Irradiation history of Itokawa regolith material deduced from noble gases in the Hayabusa samples

Nagao, Keisuke*; Okazaki, Ryuji*; Nakamura, Tomoki*; Miura, Yayoi*; Osawa, Takahito  ; Bajo, Kenichi*; Matsuda, Shintaro*; Ebihara, Mitsuru*; Ireland, T.*; Kitajima, Fumio*; Naraoka, Hiroshi*; Noguchi, Takaaki*; Tsuchiyama, Akira*; Yurimoto, Hisayoshi*; Zolensky, M.*; Uesugi, Masayuki*; Shirai, Kei*; Abe, Masanao*; Yada, Toru*; Ishibashi, Yukihiro*; Fujimura, Akio*; Mukai, Toshifumi*; Ueno, Munetaka*; Okada, Tatsuaki*; Yoshikawa, Makoto*; Kawaguchi, Junichiro*

A steroid surface materials record regolith processes and a history of cosmic-ray irradiation. Noble gas isotopes in three rocky grains from Itokawa have been determined. High concentrations of solar He, Ne, and Ar, as high as those in lunar soils, are released at variable temperatures from each sample. The isotopic compositions are essentially identical to those of solar wind but distinguishable in He relative abundance. These noble gas characteristics can be explained by repeated implantation and preferential loss of solar He by removal of weathered He-rich rim on the grain surface through friction among regolith grains on Itokawa. Residence time of regolith materials on Itokawa is alculated to be shorter than 10 Myr, suggesting that regolith materials of small asteroids would escape easily to space.