The Evolution of the Atmosphere L.O: To know how the atmosphere has been formed and what it is made up of. Tuesday, 18 September 2018

The Atmosphere today Nitrogen 78.084% 0.0314% Carbon Dioxide 20.9476% Oxygen Argon 0.934% Neon 0.001818% Methane 0.0002% 0.000524% Helium 0.000114% Krypton Hydrogen 0.00005% 0.0000087% Xenon The Earth's atmosphere has been roughly as it is now for the last 200 million years or so. The main gases in the atmosphere are nitrogen and oxygen. Nitrogen is by far the most abundant gas — about four-fifths (80%) of the atmosphere is nitrogen. About one-fifth (20%) of the atmosphere is oxygen. There are small amounts of other gases in the atmosphere too. These include carbon dioxide, water vapour and noble gases.

The earth’s early Atmosphere The Earth's surface was originally molten for many millions of years. It was so hot that any atmosphere just ‘boiled away' into space. Eventually things cooled down a bit and a thin crust formed, but volcanoes kept erupting. There was intense volcanic activity for the first billion years after the Earth was formed, and the volcanoes gave out lots of gas. Scientists think this was how the oceans and atmosphere were formed.

The earth’s early Atmosphere There are lots of different theories, but the most popular theory suggests that the early atmosphere was probably mostly carbon dioxide (CO2), with virtually no oxygen (O2). There was probably water vapour too, and small amounts of methane (CH4) and ammonia (NH3). This is quite like the atmospheres of Mars and Venus today. As the Earth cooled, the water vapour in the atmosphere condensed, forming the oceans.

The earth’s early atmosphere

Changes to the atmosphere Although the early atmosphere was mostly carbon dioxide, it didn't stay that way for long. Most of the carbon dioxide was gradually removed from the atmosphere. This happened in a number of ways. CO2 CO2 CO2 CO2 CO2

Absorption by the oceans The oceans are a natural store of carbon dioxide. When the oceans formed, a lot of the carbon dioxide from the atmosphere dissolved into them. CO2 CO2 CO2

Absorption by plants & algae Green plants and algae evolved over most of the Earth. They absorbed some of the carbon dioxide in the atmosphere and used it for a process called photosynthesis. Photosynthesis produces oxygen as a by-product, so when the green plants and algae evolved, carbon dioxide was gradually removed from the atmosphere and oxygen was added. CO2 CO2 CO2 CO2 O2 O2

Locking away the carbon Some of the carbon was locked away in fossil fuels such as coal and oil. These fossil fuels contain carbon and hydrocarbons that are the remains of plants and animals. Limestone containing the fossilised remains of early marine organisms.

Locking away the carbon When the plants and algae that had absorbed the carbon dioxide died, they were buried under layers of sediment, along with the skeletons and shells of marine organisms that had slowly evolved. Some of the carbon inside these marine organisms was locked away as insoluble carbonates in sedimentary rocks. For example, limestone is a sedimentary rock that is formed from the shells and skeletons of marine organisms. A lot of the carbon from the carbon dioxide in the early atmosphere is now locked away in limestone.

Problems with increasing CO2 There is virtually no carbon dioxide left in the atmosphere now, but when we burn fossil fuels today, this ‘locked-up' carbon is released and the concentration of carbon dioxide in the atmosphere rises. As the world has become more industrialised, more fossil fuels have been burnt in power stations and in car engines. This carbon dioxide is thought to be altering our planet in a couple of ways. CO2 CO2 CO2

Global Warming An increase in carbon dioxide is causing global warming — an increase in the average temperature of the Earth. Global warming is a type of climate change and it could lead to dramatic changes in our weather. Global warming = The increase in the average temperature of the Earth. Climate change = Any change in the Earth’s climate. E.g. global warming, changing rainfall patterns etc.

Acidic Oceans The oceans are a natural store of carbon dioxide — they absorb it from the atmosphere. However, the extra carbon dioxide we're releasing is making them too acidic. This could lead to the death of many marine organisms including corals and shellfish. It also means that in the future the oceans won't be able to absorb any more carbon dioxide and this could accelerate global warming.

Fractional distillation of air You can fractionally distil air to get a variety of products (e.g. nitrogen and oxygen) that can be used as raw materials in industry. The air is first filtered to remove dust. It's then cooled to around -200 °C and becomes a liquid. During cooling water vapour condenses and is removed. Carbon dioxide freezes, so that can be removed too.

Fractional distillation of air The remaining gases are separated by fractional distillation. The fractional distillation of air is possible because the gases in air have different boiling points. This means that they will condense at different temperatures. The liquefied air (with the water vapour and CO2 removed) enters the fractionating column and is heated slowly so that it vaporises again. The gas then travels up the column and gradually cools. The different gases are extracted at the different levels in the column where they condense. Oxygen and argon come out together so another column is used to separate them.

Diagram of fractional distillation of air Can you label the diagram using the steps you copied out? ??? ??? ??? ??? ??? ??? ??? ??? ???

Quiz questions What are the two most abundant gases found in the Earth's atmosphere today? Describe the likely composition of the Earth's early atmosphere. How did the Earth's oceans form? Why did the evolution of green plants and algae lead to a decrease in the concentration of carbon dioxide in the atmosphere? What happened to most of the carbon from the carbon dioxide in the early atmosphere? Name the gas released when fossil fuels are burnt that is thought to be contributing to global warming. How is the increase in the concentration of this gas in the atmosphere affecting the Earth's oceans? Name the process that can be used to separate air into its component gases. [Higher] 6.

Each underlined part is worth 1 mark. Give them a mark out of 16! Answers Nitrogen and oxygen [2 marks] The early atmosphere was probably mostly carbon dioxide with virtually no oxygen. There may also have been water vapour and small amounts of methane and ammonia. [5] When the Earth began to cool, the water vapour in the air condensed to form the Earth's oceans. [2] Green plants and algae absorb carbon dioxide and use it in photosynthesis. [2] It has been locked away in sedimentary rocks and fossil fuels or it has dissolved in the oceans. [2] Carbon dioxide / CO2 [1] The oceans are absorbing more carbon dioxide which is causing them to become more acidic. [1] Fractional distillation [1] 6.