Atmosphere

• The earth's atmosphere is composed of about 80% nitrogen, 20% oxygen, and less than 1% of other gases like carbon dioxide, water vapor, and noble gases
• How the earth's atmosphere evolved

  1. Volcanic activity produced CO2, water vapor, nitrogen, methane, ammonia

  2. Water vapor condensed into oceans, allowing CO2 to dissolve

  3. Algae and plants evolved, using CO2 for photosynthesis and producing O2

  4. Organic matter from dead algae/plants became buried and formed fossil fuels
• The early atmosphere was mostly CO2, similar to Mars and Venus
• Photosynthesis takes in CO2 and produces O2
• As algae and plants photosynthesized

  Atmospheric CO2 levels declined and O2 levels increased
• More complex life like animals could evolve due to the increased O2 levels
• Types of sedimentary rocks/fossil fuels formed from buried organic matter

  • Crude oil
  • Natural gas
  • Coal
  • Limestone
• The exact substance formed depends on the type of organism being buried and the conditions involved
• This process of atmospheric evolution occurred over the past 4.6 billion years
• The details of this process are considered a theory, as there is not as much evidence as we might like due to the long timescale involved