Photochemical Equilibrium and Ozone Balance Equation in the Nighttime Hydroxyl Layer on Mars

Combined measurements of ozone (O3) and vibrationally excited hydroxyl (OH*) emissions allow determination of atomic oxygen (O) and hydrogen (H) concentrations, which are otherwise difficult to measure directly. This method is applicable only when ozone photochemical equilibrium (OPE) conditions are met. This paper is the first to investigate OPE in the nighttime atmosphere of Mars and its relationship to OH* emissions. Based on numerical modeling using data from Mars Climate Database (MCD), the spatiotemporal distributions of ozone deviations from the equilibrium state and the ratio of ozone lifetimes at current and equilibrium concentrations are analyzed. Two main OPE criteria are derived. The results show that OPE is satisfied over wide regions (65–90 km) in the second half of the Martian year (Ls = 180°–360°), especially at polar and midlatitudes. However, the regions with observed OH* concentrations ([OH*] $$ \geqslant $$ 100 cm–3) and satisfied OPE criteria are limited to the first half of the year (Ls = 0°–180°) and altitudes of 50–70 km. This indicates the difficulty of applying the method for retrieving O and H concentrations from OH* under Martian conditions without additional adaptation of the equilibrium criteria. Therefore, we test a criterion derived from the basic principles of photochemistry of the terrestrial mesosphere, mesopause, and lower thermosphere. The study highlights the need for further observations and refined models to correctly interpret chemical processes in the Martian atmosphere.