Degassing history and carbon cycle of the Earth: From an impact-induced steam atmosphere to the present atmosphere

The recent theoretical studies on the formation and evolution of the atmosphere and oceans of the Earth are reviewed. Impact degassing during accretion of the Earth would probably generate an impact-induced steam atmosphere on the proto-Earth. At the end of accretion, the steam atmosphere became unstable and condensed to form the proto-ocean with almost the present mass of ocean. The steam atmosphere would have thus evolved to the proto-CO₂ atmosphere during the earliest history of the Earth because CO in the proto-atmosphere may be photochemically converted to CO₂. However, CO₂ in the proto-atmosphere has decreased with time through the global carbon cycle which may have stabilized the terrestrial environment against an increase in the solar luminosity. The continental growth during Hadean and Archean would therefore have a significant influence on the carbon cycle and the surface temperature. It is also suggested that the continental growth is a necessary condition for the terrestrial environment to evolve to the present state. Both the impact degassing and the subsequent continuous degassing are suggested to have played a major role in the formation and evolution of the atmosphere and ocean. In particular, most of N₂ may have been produced by the impact degassing during accretion, and the contribution of the subsequent continuous degassing is at most 10% for N₂. As a consequence, after the CO₂ level decreased to less than 1 bar, the atmosphere may have been at about 1 bar and composed mainly of N₂ for most of the subsequent history of the Earth.