Other
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.p41a..04k&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #P41A-04
Other
5405 Atmospheres: Composition And Chemistry, 5407 Atmospheres: Evolution, 5435 Ionospheres (2459), 6225 Mars, 2419 Ion Chemistry And Composition (0335)
Scientific paper
Self-consistent models for 11 neutral and 18 ion species from 80 to 300 km on Mars have been developed by solving the continuity equations including ambipolar diffusion for ions. The models were calculated for the conditions of the HST, FUSE, and Mariner 6, 7 observations of D, H2, and H, respectively, when solar activity index was 25, 61, and 88 on Mars orbit, respectively. Special care was taken to simulate the processes of H2 and HD dissociation in the reactions with CO2+, O+, CO+, N2+, N+, Ar+, O(1D), and by photoelectrons. Thermal and nonthermal escape velocities were used as the upper boundary conditions for H2, H, HD, D, and He. The calculated ion density profiles at various solar activity and the column reaction rates provide complete quantitative information for behavior of each ion, its formation and loss. The HCO+ ion is abundant in Mars' ionosphere because it is a final product of many reactions of other ions with H2 and does not react with neutral species. The H2 and D mixing ratios of 15 ppm and 11 ppb chosen to fit the FUSE and HST observations of H2 and D, respectively, result in (HD/H2)/(HDO/H2O) = 0.41. This value agrees with the depletion of D in H2 because of the smaller HDO photolysis cross section, the preferential condensation of HDO above the hygropause, and the fractionation in chemical reactions that result in the formation of H2. Therefore the controversial problem of deuterium fractionation is solved throughout the atmosphere. Isotope fractionation factor for hydrogen escape is equal to 0.055, 0.082, and 0.167 for low, medium, and high solar activity, respectively, and the solar cycle mean value is 0.105. The polar caps shrink or dissappear at high obliquity, and water in the polar caps is in isotopic equilibrium with the atmospheric water. Using the water amount of 14 m in the polar caps, the fractionation factor, the present D/H ratio and that at the end of hydrodynamic escape (5.5 and 1.9 times the terrestrial ratio, respectively), the calculated loss of water for the last 3.8 Byr is equal to a global ocean 30 m deep. Delivery of water by comets was ≈0.5 m in that period and could not significantly affect the D/H ratio. The existence of liquid ocean on early Mars is not sufficient to drive hydrodynamic escape, and that of the initially accreted H2 and H2 released in the reaction Fe + H2O -> FeO + H2 is more probable. Using the known fractionation factor of 0.8 for hydrodynamic escape, the total loss of water in hydrodynamic escape is equivalent of an ocean of more than 1.3 km deep. This means that the initial amount of water on Mars exceeded that on Earth scaled to the mass and size of Mars.
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