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Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples |
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| Authors: | Nagao Keisuke Okazaki Ryuji Nakamura Tomoki Miura Yayoi N Osawa Takahito Bajo Ken-ichi Matsuda Shintaro Ebihara Mitsuru Ireland Trevor R Kitajima Fumio Naraoka Hiroshi Noguchi Takaaki Tsuchiyama Akira Yurimoto Hisayoshi Zolensky Michael E Uesugi Masayuki Shirai Kei Abe Masanao Yada Toru Ishibashi Yukihiro Fujimura Akio Mukai Toshifumi Ueno Munetaka Okada Tatsuaki Yoshikawa Makoto Kawaguchi Junichiro |
| Institution: | Geochemical Research Center, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan. nagao@eqchem.s.u-tokyo.ac.jp |
| Abstract: | Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray-produced (21)Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system. |
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