Hydrogen implantation in silicates: The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space

Hydroxyl on the lunar surface revealed by remote measurements has been thought to originate from solar wind hydrogen implantation in the regolith. The hypothesis is tested here through experimental studies of the rate and mechanisms of OH bond formation due to H + implantation of amorphous SiO 2 and olivine in ultrahigh vacuum. The samples were implanted with 2–10 keV H + , in the range of solar wind energies, and the OH absorption band at ~2.8 µm measured by transmission Fourier transform infrared spectroscopy. For 2 keV protons in SiO 2 , the OH band depth saturated at fluences F ~5 × 10 16 H + /cm 2 to a maximum 0.0032 absorption band depth, corresponding to a column density η s = 1.1 × 10 16 OH/cm 2 . The corresponding values for 5 keV protons in olivine are >2 × 10 17 /cm 2 , 0.0067, and 4.0 × 10 16 OH/cm 2 . The initial conversion rate of implanted H + into hydroxyl species was found to be ~90% and decreased exponentially with fluence. There was no evidence for molecular water formation due to proton irradiation. Translating the laboratory measurements in thin plate samples to the granular lunar regolith, it is estimated that the measurements can account for a maximum of 17% relative OH absorption in reflectance spectroscopy of mature soils, consistent with spacecraft observations in the infrared of the Moon.