Space weathering acts strongly on the uppermost surface of Ryugu
M. Matsuoka, Eiichi Kagawa, Kana Amano, Tomoki Nakamura, Eri Tatsumi, Takahito Osawa, T. Hiroi, R. E. Milliken, D. L. Domingue, D. Takir, R. Brunetto, A. Barucci, K. Kitazato, Seiji Sugita, Yuri Fujioka, O. Sasaki, S. Kobayashi, Takahiro Iwata, Tomokatsu Morota, Y. Yokota, Toru Kouyama, Rie Honda, Shingo Kameda, Yuichiro Cho, Kazuo Yoshioka, Hirotaka Sawada, Masahiko Hayakawa, Naoya Sakatani, Manabu Yamada, Hidehiko Suzuki, Chikatoshi Honda, Kazunori Ogawa, K. Shirai, C. Lantz, Stefano Rubino, Hisayoshi Yurimoto, T. Noguchi, Ryuji Okazaki, Hikaru Yabuta, Hiroshi Naraoka, Kanako Sakamoto, Shogo Tachibana, 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
Abstract
Abstract Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.