Composition and Evolution of the Earth's Atmosphere (C9.1)
Current composition of the atmosphere
- Nitrogen (N₂): ~78%
- Oxygen (O₂): ~21%
- Argon (Ar): ~1%
- Carbon dioxide (CO₂): ~0.04% (400 ppm)
- Water vapour: variable (0–4%)
- Other noble gases: trace amounts
Early Earth atmosphere
About 4.6 billion years ago, the Earth was newly formed, very hot, with intense volcanic activity.
Early atmosphere contained:
- Mostly carbon dioxide (CO₂)
- Water vapour (H₂O)
- Nitrogen (N₂) — from volcanic outgassing
- Small amounts of methane (CH₄), ammonia (NH₃), hydrogen sulfide (H₂S)
- Virtually no oxygen (O₂)
This is similar to the current atmospheres of Mars and Venus (mostly CO₂).
How the atmosphere evolved
Phase 1 — Oceans formed:
As Earth cooled, water vapour condensed → seas formed. CO₂ dissolved into the oceans → reduced atmospheric CO₂. Some CO₂ ended up as carbonate rocks (limestone — CaCO₃).
Phase 2 — Photosynthesis (cyanobacteria, ~3.5 billion years ago):
First photosynthesising organisms (cyanobacteria) produced O₂:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
CO₂ removed; O₂ released. Over hundreds of millions of years, O₂ accumulated.
Phase 3 — O₂ reacted with iron:
Initially, O₂ produced was absorbed by iron compounds in oceans (forming iron oxide — "banded iron formations"). Once iron was saturated, O₂ began to accumulate in atmosphere.
Phase 4 — Ozone layer formed:
O₃ (ozone) formed from O₂ in upper atmosphere → absorbed UV radiation → allowed complex life to colonise land.
Phase 5 — Nitrogen accumulated:
N₂ released by volcanic activity and by denitrifying bacteria. N₂ is unreactive → accumulated over billions of years.
Evidence limitations
These events happened billions of years ago. Evidence is indirect and incomplete — rock records, fossil microorganisms, chemical signatures. There is still scientific debate about precise timings and mechanisms.
Common exam errors
- Saying the early atmosphere was mainly nitrogen — it was mainly CO₂.
- Forgetting that oxygen was produced by photosynthesis (cyanobacteria), not volcanic activity.
- Saying CO₂ is currently 21% — that is oxygen; CO₂ is only ~0.04%.
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