Higher and foundation tier

The Earth's early atmosphere

The planet Venus is often called Earth's twin planet. The two planets are approximately the same size and Venus is our next door neighbour in the solar system. Venus is often described as being one of the most beautiful planets to look at through a telescope. However a holiday to Venus would be a one way trip! If you set foot on the surface of Venus you would be boiled alive by surface temperatures in excess of 4500C, squashed flat by the pressure caused by a thick and dense atmosphere, where the surface pressure is equivalent to being 1000 metres below the surface of the ocean on Earth. To make matters worse it also rains concentrated sulfuric acid so you would be corroded to death as well!

Earth and Venus

Geological time

One of the most difficult concepts to grasp is the idea of geological time. We are not aware of say an oak tree growing, simply because this process happens on a different time scale from the one you operate in. When perhaps you discuss an event in history, say when Julius Caesar invaded Britain on August 26th 55 BC you could probably relate to this event. However if you were asked to describe the people and animals in Britain say 300 000 years ago or 1 million years ago or 100 million years ago then that would probably be much more difficult task.

One of the basic mistakes many students make is that they assume the Earth as it is today has always been like this, in reality nothing could be further from the truth! The plants and animals that were alive when the Earth was very young, for example when it was less than 1 billion years old, and the plants and animals that are alive today are very different. One of the main reasons for this difference is that the atmosphere of the Earth has changed massively throughout its 4.6 billion year life span.

Where do planets get their atmospheres from?

Earth's atmosphere is produced from the gases released by erupting volcanoes When the Earth first formed 4.6 billion years ago the surface was subjected to intense volcanic activity and at the same time was subjected to a massive bombardment from space by asteroids and comets which resulted in the surface temperature getting so high that the entire planet melted to form a molten ball of rock travelling trough space. The fact that the whole planet was in a liquid state allowed all the heavy elements to sink to the planets core, which is why we have a core made up of heavy metals such as iron and nickel. Once this bombardment from space decreased and the volcanic activity dropped it allowed the surface to cool and solidify and form a solid crust. But where did the atmosphere come from?

It is thought that the atmospheres of planets comes from the gases produced by erupting volcanoes. Volcanoes release large amounts of carbon dioxide, water vapour, methane, ammonia and nitrogen gases into the atmosphere. During the first 1000 million years (1 billion years) of the Earth's existence there was intensive volcanic activity which created the gases which made up our early atmosphere. It would seem logical to assume that the early atmosphere of Earth was similar to that of Venus and Mars, but this raises even more interesting questions. How and why did the Earth's atmosphere change so much? What was different about Earth that allowed its atmosphere to change and evolve over time?

A changing atmosphere

The atmospheres of Mars and Venus are almost entirely made up of carbon dioxide gas. The atmosphere of Venus is 96.5% carbon dioxide, 3.5% nitrogen with trace amounts of gases such as argon, sulfur dioxide, carbon monoxide and water vapour. The atmosphere of Earth at the moment is 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide with trace amounts of gases such as methane, krypton and water vapour. So how did the atmosphere of Earth, which was similar to that of Mars and Venus, change and develop into the atmosphere we have today?

% composition of Mars, Earth and Venus atmospheres

algae growing in a test-tube

The early atmosphere

After several hundred million years the Earth's surface had cooled and this allowed the steam in the atmosphere to condense into the early seas and oceans. With liquid water on the surface it was not long before the first living micro-organisms appeared in the early seas and oceans. As simple plants like algae evolved and appeared in the oceans about 2.7 billion years ago, photosynthesis began and the slow but gradual changes in the atmosphere began. Make no mistake this did not happen overnight- it took hundreds of millions of years.
However once life in the form of algae and simple bacteria such as cyanobacteria appeared then the atmosphere gradually starts to change. These algae and bacteria can undergo photosynthesis which will remove the gas carbon dioxide gas from the atmosphere and also release the first oxygen into the atmosphere. The equations below show how photosynthesis removes the gas carbon dioxide and releases oxygen gas into the atmosphere.
carbon dioxide(g) + water(l) → glucose(aq) + oxygen(g)
6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g)
Over the next billion years as the algae and bacteria continue to photosynthesise the amount of oxygen present in the atmosphere begins to rise and the amount of carbon dioxide begins to fall. Another way the carbon dioxide level dropped was that it simply dissolves in the newly formed seas and oceans.
Carbon dioxide gas dissolves in seawater to form soluble calcium hydrogen carbonates that ultimately ends up in the bodies of many marine organisms. When they organism die they will sink to the ocean floor and can become trapped in sediments and ultimately form sedimentary rock such as limestones (calcium carbonates). The fossil fuels oil and gas can also form from the remains of these marine organisms given the right conditions. This means that carbon which was once in the atmosphere is now trapped underground in fossil fuels and sedimentary rocks. This along with photosynthesis is the main route for getting carbon dioxide out of the atmosphere. This would also help explain why Venus still has an atmosphere consisting mainly of carbon dioxide (up to 96.5%). Venus has a runaway greenhouse effect which resulted in very high surface temperatures means there is no liquid water on the surface and so life failed to start on Venus.

Ozone layer protects living organisms from harful UV radiation The oxygen gas released by photosynthesis can also lead to the formation of an ozone layer. Ozone is gas and simply consists of 3 oxygen atoms bonded together, O3, it is a highly reactive molecule and is found high up in the atmosphere where there is very little for it to react with. However ozone does have one particularly useful property- it absorbs harmful ultraviolet light from the Sun. The formation of an ozone layer would allow larger and more complex organisms to evolve and for life to move from the oceans onto the land.

With the movement and evolution of plants and animals onto land and life in the oceans also growing and evolving the atmosphere is becoming more and more polluted with oxygen. The amount of carbon dioxide gas is starting to decrease rapidly. As plants on land and microbes in the oceans live then die they are removing carbon dioxide as they end up forming fossil fuels such as coal, oil and gas.

The oxygen gas produced by photosynthesis can also react with methane and ammonia released by volcanic eruptions to form nitrogen gas. Nitrogen gas is very unreactive and its concentration in the atmosphere will build up slowly, but being so unreative once it is in the atmosphere it is likely to stay there for a long period of time. So eventually levels of nitrogen gas build up and around 200 million years ago the composition of the atmosphere reached the composition we have today, 78% nitrogen, 21% oxygen with small amounts of other gases, mainly argon (0.9%), carbon dioxide gas (0.04%) and water vapour.

Key points

The image below is a brief summary of the processes that have changed the atmosphere over the last 4.6 billion years. summary of atmosphere evolution

Practice questions

Check your understanding - Questions on evolution of the atmosphere