Higher and foundation tiers
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! You maybe wondering at this point what has this to do with the Earth's atmopshere- well read on to find out more!
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 to describe what Earth was like 1 million years ago or 100 million years ago then that would probably be much more difficult task. This is simply because these changes and events which have happened on Earth have taken place over such vast timescales.
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.
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 through
space. The fact that the whole planet was in a liquid state allowed all the heavy elements to sink to the
planet's 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 to 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, since these planets also had intense volcanoic activity when they first formed, but this raises even more interesting questions. Why for example is the atmospere of Earth so different from that found on Mars and Venus if they were formed in the same way? 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 consists of approximately 96.5% carbon dioxide and 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 approximately 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?
After several hundred million years the Earth's surface had
cooled and this allowed the steam in the
atmosphere to condense and form the
early seas and oceans. With
liquid water on the surface it was not long before the first living
microorganisms appeared in the early
seas and oceans. As simple
plant 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 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.
The oxygen gas released by
photosynthesis can also lead to the formation of an ozone layer.
Ozone is a gas and simply consists of 3
oxygen atoms bonded together; its molecular formula is O3. Ozone is a
highly reactive toxic gas which 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 unreactive 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.
The image below is a brief summary of the processes that have changed the atmosphere over the last 4.6 billion years -study it carefully!
