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The early atmosphere

Scientists believe that the Earth was formed about 4.5 billion years ago. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence. Its early atmosphere was probably formed from the gases given out by volcanoes. The greatest proportion of volcanic gases are made up of carbon dioxide, water vapour and ammonia.

As the Earth cooled down, most of the water vapour condensed and formed the oceans.

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

Scientists can’t be sure about the early atmosphere and can only draw evidence from other sources. For example, volcanoes release high quantities of carbon dioxide. Iron-based compounds are present in very old rocks that could only have formed if there was little or no oxygen at the time.

Changes in the atmosphere

So how did the proportion of carbon dioxide, methane and ammonia in the atmosphere go down, and the proportion of oxygen go up?

• The proportion of oxygen increased because of photosynthesis by plants.
• The proportion of ammonia decreased as it reacted with the newly formed oxygen in the atmosphere to form nitrogen and water vapour.
• The proportion of methane decreased as it reacted with the newly formed oxygen to form carbon dioxide and water.

The proportion of carbon dioxide went down because:

• it was locked up in sedimentary rocks formed from marine animal shells (such as limestone and chalk) and in fossil fuels
• it was absorbed by plants for photosynthesis
• it dissolved in the oceans

The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing.

The modern atmosphere

The air is a mixture of gases. The amount of water vapour in the air varies from place to place, and day to day. For this reason, the proportions of the gases in the air are usually given for dry air.

Some of the gases in the air are elements:

• nitrogen, N₂
• oxygen, O₂
• argon, Ar
• neon, Ne

Note that these gases can be extracted from the air for various uses using fractional distillation.

Some are compounds, including:

• carbon dioxide, CO₂
• water vapour, H₂O