Evolution of the Earth's atmosphere

In a nutshell

The Earth's atmosphere was formed billions of years ago. The composition of gases in the atmosphere evolved by a decrease in carbon dioxide and an increase in oxygen to form the atmosphere that exists today.

Atmosphere

Definition

The atmosphere is a layer of gases that surrounds a planet.

The two most abundant gases in the Earth's atmosphere are nitrogen and oxygen. Other gases include carbon dioxide, water vapour and the noble gases

Note: Noble gases are the elements in group $0$ of the periodic table.

atmospheric gas	PERCENTAGE ABUNDANCE ( $\%$ )
Nitrogen	$78$
Oxygen	$21$
Other gases	$1$

Formation of the Earth's atmosphere

The composition of gases that exist in the atmosphere today are different to the the atmospheric gases formed billions of years ago.

1.	Billions of years ago, the Earth was molten and there was no atmosphere.
2.	As the Earth started to cool down, land masses began to form.
3.	Ruptures in the Earth's crust (volcanoes) were formed.
4.	Molten rock, ash and gases erupted from these volcanoes.
5.	The gases which erupted from the volcanoes were: carbon dioxide, hydrogen, nitrogen and methane.
6.	Water vapour condensed to form the oceans, as the Earth began to cool down.

Chemistry; Fuels and Earth science; KS4 Year 10; Evolution of the Earth's atmosphere

The gases which erupted from the volcanoes were: carbon dioxide, hydrogen, nitrogen and methane.

Evolution of the Earth's atmosphere

When the Earth's atmosphere was first formed, the percentage abundance of carbon dioxide was high and there was no oxygen. Today, the percentage abundance of carbon dioxide is less than $1\%$ and the percentage abundance of oxygen is $21\%$ , as evolution of the Earth's atmosphere involved the formation of oxygen and increased removal of carbon dioxide from the atmosphere.

Increase of oxygen

Oxygen was formed by photosynthesis. Photosynthesis is the process in which plants and algae convert carbon dioxide and water into glucose and oxygen, using light energy from the sun.

$carbon ~ dioxide + water \rightarrow glucose + oxygen\newline \ \newline 6CO_2 + 6H_2O\rightarrow C_6H_{12}O_6+6O_2$

Decrease of carbon dioxide

Plants and algae remove carbon dioxide from the atmosphere by using it up in photosynthesis. The formation of carbonates was another way in which carbon dioxide decreased in the atmosphere.

1.	Carbon dioxide dissolved in the oceans, forming carbonic acid.
2.	Carbonic acid released carbonate ions.
3.	Carbonate ions combined with metal ions, forming insoluble compounds.
4.	Marine organisms used up the carbonate compounds to make shells and skeletons.