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May 18, 2011

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Giant storm grips Saturn as scientists get unprecedented view

May 19, 2011
Courtesy of the European Southern Observatory
and World Science staff

Eu­ro­pe­an and Amer­i­can as­tro­no­mers have teamed up to study a rare storm on Sat­urn in more de­tail than ev­er be­fore pos­si­ble. 

The ringed plan­et’s gas­e­ous sur­face nor­mally looks plac­id. But once ev­ery 30 Earth years or so—one Sa­turn year—as spring washes over its north­ern half, some­thing stirs deep be­low the clouds, sci­en­tists say, lead­ing to a great plan­et-wide dis­turb­ance. 

There’s now “a gi­gantic, vi­o­lent and com­plex erup­tion of bright cloud ma­te­ri­al, which has spread to en­cir­cle the en­tire plan­et,” said Leigh Fletch­er of the Un­ivers­ity of Ox­ford, U.K., lead au­thor of a study on the events to ap­pear May 19 in the re­search jour­nal Sci­ence.

Ther­mal in­fra­red im­ages of Sat­urn from the Very Large Tele­scope (cen­ter and right) and an am­a­teur visible-light im­age (left) from Trev­or Bar­ry (Bro­ken Hill, Aus­tral­ia) ob­tained on Jan. 19 dur­ing the ma­ture phase of the north­ern storm. The sec­ond im­age is tak­en at a wave­length that re­veals the struc­tures in Sat­urn’s low­er at­mos­phere, show­ing the churn­ing storm clouds and the cen­tral cool­er vor­tex. The third im­age is sen­si­tive to much high­er al­ti­tudes in Sat­urn’s nor­mal­ly peace­ful strat­o­sphere and shows "bea­cons" of in­fra­red emis­sion flank­ing the cen­tral cool re­gion over the storm. (Cred­it: ESO/Ox­ford U./L. N. Fletcher/T. Bar­ry)

European and American astronomers have teamed up to study a rare storm on Saturn in more detail than ever before possible. The ringed planet’s gaseous surface normally looks placid. But about once every 30 years, as spring washes over its northern half—the Saturnian year lasts three Earth decades—something stirs deep below the clouds, scientists say. This, they add, leads to a dramatic planet-wide disturbance. There’s now “a gigantic, violent and complex eruption of bright cloud material, which has spread to encircle the entire planet,” said Leigh Fletcher of the University of Oxford, U.K., lead author of a study on the events to appear May 19 in the research journal Science. An instrument on NASA’s Cassini spacecraft, orbiting the giant planet, first detected the storm, which amateur astronomers also tracked last December. Astronomers analyzed it in further detail with an infrared camera on the European Southern Observatory’s Very Large Telescope at Cerro Paranal, Chile. Only the sixth such storm spotted since 1876, it’s also the first to be observed by an orbiting craft and to be studied in the thermal infrared light, which reveals temperature variations in the storm. Previous studies “have only been able to use reflected sunlight, but now, by observing thermal infrared light for the first time, we can reveal hidden regions of the atmosphere and measure the really substantial changes in temperatures and winds associated with this event,” Fletcher said. The storm may have originated deep down in the water clouds where a thunderstorm-like process triggered a huge “convective plume,” scientists say: just as hot air rises above a heater, this mass of gas would have headed up and punched through Saturn’s usually calm upper atmosphere. The huge disturbances interact with the circulating winds moving east and west and cause wild temperature swings high up in the skies. “Our new observations show that the storm had a major effect on the atmosphere, transporting energy and material over great distances, modifying the atmospheric winds — creating meandering jet streams and forming giant vortices — and disrupting Saturn’s slow seasonal evolution,” adds Glenn Orton of the Jet Propulsion Laboratory in Pasadena, Calif., another of the researchers. Some unexpected features seen through the Very Large Telescope have been dubbed “stratospheric beacons”—very strong temperature changes in the stratosphere, 250 to 300 km (155 to 190 miles) above lower atmosphere cloud tops, that show how far upward the storm’s effects extend. Saturn’s stratosphere is normally around minus 130 degrees Celsius (minus 200 Fahrenheit) at this season but the beacons are measured to be 15-20 degrees Celsius warmer. That means that though they’re invisible in to the unaided eye, they can outshine the rest of the planet in the thermal infrared light, astronomers say. They had never been detected before, so researchers aren’t sure how common they are. “We are continuing to observe this once-in-a-generation event,” Fletcher said.