Just over two decades ago, snapshots of the distinctive clouds in Saturn's equatorial region showed a jetstream that sped along at a bruising 1,700 kilometers (1,050 miles) per hour.

Now the winds have slowed to a relatively pedestrian 1,100 kph (690 mph), according to astronomers. Outside the equatorial belt, the planet's wind speeds appear not to have changed.

Their study compares pictures taken during the epic Voyager-1 flyby of Saturn in 1980-1 with images taken by NASA's orbiting telescope, the Hubble, between 1994-2002.

The discovery has thrown up useful evidence about how the weather systems function on the gas giants, the huge, enigmatic planets that dominate the chilly outer reaches of the Solar System.

Saturn is almost as large as Jupiter and they both have powerful winds in the equator. But there the similarity ends.

Jupiter's jetstreams vary far less in speed than Saturn's -- they are only believed to oscillate by around 10 percent.

Saturn orbits much farther from the Sun than Jupiter, and so gets less light and warmth than the jovian king.

Despite this, sunlight is a much bigger factor in shaping Saturn's weather than it is in Jupiter's, according to the study, which is published on Thursday in Nature, the British weekly science journal.

This is because Saturn is tilted at a sharp angle. As it crawls along its long circuit around the Sun -- each orbit takes more than 29 Earth years -- different areas of the planet are progressively warmed by the distant solar rays and then cool again.

It takes a long time for this sunlight, coupled with relative proximity to the Sun, to affect the weather patterns. But when it does, the change is dramatic.

Another likely factor in the weather mechanism is the shadow cast by Saturn's broad and beautiful system of rings, which again prevents large regions from receiving sunlight for prolonged periods.

"All these differences between the planets need to be considered when developing future general circulation models for the jet origin in the giant planets," says the study.

The lead authors are Agustin Sanchez-Lavega of the University of the Basque Country in Bilbao, Spain, and Richard French of Wellesley College, Massachusetts.

Saturn's atmosphere is comprised mainly of hydrogen and helium, which means it is a very light planet. It is more than nine times bigger than Earth, but just a ninth of its density.

SPACE.WIRE