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Developing space weathering on the asteroid 25143 Itokawa

Abstract

Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called ‘space weathering’—it makes the visible and near-infrared reflectance spectrum of a body darker and redder1. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering2. Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal3. A simple calculation4 suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits.

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Figure 1: A monochromatic image of asteroid Itokawa with NIRS observation points indicated.
Figure 2: Variations in brightness and redness of NIRS spectra near Tsukuba and Sagamihara.
Figure 3: Comparison of reflectance spectra of two areas on Itokawa with an ordinary chondrite spectrum.
Figure 4: Estimation of the degrees of space weathering of two areas on Itokawa.

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Acknowledgements

The Alta'ameem meteorite sample (USNM 5964) was loaned from the National Museum of Natural History, Smithsonian Institution. We thank T. McCoy for assistance. The reflectance spectrum of the Alta'ameem sample was obtained at the Reflectance Experiment Laboratory (RELAB), a multiuser facility operated under the NASA Planetary Geology and Geophysics Program. The manuscript benefited from a review by B. Hapke. Author Contributions T.H. contributed to the overall analyses and measurements of some meteorite samples in this work. M.A., K.K. and S.A. contributed to production and calibration of NIRS data as well as discussion on the NIRS data quality. K.K. and B.E.C. contributed to the photometric correction of NIRS spectra. B.E.C. and O.S.B.-J. contributed to deriving footprint positions of NIRS observations. S.A. provided gravity information. M.I. contributed to choosing the target location on Itokawa to study in this work based on AMICA colour image works. S.S. contributed to providing meteorite samples and space weathering discussions.

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Correspondence to Takahiro Hiroi.

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Hiroi, T., Abe, M., Kitazato, K. et al. Developing space weathering on the asteroid 25143 Itokawa. Nature 443, 56–58 (2006). https://doi.org/10.1038/nature05073

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