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Density Variations of the Neon and Argon Lunar Exosphere Due to Macro-Scale Topography and Realistic Surface Temperature Variations

Presentation #101.01 in the session Moon & Earth I (Oral Presentation)

Published onOct 23, 2023
Density Variations of the Neon and Argon Lunar Exosphere Due to Macro-Scale Topography and Realistic Surface Temperature Variations

We present results from our Monte Carlo simulation of the lunar exosphere that includes measured surface temperature and topography (altitude, slope, and azimuth) measured by Diviner and LOLA, respectively, two instruments onboard the Lunar Reconnaissance Orbiter (LRO). The temperature maps allow us to follow the realistic evolution in time of each 1 deg x 1 deg cell on the lunar surface and to accurately track the diurnal excursion of surface temperature, which may differ between two points at the same longitude because of thermal inertia or shadowing. The topography maps allow us to account for the variations in altitude and slope of the lunar surface, to see, for example, if molecules released from a wall on a crater have a higher chance to being ejected within the crater itself. We simulate two exospheric species detected in the lunar exosphere: argon-40, an endogenic species (it comes from outgassing of the subsurface, and it is a tracer of potassium-40) that adsorbs at the cold lunar surface; and neon, a gas of solar wind origin that does not condense on the lunar surface. We ran four types of models: a) smooth Moon and an analytical formula for the surface temperature that depends on the solar zenith angle; b) smooth Moon and a single Diviner temperature map; c) smooth Moon and multiple Diviner maps (to simulate a lunation); d) and multiple Diviner maps and rough Moon (with topography). The simulations show a marked dependence of exospheric density of both neon and argon on the surface temperature, and, to a lesser extent, on topography. Conversely, the topography plays an important role in the distribution of adsorbed molecules. The modeled densities are compared with those measured in situ by the mass spectrometers LACE (Apollo 17) and LADEE, and support ongoing measurements of the lunar exosphere by the LAMP UV spectrograph onboard LRO.

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