Lunar Atmosphere: Expectations and Goals for the LADEE Neutral Mass Spectrometer

Of all of the previous attempts to make in situ measurements of the tenuous neutral atmosphere of the moon, there are only 3 success stories. One is the mass spectrometer that was left at the Apollo 17 site, which had sufficiently low artifact backgrounds at fortuitous spectral locations to permit the unambiguous identification of lunar gases: radiogenic argon-40 and helium of both radiogenic and solar wind origin; the others are the energetic neutral atom detectors on IBEX and Chandrayaan-1 which measured the high energy part of the energy distribution of escaping solar wind hydrogen. The next attempt to improve our knowledge of the moon's atmosphere is the forthcoming Lunar Atmosphere and Dust Environment Explorer (LADEE) mission that will carry a neutral mass spectrometer (NMS) as well as a complimentary ultraviolet spectrometer. The orbit of the LADEE spacecraft will be nearly equatorial, retrograde, and elliptical, with periapsis between 20 and 50 km and generally in the vicinity of the sunrise terminator. The choice of orbit parameters is based on the evidence that rapid desorption from the warming surface at sunrise forms a pocket of argon-40. By inference, as well as on the basis of simulation results, other predicted gases that may have avoided detection by the Apollo 17 mass spectrometer due to background interference should be expected to be concentrated at sunrise as well. We present simulated orbital data for a variety of potential species and for a range of solar wind speed, magnetic field intensity, and the phenomenological parameters of surface physics, that is, the accommodation and Arrhenius coefficients. In addition, we show how the latter coefficients may be determined from scale heights derived from data collected immediately before and after each orbit maintenance maneuver that changes the altitude of periapsis. Limits on the detectable fluxes of energetic reactive species ejected from the surface by solar wind and meteor impacts are also presented.