Space weathered rims found on the surfaces of the Itokawa dust particles |
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| Authors: | Takaaki Noguchi Makoto Kimura Takahito Hashimoto Mitsuru Konno Tomoki Nakamura Michael E. Zolensky Ryuji Okazaki Masahiko Tanaka Akira Tsuchiyama Aiko Nakato Toshinori Ogami Hatsumi Ishida Ryosuke Sagae Shinichi Tsujimoto Toru Matsumoto Junya Matsuno Akio Fujimura Masanao Abe Toru Yada Toshifumi Mukai Munetaka Ueno Tatsuaki Okada Kei Shirai Yukihiro Ishibashi |
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| Affiliation: | 1. College of Science, Ibaraki University, , Mito, Ibaraki, 310‐8512 Japan;2. Hitachi High‐Technologies Corporation, , Hitachinaka, Ibaraki, 312‐8504 Japan;3. Department of Earth Sciences, Tohoku University, , Sendai, Miyagi, 980‐8578 Japan;4. NASA, ARES, Johnson Space Center, , Houston, Texas, 77058 USA;5. Department of Earth and Planetary Sciences, Kyushu University, , Hakozaki, Fukuoka, 812‐8581 Japan;6. National Institute for Materials Science, , Sayo, Hyogo, 679‐5148 Japan;7. Department of Geology and Mineralogy, Kyoto University, , Sakyo, Kyoto, 606‐8502 Japan;8. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, , Sagamihara, Kanagawa, 252‐5210 Japan |
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| Abstract: | On the basis of observations using Cs‐corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2–3 nm), (2) composite rims (30–60 nm), and (3) composite vesicular rims (60–80 nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two‐layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation‐induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa. |
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