Global exosphere models of alkali gases surrounding Mercury and the Moon assume that the primary effect of the porous soil is to reduce the effective desorption rates. We demonstrate with a kinetic simulation that, following adsorption, the complicated structure of soils has two additional effects on the fate of previously released alkali atoms: (1) trapping of free atoms at lunar temperatures by microscopic shadows and inward diffusion, which becomes the primary sink mechanism, and (2) high‐energy barriers for thermal desorption compared to what would be retrieved from experiments on thin films or compacted pellets, especially when surface diffusion of adsorbates is considered. Lunar soils retain one fifth to two thirds of recycled adsorbates, depending on the assumed adsorbate mobility, photodesorption cross section, and soil thermal gradient. A transition from a retentive surface to full outgassing at T > 500 K will produce complex feedback mechanisms of alkali circulation at Mercury.

Monte Carlo simulations of gas motion inside a granular medium are presented in order to understand the interaction of lunar gases with regolith and improve models for surface-boundary exospheres, a common type of planetary atmosphere. Results demonstrate that current models underestimate the lifetime of weakly bonded adsorbates (e.g., argon) on the surface by not considering the effect of Knudsen diffusion, and suggest that thermal desorption of adsorbates should be modeled as a second-or-higher-order process with respect to adsorbate coverage. An additional discrepancy between present models and outgassing from a realistic porous boundary is found for surface-adsorbate systems containing a distribution of activation energies (e.g., water). In that case, the mobility of adsorbates between desorption events (i.e., surface diffusion), not considered in global models of the exosphere, controls their surface residence time via transitions between sites of low and high binding energy. Without mobility the equatorial surface retains more water over a lunar day because sites of low binding energy are not repopulated by motion along the grain surface when depleted. The effects of Knudsen and surface diffusion apply to other volatile species and help us partly understand why measurements of lunar exosphere constituents appear to indicate stronger bonding of gas with the lunar surface than measured in some laboratory experiments.