 Earth 's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor . The origin of the world 's oceans was condensation augmented by water and ice delivered by asteroids , protoplanets , and comets . In this model , atmospheric " greenhouse gases " kept the oceans from freezing when the newly forming Sun had only 70 % of its current luminosity . By 3 @.@ 5 Gya , Earth 's magnetic field was established , which helped prevent the atmosphere from being stripped away by the solar wind . 
