Temperature programmed desorption comparison of lunar regolith to lunar regolith simulants LMS-1 and LHS-1

Abstract

Water and molecular hydrogen evolution from Apollo sample 14163 and lunar regolith simulants LMS-1, a mare simulant, and LHS-1, a highlands simulant, were examined using Temperature Programmed Desorption (TPD) in ultra-high vacuum. LMS-1, LHS-1, and Apollo 14163 released water upon heating, whereas only the Apollo sample directly released measurable quantities of molecular hydrogen. The resulting H2O and H2 TPD curves were fit using a model which considers desorption at the vacuum grain interface, transport in the void space between grain-grain boundaries, molecule formation via recombination reactions and sub-surface diffusion. The model yielded a most probable H2O formation and desorption effective activation energy of ∼150 kJmol−1 for all samples. The probability distribution widths of the effective activation energies were ∼100–400, ∼100–350, and ∼100–300 kJmol−1 for LMS-1, LHS-1, and Apollo 14163, respectively. In addition to having the narrowest energy distribution width, the Apollo sample released the least amount to water (103 ppm) relative to LMS-1 (176 ppm) and LHS-1 (195 ppm). Since essentially no molecular hydrogen was observed from the simulants, the results indicate that LMS-1 and LHS-1 display water surface formation, binding, and transport interactions similar to actual regolith but not the desorption chemistry associated with the implanted hydrogen from the solar wind. Overall, these terrestrial surrogates are useful for understanding the surface and interface interactions of lunar regolith grains, which are largely dominated by the terminal hydroxyl sites under both solar wind bombardment and terrestrial preparation conditions.