Improved Velocity Distributions for Exospheric Na and K Surrounding the Moon

Sodium and potassium are important tracers of volatile processes on the Moon because they can be easily observed as gases surrounding the Moon both from Earth as well as from lunar orbiters. In an effort to understand the relative importance of source processes for the lunar sodium and potassium exospheres and the initial velocity distribution functions, we modeled with an exospheric transport model the expected emission line widths and Doppler shifts for different assumed velocity distribution functions, and compared these simulations to high-resolution spectroscopic measurements from Earth. Because photon-stimulated desorption dominates other processes at high lunar phases, when an Earth observer views the subsolar region, we were able to significantly constrain the velocity distribution of photodesorbed ejecta. The best Na model was provided by a T=1,500K Maxwellian drifting by 0.2-0.3 km/s in the radial direction (i.e. vertical to the surface). The emission line widths within 40 degrees of lunar phase are consistent with considerable contributions by micrometeoroid impacts and/or solar wind sputtering. Contrasting these simulations with data from the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, we demonstrate evidence of a motion of the micrometeoroid impact vaporization source during a month. Together, LADEE and ground-based observations provide improved constraints of ejection velocities of gases from mineral surfaces.