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Martian Polar Ice Cap
Phil James (University of Toledo), Todd Clancy (STScI), Steven Lee (University of Colorado), and NASA/STScI
Video: 1 MB, MPEG
These images appear to have been taken from above the Martian pole, but they were taken with the Hubble Space Telescope from Earth orbit. Each frame in this animation was constructed from three separate images taken in October 1996 and in January and March of 1997. The first image corresponds to early spring in the Northern Hemisphere when the polar ice cap extends to about 60° north latitude. The second image was taken in mid-spring and shows a smaller ice cap resulting from increasing atmospheric warming. The final image from early summer on Mars shows a vastly reduced polar cap; much of the carbon dioxide ice has sublimated, revealing the terrain beneath.
Martian Seasons
Mars has a cycle of seasons that is similar to Earth. The reason for this is that both planets are similarly tilted on their rotational axes: a 23.5° tilt for Earth and a 25.2° tilt for Mars. Because Mars takes about twice as long to orbit the Sun, each Martian season is about twice as long as the corresponding season on Earth. In addition, the Martian atmosphere is much thinner than Earth's atmosphere, so there is little insulation to protect the Martian atmosphere from temperature shifts due to changes on the surface or in the Mars-Sun distance.
The Mars-Sun distance
The Mars-Sun distance changes by 20 percent over the course of the year. When Mars is closest to the Sun, it is summer in the Southern Hemisphere and temperatures are up 35° F, producing dust storms that swirl around the planet and absorb sunlight, further heating the atmosphere. When Mars is farthest from the Sun, water-ice clouds reduce atmospheric temperatures. The dust particles in the atmosphere seed
these clouds, then fall to the ground. Competition between cloud cooling and dust heating drives annual as well as short-term climate changes on Mars.
Ellen Cohen
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Rotation of Neptune
Lawrence Sromovsky (University of Wisconsin-Madison), NASA/Space Telescope Science Institute
A time-lapse animation of the rotation of Neptune has been assembled from images taken by NASA's Hubble Space Telescope and the Infrared Telescope Facility on Mauna Kea, Hawaii. By observing the planet at different wavelengths, astronomers see different levels of the atmosphere. These images show Neptune's storms, dark spots, and its powerful equatorial jet stream.
Video: 480 kB, MPEG
Neptune's atmosphere
Neptune's atmosphere was first revealed to us when Voyager flew by the planet in 1989. Since then, the Hubble Space Telescope has provided scientists with more detailed views of the cloud structure and dynamics. Some features of the atmosphere include dark spots, methane cloud tops, and a complex circulatory system. The northern dark spot, discovered using the Hubble in 1995, may be a hole in the methane cloud tops, allowing one to see deeper into the atmosphere. As the air in the atmosphere flows over the spot, the particles condense to form methane ice crystal clouds. Neptune's equatorial jet stream is not driven by the Sun as it is on Earth, but is driven by an internal energy source. Neptune radiates about twice as much energy as it receives from the Sun. This energy may warm the clouds triggering the atmospheric circulation we observe.
The Mars-Sun distance
The Mars-Sun distance changes by 20 percent over the course of the year. When Mars is closest to the Sun, it is summer in the Southern Hemisphere and temperatures are up 35° F, producing dust storms that swirl around the planet and absorb sunlight, further heating the atmosphere. When Mars is farthest from the Sun, water-ice clouds reduce atmospheric temperatures. The dust particles in the atmosphere seed
these clouds, then fall to the ground. Competition between cloud cooling and dust heating drives annual as well as short-term climate changes on Mars.
Ellen Cohen
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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.
The Mars-Sun distance
The Mars-Sun distance changes by 20 percent over the course of the year. When Mars is closest to the Sun, it is summer in the Southern Hemisphere and temperatures are up 35° F, producing dust storms that swirl around the planet and absorb sunlight, further heating the atmosphere. When Mars is farthest from the Sun, water-ice clouds reduce atmospheric temperatures. The dust particles in the atmosphere seed
these clouds, then fall to the ground. Competition between cloud cooling and dust heating drives annual as well as short-term climate changes on Mars.
Ellen Cohen
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