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Science 30 November 2001:
Vol. 294. no. 5548, pp. 1914 - 1917
DOI: 10.1126/science.1065569
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Reports

Detection of Molecular Hydrogen in the Atmosphere of Mars

Vladimir A. Krasnopolsky,1* Paul D. Feldman2

Four hydrogen (H2) lines have been detected in a spectrum of Mars observed with the Far Ultraviolet Spectroscopic Explorer. Three of those lines are excited by the solar Lyman beta  photons. The line intensities correspond to a column H2 abundance of 1.17 (±0.13) × 1013 per square centimeter above 140 kilometers on Mars. A photochemical model for the upper atmosphere that simulates the observed H2 abundance results in an H2 mixing ratio of 15 ± 5 parts per million in the lower atmosphere. The H2 and HD mixing ratios agree with photochemical fractionation of D (deuterium) between H2O and H2. Analysis of D fractionation among a few reservoirs of ice, water vapor, and molecular hydrogen on Mars implies that a global ocean more than 30 meters deep was lost since the end of hydrodynamic escape. Only 4% of the initially accreted water remained on the planet at the end of hydrodynamic escape, and initially Mars could have had even more water (as a proportion of mass) than Earth.

1 Department of Physics, Catholic University of America, Washington, DC 20064, USA.
2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA.
*   To whom correspondence should be addressed. E-mail: vkrasn{at}altavista.com

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