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 (CH 4 ): 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