Argon‐40 in Lunar Exosphere: Observations From CHACE‐2 on Chandrayaan‐2 Orbiter. Issue 20 (19th October 2021)

• Record Type: Journal Article
• Authors: Dhanya, M. B.; Thampi, Smitha V.; Das, Tirtha Pratim; Thampi, R. Satheesh; Naik, Neha; Abhishek, J. K.; Sreelatha, P.; Pradeepkumar, P.; Padmanabhan, G. Padma; Sundar, B.; Vajja, Dinakar Prasad; Nandi, Amarnath; Yadav, Vipin K.; Nazeer, M. Mohammed; Lali, P. T.; John, Rosmy; Aliyas, A. V.; Sen, Vijay Kumar; Prabhu, M. Ram; Krishna, A. Ajay; Pant, Tarun K.; Bhardwaj, Anil
• Is Part Of: Geophysical research letters. Volume 48:Issue 20(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-19
• Subjects: Moon -- lunar exosphere -- Chandrayaan‐2 -- CHACE‐2 -- Argon‐40; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL094970
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: We report the first observation of Argon‐40 (Ar‐40) in the mid latitude regions ( − 60 ° to + 60 ° ) 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 ° to + 30 ° ) 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 ° to + 60 ° . 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