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 a tmosphere and oceans of the Earth are reviewed. Impact degassing durin g accretion of the Earth would probably generate an impact-induced ste am atmosphere on the proto-Earth. At the end of accretion, the steam a tmosphere became unstable and condensed to form the proto-ocean with a lmost 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 conve rted to CO2. However, CO2 in the proto-atmosphere has decreased with t ime through the global carbon cycle which may have stabilized the terr estrial environment against an increase in the solar luminosity. The c ontinental growth during Hadean and Archean would therefore have a sig nificant influence on the carbon cycle and the surface temperature. It is also suggested that the continental growth is a necessary conditio n for the terrestrial environment to evolve to the present state. Both the impact degassing and the subsequent continuous degassing are sug. gested to have played a major role in the formation and evolution of the atmosphere and ocean. In particular, most of N-2 may have been pro duced by the impact degassing during accretion, and the contribution o f the subsequent continuous degassing is at most 10% for N-2. As a con sequence, after the CO2 level decreased to less than 1 bar, the atmosp here may have been at about 1 bar and composed mainly of N-2 for most of the subsequent history of the Earth.