Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water

Remote sensing discoveries of hydroxyl and water on the lunar surface have reshaped our view of the distribution of water and related compounds on airless bodies such as the Moon1–3. The origin of this surface water is unclear4, but it has been suggested that hydroxyl in the lunar regolith can result from the implantation of hydrogen ions by the solar wind1,5. Here we present Fourier transform infrared spectroscopy and secondary ion mass spectrometry analyses of Apollo samples that reveal the presence of significant amounts of hydroxyl in glasses formed in the lunar regolith by micrometeorite impacts. Hydrogen isotope compositions of these glasses suggest that some of the observed hydroxyl is derived from solar wind sources. Our findings imply that ice in polar cold traps could contain hydrogen atoms ultimately derived from the solar wind, as predicted by early theoretical models of water stability on the lunar surface6. We suggest that a similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface, such as Mercury and the asteroid 4-Vesta. The past few years have witnessed a paradigm shift in our view of ‘no water’ on the Moon. This revolution started with the report of significant H in lunar volcanic glasses7 and was followed by finds of high H contents of lunar apatite8–10 and melt inclusions11. These measurements were made by secondary ion mass spectrometry (SIMS), which cannot distinguish H species, but the measured H was suggested to be most likely OH. Equally important are the discoveries of OH/H2O in the top millimetres of the lunar regolith, as revealed by reflectance spectra1–3, which can distinguish various forms of H. Subsequently, the National Aeronautics and Space Administration (NASA) Lunar Crater Observation and Sensing Satellite (LCROSS) mission delivered an impactor into a permanently shadowed lunar crater and observed the resulting plume. Results are consistent with the presence of H2O ice in cold traps at the lunar poles12 and support observations of hydrogen by Lunar Prospector’s neutron spectrometer13. Despite these discoveries of H/OH/H2O on the Moon, there has been no direct proof of OH/H2O in specific lunar regolith phases. Although measurements on bulk samples of the lunar regolith have demonstrated the presence of solar-wind H, C, N and noble gases14, it is unknown whether the solar-wind H was bounded as OH or H2O (or is present in some other less common form). As a result, the contribution of solar-wind protons to polar ice is debated13,15. Answers to these debates are important to understanding the development of the lunar regolith and space weathering on other airless bodies in the solar system (for example, 4-Vesta, Mercury).