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What fuels Saturn's superstorms? New theory explains 'great white spots'

They're massive, violent and full of lightning, and they only arrive every 20 to 30 years. What drives Saturn's storms may be the weight of water.

Cassini caputred the progression of a massive storm known as a "great white spot" on Saturn from late 2010 into 2011. NASA/JPL-Caltech/Space Science Institute

When it comes to spots in our solar system, the big red one on Jupiter tends to get all the attention. But Saturn has spots of its own, and they're no less impressive. The details of how they develop, however, have puzzled scientists. A theory published by two Caltech planetary scientists this week in the journal Nature Geoscience may shed some light on the phenomena.

Just like Jupiter's Great Red Spot, Saturn's "great white spots" are the result of storms. The lightning-filled systems rip across the planet every 20 to 30 years. The spots themselves can be as big as the Earth and the tails they leave as they rage throughout Saturn's atmosphere can be so long that they wrap around the entire planet, meeting their own heads.

Six such spots -- which are big enough to be seen from a telescope on Earth -- have been observed on Saturn in the past 140 years, according to a report about the research from the California Institute of Technology.

To figure out what takes each spot so long to form, Caltech Professor of Planetary Science Andrew Ingersoll and graduate student Cheng Li used numerical modeling. Their calculations led them to posit that the storms -- and their long incubation time -- come about as an interaction between the outer and inner atmospheres of Saturn.

When the outer layer, which is mostly hydrogen and helium, releases its relatively heavy water molecules in the form of rain, it is left lighter than the layer beneath it. This difference between the two layers keeps warm moist air from rising -- for a time. Eventually, though, the outer layer cools to the point that the warm moist air below it is drawn up through convection. This, they say, is what sparks one of the planet's massive thunderstorms, which we observe as a great white spot.

"For decades after one of these storms, the warm air in Saturn's deep atmosphere is too wet, and too dense, to rise," study leader Li told NASA, which funded the study. "The air above has to cool off, radiating its heat to space, before its density is greater than that of the hot, wet air below. This cooling process takes about 30 years, and then come the storms."

Li and co-author Ingersoll found that their proposed storm theory syncs up nicely with observations of the planet's 2010 great white spot as seen by NASA's Cassini spacecraft, which took six months to encircle the entire planet.