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GENS4001 Astronomy Part 1: The Solar System Dr Michael Burton Department of Astrophysics School of Physics, UNSW Overview 1 Star (the Sun) 8 + 1 Planets Mercury, Venus, Earth, Mars Jupiter, Saturn, Uranus, Neptune Pluto / Charon ≥ 61 Moons

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Gens4001 astronomy part 1 the solar system l.jpg

GENS4001 AstronomyPart 1:The Solar System

Dr Michael Burton

Department of Astrophysics

School of Physics, UNSW

The Solar System

Overview l.jpg

  • 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

The Solar System

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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.

Our star the sun l.jpg
Our Star, the Sun

  • Giant ball of plasma undergoing thermonuclear fusion of hydrogen in centre!

  • Photosphere - visible surface at 6000°C

  • Chromosphere

    • Jets of gas (spicules) rise along along boundaries of granules.

  • Corona - tenuous, hot (2,000,000°C) gas

    • Blends into Solar Wind

The Solar System

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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?

Earth moon l.jpg
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.

The Solar System

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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.

The Solar System

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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?

What if the moon didn t exist l.jpg

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.

What if the Moon didn’t exist?

The Solar System

The inner and outer planets l.jpg

Small (<13,000 km)

Rocky (iron core)

Thin atmospheres

Slow rotation

Short years

Few moons


No Rings

Large (>50,000 km)

Gaseous (H, He)

Thick atmospheres

Rapid rotation

Long years

Many moons



The Inner and Outer Planets

The Solar System

Slide11 l.jpg

  • 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.

Jupiter l.jpg

  • 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.

The Solar System

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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).

The Solar System

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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.