8.18–8.19The Earth's early atmosphere
Theories about what was in the Earth's early atmosphere and how the atmosphere was formed have changed and developed over time. Evidence for the early atmosphere is limited because of the time scale of 4.6 billion years.
One theory suggests that during the first billion years of the Earth's existence there was intense volcanic activity that released gases that formed the early atmosphere, and water vapour that condensed to form the oceans. At the start of this period the Earth's atmosphere may have been like the atmospheres of Mars and Venus today, consisting of mainly carbon dioxide with little or no oxygen gas.
Volcanoes also produced nitrogen, which gradually built up in the atmosphere, and there may have been small proportions of methane and ammonia.
When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated, producing sediments and reducing the amount of carbon dioxide in the atmosphere. No knowledge of other theories is required.
You should be able to, given appropriate information, interpret evidence and evaluate different theories about the Earth's early atmosphere.
The reason there is more than one theory is stated in the specification itself: the timescale is 4.6 billion years, so there is very little direct evidence, and what there is has to be inferred from rocks rather than measured. That is the sentence to reach for whenever a question asks why scientists disagree, or why a theory changed. Comparing the Earth with Mars and Venus is not proof — it is an analogy that makes one theory plausible.
Common mistake
Say "one theory suggests", not "the early atmosphere was". Marks in this section are awarded for treating these as theories supported by limited evidence, and a confident statement of fact can lose them.
- The Earth's early atmosphere
- the atmosphere during the first billion years, thought to have been mainly carbon dioxide from intense volcanic activity, with little or no oxygen, plus water vapour that condensed to form the oceans
8.20–8.21How carbon dioxide decreased
Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis. Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon.
The same equation explains both changes at once: photosynthesis takes carbon dioxide out and puts oxygen in.
You should be able to describe and explain the formation of deposits of limestone, coal, crude oil and natural gas — the four places the carbon went.
You should also be able to describe the main changes in the atmosphere over time and some of the likely causes of these changes. A full account runs: mainly carbon dioxide with little or no oxygen from volcanic activity; oceans formed and carbon dioxide dissolved, precipitating carbonates; algae and then plants photosynthesised, removing carbon dioxide and adding oxygen; nitrogen from volcanoes built up because it is unreactive; and carbon was locked away in sedimentary rocks and fossil fuels. By about 200 million years ago the proportions had reached what they are today.
Common mistake
Nitrogen became the largest part of the atmosphere partly because it is very unreactive, so it accumulated while other gases were removed. It was not produced in the greatest quantity at the start.
8.22How oxygen increased
Algae and plants produced the oxygen that is now in the atmosphere by photosynthesis, which can be represented by the equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ carbon dioxide + water → glucose + oxygen
Algae first produced oxygen about 2.7 billion years ago, and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve.
The order matters in that last sentence: the oxygen came first, and animals could only evolve once it was there.
8.23Test for oxygen
The test for oxygen uses a glowing splint inserted into a test tube of the gas. The splint relights in oxygen.
Note the difference from the hydrogen test: a burning splint for hydrogen, held at the open end; a glowing splint for oxygen, inserted into the tube. Getting the two the wrong way round is the commonest error in this sub-topic.
Before you practise
- The early atmosphere came from intense volcanic activity, and was mostly carbon dioxide with little or no oxygen.
- Volcanoes also released water vapour, nitrogen and small amounts of methane and ammonia.
- As the Earth cooled, water vapour condensed and formed the oceans.
- Carbon dioxide dissolved in the oceans and was locked into carbonate rocks and sediments — and later into fossil fuels.
- Algae and then plants produced oxygen by photosynthesis, which also removed carbon dioxide.
- Evidence for the early atmosphere is limited, so theories about it change — say that when asked how sure we can be.
- Oxygen relights a glowing splint.