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October 5, 2011 – 10am class

October 5, 2011 – 10am class. Today: Saturn, Uranus, Neptune, Pluto, Kuiper Belt, Comets. Saturn and its moons. Photo taken by Cassini Spacecraft. Orbital period = 29.5 Earth years So as we go around the Sun, we s ee the rings with different tilts. Saturn’s Rings.

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October 5, 2011 – 10am class

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

  2. Saturn and its moons Photo taken by Cassini Spacecraft

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

  4. 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

  5. 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

  6. Spacecraft View of Ring Gaps

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

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

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

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

  11. Spokes

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

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

  14. Why do the jovian planets have rings?

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

  16. 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.

  17. 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.

  18. 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.

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

  20. 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

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

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

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

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

  25. Uranus and Neptune

  26. 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

  27. 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

  28. Rotation and Magnetic Fields

  29. 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

  30. Voyager picture of Neptune:

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

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

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

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

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