The Mystery of Saturn’s Earth-Sized Cyclone, Explained

Correction appended, 6/17/15

There’s one big difference between Earth and Saturn—OK, there are a lot of big differences between Earth and Saturn, including size, chemistry, temperature, distance from the sun and number of moons (one for Earth, up to 62 for Saturn). But the difference that may be most important concerns their atmospheres: Earth has one, Saturn essentially is one, part of the solar system’s quartet of gas giants that also includes Jupiter, Uranus and Neptune.

With a vastly larger atmosphere than Earth’s, Saturn also has vastly larger storms—and none is as impressive as the huge cyclones that spin at its north pole, each as big around as the entire Earth, with winds that whip at 300 mph (483 k/h). The storms, first photographed by the Cassini spacecraft, which has been orbiting Saturn since 2004, have always been a mystery. But now, a paper published in Nature Geoscience by a team of researchers headed by planetary scientist Morgan O’Neill of MIT may explain things.

One thing O’Neill and her colleagues knew was that understanding cyclones on Earth would provide only limited help in understanding them on Saturn. The Earthly storms can’t form without a fixed surface beneath them—especially a wet, fixed surface, which provides the friction that allows winds to drag and converge and the warm water that serves as the storms’ rocket fuel.

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To understand how things work on Saturn, the researchers had to develop a computer model that recreated the planet’s gassier, drier, deeper and more turbulent atmosphere. They then ran hundreds of simulations over the course of days to try to see how cyclones could form at all and why they would converge into one super storm at the top of the planet. The computer delivered the goods.

Around the planet, the models showed, small vortices develop as a result of temperature differences in the atmosphere interacting with condensed water and ammonium hydrosulphide. The storms spin in two directions at once, with the bottom half moving one way—either clockwise or counterclockwise—and the top half moving the other. The rotation of the planet drags the storms toward the poles, in a process called beta drift. A second process, called beta gyre, surrounds each mini-cyclone, tearing it in two, with the upper half of each moving toward the equator, where they have room to disperse, and the top half continuing toward the poles, where they converge. The result: lots of mini-storms producing one massive, long-lived one at the top of the planet.

Why does any of this matter—aside from the fact that it’s an exceedingly elegant solution to an exceedingly stubborn riddle about Saturn’s behavior? For one thing, it provides some rules that help explain atmospheric behavior on other worlds. Exceedingly large planets like Jupiter are unlikely to have suprcyclones at their poles because the size of the individual storms is too small relative to the size of the overall world. Smaller gas giants like Neptune could well have polar cyclones. All that, in turn, could lead to greater understanding of exoplanets—those orbiting other stars.

Oh, and finally there’s this: Saturn’s atmosphere is just hypnotically beautiful, as this gallery of pictures suggests. Understanding how it works doesn’t increase that beauty any, but it does help you appreciate it more.

The original version of this story misidentified the gender of lead researcher Morgan O’Neill. She is a woman.

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