Atmosphere's Temperature Mystery May Help Discover Habitable Planets

It turns out that an atmospheric peculiarity that Earth shares with Jupiter, Saturn and Uranus may be more common than expected. Scientists have discovered that it's likely common to billions of planets, which may help in the search for potentially habitable worlds.

Air grows colder and thinner with altitude. Yet there's a point in Earth's atmosphere at about 40,000 to 50,000 feet where the air stops cooling and instead begins to warm again. This invisible turnaround is known as a tropopause, and it's not only existent in our own atmosphere. In the 1980s, NASA discovered tropopauses in the atmospheres of the planets Jupiter, Saturn, Uranus and Neptune along with Saturn's largest moon, Titan. All of these turnaround points occurred at roughly the same level in the atmosphere of each of these different worlds at a pressure of about .1 bar--one-tenth of the air pressure at Earth's surface.

Now it seems like these aren't the only planets with tropopauses. Scientists used an analytic model to show that at high altitudes atmospheres become transparent to thermal radiation due to the low pressure. Above the level where the pressure is about .1 bar, the absorption of visible, or ultraviolet, light causes the atmospheres of the giant planets to grow warmer as altitude increases.

"The explanation lies in the physics of infrared radiation," said Tyler Robinson, one of the researchers, in a news release.

Atmospheric gases gain energy by absorbing infrared light from the sunlit surface of a rocky planet or from the deeper parts of the atmosphere of a planet like Jupiter, which has no surface. These physics provide a rule of then which actually applies to a vast number of planetary atmospheres with stratospheric gases that absorb ultraviolet or visible light.

The findings could be used to extrapolate temperature and pressure conditions on the surface of planets. In addition, astronomers could use it to work out whether worlds are potentially habitable--the key being whether pressure and temperature conditions allow liquid water on the surface of a rocky planet.

"Then we have somewhere we can start to characterize that world," said Robinson in a news release. "We know that temperatures are going to increase below the tropopause, and we have some models for how we think those temperatures increase-so given that leg up, we can start to extrapolate downward toward the surface."

The findings are published in the journal Nature Geoscience.