Distinctive space weathering on Vesta from regolith mixing processes

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Abstract

The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite–eucrite–diogenite class of meteorites1,2,3. Processes active in the space environment produce ‘space weathering’ products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere4,5, and it has long been a mystery why Vesta’s absorption bands are so strong. Analyses of soil samples from both the Moon6 and the asteroid Itokawa7 determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely8,9,10 but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.

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Figure 1: The optical properties across a typical area on Vesta as measured by Dawn.
Figure 2: Diverse material exposed at fresh craters in a well-developed regolith on Vesta.
Figure 3: The Canuleia region mapped at different spatial resolutions by Dawn’s two optical instruments.
Figure 4: Spectroscopic comparisons of Vesta soils with similar materials on the Moon.

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Acknowledgements

We acknowledge the Dawn Instrument, Flight and Operations teams for the successful development, cruise, orbital insertion and operations of the Dawn spacecraft at Vesta. US team members are supported by the NASA Discovery Program through contract NNM05AA86C to the University of California, Los Angeles and by the NASA Dawn participating scientist programme.

Author information

C.M.P., D.T.B., M.C.D.S., S.M., L.A.M., D.W.M., V.R. and C.T.R. contributed to writing and improving the manuscript. E.A., L.L.C. and A.N. provided calibrated Dawn data. E.P. provided data searches. All authors contributed to discussion of the results.

Correspondence to C. M. Pieters.

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Pieters, C., Ammannito, E., Blewett, D. et al. Distinctive space weathering on Vesta from regolith mixing processes. Nature 491, 79–82 (2012) doi:10.1038/nature11534

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