Earth's Early Atmosphere

This lesson covers the Earth's early atmosphere as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what the early atmosphere was like, where it came from, and how the oceans formed.

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How Did the Atmosphere Form?

The Earth is approximately 4.6 billion years old. During the first billion years, there was intense volcanic activity. Scientists believe the early atmosphere was formed by volcanic degassing — gases released from volcanoes.

Evidence from Other Planets

We cannot directly sample Earth's ancient atmosphere, but we can look at the atmospheres of other rocky planets:

• Venus and Mars both have atmospheres rich in carbon dioxide (about 95–96% CO₂).
• Scientists believe Earth's early atmosphere was similar — mostly CO₂ with some other volcanic gases.

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Composition of the Early Atmosphere

The early atmosphere (around 4–3.5 billion years ago) was very different from today:

Gas	Approximate Proportion	Source
Carbon dioxide (CO₂)	Very high (~95%)	Volcanic eruptions
Water vapour (H₂O)	Very high	Volcanic eruptions
Nitrogen (N₂)	Small but increasing	Volcanic eruptions; built up over time as it is unreactive
Methane (CH₄)	Small amounts	Volcanic activity
Ammonia (NH₃)	Small amounts	Volcanic activity
Oxygen (O₂)	Virtually none	Not yet produced by photosynthesis

Key Features

• No oxygen — there were no plants or algae to produce it.
• Very high CO₂ — similar to Venus and Mars today.
• Lots of water vapour — from continuous volcanic eruptions.
• No ozone layer — without oxygen there was no ozone (O₃) to protect the surface from UV radiation.

graph TD
    V["🌋 Intense Volcanic Activity"] --> CO2["CO₂ released<br/>(~95% of atmosphere)"]
    V --> H2O["H₂O vapour released<br/>(large quantities)"]
    V --> N2["N₂ released<br/>(small amounts, building up)"]
    V --> CH4["CH₄ and NH₃<br/>(trace amounts)"]

    style V fill:#d32f2f,color:#fff
    style CO2 fill:#795548,color:#fff
    style H2O fill:#1565c0,color:#fff
    style N2 fill:#2e7d32,color:#fff
    style CH4 fill:#f57f17,color:#000

Exam Tip: You must state that the early atmosphere had little or no oxygen and was mainly carbon dioxide and water vapour. Do not confuse the early atmosphere with the modern atmosphere.

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Formation of the Oceans

As the Earth gradually cooled over hundreds of millions of years:

1. The temperature dropped below 100 °C.
2. Water vapour in the atmosphere began to condense.
3. Rain fell and collected in low-lying areas, forming the oceans.

The formation of the oceans was critical because:

• CO₂ dissolved in the new oceans, which began to reduce the amount of CO₂ in the atmosphere.
• The oceans provided an environment where early life could develop.

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Carbon Dioxide Was Removed

Several processes removed CO₂ from the early atmosphere over billions of years:

1. Dissolving in the Oceans

CO₂ dissolved in the oceans, forming a weakly acidic solution. Some of this dissolved CO₂ later reacted with minerals to form carbonate compounds (e.g. calcium carbonate, CaCO₃).

2. Locked into Sedimentary Rocks

Marine organisms (such as shellfish and coral) used dissolved CO₂ and carbonates to make their shells and skeletons (made of calcium carbonate). When these organisms died, their remains built up on the sea floor and were compressed over millions of years to form sedimentary rocks such as limestone (CaCO₃).

3. Fossil Fuels

The remains of ancient plants and marine organisms were buried and, over millions of years, formed fossil fuels — coal, oil and natural gas. The carbon from CO₂ that was once in the atmosphere became locked away underground.

Process	What Happened to the CO₂
Dissolved in oceans	CO₂ dissolved in water
Sedimentary rocks	Carbonates formed limestone, chalk
Fossil fuels	Carbon locked in coal, oil, gas

Exam Tip: You should be able to explain at least three ways CO₂ was removed from the early atmosphere: dissolving in the oceans, forming sedimentary rocks (limestone), and being stored in fossil fuels.

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Nitrogen Built Up

Nitrogen (N₂) was released by volcanoes in smaller amounts, but because it is very unreactive it was not removed by chemical reactions. Over billions of years, nitrogen gradually accumulated in the atmosphere and became the most abundant gas.

Today, nitrogen makes up about 78% of the atmosphere.

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Theories and Uncertainties

Scientists cannot be completely certain about the exact composition of the early atmosphere because:

• There were no humans around to measure it.
• The evidence is indirect — based on analysis of ancient rocks, the atmospheres of other planets, and volcanic gases.
• Different scientists may have slightly different models, but the general picture is widely agreed upon.

Exam Tip: If asked why we cannot be sure about the early atmosphere, say that it was billions of years ago, no direct measurements were taken, and the evidence is based on indirect sources such as rock analysis and comparison with other planets.

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Summary

• The early atmosphere was formed by volcanic degassing — mainly CO₂ and water vapour, with virtually no oxygen.
• As Earth cooled, water vapour condensed to form the oceans.
• CO₂ was removed by dissolving in the oceans, forming sedimentary rocks (limestone), and being locked into fossil fuels.
• Nitrogen built up over time because it is unreactive.
• Evidence for the early atmosphere comes from analysis of ancient rocks, volcanic gases, and comparison with other planets.

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Worked Examples

Worked Example 1 — Interpreting rock evidence

Question: Iron-rich sedimentary rocks older than about 2.4 billion years often contain reduced iron compounds (grey/black), whereas rocks younger than this contain oxidised iron compounds (red "banded iron formations"). What does this tell us about oxygen levels in the atmosphere?

Answer: The change from reduced to oxidised iron compounds is evidence that free oxygen (O₂) began to accumulate in the atmosphere around 2.4 billion years ago. Before that, there was so little O₂ that iron could not rust. This event is sometimes called the Great Oxidation Event.

Worked Example 2 — Comparing planets

Question: Venus has an atmosphere of ~96% CO₂ and surface temperatures over 450 °C. Mars has ~95% CO₂ but is −60 °C on average. What do these observations support about Earth's early atmosphere?