Local time dependence of sodium in the lunar exosphere from LADEE 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 (223 day) lunar exploration and technology demonstration mission. Its 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 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 presentation, we examine the local time dependence of Na in the lunar exosphere for the first time, specifically, how the Na emission varies from dusk to the subsolar point. This is something that we cannot examine with ground-based observations as it would take an Earth observer a week to cover these local times, thus convolving temporal and spatial effects.