Space weathering
The slow alteration of airless surfaces by solar wind, cosmic rays and micrometeorite impacts, which darkens and reddens their spectra.
Bodies without an atmosphere are exposed to solar-wind ions, cosmic rays and constant micrometeoroid bombardment. Over time these processes amorphise mineral surfaces, implant solar-wind gases and produce vapour deposits containing nanophase metallic iron, particles only a few nanometres to tens of nanometres across. On olivine- and pyroxene-rich surfaces this lowers albedo, reddens the spectral slope and weakens absorption bands. Lunar soils were the first well-studied case.
The effect long confused meteorite–asteroid links: S-type asteroids did not look like ordinary chondrites in the laboratory. Samples returned from Itokawa by Hayabusa in 2010 resolved this, showing LL-chondrite grains with thin weathered rims containing nanophase iron. Space weathering varies widely, however. On Vesta, Dawn found no strong lunar-style reddening; its surface is instead darkened and altered mainly by mixing with carbon-rich impactor material. Carbonaceous bodies such as Ryugu and Bennu show yet other trends, including bluing and dehydration of surfaces.
In meteorites, the record survives in gas-rich regolith breccias, such as the howardite Kapoeta, whose grains carry implanted solar-wind noble gases and sometimes solar-flare particle tracks. Recognition requires transmission electron microscopy, noble-gas analysis or reflectance spectroscopy; nothing is visible in hand specimen.
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