Neodymium‐142 deficits and samarium neutron stratigraphy of C‐type asteroid (162173) Ryugu
Z. A. Torrano, M. K. Jordan, T. D. Mock, Richard W. Carlson, Ikshu Gautam, Makiko K. Haba, Tetsuya Yokoyama, Yoshinari Abe, J. Aléon, C. M. O'd. Alexander, S. Amari, Yuri Amelin, Ken‐ichi Bajo, Martin Bizzarro, Audrey Bouvier, Marc Chaussidon, Byeon‐Gak Choi, Nicolas Dauphas, A. M. Davis, Tommaso Di Rocco, Wataru Fujiya, Ryota Fukai, Yuki Hibiya, Hiroshi Hidaka, Hisashi Homma, P. Höppe, G. R. Huss, Kiyohiro Ichida, Tsuyoshi Iizuka, T. R. Ireland, Akira Ishikawa, Shoichi Itoh, Noriyuki Kawasaki, N. T. Kita, Kouki Kitajima, T. Kleine, Shintaro Komatani, Alexander N. Krot, Ming‐Chang Liu, Yuki Masuda, K. D. McKeegan, Mayu Morita, Kazuko Motomura, Frédéric Moynier, Izumi Nakai, K. Nagashima, A. N. Nguyen, L. R. Nittler, Morihiko Onose, Andreas Pack, Changkun Park, Laurette Piani, Liping Qin, S. S. Russell, Naoya Sakamoto, Maria Schönbächler, Lauren Tafla, Haolan Tang, Kentaro Terada, Yasuko Terada, Tomohiro Usui, Sohei Wada, M. Wadhwa, R. J. Walker, Katsuyuki Yamashita, Qing‐Zhu Yin, Shigekazu Yoneda, Edward Young, Hiroharu Yui, Ai‐Cheng Zhang, Tomoki Nakamura, Hiroshi Naraoka, T. Noguchi, Ryuji Okazaki, Kanako Sakamoto, Hikaru Yabuta, Masanao Abe, Akiko Miyazaki, Aiko Nakato, Masahiro Nishimura, Tatsuaki Okada, Toru Yada, Kasumi Yogata, Satoru Nakazawa, Takanao Saiki, Satoshi Tanaka, Fuyuto Terui, Yuichi Tsuda, Sei‐ichiro Watanabe, Makoto Yoshikawa, Shogo Tachibana, Hisayoshi Yurimoto
Abstract
Abstract We report Nd and Sm isotopic compositions of four samples of Ryugu returned by the Hayabusa2 mission, including “A” (first touchdown) and “C” (second touchdown) samples, and several carbonaceous chondrites to evaluate potential genetic relationships between Ryugu and known chondrite groups and track the cosmic ray exposure history of Ryugu. We resolved Nd and Sm isotopic anomalies in small (<20 ng Nd and Sm) sample sizes via thermal ionization mass spectrometer using 10 13 Ω amplifiers. Ryugu samples exhibit resolvable negative μ 142 Nd values consistent with carbonaceous chondrite values, suggesting that Ryugu is related to the parent bodies of carbonaceous chondrites. Ryugu's negative μ 149 Sm values are the result of exposure to galactic cosmic rays, as demonstrated by the correlation between 150 Sm/ 152 Sm and 149 Sm/ 152 Sm ratios that fall along the expected neutron capture correlation line. The neutron fluence calculated in the “A” samples (2.75 ± 1.94 × 10 15 n cm −2 ) is slightly higher compared to the “C” samples (0.95 ± 2.04 × 10 15 n cm −2 ), though overlapping within measurement uncertainty. The Sm results for Ryugu, at this level of precision, thus are consistent with a well‐mixed surface layer at least to the depths from which the “A” and “C” samples derive.