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Venus shares many similarities with the Earth, but concomitantly, some of itsfeatures are extremely original. This is especially true for its atmosphere,where high pressures and temperatures are found at the ground level. In theseconditions, carbon dioxide, the main component of Venus' atmosphere, is asupercritical fluid. The analysis of VeGa-2 probe data has revealed the highinstability of the region located in the last few kilometers above the groundlevel. Recent works have suggested an explanation based on the existence of avertical gradient of molecular nitrogen abundances, around 5 ppm per meter. Ourgoal was then to identify which physical processes could lead to theestablishment of this intriguing nitrogen gradient, in the deep atmosphere ofVenus. Using an appropriate equation of state for the binary mixture CO2-N2under supercritical conditions, and also molecular dynamics simulations, wehave investigated the separation processes of N2 and CO2 in the Venusiancontext. Our results show that molecular diffusion is strongly inefficient, andpotential phase separation is an unlikely mechanism. We have compared thequantity of CO2 required to form the proposed gradient with what could bereleased by a diffuse degassing from a low volcanic activity. The needed fluxesof CO2 are not so different from what can be measured over some terrestrialvolcanic systems, suggesting a similar effect at work on Venus.

The Physical Origin of the Venus Low Atmosphere Chemical Gradient