Storm clouds gathering as spring comes to Uranus

TOM WILKIE

It's springtime on the planet Uranus. And just as April showers herald spring here in Britain, the clouds are gathering in the skies above Uranus. The difference is that the seasons on Uranus last for more than 20 of our earth years. Not even Gene Kelly would have kept on singing in the rain for that long.

The sight of a few wisps of cloud in the atmosphere of this distant gas giant planet has taken astronomers by surprise, according to Dr Heidi Hammel, of the Massachusetts Institute of Technology, who lectured on "Exploring the Giant Planets with the Hubble space telescope" at the conference.

When, a decade ago, it was visited by the Voyager space probe, the pictures revealed an apparently featureless disk, but now the Hubble Space Telescope is showing that clouds are forming, the harbinger of changing weather. "In the next few years I think there may be interesting things happening on Uranus," Dr Hammel said.

She added: "Uranus is so peculiar because its rotational axis is tipped over 90 degrees." The Earth's axis points almost vertically upwards out of the disk of the solar system whereas the axis of Uranus lies in the plane of the system. This means that, loosely speaking, for about a quarter of the orbit, the "North" Pole points at the sun, for the next quarter the equator points sunwards, for the next quarter it's the South Pole and then its the equator again.

Dr Hammel believes the reason Voyager took such disappointing pictures of the planet "just happened to be an accident of timing. Uranus has times when it is active and that depends on what season it is. We're not used to thinking about seasons that are 20 years long".

Astronomers in the 1890s had reported that they observed clouded bands on Uranus although such clouds had not been observed since, Dr Hammel said. Now the images of the Hubble telescope, which is able to picture the atmosphere of the planet developing over time are "rapidly changing our perception of these planets and how stable they are."