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

We report the first observation of Argon-40 (Ar-40) in the mid latitude regions (-60 degrees to +60 degrees) of the lunar exosphere from CHandra's Atmospheric Composition Explorer-2 (CHACE-2) experiment aboard Chandrayaan-2 orbiter. The number density of Ar-40 shows pre-sunrise, sunrise and sunset peaks as well as nightside minima, typical of a condensable gas, which is similar to the features seen at the low latitudes in previous observations. The CHACE-2 observed number densities of Ar-40 and its diurnal variation at low latitudes (-30 degrees to +30 degrees) is consistent with LACE/Apollo observations. CHACE-2 observations show Ar-40 enhancements over certain longitude sectors. In addition to KREEP region, Ar-40 bulges are observed at other longitudes, including the South Pole Aitken (SPA) terrain. The global distribution of Ar-40 shows that the sunrise peak is observed at the same local time over highlands and mare regions. These observations call for a deeper understanding of the surface-exosphere interactions and source distribution. Plain Language Summary The Moon is known to possess a tenuous atmosphere, known as surface bound exosphere. Lunar exosphere exists as a result of a dynamic equilibrium between several sources and sink processes. Noble gases serves as important tracers to understand such processes. Though, Argon-40 (Ar-40) is known to exist in lunar exosphere, the knowledge on its distribution at higher latitudes is lacking. For the first time, CHandra's Atmospheric Composition Explorer-2 (CHACE-2) experiment aboard Chandrayaan-2 orbiter has continuously observed Ar-40 in latitude range of -60 degrees to +60 degrees. It is found that the Ar-40 density variation with local solar time shows the behavior of a condensable gas, which is similar to that observed earlier at low latitudes. The distribution of Ar-40 shows spatial heterogeneity with localized enhancements over KREEP and South Pole Aitken terrain. This suggests that there may be other regions with lower activation energy as the source of Ar-40. The observed global distribution indicates that the interaction of Ar-40 with the surface are similar in low and mid latitude regions. The CHACE-2 observations hint at requirement for improvement in our understanding of the surface-exosphere interactions and source distributions of Ar-40. Key Points First observation of Argon-40 in the mid latitude exosphere of the Moon Observed nightside minimum and sunrise and sunset peaks in Ar-40 abundance is similar to that at low latitudes Enhanced Ar-40 number density is observed at few longitudes, including South Pole Aitken terrain, in addition to KREEP