Carbon Dioxide Retrievals From NOMAD-SO on ESA's ExoMars Trace Gas Orbiter and Temperature Profile Retrievals With the Hydrostatic Equilibrium Equation: 2. Temperature Variabilities in the Mesosphere at Mars Terminator

The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument scans the Martian atmosphere since 21 April 2018. In this work, we present a subset of the NOMAD SO data measured at the mesosphere. We focused on a spectral range that started to be recorded in Martian year (MY) 35. A total of 968 vertical profiles of carbon dioxide density and temperature covering MY 35 and the beginning of MY 36 were investigated until 135 degrees of solar longitude. We compared 47 profiles with co-located profiles of the Mars Climate Sounder onboard the Mars Reconnaissance Orbiter. Most profiles show a good agreement as SO temperatures are only 1.8 K higher, but some biases lead to an average absolute difference of 7.4 degrees K. The SO data set is also compared with simulations from the Global Environmental Multiscale-Mars general circulation model. Both data sets are in good agreement except for the presence of a cold layer in the winter hemisphere and a warm layer at dawn in the Northern hemisphere for solar longitudes between 240 degrees; and 360 degrees. Five profiles contain temperatures lower than the limit for CO2 condensation. Strong warm layers were found in 13.5% of the profiles. They are present mainly at dawn and in the winter hemisphere, while the Northern dusks appear featureless. The data set mainly covers high latitudes around 60 degrees and we derived some non-migrating tides. In the Southern winter hemisphere, we derived apparent zonal wavenumber-1 (WN-1) and WN-3 tidal components with a maximum amplitude of 10% and 5% at 63 km, respectively. Plain Language Summary The vertical profiles analyzed in this work extend over more than a Martian year (687 days) and are located in the mesosphere (50-100 km altitude) at the transition between the day and night sides. Temperature can sometimes be so low that CO2 ice clouds form. Here, we report some features in our data set that are likely due to those CO2 ice clouds. We also report some warm layers which appear in the winter hemisphere, and we analyzed the components of the apparent atmospheric tides at the terminator. We compared our results to highlight possible biases with data from another instrument (Mars Climate Sounder) and a model (Global Environmental Multiscale-Mars). The measured temperatures are higher than those of the model in the winter hemisphere. Compared to the other instrument, we find similar profile curves but with stronger variations in our profiles, probably due to a higher vertical resolution.