¹Micah J. Schaible, ¹Raúl A. Baragiola
¹Laboratory of Atomic and Surface Physics, University of Virginia, Charlottesville, VA

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 2–10 keV H+, in the range of solar wind energies and the OH absorption band at ~2.8 µm measured by transmission FTIR spectroscopy. For 2 keV protons in SiO2, the OH band depth saturated at fluences F ~5 × 1016 H+/cm2 to a maximum 0.0032 absorption band depth, corresponding to a column density ηs = 1.1 × 1016 OH/cm2. The corresponding values for 5 keV protons in olivine are > 2 × 1017/cm2, 0.0067 and 4.0 × 1016 OH/cm2. 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.

Reference
Schaible MJ, Baragiola RA (2014) The role of solar wind in SiOH bond formation on the surfaces of airless bodies in space. Journal of Geophysical Research: Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004650]

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