A closer look at the Martian mesosphere with the GEM-Mars GCM: a comparison with ExoMars TGO/NOMAD temperatures
Authors: Neary Lori , Trompet Loïc , Daerden Frank , Thomas Ian , Vandaele Ann Carine 
Published: July 2024

The ExoMars Trace Gas Orbiter (TGO) was launched in 2016 and began science operations in April 2018. NOMAD (Nadir and Occultation for MArs Discovery) [1] is one of four instruments onboard, made up of three spectrometers built to probe the atmosphere and surface of Mars in the infrared and ultraviolet wavelengths using solar occultation, limb and nadir viewing geometries. The main objective is to characterize the composition and structure of the Martian atmosphere, including the seasonal trends of atmospheric gases, dust and clouds.The GEM-Mars Global Circulation Model (GCM) [2,3,4,5] is a crucial part of the NOMAD mission, supporting the observational planning, data retrieval and interpretation of results. GEM-Mars is a multiscale grid-point model, representing the atmosphere from the surface up to around 150 km.NOMAD infrared solar occultation observations provide an opportunity to look more closely at the thermal structure in the mesosphere and evaluate the model performance in this region. It is an important transition zone between the lower and upper atmosphere and can be influenced by aerosols, gravity waves and thermal tides so it is useful to perform a detailed analysis with the observations to help constrain the representation of these processes in the model. Initial comparisons of the model to data were performed in [6]. In this work, we expand on this comparison and look at model-simulated quantities such as aerosols, heating rates and impacts from gravity waves.We will present an overview of the model and recent improvements, as well as an evaluation of model performance in the mesosphere using NOMAD solar occultation measurements. From this analysis, we will discuss ways to improve our simulations.References[1] Vandaele et al., 2018. NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance. Space Science Reviews 214. https://doi.org/10.1007/s11214-018-0517-2[2] Neary and Daerden, 2018. The GEM-Mars general circulation model for Mars: Description and evaluation. Icarus 300, 458&#8211;476. https://doi.org/10.1016/j.icarus.2017.09.028[3] Daerden et al., 2019. Mars atmospheric chemistry simulations with the GEM-Mars general circulation model. Icarus 326, 197&#8211;224. https://doi.org/10.1016/j.icarus.2019.02.030[4] Neary et al., 2020. Explanation for the Increase in High-Altitude Water on Mars Observed by NOMAD During the 2018 Global Dust Storm. Geophysical Research Letters 47, e2019GL084354. https://doi.org/10.1029/2019GL084354[5] Daerden et al. 2023. Heterogeneous Processes in the Atmosphere of Mars and Impact on H2O2 and O3 Abundances. Journal of Geophysical Research: Planets 128, e2023JE008014. https://doi.org/10.1029/2023JE008014[6] Trompet et al., 2023. Carbon Dioxide Retrievals From NOMAD-SO on ESA&#8217;s ExoMars Trace Gas Orbiter and Temperature Profile Retrievals With the Hydrostatic Equilibrium Equation: 2. Temperature Variabilities in the Mesosphere at Mars Terminator. Journal of Geophysical Research: Planets 128, e2022JE007279. https://doi.org/10.1029/2022JE007279