Thermodynamics of Surface-Bounded Exospheres

TL;DR

This paper derives the vertical density profiles of surface-bounded exospheres, revealing that non-classical velocity distributions can produce ground-hugging profiles similar to observed data, challenging previous interpretations.

Contribution

It introduces a thermodynamic approach to calculating exosphere density profiles for various velocity distributions, including non-Maxwellian ones.

Findings

Classical Maxwell-Boltzmann flux distribution yields standard density profiles.

Non-Maxwellian distributions can produce infinite density near the surface.

Observed profiles may be explained by a single temperature population.

Abstract

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.