Earth’s atmosphere

• 76% N2
• 23% O2 -- only planet with oxygen!
• 0.05% CO2 -- very low!
• 0.1% (desert) - 1.7% (jungle) H2O vapor -- trapped below 10 km

• large amount of O2 maintained by plant life
  • photo-synthesis
  • life sign for extra-solar planets
  • Original H, He atmosphere escaped
    • Earth’s gravity too low and temperature too high
      • atom’s speed exceeds escape velocity
    • much heavier CO2, N2 retained
      • lower thermal velocity because of higher mass
      • H2O is in danger of escaping if it gets above 10 km

    • Origin of Earth’s atmosphere

      • Original (primordial) atmosphere boiled away
        • molten primeval Earth (infall and radioactive decay)

    Previous slide

    Next slide

    Back to first slide

    View graphic version