The Chemistry of our atmosphere
The Earth is made up of three layers, Crust, Mantle and Core. The crust is
what we are on now and the solid rock at the bottom of the oceans. We have dug 3.5 km down through the crust and drilled 12 km down
but this is insignificant as the Earth is 12800 km in diameter. We know the composition of these layers by analysing rocks at the
surface, what is coming out of volcanoes and careful physics
measurements about the mass and density of the planet.
These are the summary facts of the layers:
Based on evidence that was laughed at when alive, Alfred Wegener came up with the theory of continental drift. His evidence was:
-The shape of the continents looked like a giant jigsaw puzzle. Mainly the fit between Africa and South America.
-Animals alive today on these two continents appear to have similar ancestors but evolved into different modern species e.g. Lion and Jaguar.
-Fossils in these two continents showed that they once had exactly the same species living on them.
-The rocks on the boundaries were the same.
This evidence was refuted many times, mainly as
Alfred was a meteorologist and German national between the two World
Wars. After his death and the Second World War, Cold War submarine
hunting sonar ships spotted a mountain ridge running up the North
Atlantic and they finally had an explanation for the drifting of
continents. They had found the mid ocean ridge that is a split forming
new rock as the Americas move away from Europe and Africa.
The tectonic plates (rigid slabs of rock, roughly relating to the continents) sit on the mantle and are pushed around by convection currents driven from heat deep within the Earth.
The Earth’s Atmosphere in the Past:
Although we have no absolute record of the Earth’s atmosphere when it was created, we can make scientific assumptions based on what is ejected from volcanoes today. From this, we believe that the Earth’s early atmosphere was mainly water vapour, carbon dioxide, nitrogen gas and traces of methane and ammonia.
The Earth cooled and the water vapour condensed and with the ice from comets that hit the surface, the oceans formed.
The dryer atmosphere stayed stable up to around 3.4 Billion years ago when bacteria first started to photosynthesise and compete the mammoth task of removing the carbon dioxide and putting oxygen into the air that we rely on today. Once bacteria had made a start, sea and land plants evolved.
The methane and ammonia reacted with the oxygen in the air to form more nitrogen and carbon dioxide.
Carbon dioxide started to dissolve into the seas as soon as they formed which allowed minerals to form (carbonates).
Gases in the Atmosphere today:
The composition of our atmosphere is fairly straight forward and you
really should know it. How it came to be like this is explained above,
however, a major point is explaining where all of the carbon is now. We
are made to feel that the planet is choking in carbon dioxide but the
levels are one of the lowest in Earth's history. Most of the carbon is
no longer in the atmosphere but by the process of photosynthesis, it is
in plants which in turn, is eaten by animals to make them.
About 1% of the atmosphere is made up of an element that we have not discussed before, Argon. Argon is what we call a noble gas and it, along with its chemical family are found on the far right of the periodic table in Group 0. They are no longer the "Inert Gases" as one, Xenon, will react a tiny amount in extreme circumstances, so they are Noble gases. On the whole, they will not react to form molecules and so have uses that exploit this handy tendency. Although all gases, they become more dense as you descend the group.
Helium is a lot less dense than air and as it is not at all flammable like Hydrogen, it is used in airships and party balloons.
Neon is used to light up Las Vegas. Called electrical discharge tubes, we know them as Neon lights. Finally, Argon, quite common if 1% of the entire atmosphere is made of it. To stop light bulb filaments reacting with the oxygen in the air at high temperatures, light bulbs have the air sucked out and replaced with unreactive Argon.
The Carbon Cycle:
The levels of carbon dioxide in the atmosphere have not changed a massive
amount over the last 200 million years. There is a natural equilibrium
which is demonstrated well in the picture on the left. As animals are
breathing out carbon dioxide, plants are taking it in during
photosynthesis. It is buried as rocks or to become fossils then released
when we burn them. Also, carbon dioxide dissolves into the oceans and it
provides the carbonates which make shells which are buried to produce
limestone which is heated to produce quicklime and release the carbon
dioxide back into the atmosphere when we make cement and so on and so on.
The most important concept to take away from this is that the balance is very fine. If we produce more carbon dioxide by burning fossil fuels at the same time that we are liberating carbon dioxide that was safely locked away in limestone to feed our ever increasing need for building materials at the same time that we are cutting down the rain forests which reduced the amount of photosynthesis taking place on Earth, we are shifting the carbon cycle into a new equilibrium for the first time in 200 million years.