Thermodynamics of Surface-Bounded Exospheres

Neutral exospheres of the Moon, Mercury, and several other solar system bodies consist of particles on ballistic trajectories. Here, the vertical density profile of a surface-bounded exosphere is calculated using thermodynamic averages of an ensemble of ballistic trajectories. When the initial velocities follow the so-called "Maxwell-Boltzmann Flux" (MBF) distribution, the classical density profile results. For many other probability distributions, the density approaches infinity near the surface and has a more ground-hugging vertical profile than the classical solution. Even MBF on a rough surface results in a ground-hugging component. Observed vertical density profiles that were interpreted as a superposition of a hot and a cold (ground hugging) population may in fact be consistent with a population at a single temperature.