Chemical pathway analysis of the Martian atmosphere: CO2-formation pathways
Highlights
► The first automated quantified chemical pathway analysis of the martian atmosphere with respect to CO2 is presented. ► All dominant pathways related to CO2-production have been quantified as a function of altitude. ► Their contributions to the atmospheric CO2 abundance of individual pathways vary considerably with altitude. ► Results endorse the importance of transport processes in governing the stability of CO2 in the martian atmosphere. ► An unknown chemical pathway contributing approximately 8% to global CO2-production has been identified.
Introduction
One of the fundamental questions of planetary science concerns the photochemical stability of CO2-dominated atmospheres in our Solar System, especially on Mars (95.32%, Owen et al., 1977). There, CO2 is photolyzed by solar UV radiationwhere atomic oxygen subsequently forms O2. The termolecular formation reactionis not fast enough to compensate for the effective CO2-destruction by photolysis. If one takes only the reaction products into account, one would therefore expect an atmosphere rich in CO and O2, in contradiction to observations. This suggests that other mechanisms stabilize the observed CO2 content of the Martian atmosphere. A first step in understanding the persistence of CO2 in the Martian atmosphere was taken by McElroy and Donahue, 1972, Parkinson and Hunten, 1972, who proposed chemical pathways involving Ox and HOx chemistry reproducing CO2 from CO and O. These pathways can be understood as sets of chemical reactions, where molecules from the Ox-family (i.e. O and O3) and the HOx-family (i.e. H, OH, and HO2) or the HOx-family only, are acting as catalysts. This means that there is no net production or consumption of the catalyst species by the chemical pathways. Such chemical pathways can therefore provide efficient alternative routes for CO2-production, even if the catalyst species are only present in trace amounts. Further improvements in photochemical models were made by the investigation of NOx (e.g., Krasnopolsky, 1993; Nair et al., 1994), and heterogeneous chemistry on dust and ice cloud particles (e.g., Anbar et al., 1993; Atreya and Gu, 1994; Krasnopolsky, 1993; Lefèvre et al., 2008).
Several methods have been applied in order to gain more insight about chemical reaction systems in general. A variety of powerful methods use sensitivity analysis (for reviews, see Rabitz et al., 1983; Saltelli et al., 2005; Turányi, 1990), which aim at understanding the effects of uncertainties (e.g. in chemical reaction rate coefficients) on the chemical system. However, it is not possible to construct chemical pathways by sensitivity analysis methods only. The identification of chemical pathways is in general a demanding task and their manual construction is only possible for pathological examples or specific problems as e.g. methane photo-oxidation in Earth’s atmosphere (Johnston and Kinnison, 1998). Algorithms which take only stoichiometric information into account, were developed by e.g. Milner, 1964, Clarke, 1988, Schuster and Schuster, 1993. However, since these algorithms do not account for reaction rates (kinetic information), they cannot provide any quantitative information for individual chemical pathways. Therefore, using such methods it is not possible to determine, which chemical pathways dominate the reaction system. Moreover, these methods are usually not applicable to large reaction networks, because the number of pathways generally increases progressively with increasing number of reactions (“combinatorial explosion”).
The algorithm used in this study (PAP – Pathway Analysis Program) takes stoichiometric as well as kinetic information (i.e. reaction rates) into account and is capable of identifying and quantifying all significant chemical pathways for a given chemical reaction system. Developed by Lehmann (2004), it has been applied to Earth’s stratospheric ozone-chemistry (Lehmann, 2004; Grenfell et al., 2006), Earth’s mesospheric ion-chemistry (Verronen et al., 2011), and Mars’ near surface atmospheric chemistry (Stock et al., 2011).
However, quantification of all dominant chemical pathways forming CO2 in the whole Martian atmosphere is still lacking. In order to determine the contributions of individual chemical pathways to the altitude-dependent CO2-production, we apply the PAP algorithm to the results of the Caltech/JPL photochemical 1-D model of the Martian atmosphere. From this we derive the global mean CO2-production by each of these pathways.
Section snippets
The algorithm
The Pathway Analysis Program (Lehmann, 2004) enables the identification and quantification of chemical pathways in arbitrary given reaction systems. For this purpose, starting with individual reactions as pathways, longer pathways are formed step by step by connecting shorter pathways at so-called ‘branching-point species’. At each step this branching-point species is chosen to be the species with the shortest lifetime with respect to the pathways formed in the previous steps. The algorithm
Results and discussion
Fig. 1 shows the total CO2-production and loss rate due to all chemical reactions. The production rate profile indicates, that there are two different chemical regimes in the Martian atmosphere. In the lower part of the atmosphere, i.e. z < 86 km, CO2 is mainly formed from CO viaIn the upper region of the atmosphere (z > 86 km) ionic chemistry becomes important. Here, CO2 is mainly formed from by charge-exchange ionizationThe destruction of CO2 takes place mainly due to
Summary and conclusions
In order to address the CO2 stability problem of the Martian atmosphere, we have applied for the first time the Pathway Analysis Program (PAP) to the modified Caltech/JPL photochemical column model of the Martian atmosphere. All dominant CO2-production-pathways in the underlying reaction network throughout the lower to middle atmosphere, where CO2-production is efficient, have been identified and quantified. Our results are in good agreement with previous studies (e.g. McElroy and Donahue, 1972
Acknowledgment
This research has been partly supported by the Helmholtz Association through the research alliance “Planetary Evolution and Life”.
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