A Northern Spring

As season changes, Uranus turns a paler face toward Earthbound viewers.

Studying the seasons on the planet Uranus takes a while. One year for the distant, bluish gas giant - the time it takes it to go once around the sun - is 84 Earth years.

Photographs from the Hubble Space Telescope provide new details for astronomers to scrutinize for clues about shifting conditions on Uranus, an odd ice planet that is tipped on the side, relative to the sun.

The summer solstice in its northern hemisphere -when the sun will be shining almost directly down on the north pole and almost all of the southern hemisphere will be in darkness - will occur in 2028. The northern hemisphere is now emerging into sunlight.

Long-term brightness measurements suggest the northern hemisphere of Uranus is brighter than the southern hemisphere.

'' Is that due to a difference in the thickness of the clouds?'' said Heidi B. Hammel, vice president for science at the Association of Universities for Research in Astronomy.

'' The chemistry of the clouds? The dynamics in the clouds triggered by sunlight? Some complicated combination of all of the above? We frankly don't know.''

Last year, planetary scientists agreed to put a mission to Uranus at the top of their list during a once-a-decade survey of priorities, perhaps including an orbiting spacecraft and an atmospheric probe. [ Kenneth Chang ]


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