A PHOTOCHEMICAL MODEL OF THE MARTIAN ATMOSPHERE

The factors governing the amounts of CO, O2, and O3 in the martian atm osphere are investigated using a minimally constrained, one-dimensiona l photochemical model. We find that the incorporation of temperature-d ependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agre ement between models and observations of CO, O2, O3, and the escape fl ux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH a nd OH + HO2 within their specified uncertainties. Similar revisions ha ve been suggested to resolve discrepancies between models and observat ions of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atom ic hydrogen escape, it is much larger than recent calculations of the exopheric escape rate for oxygen. Weathering of the surface may accoun t for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian u pper atmospheric mission. We also consider the possibility that HO(x) radicals may be catalytically destroyed on dust grains suspended in th e atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much hi gher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observ ation. (C) 1994 Academic Press, Inc.