Loïc Trompet

The vertical structure of the Martian atmosphere is a key constraint for our understanding of the present-day climate of Mars. In recent years, several studies using high-spectral-resolution infrared solar occultation observations from the ExoMars Trace Gas Orbiter (TGO) have provided measurements of vertical profiles of CO ₂ density and temperature. In this study, we perform intercomparisons of these datasets to assess their mutual consistency and identify systematic differences. Overall, we find that all datasets exhibit consistent temporal and spatial variability in these parameters, indicating that TGO observations provide a robust picture of the vertical structure of the Martian atmosphere. While the details of the comparisons depend on the specific datasets and altitude ranges considered, differences are generally smaller than 5 K in temperature and 10% in CO ₂ density. This study forms part of a broader effort to combine ExoMars TGO and MAVEN observations to improve our understanding of the Martian upper atmosphere. The inter-dataset differences and scaling factors estimated here can be combined with similar metrics derived in companion papers in this collection (Jones et al. 2026; Thiemann et al. 2026) and to relate TGO and MAVEN datasets.

Aims . We use carbon monoxide (CO), a long-lived species in the Martian atmosphere, as a tracer to constrain atmospheric transport by characterizing its vertical distribution from the near-surface to an altitude of about 120 km. Methods . We retrieved CO vertical profiles from solar occultation (SO) spectra acquired by the SO channel of the Nadir and Occultation for MArs Discovery (NOMAD) instrument on board the ExoMars Trace Gas Orbiter (TGO). The dataset covers L _s = 160° in Mars year (MY) 34 to L _s = 120° in MY 36, enabling analyses over broader altitude and seasonal ranges than in previous works. We validated the retrievals using co-located measurements from the from Atmospheric Chemistry Suite (ACS) instrument on board TGO. Results . The retrieved CO volume mixing ratio (VMR) is enhanced above 40 km at both poles during seasonal spring and autumn. In summer and winter, the enhancement is largely confined to the winter polar region. These variations are in quantitative agreement with earlier observations and are consistent with seasonal changes in the meridional circulation. We also find larger CO number densities near the morning terminator during local winter in the 80–100 km altitude range, consistent with a modulation of the transport by a semidiurnal tide. During the MY 34 planet-encircling dust event, enhanced CO VMRs are confirmed at 80–110 km in altitude, but no corresponding enhancement in the CO number density is seen, suggesting that the apparent VMR variation is driven primarily by changes in atmospheric dynamics, rather than by photochemistry.

Introduction and GoalThe upper atmosphere of Mars, spanning the mesosphere (60–120 km) and the thermosphere up to the exobase (180–230 km), remains one of the least characterized regions of the planet [1]. Its structure is shaped by solar forcing, upward‑propagating waves, and dust‑driven variability originating in the lower atmosphere; however, the relative importance of these processes and their vertical coupling are not yet fully understood [2]. Two recent missions provide complementary datasets: NASA’s MAVEN mission, operational since 2014, focuses on upper‑atmosphere processes and atmospheric escape, while ESA–Roscosmos’ Trace Gas Orbiter (TGO), in routine operation since 2018, carries the NOMAD and ACS instruments, which retrieve atmospheric profiles from near‑surface layers to the upper thermosphere.In this work, we combine MAVEN and TGO observations with the primary objective of producing a comprehensive climatology of the thermal structure of the Martian upper atmosphere. This study is part of a broader andn recent project funded by the International Space Science Institute (ISSI) entitled “A multi-mission approach to close the gaps in understanding of the structure and variability in the Mars upper atmosphere”. Most TGO datasets concentrate below 100 km, whereas MAVEN primarily samples thermospheric altitudes. Because TGO data extend to lower altitudes, we actually present results spanning from the troposphere to the exosphere. The combined dataset integrates nine types of measurements from both missions, including in situ observations (NGIMS/MAVEN) and remote‑sensing measurements from NOMAD, ACS, EUVM, and IUVS (solar and stellar occultations, as well as dayglow). Together, these datasets cover multiple Martian years and an altitude range of 30–180 km, providing unprecedented vertical and temporal coverage. Exploiting this complementarity, particularly the vertical coupling between the lower and upper atmosphere, is also a major scientific goal of the present research.In this climatological study, we explicitly exclude a set of short‑lived and sporadic phenomena, such as dust storms, solar eruptions, and small‑scale atmospheric waves. Our focus is on the systematic variations associated with the following geophysical variables: altitude, latitude, season, local time, solar cycle, and longitude.Datasets, Synergies, and ChallengesThe datasets exhibit highly diverse characteristics, including field of view, spatial and temporal coverage, sensitivity, and noise levels, posing challenges for the synergetic analysis pursued here. To address coverage differences, we devoted substantial effort to designing appropriate binning strategies and averages across geophysical variables. Regarding sensitivity and retrieval performance, we used nominal uncertainties but filtered out the highest‑uncertainty percentile, and we developed a unified quality‑control framework applied to the nominal errors of each experiment. This framework aims to eliminate spurious values and identify potential biases, as revealed through cross‑comparisons within well‑defined geophysical bins. We also performed a variance‑partitioning analysis as a function of geophysical forcings, enabling us to distinguish intrinsic atmospheric variability from instrumental effects. Two vertical coordinates were used: altitude above the Martian aeroid and a modified‑CO₂ scale‑height coordinate, which is more suitable for upper‑atmosphere comparisons. Special attention was given to solar‑occultation datasets from both MAVEN and TGO (terminator measurements at dawn and dusk) owing to their synergistic local‑time coverage and excellent vertical resolution.Selected ResultsWe will present a representative sample of this extensive analyses across this broad parameter space, including global composite temperature profiles, deviations from mean distributions, multi‑year time series, and an assessment of the dominant sources of thermal variability. Our results reveal substantial atmospheric variability, highly non‑linear with respect to geophysical parameters, and a hierarchy of dominant processes that varies with altitude and which shapes the global thermal structure.In particular we found that thermospheric temperatures respond strongly to solar flux, with variations exceeding 50 K above 150 km and decreasing toward 100 km, being very small below this altitude. In contrast, mesospheric temperatures are dominated by global circulation, producing seasonal and latitudinal gradients of 20–40 K. We will show how the combined datasets also reveal interhemispheric asymmetries, the impact of dust storms at multiple altitudes, and the propagation of atmospheric waves.Comparisons with MarsPCM and MCD simulations demonstrate that the model reproduces very well the mean vertical structure as well the seasonal and latitudinal variations observed, but underestimates the variability by factors of 1.5–2 and exhibit a few biases in the mesopause and upper thermosphere. In particular, tidal amplitudes in the mesosphere are weaker in the models, the lower‑thermosphere thermal gradient occurs at lower altitudes than what is observed, and the coldest thermospheric temperatures predicted by the models are significantly colder than those reported by EUVM.The resulting MAVEN–TGO climatology provides a robust reference for future measurements, model validation, and studies of Martian atmospheric dynamics and evolution. References[1] Bougher et al., JGR-Planets, 2017 [2] López-Valverde et al., Space Sci. Rev., 2018 [3] Giuranna et al., Space Sci. Rev., 2025 [4] Heavens et al., Nature, 2018 [5] Yiğit et al., Nature Geoscience, 2023 [6] Jakosky et al., Science, 2015 [7] Korablev et al., Space Sci. Rev., 2018 [8] Vandaele et al., Space Sci. Rev., 2018

Gravity waves play an important role in the dynamical forcing of the Martian climate. If the horizontal resolution of a Global Climate Model (GCM) is larger than the scale of these waves, their impact should be parameterised to simulate their effects on the mean flow. Typically, the parameterisations derive from Earth models and are not well constrained.The GEM-Mars GCM (Neary and Daerden, 2018; Daerden et al., 2023) has two schemes: one to simulate the impact of gravity waves from orographic sources such as mountains and topography (McFarlane, 1987), and one for non-orographic sources such as convective forcing and baroclinic waves (Hines, 1997a; 1997b; 2004; Charron et al., 2002). Both schemes calculate a deceleration of the grid-scale winds, often termed “gravity wave drag”. This can have an indirect effect on temperature and on the distribution of trace gases.The schemes include some “tuning” parameters that can be adjusted to modify when and where this drag occurs. For example, in the non-orographic scheme, the level at which waves are expected to be launched and the lower bound vertical wavenumber, which limits the maximum vertical wavelength of the spectrum allowed, can be adjusted.Using estimates of wave activity derived from perturbations in observed temperature profiles from the ExoMars Trace Gas Orbiter and MAVEN (Neary et al. submitted), we will explore how these tuning parameters affect the resulting gravity wave drag climatology in GEM-Mars. By comparing model output with observational constraints across latitude, season, and local time, we aim to identify parameter settings that better reproduce the observed distribution and intensity of wave activity. This approach will help assess how well the current parameterisations capture the Martian atmosphere and where further refinement is needed.References:Charron, M. et al., 2002. Intercomparison of gravity wave parameterizations: Hines Doppler-spread and Warner and McIntyre ultra-simple schemes. Journal of the Meteorological Society of Japan 80, 335–345.Daerden, F. 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/10.1029/2023JE008014Hines, C., 1997a. Doppler-spread parameterization of gravity-wave momentum deposition in the middle atmosphere: Part 1 Basic Formulation. Journal of Atmospheric and Solar-Terrestrial Physics 59.Hines, C.O., 1997b. Doppler-spread parameterization of gravity-wave momentum deposition in the middle atmosphere. Part 2: Broad and quasi monochromatic spectra, and implementation. Journal of Atmospheric and Solar-Terrestrial Physics 59, 387–400.Hines, C.O., 2004. The Doppler spread theory and parameterization revisited. Journal of Atmospheric and Solar-Terrestrial Physics 66, 949–956. https://doi.org/10.1016/j.jastp.2004.02.005McFarlane, N.A., 1987. The effect of orographically excited gravity wave drag on the general circulation of the lower stratosphere and troposphere. Journal of the Atmospheric Sciences 44, 1775–1800.Neary, L., Daerden, F., 2018. The GEM-Mars general circulation model for Mars: Description and evaluation. Icarus 300, 458–476. https://doi.org/10.1016/j.icarus.2017.09.028Neary, L., et al., 2026. A multi-mission climatology of gravity waves in the Martian mesosphere and thermosphere. Submitted to Space Science Reviews.

Carbon monoxide (CO) is a minor trace gas of the Martian atmosphere that plays a key role in the photochemical cycle of carbon. CO is produced in the upper atmosphere by the carbon dioxide (CO2) photolysis, while its recycling through CO2 reformation occurs via catalytic reaction with the odd hydrogen radicals (HOx) produced by the water vapor (H2O) photolysis (see [1, 2] and references therein). As non-condensable species, CO column-averaged volume mixing ratio (VMR) is expected to show seasonal and spatial gradients that closely follow the seasonal CO2 condensation/sublimation processes at the poles and related surface pressure variation. Given its relatively long lifetime (~6 years), CO is often used as a tracer of the atmospheric dynamical patterns.Mars CO column abundance observations from space are relatively sparse, with Mars Express OMEGA [3] and PFS instruments [4, 5], and CRISM aboard the Mars Reconnaissance Orbiter [6, 7]. More recently, the ExoMars Trace Gas Orbiter NOMAD-LNO spectrometer has provided a climatology of CO VMR at a global scale covering slightly more than a full Martian Year (MY 34, Ls=150° to MY 35, Ls=241°; March 2018-July 2020) of observations [8]. The globally averaged CO VMR derived from these data sets is found to range from ~800 to 1000 ppm with corresponding uncertainties of 20-45%.Here we present the seasonal and spatial CO column abundance distributions derived from NOMAD-LNO dayside nadir observations over the full mission period (MY 34-38). LNO (Limb Nadir and solar Occultation) is one of the two echelle grating infrared spectrometers installed on the NOMAD (Nadir and Occultation for MArs Discovery) instrument aboard the ESA ExoMars Trace Gas Orbiter (TGO) [9]. In these spectrometers, the echelle grating is combined with an Acousto-Optic Tunable Filter (AOTF) for the spectral window selection [10]. As for our LNO H2O column retrieval [11], the LNO nadir CO column-averaged VMRs are retrieved using the Optimal Estimation scheme [12] implemented in the ASIMUT-ALVL radiative transfer tool [13]. ASIMUT-ALVL is applied separately to LNO reflectance factor spectra from grating orders 186-191 covering the ~4180-4330 cm-1 wavenumber range. Altitude, pressure, temperature, CO, dust, and water ice a priori vertical profiles are extracted at the spectra locations from the GEM-Mars General Circulation Model [14]. Scattering by dust and water ice particles is taken into account in the forward simulations by using the LIDORT radiative transfer model [15]. Our retrieval also includes the new evaluations of the LNO AOTF function and its central position temperature dependence [11].In this presentation, we will first discuss the consistency of the retrieval results between the selected diffraction orders and the impact of the CO absorption lines saturation on the retrieval results. The retrieved CO VMR seasonal, latitudinal, and inter-annual patterns will be then assessed over the full mission period and our data set will be compared to other available observational data sets (CRISM, PFS, and LNO from [7], [5], and [8], respectively), and to output from the GEM-Mars General Circulation Model. The correlation between LNO CO and H2O column abundances will be also investigated.AcknowledgementsThe NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA) with co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). This project acknowledges funding by: the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000140753, 4000140863); by the Spanish Ministry of Science and Innovation (MCIU) and European funds (grants PGC2018-101836-B-I00 and ESP2017-87143-R; MINECO/FEDER), from the Severo Ochoa (CEX2021-001131-S) and from MCIN/AEI/10.13039/501100011033 (grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00); by the UK Space Agency (grants ST/V002295/1, ST/V005332/1, ST/X006549/1, ST/Y000234/1 and ST/R003025/1); and by the Italian Space Agency (grant 2018-2-HH.0).References[1]McElroy M. B., and T. M. Donahue (1972), Stability of the Martian atmosphere, Science 177, 986–988, DOI: 10.1126/science.177.4053.986[2]Vandaele, A. C., et al. (2024), Composition and Chemistry of the Martian Atmosphere as Observed by Mars Express and ExoMars Trace Gas Orbiter, Space Science Reviews, 220:75, https://doi.org/10.1007/s11214-024-01109-7[3]Encrenaz, T., et al. (2006), Seasonal variations of the martian CO over Hellas as observed by OMEGA/Mars Express, Astronomy and Astrophysics, 459, 265–270, DOI: 10.1051/0004-6361:20065586[4]Sindoni, G., et al. (2011), Observations of water vapor and carbon monoxide in the Martian atmosphere with the SWC of PFS/MEX. Planetary and Space Science, 59, 149-162, DOI: 10.1016/j.pss.2010.12.006[5]Bouche, J., et al. (2021), Seasonal and spatial variability of carbon monoxide (CO) in the Martian atmosphere from PFS/MEX observations, Journal of Geophysical Research: Planets, 126, e2020JE006480, https://doi.org/10.1029/2020JE006480[6]Smith, M. D., et al. (2009), Compact Reconnaissance Imaging Spectrometer observations of water vapor and carbon monoxide, J. Geophys. Res., 114, E00D03, https://doi:10.1029/2008JE003288[7]Smith, M. D., et al. (2018), The climatology of carbon monoxide and water vapor on Mars as observed by CRISM and modeled by the GEM-Mars general circulation model, Icarus, 301, 117-131, https://doi.org/10.1016/j.icarus.2017.09.027[8]Smith, M. D., et al. (2021), The climatology of carbon monoxide on Mars as observed by NOMAD nadir-geometry observations, Icarus, 362, 114404, https://doi.org/10.1016/j.icarus.2021.114404[9]Vandaele, A. C., et al. (2015), Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission, Planetary and Space Science, 119, 233-249, https://doi.org/10.1016/j.pss.2015.10.003[10]Thomas, I. R., et al....

Hydrogen chloride (HCl) is one of the most recent trace species discovered in the Martian atmosphere, renewing interest in the study of its climatology. Since its unambiguous detection by the Trace Gas Orbiter (TGO) in 2021 [1,2], several studies have focused on characterizing the spatial distribution and variability of this trace species and on understanding the processes controlling its presence in the atmosphere. Observations from the two TGO instruments capable of detecting HCl in solar occultation geometry, NOMAD and ACS, have consistently reported detections mainly during the second half of the Martian year, corresponding to the perihelion season and southern summer. These enhanced abundances are followed by a rapid decline toward the end of the year. During aphelion and northern summer, however, HCl detections from orbit remain rare and close to instrumental detection limits. This pattern has been observed repeatedly across multiple Martian years, with no noticeable interannual variability.Several hypotheses have been proposed to explain the sources and sinks of HCl, its strong seasonal variability, and its preferential occurrence during perihelion. One possible source mechanism involves interactions between the surface and the atmosphere through the lifting of chlorine-bearing dust and subsequent reactions releasing chlorine species into the atmosphere. On the other hand, heterogeneous uptake by water ice particles has been proposed as a plausible sink for HCl [3, 4]. However, systematic observations of temporally and spatially collocated HCl and aerosol (dust and water ice) profiles are needed in order to provide further constraints on these hypotheses.In this work, we present a climatology of HCl derived from four Martian years of NOMAD Solar Occultation observations. This instrument is an excellent asset for the detection and systematic monitoring of trace species in the Martian atmosphere. It provides high spectral resolution (λ/∆λ ∼ 17000) and high sensitivity thanks to the excellent signal-to-noise ratio achieved in the solar occultation geometry [5]. We investigated the seasonal and latitudinal variability of HCl vertical profiles throughout 5245 solar occultations, of which 291 profiles showed reliable HCl detections. We focused the analysis on the vertical structure and local time dependence of the retrieved profiles during the perihelion season, in addition to the study of correlations between HCl and other atmospheric species such as water vapor and aerosols. Our analysis confirms that HCl is primarily observed during southern summer, in agreement with previous studies analyzing orbital [1,2,6] and ground-based observations [7], while no robust detections are identified during the aphelion at tangent altitudes with optimum transmittance levels. Our results show that HCl is generally found at altitudes where water vapor abundances are enhanced, and both species exhibit a strong positive correlation, in agreement with previous studies [8]. We also observe a remarkably similar vertical structure of HCl at the morning and evening terminators. Finally, using collocated aerosol observations, we compare HCl distributions with dust and water ice abundances, finding a positive correlation with dust abundance but no clear correlation with water ice abundance.References:[1] Korablev, O., Olsen, K. S., Trokhimovskiy, A., Lef`evre, F., Montmessin, F., Fedorova, A. A., . . . others (2021). Transient HCl in the atmosphere of Mars. Science Advances, 7 (7),eabe4386[2] Aoki, S., Daerden, F., Viscardy, S., Thomas, I. R., Erwin, J. T., Robert, S., . . . others (2021). Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. Geophysical Research Letters, 48 (11), e2021GL092506[3] Taysum, B. M., Palmer, P. I., Olsen, K., Luginin, M., Ignatiev, N., Trokhimovskiy, A., . . . Korablev, O. (2024). Observed seasonal changes in Martian hydrogen chloride explained by heterogeneous chemistry. Astronomy & Astrophysics, 687 , A191.[4] Luginin, M., Trokhimovskiy, A., Taysum, B., Fedorova, A. A., Korablev, O., Olsen, K. S.,. . . Lefevre, F. (2024). Evidence of rapid hydrogen chloride uptake on water ice in the atmosphere of Mars. Icarus, 411 , 115960.[5] Vandaele, A. C., Lopez-Moreno, J.-J., Patel, M. R., Bellucci, G., Daerden, F., Ristic, B., . . . others (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 (5), 1–47.[6] Olsen, K. S., Fedorova, A. A., Kass, D. M., Kleinb¨ohl, A., Trokhimovskiy, A., Korablev, O. I., . . . others (2024). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 1. Climatological outlook. Journal of Geophysical Research: Planets, 129 (8), e2024JE008350[7] Faggi, S., Aoki, S., Liuzzi, G., Villanueva, G., Sagawa, H., Mumma, M. J., . . . Viscardy, S. (2025). Following the HCl cycle over three Martian seasons in Mars year 36. Journal of Geophysical Research: Planets, 130 (11), e2025JE009105.[8] Olsen, K., Fedorova, A., Kass, D., Kleinb¨ohl, A., Trokhimovskiy, A., Korablev, O., . . . others (2024). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 2. Quantitative correlations. Journal of Geophysical Research: Planets, 129 (8), e2024JE008351.

The science phase of the ExoMars Trace Gas Orbiter (TGO) mission began on 21st April 2018, and since then the NOMAD suite of 3 spectrometers [1] has acquired around 100 million spectra, primarily of the Martian atmosphere and surface. These cover ultraviolet-visible and infrared wavelengths with different observation geometries. The 3 spectrometers are:1) Solar Occultation (SO): 2.2-4.3 µm (2,300-4,600 cm-1), spectral resolution 0.1-0.23 cm-1 [2].2) Limb, Nadir and Occultation (LNO): 2.2-3.8 µm (2,600-4,600 cm-1), spectral resolution 0.15-0.28 cm-1 [2], operating primarily in nadir viewing mode.3) Ultraviolet-Visible (UVIS): 200-650 nm (15,000-50,000 cm-1), spectral resolution 1.2-1.7 nm [3], operating in solar occultation, nadir and limb viewing modes. Together, the spectrometers are able to map the vertical distributions and/or column densities (where appropriate) of many properties and constituents of the Martian atmosphere, including H2O [4] and HDO [5], CO [6], O3 [7], HCl [8], dust/aerosol particle size [9], CO2 and H2O clouds [10], CO2 temperature and pressure [11], dayglow [12] and nightglow [13] emissions, surface ice/frost [14], plus the reflectance properties of Phobos and Deimos [15]. Also, stringent detection limits can be placed on other gases that have not yet been detected, e.g. CH4 [16]. Calibrated NOMAD spectra and derived climatologies are available to the scientific community in multiple sources, for example: the ESA Planetary Science Archive, Europlanet’s VESPA portal, and many online authors’ repositories. Likewise, modelling tools (for example the NASA Goddard Planetary Spectrum Generator), calibration routines, observation planning information and nadir footprints exist online, are available in journal articles, or can be found on the NOMAD website at nomad.aeronomie.be. Work is ongoing to consolidate all the available datasets and tools, including python code to find and plot online datasets, forward-model the SO and LNO spectrometers, and to understand the instrument and associated uncertainties, which we will describe here. Also, documentation regarding recent calibration improvements will be highlighted and made available online for the community. References[1] Vandaele, A.C., et al; NOMAD, an integrated suite of three spectrometers for the ExoMars Trace Gas mission: technical description, science objectives and expected performance. (2018) Space Sc. Rev. 214, 80. https://doi.org/10.1007/s11214-018-0517-2[2] Liuzzi G., et al; Methane on Mars: New insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration. (2019) Icarus 321, 671-690. https://doi.org/10.1016/j.icarus.2018.09.021[3] Willame, Y., et al; Calibration of the NOMAD-UVIS data. (2022) Planet. Space Sci., 218, 105504. https://doi.org/10.1016/j.pss.2022.105504[4] Hendrick, F., et al; New Climatology of Martian Water Vapor Column Abundance Derived from NOMAD LNO Nadir Observations Over Martian Years 34–38. (2026) J. Geophys. Res. Planets. https://doi.org/10.1029/2025JE009592 [5] Villanueva, G., et al; The deuterium isotopic ratio of water released from the Martian caps as measured with TGO/NOMAD. (2022) Geophys. Res. Lett., 49(12). https://doi.org/10.1029/2022GL098161[6] Modak, A., et al; Martian Atmospheric CO Vertical Profiles From NOMAD Observations During the First Year of TGO Operations. (2023) J. Geophys. Res. Planets. https://doi.org/10.1029/2022JE007282[7] Mason, J. P., et al; Climatology and Diurnal Variation of Ozone Column Abundances for 2.5 Mars Years as Measured by the NOMAD-UVIS Spectrometer. (2024) J. Geophys. Res. Planets. https://doi.org/10.1029/2023JE008270[8] Aoki S., et al; Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. (2021) Geophys. Res. Letters 48(11). https://doi.org/10.1029/2021gl092506[9] Flimon, Z., et al; Aerosol Climatology on Mars as Observed by NOMAD UVIS on ExoMars TGO in Journal of Geophysical Research: Planets. (2025) J. Geophys. Res. Planets. https://doi.org/10.1029/2024JE008303[10] Liuzzi, G., et al; First Detection and Thermal Characterization of Terminator CO2 Ice Clouds with ExoMars/NOMAD. (2021) Geophys. Res. Letters 48(2). https://doi.org/10.1029/2021gl095895[11] Trompet, L., et al; Carbon dioxide retrievals from NOMAD-SO on ESA’s ExoMars Trace Gas Orbiter and temperature profiles retrievals with the hydrostatic equilibrium equation. II. Temperature variabilities in the mesosphere at Mars terminator. (2023) J. Geophys. Res. Planets. https://doi.org/10.1029/2022JE007279[12] Soret, L., et al; The Martian oxygen green line dayglow: response to solar activity. (2025) Icarus. https://doi.org/10.1016/j.icarus.2025.116707[13] Gérard, J.-C., et al; Observation of the Mars O2 visible nightglow by the NOMAD spectrometer onboard the Trace Gas Orbiter. (2023) Nature Astron. https://doi.org/10.1038/s41550-023-02104-8[14] Valantinas, A., et al; Evidence for transient morning water frost deposits on the Tharsis volcanoes of Mars. (2024) Nature Geoscience. https://doi.org/10.1038/s41561-024-01457-7[15] Mason, J. P., et al; Ultraviolet and Visible Reflectance Spectra of Phobos and Deimos as Measured by the ExoMars-TGO/NOMAD-UVIS Spectrometer. (2023) J. Geophys. Res. Planets. https://doi.org/10.1029/2023JE008002[16] Knutsen, E.W., et al; Comprehensive Investigation of Mars Methane and Organics with ExoMars/NOMAD. (2021) Icarus 257. https://doi.org/10.1016/j.icarus.2020.1142

We generate climatologies of ice cloud and dust optical depth derived from the NOMAD/UVIS nadir measurements onboard ExoMars Trace Gas Orbiter (TGO) for almost four Martian Years (MY), from mid MY34 to mid MY38. Our retrieval algorithm has been updated with new results for aerosol models and surface reflectance. It now uses 3 dust particle sizes: the nominal R _eff = 1.5 μm, along with two larger sizes of 2.0 and 2.5 μm. The inclusion of the two larger sizes improves the retrieval during large dust loading events. We present the retrieved zonal‐mean distribution of water ice cloud optical depth (OD), as well as spatial distributions for portions of the MY that highlight distinct cloud features, such as the Aphelion Cloud Belt (ACB), orographic clouds and the edges of polar clouds. The diurnal trend of ice cloud optical depth during the aphelion season is shown and agrees with recent results. We also present the retrieved seasonal zonal averaged distribution of the dust OD. We have compared our aerosol OD results (i.e., dust + ice clouds) with the measurements of MastCam and MastCam‐Z onboard Curiosity and Perseverance rovers serving as ground truth validation: UVIS shows a good qualitative agreement, reproducing usually well the seasonal variations. Quantitatively, we observe UVIS values to be usually lower or similar, than MastCams' ones. We also provide and consider retrieval sensitivities of our results (altitude sensitivity, altitude profile, particle size, scattering properties) to potentially explain the differences between our nadir retrievals with the direct extinction measurements from the rovers.

We present a new climatology of Martian water vapor column abundance derived from nadir dayside observations made by the Nadir and Occultation for MArs Discovery (NOMAD) instrument suite as part of the ExoMars Trace Gas Orbiter (TGO) mission. H ₂ O vertical column densities are retrieved by applying an Optimal Estimation inversion scheme to the near‐infrared reflectance factor spectra acquired by the NOMAD Limb, Nadir and Occultation (LNO) channel over Martian Years 34–38 (April 2018–June 2025). By correcting for saturated absorption lines and using an improved Acousto‐Optical Tunable Filter (AOTF) calibration, H ₂ O vertical columns retrieved from diffraction orders 167 and 169 quantitatively agree with co‐located MGS/TES (Thermal Emission Spectrometer) and EMM/EMIRS (Emirates Mars Infrared Spectrometer) independent observations, in terms of seasonal and latitudinal variations. A good agreement is also obtained with the GEM‐Mars General Circulation Model, except at high northern latitudes during the Aphelion season, where the current GEM‐Mars version underestimates the H ₂ O summer peak. The geographical distribution of the retrieved H ₂ O column abundances is characterized over the different Martian seasons and shows the typical patterns observed by previous space‐borne infrared instruments, such as the Pole‐to‐Pole transport and some local enhancements correlated with the topography. Our retrievals nevertheless indicate a more marked difference between Northern and Southern Hemisphere H ₂ O column summer peaks. The seasonal cycle is also found to be stable from year‐to‐year, except during and after the Martian Year 34 global dust storm, where significantly smaller column abundances are observed compared to the same period in Martian Years 35–37.

The SO channel of the NOMAD instrument on board ESA’s Trace Gas Orbiter (TGO) has a spectral range extending from 2.2 to 4.2 µm (2400 cm-1 to 4600 cm-1). By solar occultation, the intense ν1+ν3 band of CO2 (around 2.7 µm or 3710 cm-1) is suitable for deriving CO2 density and temperature in the upper thermosphere of Mars at altitudes around 140 to 190 km. The lower altitude limit is due to the saturation of the CO2 molecular lines in that band. The retrieval algorithm is identical to the one described in Trompet et al. (2023) and relies on the calibration method outlined in Liuzzi et al. (2019), which was further improved in Villanueva et al. (2022). The CO2 density profiles are regularized using a Tikhonov method, and the temperature profiles are derived assuming hydrostatic equilibrium. A total of 5700 profiles were derived from April 21, 2018 (MY 34, LS 163°) to June 30, 2025 (MY 38, LS 104°).Datasets of the Martian upper thermosphere at the terminator are rather sparse, being limited to observations from the Extreme UV monitor (EUVM - Thiemann et al., 2018) on board NASA’s MAVEN orbiter and the MIR channel of the Atmospheric Chemistry Suite (ACS-Belyaev et al., 2022) also on board TGO, which uses the same CO2 band at 2.7 µm. Despite this limited coverage, some collocated profiles suitable for comparisons are found amongst the datasets of EUVM, ACS-MIR, and NOMAD-SO.Kumar et al. (2024) already derived characteristics of thermal tides for six sets of EUVM measurements. However, extending those measurements helps to confirm those characteristics and infer further information on thermal tides through comparison with the Mars Climate Database (MCD - Gonzalez-Galindo et al., 2015). The tides simulated by the MCD are in good agreement with those derived from TGO and MAVEN, with a still weaker amplitude likely due to the averaging performed within the MCD dataset. The datasets of both EUVM and NOMAD show the presence of a thermospheric polar warming at aphelion (Thiemann et al., 2024). In addition, the averaged profiles of NOMAD are compared to those of the Venus thermosphere derived from the SOIR instrument (Mahieux et al., 2023).References:Belyaev et al. (2022), JGR: Planets, 127 (10), https://doi.org/10.1029/2022JE007286Gonzalez-Galindo et al. (2015), 120 (11), https://doi.org/10.1002/2015JE004925Kumar et al. (2024), JGR: Planets, 129 (4), https://doi.org/10.1029/2023JE007887Liuzzi et al. (2019), Icarus (321), https://doi.org/10.1016/j.icarus.2018.09.021Mahieux et al. (2023), Icarus, 405, https://doi.org/10.1016/j.icarus.2023.115713Thiemann et al. (2018), JGR: Planets, 123 (9), https://doi.org/10.1029/ 2018JE005550Thiemann et al. (2024), GRL, 51 (5), https://doi.org/10.1029/2023GL107140Trompet et al. (2023), JGR: Planets, 128 (3), https://doi. org/10.1029/2022JE007277Villanueva et al. (2022), JRL, 49 (12), https://doi. org/10.1029/2022GL098