Uranus
(NASA/JPL-Caltech)
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Formation Uranus is the seventh planet from the Sun and the third largest in diameter. It is about four times the diameter of Earth. Technically, it is a gas giant although it is often referred to as an "Ice Giant". Like Neptune, it is primarily composed of hydrogen and helium, with lower concentrations of water, ammonia, methane and traces of other hydrocarbons. It is thought to have formed from an accumulation of material orbiting the Sun approximately 4.5 billion years ago. Recent modelling suggests that Uranus (and Neptune) may have formed closer to the Sun, then migrated to the current orbit.
Uranus is just barely visible to the unaided eye under ideal conditions. There are early sightings of Uranus by observers who thought it was a faint star. The identification of Uranus as a planet is credited to William Herschel in 1871. |
(NASA/Lunar and Planetary Institute)
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Structure, Atmosphere and Magnetoshpere
Uranus has not been well-studied, especially compared to the inner six planets. It has been visited by only one interplanetary probe - Voyager 2 in 1986. Our current standard model suggests that Uranus has a small rocky core (4% of total mass) surrounded by an icy mantle comprised of water, ammonia and methane ices (92% of total mass). The surface is relatively featureless in visible light and has a blue/green hue, with a thin atmosphere of hydrogen and helium (4% of total mass). The ice in the mantle is not like ice on Earth. Even though it has a crystal structure, it is fluid and hot (thousands of degrees). This special type of ice (Ice 18, or “super-ionic ice”) exists only under extreme pressure like that found within Uranus’s interior. These extreme conditions likewise may free carbon atoms from the methane molecules, which could then crystalize as diamonds raining through the icy mantel. Some physicists theorize that there are “diamond-bergs” coursing through the Uranian ocean of super-ionic ice. |
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Uranus’ internal heat is dramatically lower than that of other giant planets. The reason for this is not understood, but it appears to be the main reason that Uranus holds the record for the coldest temperatures among the planets (-224 degrees Celsius).
Uranus’ atmosphere shows faint, wispy cloud structures, with a brighter “cap” at the poles, darker bands at the equator and a brighter “collar” separating the two. There appears to be seasonal climate variation, although data describing this variation is limited because the Uranian year is so long (84 Earth years). |
(Lawrence Sromovsky, UMW/WW Keck Observatory)
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Uranus has an extreme axial tilt of nearly 98 degrees, meaning that its poles are positioned near the equator of the other planets, with the “north” pole slightly below the planetary plane. Uranus “rolls” around the Sun in its orbit. It also spins “backward” (like Venus) relative to the other planets. A day on Uranus is about 17 Earth hours. During the 84 year orbit around the Sun, each pole is in continuous darkness about half the time.
Uranus has a faint system of 13 rings. They are formed from relatively large rocky bodies (20 cm - 20 m in diameter) and most of the rings are only a few kilometers wide. Oddly, Uranus’ magnetic poles are inclined 59 degrees relative to its spin axis. The north pole is 10-11 times stronger than the south pole. The reason for this unusual configuration is unknown. Uranus’ extreme axial tilt, the orientation of the magnetic dipole and the interaction with solar wind make for a highly complex “corkscrew” magnetosphere. |
Magnetosphere of Uranus. The planet orbits in the direction of the yellow arrow, but rotates backward. The magnetic pole is inclined 59 degrees (blue arrow) to the planet's spin axis. (NASA Scientific Visualization Studio)
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Moons Uranus has 27 known moons. The largest of them is less than half the size of our moon. They are made from rock and ice in approximately equal proportions. They are named after characters in the literary works of William Shakespeare and Alexander Pope. The plane of the moons' orbit is in line with Uranus’ equator, so they too are orbiting nearly at right angles to the planetary plane.
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Missions Uranus has had only one close visit from an interplanetary probe. Voyager 2 (shown) came within 81,500 km. in 1986. It collected data on the composition of the upper atmosphere, and it discovered 10 new moons and two additional rings before speeding on to Neptune. Several future missions to Uranus have been proposed by NASA/JPL and the European Space Agency.
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