Faint young sun paradox

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The faint young sun paradox describes the apparent contradiction between observations of liquid water early in 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 standard solar model describes the history and evolution of stars. An aspect of this model is that stars similar to the sun should gradually brighten over their life time (excluding a very bright phase just after formation). This prediction is supported by the observation of lower brightness in young stars of solar type. However, with the predicted brightness 4 billion (10⁹) years ago and conditions like the modern Earth, any liquid water exposed to the surface would quickly freeze solid. This contradicts geological observations of sedimentary rocks which require the presence of flowing liquid water to form.

Most scientists confronting this contradiction propose that the answer is to posit some form of super greenhouse environment early in the Earth's history. It is possible that outgassing from volcanoes during this period led to extraordinary concentrations of carbon dioxide, methane and other greenhouse gases. Because free oxygen was very rare during this period, these gases may have persisted in the atmosphere for longer than during modern atmospheric conditions. Such greenhouse gases may have increased the impact of the greenhouse effect to levels capable of sustaining liquid water even in the presence of significantly reduced solar input.

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