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Common 0.1 bar tropopause in thick atmospheres set by pressure-dependent infrared transparency

Abstract

A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1 bar in the atmospheres of Earth1, Titan2, Jupiter3, Saturn4, Uranus and Neptune4, despite great differences in atmospheric composition, gravity, internal heat and sunlight. In all of these bodies, the tropopause separates a stratosphere with a temperature profile that is controlled by the absorption of short-wave solar radiation, from a region below characterized by convection, weather and clouds5,6. However, it is not obvious why the tropopause occurs at the specific pressure near 0.1 bar. Here we use a simple, physically based model7 to demonstrate that, at atmospheric pressures lower than 0.1 bar, transparency to thermal radiation allows short-wave heating to dominate, creating a stratosphere. At higher pressures, atmospheres become opaque to thermal radiation, causing temperatures to increase with depth and convection to ensue. A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause. We reason that a tropopause at a pressure of approximately 0.1 bar is characteristic of many thick atmospheres, including exoplanets and exomoons in our galaxy and beyond. Judicious use of this rule could help constrain the atmospheric structure, and thus the surface environments and habitability, of exoplanets.

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Figure 1: Temperature–pressure profiles for worlds in the Solar System with thick atmospheres1,2,3,4,28.
Figure 2: Schematic diagram of thermal structure in a thick planetary atmosphere with a stratospheric inversion.
Figure 3: Tropopause grey infrared optical depth and pressure.

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Acknowledgements

This work was performed as part of the NASA Astrobiology Institute’s Virtual Planetary Laboratory, supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under solicitation No. NNH05ZDA001C. T.D.R. gratefully acknowledges support from an appointment to the NASA Postdoctoral Program at NASA Ames Research Center, administered by Oak Ridge Associated Universities. D.C.C. was also supported by NASA Exobiology/Astrobiology grant NNX10AQ90G. The authors thank the late C. Leovy for discussions in which he was supportive of pursuing the idea that a 0.1 bar tropopause constitutes an emergent law.

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T.D.R. and D.C.C. made equally important contributions to the project and co-wrote the paper. T.D.R. generated the model outputs.

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Correspondence to T. D. Robinson.

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The authors declare no competing financial interests.

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Robinson, T., Catling, D. Common 0.1 bar tropopause in thick atmospheres set by pressure-dependent infrared transparency. Nature Geosci 7, 12–15 (2014). https://doi.org/10.1038/ngeo2020

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