The composition and evolution of the Earth's atmosphere

For 200 million years, the proportions of different gases in the atmosphere have been much the same as they are today:

• about four-fifths, approximately 80%, nitrogen • about one-fifth, approximately 20%, oxygen • small proportions of various other gases, including carbon dioxide, water vapour and noble gases.

Both forms are worth having — the fractions and the percentages — because questions here often ask for a ratio or a fraction rather than a percentage.

Theories about what was in the Earth's early atmosphere and how the atmosphere was formed have changed and developed over time. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.

One theory suggests that during the first billion years of the Earth's existence there was intense volcanic activity that released gases that formed the early atmosphere, and water vapour that condensed to form the oceans. At the start of this period the Earth's atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.

Volcanoes also produced nitrogen, which gradually built up in the atmosphere, and there may have been small proportions of methane and ammonia.

When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated, producing sediments and reducing the amount of carbon dioxide in the atmosphere. No knowledge of other theories is required.

You should be able to, given appropriate information, interpret evidence and evaluate different theories about the Earth's early atmosphere.

The reason there is more than one theory is stated in the specification itself: the timescale is 4.6 billion years, so there is very little direct evidence, and what there is has to be inferred from rocks rather than measured. That is the sentence to reach for whenever a question asks why scientists disagree, or why a theory changed. Comparing the Earth with Mars and Venus is not proof — it is an analogy that makes one theory plausible.

Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis, which can be represented by the equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ carbon dioxide + water → glucose + oxygen

Algae first produced oxygen about 2.7 billion years ago, and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.

The order matters in that last sentence: the oxygen came first, and animals could only evolve once it was there.

Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis. Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon.

The same equation explains both changes at once: photosynthesis takes carbon dioxide out and puts oxygen in.

You should be able to describe and explain the formation of deposits of limestone, coal, crude oil and natural gas — the four places the carbon went.

You should also be able to describe the main changes in the atmosphere over time and some of the likely causes of these changes. A full account runs: mainly carbon dioxide with little or no oxygen from volcanic activity; oceans formed and carbon dioxide dissolved, precipitating carbonates; algae and then plants photosynthesised, removing carbon dioxide and adding oxygen; nitrogen from volcanoes built up because it is unreactive; and carbon was locked away in sedimentary rocks and fossil fuels. By about 200 million years ago the proportions had reached what they are today.