The outer limits of the lunar sodium exosphere

Abstract

A new wide-angle coronagraphic-type imaging system used for the lunar eclipse of 16 July 2000 resulted in detections of the lunar sodium exosphere out to ∼20 lunar radii, approximately twice the size recorded with narrower fields of view during previous eclipses. These measurements and subsequent modeling provide a unique constraint on the fastest atoms ejected from the lunar surface that form the lunar exosphere, indicative of the most energetic space weathering processes acting on the lunar surface. At most, only a small fraction of the atoms are ejected from the surface with speeds faster than escape speed of 2.4 km s-1, meaning solar photon radiation pressure largely contributes to the escape of sodium atoms which form the comet-like tail. The total rate of sodium ejection from the surface for speeds >2.0 km s-1 is comparable to estimates from previous lunar eclipse observations and earlier images of the lunar sodium tail.