The Evolution of the Atmosphere

• Gases in the atmosphere today


  • Nitrogen (78%)
  • Oxygen (21%)
  • Carbon dioxide
  • Water vapor
  • Noble gases (e.g. argon)
• The gases in the atmosphere have stayed pretty constant for around 200 million years
• Early atmosphere


  Consisted mainly of carbon dioxide with little or no oxygen, similar to the atmospheres of Mars and Venus today
• Formation of early atmosphere


  Volcanoes released gases including water vapor, carbon dioxide, nitrogen, methane, ammonia
• As the Earth cooled


  Water vapor condensed to form the oceans
• Carbon dioxide dissolved in the oceans


  Formed a weak acid that reacted with minerals to form carbonate rock sediments
• Carbon dioxide used by organisms


  To make corals and shells, forming limestone sedimentary rock
• Photosynthetic algae evolved


  Produced oxygen that entered the atmosphere
• Plants evolved


  Increased the amount of oxygen in the atmosphere
• Photosynthesis produces oxygen and traps carbon in fossil fuels, causing carbon dioxide levels to fall
• The level of oxygen reached a point where animals could evolve