Faint young Sun paradox: Wikis

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The faint young Sun paradox or 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[1].

According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their life time[2]. 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. The geological record however shows a continually relatively warm surface during the complete early Temperature record of the earth with the exception of a cold phase about 2.4 billion years ago. Water-related sediments have been found that date to as early as 3.8 billion years ago[3]. Hints on early life forms are as early as 3.5 billion years[4], the basic carbon isotopy is very much in line with what is found today[5]. A regular change between ice ages and warm periods is only to be found since one billion years.

Explanations of the paradox take into account either greenhouse effects or astrophysical deliberations, or a combination of the two.

Contents

Greenhouse Hypothesis

Earth's atmosphere may have contained more greenhouse gases early on. Carbon dioxide concentrations may have been higher without plant photosynthesis converting it to oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce carbon dioxide, may have been more prevalent as well. [6]

Following the accretion of continents after about 1 billion years[7], Walter and others assume a non-biological version of the carbon cycle to provide a negative temperature feedback. Carbon dioxide dissolves in liquid water and combines with metal ions derived from silicate weathering to produce carbonates. During ice ages this part of the cycle would shut down. Volcanic carbon emissions would restart a warming cycle due to the greenhouse effect. [8] [9]

In 2009 Ueno et al. [10] proposed, based on analysis of geological sulfur isotopes, that carbonyl sulfide (OCS) was present in the Archean atmosphere. Carbonyl sulfide is an efficient greenhouse gas and Ueno et al. estimate that the additional greenhouse effect would have been sufficient to prevent the Earth from freezing over.

There may have been a number of periods when the Earth's oceans froze over completely (Snowball Earth hypothesis). The most recent period may have been ~630 million years ago. Afterwards, the Cambrian explosion of new multicellular life forms started.

Astronomical and Geochemical Evidence and Hypothesis

Phanerozoic Climate Change

A minority view, propounded by Nir Shaviv uses climatical influences of Solar Wind and - along the hypothesis of Henrik Svensmark - a cooling effect of Cosmic Rays to explain the paradox[11].

According to Shaviv, the early sun had emitted a stronger solar wind with a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice-free Earth. The temperature minimum around 2.4 billion years goes along with a cosmic ray flux modulation by a variable star formation rate in the Milky Way Galaxy.

The reduced solar impact later results into a stronger impact of cosmic ray flux (CRF), which is hypothesised to lead to a relationship with climatical variations.

See also

References

  1. ^ Sagan, C., Mullen, G., 1972, “Earth and Mars: Evolution of Atmospheres and Surface Temperatures”, Science, 177, 52–56.
  2. ^ Gough, D.O., 1981, “Solar Interior Structure and Luminosity Variations”, Solar Phys., 74, 21–34
  3. ^ B. Windley: The Evolving Continents. Wiley Press, New York 1984.
  4. ^ J. Schopf: Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton University Press, Princeton, N.J., 1983
  5. ^ Celestial climate driver: a perspective from four billion years of the carbon cycle Geoscience Canada, March, 2005 by Jan Veizer
  6. ^ 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]
  7. ^ Veizer, J.: 1976, in B. F. Windley (ed.), The Early History of the Earth, John Wiley and Sons, London, p. 569.
  8. ^ Before fossil fuels, Earth’s minerals kept CO2 in check, Article on www.hawaii.edu
  9. ^ A negative feedback mechanism for the long-term stabilization of the earth's surface temperature JCG WALKER, PB HAYS, JF KASTING - JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. CIO, PAGES 9776-9782, OCTOBER 20, 1981 (pdf)
  10. ^ Ueno et al. Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox, PNAS [1]
  11. ^ [2] Shaviv, N. J. (2003), Toward a solution to the early faint Sun paradox: A lower cosmic ray flux from a stronger solar wind, J. Geophys. Res., 108(A12), 1437, doi:10.1029/2003JA009997

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

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