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Uranus in the Sky. Uranus. As seen by Voyager. Uranus Highlights. Accidently discovered in March 1781 by William Herschel Fits Bode ’ s Law! Visited by Voyager 2 in 1986 Suspicious lack of internal heat – probably leads to bland appearance of atmosphere Composition: ~75% H, ~25% he

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Uranus

As seen by Voyager


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Uranus Highlights

  • Accidently discovered in March 1781 by William Herschel

    • Fits Bode’s Law!

  • Visited by Voyager 2 in 1986

  • Suspicious lack of internal heat – probably leads to bland appearance of atmosphere

  • Composition: ~75% H, ~25% he

  • Probably a small rocky core (3% of total mass)

  • Ice-rock “slush” interior

  • Bluish color derives from methane which is a good absorber of red light


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Discovery of Uranus

William Herschel at left discovered Uranus by accident. He used the 6-inch telescope at right, which he built himself.





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Oddball Properties of Uranus

  • Magnetic field:

    • About same Earth’s field

    • BUT, dipole inclined by 60o and offset from planet center by 1/3 of RU

      Caught during a field reversal???

  • Tilt:

    • Extreme tilt remains unexplained

    • Each pole spends 42 years in light and then 42 in darkness



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Rings of Uranus

  • Discovered March 1977 during a stellar occultation

  • Nine narrow rings ranging in width 10-100 km

  • Thin and faint with particles of size cm to several meters

  • Dust fills inter-ring gaps

    • Requires replenishment

  • Moons Cordelia and Ophelia act to shepherd some rings





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Share Question

Seasons on Uranus are

a) as long as Uranus' year

b) non-existent

c) one fourth of Uranus' year, or about 20 Earth years long

d) four times Uranus' year, or about 320 Earth years long


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Discovery of Neptune

  • By 1821, the orbit of Uranus clearly deviated from its predicted path which couldn’t be explained by influence of known planets.

    • Evidence for another planet!

  • “Discovered” by Adams (English, late 1845) and Leverrier (France, 1846) independently

  • 1st observed by Galle (German) on 9/23/1846






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Neptune’s Atmosphere

  • Belts

  • Zones

  • Clouds

  • Wind - 2200kph at the equator

  • Storms - ovals and Great Dark Spot

  • Great Dark Spot:

    • High pressure region in S. hemisphere, about size of Earth

    • By 1994, Spot had disappeared, and a new Dark Spot had formed at N. latitudes (HST)



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Neptune’s Great Dark Spot



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Neptune’s Magnetic Field

  • Surface field similar to Earth’s in strength

  • Magnetic axis tilted by 47o

  • Dipole offset by 1/2 RN

    Unlikely that both Uranus and Neptune are undergoing field reversals at the same time!



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Internal Energy of Neptune

  • Uranus lacks convection and therefore storms

  • Neptune does have internal heat like Jupiter and Saturn (Neptune emits 60% more than absorbs)

  • A MYSTERY!

    How can Uranus and Neptune be so different when so similar in size and mass?

    [Inhibition of convection in Uranus?]


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Rings of Neptune

Voyager 2 found rings, although there had been previous evidence for them

  • Dark and faint

  • Little intergap dust

  • Rings likely composed of dust themselves

  • Bright arcs in out ring (effect of shepherding?)




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