Chapter 6: Planet Earth

Earth Characteristics

Earth as a Planet

  • We know a tremendous amount about Earth - the study of geology.
  • Earth is also the closest planet to us.
  • We use Earth’s general properties, structures, and physical processes as a guide to understanding the other planets of the solar system.

Earth’s Shape

  • In Chapter 2 we learned:
  • Aristotle noted evidence that Earth is a sphere.
  • Eratosthenes measured Earth’s radius.
  • However, Newton’s laws of motion and gravity (Chapter 3) show that the spinning Earth is not a perfect sphere, but has a bulging equator.
  • Radius of Earth is about 6,356.8 km
  • Planets, including Earth, also have equatorial bulges caused by their rotation which makes the radius different at certain parts of the planet.

Earth’s Composition

  • We can analyze Earth’s surface to learn its composition.
  • Earth’s radius = 6,378 km, but the deepest we have drilled into Earth is only 12 km < 0.2% of Earth’s radius.
  • Therefore, we have not explored inside Earth.
  • We must use indirect methods to learn about the Earth’s interior.

Average Density of Earth

  • We know Earth’s radius, beginning with the method of Eratosthenes.
  • We can measure the acceleration of gravity at Earth’s surface and use Newton’s laws of motion and gravity to find Earth’s mass.
  • We combine Earth’s mass and radius to find Earth’s average density where Earth’s mass is in grams and Earth’s radius is in centimeters.
  • To explain Earth’s high average density the interior of Earth must contain much denser matter.
  • The calculation in the text finds that about 50% of Earth’s volume is iron and nickel.
  • This is a conclusion from logic, not a direct measurement of Earth’s interior.

Earth’s Interior

  • The model that Earth’s interior is 50% iron needs to be tested - the Scientific Method outlined in the Preview.
  • One way to test Earth’s interior is by shaking it - using earthquakes that send waves through Earth.


  • Every earthquake sends out two kinds of seismic waves:
  • P waves - “P” stands for “primary” because they travel fastest and arrive first at a distant location, but “P” can also represent “pressure” or pushing.
  • S waves - “S” stands for “secondary” because they arrive after the “P” waves since they are slower.
  • Detectors located around Earth record the arrival time and the strength of the seismic P and S waves of every earthquake.
  • P waves travel through solids, liquids, and gasses (Ex. Knocking a door).
  • S waves travel only through solids because the matter must be connected.
  • The wave’s speed \rightarrow matter’s density.

Earth’s Interior Structure

  • Analyzing the P and S waves we learn the structure of Earth’s interior.

Crust - solid surface of Earth, extending to depths of 20 to 70 km.

Mantle - solid, denser rock below the crust to a depth of about 2,900 km.

  • A molten outer metallic (iron + nickel) core.
  • A solid inner metallic (iron + nickel) core, solid because of the greater pressure.

Why is Earth’s Interior Hot?

  • Earth’s core temperature is estimated to be 6,500 K - hotter than the Sun’s surface!
  • Earth was very hot when it formed billions of years ago
  • And radioactive elements in the core, like uranium, release heat over billions of years.
  • And Earth’s crust acts like a blanket, slowing the interior’s heat from escaping to space.
  • This could happen in other planets.
  • Why is Earth’s Core Made of Iron?
  • A solid Earth should be a mix of different elements - not separated rock and iron.
  • However, the young Earth was so hot that it was completely molten - fluid.
  • In a fluid, the denser matter sinks down and the less dense matter floats to the top - called differentiation.

How Old is Earth?

  • To measure Earth’s age we use isotopes (see Chapter 4) that are radioactive - change spontaneously from one chemical element into another.
  • Radioactive potassium-40 (19 p+ + 21 n0)
  • Calcium-40 (20 p+ + 20 n0)
  • Argon-40 (18 p+ + 22 n0)
  • Potassium’s rate of change = half-life = 1.28 billion years.
  • Knowing the half-life, we find a rock’s age by measuring the amount of (argon-40/amount of potassium-40).
  • This is the time since the rock cooled and became solid, trapping the argon gas.
  • Oldest rocks \rightarrow Earth formed almost 4.6 billion years ago - “deep time”.

Dynamic Earth

  • Earth is so old that features we consider permanent may be quite “new” and have changed greatly over that time.
  • What could cause Earth to change?
  • Erosion by water, ice, and wind.
  • Motion in Earth’s interior.
  • Recall Earth’s interior is very hot.
  • Fundamental laws of energy \rightarrow heat goes from hot regions to cooler regions.
  • How does the heat from Earth’s core come to the surface?
  • Radiation? NO - Earth is solid so it doesn’t go inside the Earth.
  • Conduction? NO - Very inefficient.
  • Convection? YES - Hotter material travels to a cooler region.
  • Speed is about 2 cm/year. Takes 50-200 million years to go through one cycle.

Continental Drift - “Plate Tectonics”

  • Earth’s lower density crust “floats” on the higher density mantle.
  • Convection in the mantle pushes the rigid crust, breaking it up into pieces - rifting.
  • Moving crust causes earthquakes.
  • Crust pieces can move apart.
  • Crust pieces can collide.
  • Mantle material can reach the surface as volcanic plumes.
  • The idea of continental drift began long ago from the shape of South America and west Africa.
  • The idea was rejected and revived many times over the centuries.
  • The debate was settled when rocks from South America and west Africa were found to have the same ages and composition.
  • We can measure the motions of the continents to be up to 10 cm/year. Using this speed we can work out the arrangement of the continents millions of years ago.
  • We will look for evidence of plate tectonics on other planets.

Earth’s Magnetic Field

  • Earth has a global magnetic field with north and south magnetic poles, although they are not at the north and south rotation poles.
  • Earth’s magnetic field is NOT permanent:
  • The magnetic poles are moving constantly.
  • Lava records that the direction to the poles has flipped many times over millions of years N \rightarrow S \rightarrow N \rightarrow S …
  • Earth’s magnetic field is created by an electric current in the core - a dynamo.
  • Earth’s core is made of iron so it conducts electricity.
  • Earth’s outer core is molten so it flows.
  • Earth rotates driving a current in the core.
  • Why Earth’s magnetic field flips is still being studied.
  • Do other planets have magnetic fields?
  • Earth’s magnetic field protects Earth from charged cosmic particles.
  • We observe this in the aurora.

Earth’s Atmosphere

  • We live in Earth’s atmosphere, so it is easy to study.
  • At Earth’s surface each cm3 of air contains about 1019 molecules.
  • At every altitude the air pressure exactly balances the weight of the air above it.
  • Therefore the air density decreases with increasing altitude.
  • Air temperature also varies with altitude, but it depends on the absorption and emission of energy from radiation and cosmic rays.
  • Low atmosphere - “Troposphere”. The temperature decreases with altitude.
  • Higher in the atmosphere - “Stratosphere” the temperature increases with altitude.
  • Still higher in the atmosphere the temperature decreases and then increases.
  • At Earth’s surface we can easily measure the composition of the air.
  • Atmosphere made of mainly Nitrogen and Oxygen.
  • At higher altitude other molecules become important - ozone = O3, which is important because it absorbs harmful ultraviolet sunlight and protects Earth’s surface.

Greenhouse Effect

  • In chapter 4 we learned that energy is conserved: absorbed - emitted.
  • Earth’s surface temperature is et by energy absorbed from sunlight = the energy emitted by the warmed Earth.
  • Without an atmosphere Earth’s surface temperature would be -20o C = 253 K, the same as the Moon’s surface temperature.
  • Earth’s surface is much warmer, average is about 15o C, because the atmosphere slows Earth’s infrared emission from escaping to space.
  • This is also how a greenhouse works, so it is called the greenhouse effect.
  • The carbon dioxide (CO2) and water vapor (H2O) molecules are major absorbers.
  • Do other planets have the greenhouse effect?

Origin of Earth’s Atmosphere

  • Our atmosphere is 78% N2 and 21% O2.
  • Is this Earth’s original atmosphere? No.
  • There are several ideas about how Earth’s original atmosphere formed:
  • Volcanic eruptions emitted gas trapped inside Earth.
  • Object impacting Earth freed trapped gas.
  • Impacting comets vaporize their frozen gas.
  • All explanations form an atmosphere with carbon dioxide (CO2), methane (CH4), ammonia (NH3) … very different from now.
  • What changed the composition?
  • Ultraviolet sunlight broke molecules apart, some H drifted into space leaving C, N, and to form new molecules.
  • Water vapor condensed into rain and washed molecules like CO2 out of the atmosphere.
  • Planet life began in the ponds, lakes, and oceans (we don’t know how) and created oxygen.
  • Plants use sunlight energy + CO2 \rightarrow O2
  • Over billions of years Earth’s atmosphere gradually changed to its current composition.

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