Higher and foundation tier
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. 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 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!
One of the most difficult concepts to grasp is geological time. We are not aware of say an oak tree growing, 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 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, 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 Earth has changed massively throughout its 4.6 billion year life span.
When the Earth first formed 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 middle, which is why we have a core made up of heavy metals such as iron and nickel. Once this bombardment from space decreased and volcanic activity dropped it allowed the surface to cool and solidify. 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 did Earth's atmosphere change so much? What was different about Earth that allowed its atmosphere to change and evolve over time?
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?
The oxygen gas released by photosynthesis can also lead to the formation of an ozone layer. Ozone is simply 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 and the amount of carbon dioxide 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. 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.
The image below is a brief summary of the processes that have changed the atmosphere over the last 4.6 billion years.