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EARTH
Unit 9.1
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UNIQUE EARTH
Earth has a long and complex history.
Time and again, the splitting and collision of
continents has resulted in the formation of new
ocean basins and the creation of great mountain
ranges.
Furthermore, the nature of life on our planet has
experienced dramatic changes through time.
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UNIQUE EARTH
There is only one place in the universe, as far as
we know, that support life – a modest-sized
planet called Earth that orbits an average-sized
star, the Sun.
Life on Earth appears to be all around us; it is
found in boiling mudpots and hot springs, in the
deep abyss of the ocean, and even under the
Antarctic Ice Sheet.
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UNIQUE EARTH
Living space on our planet, however, is
significantly limited when we consider the needs
of individual organisms, particularly humans.
The global ocean covers 71% of Earth’s surface,
but only a few hundred meters below the water’s
surface, pressures are so intense that humans
cannot survive without an atmospheric diving
suit.
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UNIQUE EARTH
In addition, many continental areas are too steep,
too high, or too cold for us to inhabit.
Nevertheless, based on what we know about
bodies in the solar system – and the hundreds of
planets recently discovered orbiting around other
stars – Earth is still by far the most
accommodating.
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UNIQUE EARTH
What fortuitous events produced a planet so
hospitable to life?
Earth was not always as we find it today.
During its formative years, our planet became hot
enough to support a magma ocean.
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UNIQUE EARTH
Earth also survived a several hundred million
year period of extreme bombardment by
asteroids, to which the heavily cratered
surfaces of Mars and Moon testify.
The oxygen rich atmosphere that makes higher
life forms possible developed relatively recently.
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UNIQUE EARTH
Serendipitously, Earth seems to be the right
planet, in the right location, at the right time.
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THE RIGHT PLANET
If the Earth was more massive, gravity would be
stronger, resulting in the retention of high levels
of ammonia and methane.
If the Earth was smaller, oxygen, water vapor, and
other gases would have escaped.
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THE RIGHT PLANET
If the Earth did not have a lithosphere and
asthenosphere, it wouldn’t have motion from
tectonic plates – this tectonic activity creates
landmasses.
Also, the molten outer core is the reason why we
have a magnetic field that protects Earth’s life
forms.
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THE RIGHT LOCATION
If the Earth were 10% closer, our atmosphere
would be like the one on Venus – too hot.
If the Earth were 10% farther, it would be too
cold.
Earth’s proximity to an average sized star has
allowed enough time for the evolution of humans.
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THE RIGHT TIME
Last, but not least, timing is a critical factor.
The first organisms to live on Earth were
extremely primitive and came into existence
roughly 3.8 billion years ago.
From that point, innumerable changes occurred.
Let’s look at 2 major Earth-altering events.
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THE RIGHT TIME
The first event is the development of the Earth’s
atmosphere.
Earth’s primitive atmosphere consisted of mostly
methane, water vapor, ammonia, and carbon
dioxide.
Due to photosynthesis, free oxygen came into
existence, resulting in the flourishing of various life
forms.
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THE RIGHT TIME
The second event occurred about 65 million
years ago, our planet was struck by an asteroid
10 km wide.
We learned of this as the cause of the
Cretaceous Extinction, which led to the demise of
nearly ¾ of all plant and animals species,
including the dinosaurs.
Eventually, mammals that survived flourished to
become the dominant species.
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THE RIGHT TIME
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JUST RIGHT
As various observers have noted, Earth
developed under “just right” conditions to
support life.
Astronomers refer to this as the Goldilocks
scenario:
Venus is too hot, Mars is too cold, Earth is just
right.
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VIEWING EARTH’S HISTORY
We will be focusing on the origin and the
evolution of Earth.
Researchers utilize many tools to interpret clues
about Earth’s past.
Using tools as well as clues contained in the rock
record, scientists continue to unravel many
complex events of the geologic past.
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