DPS 2001 meeting, November 2001
Session 44. Mars Atmosphere III: Aeronomy
Oral, Chair: M. Mendillo, Friday, November 30, 2001, 11:30am-12:30pm, Regency GH

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[44.01] First Detection of Molecular Hydrogen in the Atmosphere of Mars: Implications for Evolution of Water

V. A. Krasnopolsky (Catholic University of America), P. D. Feldman (Johns Hopkins University)

We observed Mars using the Far Ultraviolet Spectroscopic Explorer (FUSE) and obtained a high-quality spectrum at 904-1186 Å with resolution of 0.2 Å. The spectrum consists of many lines of H, O, N, C, Ar, He (in the second order), O+, N+, C+, and bands of CO and N2. Four H2 lines at 1071.62, 1090.45, 1118.61, and 1166.76 Å were detected. Their intensities (~0.3 R) were corrected for air mass, contribution of the airglow at Mars' limb, selfabsorption, and resulted in the H2 column abundance of (1.56±0.17)x1013 cm-2 above 140 km. We developed a model which calculates self-consistently the density profiles of CO2, N2, CO, O, H2, H, HD, D, and 14 ions at 80-300 km for low, medium, and high solar activity. H2 forms in the lower atmosphere and is delivered to the upper atmosphere by eddy and molecular diffusion. Removal of H2 from the upper atmosphere is due to the reactions with CO2+, O+, CO+, N2+, N+, O(1D), ionization, and photoelectron dissociation. The observed H2 abundance corresponds to the H2 mixing ratio of 15±4 ppm in the lower atmosphere, which is smaller by a factor of 2.7 than the predictions of the recent models.

By fitting the HST observation of D, our model gives HD/H2=0.29 HDO/H2O which is in reasonable agreement with the recent data on deuterium fractionation in the lower atmosphere. The isotope fractionation factor for hydrogen escape varies from 0.053 at solar minimum to 0.128 at solar maximum with a mean value of 0.09. Coupled with the abundance of ice of ~14 m in the polar caps and the D/H ratio of 1.9 at the end of hydrodynamic escape, this results in a loss of ~30 m of water in the last 4 Ga. If the initial D/H was at the terrestrial value, then more than 1 km of water was lost by hydrodynamic escape. Most probably, the initially accreted H2 and that released in the reaction of Fe + H2O could escape hydrodynamically. This supports the hypothesis that initially Mars was even more rich in water than Earth. An alternative interpretation, with no hydrodynamic escape, results in a loss of ~90 m of water in the last 4 Ga.

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