The Earth formed about 4.5 billion years ago. Scientists believe that its early atmosphere was produced by volcanic activity.
The early atmosphere probably contained:
Scientists cannot be sure about the early atmosphere, since it formed so long ago. They can only analyse evidence from other sources.
Some ancient rocks contain iron compounds that break down in the presence of oxygen. They could only have formed if there was little or no oxygen at the time.
A volcano releases large volumes of carbon dioxide and water vapour. Since the early atmosphere was produced by volcanic activity, it was likely to have contained a large amount of carbon dioxide and water vapour.