October 5, 2011 – 10am class - PowerPoint PPT Presentation

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

see the rings with different tilts

• 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

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

Snowflakes to Boulders

• Some small moons create gaps within rings.

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

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

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

• They formed from dust created in impacts on moons orbiting those planets.

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.

• 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 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.

• Titan:
• Only moon in the solar system with an atmosphere
• 1.5x pressure of the Earth’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

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

• Almost all of them show evidence of past volcanism and/or tectonics.

• Ice fountains of Enceladus suggest it may have a subsurface ocean.

Probably debris from moons,

not solar wind excite Saturn’s aurorae

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

Rotation and Magnetic Fields

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

Voyager picture of Neptune:

Neptune

Has a big storm,

Similar to Jupiter’s

Big Red Spot 

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

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

Triton:

Retrograde, inclined orbit

- captured?

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