Saturn's tempests, which can grow into bright bands that encircle the entire planet, are on a natural timer that is reset by each subsequent storm, according to a press release from NASA's Jet Propulsion Laboratory.
During 140 years of telescope observations, great storms have erupted on Saturn six times. Cassini and observers on Earth tracked the most recent of these storms from December 2010 to August 2011. During that time, the storm exploded through the clouds, eventually winding its way around Saturn.
In a paper published online on Tuesday in the journal Nature Geoscience, scientists describe the effect they believe is responsible for the periodic outbursts. The basic idea is that water vapor is heavier than the hydrogen and helium that make up the bulk of Saturn's atmosphere, so once each giant storm dumps its huge mass of rain, the air within the clouds is left lighter than the atmosphere below. For a time, this situation shuts off the process of convection - in which warm, moist air rises, and cool, dense air sinks - that creates new clouds and storms.
"For decades after one of these storms, the warm air in Saturn's deep atmosphere is too wet, and too dense, to rise," said study lead Cheng Li, a graduate student at the California Institute of Technology in Pasadena, according to the press release. "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."
The researchers suggest Saturn's extra-wet interior might explain why the planet has such epic tantrums, compared to a less-wet planet like Jupiter. If Saturn's deep atmosphere were drier, scientists would expect continuous, smaller storms, as observed on Jupiter, Li said. Instead, Saturn's outbursts are episodic and quite explosive.
"Previous studies using spectroscopy have shown that Saturn's interior is enriched in methane and other volatiles, by two or three times, compared to Jupiter," said co-author Andrew Ingersoll, a member of the Cassini science team, also at Caltech, according to the press release. "From there, it's a short leap to expect that Saturn is also rich in oxygen, which is also a volatile and a big part of every H2O molecule." (Volatiles are elements and chemical compounds that change from solid to liquid or gas at relatively low temperatures.)
Scientists are interested in understanding the amount of oxygen and other volatile ingredients in Saturn and Jupiter. These ingredients provide important clues about the formation of the two planets -which are thought to have formed before all the others - and conditions in the early solar system.
The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. JPL, a division of Caltech, manages the mission for NASA's Science Mission Directorate in Washington.
