Coupled H, H2, OH, and H2O lunar exosphere simulation framework and impacts of conversion reactions

Numerical models of the lunar exosphere have been available for several decades to allow investigations of the Moon's tenuous atmosphere beyond the few measurements made by satellite or ground-based experiments. We present a novel approach to modeling a system of elements, namely the hydrogen-bearing species H, H2, OH, and H2O, in a multi-element simulation, implementing exospheric and geochemical conversion reactions to connect the different species through their loss and source processes. The model resembles the lunar water cycle, including several source and sink mechanisms like the solar wind proton influx and the eventual loss through Jean's escape, photodissociation, or cold trapping. Unknown and uncertain parameters have been modeled using probability distributions that propagate their uncertainty through the system to allow subsequent statistical analysis of the predictions. In eight studies we varied the reactivity of the lunar environment and investigated the response of the exosphere's densities and dynamics. The results show that the new framework is able to reproduce many published values for the surface number densities and reported dynamics like the H2 dawn-dusk asymmetry, even with a very simple approach to many of the complex input parameters. The most diverging results can be found for the hydroxyl and water predictions. Nonetheless, we show that the multi-element approach using conversion reactions offers an alternative explanation to the observed H and H2 exosphere without having to assume these volatiles as condensable species.