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!
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?
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?
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.
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.
- Intense volcanic activity during the first one billion years
of Earth's existence formed the early atmosphere.
- The Earth's 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's surface had cooled down enough to allowed
water vapour 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 atmosphere.
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 and 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
Next