LOS ANGELES - It began as a bright white dot in Saturn's northern hemisphere. Within days, the dot grew larger and stormier.
Soon the tempest enveloped the ringed planet, triggering lightning flashes thousands of times more intense than on Earth.
The international Cassini spacecraft and ground telescopes have been tracking the turbulence since December, visible from Earth as a type of storm known as a "great white spot."
"It's still going like crazy," said Cassini project scientist Linda Spilker of the NASA Jet Propulsion Laboratory.
Such planetwide weather disturbances are rare on Saturn, where the atmosphere is typically hazy and calm.
Since 1876, astronomers have observed only five other mega-storms on Saturn.
"This is a one-of-a-kind storm," said Andrew Ingersoll, a self-described planetary weatherman at the California Institute of Technology, who was part of the discovery team.
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Scientists have long studied weather on other planets. One of the solar system's most famous landmarks is Jupiter's Great Red Spot, a hurricanelike storm that has been raging for centuries. Landers and rovers on Mars often carried weather stations, dodged dust storms and sought favorable places to park during the winter.
An instrument aboard Cassini, which is orbiting Saturn, first picked up radio outbursts from a lightning storm Dec. 5. Around the same time, amateur astronomers saw a bright point in Saturn's northern half.
Further observations confirmed it was a brewing storm.
The storm system, which occurred during the start of spring on Saturn, grew in size and intensity, eventually stretching around the planet.
Scientists don't know exactly what stirs up the storms but think they could be linked to the change of seasons.
At the height of this storm, Cassini detected 10 lightning strikes per second. Scientists said the electrical activity emitted by the bursts was 10,000 times stronger than lightning on Earth.
The findings were described in two papers published in today's issue of the journal Nature.
The new work represents "some of the most detailed observations so far of such a dramatic event," Peter Read of the University of Oxford wrote in an accompanying editorial.
Saturn's violent weather differs from Earth and Jupiter in significant ways. Lightning storms on Earth tend to be localized and short-lived, lasting only a few hours. Jovian storms can last for days, and lightning is far more common there than on Saturn.
