Irradiation of olivine by 4 keV He+: Simulation of space weathering by the solar wind

We have studied the effects of 4 keV He+ ion irradiation on olivine while measuring, in situ, changes in the near-infrared (NIR) reflectance and in the chemical composition of the surface. The observed changes in reflectance are reddening and the attenuation of the Fe-3d absorption bands in the NIR. Spectral reddening of irradiated olivine powder (<45 ¿um) correlates with the amount of metallic iron formed by ion impact, consistent with the idea that space-weathering effects in the reflectance of olivine-bearing S type asteroids are due to the formation of metallic iron. The metallization rate of the powder is about half that of a sectioned rock of olivine, which we propose is a consequence of redeposition of sputtered material. The NIR spectral changes observed in ion irradiation experiments are similar to those observed in our previous experiments on vapor redeposition, indicating that different space weathering mechanisms can lead to similar final effects on reflectance. Finally, we estimate that at 1 AU the spectral reddening caused by the solar wind saturates approximately 2 orders of magnitude faster than comparable reddening caused by micrometeorite impacts.