Investigation of Water Interactions With Apollo Lunar Regolith Grains

Desorption activation energies of chemisorbed water on Apollo lunar Samples 14163 and 10084 were determined by temperature program desorption (TPD) experiments conducted under ultrahigh vacuum conditions. Desorption at the grain/vacuum interface and desorption/transport of water though the porous medium with readsorption were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded desorption activation energies and site probability distributions. Highland sample 14163 exhibited a broad distribution of binding site energies peaking at 60 kJ mol(-1), while mare sample 10084 exhibited a narrower distribution of binding site energies peaking at 65 kJ mol(-1). The highland sample adsorbed approximately 30% more water than the more space weathered and mature mare sample, suggesting maturity may not be a good predictor of the degree of molecular water uptake on lunar regolith. Water desorption from the lunar surface over a typical lunar day was simulated with the measured coverage-dependent activation energies of the mare and highland samples. The resulting desorption profile of water through a lunar temperature cycle is in general agreement with Lunar Reconnaissance Orbiter (LRO) Lyman-alpha Mapping Project (LAMP) spacecraft-based observations of trends for both highland and mare assuming similar to 1% submonolayer coverage and that photon stimulated desorption is neglected. Plain Language Summary Water on the Moon has been a source of intrigue throughout the years and especially now with recent space mission results that have definitely shown water ice to be present. Understanding the origin and transport of water is interesting not only from a fundamental science perspective but also from an exploration objective as well. Water is a nececsary resource for the planned human exploration and habitation on the Moon. As such, knowledge of water binding energies on lunar regolith is crucial to understanding the transport of water on the lunar surface. Our results indicate that lunar regolith has a wide distribution of binding energies with a majority of sites that allow for water migration during a typical lunar day. In addition, lunar mare soil adsorbs less water than lunar highlands. Ultimately, quantification of water binding energies and total binding sites will allow for the prediction of where and how much water can accumulate on the lunar surface, such as in permanently shadowed and polar regions that can be extracted for human use.