October 5, 2011 – 10am class

1. October 5, 2011 – 10am class Today: Saturn, Uranus, Neptune, Pluto, Kuiper Belt, Comets

2. Saturn and its moons Photo taken by Cassini Spacecraft

4. Orbital period = 29.5 Earth years So as we go around the Sun, we see the rings with different tilts

5. Saturn’s Rings • Very thin – 10-20 METERS thick • Icy particles (water ice) 1cm – 5m • Rings were once thought to be moons which are inside the “Roche limit” • Cassini’s divisions: Gaps in the rings, caused by resonances with the moons of Saturn Earth-based Telescope picture of Saturn

6. Roche Limit A large moon can be torn apart by the “tidal” forces of the planet: The gravity near the planet is larger than the gravity on the far side of the moon, away from the planet When this tidal force is greater than the self-gravity of the moon, the moon disintegrates

7. Spacecraft View of Ring Gaps

8. Artist’s Conception of Rings Close-Up Snowflakes to Boulders

9. Gap Moons • Some small moons create gaps within rings.

10. Shepherd Moons • A pair of small moons can force particles into a narrow ring.

11. Resonance Gaps • Orbital resonance with a larger moon can also produce a gap.

12. Spokes

13. How do other jovian ring systems compare to Saturn’s?

14. Jovian Ring Systems • All four jovian planets have ring systems. • Others have smaller, darker ring particles than Saturn.

15. Why do the jovian planets have rings?

16. Why do the jovian planets have rings? • They formed from dust created in impacts on moons orbiting those planets. How do we know?

17. How do we know? • Rings aren’t leftover from planet formation because the particles are too small to have survived for so long. • The ring particles are constantly ground down by collisions with other ring particles and micrometeorites, and then swept into the planet by the pressure of light • There must be a continuous replacement of tiny particles. • The most likely source is impacts with jovian moons.

18. Ring Formation • Jovian planets all have rings because they possess many small moons close in. • Impacts on these moons are random. • Saturn’s incredible rings may be an “accident” of our time.

19. What have we learned? • What are Saturn’s rings like? • They are made up of countless individual ice particles. • They are extremely thin with many gaps. • How do other jovian ring systems compare to Saturn’s? • The other jovian planets have much fainter ring systems with smaller, darker, less numerous particles. • Why do the jovian planets have rings? • Ring particles are probably debris from moons.

20. Saturn’s Moons • Titan: • Only moon in the solar system with an atmosphere • 1.5x pressure of the Earth’s atmosphere

21. Titan’s Atmosphere • Titan is the only moon in the solar system to have a thick atmosphere. • It consists mostly of nitrogen with some argon, methane, and ethane. • ethane is a greenhouse gas  Titan is warmer than it would be without ethane but still 180C

22. Titan’s Surface • Huygens probe provided first look at Titan’s surface in early 2005. • It found liquid methane and “rocks” made of ice.

23. Medium Moons of Saturn • Almost all of them show evidence of past volcanism and/or tectonics.

24. Medium Moons of Saturn • Ice fountains of Enceladus suggest it may have a subsurface ocean.

25. Aurorae on Saturn Probably debris from moons, not solar wind excite Saturn’s aurorae

26. Uranus and Neptune

27. Uranus and Neptune • Uranus was discovered in March 1781 by William Herschel and was the first planet discovered with a telescope • Neptune was discovered in 1846, after astronomers Adams and Leverrier predicted its existence, based on irregularities in Uranus’ orbit • Uranus and Neptune are similar in size, composition and internal structure • Both Uranus and Neptune appear bluish because of methane in their atmosphere

28. Uranus • The only spacecraft to visit Uranus was Voyager 2 in 1986 • Uranus’ rotational axis is tilted by 90 degrees  probably the result of a really big collision • Uranus’ magnetic field is tilted 60 degrees with respect to its rotational axis

29. Rotation and Magnetic Fields

33. SEASONS on Uranus are EXTREME: • Winter solstice (which last occurred in 1985 AD): • The north pole is pointed almost directly away from the Sun. • The northern hemisphere experiences perpetual darkness. • The southern hemisphere experiences perpetual sunlight. • Spring equinox (2006 AD): • The rotation axis is perpendicular to the Uranus-Sun direction. • From any point on Uranus, the Sun rises in the east and sets in the west 8 1/2 hours later. • Summer solstice (2027 AD): • The north pole is pointed almost directly toward the Sun. • The north experiences perpetual sunlight. • The south experiences perpetual darkness

34. Voyager picture of Neptune:

35. Neptune Has a big storm, Similar to Jupiter’s Big Red Spot 

36. Neptune’s clouds, as photographed by Voyager in 1989:

38. Neptune’s biggest moon is Triton: (picture from Voyager 1989)

39. Triton: Retrograde, inclined orbit - captured? 38 oK