GCSE Chemistry · AQA 8462 · Topic 4.9.1
The composition and evolution of the Earth's atmosphere
Key points
- Today's atmosphere is about four-fifths nitrogen and one-fifth oxygen, with small proportions of carbon dioxide, water vapour and noble gases.
- Evidence about the early atmosphere is limited because of the huge time scale (about 4.6 billion years).
- One theory: intense volcanic activity released mostly carbon dioxide, with water vapour and small amounts of methane, ammonia and nitrogen; similar to the atmospheres of Mars and Venus today.
- As the Earth cooled, water vapour condensed to form the oceans. Carbon dioxide dissolved in the oceans and formed carbonate precipitates and sediments.
- Algae and later plants produced oxygen by photosynthesis: . Oxygen built up gradually so animals could evolve.
- Carbon dioxide levels fell further as carbon was locked into sedimentary rocks (limestone) and fossil fuels (coal, crude oil, natural gas).
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The Earth is about 4.6 billion years old, so no one measured its first atmosphere. Scientists use evidence from rocks and from other planets to build a theory of how the atmosphere changed into the one we breathe today.
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