Study Notes on Atmospheric Composition and Changes

Proportion of Gases in the Atmosphere

  • Approximately 78% of the atmosphere is nitrogen.

  • About 21% of the atmosphere is oxygen.

  • Small proportions of other gases include:

    • Carbon dioxide

    • Water vapor

    • Noble gases (e.g., argon)

Historical Changes in the Atmosphere

  • The composition of the atmosphere remained relatively constant for around 200 million years.

  • Major changes occurred prior to this stable period.

  • The age of the Earth is approximately 4.6 billion years, but the early atmosphere details are uncertain.

Formation of the Early Atmosphere

  • During the first billion years of Earth's existence:

    • Intense volcanic activity was prevalent.

    • Volcanoes released gases that contributed to the formation of the early atmosphere, including water vapor.

  • As the Earth cooled:

    • Water vapor condensed to form oceans.

    • Significant amounts of carbon dioxide were released by volcanoes.

Composition of the Early Atmosphere

  • Initially, the Earth's atmosphere was mainly composed of carbon dioxide.

  • Included little or no oxygen, akin to the atmospheres of Mars and Venus today.

    • Volcanic gases included:

    • Nitrogen, which gradually accumulated in the atmosphere.

    • Small amounts of methane and ammonia.

Key Differences Between Early and Current Atmosphere

  • The early atmosphere had:

    • High levels of carbon dioxide compared to today.

    • Very low levels of oxygen, in contrast to the current 21% oxygen.

Mechanisms of Change in Atmospheric Composition

  • Once oceans formed:

    • Some carbon dioxide dissolved in oceans, forming weak acids.

    • Reactions with minerals in the sea produced precipitates that formed sedimentary rocks (carbonate rock) on the seabed.

    • Some carbon dioxide contributed to the formation of corals and shells of marine organisms (e.g., mussels).

    • When these organisms died, they formed limestone sedimentary rock, further removing carbon dioxide from the atmosphere.

Evolution of Oxygen in the Atmosphere

  • Approximately 2.7 billion years ago:

    • Photosynthetic algae evolved in oceans.

    • Photosynthesis process:

    • Produces oxygen and absorbs carbon dioxide.

  • Over the next billion years:

    • Evolution of plants led to an increase in atmospheric oxygen levels.

    • Eventually, oxygen levels reached a threshold facilitating the evolution of animals.

Photosynthesis Equation

  • The equation of photosynthesis includes:

    • Carbon dioxide and water are converted into glucose and oxygen by using sunlight.

    • This process also leads to the trapping of carbon in fossil fuels.

    • Result: Reduction in atmospheric carbon dioxide levels.

Additional Notes

  • The topic of the atmosphere is commonly examined in assessments.