Darkening of silicate rock powders by solar wind sputtering

Hydrogen ion irradiation of powdered igneous rocks, including Apollo rocks, has been observed in the laboratory to darken the powders and to make their optical properties similar to the Moon's. An extensive series of investigations shows that this darkening is not spurious. These results are consistent with those of other investigators, including Nash (1967). Darkening of lunar igneous rock powders by the formation of solar wind-sputtered glass films is a real process which occurs on the Moon. The time scale for darkening of undisturbed lunar soil is of the order of 50000–100000 yr. Comparison of the rates of the formation of glasses on the lunar surface by solar wind sputter-deposition, meteorite impact melting and impact vaporization-deposition indicates that these processes are of comparable importance under the present flux of meteorites. Thus the formation of glass by sputter-deposition must be regarded as a major process on the lunar surface.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.