Two sides of Uranus, imaged through coloured filters, showing long-lived clouds drifting across the surface (Image: Lawrence Sromovsky, UW-Madison)

Uranus may not be “the most boring planet” in the solar system after all – new images suggest the outer planet experiences weird seasonal weather patterns.

When the space probe Voyager 2 visited the planet in 1986, its surface was virtually featureless. But that view has changed dramatically with recent images showing a large number of dynamic storm systems.

In fact, a single image of Uranus taken in 2004 shows 18 distinct cloud systems – eight more than Voyager saw during its entire months-long flyby. And one set of images taken with the Keck II telescope in Hawaii in summer 2004 shows an extremely bright cloud reaching up high above the planet’s opaque methane layers in its southern hemisphere.


“We have never seen such vigorous convective activity in the southern hemisphere before,” says Imke de Pater of the University of California, Berkeley, who used the Keck II telescope to obtain the images. The difference may be a reflection of the change of seasons, but since the Uranian year is as long as 84 Earth years it will take several decades to confirm this.

Midsummer days

It was midsummer in the planet’s southern hemisphere when Voyager visited, and because Uranus is tipped over on its side – its axis is inclined 98 degrees compared with Earth’s 22.5 degrees – the entire hemisphere was in constant sunlight. Now, the spring equinox is approaching, so the Sun is nearly above the equator and the whole planet is going through a day-night cycle. The resulting changes in warming may be driving an increasingly active weather system, de Pater and colleague Heidi Hammel suggest.

Until 10 years ago, the only pictures sharp enough to show any cloud features were those from Voyager. But then images from the Hubble Space Telescope began to exceed Voyager’s resolution, and over the past four years images from the 10-metre Keck telescope, using its new adaptive optics system and an infrared camera, have done even better.

These observations have shown that some storm systems on Uranus come and go within days, while others can persist for years. The fastest winds ever recorded, seen in late 2003, were up to 420 kilometres per hour.

Observations by a group at the University of Wisconsin-Madison, also using Keck, capture dramatic weather too. These include large cloud complexes that may be driven by hurricane-like vortices. However, as the seventh planet from the Sun – receiving less solar energy – Uranus’s storms are likely to be much milder than those on Earth, says team member Lawrence Sromovsky.

Layer of chunks

The changing angle of view as Uranus slowly moves around the Sun is also making it possible to see the planet’s thin rings more clearly than ever before.

In 2007, the rings will be exactly edge-on as seen from Earth. That has made it possible to see clearly a diffuse inner ring, a thousand times fainter than the outer ten rings, which had only been glimpsed once before in a single Voyager image.

But most surprising, de Pater says, is the discovery that the rings of Uranus, unlike those of any other planet, are composed of a single layer of chunks, and apparently contain hardly any dust-sized particles. This conclusion came from detailed analysis of the changes in the polarisation of light reflected from the rings, as observed from the Keck telescope.

There is no reason, either physically or theoretically, why the rings should not form a single layer, says de Pater. “But we haven’t seen this anywhere else in the solar system, so it is a surprise.”

The findings were presented on Wednesday at the American Astronomical Society’s Division of Planetary Sciences meeting.