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

Abstract

We report the first observation of Argon-40 (Ar-40) in the mid latitude regions (urn:x-wiley:00948276:media:grl63161:grl63161-math-0001 to urn:x-wiley:00948276:media:grl63161:grl63161-math-0002) 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 (urn:x-wiley:00948276:media:grl63161:grl63161-math-0003 to urn:x-wiley:00948276:media:grl63161:grl63161-math-0004) 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.