Photochemistry of the Martian Atmosphere (Mean Conditions)

Abstract

The objective of this paper is to develop a self-consistent model which agrees with measurements and predicts the density profiles of important photochemical products. This model includes the upper atmosphere and ionosphere to account for H, H 2, and odd nitrogen profiles. The derived O and CO mixing ratios at 135 km agree with the measured values of 1% for both species while the measurements of O + 2/CO + 2 favor O/CO 2 = 2%. The NO global mean densities are rather stable above 120 km and may vary from 8 × 10 4 to 1.5 × 10 6 cm -3 at 80 km depending on small variations of odd nitrogen chemistry. The water vapor profile below 60 km is not sensitive to photochemistry and is determined by mixing in the lower troposphere and by the temperature profile above the condensation level. The O 2 mixing ratio is fixed at the surface in the model, and the basic experimental data used for comparison with the calculations are the CO mixing ratio, the ozone abundance given by the 1.27-μm dayglow intensity and the absorption at 9.7 μm, and the hydrogen escape rate. Considering only pure CO 2-H 2O gas phase chemistry odd hydrogen reactions are so strong that they imply CO and O 3 densities lower than measured. Nitrogen chemistry may contribute substantially to the balance of other components, sulfur chemistry may be also important despite the strict upper limit ƒ SO 2 < 3 × 10 -8, and chlorine chemistry may be neglected. The main role of heterogeneous chemistry is to reduce the effect of odd hydrogen reactions and to increase CO and O 3 up to the measured values. Models which fit the full range of the experimental data are based on (1) the water vapor profile measured by the Phobos orbiter, (2) reduced values of σ (H 2O) near 190 nm, (3) a sink of odd hydrogen from collisions with ice particles in haze layers with an efficiency of 0.1, and (4) nitrogen or sulfur chemistry, the latter with possible presence of sulfur components with a mixing ratio of 10 -8.