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Uranus

January 24, 1986, marks an auspicious occasion, for it was then that Voyager 2 flew within 50,600 miles of the cloud tops of Uranus, thus becoming the only spacecraft to have visited this planet.

a. Image
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Image of Uranus at a distance of 60,000 miles, taken by Voyager 2 about 21 minutes before its closest approach

It takes Uranus, the third largest planet, by density, 84 years to orbit the Sun, and Voyager 2 measured a day on Uranus at 17 hours, 14 minutes.

b. Image
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Unlike the other planets in the solar system, Uranus is tipped on its side so that, during its orbit of the Sun, Uranus's north and south poles are alternately exposed to sunlight. Voyager discovered that the hemisphere exposed to sunlight emitted large amounts of ultraviolet light, or "dayglow." Surprisingly, Voyager measured temperatures at the poles and the equator to be similar, although the polar regions receive direct solar heat and there is no other significant internal heat.

c. Image
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Magnetic field and axis of rotation of Uranus

Earth's magnetic field is oriented at 12 degrees from the rotational axis. Before Voyager 2 explored Uranus, there was no direct evidence of its magnetic field. However, Voyager 2 discovered that not only does Uranus have a strong magnetic field, but that the magnetic axis is offset by 59 degrees from the rotational axis and is tilted from the center of the planet by 30% of its radius, so that the magnetic poles are closer to the equator. The orientation of Uranus causes its magnetotail to be wound like a corkscrew behind the planet. Furthermore, from the orientation, it appears that the magnetic field is generated somewhere in the middle of the interior of Uranus, where there is sufficient pressure for water to conduct electricity.

d. Image
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e. Image
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Animation of Uranus's magnetosphere. Courtesy of Ralph McNutt.

Uranus's atmosphere is composed mainly of hydrogen and helium. Some previous studies had suggested that the helium component might be as much as 40%, so it came as a surprise when Voyager determined that it was only about 15%. About 2% of the atmosphere is made up of methane, which absorbs red light; hence Uranus appears blue-green.

f. Image
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Left image is a composite using blue, green, and orange filters, showing Uranus as we would see it, with a clear atmosphere. The right image exaggerates the contrasts, by using ultraviolet, violet, and orange filters. The dark polar hood is over Uranus's south pole.

Before the Voyager encounter, we knew that Uranus had at least five moons: Miranda, Ariel, Umbriel, Titania, and Oberon. Voyager obtained high-resolution images of all these moons and, in addition, discovered 10 new moons, the largest of which was named Puck.

g. Image
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Montage of Voyager 2 images showing the five original moons, which, traveling counterclockwise from the left, are Ariel, Miranda, Titania, Oberon, and Umbriel.

The smallest, innermost, and certainly the strangest of the 5 large moons is Miranda. Miranda's diameter is about 300 miles, only 1/7th the size of our Moon, yet it contains fault canyons 12 miles deep and cliffs up to 12 miles high. Researchers were amazed to discover this much geologic activity on a moon whose temperature is only -355 degrees Fahrenheit. They surmise that the gravitational pull from Uranus may have caused some form of tidal heating.

h. Image
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Close-up view of Miranda, from 19,000 miles, showing fractures, grooves, and craters

Umbriel has the oldest, darkest surface, covered with craters and showing little evidence of tectonic activity. Oberon's surface is similar. Titania, on the other hand, has huge fault systems and canyons, possibly caused by subsurface water's expanding while the interior of the moon froze. Ariel has the brightest, youngest surface of the five original moons. Icy material appears to have flowed through its fault valleys, and it seems that the older, deeper craters have been destroyed, leaving only smaller, younger ones.

i. Image
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j. Image
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i. Voyager 2 image of Uranus' satellite Oberon from 663,000 km. The dark spot to the right of center is the 206 km diameter crater Hamlet, centered at 46 S, 44E. Oberon, the most distant major satellite from Uranus has a diameter of 1523 km. North is at 11:30.
j. Voyager 2 image of Uranus' satellite Ariel taken through the clear filter from a distance of 130,000 km. Ariel's surface displays heavily cratered areas, complex rifts, and smooth plains, indicating a complicated tectonic history. This image has a resolution of 2.4 km and shows the southern portion of the 1,160 km diameter satellite. At the lower left is the 71 km diameter crater, Domovoy, centered at 72 S, 340 E. North is at 1:00.

Voyager 2 measured and imaged all nine of the previously known rings of Uranus. In addition, Voyager discovered a new ring and a system of dust rings that is only visible when backlit by the Sun. It appears the rings are younger than Uranus and were possibly formed by the disintegration of a moon hit at high velocity or destroyed by the effects of gravity.

k. Image
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This image of Uranus's rings was taken while Voyager 2 was in the shadow of the planet.

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