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Seasons on Uranus
Erich Karkoschka (University of Arizona) and NASA/STScI
Video: 3 MB, MPEG
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
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Seasons on Saturn
Bryan Preston (NASA/STScI)
Video: 269 kB, MPEG
As Saturn revolves around the Sun on its 29-year orbit, its ring system changes orientation for observers on Earth. At times, the rings are edge-on and almost invisible, other times they are tilted, giving us a more face-on view. Like Earth, Saturn's rotational axis is tilted with respect to its orbital plane. This tilt is 27°, similar to the 23° tilt of Earth. As Saturn makes its way around the Sun, sometimes the northern hemisphere is tilted toward the Sun, sometimes the southern hemisphere receives more direct sunlight. The rings are aligned with the Saturnian equator. Therefore, when there is an equinox on Saturn (that is, the Sun is directly over the equator), the rings appear as a thin, almost invisible line. As Saturn approaches its solstices, the planet is tilted toward or away from us, allowing us to see more of the ring system. These Hubble Space Telescope images show Saturn's rings from an equinox (edge-on) to the Northern Hemisphere winter solstice (face-on). The images were taken between 1996 and 2000.
Saturn's Rings
Saturn's rings were discovered over 300 years ago. They are composed of mainly dusty water ice particles that range from grain size up to huge boulders. The system of rings is about 70,000 km (45,000 miles) wide and only about 10 meters (30 feet) thick. These particles all orbit Saturn, constantly colliding and bumping into one another. The rings maintain their shape because of the gravitational influence of Saturn and its many moons.
Ellen Cohen
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