Lunar Atmosphere, Effects of Cometary Impacts

Introduction

The lunar atmosphere is generally very tenuous (Grava and Retherford 2017). Intermolecular collisions are so infrequent that the Moon may be thought of as hosting multiple, non-interacting atmospheres (Stern 1999), with each constituent species forming its own atmosphere controlled by physical processes specific to that species. This virtually collisionless situation arises because the atmospheric loss rate is high and the source rate is usually low. Small meteorite or micrometeorite impacts may episodically deliver small bursts of gas, but do not fundamentally alter the nature of the lunar exosphere. Occasionally, however, an episodic event may deliver enough gas to locally – or even globally – transform the baseline exosphere into a transient, collisional-to-rarefied atmosphere in which volatile transport, loss and sequestration occur in qualitatively different ways. Comets and volatile-rich asteroids are considered likely sources of such atmospheres (Morgan and...