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 SiO2 and olivine in ultrahigh vacuum. The samples were implanted with 2-10keVH(+), in the range of solar wind energies, and the OH absorption band at similar to 2.8 mu m measured by transmission Fourier transform infrared spectroscopy. For 2keV protons in SiO2, the OH band depth saturated at fluences F similar to 5x10(16)H(+)/cm(2) to a maximum 0.0032 absorption band depth, corresponding to a column density (s)=1.1x10(16) OH/cm(2). The corresponding values for 5keV protons in olivine are >2x10(17)/cm(2), 0.0067, and 4.0x10(16) OH/cm(2). The initial conversion rate of implanted H+ into hydroxyl species was found to be similar to 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.