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The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2

Nature Geoscience volume 10, pages 473–477 (2017)Cite this article

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Abstract

With temperatures around 700 K and pressures of around 75 bar, the deepest 12 km of the atmosphere of Venus are so hot and dense that the atmosphere behaves like a supercritical fluid. The Soviet VeGa-2 probe descended through the atmosphere in 1985 and obtained the only reliable temperature profile for the deep Venusian atmosphere thus far. In this temperature profile, the atmosphere appears to be highly unstable at altitudes below 7 km, contrary to expectations. We argue that the VeGa-2 temperature profile could be explained by a change in the atmospheric gas composition, and thus molecular mass, with depth. We propose that the deep atmosphere consists of a non-homogeneous layer in which the abundance of N2—the second most abundant constituent of the Venusian atmosphere after CO2—gradually decreases to near-zero at the surface. It is difficult to explain a decline in N2 towards the surface with known nitrogen sources and sinks for Venus. Instead we suggest, partly based on experiments on supercritical fluids, that density-driven separation of N2 from CO2 can occur under the high pressures of Venus’s deep atmosphere, possibly by molecular diffusion, or by natural density-driven convection. If so, the amount of nitrogen in the atmosphere of Venus is 15% lower than commonly assumed. We suggest that similar density-driven separation could occur in other massive planetary atmospheres.

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Figure 1: Vertical structure of the atmosphere of Venus.
Figure 2: The VeGa-2 spacecraft.
Figure 3: Vertical profile of potential temperature θ computed from temperatures measured by VeGa-2.
Figure 4: Meridional distributions of the turbulent mixing coefficient and averaged stream function.

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Acknowledgements

The authors thank L. Zasova for providing the VeGa-2 probe temperature profile, and J. Bellan for mentioning barodiffusion and useful discussion on this phenomenon. S.L. acknowledges the support of the Centre National d’Etudes Spatiales. G.S. acknowledges the support of the Keck Institute for Space Studies under the project ‘Techniques and technologies for investigating the interior structure of Venus’.

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Authors and Affiliations

  1. Laboratoire de Météorologie Dynamique (LMD/IPSL), Sorbonne Universités, UPMC Univ. Paris 06, ENS, PSL Research University, Ecole Polytechnique, Université Paris Saclay, CNRS, 75252, Paris, France

    Sebastien Lebonnois

  2. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, California, 90095, USA

    Gerald Schubert

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Correspondence to Sebastien Lebonnois.

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Lebonnois, S., Schubert, G. The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2. Nature Geosci 10, 473–477 (2017). https://doi.org/10.1038/ngeo2971

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