The Origin of Earth

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The Origin of Earth

• Six facts must be considered when formulating a
  hypothesis about the origin of Earth
• Planets revolve around the sun in the same
  direction.
• The orbits are all nearly circular
• The orbits are in the same flat plane.
• The sun rotates in almost the same plane as the
  planets and in the same direction as the they
  revolve.
• Most of the planets rotate in the same direction
  as the sun
• Seven of the nine planets have moons.

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Protoplanet Hypothesis

• 5 billion years ago a cloud of gas and dust at
  least 10 billion kilometres in diameter rotated
  slowly in space.
• As time passed the cloud shrank under its own
  gravity.
• Most of the material condensed at the center.
• The compression made the interior so hot that
  fusion began and the core of the cloud formed the
  sun.
• Friction caused eddies in the disk rotating
  around the core, which condensed into the
  protoplanets and later the planets and moons.

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Origin of the Oceans

• When Earth first formed, it had neither oceans
  nor atmosphere
• The protoplanet heated up b/c of the radioactive
  minerals, compression, and meteorites.
• Iron melted and sank to the centre of Earth,
  forming the core.
• As the molten iron sank, it melted other
  materials that released water and gases.
• Steam and gases escaped to the surface in
  volcanic eruptions.
• The steam condensed into water that accumulated
  as oceans.

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Origin of the Atmosphere

• The atmosphere contains 78 N2 and 21 O2
• The remaining 1 is mostly other gases, such as
  Ar, CO2, He, and H2O
• The original atmosphere is thought to have come
  from volcanoes, much like the gases now emitted,
  which contains no O2
• The atmosphere first free O2 likely came from the
  breakup of water molecules by sunlight in the
  upper atmosphere
• When green plants evolved, they added more O2 by
  photosynthesis.

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• When Earth formed 5 billion years ago, much of
  the planet was molten (liquid)
• Heavier elements such as iron and nickel (atomic
  s 26 and 28) sank to the middle of the earth and
  lighter elements such as oxygen and silicon
  (atomic s 8 and 16) floated to the top.

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Layers of Earth
Iron and nickel
Oxygen and silicon
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Partially molten

• Continental crust is 70 km thick and oceanic
  crust is 10 km thick.
• The crust and upper mantle together make up the
  lithosphere. The lithosphere is broken up into
  plates which move over millions of years
• Lithospheric plates float on the partially molten
  asthenosphere. Convection of melted rock is due
  to decay of radioactive isotopes and results in
  plate movement.