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.
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!
Geological time
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.
Where do planets get thier atmospheres from?
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?
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?
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 carbon dioxide gas from the
atmosphere and also release the first oxygen 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 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 explain why Venus still has an
atmosphere consisting mainly of carbon dioxide (up to 96.5%), Venus has a runaway greenhouse effect which means there is
no liquid water on the surface and so life failed to start on Venus.
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.
key points
The image below is a brief summary of the processes that have changed the atmosphere over the last 4.6 billion years.
- Intense volcanic activity during the first one billion years of Earth's existence formed the early atmosphere.
- The early atmosphere was mainly carbon dioxide, much like the atmospheres of Mars and Venus today. There was small amounts
of other gases, mainly water vapour, methane, ammonia and nitrogen. There was little or no oxygen gas.
- About 4 billion years ago the Earth cooled down, this allowed water vpour in the atmosphere to condense and the early seas and
oceans were formed.
- Carbon dioxide gas dissolved in the early oceans, this reduced the amount
of CO2 in the atmopshere.
Sea creatures used the dissolved carbon dioxide to form shells of calcium carbonate.
When these animals died their bodies sank to the
sea bed and eventually formed sedimentary rock.
- Bacteria nd simple plants living in the early oceans started to photosynthesise. This further reduced the amount of
CO2 and slowly started to raise oxygen levels.
Practice questions
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