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

Abstract

Planetary atmospheres are generally characterized by a compositional boundary, called the homopause, below which gases are well-mixed by eddy diffusion (homosphere) and above which gases are diffusively separated according to their own scale heights by molecular diffusion (heterosphere). In the heterosphere, the mixing ratio of lighter species is expected to increase with altitude above the homopause. The location of the homopause altitude influences the thermospheric composition and thereby the escape of species to space. In addition, the fractionation between the homopause and the exobase determines the relative abundance of species that escape to space and the total atmospheric loss from the isotope record (Chassefière &

Seasonal and Dust‐Related Variations in the Dayside Thermospheric and Ionospheric Compositions of Mars Observed by MAVEN/NGIMS

“…Variations in the number densities of CO 2 , N 2 , and O at 200 km in the afternoon are factors of ∼9, ∼3, and ∼4, respectively, based on the median densities between MY 33 L s ∼ 50 and MY 33 L s ∼ 240. The observed sinusoidal seasonal trend of CO 2 at a constant altitude of 200 km has good agreement with previous observations of CO 2 in the thermosphere and mesosphere (Gröller et al., 2018; Forget et al., 2009; Slipski et al., 2018; Yoshida et al., 2020). The variation in CO 2 density is also larger than that in N 2 density at 140 km altitude (Yoshida et al., 2020).…”

“…The observed sinusoidal seasonal trend of CO 2 at a constant altitude of 200 km has good agreement with previous observations of CO 2 in the thermosphere and mesosphere (Gröller et al., 2018; Forget et al., 2009; Slipski et al., 2018; Yoshida et al., 2020). The variation in CO 2 density is also larger than that in N 2 density at 140 km altitude (Yoshida et al., 2020). These facts suggest that the variation in densities in the upper thermosphere is associated with that in the lower thermosphere and mesosphere.…”

“…The observed variabilities in the ion composition suggest that the effects of changes in homopause altitude on neutral and ion compositions at a certain pressure are not as obvious as the results in Chaufray et al (2014). According to Yoshida et al (2020), the dayside homopause altitude is suggested to be located around a certain pressure level. The homopause pressure level can be estimated by extrapolating the N 2 /CO 2 profile and finding the pressure level where the N 2 /CO 2 ratio is equal to the lower atmospheric ratio (cf.…”

“…According to Yoshida et al. (2020), the dayside homopause altitude is suggested to be located around a certain pressure level. The homopause pressure level can be estimated by extrapolating the N 2 /CO 2 profile and finding the pressure level where the N 2 /CO 2 ratio is equal to the lower atmospheric ratio (cf.…”

“…Recently, the homopause of Mars was observed to vary between altitudes of 60 and 140 km (Jakosky et al., 2017; Slipski et al., 2018). Additionally, seasonal variations in the N 2 /CO 2 ratio at an altitude of 140 km in the lower thermosphere were investigated by the Imaging UltraViolet Spectrograph (IUVS) aboard MAVEN (Yoshida et al., 2020). The N 2 /CO 2 ratio at 140 km showed a maximum near aphelion and a minimum near perihelion and varied by a factor of two.…”