9 - Chemistry of the Atmosphere

• Around 78% of the atmosphere is nitrogen. Around 21% is oxygen. The rest contains small proportions of other gases e.g. carbon dioxide, water vapour, and other noble gases such as argon.
• The gases in the atmosphere have stayed relatively constant for around 200 million years
• The earth is around 4.6 billion years old.
• During the first billion years of the earth, scientists believe there was intense volcanic activity that released the gases of the atmosphere, one of which being water vapour. As the earth cooled, the water vapour condensed to form the oceans. The volcanoes had also released huge amounts of carbon dioxide.
• In the early stage of the earth, the atmosphere consisted mainly of carbon dioxide with little or no oxygen, very similar to the atmosphere of planets such as mars or venus today.
• Volcanoes released many gases including:

  • Nitrogen
  • Water vapour
  • Carbon dioxide
  • Methane
  • Ammonia

  Which all built the earth's early atmosphere.
• The atmosphere of the early earth is very different to today's earth's atmosphere. The early earth consisted of mainly carbon dioxide and little nitrogen, whilst today it consists of mainly nitrogen, some oxygen, and few other gases.

  
• Oceans were formed by condensation of water vapour, and some carbon dioxide dissolved into these oceans to form a weak acid, reacting with minerals in the sea to form precipitates which over time formed sediments of carbonate rock on the sea bed.
• Some of the carbon dioxide in the oceans made corals and shells of organisms. Upon death, these would form limestone and removed carbon dioxide from the atmosphere.
• Around 2.7 billion years ago, photosynthetic algae evolved in oceans, and through photosynthesis they produced oxygen which began to enter the atmosphere. This allowed plants to evolve and increased the amount of oxygen in the atmosphere, eventually also allowing animals to evolve.
• Through photosynthesis, carbon is absorbed and can be trapped in fossil fuels, which caused the levels of carbon dioxide in the early atmosphere of the earth to fall.