Causes of epic storms on Saturn unravelled

Scientists have finally decoded the long-standing mystery of why Saturn seethes with enormous storms every 30 years.

After analysing data from NASA's Cassini mission, they found that violent windstorms which can grow into bright bands that encircle the entire planet are on a natural timer that is reset by each subsequent storm.

In 140 years of telescope observations, great storms have erupted on Saturn six times.

According to the scientists from California Institute of Technology, the idea is that water vapour is heavier than the hydrogen and helium that make up the bulk of Saturn's atmosphere.

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 Cheng Li, graduate student at California Institute of Technology.

"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," he added.

Li thinks the episodic nature of the storms indicates Saturn's deep atmosphere contains more water, relative to the other atmospheric constituents, than Jupiter.

The researchers suggest Saturn's extra-wet interior might explain why the planet has such epic tantrums, whereas Jupiter does not.

"If Saturn's deep atmosphere were drier, scientists would expect continuous, smaller storms as observed on Jupiter. Instead, Saturn's outbursts are episodic and quite explosive," the authors noted.

Other observations by ground and space-based telescopes have hinted at a wet interior for Saturn.

Scientists are now 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 paper was published online in the journal Nature Geoscience.