8.19 - Composition of Earth's early atmosphere

The early-atmosphere model

The early atmosphere is thought to have contained a large amount of carbon dioxide, water vapour, small amounts of other gases, and little or no free oxygen gas. Its composition changed over time; this is a broad model, not a measured recipe with known exact percentages.

“Little or no oxygen” means little or no O₂ gas. It does not mean there were no oxygen atoms: carbon dioxide (CO₂) and water vapour (H₂O) contain oxygen atoms chemically combined with other elements. Water vapour is water in the gas state.

Observations and interpretations

Scientists cannot collect an air sample from the time Earth first formed. They reconstruct past conditions from indirect evidence, including ancient minerals and gases released from volcanoes today. An observation is what is measured or found; an inference is the explanation drawn from it.

Modern volcanoes release water vapour and carbon dioxide. Together with evidence that early Earth was volcanically active, this supports those gases being supplied to the early atmosphere. Modern emissions are not a direct sample of ancient air: volcanic gas composition, conditions and removal processes can differ. The presence of a gas in one modern sample alone cannot establish its ancient atmospheric percentage.

Some ancient river deposits contain mineral grains that would react with oxygen during transport. Their survival supports low oxygen in the air and water at that time. The reasoning is conditional: if oxygen had been abundant for long enough, the grains would have reacted. One rock alone does not describe every place or every era.

Iron-rich red beds becoming widespread later provide further evidence of changing oxidising conditions. This does not mean every ancient iron oxide required an oxygen-rich atmosphere; evidence must be interpreted in its geological setting.