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

A time-dependent simulation of the argon-40 exosphere of the Moon shows that the semiannual oscillation of argon detected by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft is consistent with adsorptive respiration in seasonal cold traps near the lunar poles. The magnitude of the oscillation requires that high-energy adsorption sites on soil grain surfaces at polar latitudes be as free of water contamination as soils at low latitudes. This requirement is met by the combination of two generally ignored water removal mechanisms: solar wind bombardment of exposed adsorption sites and the serpentinization reaction of water with olivine. The significance of these processes is supported by the lack of evidence of water in Lunar Atmosphere and Dust Environment Explorer data, which, in turn, establishes an upper bound for exospheric transport of water to polar traps at less than 10(14) molecules/Ga. Plain Language Summary The neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft recorded a gradual rise and then fall of atmospheric argon-40 that is consistent with a 140-day segment of a semidraconic oscillation. The semidraconic oscillation of argon is important because its existence has harsh implications for the accumulation of water in polar cold traps. Simulations show that the existence of the oscillation implies respiration of argon atoms in seasonal cold traps near both poles, which in turn requires that polar soil grain surfaces have significant areas of high-energy adsorption sites that are not contaminated by water molecules. The paper argues that such extreme cleanliness can be explained by two previously ignored processes. One is surface scouring by solar wind bombardment of the lunar surface, which leads to escape or scatter to lower latitudes. The other is sequestration of water by the reaction of olivine with water, a process that is known as serpentinization. Coupled with meteoritic gardening, these process are capable of removing water from the lunar atmosphere faster than current estimates of water sources. This conclusion does not preclude the assimilation of water in permanent traps, but it severely reduces the amount of water available for assimilation.

The neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft collected a trove of exospheric data, including a set of high-quality measurements of radiogenic Ar-40 over a period of 142days. Data synthesis studies, using well-established exosphere simulation tools, show that the LADEE argon data are consistent with an exosphere-regolith interaction that is dominated by adsorption and that the desorption process generates the Armand distribution of exit velocities. The synthesis work has uncovered an apparent semiannual oscillation of argon that is consistent with temporal sequestration in the seasonal cold traps created at the poles by the obliquity of the Moon. In addition, the LADEE data provide new insight into the pristine nature of lunar regolith, its spatially varying sorption properties, and the influence of sorption processes on the synodic oscillation of the argon exosphere.