Elsevier

Icarus

Volume 219, Issue 1, May 2012, Pages 13-24
Icarus

Chemical pathway analysis of the Martian atmosphere: CO2-formation pathways

https://doi.org/10.1016/j.icarus.2012.02.010Get rights and content

Abstract

The chemical composition of a planetary atmosphere plays an important role for atmospheric structure, stability, and evolution. Potentially complex interactions between chemical species do not often allow for an easy understanding of the underlying chemical mechanisms governing the atmospheric composition. In particular, trace species can affect the abundance of major species by acting in catalytic cycles. On Mars, such cycles even control the abundance of its main atmospheric constituent CO2. The identification of catalytic cycles (or more generally chemical pathways) by hand is quite demanding. Hence, the application of computer algorithms is beneficial in order to analyze complex chemical reaction networks. Here, we have performed the first automated quantified chemical pathways analysis of the Martian atmosphere with respect to CO2-production in a given reaction system. For this, we applied the Pathway Analysis Program (PAP) to output data from the Caltech/JPL photochemical Mars model. All dominant chemical pathways directly related to the global CO2-production have been quantified as a function of height up to 86 km. We quantitatively show that CO2-production is dominated by chemical pathways involving HOx and Ox. In addition, we find that NOx in combination with HOx and Ox exhibits a non-negligible contribution to CO2-production, especially in Mars’ lower atmosphere. This study reveals that only a small number of chemical pathways contribute significantly to the atmospheric abundance of CO2 on Mars; their contributions to CO2-production vary considerably with altitude. This analysis also endorses the importance of transport processes in governing CO2-stability in the Martian atmosphere. Lastly, we identify a previously unknown chemical pathway involving HOx, Ox, and HO2-photodissociation, contributing 8% towards global CO2-production by chemical pathways using recommended up-to-date values for reaction rate coefficients.

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 radiationCO2+hνCO+O,where atomic oxygen subsequently forms O2. The termolecular formation reactionCO+O+MCO2+Mis 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 viaCO+OHCO2+H.In the upper region of the atmosphere (z > 86 km) ionic chemistry becomes important. Here, CO2 is mainly formed from CO2+ by charge-exchange ionizationCO2++OCO2+O+.The 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”.

References (28)

  • R. Lehmann

    An algorithm for the determination of all significant pathways in chemical reaction systems

    J. Atmos. Chem.

    (2004)
  • F. Lefèvre

    Heterogeneous chemistry in the atmosphere of Mars

    Nature

    (2008)
  • M.B. McElroy et al.

    Stability of the martian atmosphere

    Science

    (1972)
  • P.C. Milner

    The possible mechanisms of complex reactions involving consecutive steps

    J. Electrochem. Soc.

    (1964)
  • Cited by (0)

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