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Earth encounters as the origin of fresh surfaces on near-Earth asteroids


Telescopic measurements of asteroids’ colours rarely match laboratory reflectance spectra of meteorites owing to a ‘space weathering’1,2 process that rapidly3 reddens asteroid surfaces in less than 106 years. ‘Unweathered’ asteroids (those having spectra matching the most commonly falling ordinary chondrite meteorites), however, are seen among small bodies the orbits of which cross inside Mars and the Earth. Various explanations have been proposed for the origin of these fresh surface colours, ranging from collisions4 to planetary encounters5. Less reddened asteroids seem to cross most deeply into the terrestrial planet region, strengthening6 the evidence for the planetary-encounter theory5, but encounter details within 106 years remain to be shown. Here we report that asteroids displaying unweathered spectra (so-called ‘Q-types’7) have experienced orbital intersections closer than the Earth–Moon distance within the past 5 × 105 years. These Q-type asteroids are not currently found among asteroids showing no evidence of recent close planetary encounters. Our results substantiate previous work5: tidal stress8, strong enough to disturb and expose unweathered surface grains, is the most likely dominant short-term asteroid resurfacing process. Although the seismology details are yet to be worked out, the identification of rapid physical processes that can produce both fresh and weathered asteroid surfaces resolves the decades-long9 puzzle of the difference in colour of asteroids and meteorites.

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Figure 1: Reflectance spectra properties of ordinary chondrite meteorites compared with asteroids grouped according to taxonomic types.
Figure 2: Distributions for the orbit intersection distances for weathered and fresh asteroids.
Figure 3: Comparison of typical long-term orbital evolution behaviour for objects inside the orbits of Earth and Mars.


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R.P.B. is a Chercheur Associé at the Observatoire de Paris IMCCE and thanks IMCCE and LESIA for their collaboration and hospitality during the initiation of this analysis. We thank C. Chapman, B. Clark, D. Richardson and D. Nesvorný for their constructive reviews and comments. Observations reported here were obtained at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement NCC 5-538 with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. F.E.D. acknowledges funding from the Fulbright Program. This material is based upon work supported by the National Science Foundation under grant 0506716 and NASA under grant NAG5-12355. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or NASA.

Author Contributions R.P.B. and A.T.T. served as principal investigators for a joint observing programme to acquire the near-infrared spectral data. Most data were acquired by R.P.B., C.A.T. and F.E.D., while F.E.D. performed most of the processing and taxonomic evaluation. Processing routines were developed by S.J.B., A.S.R. and R.P.B. The scientific analysis was directed by R.P.B. with the first stages performed by S.M. with input from M.B. and P.V. A.M. performed all of the orbital integrations. R.P.B. and A.M. worked jointly to draft the manuscript with all authors reviewing and contributing to its final form.

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Correspondence to Richard P. Binzel.

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Binzel, R., Morbidelli, A., Merouane, S. et al. Earth encounters as the origin of fresh surfaces on near-Earth asteroids. Nature 463, 331–334 (2010).

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