OUTLINE OF LECTURE 25

THE EARTH AS A PLANET (PART I) 

I. General Characteristics of the earth:
  • Semimajor axis 1AU; Orbital Period 1 year
  • Sidereal Rotation period: 23H 56 m
  • Radius: RE = 6378 km.
  • Mass: ME = 5.98 x 1024 kg
  • Density: 5500 kg/m3; 5.5 x that of water. 
  • Escape velocity 11 km/sec.
  • Atmospheric Pressure 1.00 bar.
  • Atmospheric Composition: 78% N2,  21% O2,  1% other gases.

 

II. Internal Structure of the Earth

  • Basic divisions
    • Inner Core--radius ~1200 km
    • Outer Core--1200-3500 km
    • Mantle--3500 km to near surface
    • Crust--ocean crust 5-10 km thick. Continental crust 20-70 km thick.
  • Differences between the layers--composition, physical state, density.
    • Inner Core--solid; Fe, Ni, S.  High density and Temperature ~5000K
    • Outer Core--thick liquid. Fe, Ni, S. High density  and Temp. ~5000K
    • Mantle -- thick liquid. Silicates; igneous rocks. Density 4-5; temperature 103 K.
    • Crust--Mg, Si, Al, compounds.
  • Source of Heat Energy to keep earth molten:
    • Pressure isn't enough to keep it hot
    • If earth had cooled from molten state (which it was once in at time of its formation), it would have cooled in about 105 years (Buffon was first to do this). But since earth is so much older than this, the internal heat can't be left over from this early state.
    • Source is naturally occurring radioactivity---radioactive elements are found in trace amounts in all rock; their energy keeps the earth hot inside.
  • Use of seismic waves to detect discontinuities in density and physical state.
    • P waves -- "Pressure" waves. Transmitted by solids and liquids.
    • S waves -- "Shear" waves. Transmitted only by solids.
    • "Echoes" off density boundaries. 
  • Generation of the earth's magnetic field--dynamo theory.
    • Requisites for a magnetic field:
      • 1. Electrically conductive core
      • 2. Fluid core so that currents can be established.
      • 3. Sufficiently rapid rotation.
    • Effects of Magnetic field: Earth's Magnetosphere (region of magnetic influence)
      • Trapped solar-wind particle belts--Van Allen Belts around the earth.

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Copyright 2007, Laurence A. Marschall, Gettysburg College