Speed and angular distribution of photodesorbed sodium at Moon and Mercury

The release of sodium from regolith was simulated in order to elucidate the physical processes that generate surface-boundary exospheres in the inner solar system. Given the absence of laboratory experiments in relevant powders, a kinetic model used findings from experiments of Electron Stimulated Desorption (ESD) and Photon Stimulated Desorption (PSD) of adsorbates on single crystals to predict the angular distribution of photodesorption products from granular media. The regolith was simulated as a computer-generated sphere packing with grain size distributions selected from Luna and Apollo samples, while the effect of roughness at larger scales was also considered. The predicted angular distribution with this method varied as a function of solar inclination angle. Results were adopted into a global model of the sodium exosphere and compared to measurements from the Moon and Mercury.  Results indicate that small deviations of the speed distribution from a Maxwellian suffice to explain the lunar sodium measurements. However, measurements at Mercury appear to indicate a somewhat cooler speed distribution for PSD with a suppressed high-speed tail. We hypothesize that this difference in the sodium speed distribution from PSD between Mercury and the Moon could be attributed to the lower surface abundance of sodium adsorbates at the Moon.