Lunar Argon Cycle Modeling

Introduction: The idea of stable deposits of volatiles in cold permanently shaded regions (PSRs), first advanced by [1], has been revived since observations by Lunar Prospector suggested large amounts of hydrogen in the polar caps. Previous studies have mainly focused on hydrogen (and water) [2] but other important volatiles such as argon could also be trapped. Temperatures in the coldest regions (Amundsen and Nansen F craters) could reach 30 – 40 K [3], low enough to hold argon and create a reservoir of trapped argon [4]. Such deposits could explain a temporal enhancement of argon in the lunar exosphere observed by Apollo 17 mission which may have resulted from a seismic event releasing argon in a cold region [5]. Argon is unique because, contrary to water vapor, argon has been clearly identified in the lunar exosphere and measurements of its diurnal variation are available [5]. New measurements of the argon exosphere are planned with the LRO/LAMP instrument which will further characterize its spatial and temporal variations. In the following we present the goals and the method of a project, recently funded by NASA’s LASER program, to describe the transport of argon from the lunar surface to the cold traps or to space and the possibility of argon frost in the coldest regions on the Moon.