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Science 6 January 2006:
Vol. 311. no. 5757, p. 38
DOI: 10.1126/science.1118412
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Technical Comments

Response to Comment on "A Hydrogen-Rich Early Earth Atmosphere"

Feng Tian,1* Owen B. Toon,1,2 Alexander A. Pavlov1

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.

1 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA.
2 Program in Atmospheric and Oceanic Science, University of Colorado, Boulder, CO 80309, USA.

* To whom correspondence should be addressed: E-mail: tian{at}colorado.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)