Discussion Notes to be used in the Help Session for Midterm 2. Warning: this is what I could squeeze onto 6 viewgraphs… just some of the high points of what is covered on the midterm. You will need to know more than this to get a good score on the midterm.

Overview of Solar System

• The solar system is a disk
• Rotation of sun, orbits of planets all in same direction.
• Most planets rotate in this same sense. (Venus, Uranus, Pluto are exceptions).
• Angular momentum of pre-solar gas cloud.
• Terrestrial vs. Giant planets
• High vs. low density
• Rocks vs. mostly gas
• Composition
• heavy elements vs. primarily H/He
• Difference due to distance from Sun.
• In the Solar Nebula, while the planets were forming:

Presence of ice èmore material for core ècould gravitationally attract largemasses of hydrogen & helium gas.

Terrestrial Planets

Earth

• Differentiated:
• Iron/nickel core
• Mantle of lighter rock
• Thin crust on top
• Plate Tectonics
• Plates formed at rifts, usually in mid-ocean.
• Drift & collide
• Subduction
• Volcanic activity
• Mountain building
• Evolution of atmosphere
• Thick CO₂ è life è N₂, O₂
• Current global warming
• Greenhouse effect
• Man-made CO₂

(Moon)

• Impact craters as clocks
• Old highlands (4.1-4.4 billion yrs)
• Heavily cratered
• Maria (3.3- 3.8 billion yrs)
• Fewer craters
• Rocks from each brought back by Apollo astronauts.
• Age dating
• Chemical composition
• Tidally locked to Earth
• Formation of Moon
• 4 theories (know about them)
• Giant Impact is current favorite.

Mercury

• Closest to Sun, eccentric orbit.
• Airless, heavily cratered.
• Hot, but (slightly) colder than Hell.
• Very dense - mostly iron-nickel core.
• Geologically dead (probably)
• But rupes è shrinkage at early time.
• Rotates in 2/3 of its orbital period
• Tidal locking with a twist.

Venus

• Differentiated like Earth
• But no tectonic plates.
• Surface mostly studied by radar
• Large volcanoes
• “Continents” pushed up by tectonic flows in mantle.
• Recent lava flows, constant resurfacing.
• Crater density èvery young surface
• only 800 million yrs old.
• Thick CO₂ atmosphere
• High Surface temperature
• Result of runaway greenhouse effect.
• Keeps surface very hot (900F).
• Lead, brimstone (sulfer) are molten.
• Retrograde rotation
• Probably due to giant impact.

Mars

• 50% smaller diameter than Earth
• 1.5 times further from Sun.
• Many visits by spacecraft.
• Small metal core, but much activity in mantle.
• Gigantic volcanoes.
• 50% highland “continents”
• Tharsis bulge.
• Cracked open to form Valles Marineris.
• 50% low-lying lava plains.
• 4 billion yrs old (crater counts)
• Atmosphere
• CO₂, like Venus, but very thin.
• Runaway refrigerator effect
• Atmosphere gradually escaped
• Could not retain heat
• Water froze out
• even less heat retained
• Life?
• Viking landers found no sign.
• In meteorites from Mars? Very questionable.

The Giant Planets
Jupiter – Saturn – Uranus – Neptune

• 14-300 x more massive than Earth.
• Massive H, He atmospheres
• By far the most abundant elements in the solar system.
• On top of rock/ice core with 10-15 x mass of Earth.
• Lots of weather on Jupiter & Saturn.
• Ammonia (NH₃) clouds.
• Strong winds at different latitudes.
• (differential rotation)
• Cyclonic storms
• Great Red Spot
• 2 x size of Earth
• 400 yrs so far
• Investigated by Galileo probe.

Uranus, Neptune have methane reflective layers (blue-green color).

• Neptune has high altitude clouds of methane ice crystals.

Moons

Jupiter’s Galilean moons, as we get closer to Jupiter:

• Callisto – ice, geologically dead.
• Ganymede – ice, but geologically active.
• Europa – rock, but covered by ice pack over liquid water.
• Io – rock, extreme volcanic activity.
• Gradient of properties due to increased tidal effects & heating from Jupiter.
• Jupiter’s 24 other moons are much smaller.

Saturn: 31 known moons

• largest is Titan
• N₂ atmosphere.
• Similar to Earth’s, but very cold (ethane oceans).
• Cassini/Huygens probe to land in 2004.

Triton

• Neptune’s largest moon.
• Retrograde orbit.
• 75% rock, 25% ice.
• Very thin N₂ atmosphere.

Pluto (& Charon)

• No spacecraft visits, so little is known
• Pluto probably quite similar to Triton.
• Charon is half as big as Pluto.
• Debate about whether Pluto should be called a planet.
• Very low mass.
• Eccentric, tilted orbit.
• Similar to some comets.

Rings

• All 4 giant planets have rings.
• Rings form inside Roche limit:
• P² = a³ èdifferent parts of a moon try to move in orbits with different periods.
• This tears bodies apart unless gravity (+ internal tensile strength) can hold them together.
• For orbits inside Roche limit, prospective moons are torn apart.
• Jupiter, Uranus, Neptune have very thin rings. Saturn has much larger rings.
• Shepherd satellites
• Moons sweep out divisions, contain rings through gravitational resonances.
• Rings made of gravel and small bits of ice.

Asteroids

• Small rocky bodies in orbit about sun.
• Left over from formation of Solar System.
• Most, but not all, in asteroid belt.
• Some cross Earth’s orbit
• Meteorites
• Asteroids that hit Earth and don’t burn up in atmosphere.
• Analyzing them è
• Age of solar system (4.5 billion yrs)
• èInitial chemical composition of solar system.

Comets

• Mostly ice
• Some on highly eccentric orbits
• Spectacular tails when close to Sun.
• Melted ice is driven off by solar radiation, solar wind.
• Most come from Oort Comet Cloud at edge of solar system.
• Some from Kuiper Belt, just beyond Pluto.