History of Earth 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 atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as "impact degassing" in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as the Earth formed. The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases. Planetesimals
Snowball Earth with methane in the atmosphere, oxidizing it to carbon dioxide, a much weaker greenhouse gas, and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic. Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this 'negative feedback' process limits the magnitude of cooling. During the Cryogenian period, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high