The Evolution of the AtmosphereAQA GCSE Chemistry: Revision notes
Section 1
What was the composition of Earth's early atmosphere?
Earth's early atmosphere, which formed approximately 4.6 billion years ago, was very different from today's atmosphere. The early atmosphere was composed of:
- Carbon dioxide (CO₂) – the main component
- Water vapour (H₂O) – present in large quantities
- Ammonia (NH₃) – small amounts
- Other gases – including methane and nitrogen compounds in trace amounts
- Oxygen (O₂) – very little or none present
This atmosphere was produced by outgassing from the Earth's interior during volcanic activity. The absence of oxygen is crucial to understand, as this indicates that the early atmosphere was a reducing atmosphere (lacking oxygen). Life as we know it could not have existed in these conditions.
Examiners want you to state clearly that the early atmosphere had little or NO oxygen – this contrast with today is a key point. Always mention CO₂ and H₂O as the main components.
Section 2
How did water vapour and carbon dioxide leave the atmosphere?
Two critical processes removed these gases from the early atmosphere:
Water vapour removal:
- As the Earth cooled, water vapour condensed to form liquid water
- This process occurred over millions of years
- The condensed water accumulated to form the oceans
- Once oceans existed, water vapour continued to cycle but no longer accumulated in the atmosphere
Carbon dioxide removal:
- CO₂ dissolved in the oceans when water was present
- Dissolved CO₂ reacted with minerals to form carbonates
- Carbonates became locked in sedimentary rocks (like limestone and chalk) through chemical precipitation
- Some carbon became trapped in fossil fuels (coal, oil, natural gas) as dead organisms accumulated on the ocean floor over millions of years
- This process meant CO₂ was removed from the atmosphere and stored in the Earth's crust for very long periods
These two processes fundamentally altered atmospheric composition and made the planet habitable.
Think of the early atmosphere as a polluted room: water vapour is like steam that condenses on windows and drains away, while CO₂ is like smoke that dissolves into a damp cloth (the ocean) and gets trapped there permanently.
When you see a question asking 'where did atmospheric CO₂ go?', the complete answer should include both pathways: dissolved in oceans → formed carbonates in sedimentary rocks (like limestone) AND incorporated into fossil fuels through dead organisms.
Section 3
What role did photosynthesis play in atmospheric change?
Photosynthesis was the key process that introduced oxygen to the atmosphere:
- Early photosynthetic organisms (simple algae and later plants) appeared in the oceans and on land over billions of years
- These organisms used light energy to convert CO₂ and water into glucose and oxygen
- The oxygen released as a waste product gradually accumulated in the atmosphere
- This process is called the Great Oxygenation Event (occurring around 2.4 billion years ago)
- Over time, photosynthesis increased atmospheric oxygen from nearly 0% to approximately 21% (today's level)
- The build-up of oxygen was slow and took hundreds of millions of years
Why photosynthesis mattered for life:
- Oxygen enabled the evolution of complex organisms that required aerobic respiration
- The accumulating oxygen also formed the ozone layer (O₃), which protects Earth from harmful ultraviolet radiation
- This protection allowed life to eventually move from oceans to land
Without photosynthesis, Earth's atmosphere would never have developed the oxygen-rich composition we depend on today.
Students often forget to explain WHY oxygen accumulated – you must state that photosynthesis produced it faster than it was consumed, allowing gradual atmospheric build-up over billions of years.
Examiners expect you to link photosynthesis to oxygen production AND explain the consequence (enabling complex life and forming the ozone layer). This shows understanding of atmospheric change, not just memorisation.
Section 4
How was ammonia converted to nitrogen?
The early atmosphere contained small amounts of ammonia (NH₃), but this gas is no longer a significant component of our atmosphere. Several processes converted ammonia:
Chemical conversion:
- Ammonia in the atmosphere was converted to nitrogen (N₂) and water through chemical reactions
- These reactions were triggered by energy from ultraviolet radiation and lightning
- The process occurred over billions of years as atmospheric conditions changed
Biological conversion:
- Denitrifying bacteria in soil and water converted nitrogen compounds back to N₂ gas
- This bacterial process became increasingly important as bacteria evolved
- The nitrogen cycle became established, continuously cycling nitrogen between atmosphere, organisms, and soil
Result:
- Ammonia essentially disappeared from the atmosphere and was replaced by nitrogen gas
- Nitrogen now comprises approximately 78% of the atmosphere
- Nitrogen is relatively inert and does not react easily, making it stable in the atmosphere
This conversion was essential for making the atmosphere suitable for life, as ammonia is toxic to most organisms.
Remember: ammonia was removed and replaced by nitrogen. The question is not why nitrogen is present (it wasn't in large amounts initially), but rather how ammonia (which WAS present) disappeared.
Section 5
What is the current composition of Earth's atmosphere?
After billions of years of change, the atmosphere has reached a composition that has remained broadly stable for approximately 200 million years:
| Gas | Percentage (%) | Notes |
|---|---|---|
| Nitrogen (N₂) | ~78 | Inert; most abundant; forms from ammonia conversion |
| Oxygen (O₂) | ~21 | Produced by photosynthesis; essential for respiration |
| Argon (Ar) | ~1 | Inert noble gas; present from Earth's formation |
| Carbon dioxide (CO₂) | ~0.04 (small amount) | Locked in rocks and fossil fuels; now increasing due to human activity |
| Water vapour (H₂O) | Variable (small amount) | Cycles between atmosphere, oceans, and organisms |
| Other gases | Trace amounts | Including neon, helium, and methane |
Key points about current composition:
- The atmosphere is now oxidising (rich in oxygen), not reducing
- Nitrogen and oxygen make up approximately 99% of the atmosphere
- CO₂ and water vapour are present in much smaller quantities than in the early atmosphere
- The stability over the last 200 million years suggests the atmosphere has reached a long-term equilibrium
- This composition supports complex life and diverse ecosystems
If asked to describe the change from early to current atmosphere, you should state: early = mainly CO₂ and H₂O with little O₂ (reducing); current = mainly N₂ and O₂ (oxidising). This clear contrast demonstrates understanding of atmospheric evolution.
Must Know
- Early atmosphere composition: Mainly CO₂ and H₂O, small amounts of ammonia and other gases, very little or NO oxygen – this is fundamentally different from today
- Water and CO₂ removal: Water vapour condensed to form oceans; CO₂ dissolved in oceans and became locked in sedimentary rocks and fossil fuels through chemical and biological processes
- Oxygen production: Photosynthesis by algae and plants produced oxygen that accumulated over billions of years, gradually creating an oxygen-rich atmosphere
- Ammonia conversion: Ammonia was converted to nitrogen gas through chemical reactions (ultraviolet radiation and lightning) and bacterial processes, removing it from the atmosphere
- Current composition: Approximately 78% nitrogen, 21% oxygen, ~1% argon, and small amounts of CO₂ and water vapour
- Atmospheric stability: The composition has been broadly stable for approximately 200 million years, indicating a balanced equilibrium state
That's the notes covered.
Carry on to the next subtopic.