Ultraviolet NO and Visible O2 Nightglow in the Mars Southern Winter Polar Region: Statistical Study and Model Comparison.

The Mars NO and the O2 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 O2 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 O2 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. Plain Language Summary: Nightside emissions of the Martian upper atmosphere are excited by the recombination of atoms created on the dayside. For the first time, the NO ultraviolet emission and the O2 visible emission have been observed simultaneously with the UV‐visible (UVIS) channel of the Nadir and Occultation for MArs Discovery (NOMAD) spectrometer onboard Trace Gas Orbiter (TGO). They are used as tracers of the atmospheric dynamics to demonstrate the summer to winter pole circulation that governs the Mars upper atmosphere. We show that the two nightglow layers occur between 30 and 60 km and that they are separated by ∼10 km. We compare our results with simulations from the Mars Planetary Climate Model. Simulations manage to 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 altitude distribution of the nitrogen density might not be properly modeled. Key Points: For the first time, NOMAD/UVIS simultaneously monitors both the NO UV and O2 visible nightglow in the Mars southern winter atmosphereThe NO emitting layer is consistently located ∼10 km above the O2 nightglow layer (∼55 and ∼45 km, respectively)Brightness is overall well predicted but the altitude difference between the two polar emission layers is overestimated by the MPCM v6.1 model [ABSTRACT FROM AUTHOR]