Modeling the Impact Ejected Dust Contribution to the Lunar Exosphere: Results from Experiments and Ground Truth from LADEE

A considerable body of evidence indicates the presence of lofted regolith dust above the lunar surface. These observations range from multiple in-situ and orbital horizon glow detections to direct measurement of dust motion on the surface, as by the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment. Despite this evidence, the specific mechanisms responsible for the lofting of regolith are still actively debated. These include impact ejection, electrostatic lofting, effects of high energy radiation, UV/X- rays, and interplay with solar wind plasma. These processes are highly relevant to one of the two main scientific objectives of the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission (due to launch September, 2013): to directly measure the lunar exospheric dust environment and its spatial and temporal variability towards the goal of better understanding the dust flux. Of all the proposed mechanisms taking place on the lunar surface, the only unequivocal ongoing process is impact cratering. Hypervelocity impact events, which mobilize and redistribute regolith across planetary surfaces, are arguably the most pervasive geologic process on rocky bodies. While many studies of dust lofting state that the impact flux rate is orders of magnitude too low to account for the lunar horizon glow phenomenon and discount its contribution, it is imperative to re-examine these assumptions in light of new data on impact ejecta, particularly from the contributions from mesoscale (impactor size on the order of grain size) and macroscale (impactor > grain size) cratering. This is in large part due to a previous lack of data, for while past studies have established a canonical ejecta model for main-stage ejection of sand targets from vertical impacts, only recent studies have been able to begin quantitatively probing the intricacies of the ejection process outside this main-stage, vertical regime. In particular, it is the high-speed early-time ejecta that will reach significant altitude above the surface and remain aloft ballistically for hours. In addition, ejecta dynamics in the transition regime between microcratering and macro scale events is not yet well understood. As such, there is no currently accepted encompassing model of impact ejecta delivery to the lunar exosphere. It is important to note that the work described here is not to duplicate or exclude other lofting mechanisms -- in reality, the lofting of dust is almost definitely a complex combination of processes -- but instead to provide essential constraints on the impact contribution. This study attempts to constrain the expected contributions from cratering to the lunar exosphere by assessing the ejecta 'background' signal lofted above the surface and the effects of transient focused events (meteor showers) which can produce significant increases in ejecta. In particular, this work couples scaling of previous ejecta studies with Monte-Carlo and ballistics models and will present LADEE data analysis (particularly from the UVS and LDEX instruments) and interpretation in context of constraining the ejected mass distribution. These results are relevant to both our understanding of exospheric dust and for constraint of hazards for future human habitation.