GENS4001 The Solar System 1

GENS4001 AstronomyPart 1:

The Solar SystemDr 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

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 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 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.

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 didn’t 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 (<13,000 km)• Rocky (iron core)• Thin atmospheres• Slow rotation• Short years• Few moons• Warm• No Rings

• Large (>50,000 km)• Gaseous (H, He)• Thick atmospheres• Rapid rotation• Long years• Many moons• Cold• Rings

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).

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.