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Learning objectivesAfter this topic, you should know:

● a theory about how our atmosphere developed

● how to interpret evidence and evaluate different theories about the Earth’s early atmosphere, given appropriate information.

Scientists think that the Earth was formed about 4.6 billion years ago. They think that to begin with, it was a molten ball of rock and minerals. For its first billion years it was a very hot, turbulent place. The Earth’s surface was covered with volcanoes belching fire and gases into the atmosphere.

The Earth’s early atmosphereThere are several theories about the Earth’s early atmosphere, although there is little direct evidence to draw on from billions of years ago. However, scientists have reconstructed what they think the atmosphere must have been like, based on evidence from gas bubbles trapped in ancient rocks. They also use data gathered from the atmospheres of other planets and their moons in the solar system.

One theory suggests that volcanoes released carbon dioxide, CO2, water

vapour, H2O, and nitrogen, N

2, and that these gases formed the early

atmosphere. Water vapour in the atmosphere condensed as the Earth gradually cooled down, and fell as rain. Water collected in hollows in the crust as the rock solidified and the first oceans were formed. Another theory speculates that comets could also have brought water to the Earth. As icy comets rained down on the surface of the Earth, they melted, adding to its water supplies.

As the Earth began to stabilise, the atmosphere was probably mainly carbon dioxide. There could also have been some water vapour and nitrogen gas, and traces of methane, CH

4, and ammonia, NH

3. There

would have been very little or no oxygen at that time. This resembles the atmospheres that are known to exist today on the planets Mars and Venus. Our nearest neighbours have atmospheres made up mainly of carbon dioxide with little or no oxygen.

After the initial violent years of the history of the Earth, the atmosphere remained quite stable. That is until life first appeared on Earth.

Oxygen in the atmosphereThere are many theories as to how life was formed on Earth billions of years ago. Scientists think that life began about 3.4 billion years ago, when the first simple organisms, similar to bacteria, appeared. These could use the breakdown of chemicals as a source of energy.

Then, about 2.7 billion years ago, bacteria and other simple organisms, such as algae, evolved. Algae could use the energy from the Sun to make their own food by photosynthesis. This produced oxygen gas as a waste product. Over the next billion years or so, the levels of oxygen rose steadily as the algae and bacteria thrived in the seas. More and more plants evolved – all of them were photosynthesising, removing carbon dioxide, and making oxygen.

Figure 1 Volcanoes moved chemicals from inside the Earth to the surface and the newly forming atmosphere

Figure 2 The surface of one of Jupiter’s moons, Io, with its active volcanoes releasing gases into its sparse atmosphere. This is likely to be what our own Earth was like billions of years ago

11.1 History of our atmosphere 11 The Earth’s atmosphereC

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(energy from sunlight)carbon dioxide + water glucose + oxygen

6CO2 + 6H

2O C

6H

12O

6 + 6O2

As plants evolved, they successfully colonised most of the surface of the Earth. So the atmosphere became richer in oxygen. This made it possible for the first animal forms to evolve. These animals could not make their own food like the algae and plants could. They relied on the algae and plants for their food and on oxygen to respire.

Study tipRemember that oxygen was not one of the gases in the Earth’s original atmosphere. It was only made after the first simple organisms that could carry out photosynthesis had evolved.

1 Name and give the chemical formula of five gases that scientists speculate were found in the Earth’s early atmosphere. Display your answer in a table. [5 marks]

2 Describe how the Earth’s early atmosphere was probably formed during its first billion years of existence. [2 marks]

3 a Suggest why scientists believe there was no life on Earth for its first billion years. [1 mark]

b Suggest two possible sources of the water that collected and formed our early oceans. [2 marks]

4 Explain how the levels of oxygen in our atmosphere increased and why this was significant in the history of the Earth. Include any relevant chemical equations in your answer. [6 marks]

5 H Over a period of time, the algae in an ancient sea made 270 tonnes of glucose during photosynthesis.a Express 270 tonnes in grams using standard form.

(1 tonne = 1000 kg) [1 mark]b Using the equation for photosynthesis at the top of this page,

calculate the mass of oxygen gas produced by the algae over this period of time. Give your answer to 2 significant figures. [4 marks]

Figure 4 Not only do bacteria such as these not need oxygen – they die if they are exposed to it. However, they can survive and breed in rotting tissue and other places where there is no oxygen

11.1 History of our atmosphere

● The Earth’s early atmosphere was formed by volcanic activity.

● It probably consisted mainly of carbon dioxide. There may also have been nitrogen and water vapour, together with traces of methane and ammonia.

● As plants spread over the Earth, the levels of oxygen in the atmosphere increased.

Key points

Go furtherEvaluating alternative theories

Scientists have found evidence from some of the oldest rocks on Earth that question assumptions that the early gases originated from volcanoes. Some scientists suggest that the mixture of gases could have been formed from solar debris, similar to comets, smashing into the Earth and vaporising around 500 million years after its formation.

Figure 3 Some of the first photosynthesising bacteria probably lived in colonies like these stromatolites. They grew in water and released oxygen into the early atmosphere

On the other hand, many of the earliest living microorganisms could not tolerate a high oxygen concentration, because they had evolved without it. They largely died out, as there were fewer places where they could survive.

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Learning objectivesAfter this topic, you should know:

● the main changes in the atmosphere over time and some of the likely causes of these changes

● the relative proportions of gases in our atmosphere now.

C11.2 Our evolving atmosphereScientists think that the early atmosphere of the Earth contained a great deal of carbon dioxide. Yet the Earth’s atmosphere today only has around 0.04% of this gas. So where has it all gone? The answer is mostly into living organisms and into materials formed from living organisms. As you saw in the Topic C13.1, algae and plants decreased the percentage of carbon dioxide in the early atmosphere by photosynthesis.

Carbon ‘locked into’ rockCarbon dioxide, along with water, is taken in by plants and converted to glucose and oxygen during photosynthesis. The carbon in the glucose can then end up in new plant material. When animals eat the plants, some of this carbon can be transferred to the animal tissues, including their skeletons and shells.

Over millions of years, the skeletons and shells of huge numbers of these marine organisms built up at the bottom of vast oceans. There they became covered with layer upon layer of fine sediment. Under the pressure caused by being buried by all these layers of sediment, eventually the deposits formed sedimentary carbonate rocks such as limestone, a rock containing mainly calcium carbonate, CaCO

3.

Some of the remains of ancient living things (animals and plants) were crushed by large-scale movements of the Earth and were heated within the Earth’s crust over very long periods of time. They formed the fossil fuels coal, crude oil, and natural gas.

● Coal is classed as a sedimentary rock, and was formed from thick deposits of plant material, such as ancient trees and ferns. When the plants died in swamps, they were buried, in the absence of oxygen, and compressed over millions of years.

● Crude oil and natural gas were formed from the remains of plankton deposited in muds on the seabed. These remains were covered by sediments that became layers of rock when compressed over millions of years. The crude oil and natural gas formed is found trapped beneath these layers of rock.

In this way, much of the carbon from the old carbon dioxide-rich atmosphere became locked up within the Earth’s crust in rocks and fossil fuels.

Carbon dioxide gas was also removed from the early atmosphere by dissolving in the water of the oceans. It reacted, for example, with metal oxides, and made insoluble carbonate compounds. These fell to the seabed as sediments and helped to form more carbonate rocks.

Over the past 200 million years, the level of carbon dioxide in the atmosphere has not changed much. This is due to the natural cycle of carbon in which carbon moves between the oceans, rocks, and the atmosphere.

Shelly carbonates

Carry out a test to see if crushed samples of shells contain carbonates. Think of the reaction that all carbonates undergo with dilute acid. How will you test any gas given off?

● Record your findings.

Figure 1 There is clear fossil evidence in carbonate rocks of the organisms which lived millions of years ago

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Ammonia and methaneVolcanoes also produced nitrogen gas, which gradually built up in the early atmosphere, and there may have also been small proportions of methane and ammonia gases.

Any methane and ammonia found in the Earth’s early atmosphere reacted with the oxygen, formed by the evolving algae and plants:

CH4 + 2O

2 → CO

2 + 2H

2O

4NH3 + 3O

2 → 2N

2 + 6H

2O

This removed the methane and ammonia from the atmosphere. However, the levels of nitrogen gas, N

2, in the atmosphere could build up,

as nitrogen is a very unreactive gas.

The atmosphere todayBy 200 million years ago, the proportions of gases in the Earth’s atmosphere had stabilised. These were much the same as they are today.

Look at the percentage of gases in the atmosphere today in the pie chart in Figure 2.

1 Complete the table to show the percentage proportions of gases in the Earth’s atmosphere today. [3 marks]

Nitrogen Oxygen Argon Carbon dioxide Other gases

2 Explain the origins of nitrogen gas in the early atmosphere and suggest why its percentage of the composition of air remains so high. [4 marks]

3 Explain how most of the carbon dioxide in the Earth’s early atmosphere was removed to arrive at a level of around 0.04% of today’s atmosphere. [4 marks]

The noble gases are all found in air, with argon, Ar, the most abundant at about 0.9%. Neon, Ne, krypton, Kr, and xenon, Xe, together make up less than 0.1% of clean, dry air.

Figure 2 The relative proportions of nitrogen, oxygen, and other gases in the Earth’s atmosphere. The Earth’s atmosphere also contains water vapour but the percentage in the atmosphere varies

nitrogen 78%

oxygen 21%

argon 0.9%

carbon dioxide 0.04%

trace amounts of other gases

● Photosynthesis by algae and plants decreased the percentage of carbon dioxide in the early atmosphere. The formation of sedimentary rocks and fossil fuels that contain carbon also removed carbon dioxide from the atmosphere.

● Any ammonia and methane was removed by reactions with oxygen, once oxygen had been formed by photosynthesis.

● Approximately four-fifths (about 80%) of the atmosphere today is nitrogen, and about one-fifth (about 20%) is oxygen.

● There are also small proportions of various other gases, including carbon dioxide, water vapour, and noble gases.

Key points

Synoptic linkTo see where the noble gases are situated in the periodic table, look back to Topic C2.2.

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