"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.

Click on image to enlarge. High res images online at NASA JPL

"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, and instead of simply fading away, many storms on the giant planets end their lives by merging. How they form, however, is still uncertain," said Ingersoll.

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 through 20, 2004.

The northern storm moved about twice as fast as the southern storm, 11 meters per second versus 6 meters per second (25 miles per hour versus 13 miles per hour) 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 move slowly west, storms at Saturn's equator move east at speeds up to 450 meters per second (1,000 miles per hour), which is 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," said Ingersoll, "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."

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 Cassini's four-year Saturn tour. "I'm optimistic because these images are already so good. The best is yet to come," said Ingersoll.

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