi University, Kochi 780-8520, Japan. 6. Aichi Toho University, Nagoya 465-8515, Japan. 7. The Côte d'Azur Observatory, 06304 Nice Cedex 4, France. 8. Physics Institute, University of Bern, NCCR PlanetS, Gesellschaftsstrasse 6, 3012, Bern, 25 Switzerland. 9. Tokyo University of Science, Noda 278-8510, Japan. 10. The University of Tokyo, Tokyo 113-0033, Japan. 11. Rikkyo University, Tokyo 171-8501, Japan. 12. University of La Laguna, 38205 San Cristóbal de La Laguna, Spain. 30 13. National Institute of Advanced Industrial Science and Technology, Tokyo 135-0064, Japan. 14. The University of Aizu, Aizu-Wakamatsu 965-8580, Japan. 15. Nagoya University, Nagoya 464-8601, Japan. 16. Research and Development Directorate, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan. 35 17. University of Liverpool, Liverpool L3 5TQ, United Kingdom. 18. National Astronomical Observatory of Japan, Mitaka 181-8588, Japan. 19. SOKENDAI (The Graduate University for Advanced Studies), Hayama 240-0193, Japan. 20. Instituto de Astrofísica de Canarias, La Laguna, Tenerife, E38205, Spain. 21. JAXA Space Exploration Center, Japan Aerospace Exploration Agency, Sagamihara 252- 40 5210, Japan. *Corresponding author: E-mail: [email protected] †Current affiliation: Mitsubishi Electric Corporation, Kamakura 247-8520, Japan. ‡Current affiliation: NEC Corporation, 1-10 Nisshin-cho, Fuchu, Tokyo 183-0036, Japan. 45 §Deceased. 1 Submitted Manuscript Abstract: The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor (SCI). The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim and a central pit. Images of the impact and resulting ejecta 5 were recorded by the Deployable CAMera 3 (DCAM3) for >8 minutes, showing the growth of an ejecta curtain and deposition of ejecta onto the surface. The ejecta curtain, outer edge of ejecta, was asymmetric, heterogeneous, and never fully detached from the surface. The artificial crater was formed in the gravity-dominated regime: the gravity limited the crater growth; we discuss the implication of this observation for Ryugu’s surface age. 10 Main Text: The Hayabusa2 spacecraft reached the near-Earth asteroid (162173) Ryugu in June 2018, showing that this small asteroid is covered with boulders with sizes up to ~160 m (1, 2). The surface is covered with a regolith layer composed of boulders and granules under microgravity conditions (gravitational acceleration about 1×10-4 m s-2), which was expected to have strength originating from cohesion forces between regolith grains, with the maximum 15 theoretical strength of the surface layer predicted to be 1 kPa (3). Impact craters formed under those conditions should be limited in size by this surface strength, which reduces the expected crater diameter compared to a strengthless surface (4). Crater sizes produced by hypervelocity impacts on small bodies can be predicted using a variety of crater scaling laws. The number and sizes of craters on asteroids (including Ryugu) can be used to date their surfaces, but different 20 crater scaling laws give ages that differ by more than an order of magnitude (1). Hayabusa2 spacecraft was equipped with a Small Carry-on Impactor (SCI) designed to launch a 2kg copper projectile at a velocity of 2 km s-1 to form an artificial impact crater (referred to hereafter as the SCI crater) on the surface of Ryugu. The SCI crater was intended to expose the asteroid subsurface for remote sensing and sample collection (5, 6). The Deep Impact 25 spacecraft performed an impact experiment on a small body and formed an artificial impact crater on the nucleus of the comet 9P/Tempel 1(7-9). The SCI impact can be used to test crater scaling laws, for the crater size and ejecta velocity distribution, which were constructed by laboratory experiments (10–12), and be compared with numerical simulations of the impact cratering processes in a microgravity environment (13). 30 The SCI impact operation was carried out on 5 April 2019 (14). The impact and evolution of impact ejecta were simultaneously observed by Hayabusa2’s Deployable CAMera 3 (DCAM3) (14). About 3 weeks after the SCI impact, Hayabusa2 searched for evidence of the impact crater using its Optical Navigation Camera-Telescopic (ONC-T) from an altitude of 1.7 km with a resolution of ~18 cm/pixel. The fresh crater was identified at latitude 7.9° N and longitude 35 301.3° E (Figure 1A-B), ~20 m from the aiming point (which was latitude 6.00° N, longitude 303.00° E). This location is in a northern part of Ryugu’s equatorial ridge (2). The impact angle was estimated to be ~60° from the local horizontal surface (14). Comparison of ONC-T images taken from the same altitude before and after the impact (Fig. 1A-B) shows the formation of an artificial impact crater. A dozen boulders with sizes of at least 40 tens of cm were driven away from the surface, and a large block with a size of 5 m (which we refer to as the Mobile Block, MB) was excavated and moved ~3 m northwest and >1 m above the initial surface by the impact. Before the impact, MB was adjacent to another large block (Stable Block, SB) (Fig. 1A), but SB was hardly moved by the SCI impact. SB might be part of a larger block buried below the crater floor. We speculate that SB and its subsurface extension 45 halted the crater growth toward the southeast. Alternatively, SCI may have impacted closer to MB than SB, allowing the higher pressure to move MB but not SB. 2 Submitted Manuscript To investigate the crater morphology profile, we generated a Digital Elevation Map (DEM) (Fig. 2A) from ONC-T images (14). This shows an elevated rim with a height of ~40 cm surrounding the SCI crater (Fig. 2B). The elevated rim could be composed of ejecta deposits and/or structural uplifts. We used the DEM to produce an ortho-corrected (a geometrically 5 accurate) image on which we traced the top of the rim to determine the crater shape (Fig. 1C). The SCI crater is semi-circular with a rim to rim diameter �rim of 17.6 ± 0.7 m. This shape is not due to the oblique impact but the effect of a large block (possibly SB) southeast of the impact point (Fig. 1B) (14). Impact craters are usually symmetrical at this impact angle, so we assume that the SCI impact point is the center of the semicircular crater rim. A crater’s apparent diameter 10 � is conventionally measured at an initial surface elevation. Using the DEM, we determine the SCI crater to have D = 14.5 ± 0.8 m (Fig. 2A). There is a small pit close to the impact point on the crater floor. The pit entrance is at a depth of 1.7 m from the initial surface (Fig. 2B); the diameter of the pit is ~3 m and its depth is 0.6 m. The pit has a conical shape similar to the simple craters formed in laboratory experiments (10, 15 15). Central pits are commonly observed in craters smaller than a few hundred meters on the Moon because the lunar subsurface has a layered structure consisting of a relatively hard layer covered with softer cohesionless regolith (16).