The Earth's Early Atmosphere and How It Changed

The atmosphere is the layer of gases surrounding the Earth. For about 200 million years its composition has stayed much as it is now, so this is the mixture you are expected to know for the exam.

The two main gases are nitrogen and oxygen. A useful way to remember the split is as fractions: about four fifths nitrogen and one fifth oxygen.

Gas	Approximate proportion	Fraction
Nitrogen (N₂)	about 80%	~4/5
Oxygen (O₂)	about 20%	~1/5
Other gases	small amounts	small

The other gases in that final row are carbon dioxide, water vapour and the noble gases (mainly argon). Each is present only in a small proportion, but carbon dioxide and water vapour matter a great deal because they trap heat and help keep the Earth warm.

Learn the composition as roughly 80% nitrogen and 20% oxygen. Writing the fractions "four fifths" and "one fifth" is also accepted.

The Earth formed about 4.6 billion years ago, and there were no instruments or observers to record what the air was like. Because the time scale is so vast and direct evidence is scarce, scientists cannot be certain about the early atmosphere.

This is why the specification asks you to evaluate theories rather than simply state facts. When you read a claim about the early atmosphere, treat it as the best current explanation supported by evidence, not as a proven certainty.

The huge 4.6-billion-year time scale means evidence is limited, so theories about the early atmosphere are uncertain and may change as new evidence appears.

You only need to know the one theory set out in the next slides. You are not required to learn any alternative theories, but you should be able to interpret evidence you are given and comment on how strong it is.

The leading theory is that during the Earth's first billion years there was intense volcanic activity. This activity released the gases that formed the early atmosphere and also released water vapour.

The gases from the volcanoes are thought to have produced an atmosphere that was mainly carbon dioxide, with little or no oxygen. This is similar to the atmospheres of Mars and Venus today, which supports the theory. Volcanoes also released nitrogen, which gradually built up in the atmosphere, and possibly small amounts of methane and ammonia.

As the Earth cooled, two changes lowered the amount of carbon dioxide:

• Water vapour condensed to form the oceans as temperatures fell below the boiling point of water.
• Once the oceans existed, carbon dioxide dissolved in them, and carbonates were precipitated as sediments on the sea bed.

Early atmosphere: mainly CO₂, little or no O₂, from volcanic activity, like Mars and Venus today. Water vapour condensed to form the oceans.

There was almost no oxygen in the early atmosphere. Oxygen appeared because living things began to carry out photosynthesis, the process that uses light energy to turn carbon dioxide and water into glucose and oxygen.

The balanced symbol equation for photosynthesis is:

$6C{O}_2+6H_{2}O\stackrel{\text{light}}{\to }{C}_6{H}_{12}{O}_6+6O_2$

In words: six molecules of carbon dioxide react with six molecules of water to make one molecule of glucose and six molecules of oxygen, using light energy.

The story unfolds over a huge span of time:

• Algae first evolved and began producing oxygen about 2.7 billion years ago.
• Over the next billion years, plants evolved as well and continued to release oxygen.
• Oxygen slowly built up in the atmosphere until it reached a level that allowed animals to evolve.

So the rise of oxygen and the appearance of animal life are directly linked to photosynthesis by algae and plants.

The early atmosphere was rich in carbon dioxide, yet today only a small proportion remains. Three processes removed it, and it helps to think of them as ways of moving carbon out of the air and locking it away.

Process	What happens to the carbon dioxide
Photosynthesis	Algae and plants convert CO₂ into glucose, so carbon is stored in living tissue
Dissolving in oceans	CO₂ dissolves in seawater, and carbonates precipitate as sediment
Sedimentary rocks and fossil fuels	Carbon becomes locked into rocks and fuels formed from dead organisms

The third row matters most for the exam because it explains why carbon is held out of the atmosphere for millions of years. Shelled sea creatures and precipitated carbonates built up on the sea bed and were compressed into sedimentary rocks. The remains of other organisms became fossil fuels. In both cases the carbon that was once in the air is trapped in the ground.

Carbon dioxide decreased because photosynthesis, dissolving in the oceans, and the formation of sedimentary rocks and fossil fuels all removed carbon from the atmosphere.

The carbon locked out of the atmosphere ended up in familiar carbon stores. You should be able to describe how each one formed.

Limestone is a sedimentary rock made largely of calcium carbonate. It formed from the shells and skeletons of tiny sea creatures. When these organisms died, their carbonate shells sank, built up in layers on the sea bed, and were compressed over millions of years into rock.

Coal formed from the remains of plants. Dead plant material, such as that in ancient swamps, was buried before it could fully decay. Heat and pressure over millions of years turned it into coal.

Crude oil and natural gas formed from the remains of tiny plankton (marine organisms) that were buried in mud on the sea bed. Over millions of years, heat and pressure, in the absence of oxygen, turned these remains into crude oil and natural gas, which became trapped in porous rocks.

Deposit	Formed from	Type of carbon store
Limestone	Shells/skeletons of sea creatures	Sedimentary rock
Coal	Buried plant material	Fossil fuel
Crude oil	Buried marine plankton	Fossil fuel
Natural gas	Buried marine plankton	Fossil fuel

1. Getting the nitrogen and oxygen proportions the wrong way round

The atmosphere is about 80% nitrogen and 20% oxygen, not the other way round. A quick check: nitrogen is the larger share, oxygen the smaller.

2. Saying the early atmosphere had lots of oxygen

The early atmosphere had little or no oxygen. Oxygen only built up later through photosynthesis. Comparing it to Mars and Venus (mainly CO₂) helps you remember this.

3. Forgetting light in the photosynthesis equation

Photosynthesis needs light energy. Write it over the arrow: $6C{O}_2+6H_{2}O\stackrel{\text{light}}{\to }{C}_6{H}_{12}{O}_6+6O_2$. Also check the equation balances: 6 carbon, 12 hydrogen and 18 oxygen atoms on each side.

4. Giving only one reason carbon dioxide decreased

There are three: photosynthesis, dissolving in the oceans, and locking carbon into sedimentary rocks and fossil fuels. Questions worth several marks expect more than one.

5. Muddling how the fuels and limestone formed

Coal comes from land plants; crude oil and natural gas come from marine plankton; limestone comes from the shells of sea creatures. Do not swap these origins.