May 6, 2011, 12:00 pm - 1:00 pm

May 6, 2011, 12:00 pm - 1:00 pm

Lunar Pickup Ions: Exospheric or Surface Origin?

Dr. Menelaos Sarantos, NASA Goddard Space Flight Center/UMBC

Our Moon is the nearest to Earth example of a common type of atmosphere, a Surface Boundary Exosphere. Many of its expected neutral constituents remain undetected because their modeled abundances are small and they cannot be easily observed from Earth. Lunar ions have been observed by spaceborne ion spectrometers, but it is unclear whether these ions originate from the exosphere, or whether they are ejected from the surface during its bombardment by solar wind ions and electrons. Such measurements have raised the questions: are we primarily measuring the escape products of photoionization with pickup ions? If so, can we constrain exosphere levels for undetected neutral species given the detected ion levels? To begin answering these questions, a model of lunar exospheric constituents was validated using existing observations (detections for He, Na, and K; upper limits for other species). Then, this neutral model was combined with ion transport models to quantify densities and fluxes of lunar pickup ions from exospheric sources, and to compare these estimates to rates of secondary ions emitted from the surface by sputtering and electron stimulated desorption. Mass and energy spectra that an ion spectrometer would see from orbit were simulated for ten ion species and for different pointing geometries. The simulations that will be presented enable the analysis of scientific data from JAXA's Kaguya and inform the data acquisition planning for NASA's ARTEMIS and LADEE spacecraft.