For the last decade, astronomers have struggled to figure out what was causing the mysterious massive cyclones seen on Saturn. Now, new research has revealed that multiple small thunderstorms are to blame.
These cyclones, first observed in 2008 by NASA's Cassini spacecraft, are each as wide as the Earth and whip up roughly 300 mph winds.
While cyclones on Earth are fueled by the heat and moisture of the oceans, no such bodies of water exist on Saturn.
"There's no surface at all - it just gets denser as you get deeper," Morgan O'Neill from MIT, the study's lead author, said in a statement. "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?"
According to new findings published in the journal Nature Geoscience, over time, small, short-lived thunderstorms across the planet may build up angular momentum, or spin, within the atmosphere - ultimately stirring up a massive polar vortex, or cyclone, that can last for years.
To arrive at this conclusion, a team of scientists from the Massachusetts Institute of Technology (MIT) developed a model of Saturn's atmosphere and simulated the long-term effect of multiple small thunderstorms forming across the planet.
Despite the fact that each thunderstorm is relatively small, collectively, they pull so much air towards the planet's poles that it generates enough energy to cause a massive, long-lasting cyclone.
However, whether a cyclone eventually develops depends on two factors, researchers say. First, the size of the planet relative to the size of an average thunderstorm on it is a factor, and how much storm-induced energy is in the planet's atmosphere also plays a part.
Specifically, the larger an average thunderstorm compared to a planet's size, the more likely a polar cyclone is to develop.
"Before it was observed, we never considered the possibility of a cyclone on a pole," said O'Neill. "Only recently did Cassini give us this huge wealth of observations that made it possible, and only recently have we had to think about why [polar cyclones] occur."
But this still does not answer the question of how these thunderstorms form in the first place, given that Saturn lacks an essential ingredient for brewing up such storms: water. The MIT team believes the solution is a phenomenon called "beta drift," by which a planet's spin causes small thunderstorms to drift toward the poles. Beta drift can even drive the motion of hurricanes on Earth, without needing water.
When a storm forms, it spins in one direction at the surface, and the opposite direction toward the upper atmosphere.
"The whole atmosphere is kind of being dragged by the planet as the planet rotates, so all this air has some ambient angular momentum," O'Neill explained. "If you converge a bunch of that air at the base of a thunderstorm, you're going to get a small cyclone."
The team ran hundreds of simulations for hundreds of days each, and found that beta drift allowed the thunderstorms to build up enough air circulation to trigger a large cyclone.
Interestingly, the researchers surmise that based on the two aforementioned parameters (average thunderstorm size relative to planet's size, and the energy within a planet's atmosphere), Neptune, another gas giant, may also experience similar polar cyclones - albeit on a fleeting basis.
The team plans to explore this possibility in the future, as well as use their study model to gauge atmospheric conditions on exoplanets with detectable cyclones or hotspots.
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