The Earth's Atmosphere — Past and Present

This lesson covers the composition of the Earth's atmosphere, how it has changed over time, and the evidence for these changes, as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 9. You need to know the approximate composition of today's atmosphere, what the early atmosphere was like, and the processes that caused it to change.

---

The Current Atmosphere

The Earth's atmosphere today has a very stable composition. The main gases are:

Gas	Approximate percentage
Nitrogen (N₂)	~78%
Oxygen (O₂)	~21%
Argon (Ar)	~0.9%
Carbon dioxide (CO₂)	~0.04%
Water vapour (H₂O)	Variable (0–4%)
Other trace gases	Very small amounts (neon, helium, methane, etc.)

Exam Tip: You must know the approximate percentages of nitrogen (~78%), oxygen (~21%) and carbon dioxide (~0.04%). These numbers come up very frequently in exam questions.

---

The Early Atmosphere

The Earth is approximately 4.6 billion years old. For the first billion years or so, the atmosphere was very different from today. Scientists believe the early atmosphere was similar to the atmospheres of Mars and Venus today.

Composition of the Early Atmosphere

Gas	Approximate proportion	Source
Carbon dioxide (CO₂)	Very high (~95%)	Volcanic eruptions
Water vapour (H₂O)	High	Volcanic eruptions
Nitrogen (N₂)	Small but increasing	Volcanic eruptions; released by reactions of ammonia
Ammonia (NH₃)	Small amounts	Volcanic eruptions
Methane (CH₄)	Small amounts	Volcanic eruptions
Oxygen (O₂)	Little or none	Not yet produced by living organisms

The early atmosphere was produced mainly by volcanic activity. Volcanoes released huge volumes of CO₂, water vapour, nitrogen and other gases.

Exam Tip: The key point about the early atmosphere is that it was mainly carbon dioxide with very little or no oxygen. This is the opposite of today.

---

How the Atmosphere Changed

The transformation from the early CO₂-rich atmosphere to today's nitrogen-oxygen atmosphere happened over billions of years through several key processes.

Stage 1: Formation of the Oceans

• As the Earth cooled over millions of years, the water vapour in the atmosphere condensed to form the oceans.
• This was a crucial step because the oceans then acted as a reservoir for dissolving CO₂.

Stage 2: Carbon Dioxide Was Removed

CO₂ was removed from the atmosphere by three main processes:

1. Dissolving in the oceans: CO₂ is soluble in water. As the oceans formed, vast amounts of CO₂ dissolved in the seawater.

2. Photosynthesis by early organisms: The first photosynthetic organisms (cyanobacteria and later algae and plants) appeared about 2.7 billion years ago. They used CO₂ and released oxygen:

   carbon dioxide + water → glucose + oxygen

   6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

3. Formation of sedimentary rocks and fossil fuels:

   • Dissolved CO₂ reacted with metal ions in the sea to form insoluble carbonates, which sank and formed sedimentary rocks such as limestone (calcium carbonate, CaCO₃).
   • The shells and skeletons of marine organisms (made of calcium carbonate) also accumulated on the sea floor and eventually formed limestone.
   • Dead organisms that were buried and compressed over millions of years formed fossil fuels (coal, oil, gas), locking carbon away underground.

Stage 3: Oxygen Levels Increased

• Photosynthetic organisms gradually produced more and more oxygen.
• At first, the oxygen reacted with iron and other metals in the rocks and oceans (forming iron oxide — this is why some ancient rocks are red).
• Once the rocks and oceans were "saturated" with oxygen, it began to accumulate in the atmosphere.
• Oxygen levels reached roughly their current level (~21%) about 400–500 million years ago.

Stage 4: Nitrogen Accumulated

• Nitrogen was released by volcanoes and by reactions involving ammonia.
• Unlike CO₂ and O₂, nitrogen is very unreactive — it does not dissolve easily in water and does not react with rocks.
• Because nitrogen was not removed, it gradually accumulated to become the most abundant gas in the atmosphere (~78%).

---

How the Atmosphere Changed — Summary Diagram

flowchart TD
    A["Early Earth<br/>Atmosphere: mainly CO₂,<br/>water vapour, N₂, NH₃, CH₄<br/>No oxygen"] --> B["Earth cools<br/>Water vapour condenses<br/>→ Oceans form"]
    B --> C["CO₂ dissolves in oceans<br/>CO₂ used by photosynthetic<br/>organisms (cyanobacteria)"]
    C --> D["Oxygen released by<br/>photosynthesis<br/>O₂ levels rise"]
    C --> E["CO₂ locked away in<br/>sedimentary rocks (limestone)<br/>and fossil fuels"]
    D --> F["Present atmosphere<br/>~78% N₂, ~21% O₂,<br/>~0.04% CO₂, ~1% Ar"]
    E --> F
    G["N₂ unreactive<br/>→ accumulates over time"] --> F

---

Evidence for Changes in the Atmosphere

How do scientists know what the early atmosphere was like? Several lines of evidence are used:

Evidence	What it tells us
Volcanic gas analysis	Modern volcanoes release CO₂, water vapour, N₂ and SO₂ — similar to what early volcanoes would have emitted
Comparison with Mars and Venus	These planets have atmospheres of ~95% CO₂, suggesting Earth's early atmosphere was similar before life changed it
Ancient sedimentary rocks	Limestone and other carbonate rocks show that CO₂ was once abundant (it was locked into these rocks)
Fossil record	Fossils of early photosynthetic organisms (stromatolites — fossilised cyanobacteria) date back ~2.7 billion years
Ice cores	Air bubbles trapped in ancient ice (from Antarctica and Greenland) provide a record of atmospheric composition going back ~800,000 years
Banded iron formations	Ancient rock layers containing iron oxide show that oxygen was first produced in the oceans, reacting with dissolved iron before accumulating in the air

Exam Tip: The composition of the early atmosphere is still uncertain — there are no rocks from the first billion years of Earth's history. Scientists use the evidence above to develop models, but these may change as new evidence is found. Be prepared to discuss the limitations of the evidence.

---

Why the Atmosphere Is Now Stable

The current atmosphere has been relatively stable for about 200 million years. This is because:

• The rates of processes that add oxygen (photosynthesis) roughly balance those that remove it (respiration, combustion).
• The rates of processes that add CO₂ (respiration, combustion, volcanic activity) roughly balance those that remove it (photosynthesis, dissolving in oceans).

However, human activities are now disrupting this balance — this is covered in the next lesson on climate change.

---