STABILITY OF THE MARTIAN ATMOSPHERE - IS HETEROGENEOUS CATALYSIS ESSENTIAL

A comprehensive homogeneous gas phase photochemical model is developed to study the problem of stability of the Martian atmosphere. The one- dimensional model extends from the ground up to 220 km, passing throug h the homopause at 125 km. The model thus couples the lower (neutral) atmosphere to the ionosphere above which provides significant downward flux of carbon monoxide and oxygen atoms. It is concluded on the basi s of currently accepted values for globally and seasonally averaged wa ter vapor abundance, dust opacity and the middle atmospheric eddy mixi ng coefficient, as well as the relevant laboratory data (particularly the temperature dependence of CO2 absorption cross section and the rat e constant for CO + OH reaction), that the rate of re-formation of car bon dioxide exceeds its photolytic destruction rate by about 40%. Furt hermore, it is found that this result is virtually independent of the choice of eddy mixing coefficient, unless its value in the middle atmo sphere exceeds 10(8) cm2s-1 or is far smaller than 10(5) cm2s-1, or th e dust opacity, unless it exceeds unity, or the water vapor mixing rat io at the surface, unless it is far smaller (less-than-or-equal-to 1 p pm) or far greater (greater-than-or-equal-to 500 ppm) than the average value (approximately 150 ppm). Since none of these extremes represent globally and seasonally averaged conditions on Mars, we propose that the present model requires existence of a mechanism to throttle down t he recycling rate of carbon dioxide on Mars. Therefore, it is suggeste d that a heterogeneous process which provides a sink to the species th at participate in the recycling of CO2, i.e., H2O, H2O2, OH, CO or O, in particular, may be necessary to bring about the balance between the CO2 recycling rate and its photolytic destruction rate. Aerosols of d ust or ice (pure or doped water or carbon dioxide ice present in the a tmosphere of Mars) can provide the appropriate adsorption sites for th e above heterogeneous process. Despite our conclusion that some hetero geneous process may be needed, it is important to recognize that one-d imensional models can only provide first-order results which, most lik ely, represent globally and seasonally averaged conditions. However, i t is only after actual temporal, latitudinal and longitudinal variatio ns of relevant atmospheric parameters are included in the model that o ne can determine fully whether the problem of atmospheric stability st ill continues to persist and whether some heterogeneous process is req uired to correct it.