Laboratory simulations of solar wind irradiation of olivine, a constituent of ordinary chondritic meteorites and S-type asteroids, show a dramatic chemical reduction of surface iron and, to a lesser extent, silicon. Earth's atmosphere oxidizes any reduced iron instantaneously, whereas irradiated material on the surface of an asteroid would remain reduced. Changes in optical reflectance, due to metallization of the surface, provide a possible explanation for differences in the near-infrared spectra of ordinary chondrite meteorites and their likely parent bodies, the S(IV) asteroids. ¿ 1999 American Geophysical Union