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Many planets, similar tropopauses

Despite numerous differences, Earth and many of its neighbors share an atmospheric peculiarity.

In the troposphere, the lowest layer of Earth's atmosphere (extending to 10–15 km above the surface), temperature decreases with altitude. But in the enveloping layer, the stratosphere, that trend changes direction. At the boundary between the two, called the tropopause, the temperature is at a minimum and, in the global average, atmospheric pressure is about 0.1 bar. (See the articles by Raymond T. Pierrehumbert in Physics Today, January 2011, page 33 and by Bjorn Stevens and Sandrine Bony in Physics Today, June 2013, page 29.) Curiously, such temperature minima have also been found in the atmospheres of Jupiter, Saturn, Uranus, Neptune, and the Saturnian moon Titan, all at roughly the same pressure despite significant differences in solar irradiation, atmospheric composition, and gravity. Tyler Robinson (now at NASA’s Ames Research Center) and David Catling (University of Washington) show how a simple physical model helps explain that commonality. At low altitudes and higher pressures, atmospheres are opaque to long-wavelength thermal IR radiation and are heated from below; the result is convective mixing. At lower pressures, radiative heat transfer at shorter wavelengths dominates. The researchers note that although the various atmospheres differ in their details, they are all generally thick, which leads to a generic pressure-squared dependence for molecular IR absorption. Because of that scaling, stratospheric temperature inversions ascend from a relatively narrow range of parameter space around the observed pressure of 0.1 bar. That general rule, the pair notes, could provide useful insights into exoplanets and exomoons. (T. D. Robinson, D. C. Catling, Nat. Geosci., in press.)—Richard J. Fitzgerald

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