Ozone chemical equilibrium near the Martian nighttime OH* layer: observational constraints

Abstract Simultaneous measurements of ozone concentrations and vibrationally excited hydroxyl (OH*) emissions provide a method for determining atomic oxygen (O) and atomic hydrogen (H) concentrations, which are otherwise difficult to measure directly. This approach requires that the photochemical equilibrium conditions for ozone are satisfied. In this study, we use model simulations to examine the spatio-temporal distribution of ozone photochemical equilibrium in the nighttime Martian atmosphere and evaluate its relationship to detectable OH* emission layers. Our analysis reveals that ozone mostly maintains chemical equilibrium over large latitude–altitude regions during the second half of the Martian year ( Lₛ = 180°–360°). However, these equilibrium conditions are partial consistent with observable OH* emissions only in limited areas: at high latitudes (60–70 km) during Lₛ = 90°–120°, in northern mid-latitudes (45–50 km) during Lₛ = 80°–135°, at equatorial latitudes (50–60 km) during Lₛ = 80°–135°, and in southern mid-latitudes (55–65 km) during Lₛ = 45°–90°. Graphical Abstract