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# 7.30 Uranus

The 7th planet from the Sun, Uranus was discovered in 1781 by then amateur astronomer William Herschel. The blue world is perhaps best known for the unique axis of rotation: it orbits on its side compared to all the other planets. Most planets, like the Earth, have low obliquities, with their north pole pointing more or less "upward" (out of the plane of the Solar System), at a steep angle to the ecliptic. Uranus, on the other hand, has its pole lying nearly in the plane of the planets as it orbits around the Sun. This means that Uranus’ north and south poles alternately point almost directly at the Sun during its 84-year orbit. Seasons on the Earth, and on any planet, are caused by the planet's obliquity, and Uranus' unusual tilt caused it to have a very unusual seasonal cycle.

Uranus revolves around the Sun once every 84 Earth years. Its average distance from the Sun is roughly 3 billion km (about 20 AU). Click here for original source URL.

When the north pole of Uranus points almost directly toward the Sun, the southern hemisphere is in a long, dark winter, lasting for a quarter of the planet's revolution, or about 21 years! After the south polar winter and north polar summer, the Sun shines on the equatorial regions, providing each point on the planet both day and night during the planet's 17-hour rotation. During this season, the situation is much like that on the Earth during the fall and spring, although much colder (Uranus receives 400 times less sunlight than the Earth). After 21 more years, the south pole points approximately toward the Sun, and the southern hemisphere experiences a 21-year summer. Despite these long, intense seasons, the Voyager 2 spacecraft measured the temperature at the equator to be about the same as both of the poles — even with one pole in the midst of a 21-year-long summer and the other in a similarly long winter. The reasons for this uniformity are still a mystery but it presumably has something to do with very efficient circulation patterns within the atmosphere that smooth out the temperature.

William Herschel. Click here for original source URL

Uranus' magnetic field is also an anomaly. Not only is it tilted 60º from the planet's spin axis, it's also offset considerably from the center of the planet. The combination of this orientation and Uranus's unique rotation creates an unusual magnetic field with lines in the magnetotail (the part of Uranus' magnetic field that extends away from the Sun) twisting into corkscrew shapes. The cause of this bizarre magnetic field is also unknown.

What causes the extreme obliquity of Uranus? There are currently two different models. On one hand, scientists speculate that Uranus was hit by an unusually large planetesimal that skewed its rotation axis. A similar collision probably occurred in Earth's history, forming our Moon. The difference in outcome was probably a result of the different geometry of the two collisions. Earth may have been hit head-on, disrupting both bodies and blasting mantle material into orbit. Perhaps Uranus was merely grazed by an impact. This hypothesis is tentative, because the only evidence we have is the peculiarity of Uranus's rotation. Most of the other planets show a high degree of regularity in their orbits: they have small axial tilts, prograde rotation, and nearly circular orbits. However, a few large collisions early in the history of the Solar System may have caused other irregularities (see the article on Venus) and given some planets distinct "personalities.”

An alternative theory explains Uranus' tilt as the product of gravitational interactions rather than of collisions. If the early Solar System was much more compact, with the giant planets migrating to, rther than forming in, their current positions, it is possible that Saturn and Jupiter temporarily had a 2:1 orbital resonance. Some models show that such a resonance could have caused Uranus to get torqued onto its side. While this model is less popular, it is able to explain, unlike other models, why the moons of the outer planets typically orbit their equators rather then orbiting in the disk of the Solar System as might otherwise be expected.

Uranus in natural colors. Click here for original source URL.

While it's tilt remains one of the great mysteries of the Solar System, Uranus itself is a fairly mundane ice giant. With an orbit of 84.3 years and a typical distance of 19.2 A.U. from the Sun, Uranus is so cold that many terrestrial gases exist as ices in its blue atmosphere. At 14.5 Earth masses, it has a small rock core surrounded by an atmosphere of primarily hydrogen and helium, but also ammonia, methane and hydrocarbons.

Like the other giant outer planets, Uranus has rings made of ice and dust. These rings are extremely thin and very dark. The rings are so dark (they have such a low albedo) because over time the solar radiation incident on the rings ice has caused chemical reactions that build up dark compounds on the icy surface.

Uranus also has several moons, all named after characters in the works of Shakespeare and Alexander Pope. While over 25 satellites are known, the most important are the largest five (from closest to furthest): Miranda, Ariel, Umbriel, Titania, and Oberon. Each of these moons is large enough to be spherical in shape due to its own self-gravity, and they may be captured Kuiper Belt objects. All but Miranda (and maybe Ariel) are actually bigger than Pluto!

Uranus was imaged by the Voyager 2 spacecraft during its 1986 visit, which coincided with the planets northern winter solstice. Since then it has been regularly observed by ground and space-based telescopes at Earth. It passed through it's orbital equinox in 2007, and is now headed toward northern summer solstice in 2028.