title: Detection of molecular hydrogen in the atmosphere of Mars.
abstract: 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) x 10(13) 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.
authors.affiliation: Department of Physics, Catholic University of America, Washington, DC 20064, USA. vkrasn@altavista.com
authors.forename: V A
authors.initials: VA
authors.lastname: Krasnopolsky
authors.forename: P D
authors.initials: PD
authors.lastname: Feldman