The Physical Origin of the Venus Low Atmosphere Chemical Gradient

TL;DR

This study investigates the physical processes behind the nitrogen and carbon dioxide gradient in Venus's deep atmosphere, using simulations and equations of state, suggesting volcanic degassing as a plausible cause.

Contribution

It provides a physical explanation for the nitrogen gradient in Venus's atmosphere by analyzing diffusion and phase separation, integrating molecular dynamics and supercritical fluid models.

Findings

Molecular diffusion is inefficient in Venus's deep atmosphere.

Phase separation of N2 and CO2 is unlikely under Venus conditions.

Volcanic degassing could plausibly produce the observed nitrogen gradient.

Abstract

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.