The surface‐bounded atmospheres of Mercury and the Moon

Surface-bounded exospheres have been detected at the Moon, Mercury, and Europa and almost certainly exist about other objects. Historically, the first of these systems to be observed was the lunar exosphere, where He and Ar were detected by the Apollo spacecraft. The Hermean exosphere is archetypical of these systems in that it is part of a coupled system including the surface and magnetosphere interacting dynamically with the solar wind and fields. Studies of the Hermean exosphere heretofore have neglected or only superficially considered these interactions. We will review the current state of knowledge of the exospheres of Mercury and the Moon and discuss areas in which our knowledge is most incomplete. We will focus on the exosphere as part of a coupled system including the surface at its base and the particle, field, and interplanetary environment as both a source and sink for neutrals. Apollo era instruments made unambiguous detections of 36Ar, 40Ar, and He and placed stringent upper limits on other species [Hodges, 1975; Hodges and Hoffman, 1973]. Post-Apollo work began with the discovery of Na and K using ground-based techniques [Potter and Morgan, 1988]. There have also been attempts to use the International Ultraviolet Explorer and the Hubble Space Telescope to detect additional species. The Mariner 10 mission included two encounters with Mercury and one distant flyby in 1974–1975: (1) March 29, 1974 (nightside pass with closest approach at 723-km altitude), (2) September 21, 1974 (distant dayside pass), and (3) March 16, 1975 (nightside pass with closest approach at 327-km altitude). Since then there has been no other spacecraft mission to Mercury. In the 2 decades since the Mariner 10 mission, slow but steady progress has been made in understanding the Hermean system, but our knowledge of this innermost terrestrial planet is extremely limited. It is clear that many theories about the origin and evolution of Mercury that were proposed before Mariner 10 are inconsistent with what we now know about the system. Substantial progress will be made only by returning to the planet. This review aims to present a tool for mission planning as well as an introduction and overview of the exosphere.