Erich Karkoschka (University of Arizona) and NASA/STScI

Since the Voyager 2 fly-by in 1986, Uranus was thought to be a featureless blue ball. However, NASA's Hubble Space Telescope (HST) has revealed a dynamic world with bright cloudtops and a fragile ring system. While the HST did not discover these features, it shed new light on their importance. This time-lapse movie from the HST shows seasonal changes on the planet, including atmospheric variations as well as the wobble of the ring system. This wobble is thought to result from the gravitational tug of Uranus's many moons which affect the billions of tiny pebbles that constitute its rings. This movie zooms in on Uranus showing its moons revolving around the planet. Then, time speeds up and we see the wobble of the rings. Finally, we slow down again (note the orientation of the planet change as it orbits from 1994 to 1998) and we see to motion of the clouds and rings highlighted.

Seasonal variations on Uranus

Unlike Jupiter and Saturn, the clouds on Uranus appear relatively featureless. One reason for this is may be due to the planet's tilt. On Earth, the northern and southern mid-latitudes have seasons because our planet's rotational axis is tilted 23° with respect to the plane of its orbit. Uranus is an extreme case. It's rotational axis is tilted 98°! Therefore, as Uranus orbits the Sun, the Sun shines directly down on the north pole, then the northern latitudes, the equator, the southern hemisphere, the south pole, and so on. In the 1990s, the Sun was shining down on the southern mid-latitudes. In 2007, the Sun shined directly over Uranian equator. Because the Sun drives the weather, Uranus is a strange place with 20-year-long seasons. For a quarter of the Uranian year (one Uranian year is 84 Earth years), the Sun shines over each pole, leaving the opposite side of the planet in frigid darkness. As the atmosphere is warmed by sunlight, the methane clouds condense as warm bubbles of gas well up from deep in the atmosphere.

Ellen Cohen