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UA scientists look closely at Uranus

By Sean McLachlan
Arizona Daily Wildcat
March 30, 1999
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Arizona Daily Wildcat

Photo courtesy of the Space Telescope Science Institute New Hubble Space Telescope images are giving scientists their first look at seasonal changes on Uranus. Bright spots on the planet's atmosphere are believed to be clouds of crystallized methane. The dots around the planet are some of Uranus' many moons.

Here on Earth, springtime brings images of sunny skies, mild weather and long, romantic walks.

On Uranus, springtime brings 500 mph jet streams and mild, -300 degree temperatures - leading two University of Arizona astronomers to investigate seasonal changes on Uranus using the Hubble Space Telescope images.

The only other detailed photos of Uranus were taken in 1986 when Voyager 2 passed by on its way to the outer limits of the solar system. At that time, the northern hemisphere was shrouded in darkness.

"We expect to see differences in the two hemispheres - one that has been in the sunlight for the last 40 years, and one that is just beginning to see sunlight," said Martin Tomasko, UA Lunar and Planetary Laboratory research professor.

One major difference is a system of giant jet-streams pushing through Uranus' cloud layers at hundreds of miles an hour. Another is several isolated, bright clouds - measuring about 600-1200 miles across - relatively small by Uranus' standards.

Tomasko and his fellow researcher, Erich Karkoschka, believe that the clouds are made up of methane crystals, condensed as warm bubbles of gas rise up from the lower atmosphere.

"Compared to what we have seen on Voyager, these are really big, big changes," said Karkoschka, a senior research associate at the Lunar and Planetary Laboratory.

Uranus, an immense ball of hydrogen and helium, is four times the diameter of Earth and 20 times as far away from the Sun.

The planet's northern hemisphere is tilting towards the Sun, creating springtime for that part of the planet much like the Earth.

And the event is providing a good opportunity for scientists to study how seasons change on the planet. Uranus has a longer year than Earth, making each season 21 years long.

Uranus is unique among the planets because it is tipped on its side, with one hemisphere constantly in darkness until the change of seasons brings it into sunlight.

There is no "day" or "night" like on Earth. Instead, only the change of seasons brings about light and dark.

When the south pole is facing the Sun, the southern hemisphere is bathed in light and the northern hemisphere doesn't see daylight for decades. As the planet slowly tilts, the northern hemisphere sees the Sun and the southern hemisphere gets plunged into darkness.

The Hubble telescope has also given astronomers a clearer view of the planet's rings and moons.

Uranus is not as spherical as other planets, and the strange gravity field it produces gives the rings a pronounced "wobble," Karkoschka said.

Tomasko and Karkoschka began working with Hubble Space Telescope images in 1994. Karkoschka has also used the Hubble telescope to find evidence of water ice on Uranus' five largest moons.