Earth Atmosphere’’


Catling speculates that the exobase of early Earth was hot and that the ancient nonthermal escape rate was more than 1000 times the present rate. However, low oxygen and high carbon dioxide on early Earth yields a cold exobase, and nonthermal escape rates are limited and cannot balance the volcanic outgassing of hydrogen. A lthough the supply of organic com-pounds by hydrothermal vents and me-teorites to the prebiotic Earth remains an interesting subject of study, our modeling results (1) suggest that research into the ori-gin of life should be refocused on chemistry in the atmosphere, in the global oceans, and at the interface between the atmosphere and ocean. The abundance of hydrogen in the atmo-sphere is currently limited by oxidation, not escape. We found that the hydrogen escape rate would have been È1000 times as high on prebiotic Earth as it is today (È1 to 3 108 cm–2 s–1) (2) and would have been the major sink for hydrogen. Therefore, our model sup-poses that hydrogen escape was energy-limited rather than diffusion-limited. The current high exobase temperature on Earth is caused by absorption of sunlight by oxygen and the lack of an effective radiator. Catling (3) speculates that a high exobase tem-perature on early Earth could be caused by other gases. However, other gases are as likely to cool the atmosphere as they are to warm it. Early Earth analogs include Titan, Venus, and Mars, all of which have cold exobases (4). The model cited by Catling (5) is not appropriate for early Earth (6) because it contains too much O 2 and not enough C

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