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Uranus and Neptune

Uranus and Neptune. Astronomy 311 Professor Lee Carkner Lecture 19. Uranus -- God of the Sky. He gives his name to Urania, the Greek muse of astronomy. Discovery of Uranus. The other planets can only be seen with a telescope

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Uranus and Neptune

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  1. Uranus and Neptune Astronomy 311 Professor Lee Carkner Lecture 19

  2. Uranus -- God of the Sky • He gives his name to Urania, the Greek muse of astronomy

  3. Discovery of Uranus • The other planets can only be seen with a telescope • Uranus was discovered in 1781 by William Herschel while surveying the sky • Herschel had built a very high quality telescope and was systematically observing the brighter stars when he found Uranus

  4. Observing Uranus • Most of our information about Uranus comes from Voyager 2 and HST • No future missions planned

  5. Uranus Facts • Size: • Orbit: 19.22 AU • Description: blue-green, featureless, tilted on its side

  6. The Rotation of Uranus • The tilt of Uranus’s axis is 98 degrees • Extreme tilt may be due to a large impact when Uranus was forming • The large tilt produces seasons where half of the planet is in sunlight and half in darkness for long periods of time

  7. Seasons on Uranus

  8. Composition of Atmosphere • Hydrogen: 84 % • Helium: 14 % • Methane (CH4): 2% • The large amount of methane gives Uranus its bluish color

  9. Structure of Atmosphere • Ammonia, Ammonium hydrosulfide and water have frozen out in the lower atmosphere where we can’t see them • Careful observations have determined that Uranus does not have alternating zones and bands • Winds mostly blow east

  10. Uranus’s Rings • As Uranus moved past a star, the star dimmed several times before being occulted by the planet • Rings are composed of dark material • They reflect very little light and are difficult to observe at optical wavelengths

  11. The Moons of Uranus • Uranus has 5 major moons and 22 minor moons • Moons are composed of mixture of ice and rock • Two of the moons shepherd the Epsilon ring • The other rings may also have shepherd moons that are too small to see

  12. Radiation Darkening • Why are the moons and rings of Uranus (and Neptune) so dark? • Impacts by high energy electrons from the magnetosphere break off the carbon atoms • Carbon soot builds up on the ice making it dark

  13. Magnetic Fields Fields on Uranus and Neptune • May be formed by motions of a liquid water mantle containing ions • The centers of the magnetic fields are offset from the center of the planet • How can the dynamo effect produce an off-centered field?

  14. Rotation and Magnetic Axis

  15. Determining Mass • How are the masses of planets determined? • We can measure the period and the orbital radius of a moon or spacecraft • The relationship between them depends on mass

  16. Neptune -- God of the Sea • The name is appropriate due to Neptune’s blue-green color

  17. The Discovery of Neptune • Was an undiscovered planet altering the orbit? • In 1846 J. G. Galle used Le Verrier’s calculations to find Neptune after a 30 minute search

  18. Observing Neptune • Neptune shows no detail from ground-based telescopes • Best images from Earth from the Hubble Space Telescope

  19. Neptune Facts • Size: ~4 Earth diameters • Orbit: 30.11 AU • Description: more distant, cloudier Uranus

  20. Neptune’s Atmosphere • Neptune has visible storms like Jupiter, but they appear to be short lived • The white features in Neptune’s atmosphere are high altitude methane clouds • All seem to be moving east rather than in opposite directions

  21. Composition and Heating • 84 % Hydrogen • 14 % Helium • 2 % Methane • Neptune may be still contracting

  22. The Rings of Neptune • But stars were dimming by different amounts • Caused by the gravity of a near-by moon • Inner narrow ring has shepherd moons

  23. The Moons of Neptune • Triton’s orbit is unstable, in 100 million years it will be inside of the Roche limit giving Neptune a spectacular ring system • Triton may be a captured Kuiper Belt object

  24. Interiors • We can model each planet with a similar interior • Mantle of water and ammonia (Windex) • The two planets have relatively more heavy elements and less hydrogen than Jupiter and Saturn • They also do not have enough gravity to produce liquid metallic hydrogen

  25. The Interiors of Uranus and Neptune

  26. The Formation of Uranus and Neptune • At 20-30 AU the planetesimals were fewer and more widely dispersed than at 5-10 AU • By the time they formed much of the hydrogen and helium was dispersed

  27. Next Time • Read Chapter 11.5 and 12.5

  28. Summary • Information comes from Voyager and HST • Blue-green in color with white clouds • Caused by methane • Radiation darkening produces dark, soot colored rings and moons • Interior composed of rocky core, water and ammonia mantle and hydrogen atmosphere • Offset magnetic field • Formed more slowly than Jupiter and Saturn and so captured less gas

  29. Summary: Uranus • Discovered by survey (1781) • Faint cloud patterns • Due to low internal heat • Tilted on its axis • Causing non-uniform solar illumination

  30. Summary: Neptune • Discovered through use of Newton’s laws (1846) • Most distant gas giant • Has more internal heat and stronger cloud features than Uranus

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