Solar wind as the origin of rapid reddening of asteroid surfaces

Abstract

A comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much ‘redder’, with the spectral difference explained by ‘space weathering’1,2, though the actual processes and timescales involved have remained controversial3,4. A recent study5 of young asteroid families concluded that they suffered only minimal space weathering. Here we report additional observations of those families, revealing that space weathering must be a very rapid process—the final colour of a silicate-rich asteroid is acquired shortly after its ‘birth’ (within 106 years of undergoing a catastrophic collision). This rapid timescale favours solar wind implantation as the main mechanism of space weathering, as laboratory experiments have shown that it is the most rapid of several competing processes. We further demonstrate the necessity to take account of composition when evaluating weathering effectiveness, as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance. The rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface-particle-size properties, rather than to different relative ages. Apparently fresh surfaces most frequently seen among small near-Earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters6,7.

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Figure 1: The relationship between the spectral slope (visible wavelengths) of S-type asteroid families and their ages, as observed.
Figure 2: The relationship between the slope of S- and A-type asteroids and their composition.
Figure 3: The relationship between the spectral slope (visible wavelengths) of S-type asteroid families and their ages, after being corrected for composition.

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Acknowledgements

The visible data were based on observations at the NTT and VLT (European Southern Observatory, ESO, Chile) and the TNG (La Palma, Canary Islands). The near-infrared data were acquired by the authors operating as Visiting Astronomers at the IRTF, which is operated by the University of Hawaii under Cooperative Agreement no. NNX08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program. This Letter is based on work supported by the National Science Foundation (grant 0506716) and NASA (grant NAG5-12355). Any opinions, findings, and conclusions or recommendations expressed here are those of the authors and do not necessarily reflect the views of the National Science Foundation or NASA.

Author Contributions P.V. performed the quantitative analysis that solidified the results of this paper and led the formulation of possible explanations. P.V., R.P.B. and A.R. served as principal investigators to acquire the visible and near-infrared data. Most data were acquired by P.V., R.P.B and A.R. P.V. and R.P.B. worked jointly to write the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to P. Vernazza.

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Vernazza, P., Binzel, R., Rossi, A. et al. Solar wind as the origin of rapid reddening of asteroid surfaces. Nature 458, 993–995 (2009). https://doi.org/10.1038/nature07956

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