The composition and evolution of the Earth's atmosphere

• What are the main gases in the atmosphere and their approximate proportions?

  ~80% nitrogen and ~20% oxygen
• What does the symbol ∼ represent in the context of gas proportions?

  It means around or approximately
• Why is evidence about the Earth's early atmosphere limited?

  Due to the time scale of 4.6 billion years and evolving theories
• What was one theory about the Earth's early atmosphere during the first billion years?

  Intense volcanic activity released gases that formed the early atmosphere
• How did the early atmosphere compare to that of Mars and Venus?

  It may have been mainly CO₂ with little or no O₂
• What gases did volcanoes produce that contributed to the atmosphere?

  Nitrogen (N₂), methane (CH₄), and ammonia (NH₃)
• What happened to water vapor in the early atmosphere?

  It condensed to form the oceans
• How did CO₂ levels change due to the formation of oceans?

  CO₂ dissolved in water and carbonates were precipitated
• What is the only theory of the early atmosphere that students need to know?

  The theory of intense volcanic activity releasing gases
• What process produced the oxygen now found in the atmosphere?

  Photosynthesis by algae and plants
• Write the equation for photosynthesis.

  6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• When did algae first produce oxygen?

  About 2.7 billion years ago
• How did the percentage of oxygen change over the next billion years?

  It gradually increased to enable animal evolution
• How did algae and plants decrease the percentage of CO₂ in the atmosphere?

  By photosynthesis
• What geological processes contributed to the decrease of CO₂ in the atmosphere?

  Formation of sedimentary rocks and production of fossil fuels
• What types of sedimentary rocks contain carbon?

  Limestone and coal
• How are fossil fuels produced from dead plants and animals?

  Through the process of decay