Case closed! Astronomers solve atmospheric mystery

The Sun is just below the horizon in this photo and creates an orange-red glow above the Earth’s surface, which is the troposphere, or lowest layer of the atmosphere. The tropopause is the brown line along the upper edge of the troposphere. Above both are the stratosphere, higher atmospheric layers, and the blackness of space.

The reasoning behind why our planet’s atmosphere grows colder and thinner with altitude, yet at around 40,000 to 50,000 feet above it begins to become warmer, has been solved by astronomers at the University of Washington.

The peculiarity, which was at first discovered by Leon Teisserenc de Bort in 1902 using instrument-equipped balloons, is shared with our Solar System’s gas giants – Jupiter, Saturn, Uranus and Neptune – and could also give us a leg-up by assisting in our search for potentially habitable worlds.

We know that air is meant to grow colder and thinner the higher up in the atmosphere we move. Yet what Teisserenc de Bort found led him to coin the term tropopause; the odd atmospheric temperature turnaround sandwiched in between the upper layer of atmosphere we know today as the stratosphere and the lower layer, the troposphere.

And Teisserenc de Bort wasn’t imagining things. The 1980s saw NASA spacecraft uncover these tropopauses in the atmospheres of our gaseous neighbours – Jupiter, Saturn, Uranus and Neptune as well Titan; the largest of the ringed planet’s moons. What’s more is that even though these worlds are different, their atmospheres began to warm up at around the same level – at a pressure of about 0.1 bar, which equates to around one-tenth of Earth’s air pressure.

The oddity can also be found in the atmospheres of the Solar System’s gas giants as well as Saturn’s moon, Titan.

According to postdoctoral researcher Tyler Robinson at the University of Washington alongside Professor of Earth and Space Sciences, David Catling, the explanation is really just a matter of basic physics. And they reckon that these tropopause are much more common to the billions of thick-atmosphere planets that reside in our Galaxy than originally thought.

“The explanation lies in the physics of infrared radiation,” explains Robinson. “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.”

Armed with an analytic model, the scientists illustrated that, at high altitudes, planetary atmospheres become transparent to hot radiation since there is low pressure at these heights. The atmosphere above that with a pressure of 0.1 bar absorbs visible and ultraviolet light from the Sun, causing it to heat up the higher up in a planet’s atmosphere – in particular the aforementioned – we move.

With this new finding, the University of Washington astronomers say, figuring out the temperature and pressure conditions on the surface of exoplanets could tell us if life could exist on them. The large hint would be if the pressure and temperature of a particular world would allow for the existence of liquid water on the surface of a rocky planet.

“[We then] have somewhere we can start to characterise that world,” adds Robinson who is enthralled by the fact that the most basic of physics not only explains the atmospheres of planets in our Solar System, but also could help in the search for life elsewhere. “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, we can start to extrapolate downward toward the surface.”

Images courtesy of NASA Johnson Space Center