Model-observation discrepancies in protonated species in Mars’ ionosphere from MAVEN/NGIMS

The Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has measured ions with mass-to-charge ratios between 2 and 150 in the Martian ionosphere. Among these observations, protonated species at 31, 33, and 41 atomic mass units (amu) exhibit densities substantially higher than those predicted by existing photochemical models. In this study, we investigate these model-observation discrepancies using a combination of photochemical modeling and NGIMS measurements from the dayside ionosphere.

Photochemical models predict ArH⁺ densities at 41 amu that are more than an order of magnitude lower than NGIMS observations, while modeled densities of HNO⁺ and HO₂⁺ at 31 and 33 amu are underestimated by approximately three orders of magnitude. Analysis of vertical density profiles reveals strong similarities among the 31, 32, and 33 amu channels, as well as among the 41, 42, 43, and 44 amu channels. These similarities cannot be fully accounted for by known chemical pathways or contributions from oxygen isotopes. Instead, our results indicate that instrumental effects, specifically mass channel cross-talk from the dominant 32 and 44 amu species, provide a plausible explanation for the anomalously high densities reported at 31, 33, and 41 amu. These findings highlight the importance of carefully accounting for instrumental artifacts when interpreting ion composition measurements in the Martian ionosphere.