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Edited by* Ikuo Kushiro, University of Tokyo, Tokyo, Japan, and approved January 28, 2012 (received for review October 4, 2011)
Abstract
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.
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: eizonak{at}misasa.okayama-u.ac.jp.
Author contributions: E.N. undertook the initial analysis; A.M. and T. Moriguti prepared reference materials; K.K. coordinated the flow of the analytical work through the laboratories and performed the FIB slabbing of the grains; R.T. analyzed oxygen isotopes for the reference materials; T.K. carried out in situ oxygen-isotope analyses; T.T. carried out petrographic and mineralogical investigations; C.S. evaluated adhered objects; H.K. assisted in the electron and ion probe work; T.O. determined elemental compositions; Y.Y. documented the sample status; T.Y., M.A., A.F., M.U., T. Mukai, M.Y., and J.K. worked on sample preparation; and E.N., A.M., T. Moriguti, K.K., R.T., T.K., and T.T. wrote the paper.
The authors declare no conflict of interest.
*This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116236109/-/DCSupplemental.
Freely available online through the PNAS open access option.
Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft
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