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