Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft

• Published in: Proceedings of the National Academy of Sciences - PNAS 2012-03, Vol.109 (11), p.E624-E629
• Main Authors: Nakamura, Eizo, Makishima, Akio, Moriguti, Takuya, Kobayashi, Katsura, Tanaka, Ryoji, Kunihiro, Tak, Tsujimori, Tatsuki, Sakaguchi, Chie, Kitagawa, Hiroshi, Ota, Tsutomu, Yachi, Yusuke, Yada, Toru, Abe, Masanao, Fujimura, Akio, Ueno, Munetaka, Mukai, Toshifumi, Yoshikawa, Makoto, Kawaguchi, Jun’ichiro
• Format: Article
• Language: English
• Subjects: Asteroids; Chemical composition; Collisions; craters; Dust; glass; Isotopes; particles; Particulates; Physical Sciences; PNAS Plus; Spacecraft
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1116236109

Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid’s surface. We describe impact-related phenomena, ranging in scale from 10-9 to 104 meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.