AQA Combined Science (8464) 5.9.1.2–5.9.1.4 · Process
The Earth's early atmosphere and how it changed
Four billion years ago the air had almost no oxygen. Where did today's oxygen come from, and where did all that carbon dioxide go?
Combined Science · Higher tier
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Venus and Mars may still have the kind of air Earth started with.
Scientists think Earth’s air, about 4 billion years ago, was mainly carbon dioxide, like theirs. Compare it with Earth’s air today.
If you had to guess, is most of the carbon from all that carbon dioxide now locked underground, or inside living things?
AQA examines one theory of how Earth's atmosphere formed and changed.
Run the clock · the first billion years
Volcanoes everywhere
During its first billion years the Earth has intense volcanic activity. The volcanoes release the gases that form the early atmosphere.
Which gas is missing from what the volcanoes release?
Equation
carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Spot the flaw
ExplainTo explain where the oxygen came from, a student writes: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. What is wrong?
Build the chain
How crude oil and natural gas formed
Coal formed from plants on land. Crude oil and natural gas formed in the sea. Put the links in order.
4 links belong. 3 do not.
The links
Your chain · 0 of 4
No sample of the early atmosphere survives: evidence is limited because of the 4.6-billion-year time scale. So scientists build the theory from indirect evidence, such as the minerals in the oldest rocks, the gases volcanoes give out today and the atmospheres of other planets. Theories about the early atmosphere have changed over time as new evidence arrived. In the exam you may be given evidence and asked to evaluate a theory: what supports it, what weakens it, then a judgement.
Weigh the evidence
Was early Earth's air like Mars's air today?
Decide what each piece of evidence does to that idea.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- The changes in order, each with its cause: oceans, then algae, then plants.
- Explain
- Give the reason, and link each step to the next with so or because.
- Give
- A short answer: a reason or a fact, no explanation needed.
- Evaluate
- Evidence for, evidence against, then your judgement.
Key note · AQA 5.9.1.2–5.9.1.4 (8464)
The Earth's early atmosphere and how it changed
- Early atmosphere (one theory): gases released by volcanoes; mainly carbon dioxide, with water vapour and some nitrogen, little or no oxygen, perhaps some methane and ammonia.
- Oceans formed when the Earth cooled and water vapour condensed; carbon dioxide dissolved and carbonates formed sediments.
- Oxygen increased from about 2.7 billion years ago, by photosynthesis by algae and later plants, to a level that let animals evolve.
- Carbon dioxide decreased: dissolved in the oceans, photosynthesis, and locked in sedimentary rocks (limestone) and fossil fuels (coal, crude oil, natural gas).
- Nitrogen built up: released by volcanoes and unreactive, so little was removed.
- Evidence is limited because of the 4.6-billion-year time scale.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure what took the carbon dioxide out of the air?
The air bar in Run the clock is a sketch: it shows which way each gas changed, not measured percentages, and in reality the stages overlapped rather than following one at a time. This is the one theory AQA examines; scientists still debate its details.