Warming Early Earth and Mars

Life on Earth is thought to have started about 4 billion years ago, yet the sun's luminosity at that stage of its evolution would have been 25 to 30% lower than today. This primordial solar luminosity would have been insufficient to heat the Earth enough to allow water to be in its liquid phase and so enable the formation of life, a dilemma that has been called the early faint sun paradox. In his Perspective, Kasting reviews the background to this paradox and comments on the results of Sagan and Chyba in the same issue, which are presented as a resolution to the dilemma. Sagan and Chyba ([p. 1217][1]) suggest that absorption of solar ultraviolet radiation by high-altitude organic particulates may have permitted sufficient ammonia concentration in the early atmosphere to allow greenhouse warming of the Earth. Kasting comments on this suggestion and discusses its implications. [1]: http://www.sciencemag.org/cgi/content/short/276/5316/1217