Photos: Saturn's global storm

NASA released fresh imagery Thursday of a mega-storm that encircled Saturn for more than six months earlier this year, becoming the longest-lived weather system ever observed on the ringed planet.

The Cassini spacecraft circling Saturn also became the first probe to ever observe such a large storm from nearby. Previous system were studied from telescopes on or near Earth.

The storm emerged Dec. 5, 2010, and stretched around the planet by the end of January, according to NASA. Its north-south width eventually reached 9,000 miles.

Cassini's radio and plasma wave instrument also detected lightning from the storm, confirming it was a "convective" weather system, NASA said.

"The Saturn storm is more like a volcano than a terrestrial weather system," said Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena. "The pressure builds up for many years before the storm erupts. The mystery is that there's no rock to resist the pressure - to delay the eruption for so many years."

The storm fizzled in June, but its clouds are still present in Saturn's hydrogen-helium atmosphere today. At its peak, it covered about 2 billion square kilometers, researchers said.

"This new storm is a completely different kind of beast compared to anything we saw on Saturn previously with Cassini," said Kunio Sayanagi, a Cassini imaging team associate and planetary scientist at the University of California, Los Angeles. "The fact that such outbursts are episodic and keep happening on Saturn every 20 to 30 years or so is telling us something about deep inside the planet, but we have yet to figure out what it is."

Scientists are examining data preceding the storm's appearance to determine what may have triggered its development.

Credit: NASA/JPL-Caltech/Space Science Institute

The subtle light dot at the upper center of this image captured Dec. 5, 2010, was the genesis of a global storm that encircled the planet in a matter of weeks.

These images were taken by Cassini in spring 2011 and used to help scientists measure wind speeds in Saturn's atmosphere and inside the giant storm.

This false color snapshot from Cassini shows the head of the storm sweeping through the northern hemisphere of Saturn. The blue line represents Saturn's rings, which are viewed nearly edge-on in this image.

Scientists created this composite image from 12 pictures taken Jan. 12 as Cassini was 684,000 miles from Saturn. White and blue clouds near Saturn's equator represent high clouds and hase, while red and orange colors indicate clouds deeper in Saturn's atmosphere.

This high-resolution mosaic of Cassini imagery shows an up-close view of the churning mega-storm.

A false-color mosaic shows additional details not seen in true-color imagery.

Cassini gathered images for these composite images one Saturn day apart, showing the changes and movements of the storm. A Saturn day lasts about 10-and-a-half hours.