Effects of regolith porosity on exospheric gas release and retention

The release of gases from regolith is studied with a Monte Carlo model in order to improve models of surface-boundary exospheres in the inner solar system. This kinetic model quantifies the competition between adsorption, desorption, and Knudsen and surface diffusion in three-dimensional granular media. The regolith is simulated as a computer-generated sphere packing with grain size distributions selected from lunar samples, whose voids present realistic paths for mass transport of trapped gases. In this work previous studies of exemplary gas-surface systems (argon, water, sodium) are extended by building sphere packings of varying macroporosity (0.4-0.6). Furthermore, the new calculations include light scattering and heat transfer. Results quantify the effect of regolith porosity on 1) gas residence time, 2) gas angular distribution, and 3) the magnitude of grain-to-grain temperature variations in vacuum. The findings are useful for understanding measurements from laboratory experiments as well as spaceborne measurements (e.g., LADEE, MESSENGER, Bepi Colombo).