Uranus and Neptune
Outer Worlds of the Solar System

Feb. 5, 2005, jeb PRELIMINARY

OUTLINE
• Discovery and Exploration
• Basic Properties of Uranus and Neptune
• Rotation of Uranus
• Atmospheres of Uranus and Neptune
• Magnetospheres and Internal Structure of Uranus and Neptune
• Moons of Uranus and Neptune
• Rings of Uranus and Neptune
• Golden Globe Awards for Uranus and Neptune
• Other Sources on Uranus
• Other Sources on Neptune

Discovery and Exploration

Uranus (1781)
• William Herschel, British Astronomer
  • discovered by chance while viewing sky with telescope
  • named for the mythical father of Saturn

Neptune (1846)
• Deviations in orbit of Uranus led to prediction of Neptune's orbit by two astronomers:
  • John Adams (British) and Urbain Leverrier (French)
• first observed (where predicted) by German astronomer Johann Galle

Voyager 2
• with the flybys of Uranus (1986) and Neptune (1989), our knowledge of these two outer planets expanded greatly

Basic Properties of Uranus and Neptune

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Rotation of Uranus

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• Rotation axis tilted relative to orbital plane
  • tilt angle 98^o
  • consequence:
    • north pole points directly at the Sun at one point in orbit
      • south total darkness
    • south pole points directly at the Sun 1/2 year later
      • north total darkness
• Very strange seasons:
  • poles have summers with Sun directly overhead
    winters with Sun never appearing
  • equator
• Moons also orbit in the skewed equatorial plane
• No other planet has such a large tilt

• Catastrophic event?
• Grazing collision with another planet sized body?

Atmospheres of Uranus and Neptune

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• 84% molecular hydrogen
• 14% helium
• methane (2% on Uranus and 3% on Neptune)
• (very little ammonia)
  • frozen out (much colder than Jupiter and Saturn)

• Uranus and Neptune are a bluish color (Uranus blue-green)
  • due to methane in the atmosphere
  • methane absorbs out the red

• Uranus
  • Featureless appearance
    • no internal heat source
  • Flows below surface
    • very efficient energy transfer
    • south pole (currently in total darkness) only a few degrees colder than north pole
• Neptune
  • Cloud and band structure more easily seen
    • internal heat source -> convection
  • Cloud streaks
  • Upper atmosphere slightly warmer than Uranus
  • three features studies by Voyager 2:
    • Great Dark Spot, scooter, and small dark spot
  • Great Dark Spot
    • similar to Great Red Spot on Jupiter
    • but did not survive
      • discovered by Voyager 2 (1989)
      • gone in 1994, although other features have appeared ( Hubble Space Telescope)

Magnetospheres and Internal Structure of Uranus and Neptune

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Strength of Magnetic Field

• internal magnetic fields 100 x's Earth
  • 1/10 of Saturn
• at cloud tops (4 x's Earth's radius) comparable to field strength at surface of Earth

Tilt and offsets of magnetic fields

• Uranus and Neptune magnetic fields are tilted to large angles relative to the rotation axis
  • 59^o on Uranus
  • 46^o on Neptune
• Uranus and Neptune magnetic fields are not centered on center of planet
  • large offsets

Internal Structure

• Rocky cores
  • about 10 times the Earth's mass
• no metallic hydrogen (found in Jupiter and Saturn)
• source of magnetic field differs from Jupiter and Saturn (where metallic hydrogen is responsible)
  • frozen out ammonia -> ionic layer?

Moons of Uranus and Neptune

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Uranus