Day/Night Differences in Molecular Oxygen in the Martian Upper Atmosphere

We use the extensive stellar occultation data set of the Imaging Ultraviolet Spectrograph aboard the Mars Atmosphere and Volatile EvolutioN spacecraft to determine the first quantification of vertical variation in O₂ mole fraction separately for day and night in the ∼90–130 km altitude range. The upper atmospheric O₂ variation is expected to be due to the interplay between diffusion and advection because of its long photochemical lifetime. It is therefore a useful tracer of the state of atmospheric mixing and circulation. The altitude-averaged mixing ratio is measured to be 2.69(±0.03) × 10⁻³ for the nightside and 2.05(±0.03) × 10⁻³ for the dayside. The average O₂ mole fraction for day and night are nearly identical below 105 km, consistent with the value of 1.61 × 10⁻³ derived from the Mars Curiosity Rover/Sample Analysis at Mars near-surface measurements. At higher altitudes, dominated by molecular diffusive separation, the measured O₂ mole fraction demonstrates a vertical gradient with a local time dependence. The nightside mole fraction is a factor of 1.37 ± 0.04 larger than the dayside value at ∼125 km. This nightside enhancement is explained in terms of the relative role of solar-driven rapid horizontal winds at high altitudes and slower vertical diffusion, resulting in a nightside (dayside) downward (upward) diffusive flux. Using the 1-D diffusion model, the measured profiles correspond to a vertical eddy diffusion coefficient K = 3.5(±1.5) × 10⁶ cm²/s. The Mars Climate Database predicts comparable but lower day-night differences in oxygen mole fraction due to an overestimated K = 7.0(±1.0) × 10⁶ cm²/s, which affects atmospheric mixing as well as the rate of atmospheric escape to space.