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

• DOI: 10.1126/science.276.5316.1217
• Publication Date: 2002-07-27T09:37:20Z
• Authors: Carl Sagan; Christopher Chyba
• Keywords: Greenhouse Effect; Photolysis; Atmosphere; Earth, Planet; Ultraviolet Rays; Carbon Dioxide; 01 natural sciences; 7. Clean energy; Models, Chemical; Ammonia; 13. Climate action; 0103 physical sciences; Solar System; Evolution, Planetary; Methane

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.