Darkening of silicate rock powders by bombardment with 2‐ to 16‐kev protons was found to be slight unless proton flux or total proton incident‐power density is sufficient to produce sample surface temperature in excess of about 150°C. Darkening then increases with incident power density for a given proton dose; i.e., the darkening is rate dependent as well as dose dependent. The darkening is due to contamination by (1) carbon from irradiation decomposition of hydrocarbon vacuum contaminants and/or (2) metal atoms deposited on the sample surface from sputtering of ion source components. The photometric properties of the contamination‐darkened samples are similar to those reported for samples darkened with hydrogen ions by Hapke and by Wehner et al.; this result suggests that contamination darkening may have caused the photometric modifications that they report; a review and comparison of experimental apparatus and procedures is presented. The experimental conditions under which silicate powders darken by proton or hydrogen‐ion bombardment are shown to be unrealistic for simulation of solar‐wind darkening of the moon's surface. It is suggested, however, that deposition of solar carbon on the moon may occur in sufficient amounts to produce darkening.