Resurfacing processes on asteroid (162173) Ryugu caused by an artificial impact of Hayabusa2's Small Carry-on Impactor

Honda, Rie; Arakawa, Masahiko; Shimaki, Yuri; Shirai, Kei; Yokota, Yasuhiro; Kadono, Toshihiko; Wada, Koji; Ogawa, Kazunori; Ishibashi, Ko; Sakatani, Naoya; Nakazawa, Satoru; Yasui, Minami; Morota, Tomokatsu; Kameda, Shingo; Tatsumi, Eri; Yamada, Manabu; Kouyama, Toru; Cho, Yuichiro; Matsuoka, Moe; Suzuki, Hidehiko; Honda, Chikatoshi; Hayakawa, Masahiko; Yoshioka, Kazuo; Hirata, Naru; Hirata, Naoyuki; Sawada, Hirotaka; Sugita, Seiji; Saiki, Takanao; Imamura, Hiroshi; Takagi, Yasuhiko; Yano, Hajime; Okamoto, Chisato; Tsuda, Yuichi; Iijima, Yu-ichi
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The resurfacing process on Ryugu accompanying the artificial impact crater formation by Hayabusa2's Small Carry-on Impactor (SCI) was studied by comparing pre- and post-impact images of this region captured by an optical navigation camera. Three different aspects of the resurfacing process were examined: the crater rim profiles, the motion of boulders and the appearance of new boulders, and the motion vectors of Ryugu's surface around the SCI crater. The averaged crater rim height, h, was derived as follows: h = hr exp [-(r/Rrim - 1)/λrim], where Rrim is the SCI crater rim radius of 8.8 m, the fitted parameter, hr, is 0.475 m, and the λrim is 0.245. The ejecta blanket thickness of the SCI crater was thinner than that estimated from both the observation of natural craters and the crater formation theory. However, this discrepancy of the ejecta blanket thickness was resolved by taking into account the new boulders appearing in the post-impact images in the volume. The motion of the discovered boulders could be classified by its mechanisms as follows: a dragging motion created by excavation flow during the crater formation, a pushing motion created by falling-back ejecta, a dragging motion created by the slight motion of the Okamoto boulder, and a motion caused by seismic shaking induced by the SCI impact itself. The seismic shaking caused boulders to move farther than 3 cm from the original site in most of the region within 15 m distance from the SCI crater center, where the maximum acceleration of the impact induced seismic waves 7 times larger than the surface gravity of Ryugu based on the laboratory experiments (Matsue et al. [2020] Icarus, 338, 113520), and the evidence of the seismic shaking for boulders with a movement of >3 cm was detected in about 10% of the boulders in the region between 15 m and 30 m from the crater center, which region was inferred to experience acceleration larger than the Ryugu's surface gravity based on previous laboratory experiments (Matsue et al. [2020] Icarus, 338, 113520).
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