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Chapter 7
Earth as a Planet
What do we know about the
structure, composition, and
history of the planet Earth
that we can use in the study of
other planets?
How have extraterrestrial
factors influenced the Earth?
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Chapter 7 Outline
• The overall structure of Earth
– Below the surface
– At the surface
– Atmosphere
• Life on Earth
• Extraterrestrial influences
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7.1 The Global Perspective
The Earth is a terrestrial planet (composed primarily of heavy
Elements) with a temperature suitable for liquid water.
Semimajor axis
Period
Mass
Diameter
Escape velocity
Rotation period
Surface area
Atmospheric pressure
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1.00 AU
1.00 year
1.00 Mearth = 5.98 x 1024 kg
12,756 km
11.2 km/s
Sidereal day = 23h 56m 4s
5.1 x 108 km2
1.00 bar
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7.1.1 Earth’s Interior
• The interior is difficult to study.
– What’s the deepest hole bored in the Earth?
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Seismic Waves
• We learn about the structure
of the Earth’s interior by
observing the behavior of
seismic waves.
– Earthquakes
– Impacts
– Explosions
• Two types of waves
propagate in the Earth
– P wave: compression wave
(like sound)
– S wave: transverse wave (like
vibrating string)
• Evidence for a liquid core.
• Evidence of layers implies
that the Earth differentiated.
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7.1.2 Magnetic Field and Magnetosphere
•
The Earth’s magnetic field is the result of
1. The Earth’s rotation
2. Electric charges moving within the core  core must be molten.
•
The north and south magnetic poles lie near the north and
south poles.
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The
Magnetosphere
• The magnetosphere is the region where the Earth’s
magnetic field dominates other magnetic fields
(primarily the Sun’s).
• Charged particles from the Sun become trapped in
the magnetosphere.
• The flow of charged particles from the Sun is called
the solar wind.
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The Magnetosphere (cont’d)
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Aurora
• Charged particles spiral down and excite atoms of gas in the
upper atmosphere. The atoms then give off light of particular
colors:
• red – hydrogen, green – helium
• The particles are able to spiral down at the North and South
magnetic poles, hence the aurora borealis and aurora australis.
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Aurora Borealis: The Northern Lights
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7.2 The Crust of the Earth
• The Earth’s crust is the outer layer we live on,
extending down about 10km.
• The crust is composed of 4 types of rock:
• Igneous: rock that has cooled
from a molten state.
• Sedimentary: rock built up by
layering.
• Metamorphic: igneous or
sedimentary rock chemically
altered by pressure or
temperature.
• Primitive: unaltered from
formation of the solar sys.
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7.2.2 Plate Tectonics
• Theory that explains the
motions of the Earth’s crust
produced by heat-driven
currents in the mantle.
• Also known as continental
drift.
• The Earth’s surface can be
divided into regions (plates)
that tend to slide, push, or
pull against each other,
producing earthquakes,
volcanoes, mountains, and
rifts.
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7.2.3 Rift and Subduction Zones
• Rift zone: where 2 plates are pulling apart,
forming new crust in the gap.
• Subduction zone: where 2 plates push together,
one sliding beneath the other.
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7.2.4 Fault Zones and Mountain Building
• Fault zone: where 2 plates are sliding past each
other. Plates can move at several cm per year.
Usually active earthquake areas.
• San Andreas fault is one of the
most famous fault zones.
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Mountains
• When two crustal plates
collide, high mountains can
be formed by lifting and
folding of the crust.
–
–
–
–
Alps
Himalayas
Rocky mountains
Andes
• On Earth, erosion will
sculpt the mountains to
sharp peaks.
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7.2.5 Volcanoes
• Volcanoes occur when molten
magma pushes up through the
crust to the surface.
• Sometimes volcanoes involve
dramatic explosions.
• Often they are less dramatic,
involving a more or less
constant release of lava.
• Hawaiian islands lie over a
hot spot that changes position
over time as the ocean floor
moves.
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7.3 The Earth’s Atmosphere
• “Ocean of air” in which we live.
• The pressure of the atmosphere is produced by the weight of
the air above us.
• Pressure at surface is 1 bar.
• We live in the troposphere.
• Above that are the
stratosphere, mesosphere,
and ionosphere.
• The ozone layer in the
stratosphere absorbs UV
radiation.
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7.3.2 Atmospheric Composition and Origin
• The composition is summarized in the table below.
• This isn’t the whole story.
• Water in the oceans
• Carbon dioxide locked in carbonaceous matter.
Nitrogen (N2)
78%
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Oxygen (O2)
21%
Argon (A)
1%
Water vapor (H2O)
Trace
Carbon dioxide (CO2)
Trace
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7.3.3 Weather and Climate
• I’ll skip this section.
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7.4 Life and Chemical Evolution
• Earth is the only place
we know has life.
• Life on Earth dates
back to at least 3.5
billion years ago.
• Early life was plantlike, taking in CO2 and
releasing O2.
• When enough oxygen
was present, animals
could evolve.
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7.4.3 The Greenhouse Effect and
Global Warming
• The surface of the Earth is heated by light from the
Sun, mostly visible light.
• The hot surface radiates back infrared radiation,
some of which escapes into space, cooling the Earth.
• Carbon dioxide absorbs infrared radiation.
• Like a blanket, CO2 in the atmosphere can stop the
infrared radiation from escaping, trapping heat and
causing temperatures to rise.
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7.5 Cosmic Influences on the Evolution
of Earth
• If the Earth has been influenced by extra-terrestrial
phenomena, where is the evidence?
• Why isn’t the Earth cratered like the Moon?
– Could the impacts have missed the Earth and hit the
Moon?
– Maybe the objects burned up in the atmosphere?
• Possible for small meteors, but not the large objects that
produce large craters.
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7.5.2 Recent Impacts
• On June 30, 1908, a huge explosion occurred in the
region of Tunguska River, Siberia.
• The shock wave flattened more than 1000 sq. km of
forest and herds of animals were killed.
• The blast was equivalent to 15 megatons of TNT.
• No crater was formed.
• This event is believed to be due to a stony projectile
of about 100,000 tons that disintegrated and
exploded in the atmosphere.
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Arizona – Meteor Crater
• Northern Arizona
• Impact 50,000 years ago
• Iron-nickel meteorite
• Weighing several hundred
thousand tons
• Estimated size ~ 150 feet across
• Hurtling at about 40,000 miles per
hour
• Explosive force greater than 20
million tons of TNT.
• Crater 700 feet deep, 4000 feet
across.
• Today: crater is 550 feet deep, and
2.4 miles in circumference.
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Impacts
• There is evidence that the Earth has been and
continues to be hit by large objects.
• Over periods of about 100 million years, the Earth’s
crust is (almost) completely recycled, removing
evidence of old craters.
• Over shorter periods, erosion of craters makes them
harder to spot. With modern imaging, more than
150 craters have been found.
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Known Craters on Earth
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7.5.3 Extinction of the Dinosaurs
• Very large impacts can have global
consequences.
• 65 million years ago an impact on
the Yucatan coast occurred.
• Evidence of the impact:
– Outline of a large crater (200km in
diameter) near Chicxulub, Mexico of
the right age
– Global sediment layers of that age
with an excess of the element
iridium.
• Hypothesis: This impact put
enough material in the atmosphere
to block the Sun and kill much of
the plant and animal life on Earth
(mass extinction).
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• Explosive power of 5 billion
Hiroshima nuclear bombs, about
50,000 trillion tons of TNT!
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Chicxulub Gravity Profiles
• These images show the small changes in gravity (few parts
per 100,000) outlining the crater.
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7.5.5 Impacts in Our Future?
• Periodic large impacts may be the reason for other
past mass extinctions.
• The evidence is that impacts have occurred in the
past, and that some were large enough to cause
global consequences.
• There is reason to believe it can happen again!
– Small sized impact could destroy a city
– Large impact could have global effects; could destroy
human civilization.
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What’s Being Done
• In the 1990’s Congress funded a program to search
for and track potentially dangerous objects, objects
in so-called Earth-crossing orbits.
– Objects bigger than 10km have been identified.
– About half of the objects bigger than 1km have been
found. (Estimated total is 1100 objects.)
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Discussion Question
• Rate the importance (in $) of
finding objects with Earthcrossing orbits?
• For comparison, U.S.
government spends:
– Space station: $10 billion/yr
– All NASA: $16 billion/yr
– Medical research: $100
billion/yr
– War on terrorism: $100
billion/yr
• How much taxpayer money
should be spent?
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