Effects of Macro-Scale Topography and Realistic Surface Temperature Variations on the Lunar Exosphere

The structure of the tenuous lunar atmosphere (exosphere) depends on its interactions with the lunar surface. In this regard, surface temperature is a critical parameter. For example, it determines the residence time of molecules on surface grains and the lateral and vertical migration on such grains, affecting the short- and long-term sequestration of volatiles on the lunar surface. Surface temperature also affects the migration of volatiles in the lunar exosphere: the warmer the surface, the longer the hop between one encounter with the surface and the next. Topography also plays a role. Micro-scale topography, in particular micro-scale shadows from nearby grains, has been shown to play a role in the adsorption and transport of water molecules. However, the effect of macro-scale topography (crater wall, mountains, etc.) on the exosphere dynamics has not been investigated in detail so far.

Here we present preliminary results from our Monte Carlo simulation of the lunar exosphere that incorporates realistic relief and temperature maps from two instruments onboard the Lunar Reconnaissance Orbiter (LRO). Past simulations of the lunar exosphere used a smooth lunar surface and, for the lunar surface temperature, either an analytic function or a map from LRO’s Diviner radiometer at a given lunar phase. In these new simulations we incorporate topography (altitude and slope) from the LOLA altimeter, and 48 different lunar surface temperature maps from the Diviner radiometer. These temperature maps allow us to follow the realistic evolution in time of each 1 deg x 1 deg cell on the lunar surface, with a temporal resolution of half a lunar hour (36 hours).

These new simulations are performed for two exospheric species that have been detected in the lunar exosphere: argon, an endogenic species (i.e. outgassing from the lunar interior) that adsorbs at the cold lunar surface; and neon, a gas of solar wind origin that does not condense on the lunar surface.

We compare modeled densities with those from two other codes: one with topography (altitude) and slope but uniform temperature throughout the lunar surface, and one without topography (i.e. a smooth Moon) but with the 48 temperature maps from Diviner, isolating therefore the effects of topography and surface temperature, respectively.

The modeled densities are compared with those measured in situ by LACE and LADEE’s mass spectrometers, and will support ongoing measurements of the lunar exosphere by the UV spectrograph LAMP onboard LRO.