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Appears in Collections:Biological and Environmental Sciences Journal Articles
Peer Review Status: Refereed
Title: Boulder size and shape distributions on asteroid Ryugu
Author(s): Michikami, Tatsuhiro
Honda, Chikatoshi
Miyamoto, Hideaki
Hirabayashi, Masatoshi
Hagermann, Axel
Irie, Terunori
Nomura, Keita
Ernst, Carolyn M
Kawamura, Masaki
Sugimoto, Kiichi
Tatsumi, Eri
Morota, Tomokatsu
Hirata, Naru
Noguchi, Takaaki
Cho, Yuichiro
Keywords: Asteroids
Impact processes
Geological processes
Asteroid Ryugu
Issue Date: Oct-2019
Date Deposited: 17-Jun-2019
Citation: Michikami T, Honda C, Miyamoto H, Hirabayashi M, Hagermann A, Irie T, Nomura K, Ernst CM, Kawamura M, Sugimoto K, Tatsumi E, Morota T, Hirata N, Noguchi T & Cho Y (2019) Boulder size and shape distributions on asteroid Ryugu. Icarus, 331, pp. 179-191.
Abstract: In 2018, the Japanese spacecraft Hayabusa2, arrived at the small asteroid Ryugu. The surface of this C-type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study. Using a few hundred Optical Navigation Camera (ONC) images with a pixel scale of approximately 0.65 m, we focus on boulders greater than 5m in diameter. Smaller boulders are also considered using five arbitrarily chosen ONC close-up images with pixel scales ranging from 0.7 to 6 cm. Across the entire surface area (~2.7 km2) of Ryugu, nearly 4400 boulders larger than 5m were identified. Boulders appear to be uniformly distributed across the entire surface, with some slight differences in latitude and longitude. At ~50 km−2, the number density of boulders larger than 20m is twice as large as on asteroid Itokawa (or Bennu). The apparent shapes of Ryugu's boulders resemble laboratory impact fragments, with larger boulders being more elongated. The ratio of the total volume of boulders larger than 5m to the total excavated volume of craters larger than 20m on Ryugu can be estimated to be ~94%, which is comparatively high. These observations strongly support the hypothesis that most boulders found on Ryugu resulted from the catastrophic disruption of Ryugu's larger parent body, as described in previous papers (Watanabe et al., 2019; Sugita et al.,2019). The cumulative size distribution of boulders larger than 5 m has a power-index of −2.65 ± 0.05, which is comparatively shallow compared with other asteroids visited by spacecraft. For boulders smaller than 4 m, the power-index is even shallower and ranges from −1.65 ± 0.05 to −2.01 ± 0.06. This particularly shallow power-index implies that some boulders are buried in Ryugu's regolith. Based on our observations, we suggest that boulders near the equator might have been buried by the migration of finer material and, as a result, the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes.
DOI Link: 10.1016/j.icarus.2019.05.019
Rights: © 2019 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license (
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