Formation of Earth's Atmosphere (AQA GCSE Chemistry): Revision Notes
The early atmosphere and today's atmosphere
What is our atmosphere like today?
Our atmosphere has remained fairly stable for about 200 million years. It contains two main gases that make up almost all of it:
- Nitrogen - about 78% (roughly 4/5 of the atmosphere)
- Oxygen - about 21% (roughly 1/5 of the atmosphere)
The remaining 1% includes small amounts of other gases like carbon dioxide and noble gases. There's also water vapour, but the amount changes depending on weather conditions.
This stable atmospheric composition is what makes Earth suitable for life as we know it. The precise balance of nitrogen and oxygen has been maintained for millions of years.
This mixture is very different from other planets. For example, Mars and Venus have atmospheres that are mostly carbon dioxide with very little oxygen or nitrogen.
How did Earth's early atmosphere form?
Earth first developed an atmosphere about 4.6 billion years ago. Because this was so long ago, scientists have limited evidence to work with. However, they have developed theories about how our atmosphere changed over time.
The most accepted theory suggests our early atmosphere formed through these four main stages:
Stage 1: The first atmosphere
Earth's very early atmosphere was mainly carbon dioxide with little or no oxygen. This made it similar to what we see on Mars and Venus today.
Stage 2: Volcanic activity
During the first billion years, volcanoes were very active. They released gases including:
- Carbon dioxide ()
- Water vapour ()
- Small amounts of methane () and ammonia ()
Stage 3: Ocean formation
As Earth cooled down, the water vapour released by volcanoes condensed. This formed the oceans we see today.
Stage 4: Carbon dioxide removal
Much of the carbon dioxide dissolved into the newly formed oceans. Some of this carbon dioxide formed carbonates, which settled as sediment on the ocean floor. This process gradually removed most of the carbon dioxide from the atmosphere.
Key differences between early and modern atmospheres
Critical Changes Over Time:
The biggest changes from Earth's early atmosphere to today are:
- Much less carbon dioxide - went from being the main gas to less than 1%
- Much more oxygen - increased from almost none to about 21%
- More nitrogen - became the dominant gas at about 78%
This transformation happened over billions of years and was essential for life as we know it to develop.
Why is this different from other planets?
Earth's atmosphere is unique in our solar system. Mars has about 95% carbon dioxide and almost no oxygen. Venus is similar with 96% carbon dioxide. This shows how special Earth's atmospheric evolution has been.
The key difference was Earth's ability to form oceans, which absorbed much of the carbon dioxide and allowed oxygen levels to increase over time.
Key Points to Remember:
- Today's atmosphere is about 4/5 nitrogen and 1/5 oxygen
- The early atmosphere was mostly carbon dioxide with little oxygen
- Volcanoes released the gases that formed the early atmosphere
- Oceans formed when water vapour condensed as Earth cooled
- Carbon dioxide dissolved in the oceans, removing it from the atmosphere
- This process took billions of years to create the atmosphere we have today