Reviewing 41 amu ions in the Martian Ionosphere

We make use of photochemical modelling and measurements by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) in an attempt to identify the ion species that dominate the sub-population of ions in the Martian ionosphere characterized by a mass of 41 atomic mass units (amu). Previous photochemical reaction networks used for the ionospheres of Mars and Venus include only a single species, namely ArH⁺, to account for ions with that mass. We highlight that the straightforward photochemical modeling of ArH⁺ yields predicted concentrations that are two orders of magnitude lower than the observed 41 amu ions densities in the dayside ionosphere of Mars. We hypothesize that the 41 amu ion population is instead dominated by protonated dicarbon monoxide, HC₂O⁺, and we advocate for experimental investigations into relevant ion-neutral reactions.