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Day-Night Differences in the Martian Upper Atmospheric Molecular Oxygen

Presentation #213.05 in the session Martian Aurora, Atmosphere, Winds, and Dust (Poster)

Published onOct 23, 2023
Day-Night Differences in the Martian Upper Atmospheric Molecular Oxygen

The Martian 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 dynamics. Though the previous studies have given a glimpse of the expected O2 abundance in the Martian upper mesosphere and lower thermosphere, none of these studies provide vertical variation in O2 for this region on a diurnal scale to give information on how the O2 mixing ratio depends on the vertical and horizontal diffusion as well as on the day/night transport. This knowledge gap is filled in by the stellar occultation observations of the Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, providing the first consistent measurement of both the dayside and nightside Martian mesosphere.

We use this extensive MAVEN/IUVS stellar occultation dataset to report the first study of vertical variation in O2 mole fraction for day and night in ~100-140 km altitude range. The average O2 mole fraction for day and night are observed to be nearly identical below 105 km, consistent with the value of 0.0016 derived from the Mars Curiosity Rover/SAM measurements. At higher altitudes, the measured O2 mole fraction shows a local time dependence with the nightside values on average a factor of two larger than the dayside values. This nightside enhancement is explained in terms of the relative role of solar driven rapid horizontal winds and slow vertical diffusion. The Mars Climate Database shows similar day-night differences in O2 mole fraction but predicts smaller magnitudes for both day and night. These data-model discrepancies in O2 diffusion and day/night transport imply lack of our current understanding on the structure and dynamics of the Martian upper mesosphere/lower thermosphere.

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