(b) The composition of the Earth's early atmosphere was very different from the composition of the Earth's atmosphere today - Edexcel - GCSE Chemistry - Question 2 - 2018 - Paper 1

The planet that has an atmosphere similar to that of the Earth's early atmosphere is Mars. Mars is known to have a thin atmosphere primarily composed of carbon dioxide, with traces of nitrogen and argon. In the past, it is believed that Mars had a more substantial atmosphere, possibly resembling Earth's early atmosphere, which contained high levels of carbon dioxide and water vapor, facilitating chemical reactions and potentially supporting primitive life forms.