Argon‐40 in Lunar Exosphere: Observations From CHACE‐2 on Chandrayaan‐2 Orbiter

Abstract

The Moon is known to possess a surface bounded exosphere (Killen & Ip, 1999;Stern, 1999). Characterization of the composition of lunar exosphere and its variability remains an outstanding problem. Noble gases serve as important tracers to understand different processes that contribute to the exosphere and their distribution in the high latitude regions remains poorly explored.Argon-40 (Ar-40) is one of the noble gas, whose presence in the lunar exosphere was first confirmed by Lunar Atmospheric composition Experiment (LACE) on Apollo-17 (Hodges & Hoffman, 1974;Hoffman et al., 1973). LACE detected Ar-40 at the landing site close to the equatorial region of the Moon. LACE observations, which lasted for nine lunations, were confined to the nightside of the Moon bounded by sunrise and sunset terminators, due to the detector saturation effects on the dayside. The observations showed that the surface number density of Ar-40 decrease through the nightside and peaks near the sunrise terminator, typical of a condensable gas that gets absorbed at the cold lunar nightside surface and gets desorbed during