Faculty Profiles - WATANABE, Sei-ichiro

Papers - WATANABE, Sei-ichiro

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Elastic Property of Returned Samples From Asteroid (162173) Ryugu

Keisuke Onodera, Yuta Ino, Satoshi Tanaka, Taichi Kawamura, Rei Kanemaru, Takuya Ishizaki, Ryota Fukai, Takeshi Tsuji, Tomoki Nakamura, Daisuke Nakashima, Masayuki Uesugi, Shogo Tachibana, Seiji Sugita, Hisayoshi Yurimoto, Takaaki Noguchi, Ryuji Okazaki, Hikaru Yabuta, Hiroshi Naraoka, Kanako Sakamoto, Toru Yada, Masahiro Nishimura, Aiko Nakato, Akiko Miyazaki, Kasumi Yogata, Masanao Abe, Tatsuaki Okada, Tomohiro Usui, Makoto Yoshikawa, Takanao Saiki, Satoru Nakazawa, Fuyuto Terui, Sei-ichiro Watanabe, Yuichi Tsuda

ESS Open Archive 2023.5

Language：English Publishing type：Research paper (other academic) Publisher：Authorea, Inc.

<p id="p1">The elastic property of asteroids is one of the paramount parameters forunderstanding their physical nature. For example, the rigidity enablesus to discuss the asteroid’s shape and surface features such as cratersand boulders, leading to a better understanding of geomorphological andgeological features on small celestial bodies. The sound velocity allowsus to construct an equation of state that is the most fundamental stepto simulate the formation of small bodies numerically. Moreover, seismicwave velocities and attenuation factors are useful to account forresurfacing caused by impact-induced seismic shaking. The elasticproperty of asteroids thus plays an important role in elucidating theasteroid’s evolution and current geological processes. The Hayabusa2spacecraft brought back the rock samples from C-type asteroid (162173)Ryugu in December 2020. As a part of the initial analysis of returnedsamples, we measured the seismic wave velocity of the Ryugu samplesusing the pulse transmission method. We found that P- and S-wavevelocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s,respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. Acomparison of the derived parameters with those of carbonaceouschondrites showed that the Ryugu samples have a similar elastic propertyto the Tagish Lake meteorite, which may have come from a D-typeasteroid. Both Ryugu and Tagish Lake show a high degree of aqueousalteration and few high-temperature components such as chondrules,indicating that they formed in the outer region of the solar system.</p>

DOI： 10.22541/essoar.168500248.80647094/v1

Re-distribution of volatiles on the airless surface of the C-type carbonaceous asteroid Ryugu. International coauthorship International journal

Toru Matsumoto, Takaaki Noguchi, Akira Miyake, Yohei Igami, Mitsutaka Haruta, Yusuke Seto, Masaaki Miyahara, Naotaka Tomioka, Hikaru Saito, Satoshi Hata, Dennis Harries, Aki Takigawa, Yuusuke Nakauchi, Shogo Tachibana, Tomoki Nakamura, Megumi Matsumoto, Hope Ishii, John Bradley, Kenta Ohtaki, Elena Dobrică, Hugues Leroux, Corentin Le Guillou, Damien Jacob, Francisco de la Peña, Sylvain Laforet, Maya Marinova, Falko Langenhorst, Pierre Beck, Thi Phan, Rolando Rebois, Neyda Abreu, Jennifer Gray, Thomas Zega, Pierre-Marie Zanetta, Michelle Thompson, Rhonda Stroud, Katherine Burgess, Brittany Cymes, John Bridges, Leon Hicks, Martin Lee, Luke Daly, Phil Bland, Michael Zolensky, David Frank, James Martinez, Akira Tsuchiyama, Masahiro Yasutake, Junya Matsuno, Shota Okumura, Itaru Mitsukawa, Kentaro Uesugi, Masayuki Uesugi, Akihisa Takeuchi, Mingqi Sun, Satomi Enju, Tatsuhiro Michikami, Hisayoshi Yurimoto, Ryuji Okazaki, Hikaru Yabuta, Hiroshi Naraoka, Kanako Sakamoto, Toru Yada, Masahiro Nishimura, Aiko Nakato, Akiko Miyazaki, Kasumi Yogata, Masanao Abe, Tatsuaki Okada, Tomohiro Usui, Makoto Yoshikawa, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Satoru Nakazawa, Sei-ichiro Watanabe, Yuichi Tsuda

2023.3

Crack Orientation of Boulders on Ryugu: Meridional Preference and Exfoliation

Sho SASAKI, Shiho KANDA, Hiroshi KIKUCHI, Tatsuhiro MICHIKAMI, Tomokatsu MOROTA, Chikatoshi HONDA, Hideaki MIYAMOTO, Ryodo HEMMI, Seiji SUGITA, Eri TATSUMI, Masanori KANAMARU, Sei-ichiro WATANABE, Noriyuki NAMIKI, Patrick MICHEL, Masatoshi HIRABAYASHI, Naru HIRATA, Tomoki NAKAMURA, Takaaki NOGUCHI, Takahiro HIROI, Naoya SAKATANI, Koji MATSUMOTO, Hirotomo NODA, Shingo KAMEDA, Tohru KOUYAMA, Hidehiko SUZUKI, Manabu YAMADA, Rie HONDA, Yuichiro CHO, Kazuo YOSHIOKA, Masahiko HAYAKAWA, Moe MATSUOKA, Rina NOGUCHI, Hirotaka SAWADA, Yasuhiro YOKOTA, Makoto YOSHIKAWA

Journal of Evolving Space Activities 2023

Early fluid activity on Ryugu: perspectives from oxygen, carbon, and 53Mn-53Cr isotopes Reviewed International coauthorship International journal

Kaitlyn McCain, Nozomi Matsuda, Ming-Chang Liu, Kevin McKeegan, Akira Yamaguchi, Makoto Kimura, Naotaka Tomioka, Motoo Ito, Naoya Imae, Masayuki Uesugi, Naoki Shirai, Takuji Ohigashi, Richard Greenwood, Kentaro Uesugi, Aiko Nakato, Kasumi Yogata, Hayato Yuzawa, Yu Kodama, Kaori Hirahara, Ikuya Sakurai, Ikuo Okada, Yuzuru Karouji, Satoru Nakazawa, Tatsuaki Okada, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Makoto Yoshikawa, Akiko Miyazaki, Masahiro Nishimura, Toru Yada, Masanao Abe, Tomohiro Usui, Sei-ichiro Watanabe, Yuichi Tsuda

2022.6

Rotational states and shapes of Ryugu and Bennu: Implications for interior structure and strength Reviewed

PLANETARY AND SPACE SCIENCE Vol. 204 2021.9

Hayabusa2 extended mission: New voyage to rendezvous with a small asteroid rotating with a short period Reviewed International coauthorship

M. Hirabayashi, Y. Mimasu, N. Sakatani, S. Watanabe, Y. Tsuda, T. Saiki, S. Kikuchi, T. Kouyama, M. Yoshikawa, S. Tanaka, S. Nakazawa, Y. Takei, F. Terui, H. Takeuchi, A. Fujii, T. Iwata, K. Tsumura, S. Matsuura, Y. Shimaki, S. Urakawa, Y. Ishibashi, S. Hasegawa, M. Ishiguro, D. Kuroda, S. Okumura, S. Sugita, T. Okada, S. Kameda, S. Kamata, A. Higuchi, H. Senshu, H. Noda, K. Matsumoto, R. Suetsugu, T. Hirai, K. Kitazato, D. Farnocchia, S.P. Naidu, D.J. Tholen, C.W. Hergenrother, R.J. Whiteley, N.A. Moskovitz, P.A. Abell

Advances in Space Research Vol. 68 ( 3 ) page： 1533 - 1555 2021.8

FIRST NIR HYPER-SPECTRAL IMAGING OF HAYABUSA2 RETURNED SAMPLES BY THE MICROMEGA MICROSCOPE WITHIN THE ISAS CURATION FACILITY Reviewed International coauthorship

Bibring J. P., Pilorget C., Okada T., Hamm V., Brunetto R., Yada T., Loizeau D., Riu L., Usui T., Moussi-Soffys A., Hatakeda K., Nakato A., Yogata K., Abe M., Aleon-Toppani A., Carter J., Chaigneau M., Crane B., Gondet B., Kumagai K., Langevin Y., Lantz C., Le Pivert-Jolivet T., Lequertier G., Lourit L., Miyazaki A., Nishimura M., Poulet F., Arakawa M., Hirata N., Kitazato K., Nakazawa S., Namiki N., Saiki T., Sugita S., Tachibana S., Tanaka S., Yoshikawa M., Tsuda Y., Watanabe S.

METEORITICS & PLANETARY SCIENCE Vol. 56 2021.8

INITIAL DESCRIPTIONS OF ASTEROID RYUGU SAMPLES RETURNED BY HAYABUSA2. Reviewed International coauthorship

Yada T., Abe M., Nakato A., Yogata K., Miyazaki A., Kumagai K., Hatakeda K., Okada T., Nishimura M., Furuya S., Yoshitake M., Iwamae A., Soejima H., Hitomi Y., Riu L., Lourit L., Pilorget C., Hamm V., Brunetto R., Bibring J. -P., Cho Y., Yumoto K., Yabe Y., Sugita S., Tachibana S., Sawada H., Sakamoto K., Hayashi T., Yamamoto D., Fukai R., Sugahara H., Yurimoto H., Usui T., Watanabe S., Tsuda Y.

METEORITICS & PLANETARY SCIENCE Vol. 56 2021.8

Ryugu: A brand-new planetary sample returned from a C-type asteroid

Toru Yada, Masanao Abe, Tatsuaki Okada, Aiko Nakato, Kasumi Yogata, Akiko Miyazaki, Kentaro Hatakeda, Kazuya Kumagai, Masahiro Nishimura, Yuya Hitomi, Hiromichi Soejima, Miwa Yoshitake, Ayako Iwamae, Shizuho Furuya, Masayuki Uesugi, Yuzuru Karouji, Tomohiro Usui, Tasuku Hayashi, Daiki Yamamoto, Ryota Fukai, Seiji Sugita, Yuichiro Cho, Koki Yumoto, Yuna Yabe, Jean-Pierre Bibring, Cedric Pilorget, Vincent Hamm, Rosario Brunetto, Lucie Riu, Lionel Lourit, Damien Loizeau, Guillaume Lequertier, Aurelie Moussi-Soffys, Shogo Tachibana, Hirotaka Sawada, Ryuji Okazaki, Yoshinori Takano, Kanako Sakamoto, Yayoi Miura, Hajime Yano, Trevor Ireland, Tetsuya Yamada, Masaki Fujimoto, Kohei Kitazato, Noriyuki Namiki, Masahiko Arakawa, Naru Hirata, Hisayoshi Yurimoto, Tomoki Nakamura, Takaaki Noguchi, Hikaru Yabuta, Hiroshi Naraoka, Motoo Ito, Eizo Nakamura, Kentaro Uesugi, Katsura Kobayashi, Tatsuhiro Michikami, Hiroshi Kikuchi, Naoyuki Hirata, Yoshiaki Ishihara, Koji Matsumoto, Hirotomo Noda, Rina Noguchi, Yuri Shimaki, Kazunori Ogawa, Kei Shirai, Koji Wada, Hiroki Senshu, Yukio Yamamoto, Tomokatsu Morota, Rie Honda, Chikatoshi Honda, Yasuhiro Yokota, Moe Matsuoka, Naoya Sakatani, Eri Tatsumi, Akira Miura, Manabu Yamada, Atsushi Fujii, Chikako Hirose, Satoshi Hosoda, Hitoshi Ikeda, Takahiro Iwata, Shota Kikuchi, Yuya Mimasu, Osamu Mori, Naoko Ogawa, Go Ono, Takanobu Shimada, Stefania Soldini, Tadateru Takahashi, Yuto Takei, Hiroshi Takeuchi, Ryudo Tsukizaki, Kent Yoshikawa, Fuyuto Terui, Satoru Nakazawa, Satoshi Tanaka, Takanao Saiki, Makoto Yoshikawa, Sei-ichiro Watanabe, Yuichi Tsuda

2021.6

Publisher：Research Square Platform LLC

<title>Abstract</title>
C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

DOI： 10.21203/rs.3.rs-608561/v1

Other Link： https://www.researchsquare.com/article/rs-608561/v1.html

Ballistic deployment of the Hayabusa2 artificial landmarks in the microgravity environment of Ryugu Reviewed International coauthorship

Shota Kikuchi, Naoko Ogawa, Osamu Mori, Takanao Saiki, Yuto Takei, Fuyuto Terui, Go Ono, Yuya Mimasu, Kent Yoshikawa, Stefaan Van Wal, Hiroshi Takeuchi, Hitoshi Ikeda, Atsushi Fujii, Yuki Takao, Tetsuya Kusumoto, Naru Hirata, Naoyuki Hirata, Kei Shirai, Toru Kouyama, Shingo Kameda, Manabu Yamada, Satoru Nakazawa, Makoto Yoshikawa, Satoshi Tanaka, Seiji Sugita, Sei-ichiro Watanabe, Yuichi Tsuda

Icarus Vol. 358 page： 114220 - 114220 2021.4

Crater depth-to-diameter ratios on asteroid 162173 Ryugu Reviewed

Rina Noguchi, Naoyuki Hirata, Naru Hirata, Yuri Shimaki, Naoki Nishikawa, Sayuri Tanaka, Takaaki Sugiyama, Tomokatsu Morota, Seiji Sugita, Yuichiro Cho, Rie Honda, Shingo Kameda, Eri Tatsumi, Kazuo Yoshioka, Hirotaka Sawada, Yasuhiro Yokota, Naoya Sakatani, Masahiko Hayakawa, Moe Matsuoka, Manabu Yamada, Toru Kouyama, Hidehiko Suzuki, Chikatoshi Honda, Kazunori Ogawa, Masanori Kanamaru, Sei-ichiro Watanabe

Icarus Vol. 354 ( 15 ) page： 114016 - 114016 2021.1

Rotational effect as the possible cause of the east-west asymmetric crater rims on Ryugu observed by LIDAR data Reviewed

Naoyuki Hirata, Noriyuki Namiki, Fumi Yoshida, Koji Matsumoto, Hirotomo Noda, Hiroki Senshu, Takahide Mizuno, Fuyuto Terui, Yoshiaki Ishihara, Ryuhei Yamada, Keiko Yamamoto, Sinsuke Abe, Rina Noguchi, Naru Hirata, Yuichi Tsuda, Sei-ichiro Watanabe

Icarus Vol. 354 page： 114073 - 114073 2021.1

Collisional history of Ryugu’s parent body from bright surface boulders Reviewed International coauthorship

E. Tatsumi, C. Sugimoto, L. Riu, S. Sugita, T. Nakamura, T. Hiroi, T. Morota, M. Popescu, T. Michikami, K. Kitazato, M. Matsuoka, S. Kameda, R. Honda, M. Yamada, N. Sakatani, T. Kouyama, Y. Yokota, C. Honda, H. Suzuki, Y. Cho, K. Ogawa, M. Hayakawa, H. Sawada, K. Yoshioka, C. Pilorget, M. Ishida, D. Domingue, N. Hirata, S. Sasaki, J. de León, M. A. Barucci, P. Michel, M. Suemitsu, T. Saiki, S. Tanaka, F. Terui, S. Nakazawa, S. Kikuchi, T. Yamaguchi, N. Ogawa, G. Ono, Y. Mimasu, K. Yoshikawa, T. Takahashi, Y. Takei, A. Fujii, Y. Yamamoto, T. Okada, C. Hirose, S. Hosoda, O. Mori, T. Shimada, S. Soldini, R. Tsukizaki, T. Mizuno, T. Iwata, H. Yano, M. Ozaki, M. Abe, M. Ohtake, N. Namiki, S. Tachibana, M. Arakawa, H. Ikeda, M. Ishiguro, K. Wada, H. Yabuta, H. Takeuchi, Y. Shimaki, K. Shirai, N. Hirata, Y. Iijima, Y. Tsuda, S. Watanabe, M. Yoshikawa

Nature Astronomy Vol. 5 ( 1 ) page： 39 - 45 2021.1

Thermophysical Properties of C-Type Asteroid 162173 Ryugu Revealed by the Thermal Infrared Imager TIR on Hayabusa2

Tatsuaki OKADA, Tetsuya FUKUHARA, Satoshi TANAKA, Makoto TAGUCHI, Takehiko ARAI, Hiroki SENSHU, Naoya SAKATANI, Yuri SHIMAKI, Hirohide DEMURA, Yoshiko OGAWA, Kohei KITAZATO, Kentaro SUKO, Tomohiko SEKIGUCHI, Toru KOUYAMA, Jun TAKITA, Tsuneo MATSUNAGA, Takeshi IMAMURA, Takehiko WADA, Sunao HASEGAWA, Jorn HELBERT, Thomas G. MUELLER, Axel HAGERMANN, Jens BIELE, Matthias GROTT, Maximilian HAMM, Marco DELBO, Naru HIRATA, Naoyuki HIRATA, Yukio YAMAMOTO, Fuyuto TERUI, Takanao SAIKI, Satoru NAKAZAWA, Makoto YOSHIKAWA, Seiichiro WATANABE, Yuichi TSUDA

TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, AEROSPACE TECHNOLOGY JAPAN Vol. 19 ( 5 ) page： 654 - 659 2021

Sample Return Accomplished: Flight Result of Asteroid Explorer Hayabusa2 International coauthorship International journal

Y. Tsuda, T. Saiki, F. Terui, S. Nakazawa, M. Yoshikawa, S. Watanabe

Proceedings of the International Astronautical Congress, IAC Vol. A3 2021

Language：English Publishing type：Research paper (international conference proceedings)

Hayabusa2 is an asteroid sample return mission developed and operated by the Japan Aerospace Exploration Agency (JAXA). Hayabusa2 visited the C-Type asteroid Ryugu in 2018, stayed in the proximity of the asteroid for 1.5 years, and returned to Earth in 2020. Hayabusa2 succeeded in delivering three surface exploration robots to the asteroid surface, performing two landing and sample collection activities, generating one artificial crater impact, and deploying three small objects into orbit around the asteroid. The terrain of Ryugu was found to be unexpectedly harsh through the in-situ observations, and the operation strategy was obliged to be changed and aligned to the Ryugu environment. The project team overcame all the difficulties through tight and collaborative works between the team s scientists and engineers, and completed the planned missions perfectly. The total of 5.4 g of Ryugu sample was confirmed to contain in the returned capsule, which is now being analysed by specifically organized initial analysis teams, and will be delivered to international researchers through AO in 2022. This paper describes the entire flight result of the Hayabusa2 mission, and summarizes the engineering and scientific accomplishments of the mission.

Scopus

Spin-driven evolution of asteroids' top-shapes at fast and slow spins seen from (101955) Bennu and (162173) Ryugu Reviewed International coauthorship International journal

Hirabayashi Masatoshi, Nakano Ryota, Tatsumi Eri, Walsh Kevin J, Barnouin Olivier S, Michel Patrick, Hartzell Christine M, Britt Daniel T, Sugita Seiji, Watanabe Sei-ichiro, Bottke William F, Scheeres Daniel J, Ballouz Ronald-Louis, Cho Yuichiro, Morota Tomokatsu, Howell Ellen S, Lauretta Dante S

ICARUS Vol. 352 page： 113946 - 113946 2020.12

Motion reconstruction of the small carry-on impactor aboard Hayabusa2 Reviewed International journal

Takanao Saiki, Yuya Mimasu, Yuto Takei, Manabu Yamada, Hirotaka Sawada, Kazunori Ogawa, Naoko Ogawa, Hiroshi Takeuchi, Akira Miura, Yuri Shimaki, Koji Wada, Rie Honda, Yasuhiro Yokota, Kei Shirai, Naruhisa Sano, Hirohito Ohtsuka, Go Ono, Kent Yoshikawa, Shota Kikuchi, Chikako Hirose, Yukio Yamamoto, Takahiro Iwata, Masahiko Arakawa, Seiji Sugita, Satoshi Tanaka, Fuyuto Terui, Makoto Yoshikawa, Satoru Nakazawa, Sei-ichiro Watanabe, Yuichi Tsuda

Astrodynamics Vol. 4 ( 4 ) page： 289 - 308 2020.12

Characterization of the Ryugu surface by means of the variability of the near-infrared spectral slope in NIRS3 data Reviewed International coauthorship

Galiano A, Palomba E, D'Amore M, Zinzi A, Dirri F, Longobardo A, Kitazato K, Iwata T, Matsuoka M, Hiroi T, Takir D, Nakamura T, Abe M, Ohtake M, Matsuura S, Watanabe S, Yoshikawa M, Saiki T, Tanaka S, Okada T, Yamamoto Y, Takei Y, Shirai K, Hirata N, Matsumoto K, Tsuda Y

ICARUS Vol. 351 page： 113959 - 113959 2020.11

Hayabusa2 Landing Site Selection: Surface Topography of Ryugu and Touchdown Safety Reviewed International coauthorship

Shota Kikuchi, Sei-ichiro Watanabe, Takanao Saiki, Hikaru Yabuta, Seiji Sugita, Tomokatsu Morota, Naru Hirata, Naoyuki Hirata, Tatsuhiro Michikami, Chikatoshi Honda, Yashuhiro Yokota, Rie Honda, Naoya Sakatani, Tatsuaki Okada, Yuri Shimaki, Koji Matsumoto, Rina Noguchi, Yuto Takei, Fuyuto Terui, Naoko Ogawa, Kent Yoshikawa, Go Ono, Yuya Mimasu, Hirotaka Sawada, Hitoshi Ikeda, Chikako Hirose, Tadateru Takahashi, Atsushi Fujii, Tomohiro Yamaguchi, Yoshiaki Ishihara, Tomoki Nakamura, Kohei Kitazato, Koji Wada, Shogo Tachibana, Eri Tatsumi, Moe Matsuoka, Hiroki Senshu, Shingo Kameda, Toru Kouyama, Manabu Yamada, Kei Shirai, Yuichiro Cho, Kazunori Ogawa, Yukio Yamamoto, Akira Miura, Takahiro Iwata, Noriyuki Namiki, Masahiko Hayakawa, Masanao Abe, Satoshi Tanaka, Makoto Yoshikawa, Satoru Nakazawa, Yuichi Tsuda

Space Science Reviews Vol. 216 ( 7 ) page： 116 2020.10

Mission Status of Hayabusa2, Science Highlights, and Outlook for Sample Analyses

Seiji Sugita, Rie Honda, Tomokatsu Morota, Shingo Kameda, Eri Tatsumi, Shogo Tachibana, Kohei Kitazato, Tatsuaki Okada, Noriyuki Namiki, Masahiko Arakawa, Patrick Michel, Deborah Domingue, Satoshi Tanaka, Makoto Yoshikawa, Sei-ichiro Watanabe, Yuichi Tsuda

2020.10

Publisher：Copernicus GmbH

&lt;p&gt;JAXA&amp;#8217;s Hayabusa2 is a sample-return mission was launched on Dec. 3, 2014 for bringing back first samples from a C-complex asteroid [1,2]. It arrived at asteroid Ryugu on June 27, 2018 and left for Earth on Nov. 13, 2019 after conducting global remote-sensing observations, two touchdown sampling operations, rover deployments, and an artificial impact experiment. We review our science results and update the mission status of Hayabusa2 in this presentation.&amp;#160;&lt;/p&gt;
&lt;p&gt;The global observations revealed that Ryugu has a top-shaped body with very low density (1.19&amp;#177;0.02 g/cc) [3], spatially uniform Cb-type spectra without strong Fe-rich serpentine absorption at 0.7-um [4], and a weak but significant OH absorption at 2.7 um [5]. Based on these observations, we proposed that Ryugu materials may have experienced aqueous alteration and subsequent thermal metamorphism due to radiogenic heating [4]. However, other scenarios, such as impact-induced thermal metamorphism and extremely primitive carbonaceous materials before extensive alteration, were also considered because there were many new properties of Ryugu whose origins are unclear. Also, numerical calculations show that impact heating can raise the temperatures high enough to dehydrate serpentine at typical collision speed in the asteroid main belt [6]. &amp;#160;&lt;/p&gt;
&lt;p&gt;Further analysis using high-resolution data obtained at low-altitude descents for both rehearsal and actual touchdown operations as well as the artificial impact experiment by small carryon impactor (SCI) and landers observations the Ryugu surface on allowed us to find out what caused the properties of Ryugu. For example, subtle but distinct latitudinal variation of spectral slope in optical wavelengths found in the initial observations [4] turned out be caused by solar heating or space weathering during orbital excursion toward the Sun and subsequent erosion of the equatorial ridge owing to slowdown in Ryugu&amp;#8217;s spin rate [7]. The SCI impact created a very large (~17 m in crest diameter) crater consistent with gravity-controlled scaling showing that Ryugu surface has very low intra-boulder cohesion and the Ryugu surface is very young and well mixed [8].&lt;/p&gt;
&lt;p&gt;Furthermore, the MASCOT lander also showed that typical boulders on Ryugu is not covered with a layer of fine regolith [9] and yet possess very low thermal inertia (282+93/-35 MKS) consistent with highly porous structure [10]. This value is consistent with the global values or Ryugu [4, 11], suggesting that the vast majority of boulders on Ryugu are very porous. However, thermal infrared imager (TIR) also found that Ryugu has a number of &amp;#8220;dense boulders&amp;#8221; with high thermal inertia (&gt;600 MKS) consistent with typical carbonaceous chondrites, showing that Ryugu&amp;#8217;s parent body must have had a large enough gravity and pressure to compress the constituent materials [11]. This observation supports that Ryugu originated from a large parent body, such as proto-Polana and proto-Eulalia, which are estimated to be ~100 km in diameter.&lt;/p&gt;
&lt;p&gt;Some of the dense boulders were also covered by multi-band images of optical navigation camera (ONC-T) and turned out to have C-type spectra with albedos much higher than the Ryugu average [12]. These spectra and albedos are similar to carbonaceous chondrites heated at low temperatures. Although the total mass of these high-albedo boulders on Ryugu is estimated to be very small (&lt; 1%), the spectral and albedo varieties are much greater than the bulk Ryugu surface and approximately follow the dehydration track of carbonaceous chondrites [12]. These spectral match supports that Ryugu materials experienced aqueous alteration and subsequent thermal metamorphism. The dominance of a high-temperature component and scarcity of lower temperature components are consistent with radiogenic heating in a relatively large parent body because large bodies would have only thin low-temperature thermal skin and large volume of high-temperature interior.&amp;#160;&lt;/p&gt;
&lt;p&gt;If radiogenic heating is really responsible for Ryugu&amp;#8217;s moderate dehydration, this may place a very important constraint on the timing of the formation of Ryugu&amp;#8217;s parent body. Because the radiogenic heat source for most meteorite parent bodies are likely extinct species, such as 26Al, the peak temperature is chiefly controlled by the timing of accretion [13]. Thus, high metamorphism temperatures (several hundred degrees in Celsius) of Ryugu&amp;#8217;s bulk materials inferred from spectral comparison with laboratory heated CM and CI meteorites [4, 12] require Ryugu&amp;#8217;s parent body formed early in the Solar System. Because Ryugu&amp;#8217;s parent body contained substantial amount of water at the time of formation, it must have been formed outside the snowline. Thus, the birth place of Ryugu&amp;#8217;s parent body would be a high-accretion-rate location outside the snowline.&lt;/p&gt;
&lt;p&gt;Recent high-precision measurements of stable isotopes of meteorites have found that there is a major dichotomy between carbonaceous chondrites (CCs) and some iron meteorites, which formed outside Jupiter&amp;#8217;s orbit, and non-carbonaceous meteorites (NCs), which formed inside Jupiter&amp;#8217;s orbit [e.g., 14]. If Ryugu belongs to CCs, then Ryugu materials could be form near Jupiter, where accretion could occur early. Thus, measurements of stable isotopes of elements, such as Cr, Ti and Mo, of Ryugu samples to be returned to Earth by the end of 2020 would be highly valuable for constraining the original locations of Polana or Eulalia, among the largest C-complex asteroids in the inner main belt.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgements:&lt;/strong&gt; This study was supported by JSPS Core-to-Core program &amp;#8220;International Network of Planetary Sciences&amp;#8221;, CNES, and Univ. Co?te d&amp;#8217;Azur.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&amp;#160; [1] Watanabe et al., SSR, 208, 3-16, 2017. [2] Tsuda et at., Acta Astronaut. 91, 356-363, 2013. [3] Watanabe et al., Science, 364, 268-272, 2019. [4] Sugita et al., Science, 364, eaaw0422, 2019. [5] Kitazato et al., Science, 364, 272-275, 2019. [6] Michel et al., Nature Comm., 11, 5184, 2020. [7] Morota et al., Science, 368, 654-659, 2020. [8] Akarawa et al. Science, 368, 67-671, 2020. [9] Jaumann et al. Science, 365, 817-820, 2019.&amp;#160; [10] Grott et al., Nature Astron. 3, 971-976, 2019.&amp;#160; [11] Okada et al., Nature, 579, 518-522, 2020. [12] Sugimoto et al. 51st LPSC, #1770, 2020.&amp;#160; [13] Grimm and McSween, Science, 259, 653-655, 1993.&amp;#160; [14] Kruijer et al., PNAS, 114, 6712-6716, 2017.&amp;#160;&lt;/p&gt;

DOI： 10.5194/epsc2020-995

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Papers - WATANABE, Sei-ichiro