Planet Earth

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Planet Earth
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Shape and Size of the Earth

• Gravity is what gives Earth its spherical shape
• Only effective if the body is of a critical size
• Critical radius is about 350 km

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Shape and Size of the Earth

• Earth is not a perfect sphere but an oblate
  spheroid that bulges out at the equator
• Particles on a spinning object fly outward as a
  result of their inertia strongest force at the
  equator because it is spinning the fastest there
• 1000 mph at the equator, 700 mph at mid-latitudes

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Composition of the Earth

• Most common elements are oxygen, silicon,
  aluminum, magnesium, and iron (oxygen and silicon
  are usually found combined together as silicates)
• Earths interior is mostly the mineral olivine
  and iron-magnesium silicate

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Density of the Earth

• Density mass/volume g/cm3
• Earths mass 6 X 1021 metric tons or 6 X 1027
  grams
• Radius 6400 km 6.4 X 108 cm
• Volume of a sphere 4/3 p R3
• So what is Earths density?
• What does this number mean?

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The Interior of the Earth

• How do we know whats inside when the deepest we
  have penetrated the Earths surface is about 7
  miles, a mere scratch in the surface
• We need indirect evidence
• Provided by seismic waves speed of the waves is
  dependent on the type of material they are
  passing through

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Using Seismic Waves

• S waves and P waves
• P waves can travel through both solids and
  liquids
• S waves can travel only through solids
• When an earthquake takes place the P waves pass
  directly through the center of the Earth, but the
  S waves are deflected indicating that there is a
  liquid portion to the core
• More complicated analysis can help determine the
  density of the material these waves are passing
  through and lead to clues to about their
  composition

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Using Seismic Waves

• Seismic studies show that Earths interior has
  four distinct regions
• Crust 20 to 70 km rocks composed mostly of
  silicates
• Mantle hot, but not quite molten composed of
  silicates mostly olivine extends roughly
  halfway to Earths center capable of slow flow
  even though it is not molten
• Outer core liquid, probably a mixture of iron,
  nickel, and perhaps sulfur
• Inner core solid, probably iron and nickel
  Why solid?

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Structure of the Earth

• Materials separate by density in a process called
  differentiation
• Takes place when a mixture of heavy and light
  material melts with the heavier materials sinking
  and lighter materials rising
• Scientists infer that this means that the Earth
  was completely molten at one time

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Heating of the Earths Core

• Near the surface temps rise about 2 K every 100
  meters you descend (if this held true all the way
  to the center the temp would be 120,000 K
• The core is estimated to be about 6500 K
• Cooling from the outside
• Radioactive material present in all rocks
  radioactive decay releases energy in the form of
  heat
• Heat loss depends on surface area amount of
  heat contained depends on the volume

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Heating of the Earths Core

• Smaller bodies cool much quicker than larger one
• Heat gets trapped in Earths interior causes the
  temperature to rise and rock to eventually melt

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Age of the Earth

• Age of a rock can be determined by measuring the
  amount of radioactive material it contains
• Radioactive atoms decay into daughter atoms
• Uranium decays into lead
• Radioactive potassium decays into argon
• Oldest rocks ages to 3.6 billion years
• Rocks from the moon and other bodies have ages
  about 4.5 billion years

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Movement in Earths interior

• Convection heated matter becomes less dense and
  rises
• Plate tectonics rifting the spreading and
  breaking apart of the crust acts like a
  conveyor belt
• Subduction one piece of crust sinking of being
  pushed beneath another piece of crust
• Called continental drift or plate tectonics

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Earths Atmosphere

• Mostly Nitrogen (78) and oxygen (21)
• Early atmosphere likely was hydrogen, methane,
  ammonia, and water vapor (methane and ammonia are
  noxious poisons)
• Solar winds and UV radiation would have destroyed
  this early atmosphere
• Secondary atmosphere - Outgassing from volcanoes
• Comets hitting the young Earth

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Earths Atmosphere

• Over the past 3 billion years the amount of
  oxygen in our atmosphere has been increasing
• This rise in oxygen paralleled the development on
  life on our planet
• Most of the free oxygen we breath today came
  water in the photosynthetic process
• The rest comes water molecules being split by UV
  radiation

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The Ozone Layer

• Some shielding of UV radiation comes from O2, but
  most comes from Ozone O3
• Located about 25 km above the surface of the
  Earth
• Life probably couldnt exist without the ozone
  layer
• Control of ozone destroying chemicals

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The Greenhouse effect

• Sunlight converted to heat at the Earths surface
• Surface radiates infrared energy that is blocked
  by the atmosphere from returning to space
• This blocking makes the surface warmer than it
  would be without this blocking effect

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The Greenhouse effect

• This does not generate heat, but merely keeps
  heat from being lost into space
• This infrared is absorbed by carbon dioxide and
  water vapor which is then re-emitted both into
  space and back to Earth
• Mars has carbon dioxide in its atmosphere but so
  little of it that it doesnt trap much heat
• Venus has a lot of carbon dioxide and traps a lot
  of heat
• Scientist prefer to use the term atmospheric
  effect

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The Earths magnetic field

• William Gilbert was the first to recognize that
  the Earth acted like a magnet (1603)
• The Chinese has used this idea hundreds of years
  earlier to invent the compass
• Magnetic lines of force the closer the lines
  the stronger the magnetic field
• Polarity gives field lines direction start at
  a north magnetic pole and end at a south magnetic
  pole

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The Earths magnetic field

• Generated by electric currents
• These currents are generated in Earths molten
  iron core
• Scientists think they come from rotational motion
  and convection
• Earths magnetic poles do no coincide with its
  axial poles
• The position of the magnetic pole changes
  slightly from year to year and even reverses
  polarity about every 200,000 years.
• These changes probably come from the irregular
  motion of the core

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Effects of the Magnetic Field

• Screens us from charged particles emitted by the
  sun
• Aurora caused by these charged particles
  colliding with nitrogen and oxygen in the upper
  atmosphere
• Magnetosphere found at the top of the
  atmosphere where it controls the structure of the
  atmosphere
• Magnetosphere traps charged particles in two
  doughnut shaped rings called the Van Allen
  Radiation belts

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Magnetosphere
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Earths motion

• The Seasons
• Coriolis effect air and ocean circulation
  also establishes the direction of the jet streams
• Precession - the very slow change in the
  direction that Earths axis is pointing
• Precession wouldnt happen if the Earth were a
  perfect sphere
• One swing in about 26,000 years
• This will eventually mean Vega replaces the North
  Star (Polaris) as the one the North pole points
  toward (14,000 AD)
• Climate change