Numerical study on simplified reaction set of ground state species in CO<sub>2</sub> discharges under Martian atmospheric conditions

Abstract

The exploration of Mars has attracted increasing interest in these years. The experiments and simulations show that strong electric field triggered by the dust storms in the Martian atmosphere may cause CO<sub>2</sub> discharge. Studies on this phenomenon will not only help deepen our comprehension on the evolution of Martian surface, but also provide a possibility to realize the <i>in-situ</i> oxygen generation on Mars based on plasma chemistry. In this paper, a zero-dimensional global model is used to simplify the complicated description of CO<sub>2</sub> chemical kinetics, therefore a reduced chemistry can be obtained for detailed numerical simulation in the near future. At the beginning of simplification, the graph theoretical analysis is used to pre-treat the original model by exploring the relationship between reacting species. Through the study of connectivity and the topological network, species such as O<sub>2</sub>, e, and CO prove to be important in the information transmission of the network. While gaining a clearer understanding of the chemistry model, dependence analysis will be used to investigate numerical simulation results. In this way a directed relation diagram can be obtained, where the influence between different species is quantitively explained in terms of numerical solutions. Users could keep different types of species correspondingly according to their own needs, and in this paper, some species with low activeness such as C<sub>2</sub>O, <inline-formula><tex-math id="M5">\begin{document}$ {\mathrm{O}}_{5}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M5.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M6">\begin{document}$ {\mathrm{O}}_{4}^{-} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M6.png"/></alternatives></inline-formula> and species with uncertainties such as <inline-formula><tex-math id="M7">\begin{document}$ {\mathrm{C}}_{2}{\mathrm{O}}_{2}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M7.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M8">\begin{document}$ {\mathrm{C}\mathrm{O}}_{4}^{+} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20210664_M8.png"/></alternatives></inline-formula> are removed from the original model. As for the reduction of specific reactions among species, the reaction proportion analysis based on the calculation of reaction rates is used to obtain the contribution of each reaction to the entire process of CO<sub>2</sub> discharge, through which the important reactions can be selected. Finally, a simplified chemistry model of CO<sub>2</sub> discharge based on Martian atmospheric conditions, including 16 species and 67 reactions, is established. The numerical simulations show that the trends of species densities based on the simplified chemistry model are highly consistent with those based on the original one, and considerations about the CO<sub>2</sub> conversion and the energy efficiency are also in line with expectations, which can help deepen the understanding of the essential process of CO<sub>2</sub> discharge under Martian atmospheric conditions. In addition, the quantitative results of the relationship between reactive species will lay a theoretical foundation for the accurate analysis of various products in Martian dust storm discharges and the realization of Mars <i>in-situ</i> oxygen generation technology based on plasma chemistry.