Ultraviolet NO and Visible O<SUB>2</SUB> Nightglow in the Mars Southern Winter Polar Region: Statistical Study and Model Comparison

The Mars NO and the O<SUB>2</SUB> nightglow are produced by the recombination of atoms produced on the dayside by photodissociation and transported to the nightside. These emissions are tracers of the summer to winter pole dynamics in the upper Mars atmosphere. The UV-visible (UVIS) channel of the Nadir and Occultation for MArs Discovery (NOMAD) spectrometer onboard Trace Gas Orbiter (TGO) is the first instrument able to simultaneously monitor both nightglow emissions. Observations by NOMAD/UVIS during the first part of the Martian year show that both the NO and O<SUB>2</SUB> nightglow emissions are enhanced near the southern winter pole. Their mean brightnesses are 15 and 108 kR, respectively. These nightglow emissions generally occur between 30 and 60 km, the NO emitting layer being consistently located ∼10 km higher than the O<SUB>2</SUB> nightglow layer. Numerical simulations with the Mars Planetary Climate Model (MPCM, v6.1) properly reproduce the nightglow brightness but tend to overestimate the NO peak altitude by ∼10 km. These results suggest that the atomic oxygen density is correctly predicted by the model but that the nitrogen density altitude distribution might not be properly modeled.