Concept

Faint young Sun paradox

The faint young Sun paradox or faint young Sun problem 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 percent as intense during that epoch as it is during the modern epoch. The paradox is this: with the young sun's output at only 70 percent of its current output, early Earth would be expected to be completely frozen - but early Earth seems to have had liquid water and supported life. The issue was raised by astronomers Carl Sagan and George Mullen in 1972. Proposed resolutions of this paradox have taken into account greenhouse effects, changes to planetary albedo, astrophysical influences, or combinations of these suggestions. It turned out that the greenhouse gas carbon dioxide contributed most. Models of stellar structure, esp. the standard solar model were already sufficiently evolved at that time to predict a brightening: It is due to the decrease in the number of particles per unit mass due to nuclear fusion in the Sun's core, from four protons and electrons each to one helium nucleus and two electrons. Fewer particles would exert less pressure. A collapse under the enormous gravity is prevented by an increase in temperature, which is both cause and effect of a higher rate of nuclear fusion. More recent modeling studies have shown that the Sun is currently 1.4 times brighter today than it was 4.6 billion years ago (Ga), and that the brightening has accelerated considerably. At the surface of the Sun, more fusion power means a higher solar luminosity (via slight increases in temperature and radius), which on Earth is termed radiative forcing. Sagan and Mullen even suggested during their descriptions of the paradox that it might be solved by high concentrations of ammonia gas, NH3. However, it has since been shown that while ammonia is an effective greenhouse gas, it is easily destroyed photochemically in the atmosphere and converted to nitrogen (N2) and hydrogen (H2) gases.

Official source

https://en.wikipedia.org/wiki/Faint_young_Sun_paradox

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