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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 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 photochemicalmodel
for the upper atmosphere that simulates the observed H2abundance results in an H2 mixing ratio of 15 ± 5 parts per millionin the lower atmosphere. The H2 and HD
mixing ratios agree withphotochemical fractionation of D (deuterium)
between H2O and H2.Analysis of D fractionation
among a few reservoirs of ice, watervapor, and molecular hydrogen on
Mars implies that a global oceanmore than 30 meters deep was lost
since the end of hydrodynamicescape. Only 4% of the initially
accreted water remained on theplanet at the end of hydrodynamic
escape, and initially Mars couldhave 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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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.
