Saturn, one of the windiest planets, has experienced an unexpected and dramatic change in its weather over the past 20 years or so.
By Dr David Whitehouse
BBC News Online science editor
Since the Voyager flybys of 1980-81, the windspeed in its equatorial regions has declined by about 40%, from 1,700 km to 1,000 km per hour.
It's not quite as breezy down there as it was
Over 100 high-resolution images taken by the Hubble Space Telescope allowed more than 300 cloud features to be monitored and wind velocities measured.
The change is probably explained by Saturn's seasonal cycle of 29.4 years (the length of time it takes to orbit the Sun), as well as shadowing by its ring system.
Between 1994 and 2002, the period when Hubble scrutinised Saturn, it was spring and early summer in its southern hemisphere.
It has been known for quite a while that Saturn is, at first glance, a less dramatic planet than its larger cousin Jupiter. It displays fewer features in its atmosphere.
The Voyager spacecraft encounters in 1980-81 showed Saturn's equatorial wind velocity was surprisingly high, about 1,700 km per hour.
Since then, not only has the windspeed decreased, but there is also evidence that the nature of the clouds in the equatorial region has altered.
The main features seen by the Voyagers were bright plumes. Hubble, observing later, has seen more large and complex cloud features.
Such changes have not been seen on Jupiter, which has been more intensively scrutinised during its 12-year orbit of the Sun.
Researchers, writing in the journal Nature, suggest two reasons for the difference.
First, Saturn's axis or rotation is inclined by 27 degrees, resulting in strong seasonal effects. Secondly, Saturn's rings periodically shade the atmosphere, reducing the amount of energy available.