Faint young Sun paradox

The faint young Sun paradox or the faint young Sun problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan and George Mullen in 1972 [ [http://www.sciencemag.org/cgi/content/abstract/177/4043/52?ck=nck Sagan, C., Mullen, G., 1972, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures”, Science, 177, 52–56.] ] .

According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their life time [ [http://adsabs.harvard.edu/abs/1981SoPh...74...21G Gough, D.O., 1981, “Solar Interior Structure and Luminosity Variations”, Solar Phys., 74, 21–34] ] . However, with the predicted brightness 4 billion (10⁹) years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. This contradicts geological observations of sedimentary rocks, which required the presence of flowing liquid water to form.

The tension between the two hypotheses stems from the incorrect assumption that atmospheric gas concentrations in the past were the same as today. First, before the advent of abundant life the atmospheric oxygen concentrations were orders of magnitude lower than today. In the presence of oxygen, methane breaks down to carbon dioxide, so in the absence of oxygen the methane concentration could be much larger than currently observed [ [http://deepblue.lib.umich.edu/bitstream/2027.42/43349/1/11084_2005_Article_BF01809466.pdf Walker, J.C.G., 1985, “Carbon Dioxide on the Early Earth”, Origins of Life, 16, 117–127] ] . Methane is a more potent greenhouse gas than carbon dioxide, so the relative abundance of atmospheric methane throughout Earth's history must be considered when modeling the temperature.

Further, the inorganic version of the carbon cycle can be expected to provide negative feedback towards an Earth with liquid water. Carbon dissolved in liquid water can form carbonic acids, which can then interact with calcium to produce calcium carbonate. If rainfall were to cease and the oceans froze over, then this part of the inorganic carbon cycle would shut down. Periodic explosions from volcanoes would then cause a net increase in the atmospheric carbon dioxide and methane levels with no liquid water to absorb these emitted gases. Eventually, the concentrations would become large enough that the surface temperature would rise due to the greenhouse effect. When the surface temperature became large enough for the oceans to melt and rainfall to resume the other half of the inorganic carbon cycle would turn on and moderate the greenhouse gas concentrations [ [http://www.hawaii.edu/cgi-bin/uhnews?20080428100407 Before fossil fuels, Earth’s minerals kept CO2 in check, Article on www.hawaii.edu] ] .

It is also noteworthy that even though evidence of flowing water exists even from very early in Earth's history, it has been speculated that there may have been a number of examples of periods when the Earth's oceans froze over completely. The most recent such period may have been ~630 million years ago, and may have been instrumental in leading the Cambrian explosion of new multicellular life forms.

ee also

* Isua greenstone belt

References

Geosphere-Biosphere Interactions and Climate By Lennart Bengtsson, Claus U. Hammer