Water Vapor

Water Vapor was first convincingly demonstrated in the atmosphere of Mars in 1964. It is well established that the H₂O vapor abundance over the entire disk of Mars is variable and sometimes undetectably small. Seasonal variations caused the abundance to vary. Typical planetary average water abundance are a few hundred parts per million of the CO₂ abundance based on the assumption of uniform mixing of these gases. However, because of the temperature gradient in the atmosphere, the water is strongly concentrated into the lowest 6 to 10 km, with even further enhancement likely in the lowest 1 or 2 km. The water vapor abundance was found to be a smooth function of latitude, ranging almost linearly from 0 near 60^O S to 10 ppm near 20^O N.

Carbon Monoxide

CO, which is an obviously likely component of any CO₂ atmosphere exposed to solar UV irradiation, was first detected and measured in 1969. Present estimates of the CO abundance are near a mole fraction of 2.7´10^-3. Photolysis of CO₂ will generate not only CO but also atomic O, which will rapidly recombine via three-body reactions to form molecular oxygen, O₂. Observers have succeeded in establishing the presence of a small but significant amount of O₂, about the mole fraction of 1.4´10^-3. In the photolysis of CO₂ the molar ratio O₂ : CO in the products is 0.5 ( CO₂ ® CO + ½O₂ ). Ozone is also present in detectable amounts in the polar regions.

Nitrogen

The search for nitrogen compounds has been under way for nearly 20 years. Numerous papers dealing with nitrogen dioxide have been published without a positive detection. The only nitrogen compound detected to date in the troposphere of Mars is the molecular nitrogen found by the Viking analytical experiments, at a level of about 2.5%.

Two distinct theories attempted to explain the red coloration of Mars on the basis of photochemical reactions of atmospheric gases. One model attributed the red color to the dimer of NO₂, nitrogen textroxide, whereas the other ascribed the red color to polymers of C₃O₂, carbon suboxide. However, spectroscopic data set an extremely low upper limit on the abundance of gaseous C₃O₂ monomer. This is some 10⁷ times less than the vapor pressure of C₃O₂ monomer in the regions of Mars surveyed.

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