The lunar atmosphere: History, status, current problems, and context

After decades of speculation and fruitless searches by observers, the lunar atmosphere was first observed by Apollo surface and orbital instruments be ginning in 1971. With the end of Apollo missions in 1972 and the terminatio n of funding for Apollo lunar ground station observations in 1977 the field withered for many years, but it has recently enjoyed a renaissance. This r enewal was initiated by the discovery of lunar atmospheric sodium and potas sium by ground-based observers and was furthered by the in situ detection o f metal ions derived from the Moon in interplanetary space, the possible di scoveries of H2O ice at the poles of the Moon and Mercury, and the detectio n of tenuous atmospheres around other remote sites in the solar system, inc luding Mercury and several Galilean satellites. In this review I attempt to summarize the present state of knowledge about the lunar atmosphere, descr ibe the important physical processes taking place within it, and compare th e lunar atmosphere with other tenuous atmospheres in the solar system.