The Earth formed approximately 4.6 billion years ago. Scientists believe the EARLY ATMOSPHERE was very different from today's.
EVIDENCE is LIMITED — there are no direct samples. Scientists piece together evidence from:
Rock records and the oldest minerals.
Studying the atmospheres of other planets (Mars and Venus have CO₂-rich atmospheres — possibly similar to early Earth).
Isotope analysis of ancient rocks.
BELIEVED COMPOSITION of the early atmosphere (~4 billion years ago):
MAINLY CARBON DIOXIDE (CO₂) — similar to Mars and Venus today.
Some water vapour (H₂O).
Some nitrogen (N₂).
Very little oxygen (O₂) — essentially none.
Possibly methane (CH₄) and ammonia (NH₃).
SOURCE: intense volcanic activity released these gases from the Earth's interior — VOLCANIC OUTGASSING.
How Oxygen Increased
The dramatic INCREASE IN OXYGEN is one of the most significant events in Earth's history.
STAGE 1 — PHOTOSYNTHESIS:
About 2.7 billion years ago, CYANOBACTERIA (blue-green bacteria) evolved — the first photosynthetic organisms.
They used CO₂ and sunlight to produce glucose and OXYGEN:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Oxygen released into the atmosphere.
STAGE 2 — OXYGEN ACCUMULATES:
Initially, oxygen was absorbed by reacting with dissolved iron in oceans → iron oxide (rusting), forming BANDED IRON FORMATIONS in rocks.
Also absorbed by reacting with surface rocks (oxidation).
Once these 'sinks' were saturated, oxygen began building up in the atmosphere.
OVER BILLIONS OF YEARS:
Oxygen levels gradually rose.
As O₂ rose, an OZONE LAYER (O₃) formed in the upper atmosphere — shielding the Earth from UV radiation.
This allowed more complex life to evolve on land (previously UV would have been lethal).
HOW CO₂ DECREASED:
Photosynthesis removed CO₂.
CO₂ dissolved in the cooling oceans.
Marine organisms used dissolved CO₂ to make calcium carbonate (CaCO₃) shells — when they died, shells became LIMESTONE rocks — locking carbon away.
Formation of Oceans and Nitrogen
FORMATION OF OCEANS:
Early Earth was very hot — water existed only as steam.
As Earth cooled below 100°C, water vapour CONDENSED → formed the oceans.
Oceans dissolved large amounts of CO₂ → dramatically reduced CO₂ in the atmosphere.
This was a major step in changing the atmosphere.
NITROGEN ACCUMULATION:
Nitrogen was present in the early atmosphere and gradually accumulated as:
CO₂ was removed (by photosynthesis and ocean absorption).
Nitrogen is very unreactive — it doesn't get removed by most geological or biological processes.
This left nitrogen as the dominant gas.
SUMMARY OF CHANGES:
Early: mainly CO₂, water vapour, some N₂, no O₂.
Oceans form: CO₂ dissolves, water vapour removed.
Cyanobacteria: photosynthesis removes more CO₂, releases O₂.
Ozone layer forms: protects surface, allows complex life.
Present: N₂ 78%, O₂ 21%, CO₂ only 0.04%.
⚠️ Common Mistake
The early atmosphere had LOTS of CO₂ and VERY LITTLE O₂ — the opposite of today. Oxygen was produced by PHOTOSYNTHESIS from cyanobacteria — not by volcanoes. Volcanoes released CO₂, water vapour and other gases but NOT oxygen.
📐 Key Equations
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis — released O₂ into atmosphere)
📌 Key Note
Early atmosphere: mainly CO₂ + H₂O vapour + some N₂, virtually no O₂. Volcanoes released gases (outgassing). O₂ increased due to photosynthesis by cyanobacteria ~2.7 bn years ago. CO₂ decreased: photosynthesis, dissolved in oceans, formed limestone. N₂ accumulated as other gases removed. Oceans formed when Earth cooled.
🎯 Matching Activity — Timeline of Atmospheric Change
Match each change to the process that caused it. — drag the symbols on the right to match the component names on the left.
CO₂ decreased — dissolved in oceans
Drop here
CO₂ decreased — locked in rocks
Drop here
O₂ increased
Drop here
N₂ became dominant
Drop here
Ozone layer formed
Drop here
Earth cooled below 100°C — water vapour condensed to form oceans
As O₂ built up, UV converted some O₂ to O₃ — shielded Earth from UV radiation
Marine organisms made CaCO₃ shells → died → shells became limestone
Nitrogen is very unreactive — accumulated as CO₂ and H₂O were removed
Cyanobacteria evolved ~2.7 billion years ago — photosynthesis released O₂
⭐ Higher Tier Only
Evaluate the quality of evidence for early atmosphere theories — no direct samples available. Miller-Urey experiment (1953): simulated early Earth atmosphere with H₂, CH₄, NH₃, H₂O + electrical sparks → produced amino acids. Suggests organic molecules can form from simple gases — but doesn't prove this is how life originated.
🎯 Test Yourself
Question 1 of 2
1. What was the main source of oxygen in Earth's early atmosphere?
2. Why did CO₂ levels decrease as Earth's oceans formed?
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