A Dynamic Fountain Model for Dust in the Lunar Exosphere

Abstract

ABSTRACT From the Apollo era there is much evidence to show that the lunar “horizon glow” and “streamers” observed at the terminator are caused by sunlight scattered by dust grains originating from the Moon’s surface. A simple dynamic dust “fountain” model has previously been proposed to explain observations of submicron dust “lofted” to altitudes of ~100 km. In this model charged dust grains follow ballistic trajectories, subsequent to being accelerated upwards through a narrow sheath region by the surface electric field. Described here are the effects of including a more realistic exponentially decaying electric field (consistent with Debye shielding of a charged surface in a plasma). Also discussed are the different mechanisms by which highly charged dust grains can be generated at the lunar surface; in particular, the possible effects of triboelectric charging are considered. Dust in the exosphere will affect the optical quality of the lunar environment for astronomical observations, as well as interfere with future robotic and human exploration activities.