The Earth formed approximately 4.6 billion years ago. Scientists cannot be certain about what gases made up the Earth’s early atmosphere. Ideas about how the atmosphere was produced and has changed have developed over time as new evidence has been discovered. There is still not enough evidence for scientists to be certain.
One theory suggests that the early atmosphere came from intense volcanic activity, which released gases that made the early atmosphere very similar to the atmospheres of Mars and Venus today. These atmospheres have:
Volcanic activity also released water vapour, which condensed as the Earth cooled to form the oceans. Nitrogen was probably also released by volcanoes which gradually built up in the atmosphere because it is unreactive.
For approximately 200 million years, the proportions of different gases in the atmosphere have been relatively stable. The pie chart below shows the percentages of gases that make up the atmosphere.
Name the most abundant gas in the modern atmosphere.