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6.3.1Global Challenges

Origins of the Atmosphere — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 8 of 12

📚 OCR Gateway GCSE Combined Science A J250 - Chemistry Hub / Topical Revision
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Part 1 of 4The early atmosphere

A molten start
  • A molten start For millions of years the Earth's surface was molten, so there was no lasting atmosphere.
Volcanic outgassing
  • Volcanic outgassing As the surface cooled and solidified, volcanoes erupted and released gases from the Earth's interior.
    • Gravity held these gases around the planet and they formed the atmosphere.
What it contained
  • What it contained The early atmosphere was mostly carbon dioxide and water vapour, with some nitrogen, methane and ammonia.
    • There was little or no oxygen.
  • It was probably similar to the atmospheres of Mars and Venus today, which are mostly carbon dioxide.
How we know
  • How we know Scientists analyse minerals in rocks to work out which gases must have been present.
    • Ideas have changed as measuring methods improved.

Part 2 of 4From steam to oceans

1Earth cools
  • Earth cools The surface temperature drops below 100 °C.
2Water vapour condenses
  • Water vapour condenses Steam turns to liquid water and falls as rain.
3Oceans form
  • Oceans form Liquid water collects in the low-lying areas of the surface.
4Carbon dioxide dissolves
  • Carbon dioxide dissolves CO2 is soluble, so a lot dissolved in the oceans.
    • Carbonates formed and were deposited as sediments, lowering the amount in the air.

Part 3 of 4Oxygen enters the air

1
Algae and primitive plants evolved around 2.7 billion years ago and began to photosynthesise.
2
Photosynthesis takes in carbon dioxide and water and produces glucose and oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2.
3
Over a very long time the carbon dioxide level fell and the oxygen level rose.
4
More oxygen allowed more complex animals to evolve.
5
Marine algae are thought to be responsible for most of the oxygen produced.

Part 4 of 4Composition then and now

TimeMain gasesComment
Early EarthCarbon dioxide, water vapour, some nitrogen, methane, ammoniaLittle or no oxygen
After oceans formedLess carbon dioxide; nitrogen becoming dominantCO2 dissolved and formed sediments
After plants and algae spreadRising oxygen, falling carbon dioxidePhotosynthesis changed the air
TodayAbout 80% nitrogen and about 20% oxygenSmall amounts of carbon dioxide, argon and other gases

Key terms

TermMeaning
AtmosphereThe layer of gases surrounding a planet.
OutgassingThe release of gases from inside the Earth by volcanoes.
PhotosynthesisThe process in which plants and algae use light to make glucose and oxygen from carbon dioxide and water.
CondensationThe change of a gas into a liquid when it cools.
SedimentMaterial, such as carbonate, that settles on the bed of a body of water and may form rock.

Avoid these mistakes

1
Saying that the early atmosphere was like today's air. It had very little oxygen and was rich in carbon dioxide.
2
Writing that oxygen came from volcanoes. It came from photosynthesis in algae and plants.
3
Forgetting to say how carbon dioxide levels fell: it dissolved in the oceans, formed carbonate sediments and was used in photosynthesis.
4
Giving a firm composition as if it were certain. Evidence is indirect, so the picture is a theory.

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