Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body

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Sakatani, N, Tanaka, S, Okada, T, Fukuhara, T, Riu, L, Sugita, S, Honda, R, Morota, T, Kameda, S, Yokota, Y et al (show 84 more authors) , Tatsumi, E, Yumoto, K, Hirata, N, Miura, A, Kouyama, T, Senshu, H, Shimaki, Y, Arai, T, Takita, J, Demura, H, Sekiguchi, T, Mueller, TG, Hagermann, A, Biele, J, Grott, M, Hamm, M, Delbo, M, Neumann, W, Taguchi, M, Ogawa, Y, Matsunaga, T, Wada, T, Hasegawa, S, Helbert, J, Hirata, N, Noguchi, R, Yamada, M, Suzuki, H, Honda, C, Ogawa, K, Hayakawa, M, Yoshioka, K, Matsuoka, M, Cho, Y, Sawada, H, Kitazato, K, Iwata, T, Abe, M, Ohtake, M, Matsuura, S, Matsumoto, K, Noda, H, Ishihara, Y, Yamamoto, K, Higuchi, A, Namiki, N, Ono, G, Saiki, T, Imamura, H, Takagi, Y, Yano, H, Shirai, K, Okamoto, C, Nakazawa, S, Iijima, Y, Arakawa, M, Wada, K, Kadono, T, Ishibashi, K, Terui, F, Kikuchi, S, Yamaguchi, T, Ogawa, N, Mimasu, Y, Yoshikawa, K, Takahashi, T, Takei, Y, Fujii, A, Takeuchi, H, Yamamoto, Y, Hirose, C, Hosoda, S, Mori, O, Shimada, T, Soldini, S ORCID: 0000-0003-3121-3845, Tsukizaki, R, Ozaki, M, Tachibana, S, Ikeda, H, Ishiguro, M, Yabuta, H, Yoshikawa, M, Watanabe, S and Tsuda, Y (2021) Anomalously porous boulders on (162173) Ryugu as primordial materials from its parent body NATURE ASTRONOMY, 5 (8). 766-+. ISSN 2397-3366, 2397-3366

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Abstract

Planetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts^1,2, and subsequent thermal and compaction processes reduce their porosity³. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. ^4–6), higher than meteorites but lower than cometary nuclei⁷, which are considered to be remnants of the original planetesimals⁸. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average⁶ and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment⁹ is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction³. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations^10,11.

Item Type:	Article
Uncontrolled Keywords:	5109 Space Sciences, 51 Physical Sciences
Divisions:	Faculty of Science & Engineering > School of Engineering
Depositing User:	Symplectic Admin
Date Deposited:	05 Jul 2021 13:21
Last Modified:	16 Jun 2026 09:58
DOI:	10.1038/s41550-021-01371-7
Related Websites:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3128851
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