If springtime on Earth were anything like it will be on Uranus, we would be experiencing waves of massive storms, each one covering the country from Kansas to New York, with temperatures of 300 degrees below zero.
A dramatic new time-lapse movie by NASA's Hubble Space Telescope shows for the first time seasonal changes on the planet. Once considered one of the blander-looking planets, Uranus is now revealed as a dynamic world with the brightest clouds in the outer Solar System and a fragile ring system that wobbles like an unbalanced wagon wheel. The clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the atmosphere of Uranus.
The movie, created by Hubble researcher Erich Karkoschka of the University of Arizona, clearly shows for the first time the wobble in the ring system, which is made of billions of tiny pebbles. This wobble may be caused by Uranus' shape, which is like a slightly flattened globe, along with the gravitational tug from its many moons.
Although Uranus has been observed for more than 200 years, "no one has ever seen this view in the modern era of astronomy because of the long year of Uranus more than 84 Earth years," said Dr. Heidi Hammel, Massachusetts Institute of Technology.
The seasonal changes on Earth are caused by our planet's rotational pole being slightly tilted. Consequently, the Earth's Southern and Northern hemispheres are alternately tipped toward or away from the Sun as the Earth moves around its orbit. Uranus is tilted completely over on its side, giving rise to extreme 20-year-long seasons and unusual weather. For nearly a quarter of the Uranian year, the sun shines directly over each pole, leaving the other half of the planet plunged into a long, dark, frigid winter.
The Northern Hemisphere of Uranus is just now coming out of the grip of its decades-long winter. As the sunlight reaches some latitudes, it warms the atmosphere. This appears to be causing the atmosphere to come out of a frigid hibernation and stir back to life. Uranus does not have a solid surface, but is instead a ball of mostly hydrogen and helium. Absorption of red light by methane in the atmosphere gives the planet its cyan color.
Uranus was discovered March 13, 1781, by William Herschel. Early visual observers reported Jupiter-like cloud belts on the planet, but when NASA's Voyager 2 flew by in 1986, Uranus appeared as featureless as a cue ball. In the past 13 years, the planet has moved far enough along its orbit for the sun to shine at mid-latitudes in the Northern Hemisphere. By the year 2007, the sun will be shining directly over Uranus' equator.
Karkoschka, Hammel and other investigators used Hubble from 1994 through 1998 to take images of Uranus in both visible and near-infrared light.
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