Rate Coefficients for the Reactions of CO2+ With O: Lessons From MAVEN at Mars

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In the lower ionosphere of Mars, the relative density distributions of the major ions CO2+, O2+, and O+ are largely determined by three reactions, including the two channels of the reaction of CO2+ with O, which yield either O2+ (R1) or O+ (R2), and the reaction of O+ with CO2, which yields solely O2+ (R3). There have been only two measurements of the rate coefficients for reactions (R1) and (R2) in the last 50 years, and they are very different (Fehsenfeld et al., 1970; Tenewitz et al., 2018). Although we have carried out a fairly thorough exploration of parameter space in our quest to fit the density profiles of the major ions and O atoms as measured by instruments on the MAVEN spacecraft, we report here only a small fraction of that exploration. In this investigation, we have used the atmosphere of Mars as a laboratory to distinguish between the two sets of rate coefficients. We find that the best fits to the O2+, CO2+, and O+ densities are for the three models in which the Fehsenfeld et al. rate coefficients are adopted, each of which has its advantages. We conclude that the favored O density profile is 1.5±0.5 times the measured profile, and that the rate coefficients measured 50 years ago are much better than those recently measured at explaining the density profiles of O2+, CO2+, and O+ in the Martian ionosphere as we know it.



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