Soon after it formed, Earth's incandescent core was enveloped by an ocean of magma. Intense bombardment by meteorites combined with the mantle's heat caused outgassing of its volatile constituent rocks. Within a few million years, a thick, dense atmosphere formed, even though a large portion of it was probably expelled by the gigantic impact that created the Moon 4.4 billion years ago.
The gases released by this mechanism probably had the same composition as those emitted by volcanic eruptions. The early atmosphere would therefore have been rich in water vapour, nitrogen (N2 ) and oxide gases like carbon dioxide (CO2 ) and sulphur dioxide (SO2 ) Oxygen was only present in trace levels at this stage. Its concentration began to rise about 2.5 billion years ago, at the same time as the concentration of CO2 was decreasing. This evolution was very probably the result of microorganisms using light (photosynthesis) or chemical reactions (chemosynthesis) as a source of energy.