Variability of helium, neon, and argon in the lunar exosphere as observed by the LADEE NMS instrument
The Neutral Mass Spectrometer (NMS) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) provided the first global characterization of He and Ar along with the discovery of Ne in the lunar exosphere. The mapping of the equatorial distribution of these noble gases revealed new selenographic and temporal variations. Helium was found to be controlled by the supply of solar wind alpha particles and by the presence of an endogenous source that supplies the exosphere at a rate of 1.9 × 1023 atoms s-1. Neon was detected over the nightside at levels comparable to He and was found to exhibit the spatial distribution of a surface accommodated noncondensable gas. The global measurements of NMS revealed the presence of a localized Ar enhancement that has never been identified before at the western maria. The variability resulting from this local enhancement is coupled to a more global but transient source.