Warming Early Earth and Mars

摘要

Sagan and Chyba (1), in their article on page 1217 of this issue, have revived an old debate about how liquid water was maintained on early Earth and Mars despite a solar luminosity 25 to 30% lower than that at present. A theory that has been popularfor some time (2) is that greatly elevated concentrations of atmospheric CO_2, produced by the action of the carbonate-silicate cycle, provided enough of a greenhouse effect to warm early Earth. However, Rye et al. (3) have placed geochernical constraints on early atmospheric CO_2 abundances that fall well below the levels needed to warm the surface.