THE modern theory of stellar evolution implies that the Sun has increased in brightness by several tens of per cent over geological time. Were all other global parameters held constant, this would imply that the mean temperature of the Earth was below the freezing point of seawater about 2×109 yr ago1. There is, however, excellent geological and palaeontological evidence that there were extensive bodies of liquid water on the Earth between 3 and 4×109 yr ago, A possible solution to this puzzle, first postulated by Sagan and Mullen1, is that the Earth's primitive atmosphere contained small quantities of NH3 and other reducing gases which significantly enhanced the global greenhouse effect. NH3 is especially effective in the 8–13 µm window in a CO2—H2O atmosphere. It was argued that plausible changes in other global parameters, such as the infrared emissivity or the Russell–Bond albedo, would have been ineffective. The increase in solar brightness over geological time seems model-invariant even under extreme model perturbation for the solar interior1, as has recently been reconsidered in a straightforward stellar structure scaling argument2, Cosmochemical considerations point strongly to a higher abundance of reduced constituents in the primitive than in the contemporary terrestrial atmosphere; and reduced atmospheric components such as NH3 and CH4 are required to understand the accumulation of prebiological organic compounds necessary for the origin of life in this same period, between 3 and 4×109 yr ago3.
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SAGAN, C. Reducing greenhouses and the temperature history of Earth and Mars. Nature 269, 224–226 (1977). https://doi.org/10.1038/269224a0
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