Insights from model-observation comparisons of CO2++ concentrations in the Martian Ionopshere

The molecular dication, CO₂⁺⁺, was detected in the ionosphere of Mars by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission [1]. This marked the first detection of a molecular dication in a planetary atmosphere. Results from photochemical models were compared with the observations with the modeled densities being significantly lower than the densities inferred from the observations. Here we show that a much better agreement between model results and observations is obtained when incorporating in the model the assumption that the ion is stable against unimolecular decay. We argue that this assumption not necessarily conflict with results from a storage ring experiment by Mathur et al. (1995) [Ref. 2]. Several modeling studies that cite [2] use a CO₂⁺⁺ lifetime against unimolecular decay of 4 s. This is, however, only a lower limit of the lifetime in question as the removal of the ions in the storage ring may have been strongly dominated by high energy collisions with residual gases. An experiment at a facility offering better (or variable) vacuum conditions could possible constraint the stability/longevity of CO₂⁺⁺.

[1] Gu, H., Cui, J., Niu, D. D., et al. 2020, E&PP, 4, 396

[2] Mathur, D., Andersen, L. H., Hvelplund, P., Kella, D., & Safvan, C. P. 1995, J Phys B At Mol Opt Phys, 28, 3415