Week 2:

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Week 2 When the world was young...
EPSC233-001 Earth Life History (Fall 2002)
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Recommended reading STANLEY Earth System
History Chapter 11.
Keywords Archean eon (4.6-2.5 billion years
ago), nebula, planets, meteorites, comets,
differentiation, oceanic crust, continental
crust, plate tectonics, mantle convection.
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Geologists seeking evidence of the oldest rocks
on earth are up against large odds...
... our planet is a huge recycling engine.
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Our solar system started with the explosion of a
supernova, the dying stage of a star. Clouds of
gas and dust were compressed by the shock wave,
giving rise to a solar nebula.
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1. a slowly rotating cloud of gases called a
   nebula starts to collapse (gravitational
   attraction)
2. the cloud continues to condense and flattens, a
   gaseous protostar and solid planetesimals grow
3. the sun lights up, solar wind vapourizes dust and
   blows it away
4. the nebula clears up a solar system is born.

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Dust accreted into larger bodies.
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Planets which grew from planetesimals which
condensed close to the sun are rocky, those far
from the sun are gas- and ice-rich
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http//nssdc.gsfc.nasa.gov/planetary/image/near_ma
thilde3.jpg
What did not grow into planets became asteroids
(rocky)...
Or comets (dirty snowballs)
http//www.jpl.nasa.gov/comet/hyakutake/gif/burger
2.jpg
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Dates from about 70 meteorites
Allende meteorite (Mexico) 4.56 B.y. old, sample
of the stuff from the sun (- H, He).
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Impacts and radioactive decay led to melting
differentiation
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Around 4.5-4.4 Ga, an impact with a Mars-sized
body had a great effect on Earths history. All
volatiles were lost from Earth, and the
Moon formed from the impactor and
terrestrial debris.
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Lunar basalts on the moon
Apollo 17, oldest lunar basalt samples 4.55 ?
0.1 b.y. and 4.60 ? 0.1 b.y.
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Initially the Moon was hot-- covered by a magma
ocean.
But, the Moon is small and cooled rapidly. It
now has no volcanic activity and is
both biologically and geologically lifeless.
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The Earth, bigger, is cooling more slowly and
remains geologically active. Crust showing scars
of early meteoritic impacts has been recycled.
Manicouagan crater 100 km diam (200 m.y. ago).
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Why is there a difference? Earth is a
geologically active planet, the Moon is not.
Top few hundred km of Earth are broken into
tectonic plates which are constantly being
created and destroyed.