Inferred contributions of the solar wind and micrometeoroids to metallic species in the lunar exosphere from LADEE data and models

Two processes that have been hypothesized as sources for metals in the lunar exosphere are micrometeoroid impact vaporization and solar wind sputtering. Measurements of exospheric potassium, titanium and magnesium line emission, obtained by the Ultraviolet and Visible Spectrometer (UVS) on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft between November 2013 and April 2014, allow us to quantify the relative roles of these processes in vaporizing lunar soils. We use information provided by the Lunar Dust Experiment (LDEX) on LADEE to seed a Monte Carlo model of exospheric transport, while the solar wind flux impinging on the lunar surface throughout the LADEE mission was observed by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission. The model output for different assumptions such as sputtering yields and micrometeoroid vapor flux is compared to the observed monthly cycle of these exospheres, especially times during passages of the Moon through the Earth's magnetosphere, to obtain the relative importance of sputtering to the overall source. Time-dependent simulation of the evolution of the atmosphere before, during, and following the Geminids meteor shower are also used to further constrain the roles of these two sources. Last, the observed monthly variation is used to obtain limits to the exogenous contribution to the atmosphere by the vaporizing micrometeoroid projectiles.