Formation and Liberation of Lunar Water by Dual Solar Wind Irradiation and Micrometeorite Impact

Recent remote infrared spectroscopic evidence of water (H₂O) and hydroxyl radicals (OH) on the lunar surface has challenged the presumption that the Moon is anhydrous. The sources and chemical processes responsible for the production of this surface water are still uncertain. The interaction of solar wind protons (H⁺) with silicates and oxides is suggested as one possible mechanism However, several laboratory simulations exploiting keV proton bombardment of minerals typical in lunar regolith and even one experiment with actual lunar regolith yielded conflicting results with only one study claiming the detection of water. Here we establish that proton implantation alone is insufficient to generate and liberate detectable surface water. As an alternative, our study reveals that water is released by micrometeorite impact and resultant thermal/shock pulses into materials doped with solar wind hydrogen. We support this conclusion with the first definitive chemical and electron microscopy analyses of water expulsion from anhydrous San Carlos olivine samples that were first exposed to deuterium ions and subsequently laser irradiated. Laser irradiation, which simulates micrometeorite impact exposure, generated water at mineral temperatures from 10 K to 300 K. Hence, we propose that micrometeorite impact of regolith materials saturated with hydrogen atoms may also contribute to the surface H₂O/OH as detected on airless bodies such as Mercury, S-type asteroids 433 Eros and 1036 Ganymed, and possibly C-type asteroids 1 Ceres and 24 Themis.