Earth as a Planet

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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.
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7.1.1 Earths Interior

• The interior is difficult to study.
• Whats the deepest hole bored in the Earth?

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Seismic Waves

• We learn about the structure of the Earths
  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 Earths magnetic field is the result of
• The Earths rotation
• Electric charges moving within the 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 Earths
  magnetic field dominates other magnetic fields
  (primarily the Suns).
• 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 (contd)
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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 Earths 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 Earths
  crust produced by heat-driven currents in the
  mantle.
• Also known as continental drift.
• The Earths 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 Earths 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 isnt the whole story.
• Water in the oceans
• Carbon dioxide locked in carbonaceous matter.

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7.3.3 Weather and Climate

• Ill 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 plant-like, taking in CO2 and
  releasing O2.
• When enough oxygen was present, animals could
  evolve.

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7.4.3 The Greenhouse Effect andGlobal 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 isnt 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 and
  continues to be hit by large objects.
• Over periods of about 100 million years, the
  Earths crust is completely recycled, completely
  removing evidence of old craters.
• Over shorter periods, erosion of craters makes
  them harder to spot. With modern imaging, 150
  craters have been found.

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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 much material
  in the atmosphere to block the Sun and kill much
  of the plant and animal life on Earth (mass
  extinction).

• Explosive power of 5 billion Hiroshima nuclear
  bombs, about 50,000 trillion tons of TNT!

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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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Whats being done

• In the 1990s 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 of finding objects with
  Earth-crossing orbits?
• Space station 10 billion/yr
• Medical research 10 billion/yr
• War on terrorism 100 billion/yr
• How much should be spent?
• Who should pay for it?