Space weathering from Mercury to the asteroid belt

The variety of evidence bearing on the nature of space weathering is review ed. The effects of space weathering include spectral darkening, reddening a nd subdued absorption bands, and the distinctive magnetic electron spin res onance caused by single-domain metallic iron particles. Ever since the Apol lo missions, two paradigms have dominated the thinking of the planetary sci ence community concerning space weathering: (I)the optical effects are caus ed by impact-vitrified glass in agglutinates, and (2) the submicroscopic me tallic iron results from the reduction of ferrous iron by the impact meltin g of minerals whose surfaces have been saturated with hydrogen from the sol ar wind. However, studies carried out since the Apollo program showed that both of these paradigms are invalid. A hypothesis first suggested by the au thor and his colleagues 26 years ago, but not generally accepted at that ti me, now appears to be essentially correct: Both the optical and magnetic ef fects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor-deposited coatings on soil particle surfaces acid inside a gglutinates. The vapor is generated by both solar wind sputtering and micro meteorite impact vaporization and injected preferentially downward into the porous regolith. The iron is reduced by a physical process, the selective loss of oxygen that occurs during deposition of the vapor, and does not req uire heating, melting, or a reducing environment. A mathematical theory tha t describes the optical effects of the submicroscopic iron quantitatively i s derived and applied to the regoliths of the Moon, Mercury and an S astero id.