Simulating the boundary conditions for the lunar exosphere with a microphysical model

Abstract:

We quantify the competition between desorption and diffusion within the top 5 mm of the soil, which supports the exosphere reservoir, with test particle trajectories in regolith beds created with a sphere packing code. Two volatiles, Ar and water, and one species that binds more strongly with the surface, Na, were studied. We considered how differences in sticking and activation energy for desorption affect the outgassing rate near dawn (sunrise). Results demonstrate that as the strength of bonds increase, Na atoms are trapped in microscopic shadows in the soil. In all cases diffusion decreased the rate of desorption from a porous medium. These simulations help us understand why exospheric measurements (e.g., LADEE) appear to indicate stronger binding energies than those measured in thin-film laboratory experiments.