Observations of the Mars O2 and NO nightglow from the TGO/NOMAD-UVIS spectrometer

Airglow observations provide a unique tool to remotely monitor the atmospheric composition and dynamics, allowing to derive densities of atmospheric constituents as well as the photochemical and chemical processes within the atmosphere. NOMAD/UVIS aboard TGO is the first spectrometer orbiting Mars remotely exploring the atmosphere across both the middle ultraviolet and visible domains. We report on nightside limb observations of two key emissions: the bright O (c Σ -> X Σ) Herzberg II visible nightglow, recently discovered in the polar winter hemisphere, and the NO ultraviolet d and g bands. From the first one, we deduce the high-latitude oxygen density variations across seasons, understand their high latitude
luminosity and explore the transition period between winter poles. We determine the O density vertical/latitudinal distribution of atomic oxygen atoms from the O brightness, assuming that the c Σ state is produced by three-body recombination of O atoms, similarly to Venus. Understanding the atomic oxygen distribution is crucial, as it directly influences infrared
radiative cooling of the atmosphere and the presence of an ozone layer near the south pole. Comparison between the observations and model simulations will quantify oxygen transport from the summer to the winter hemisphere and descending flows in the winter polar regions.
Similarly, the NO nightglow results from two-body radiative recombination of N and O atoms during their descent from the sunlit to the night side of the planet. Simultaneous and colocated
measurements of the two emissions are expected to provide key elements to solve the discrepancy in the altitude of the NO layer previously observed with MAVEN-IUVS. This combination provides an unprecedented opportunity to improve 3-D models simulating global circulation and dynamic processes. In addition, the strong visible emissions under investigation are anticipated to be visible to future astronauts on Mars. Therefore, this study also paves the way for human Mars exploration.