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Influence of subsurface diffusion on Mercury’s Sodium exosphere

Presentation #106.05 in the session Mercury (Oral Presentation)

Published onOct 23, 2023
Influence of subsurface diffusion on Mercury’s Sodium exosphere

In the 1970’s, Mariner 10 preformed the first measurement of the surface bounded exosphere, composed of hydrogen, helium and oxygen of Mercury. This was followed by intensive ground based observations that led to the discovery of sodium in the Hermean exosphere a decade later by Potter and Morgan. Nearly 30 years after the first evidence of the presence of Mercury’s sodium exosphere, many models attempted to describe it while reproducing its distinctive features such as the dusk enhancement observed by MESSENGER or the dawn-dusk asymmetry and variability. However, until today no comprehensive model has managed to agree with all the different ground based and spacecrafts observations. The tenuous atmosphere of Mercury allows for micro-meteoroid to impact its surface, forming a porous regolith and the slow rotation and orbit of the planet induces large thermal gradients in the first meter subsurface. The 3:2 spin-orbit resonance creates a periodicity of these gradients which favors subsurface diffusion at specific longitudes. This study investigates the influence of subsurface diffusion on the Hermean exosphere. Using an Exospheric Global Model (EGM), a 3-D test-particles Monte Carlo model, the collisionless Na exosphere is described using photon stimulated desorption, thermal desorption, micro-meteoroid vaporization and sputtering by impacting solar particles as the sodium production mechanisms. Atoms falling on the planet surface can either be reejected or diffuse inside the regolith. The subsurface diffusion and thermal gradient are described by a 1-D model down to 30 cm below the surface. Adsorption and desorption on and off regolith grains is also considered while subsurface diffusion is taking place. Including subsurface diffusion allows Sodium to get trapped in the regolith over periods of time depending on the thermal gradient which could explain the seasonal variability of the Mercury sodium exosphere. Developing a fully comprehensive model of Mercury’s exosphere is necessary in the perspective of the BepiColombo mission which will study this environment upon its arrival in 2025.

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