Diagram courtesy ESO, University of Oxford, L.N. Fletcher, and T. Barry

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Saturn's northern hemisphere superstorm pictured in visible light (far left) and in infrared, which reveals both surface (pictures three and five) and lower layers (two and four) of the gaseous planet.

Diagram courtesy ESO, University of Oxford, L.N. Fletcher, and T. Barry

Giant Saturn Storm Revealed; Wider Than Earth

Probe, telescope penetrate tempest for unprecedented study.

Using a distant spacecraft and a giant telescope, astronomers have unmasked the full ire of a storm so big that it encircles Saturn, a planet nearly ten times bigger than Earth.

Astronomers have been watching this northern-hemisphere storm since December 2010, when a bright plume of gas and ice bubbled up to the surface of the gaseous sphere that makes up Saturn.

The disturbance has since expanded by riding easterly winds blowing at up to about 220 miles an hour (100 meters a second). But until now very little has been known about the workings of the storm, its depth, and how it affects the ringed planet.

Now a new study, released Thursday by the journal Science, says the Saturn storm's effects reach about 370 miles (600 kilometers) into the stratosphere, according to observations made both by NASA's Cassini probe and the European Southern Observatory's Very Large Telescope array in Chile.

By comparison, thunderstorms on Earth usually top out at a height of 12.5 miles (20 kilometers)—and none of them circle our entire planet, despite its comparatively small size.

"It's hard to compare this storm to anything on Earth. It's simply gigantic," said study leader Leigh Fletcher, a planetary scientist at the University of Oxford in England. "The head met the tail in February of this year."

Saturn Storm Billions of Years in the Making

The Cassini probe, which has orbited Saturn and observed its motley system of moons for nearly seven years, sounded the first alarms of a brewing storm in December 2010. That's when unusual radio chatter—a sign of spiked lightning activity—emanated from the emerging plume of methane, ethane, and other material that make up the storm.

At the time, though, the probe wasn't able to analyze the storm's deeper layers—the necessary instrument, called the composite infrared spectrometer, must be programmed months in advance.

A second chance appeared in January 2011, however, so Fletcher brought in the cavalry. He convinced the Very Large Telescope's operators to monitor the storm's temperatures in high-resolution at the same time as Cassini observed the tempest in infrared, which allowed experts to infer wind speeds and the structure of the storm.

The combined observations suggest the storm was spawned by a layer of water clouds about 190 miles (300 kilometers) beneath the outer edge of the gas-giant planet. The cloud layer fueled an upwelling of gas and ice, followed by downwelling flanks that were about 20 degrees Celsius (36 degrees Fahrenheit) warmer than usual.

When Saturn formed about 4.6 billion years ago, it retained some heat from its formation. Fletcher said it's this warmth—not the sun's—that powers the planet's infrequent weather. In effect, the storm has been billions of years in the making.

Saturn's gaseous surface rotates every ten hours, but the planet takes about 30 years to orbit the sun. And because Saturn tilts toward the sun, like Earth, it has seasons—each lasting about 7.5 years.

The monster storm happened to usher in the second largest planet's latest spring—and an unprecedented opportunity.

"During the last spring on Saturn, we didn't have this kind of instrumentation available to us," Fletcher said. "It will be interesting to follow this storm and see if its dynamics apply to [other gas-giant] planets, like Jupiter, Uranus, and Neptune."