title: Two-dimensional description of surface-bounded exospheres with application to the migration of water molecules on the Moon.
abstract: On the Moon, water molecules and other volatiles are thought to migrate along ballistic trajectories. Here, this migration process is described in terms of a two-dimensional partial differential equation for the surface concentration, based on the probability distribution of thermal ballistic hops. A random-walk model, a corresponding diffusion coefficient, and a continuum description are provided. In other words, a surface-bounded exosphere is described purely in terms of quantities on the surface, which can provide computational and conceptual advantages. The derived continuum equation can be used to calculate the steady-state distribution of the surface concentration of volatile water molecules. An analytic steady-state solution is obtained for an equatorial ring; it reveals the width and mass of the pileup of molecules at the morning terminator.
authors.affiliation: Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, Hawaii 96822, USA.
authors.forename: Norbert
authors.initials: N
authors.lastname: Schorghofer