The Earth's early atmosphere: 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.

The idea

The Earth formed about 4.6 billion years ago, and no direct samples of its earliest atmosphere survive. Scientists therefore build models from indirect evidence, including the composition of gases released by modern volcanoes, the atmospheres of other planets, and rocks and minerals that formed long ago. One widely accepted model suggests that intense volcanic activity released carbon dioxide, water vapour and nitrogen, with little or no oxygen. As the Earth cooled, water vapour condensed to form oceans, and carbon dioxide later dissolved in the oceans and became locked in carbonate rocks and fossil fuels. Photosynthesising organisms then added oxygen. Because the evidence is indirect and spans such an enormous time scale, theories have changed as new data emerged, showing that scientific explanations are provisional and develop over time.

What earns the marks

• State that the Earth formed about 4.6 billion years ago and that direct evidence from the early atmosphere is not available.
• Describe how volcanic activity released gases such as carbon dioxide, water vapour and nitrogen, forming an early atmosphere with little or no oxygen.
• Explain that water vapour condensed as the Earth cooled to form oceans, and that carbon dioxide later dissolved in oceans or became locked in rocks and fossil fuels.
• Explain that photosynthesising organisms, including algae, later increased oxygen levels in the atmosphere.
• Recognise that theories about the early atmosphere have changed over time because evidence is limited and indirect, and new evidence can lead to revised models.

Common mistakes and how to put them right

• Stating that the early atmosphere contained large amounts of oxygen.

  Put it right: Oxygen was absent or very low until photosynthesising organisms evolved.

• Claiming that scientists know the exact composition of the early atmosphere.

  Put it right: Evidence is indirect and limited, so the composition is modelled rather than directly measured.

• Confusing the formation of oceans with the formation of the atmosphere.

  Put it right: Volcanic gases formed the early atmosphere, while cooling and condensation of water vapour formed the oceans.

Exam tips

• Use the phrase 'about 4.6 billion years ago' when dating the Earth and link the long time scale to why direct evidence is limited.
• When describing the early atmosphere, name carbon dioxide, water vapour and nitrogen, and state that oxygen was little or none.
• Explain how a change occurred, for example water vapour condensing to form oceans or photosynthesis adding oxygen, rather than listing gases without a process.