Ryugu: A brand-new planetary sample returned from a C-type asteroid
Authors: Yada Toru , Abe Masanao , Okada Tatsuaki , Nakato Aiko , Yogata Kasumi , Miyazaki Akiko , Hatakeda Kentaro , Kumagai Kazuya , Nishimura Masahiro , Hitomi Yuya , Soejima Hiromichi , Yoshitake Miwa , Iwamae Ayako , Furuya Shizuho , Uesugi Masayuki , Karouji Yuzuru , Usui Tomohiro , Hayashi Tasuku , Yamamoto Daiki , Fukai Ryota , Sugita Seiji , Cho Yuichiro , Yumoto Koki , Yabe Yuna , Bibring Jean-Pierre , Pilorget Cedric , Hamm Vincent , Brunetto Rosario , Riu Lucie , Lourit Lionel , Loizeau Damien , Lequertier Guillaume , Moussi-Soffys Aurelie , Tachibana Shogo , Sawada Hirotaka , Okazaki Ryuji , Takano Yoshinori , Sakamoto Kanako , Miura Yayoi , Yano Hajime , Ireland Trevor , Yamada Tetsuya , Fujimoto Masaki , Kitazato Kohei , Namiki Noriyuki , Arakawa Masahiko , Hirata Naru , Yurimoto Hisayoshi , Nakamura Tomoki , Noguchi Takaaki , Yabuta Hikaru , Naraoka Hiroshi , Ito Motoo , Nakamura Eizo , Uesugi Kentaro , Kobayashi Katsura , Michikami Tatsuhiro , Kikuchi Hiroshi , Hirata Naoyuki , Ishihara Yoshiaki , Matsumoto Koji , Noda Hirotomo , Noguchi Rina , Shimaki Yuri , Ogawa Kazunori , Shirai Kei , Wada Koji , Senshu Hiroki , Yamamoto Yukio , Morota Tomokatsu , Honda Rie , Honda Chikatoshi , Yokota Yasuhiro , Matsuoka Moe , Sakatani Naoya , Tatsumi Eri , Miura Akira , Yamada Manabu , Fujii Atsushi , Hirose Chikako , Hosoda Satoshi , Ikeda Hitoshi , Iwata Takahiro , Kikuchi Shota , Mimasu Yuya , Mori Osamu , Ogawa Naoko , Ono Go , Shimada Takanobu , Soldini Stefania , Takahashi Tadateru , Takei Yuto , Takeuchi Hiroshi , Tsukizaki Ryudo , Yoshikawa Kent , Terui Fuyuto , Nakazawa Satoru , Tanaka Satoshi , Saiki Takanao , Yoshikawa Makoto , Watanabe Sei-ichiro , Tsuda Yuichi 
Published: June 2021

Abstract
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.