Uranus is a relatively little explored planet of the Solar System. But incredibly detailed images from Earth are shedding new light on its mysterious windy weather. 

A pair of images of Uranus from the Keck II telescope, with the north pole to their right, reveal new details about the planet’s enigmatic atmosphere. Credit: Lawrence Sromovsky, Pat Fry, Heidi Hammel, Imke de Pater

In January 1986, NASA’s Voyager 2 probe made a close flyby of this ice giant, which is the seventh planet from the Sun. But unfortunately the pictures it sent back showed a bland, featureless atmosphere.

Today however astronomers are getting much more helpful views of weather patterns in Uranus’ cloud tops by using advanced instrumentation and special filters on a powerful telescope here on Earth. They are the sharpest pictures ever taken of the distant planet.

High-resolution infrared images, taken from the Keck II telescope on the Mauna Kea mountain peak in Hawaii, were shown off this week at a gathering of the American Astronomical Society’s Division of Planetary Sciences at Reno, Nevada.

They clearly show the bizarre and complex behaviour of weather on Uranus which appears to be one of the coldest and windiest places in the Solar System.

Uranus has a deep, blue-green atmosphere which is composed mainly of hydrogen helium and methane. Though the planet does not generate much energy, winds blow mainly from east to west and speeds of up to 900 km per hour (560 mph). Cloud-top temperatures sink to minus 218 C (-360F) which is cold enough to freeze methane.

Despite the speed of the winds, Uranus’s weather systems are probably a lot less violent than storms we see on Earth, but they behave in bizarre ways, according to Larry Sromovsky, a University of Wisconsin-Madison planetary scientist who led the new study.

He said: “Some of these weather systems stay at fixed latitudes and undergo large variations in activity. Others are seen to drift toward the planet’s equator while undergoing great changes in size and shape. Better measures of the wind fields that surround these massive weather systems are the key to unravelling their mysteries.”

Dr Sromovsky and his team spent two nights enjoying near perfect conditions on Mauna Kea capturing their images and they were delighted with the result. He says they are the “most richly detailed views of Uranus yet obtained by any instrument on any observatory. No other telescope could come close to producing this result.”

But detail that was previously hidden presents the scientists with a puzzle because the force driving the weather must be the Sun, since there is no detectable internal energy source.

Dr Sromovsky says: “But the Sun is 900 times weaker there than on Earth because it is 30 times further from the Sun, so you don’t have the same intensity of solar energy driving the system. Thus the atmosphere of Uranus must operate as a very efficient machine with very little dissipation. Yet the weather variations we see seem to defy that requirement.”

New features found by the team include a scalloped band of clouds just south of Uranus’ equator and a swarm of small convective features in the north polar regions of the planet, features that have never been seen in the southern polar regions.

Dr Sromovsky says: “This is a very asymmetric situation. There is certainly something different going on in those two polar regions.”

One possible explanation being explored by the scientists is that methane is pushed north by an atmospheric conveyor belt toward the pole where it wells up to form the convective features observed.

Co-investigator and expert in the atmospheres of the outer planets Dr Heidi Hammel, of the Association of Universities for Research in Astronomy (AURA), has seen plenty of amazing pictures before in her work with the Hubble Space Telescope. But the Keck pictures made her eyes pop.

She said: “My first reaction to these images was ‘wow’ and then my second reaction was ‘WOW’. These images reveal an astonishing amount of complexity in Uranus’s atmosphere. We knew the planet was active, but until now much of the activity was masked by noise in our data.”

Other members of the observing team were Dr Sromovsky’s colleague Pat Fry, also of UW-Madison, and Imke de Pater of the University of California at Berkeley.