Seasonal Variations of the Martian Middle‐Atmosphere CO 2 /Ar Ratio Inferred From MAVEN/NGIMS Observations

Abstract The carbon dioxide (CO 2 )/argon (Ar) ratio is a useful tracer of atmospheric circulation on Mars, but it remains observationally poorly constrained in the middle atmosphere, where large‐scale circulation occurs. In this study, we infer the CO 2 /Ar ratio at the homopause, which marks the upper boundary of the well‐mixed Martian middle atmosphere, by extrapolating observations from NASA’s Mars Atmosphere and Volatile EvolutioN mission. The inferred ratio shows a double‐peaked seasonal variation, with minima near equinoxes and maxima near solstices, broadly consistent with simulations from a Mars General Circulation Model (GCM). However, we also identify pronounced high‐latitude enhancements of this ratio during L S ∼ 270°–300°, reaching ∼80–110 compared with ∼60 predicted by the GCM. These enhancements may indicate a transient atmospheric circulation pattern not captured by the model simulations. Our results provide a new observational constraint on composition near the Martian homopause and, consequently, on middle‐atmosphere circulation, and suggest that additional atmospheric processes remain to be understood.