Seasonal and Latitudinal Variations of Dayside N₂/CO₂ Ratio in the Martian Thermosphere Derived From MAVEN IUVS Observations

The dayside N₂/CO₂ at 140 km altitude in the Martian thermosphere has been investigated by the imaging ultraviolet spectrograph (IUVS) aboard the Mars atmosphere and volatile evolution spacecraft during the period from October 2014 to May 2018. We find that N₂/CO₂ at 140 km altitude varies significantly in the range of 0.02–0.20 and shows a seasonal sinusoidal trend. The higher value appears during aphelion and the lower value during perihelion. Variations of observed N₂/CO₂ ratio at 140 km are mainly associated with CO₂ number density. Thus, while we found that N₂/CO₂ varies at a given altitude, we could not identify variations at a given pressure level. In order to reveal the drivers of N₂/CO₂ variations at 140 km, we examine the effects of surface N₂/CO₂, thermospheric temperature, and homopause altitude. The variations of homopause altitude could be the dominant driver of changes in N₂/CO₂. Inferred dayside homopause altitudes derived from IUVS observations show an anti-correlation with the trend of N₂/CO₂ at 140 km. Distributions of CO₂ density at the inferred homopause altitudes suggest that dayside homopause altitude is mainly controlled by inflation and contraction of the lower atmosphere. Additionally, N₂/CO₂ shows a clear latitudinal dependence for L_s = 80°–100°. Higher N₂/CO₂ values appear in the southern winter hemisphere, corresponding to lower homopause altitude by ∼30 km. Meanwhile, the latitudinal dependence of both N₂/CO₂ and homopause altitude disappears for L_s = 340°–360°.