Faint young Sun paradox

Unsolved problem in astronomy:

How can the early Earth have had liquid water if the Sun's output is theorized to have been only 70% as intense as it is today?

(more unsolved problems in astronomy)

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.^[1] 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^[2] and supported life.^[3]

The issue was raised by astronomers Carl Sagan and George Mullen in 1972.^[4] Proposed resolutions of this paradox have taken into account greenhouse effects, changes to planetary albedo, astrophysical influences, or combinations of these suggestions. The predominant theory is that the greenhouse gas carbon dioxide contributed most to the warming of the Earth.^[5]

^ Feulner, Georg (2012). "The faint young Sun problem". Reviews of Geophysics. 50 (2): RG2006. arXiv:1204.4449. Bibcode:2012RvGeo..50.2006F. doi:10.1029/2011RG000375. S2CID 119248267.
^ Windley, B. (1984). The Evolving Continents. New York: Wiley Press. ISBN 978-0-471-90376-5.
^ Schopf, J. (1983). Earth's Earliest Biosphere: Its Origin and Evolution. Princeton, N.J.: Princeton University Press. ISBN 978-0-691-08323-0.
^ Sagan, C.; Mullen, G. (1972). "Earth and Mars: Evolution of Atmospheres and Surface Temperatures". Science. 177 (4043): 52–56. Bibcode:1972Sci...177...52S. doi:10.1126/science.177.4043.52. PMID 17756316. S2CID 12566286.
^ David Morrison, NASA Lunar Science Institute, "Catastrophic Impacts in Earth's History", video-recorded lecture, Stanford University (Astrobiology), 2010 Feb. 2, access 2016-05-10.

[1] Feulner, Georg (2012). "The faint young Sun problem". Reviews of Geophysics. 50 (2): RG2006. arXiv:1204.4449. Bibcode:2012RvGeo..50.2006F. doi:10.1029/2011RG000375. S2CID 119248267.

[2] Windley, B. (1984). The Evolving Continents. New York: Wiley Press. ISBN 978-0-471-90376-5.

[3] Schopf, J. (1983). Earth's Earliest Biosphere: Its Origin and Evolution. Princeton, N.J.: Princeton University Press. ISBN 978-0-691-08323-0.

[science177-4] Sagan, C.; Mullen, G. (1972). "Earth and Mars: Evolution of Atmospheres and Surface Temperatures". Science. 177 (4043): 52–56. Bibcode:1972Sci...177...52S. doi:10.1126/science.177.4043.52. PMID 17756316. S2CID 12566286.

[5] David Morrison, NASA Lunar Science Institute, "Catastrophic Impacts in Earth's History", video-recorded lecture, Stanford University (Astrobiology), 2010 Feb. 2, access 2016-05-10.

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