Three months before Saturn arrival, the Cassini spacecraft caught two storms in the act of merging into one larger storm. This is only the second time this phenomenon has been observed on the ringed planet.

"Merging is one of the distinct features of storms in the giant planet atmospheres," said Dr. Andrew Ingersoll, member of the Cassini imaging team and professor of planetary science at the California Institute of Technology in Pasadena, Calif.

The storms, which appear as spots in the southern hemisphere, are seen moving west, relative to the rotation of Saturn's interior, for about a month before they merged on March 19 through 20. Credit: NASA/JPL/Space Science Institute Download larger verison here

"On Earth, storms last for a week or so and usually fade away when they enter the mature phase and can no longer extract energy from their surroundings. On Saturn and the other giant planets, storms last for months, years, or even centuries. Instead of simply fading away, many storms on the giant planets end their lives by merging. How they form, however, is still uncertain," Ingersoll said.

With diameters close to 1,000 kilometers (621 miles), both storms were seen moving west, relative to the rotation of Saturn's interior, for about a month before they merged on March 19-20, 2004.

The northern storm moved about twice as fast as the southern storm, 11 meters versus 6 meters per second (25 vs. 13 mph) respectively. They approached each other like two cars on a highway and spun around each other in a counterclockwise direction as they merged. This is the opposite of how hurricanes spin in the southern hemisphere on Earth.

Just after the merger, on March 20, the new storm was elongated in the north-south direction, with bright clouds on either end. Two days later the storm settled into a more circular shape and the bright clouds were spread around the circumference to form a halo. Whether the bright clouds are particles of a different composition or simply at a different altitude is uncertain.

Although these storms moved slowly west, others at Saturn's equator move east at speeds up to 450 meters per second (1,000 miles per hour). That is about 10 times the speed of Earth's jet streams and three times greater than the equatorial winds on Jupiter.

"Saturn is the windiest planet in the solar system," Ingersoll said. "And that's a huge mystery. We'll be getting closer to the planet all the way through June, so maybe we'll find out," he said.

Images from the Voyager spacecraft flybys of Saturn in August 1981 show storms partially merging. But to see them with Cassini, this far out from Saturn, is a mouthwatering surprise to scientists, because they will get even closer during the spacecraft's four-year Saturn tour. "I'm optimistic because these images are already so good. The best is yet to come," Ingersoll said.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini- Huygens mission for NASA's Office of Space Science, Washington. The Cassini orbiter, including the two cameras onboard the spacecraft, was designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.