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

We use the extensive stellar occultation dataset of the Imaging Ultraviolet Spectrograph21 (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft to22 determine the first quantification of vertical variation in O2 mole fraction separately for23 day and night in the ∼90-130 km altitude range. The upper atmospheric O2 variation24 is expected to be due to the interplay between diffusion and advection because of its long25 photochemical lifetime. It is therefore a useful tracer of the state of atmospheric mix-26 ing and circulation. The altitude-averaged mixing ratio is measured to be 2.69(±0.03)×27 10−3 for the nightside and 2.05(±0.03) × 10−3 for the dayside. The average O2 mole28 fraction for day and night are nearly identical below 105 km, consistent with the value29 of 1.61×10−3 derived from the Mars Curiosity Rover/SAM near-surface measurements.30 At higher altitudes, dominated by molecular diffusive separation, the measured O2 mole31 fraction demonstrates a vertical gradient with a local time dependence. The nightside32 mole fraction is a factor of 1.37±0.04 larger than the dayside value at ∼125 km. This33 nightside enhancement is explained in terms of the relative role of solar-driven rapid hor-34 izontal winds at high altitudes and slower vertical diffusion, resulting in a nightside (day-35 side) downward (upward) diffusive flux. Using the 1-D diffusion model, the measured pro-36 files correspond to a vertical eddy diffusion coefficient K = 3.5(±1.5)×106 cm2/s. The37 Mars Climate Database predicts comparable but lower day-night differences in oxygen38 mole fraction due to an overestimated K = 7.0(±1.0) × 106 cm2/s, which affects at-39 mospheric mixing as well as the rate of atmospheric escape to space.