What were the gases which initially formed the earth's atmosphere?
The early atmosphere of the earth largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen.
Initially, the earth had a thin atmosphere of hydrogen and helium, but this primordial atmosphere was stripped off due to solar winds. The present atmosphere evolved through the process of degassing, where gases and water vapour were released from the interior solid earth during the cooling of the earth. Continuous volcanic eruptions also contributed water vapour and gases to the atmosphere.
Explanation
The textbook clearly mentions two stages of atmospheric gases. First, the primordial atmosphere consisting of hydrogen and helium, which was lost due to solar winds. Second, the early atmosphere formed through degassing, which contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little free oxygen. Students should mention both stages to show complete understanding.