Local time dependence of sodium in the lunar exosphere from NASA LADEE/UVS data.

The Moon has a surface boundary exosphere, which consists of a thin, collision-less atmosphere extending down to the surface. Although surface boundary exospheres are thought to be the most common class of atmosphere for objects within our solar system, they remain poorly understood to date.

The NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) carried out a seven-month (Sep 2013 - Apr 2014) lunar exploration and technology demonstration mission. The scientific objective was to gather detailed measurements on the composition, structure, and spatiotemporal variability of the lunar atmosphere, characterizing the relevant processes, sources and sinks, and interactions with the surface. One of the instruments on-board LADEE was the Ultraviolet and Visible Spectrometer (UVS) which could observe resonant scattering of exospheric species across a wavelength range of 230-810 nm, and was able to operate in both limb- and solar-viewing modes.

Previous work by Colaprete et al. (2016) used UVS data to detect the spatiotemporal variability of two trace - but easily observed - constituents in the lunar exosphere, sodium (Na) and potassium (K). In this work, we examine the local time dependence of Na in the lunar exosphere, demonstrating how the Na emission varies from dawn to dusk across one full lunation cycle (Feb-Mar 2014). In conjunction with theoretical models, we explore the relative importance of interactions between the solar wind, meteor bombardment, and lunar surface heterogeneity in explaining the observed exospheric Na distributions.