Download The Origin of Earth

The Origin of Earth
Six facts must be considered when formulating a
hypothesis about the origin of Earth:
1. Planets revolve around the sun in the same
direction.
2. The orbits are all nearly circular
3. The orbits are in the same flat plane.
4. The sun rotates in almost the same plane as
the planets and in the same direction as the
they revolve.
5. Most of the planets rotate in the same
direction as the sun
6. Seven of the nine planets have moons.
Protoplanet Hypothesis
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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.
Origin of the Oceans
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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.
Origin of the Atmosphere
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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.
Layers of Earth
Iron and
nickel
Oxygen and
silicon
Partially molten
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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.