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Uranus is the seventh planet from the Sun; it is the third largest and fourth most massive planet in the solar system. Uranus was the first planet discovered in modern times. Though it is visible to the naked eye like the five classical planets, it was never recognised as a planet by ancient observers due to its dimness. Sir William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system. Uranus' atmosphere, although similar to Jupiter and Saturn in being composed primarily of hydrogen and helium, contains a higher proportion of "ices" such as water, ammonia and methane, along with the usual traces of hydrocarbons. It has the coldest planetary atmosphere in the solar system, with a minimum temperature of 49 K, and has a complex layered cloud structure in which water is thought to make up the lowest clouds, while methane makes up the uppermost layer of clouds. In 1986, images from the Voyager 2 space probe showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants. The wind speeds on Uranus can reach 250 m/s (560 mph).
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Ariel is the brightest and fourth-largest of the 27 known
moons of
Uranus. Discovered on 24 October 1851 by
William Lassell, it is named for a
sky spirit in
Alexander Pope's
The Rape of the Lock and
Shakespeare's
The Tempest. Like its parent planet, Ariel orbits on its side, granting it an extreme seasonal cycle. As of 2011, almost all knowledge of Ariel derives from a single
flyby of Uranus performed by the spacecraft
Voyager 2 in 1986. Ariel is the second-smallest of Uranus's five round satellites, and the second-closest to its
planet. It is believed to be composed of roughly equal parts ice and rocky material. Like all of Uranus's moons, Ariel probably formed from an
accretion disk that surrounded the planet shortly after its formation, and, like other large moons, it is likely
differentiated, with an inner core of rock surrounded by a
mantle of ice. Ariel has a complex surface comprising extensive cratered terrain cross-cut by a system of
scarps,
canyons and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to
tidal heating.
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Born in
Bolton, he made his fortune as a beer
brewer, which enabled him to indulge his interest in
astronomy. He built an observatory near
Liverpool with a 24-inch (610 mm) reflector
telescope. In 1846 Lassell discovered
Triton, the largest
moon of
Neptune, just 17 days after the discovery of Neptune itself by
German astronomer
Johann Gottfried Galle. In 1848 he independently co-discovered
Hyperion, a moon of
Saturn. In 1851 he discovered
Ariel and
Umbriel, two new moons of
Uranus.
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This highest-resolution Voyager 2 view of Ariel's terminator shows a complex array of transecting valleys with super-imposed impact craters. Voyager obtained this clear-filter, narrow-angle view from a distance of 130,000 kilometers (80,000 miles) and with a resolution of about 2.4 km (1.5 mi). Particularly striking to Voyager scientists is the fact that the faults that bound the linear valleys are not visible where they transect one another across the valleys. Apparently these valleys were filled with deposits sometime after they were formed by tectonic processes, leaving them flat and smooth. The Voyager project is managed for NASA by the Jet Propulsion Laboratory.
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