Highly porous nature of a primitive asteroid revealed by thermal imaging

Tatsuaki Okada (), Tetsuya Fukuhara, Satoshi Tanaka, Makoto Taguchi, Takehiko Arai, Hiroki Senshu, Naoya Sakatani, Yuri Shimaki, Hirohide Demura, Yoshiko Ogawa, Kentaro Suko, Tomohiko Sekiguchi, Toru Kouyama, Jun Takita, Tsuneo Matsunaga, Takeshi Imamura, Takehiko Wada, Sunao Hasegawa, Jörn Helbert, Thomas G. Müller, Axel Hagermann, Jens Biele, Matthias Grott, Maximilian Hamm, Marco Delbo, Naru Hirata, Naoyuki Hirata, Yukio Yamamoto, Seiji Sugita, Noriyuki Namiki, Kohei Kitazato, Masahiko Arakawa, Shogo Tachibana, Hitoshi Ikeda, Masateru Ishiguro, Koji Wada, Chikatoshi Honda, Rie Honda, Yoshiaki Ishihara, Koji Matsumoto, Moe Matsuoka, Tatsuhiro Michikami, Akira Miura, Tomokatsu Morota, Hirotomo Noda, Rina Noguchi, Kazunori Ogawa, Kei Shirai, Eri Tatsumi, Hikaru Yabuta, Yasuhiro Yokota, Manabu Yamada, Masanao Abe, Masahiko Hayakawa, Takahiro Iwata, Masanobu Ozaki, Hajime Yano, Satoshi Hosoda, Osamu Mori, Hirotaka Sawada, Takanobu Shimada, Hiroshi Takeuchi, Ryudo Tsukizaki, Atsushi Fujii, Chikako Hirose, Shota Kikuchi, Yuya Mimasu, Naoko Ogawa, Go Ono, Tadateru Takahashi, Yuto Takei, Tomohiro Yamaguchi, Kent Yoshikawa, Fuyuto Terui, Takanao Saiki, Satoru Nakazawa, Makoto Yoshikawa, Seiichiro Watanabe and Yuichi Tsuda
Additional contact information
Tatsuaki Okada: Japan Aerospace Exploration Agency (JAXA)
Tetsuya Fukuhara: Rikkyo University
Satoshi Tanaka: Japan Aerospace Exploration Agency (JAXA)
Makoto Taguchi: Rikkyo University
Takehiko Arai: Ashikaga University
Hiroki Senshu: Chiba Institute of Technology
Naoya Sakatani: Japan Aerospace Exploration Agency (JAXA)
Yuri Shimaki: Japan Aerospace Exploration Agency (JAXA)
Hirohide Demura: University of Aizu
Yoshiko Ogawa: University of Aizu
Kentaro Suko: University of Aizu
Tomohiko Sekiguchi: Hokkaido University of Education
Toru Kouyama: National Institute of Advanced Industrial Science and Technology (AIST)
Jun Takita: Hokkaido Kitami Hokuto High School
Tsuneo Matsunaga: National Institute for Environmental Studies (NIES)
Takeshi Imamura: University of Tokyo
Takehiko Wada: Japan Aerospace Exploration Agency (JAXA)
Sunao Hasegawa: Japan Aerospace Exploration Agency (JAXA)
Jörn Helbert: German Aerospace Center (DLR)
Thomas G. Müller: Max-Planck Institute for Extraterrestrial Physics
Axel Hagermann: University of Stirling
Jens Biele: German Aerospace Center (DLR)
Matthias Grott: German Aerospace Center (DLR)
Maximilian Hamm: German Aerospace Center (DLR)
Marco Delbo: Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange
Naru Hirata: University of Aizu
Naoyuki Hirata: Kobe University
Yukio Yamamoto: Japan Aerospace Exploration Agency (JAXA)
Seiji Sugita: University of Tokyo
Noriyuki Namiki: The Graduate University for Advanced Studies, SOKENDAI
Kohei Kitazato: University of Aizu
Masahiko Arakawa: Kobe University
Shogo Tachibana: Japan Aerospace Exploration Agency (JAXA)
Hitoshi Ikeda: Japan Aerospace Exploration Agency (JAXA)
Masateru Ishiguro: Seoul National University
Koji Wada: Chiba Institute of Technology
Chikatoshi Honda: University of Aizu
Rie Honda: Kochi University
Yoshiaki Ishihara: National Institute for Environmental Studies (NIES)
Koji Matsumoto: The Graduate University for Advanced Studies, SOKENDAI
Moe Matsuoka: Japan Aerospace Exploration Agency (JAXA)
Tatsuhiro Michikami: Kindai University
Akira Miura: Japan Aerospace Exploration Agency (JAXA)
Tomokatsu Morota: University of Tokyo
Hirotomo Noda: National Astronomical Observatory of Japan (NAOJ)
Rina Noguchi: Japan Aerospace Exploration Agency (JAXA)
Kazunori Ogawa: Kobe University
Kei Shirai: Kobe University
Eri Tatsumi: University of Tokyo
Hikaru Yabuta: Hiroshima University
Yasuhiro Yokota: Japan Aerospace Exploration Agency (JAXA)
Manabu Yamada: Chiba Institute of Technology
Masanao Abe: Japan Aerospace Exploration Agency (JAXA)
Masahiko Hayakawa: Japan Aerospace Exploration Agency (JAXA)
Takahiro Iwata: Japan Aerospace Exploration Agency (JAXA)
Masanobu Ozaki: Japan Aerospace Exploration Agency (JAXA)
Hajime Yano: Japan Aerospace Exploration Agency (JAXA)
Satoshi Hosoda: Japan Aerospace Exploration Agency (JAXA)
Osamu Mori: Japan Aerospace Exploration Agency (JAXA)
Hirotaka Sawada: Japan Aerospace Exploration Agency (JAXA)
Takanobu Shimada: Japan Aerospace Exploration Agency (JAXA)
Hiroshi Takeuchi: Japan Aerospace Exploration Agency (JAXA)
Ryudo Tsukizaki: Japan Aerospace Exploration Agency (JAXA)
Atsushi Fujii: Japan Aerospace Exploration Agency (JAXA)
Chikako Hirose: Japan Aerospace Exploration Agency (JAXA)
Shota Kikuchi: Japan Aerospace Exploration Agency (JAXA)
Yuya Mimasu: Japan Aerospace Exploration Agency (JAXA)
Naoko Ogawa: Japan Aerospace Exploration Agency (JAXA)
Go Ono: Japan Aerospace Exploration Agency (JAXA)
Tadateru Takahashi: Japan Aerospace Exploration Agency (JAXA)
Yuto Takei: Japan Aerospace Exploration Agency (JAXA)
Tomohiro Yamaguchi: Japan Aerospace Exploration Agency (JAXA)
Kent Yoshikawa: Japan Aerospace Exploration Agency (JAXA)
Fuyuto Terui: Japan Aerospace Exploration Agency (JAXA)
Takanao Saiki: Japan Aerospace Exploration Agency (JAXA)
Satoru Nakazawa: Japan Aerospace Exploration Agency (JAXA)
Makoto Yoshikawa: Japan Aerospace Exploration Agency (JAXA)
Seiichiro Watanabe: Japan Aerospace Exploration Agency (JAXA)
Yuichi Tsuda: Japan Aerospace Exploration Agency (JAXA)

Nature, 2020, vol. 579, issue 7800, 518-522

Abstract: Abstract Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.

Date: 2020
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DOI: 10.1038/s41586-020-2102-6

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