Hydroxyl (OH) production on airless planetary bodies: Evidence from H+/D+ ion-beam experiments

The hypothesis that bombardment of lunar soil with solar-wind protons might form hydroxyl (OH) and perhaps HOH has been tested by experiments with Apollo 16 (highlands) and Apollo 17 (mare) soils. Pre-dried soils (500 °C) were bombarded with 1.1 keV protons and deuterons and provide unambiguous evidence for the formation of OH or OD in both samples. This hypothesis further predicts the formation of hydroxyl (OH) on other airless planetary/asteroidal bodies, with a sufficient solar-wind flux. Deuteron implantation of unaltered lunar soils and a heat-treated plagioclase specimen cause simultaneous OH depletion and OD formation. Ion bombardment of lunar soils simulates the dynamic process of hydroxyl formation and may also deplete intrinsic OH, thereby effectively contributing to the day/night, diurnal variability of OH reported by Sunshine et al. (2009). Our results emphasize the need to use lunar soils with space-weathered exteriors in laboratory simulations of the solar wind. Infrared spectra of hydrogen ion-beam implanted soils are similar to spectra obtained at RELAB (Brown Univ.) and to those observed by remote sensing confirming the solar-wind hypothesis.