Wednesday, March 31, 1999 Published at 14:33 GMT 15:33 UK
Changing seasons on Uranus
Uranus: The atmosphere, the rings and a moon
By BBC News Online Science Editor Dr David Whitehouse
A new time-lapse movie made from Hubble Space Telescope images shows, for the first time, the cycle of decades-long seasons on the distant planet Uranus.
At present the Northern Hemisphere of Uranus is just emerging from a winter that has lasted about 20 Earth-years.
The latest Hubble pictures show that the sun is warming the atmosphere out of its cold hibernation. Slowly the giant world of mostly hydrogen and helium is stirring.
Almost featureless, Uranus was once considered one of the dullest planets. However, Hubble's movie reveals it to be a dynamic planet with the brightest clouds in the outer solar system.
Circling the planet is a ring system that can be seen in the movie to wobble like an unbalanced wheel.
Detailed observations reveal that the clouds are made of methane crystals that condense as warm bubbles of gas well up from deep in Uranus' atmosphere.
The movie was created by Hubble researcher Erich Karkoschka of the University of Arizona.
Although Uranus has been observed for more than 200 years, "no one has ever seen this view in the modern era of astronomy because of the long year of Uranus - more than 84 Earth years," says astronomer Heidi Hammel of the Massachusetts Institute of Technology.
Karkoschka, Hammel and other investigators used the Hubble Space Telescope from 1994 to 1998 to take images of Uranus in both visible and near-infrared light to make the movie.