Photochemical Stability of CO2 in the Martian Atmosphere: Reevaluation of the Eddy Diffusion Coefficient and the Role of Water Vapor

Abstract

The stability of CO2 in the Martian atmosphere against photodissociation is re-examined. A new one-dimensional model using lower thermospheric temperatures and smaller solar fluxes, which are more appropriate for the time of the Mariner 6/7 experiments, predicts smaller atomic oxygen densities in the upper atmosphere than those predicted by previous investigators. In order to explain the observed O densities in the upper atmosphere(1-2% at 135 km), our model requires a vertical eddy diffusion coefficient of ~5×107 cm2 sec-1, which is much smaller than the previously suggested values (>5×108 cm2 sec-1). In the lower atmosphere, catalytic reactions of HOx are essential for reforming CO2 from its dissociation products CO and O. Our results show that the HOx generated by the model using the global average of the observed amount of water vapor(~10 μm) catalyzes too much recombination. Diurnal variation in OH densities may help to suppress HOx catalyses, particularly at night. Martian water vapor is highly variable; therefore, the mechanism by which the CO2 level is maintained and its relationship to the water vapor distribution must be considered on a global scale. © 1989, Society of Geomagnetism and Earth, Planetary and Space Sciences. All rights reserved.