Three months before Saturn arrival, the Cassini spacecraft has
observed 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.

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.