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Saturn's cyclones: From little storms, big storms grow

The permanent, turbulent colossal storms at Saturn's poles are a mystery -- but a new model may explain what causes them.

False-colour image of the eye of the storm at Jupiter's north pole, dubbed "The Rose". NASA/JPL-Caltech/SSI

At Saturn's north and south poles, enormous storms rage around vortices, with the northern storm bordered by a strange hexagonal pattern and large enough to fit four Earths inside.

Plenty of planets have storms. Perhaps the most famous is Jupiter's Great Red Spot, yet Saturn's polar storms contain a prevailing mystery. We don't actually know what causes them. While they bear some similarities to Earth's cyclones, the latter are caused by a combination of heat and moisture from the oceans. Saturn only has very small amounts of water, never mind oceans.

A new model by researchers at Massachusetts Institute of Technology proposes that the answer could be storms -- other storms raging elsewhere on Saturn.

"There's no surface [on Saturn] at all -- it just gets denser as you get deeper," said lead author of the paper published today in Nature Geoscience Morgan O'Neill, a former MIT PhD student, now a postdoc at the Weizmann Institute of Science in Israel. "If you lack choppy waters or a frictional surface that allows wind to converge, which is how hurricanes form on Earth, how can you possibly get something that looks similar on a gas giant?"

To figure it out, the team developed a model of Saturn's atmosphere and simulated multiple small, short-lived thunderstorms forming across the planet. What they found was that the planet's rotation caused the thunderstorms to drift towards the poles in a phenomenon known as beta drift. The storms on Saturn were able to generate enough atmospheric energy at the poles to create a much bigger and longer-lived cyclone.

On Earth, beta drift drives cyclones without requiring water. The storm rotates in one direction at the surface, and the opposite direction in the upper atmosphere. Combining a circulating storm with a planet's rotation generates a spiral motion called a beta gyre. These tear the cyclone in half, with the top half heading for the equator and the bottom half making its way towards the poles.

Saturn's north pole, imaged by Cassini in May 2014. NASA/JPL-Caltech/Space Science Institute

Across hundreds of storm simulations in the model of Saturn's atmosphere, running for a period of a hundred days each, the team observed multiple thunderstorms reaching a point of beta drift, eventually accumulating enough atmospheric circulation to create the larger polar cyclones.

"Each of these storms is beta-drifting a little bit before they sputter out and die," O'Neill said. "This mechanism means that little thunderstorms -- fast, abundant, but not very strong thunderstorms -- over a long period of time can actually accumulate so much angular momentum right on the pole, that you get a permanent, wildly strong cyclone."

Two factors determine whether a cyclone develops at the poles: the size of the planet relative to the size of an average storm on that planet and how much storm-induced energy is in its atmosphere. Based on these parameters, the team determined that Neptune is likely to have polar cyclones that come and go, whereas Jupiter is unlikely to have any at all, since it is so large and most of its storms would be very small in comparison. Because Jupiter isn't tilted in a direction where we can easily see its poles, however, it's hard to tell if the model is accurate. When Jupiter probe Juno arrives in orbit around the planet in 2016, NASA scientists should receive more data about its poles.

The team also believes that the model may be used to predict and gauge atmospheric conditions on planets outside the solar system if scientists can locate cyclone-like hotspots on exoplanets.