GENS4001 Astronomy Part 1: The Solar System

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GENS4001 AstronomyPart 1The Solar System

• Dr Michael Burton
• Department of Astrophysics
• School of Physics, UNSW

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Overview

• 1 Star (the Sun)
• 8 1 Planets
• Mercury, Venus, Earth, Mars
• Jupiter, Saturn, Uranus, Neptune
• Pluto / Charon
• 61 Moons
• 100,000 (?) Asteroids, 100 million (?) Comets,
  Solar Wind

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Formation of the Solar System

• Collapse of cloud of gas, dust ice
• 4.6 billion years ago,
• Swirling, disk-shaped,
• Sun formed at centre of Solar Nebula.
• Inner planets form through accretion of dust
  particles to planitesimals into protoplanets.
• Collisions cratering dominate for 150 Myrs.
• Outer planets form through accretion of gas onto
  rocky protoplanetary cores.

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Our Star, the Sun

• Giant ball of plasma undergoing thermonuclear
  fusion of hydrogen in centre!
• Photosphere - visible surface at 6000C
• Chromosphere
• Jets of gas (spicules) rise along along
  boundaries of granules.
• Corona - tenuous, hot (2,000,000C) gas
• Blends into Solar Wind

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The Sun (continued)

• Surface features vary with 11-year cycle
• Sunspots cooler with strong magnetic field,
• Solar flare eruption from sunspot group,
• Convection cells, transporting energy outwards.
• Energy produced by thermonuclear fusion of 4
  H-atoms into He-atom at 8 million C.
• Solar Model well understood
• Fusion in core about 1/4 solar radius in size,
• Neutrinos - 1/4 predicted number?

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Earth / Moon

• Double planet system, tidally interacting.
• Plate Tectonics produces continents, oceans,
  mountains volcanoes.
• Iron rich core producing magnetic field.
• Atmosphere of 80 nitrogen, 20 oxygen
• Troposphere, stratosphere, mesosphere,
  thermosphere
• Supports life on land, oceans atmosphere.

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Earth / Moon (continued)

• Magnetosphere surrounding Earth, protecting
  atmosphere from Solar Wind.
• Collision-ejection with giant asteroid, with
  debris coalescing to form Moon.
• Weathering has erased asteroid impacts on Earth
  but past history still visible on Moon.
• Cratered highlands,
• Smooth-surfaced Maria, from lava flows.

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Earth / Moon Phenomena

• Phases of the Moon
• Orbit about Earth, and Solar illumination.
• Eclipses when Sun/Earth/Moon in line
• Lunar eclipses
• Solar eclipses (inc. annular)
• Tides
• Differential gravitational pull of near and far
  sides of Earth by Moon
• Asteroid collisions and mass extinctions?

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What if the Moon didnt exist?

• Effect on culture (romances)?
• No eclipses or phases - dark skies!
• Tides only 1/3 current size (from Sun).
• 12 hour cycle, constant level.
• Day would still be 6 hours long
• Enormous tides helped form soup for life?
• More powerful winds, mountains eroded.

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The Inner and Outer Planets

• Small (lt13,000 km)
• Rocky (iron core)
• Thin atmospheres
• Slow rotation
• Short years
• Few moons
• Warm
• No Rings

• Large (gt50,000 km)
• Gaseous (H, He)
• Thick atmospheres
• Rapid rotation
• Long years
• Many moons
• Cold
• Rings

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Mars

• No canals, but ancient river channels!
• No plate tectonics, resulting in giant shield
  volcanoes over hot spots, plus canyons.
• A few impact craters.
• Thin carbon dioxide atmosphere red dust.
• Water must once have flowed - flash floods.
• Could life have once existed?
• 2 tiny moons (Phobos, Deimos) are captured
  asteroids.

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Jupiter

• The Giant of the planets (not a failed star).
• Rapid, differential rotation.
• Belts Zones, methane ammonia clouds.
• Cyclones interface Red Spot white ovals.
• Strong magnetic fields, huge aurorae.
• Gas / Liquid Metallic H / Rocky Core.
• Thin, transient dust ring - meteor impacts.

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Moons of Jupiter

• 16 Moons with 4 giants (Galilean satellites).
• Io Volcanic, sulphur-covered, kept molten
  through tidal heating.
• Europa covered in ice with intricate pattern of
  cracks
• tidal heating supports oceans, life??
• Ganymede Rock Ice, past tectonics.
• Callisto Rock, cratered terrain (impacts).

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Comets and Asteroids

• Debris from formation of Solar System.
• Asteroids lumps of rock a few km in size
• Most in belt between Mars Jupiter.
• Comets dirty snowballs of ices and rocks
• Primordial, but transient,
• Highly elliptical orbits, from Kuiper Belt
  (50-500AU) or Oort Cloud (to 50,000AU),
• Tails only when near Sun (vaporised ice), blown
  out by the solar wind sunlight.