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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

October 5, 2011 – 10am class

Today: Saturn, Uranus, Neptune, Pluto, Kuiper Belt, Comets

saturn and its moons

Saturn and its moons

Photo taken by Cassini Spacecraft


Orbital period = 29.5 Earth years

So as we go around the Sun, we

see the rings with different tilts

saturn s rings
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


Telescope picture of


roche limit
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

gap moons
Gap Moons
  • Some small moons create gaps within rings.
shepherd moons
Shepherd Moons
  • A pair of small moons can force particles into a narrow ring.
resonance gaps
Resonance Gaps
  • Orbital resonance with a larger moon can also produce a gap.
jovian ring systems
Jovian Ring Systems
  • All four jovian planets have ring systems.
  • Others have smaller, darker ring particles than Saturn.
why do the jovian planets have rings1
Why do the jovian planets have rings?
  • They formed from dust created in impacts on moons orbiting those planets.

How do we know?

how do we know
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.
ring formation
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.
what have we learned
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.
saturn s moons
Saturn’s Moons
  • Titan:
  • Only moon in the solar system with an atmosphere
  • 1.5x pressure of the Earth’s atmosphere
titan s atmosphere
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
titan s surface
Titan’s Surface
  • Huygens probe provided first look at Titan’s surface in early 2005.
  • It found liquid methane and “rocks” made of ice.
medium moons of saturn
Medium Moons of Saturn
  • Almost all of them show evidence of past volcanism and/or tectonics.
medium moons of saturn1
Medium Moons of Saturn
  • Ice fountains of Enceladus suggest it may have a subsurface ocean.
aurorae on saturn
Aurorae on Saturn

Probably debris from moons,

not solar wind excite Saturn’s aurorae

uranus and neptune1
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
  • 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

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


Has a big storm,

Similar to Jupiter’s

Big Red Spot 



Retrograde, inclined orbit

- captured?

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