Uranus

Steven Dutch, Professor Emeritus, Natural and Applied Sciences,University of Wisconsin - Green Bay

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Vital Statistics

• Averages 1783 million miles from the Sun.
• Perihelion, 1703 million; Aphelion, 1866 million.
• Takes 84 years to circle the Sun.
• 31,763 miles in diameter, 14.5 times as massive as Earth.
• Chemically like Jupiter (88% hydrogen, 12% helium, traces of methane and acetylene in itsatmosphere) but denser, probably with a rocky core about the size of the Earth.
• Uranus rotates in 17 hours, and has the strangest rotation of any planet.
• Its rotation axis lies nearly in the plane of its orbit, so first one pole, then the other, faces the Sun for years on end. The polar regions actually get more sunlight than the equator.
• Uranus was pole-on to the Sun when Voyager visited it in 1986.
• Uranus is covered by a thick blue haze that makes it nearly featureless as seen from Earth or even from Voyager.
• Its magnetic field is unique--inclined 60 degrees to its rotation axis and off-center with respect to the planet.

Uranus as Seen From Earth

• Discovered accidentally by William Herschel in 1781.
• Can be magnitude 5.5, borderline naked-eye visibility.
• A number of observers back to the 1600's catalogued Uranus as a star, not recognizing its planetary nature.

The Moons of Uranus

Unlike the other planets, the moons of Uranus are not named from mythology. Titania,Miranda, and Oberon are from Shakespeare, Ariel and Umbriel from Alexander Pope. All are icy.

Oberon

• 362,000 miles from Uranus
• A cratered ice moon.
• Voyager saw one peak 10 km high on the edge of Oberon.
• Parts of Oberon are covered with a dark coating probably formed when water vented from within; otherwise, Oberon has no internal activity.
• 965 miles in diameter.

Titania

• 271,000 miles from Uranus
• Criss-crossed with faults and rift valleys.
• The largest of Uranus' moons, 990 miles in diameter.

Umbriel

• 165,500 miles from Uranus
• 740 miles in diameter.
• Covered with a dark coating, source unknown. Also unknown is why Umbriel alone is so dark.
• Heavily cratered, otherwise inactive.

Ariel

• 118,700 miles from Uranus
• 725 miles in diameter; is almost entirely covered with faults and rift valleys.
• Some close-up views suggest ice flow on the surface.

Miranda

• 80,400 miles from Uranus,
• 300 miles in diameter.
• Even after years of planetary imaging, Miranda is incredible. A vast network of v-shaped ridges, nicknamed the Chevron covers part of Miranda, as do two large ovals made of concentric elliptical grooves and ridges. These have been nicknamed Racetracks. These bizarre markings are totally unlike anything else in the Solar System and may be the scars left after Miranda was shattered by impact and re-assembled.
• There is also an enormous rift valley with a sheer cliff three miles high.

Newly Discovered Moons.

• 1985UI, dubbed "Puck" (not from its shape but from Shakespeare again) is 100 miles in diameter and 53,400 miles from Uranus.
• Nine other moonlets, 10-60 miles across and 30,000-47,000 miles from Uranus, were discovered by Voyager. In keeping with the other moons, they are named for Shakespearean heroines:
  • Cordelia 26 km diameter, 49752 km distance.
  • Ophelia 32 km diameter, 53764 km distance.
  • Bianca 44 km diameter, 59165 km distance.
  • Cressida 66 km diameter, 61777 km distance.
  • Desdemona 60 km diameter, 62659 km distance.
  • Juliet 85 km diameter, 64358 km distance.
  • Portia 110 km diameter, 66097 km distance.
  • Rosalind 60 km diameter, 69927 km distance.
  • Belinda 70 km diameter, 75255 km distance.
• Additional satellites have been discovered by re-examination of Voyager data and ground-based studies. The count as of 2008 is 33.

The Rings of Uranus

• Discovered in 1977 when Uranus eclipsed or occulted a star, the rings were photographed by Voyager.
• Nine were detected from Earth and at least one more by Voyager.
• The rings are dark, made of fairly large pieces (inches or feet rather than dust-sized) and very narrow--only a few miles wide. One, observed in detail, is only 2,000 feet wide.
• The tiny moons of Uranus discovered by Voyager help "shepherd" the rings, but it is still a mystery how rings so thin and sharp can exist.

References

• Robert H. Brown and Dale P. Cruickshank, 1985, The Moons of Uranus, Neptune, and Pluto. Scientific American, vol. 253, no. 1, pp. 38-47
• Cuzzi, Jeffrey N.; Esposito, Larry W., The rings of Uranus, Scientific American, p52-4+ July 1987
• Johnson, Torrence V.; Brown, Robert Hamilton; Soderblom, Laurence A., The moons of Uranus, Scientific American, v. 256 p48-60 April 1987
• Voyager 2 Uranus Encounter, Special issue of Science, vol. 233, July 4 , 1986.

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Created 20 May 1997, Last Update 11 January 2020