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The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases

Science
23 May 1997
Vol 276, Issue 5316
pp. 1217-1221

Abstract

Atmospheric mixing ratios of ∼10−5 ± 1 for ammonia on the early Earth would have been sufficient, through the resulting greenhouse warming, to counteract the temperature effects of the faint early sun. One argument against such model atmospheres has been the short time scale for ammonia photodissociation by solar ultraviolet light. Here it is shown that ultraviolet absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently that ammonia resupply rates were able to maintain surface temperatures above freezing.

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This research was supported in part by NASA grants NAGW-3273, NAGW-1896, and NAGW-1870. We thank K. J. Zahnle, J. Bada, J. Kasting, W. R. Thompson, B. N. Khare, S. L. Miller, E. E. Salpeter, S. Epstein, J. W. Schopf, D. Layzell, D. Turpin, J. C. G. Walker, S. Soter, J. Goldspiel, P. Gogarten, L. A. Wilson, G. Bowen, C. de Ruiter, R. Hargraves, and G. D. McDonald.

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Science
Volume 276 | Issue 5316
23 May 1997

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Published in print: 23 May 1997

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Carl Sagan
Prior to his death in December 1996, Dr. Sagan directed the Laboratory for Planetary Studies at Cornell University, Ithaca, NY 14853–6801, USA. Dr. Chyba is in the Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721–0092, USA. Final changes in response to editors’ and referees’ comments were made in this manuscript by Dr. Chyba subsequent to Dr. Sagan’s death.
Christopher Chyba
Prior to his death in December 1996, Dr. Sagan directed the Laboratory for Planetary Studies at Cornell University, Ithaca, NY 14853–6801, USA. Dr. Chyba is in the Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721–0092, USA. Final changes in response to editors’ and referees’ comments were made in this manuscript by Dr. Chyba subsequent to Dr. Sagan’s death.

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