A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1bar in the atmospheres of Earth, Titan, Jupiter, Saturn, Uranus and Neptune, 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 clouds. However, it is not obvious why the tropopause occurs at the specific pressure near 0.1bar. Here we use a simple, physically based model to demonstrate that, at atmospheric pressures lower than 0.1bar, 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 0.1bar 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.
- Pub Date:
- January 2014
- Astrophysics - Earth and Planetary Astrophysics
- Accepted to Nature Geoscience. Final article and Supplementary Information available at http://dx.doi.org/10.1038/ngeo2020