Astonishing Astronomy 101With Doctor Bones (Don R. Mueller,

Ph.D.)

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RPLANETARY

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Chapter 12 - Saturn The 2nd Largest Planet

Saturn

9.5 AU from the Sun.

9.5 times the Earth’s diameter.

100 times the Earth’s Mass.

Saturn

Earth

The Appearance of Saturn

• Parallel bands of cloudsSimilar to Jupiter’s, but not as

distinct.

• 96% H2 and 4% He, traces of hydrogen-rich compounds.

• Outer atmosphere has a temperature of 130 K.

• Rotates once every 10.7 hours.

• Flatter than Jupiter.

The Outer Planets

Jupiter

Saturn

Uranus

Neptune

Jupiter

Earth

Saturn

NeptuneUranus

• Far from the Sun: cold enough that water vapor condenses into ice.

• Beyond this frost line, planets are primarily composed of hydrogen and ices.

• The low temperatures allowed the inner planets to capture hydrogen and helium gas and grow to immense sizes.

• The outer planets have no surfaces:–Pressures steadily climb, turning gases into

liquids and eventually into metals.

The Outer Planets

The Interior of the Gas Giant

Saturn

The Interior of Saturn• Saturn’s average density is 0.7 kg/liter.

• Like Jupiter, Saturn is in hydrostatic equilibrium.

• Most of Saturn’s mass is concentrated at the center.

• Has pressures high enough to create liquid metallic hydrogen.

• Has a molten rocky core larger than the Earth.

• Generates more heat than it receives from the Sun.

Storms on Saturn

• Saturn, although calmer than Jupiter, also has violent storms:

• With higher wind speeds than for Jupiter.

• The storms are deeper in its atmosphere.

Magnetic Fields

• The liquid metallic hydrogen in Saturn can carry electrical currents, similar to the liquid core of the Earth.

• These currents generate very large magnetic fields.

• Saturn’s magnetic field is 500 times as strong as Earth’s.

• Saturn experiences auroras.

Saturn’s Rings

• The ring particles are made almost entirely of water ice with some contamination from dust.

• Using his telescope, Galileo Galilei (in 1610) was the first person to observe the rings of Saturn.

• Galileo saw the rings of Saturn. • Huygens observed that the

rings were detached.• Maxwell realized that the rings

were not solid and were made up of small particles.

• Saturn has three main rings• Outermost is the A ring, the

middle ring is the B ring and the inner ring is the C or crepe ring.

• The separation between rings A and B is the Cassini division.

Ring Systems

Saturn’s Shadow

Ringlets

• A closer inspection of Saturn’s rings shows that they are composed of tiny ringlets:

These might be caused by gravitational influences of very small moons, creating waves in the main rings (spiral density waves).

Larger gaps (Cassini and Enke divisions, for example) are caused by orbital resonances with some of Saturn’s moons.

All of the Gas Giants Have Ring Systems

Jupiter

Neptune

Uranus

The Origin of Planetary Rings

• If a body held together only by gravity gets too close to a planet, tidal forces pull it apart:This distance is called the Roche Limit.

• Solid bodies: rock and ice are safe, as they are held together by forces other than gravity.

• With time, the fragments of the broken satellite go orbit the planet, forming a ring.

• When a moon gets within the Roche limit of its planet it is broken and further disintegrated.

Origin of Planetary Rings: The Roche Limit

Saturn’s Moon Titan

• Saturn’s largest moon:

– Bigger than Mercury.– Thick atmosphere of

nitrogen (orange color).– Temperature of 95 K.– Atmospheric pressure

similar to that of Earth.– Huygens probe sent back

pictures of what looks like rivers and lakes of methane (CH4).

Saturn’s Moon Iapetus

• Iapetus is an odd moon

One half is white, the other is very black.

Tidally locked with Saturn, so that the same face always points toward the planet.

The leading hemisphere is dark due to deposits of dust or soot, either from early volcanoes on the moon or residue from possibly another moon’s destruction.

Chapter 13 Uranus and Neptune

Uranus Neptune

The Discovery of Uranus

• In 1781 a new planet was discovered by William Herschel.

• Originally thought to be a comet.

• Herschel named it Georgium Sidus (George’s Star) after King George III.

• The name was changed to Uranus: consistent with the mythological names of the other planets.

Neptune: A New Method of Discovery

• It was noted that Uranus did not follow its calculated orbit.

• Therefore, another planet must be influencing its orbit.

• Scientists calculated where the new planet should be. Astronomers looked and found Neptune.

• Galileo saw Neptune while looking at Jupiter’s moons, but didn’t realize what it was.

• The atmospheres of Uranus and Neptune planets are rich in hydrogen and helium.

• Their respective atmospheres have significant amounts of methane, giving them their bluish color.

• Both planets are cold:– Uranus: 80 K– Neptune: 75 K

• Densities:– Uranus: 1.3 kg/liter– Neptune: 1.6 kg/liter

The Atmospheres of Uranus and Neptune

ColdBlue

Spheres

Large amounts of methane give Uranus and Neptune their blue color.

The Interior of Uranus

Please insert figure 44.3

Storms High winds lead to storms on Neptune.

Neptune has the “Great Dark Spot,” which disappeared recently.

Uranus’s Axial Tilt

• Uranus is tipped almost 90 degrees to the ecliptic plane.

• It is possible that a collision early in its history tipped the axis, splashing out material for its moons.

• Uranus spins about an axis of an extreme tilt angle compared to its orbit around the Sun.

Planetary Tilt Angles

Sun

Planetary Tilt Angles

Sun

Orbital Locations

Odd Magnetic Fields

• Uranus and Neptune both have “odd” magnetic fields.

• Stronger than Earth’s:– Uranus: 47 times– Neptune: 25 times

• Probably generated by currents in the liquid water in their interiors.

• Tipped in odd directions.• Off-centered on the

planet.

Uranus’s Moon Miranda• Miranda appears to have been shattered by impact:– Long cracks or faults

riddle its surface.– Huge 20 km cliffs.– Rolling hills adjacent to

wrinkled terrain.• The leading hypothesis for

Miranda’s appearance is that the complex pattern arose as the result of strong tectonic activity that broke the surface into “plates.”

Neptune’s moon Triton is larger than Pluto and almost as big as Jupiter’s moon Europa.

Triton orbits backward relativeto Neptune’s forward rotation: Known as a retrograde orbit.

Neptune

Triton