NASA's Cassini spacecraft and a European Southern Observatory1 ground-based telescope tracked the growth of a giant early-spring storm in Saturn2's northern hemisphere that is so powerful it stretches around the entire planet. The rare storm has been wreaking3 havoc4（肆虐，造成破坏） for months and shooting plumes5 of gas high into the planet's atmosphere. Cassini's radio and plasma6 wave science instrument first detected the large disturbance7, and amateur（业余的） astronomers8 tracked its emergence9 in December 2010. As it rapidly expanded, its core developed into a giant, powerful thunderstorm. The storm produced a 3,000-mile-wide (5,000-kilometer-wide) dark vortex, possibly similar to Jupiter's Great Red Spot, within the turbulent（骚乱的） atmosphere.
The dramatic effects of the deep plumes disturbed areas high up in Saturn's usually stable stratosphere, generating regions of warm air that shone like bright "beacons10" in the infrared11. Details are published in this week's edition of Science Magazine.
"Nothing on Earth comes close to this powerful storm," says Leigh Fletcher, the study's lead author and a Cassini team scientist at the University of Oxford12 in the United Kingdom. "A storm like this is rare. This is only the sixth one to be recorded since 1876, and the last was way back in 1990."
This is the first major storm on Saturn observed by an orbiting spacecraft and studied at thermal13 infrared wavelengths14, where Saturn's heat energy reveals atmospheric15 temperatures, winds and composition within the disturbance.
Temperature data were provided by the Very Large Telescope (VLT) on Cerro Paranal in Chile and Cassini's composite infrared spectrometer (CIRS), operated by NASA's Goddard Space Flight Center in Greenbelt, Md.
"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 meandering16 jet streams and forming giant vortices -- and disrupting Saturn's slow seasonal17 evolution," said Glenn Orton, a paper co-author, based at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
The violence of the storm -- the strongest disturbances18 ever detected in Saturn's stratosphere -- took researchers by surprise. What started as an ordinary disturbance deep in Saturn's atmosphere punched through the planet's serene19 cloud cover to roil20 the high layer known as the stratosphere.
"On Earth, the lower stratosphere（同温层） is where commercial airplanes generally fly to avoid storms which can cause turbulence," says Brigette Hesman, a scientist at the University of Maryland in College Park who works on the CIRS team at Goddard and is the second author on the paper. "If you were flying in an airplane on Saturn, this storm would reach so high up, it would probably be impossible to avoid it."
Other indications of the storm's strength are the changes in the composition of the atmosphere brought on by the mixing of air from different layers. CIRS found evidence of such changes by looking at the amounts of acetylene（乙炔） and phosphine（磷化氢） , both considered to be tracers of atmospheric motion. A separate analysis using Cassini's visual and infrared mapping spectrometer, led by Kevin Baines of JPL, confirmed the storm is very violent, dredging up larger atmospheric particles and churning up ammonia（氨） from deep in the atmosphere in volumes several times larger than previous storms. Other Cassini scientists are studying the evolving storm, and a more extensive picture will emerge soon.
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