Investigating Significant Model-Observation Discrepancies of Protonated Species in the Martian Ionosphere

Abstract

Ions with mass-to-charge ratios from 2 to 150 have been measured in the Martian ionosphere by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Among these, protonated species at 31, 33, and 41 atomic mass units (amu) are observed at densities significantly higher than those predicted by photochemical models. In this study, we combine photochemical modeling with NGIMS measurements to investigate these discrepancies in the dayside ionosphere of Mars. For 41 amu ions, photochemical models predicts ArH⁺ densities more than one order of magnitude lower than NGIMS measurements. For 31 and 33 amu ions, modeled densities of HNO⁺ and HO₂⁺ are three orders of magnitude lower than observed values. We find that the vertical profiles reveal striking similarities among (a) the 31, 32, and 33 amu channels and (b) the 41, 42, 43, and 44 amu channels, which cannot be fully explained by interlinked chemical pathways and contributions from oxygen isotopes. Instead, we suggest that instrumental effects, specifically mass channel cross-talk from the strong 32 and 44 amu, provide a plausible explanation for the anomalously high densities reported at 31, 33, and 41 amu.