Oceans and atmosphere
See also: Origin of the world's oceans Graph showing range of estimated partial pressure of atmospheric oxygen through geologic time Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light ( atmophile ) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances. After the impact which created the moon, the molten Earth released volatile gases; and later more gases were released by volcanoes , completing a second atmosphere rich in greenhouse gases but poor in oxygen. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga. In early models for the formation of the atmosphere and ocean, the second atmosp...