The first detection of aurora in the Mars night sky was made in 2005 with the SPICAM ultraviolet spectrometer on board Mars Express. It revealed the presence of emissions from CO, CO₂⁺ and O, concentrated in regions of strong open magnetic field lines in the southern hemisphere. Further ultraviolet observations with the IUVS instrument on board MAVEN and EMUS on the Emirates Mars missions measured the N₂ Vegard-Kaplan bands, the OI 130.4 and 135.6 nm and the CI 156.1 and 165.7 nm emissions.

All space-borne instruments so far were only able to observe in the ultraviolet. Yet, laboratory simulations of the aurora at low CO₂ pressure have shown the presence of the CO₂⁺ Fox-Duffendack-Barker (FDB) emission between 300 and 500 nm with 7% of the intensity in the blue. The oxygen green and red lines were not observed. More recently, the green line at 557.7 nm has been unambiguously measured from the Martian surface with the Supercam spectrometer covering the 535-853 nm range and the Mastcam-Z imaging camera on the Perseverance rover during Solar Energetic Particle Events. It is mainly excited by collisions of energetic electrons with ambient CO₂. ESA’s M-MATISSE mission will carry the Mars Aurora and dust Camera (M-AC) equipped with a green line (557.7 nm) and a FDB channel (367.0-393.2 nm).

In contrast, the Martian ultraviolet nightglow is characterised by the presence of the nitric oxide δ and ϒ bands between 180 and 300 nm discovered with Mars Express-SPICAM. It is caused by radiative recombination of O and N atoms created on the day side. Additional emissions in the near infrared and visible ranges result from three-body recombination of O atoms in the winter polar regions. The visible contribution corresponds to the O₂ c ¹∑_{u -} X ³∑_g Herzberg II band system extending from 300 to 600 nm. It is expected to be bright enough to be observable with the naked eye in the winter polar regions from Mars orbit and from the surface.

In this presentation, we will combine observed spectral measurements with Monte Carlo modelling of the excitation rates to define the colours and the visibility of the nightside emissions.

How to cite: Gérard, J.-C. and Soret, L.: The colours of the Mars night sky: when  aurora and nightglow illuminate the atmosphere, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-170, https://doi.org/10.5194/epsc2026-170, 2026.

O₂ and NO nightglow emission on Mars from NOMAD UVIS

Nightglow observations on Mars provide important insights into the chemical and dynamical processes of the Martian atmosphere. The first visible observations of nitric oxide (NO) nightglow were reported by (Bertaux et al. 2005) using the SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument aboard Mars Express. Additional NO observations were later obtained with the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard Mars Atmosphere and Volatile Evolution (MAVEN). These visible emissions originate from the recombination of oxygen and nitrogen atoms produced on the dayside through the photodissociation of CO₂ and N₂ and subsequently transported to the nightside by atmospheric circulation. The resulting NO nightglow emissions provide valuable information on the product of the densities of atomic oxygen and nitrogen in the upper atmosphere. In the visible it is also possible to observe the O₂ Herzberg II emission, this emission was first observed by (Gérard et al. 2023; Soret et al. 2024) using the UVIS channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument.

The NOMAD spectrometer aboard the ExoMars Trace Gas Orbiter (Vandaele et al. 2018) began science operations in April 2018. In this work, we investigate limb observations from the UV‐visible (UVIS) channel of NOMAD ( Vandaele et al. 2015). Covering a wavelength range from 200 to 650 nm, UVIS enables simultaneous observation of NO δ and γ bands between 205 and 270 nm and O₂ Herzberg II emissions between 400 and 600 nm. Using dedicated observation modes, this study explores the UVIS dataset with the objective of deriving atmospheric parameters from these nightglow emissions.

The methodology consists first of retrieving the emission intensities from UVIS limb spectra. We apply the same approach as described by (Soret et al. 2024), by fitting reference spectra derived from previous missions (Venus for O₂ and MAVEN for NO) to compute the total intensity at each altitude due to the nightglow emission. From the resulting vertical intensity profiles, Chapman functions are fitted to derive the volume emission rates. Using these emission rates and the corresponding chemical relationships, we retrieve the densities of atomic oxygen and nitrogen.

The altitude probed are between 20 to 80 km with the peak emission of NO on average 20 km above the emission of O₂. All observations are in the polar regions and the limited number of limb profiles (around 50 from MY35 to MY 38) limit the interpretation for seasonality. An example showing the different vertical profiles can be found in Figure 1. Currently the density of N is not included in the model but new simulations should be provided allowing the comparison.

This work provides a climatology of NO and O₂ nightglow emissions observed simultaneously by NOMAD-UVIS, together with the corresponding atomic oxygen and nitrogen densities at coincident altitudes whenever possible. Finally, the retrieved densities are compared with predictions from global climate models, including GEM (Neary and Daerden 2018) and PCM (Millour et al. 2018). Overall, the modeled density profiles show good agreement with the observations.

Figure 1: Vertical profiles for NO and O₂ emissions on the first panel, the middle panel represents the volume emission rate and the last panel the O and N densities. In the last panel the densities for O are computed using the temperature from GEM or the MCD. In this case, the value for the O densities at the peak are similar between the models and the observations.

Bertaux, Jean-Loup, François Leblanc, Séverine Perrier, et al. 2005. “Nightglow in the Upper Atmosphere of Mars and Implications for Atmospheric Transport.” Science 307 (5709): 566–69. https://doi.org/10.1126/science.1106957.

Gérard, J. C., L. Soret, I. R. Thomas, et al. 2023. “Observation of the Mars O2 Visible Nightglow by the NOMAD Spectrometer Onboard the Trace Gas Orbiter.” Nature Astronomy 8 (1): 77–81. https://doi.org/10.1038/s41550-023-02104-8.

Millour, E., F. Forget, A. Spiga, et al. 2018. THE MARS CLIMATE DATABASE (VERSION 5.3).

Neary, L., and F. Daerden. 2018. “The GEM-Mars General Circulation Model for Mars: Description and Evaluation.” Icarus 300 (January): 458–76. https://doi.org/10.1016/j.icarus.2017.09.028.

Soret, L., F. González‐Galindo, J. ‐C. Gérard, et al. 2024. “Ultraviolet NO and Visible O₂ Nightglow in the Mars Southern Winter Polar Region: Statistical Study and Model Comparison.” Journal of Geophysical Research: Planets 129 (12): e2024JE008620. https://doi.org/10.1029/2024JE008620.

Vandaele, A. C., J. J. Lopez-Moreno, M. R. Patel, 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 (5): 80. https://doi.org/10.1007/s11214-018-0517-2.

Vandaele, Ann C., Yannick Willame, Cédric Depiesse, et al. 2015. “Optical and Radiometric Models of the NOMAD Instrument Part I: The UVIS Channel.” Optics Express 23 (23): 30028. https://doi.org/10.1364/OE.23.030028.

How to cite: Flimon, Z., Soret, L., Gérard, J.-C., and Hubert, B. and the NOMAD team: O2 and NO nightglow emission on Mars from NOMAD UVIS, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1146, https://doi.org/10.5194/epsc2026-1146, 2026.

The Curiosity rover’s ChemCam instrument suite and the Perseverance rover’s SuperCam instrument suite have been conducting routine passive UV-to-near-IR spectroscopy of the Martian sky since, respectively, 2013 and 2021. From these “passive sky” observations we retrieve column abundances of trace gases O₂ and H₂O, as well as aerosol properties, using a combination of grid-search based and Nelder Mead (also known as “downhill simplex”) optimization, and discrete-ordinates multiple-scattering radiative transfer models with correlated-k-based gas absorption. Ratios of high-elevation angle to low-elevation angle sky pointings are used to divide out most instrumental uncertainties and make weak spectral features detectable.

The latest passive sky aerosol results are described by Stcherbinine et al. (this meeting; EGU 2026). Here, we focus on the latest trace gas measurement results.

The passive sky trace gas results are important for testing hypotheses about the unexplained variability of O₂ abundances relative to argon and other inert trace gases (Trainer et al., 2019; Lo et al., 2024). They also help constrain surface-atmosphere exchange of water vapor (e.g. Savijärvi et al., 2019). Furthermore, the rapid (roughly every 14 – 21 days in most years) sampling of O₂ and water vapor abundances provided by passive sky combined with similar sampling of Ar with APXS on Curiosity (VanBommel et al. 2018, 2024) provides evidence for close correlation of water vapor with the O₂/Ar variability in northern summer (McConnochie et al., 2024).

The possibility of such a correlation was first suggested by the results of the Viking Gas Exchange experiments, which showed release of O₂ from a soil sample in response to adding water vapor (Klein, 1978; Oyama & Berdahl, 1977), and by the fact that a surface/subsurface reservoir of O atoms appears to be required because atmospheric reservoirs of O atoms are too small to supply the observed magnitude of O₂ variability (Trainer et al., 2019). Soil oxychlorines have been proposed (Trainer et al., 2019; Lo et al., 2024) as the soil reservoir for O atoms, as were superoxide ions in the context of the Viking Gas Exchange experiments (Yen et al., 2000), but specific reactions with the necessary rates to explain the apparent seasonal cycle of O₂ into and out of the soil have not been identified.

The details and timing of the O₂-to-water-vapor correlation, combined with the information about the effects of the global circulation on all trace gases that is provided by Ar measurements, could potentially constrain the mechanisms and predominant locations for the apparent O₂ cycle. However as shown in Figure 1, the apparent O₂ cycle has substantial interannual variability despite only modest interannual variability in Ar and H₂O. We have therefore been working to both extend the passive sky and APXS argon data sets and to improve data analysis methods. New developments in work include initial O₂ measurements from Perseverance-SuperCam, explicit modeling of stray light effects on gas absorption lines for both SuperCam and ChemCam, and more accurate detector background subtraction and outlier rejection for ChemCam. Even in their current form, however, the ChemCam O₂ measurements show statistically significant variability relative to argon, just as the SAM O₂ measurements in Trainer et al. (2019) do. They also show, as detailed in Figure 1, significant interannual variability, especially around northern hemisphere summer solstice, and a tendency for the timing of the northern spring-summer seasonal increase in O₂/Ar to be correlated with that of water vapor.

Table 1 shows an alternative approach to understanding the nature of the apparent O₂ cycle, in this relying on long term averages of ChemCam and SuperCam measurements, compared to long term averages from SAM mass spectrometer in-situ atmospheric samples (Trainer et al. 2019), and to other available O₂ measurements. What we see is that the measurements representative of column-averaged O₂ are all consistent with each other and significantly higher than what SAM observes. This implies that there is a net sink for O₂ near the surface at Gale crater and that that sink is relatively fast compared to vertical mixing time scales.

References: Barker, 1972, Nature, 238(5365), https://doi.org/10.1038/238447a0. Carleton and Traub, 1972, Science, 177(4053), https://doi.org/10.1126/science.177.4053.988. Fedorova et al., 2021 EGU Meeting. Hartogh et al., 2010, A & A, 521, https://doi.org/10.1051/0004‐6361/201015160. Klein, 1978, Icarus, 34(3), https://doi.org/10.1016/0019‐1035(78)90053‐2. ,Lefevre et al., 2004, JGR, 109, https://doi.org/10.1029/2004JE002268. Lo et al. 2024, PSJ 5, https://doi.org/10.3847/PSJ/ad251b.McConnochie et al., 2022, Mars Atmosphere Modeling and Observations Conference. McConnochie et al. 2024 10^th Mars Conference.  Oyama & Berdahl, 1977, JGR, 82(28), https://doi.org/10.1029/JS082i028p04669.  Stcherbinine et al., 2026, EGU Meeting.  Trainer et al. 2019, JGR12, https://doi.org/10.1029/2019JE006175, Savijärvi et al. 2019, Icarus 326, https://doi.org/10.1016/j.icarus.2019.03.020. VanBommel et al. 2018, JGR, 123, https://doi.org/10.1002/2017JE005454. VanBommel et al. 2024, 10^th Mars Conference. Yen et al, 2000, Science, 289(5486), https://doi.org/10.1126/science.289.5486.1909.

TABLE 1: Comparison of lower-atmosphere O₂ mixing ratios from various sources                                           

FIGURE 1: Comparison of trace gas seasonal cycles at Gale Crater over 5 Mars years. Individual points are retrieval results for the indicated year and solar longitude, with 1-sigma error bars. The smooth lines for O₂ are the result a LOESS regression and the shaded region around those smoothed lines represents the 1-sigma uncertainties of that LOESS regression. The “LMD model” for O₂ and Ar is from Lefevre et al. (2004), but scaled so that the average model value matches the average observed value.The argon data in the top panel is from VanBommel et al. (2024) using methodology from VanBommel et al. (2018).

How to cite: McConnochie, T. and the Co-authors: Sky Spectroscopy with the Perseverance and Curiosity Mars Rovers: Latest results for Molecular Oxygen and Water Vapor, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-312, https://doi.org/10.5194/epsc2026-312, 2026.

We present new ground-based microwave observations of the diurnal variation of mesospheric ozone made over Göttingen, Germany, over four years from March 2022 to March 2026. These data are of high quality and characterise the diurnal behaviour of the ozone mixing ratio during the solstices and equinoctial conditions. The observed diurnal variation of ozone peaks at ~75 km. At 65 km, the observed diurnal variation is less than half, whereas at 50 km it is less than a quarter of the maximum variation. In this presentation, we compare measurements taken over different periods throughout the year and conduct a comprehensive data analysis. We compare it with satellite observations for further validation.

How to cite: Dutta, M., Rezac, L., Dabrowski, B., and Hartogh, P.: New High-Sensitivity Ground-Based Microwave Observations of Mesospheric Ozone. , Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-199, https://doi.org/10.5194/epsc2026-199, 2026.

Ozone is an important trace gas for understanding the dynamics and photochemistry of planetary atmospheres and protecting life from harmful ultraviolet radiation on Earth. It is well known that ozone is efficiently destroyed by odd nitrogen (NOx) and odd hydrogen (HOx) generated by solar energetic particles (SEPs) on Earth [1]. It has never been proven that such phenomena can occur universally on the terrestrial planets. For instance, Mars lacks a global intrinsic magnetic field and has a thin CO2-dominated atmosphere, making it vulnerable to SEP impacts. Such SEP impacts are global and much closer to the surface than on Earth, as revealed by the global SEP aurora [2]. Nakamura et al. (2023) [3] predicted that such a space-atmosphere interaction induced by SEP would cause significant impacts on Mars’ atmospheric composition. Despite model predictions, observational understanding of such effects on Mars remains limited. This study aims to detect SEP-induced ozone depletion on Mars through observations.

In this study, we analyzed the absorption spectra of ozone around 200-300 nm obtained from the ultraviolet to visible (UVIS) channel of the NOMAD (Nadir and Occultation for MArs Discovery) instrument onboard the ExoMars Trace Gas Orbiter (TGO) [4]. We focused on the major SEP event in February 2024. During this event, three distinct peaks of SEP flux were observed by SEP measurements onboard MAVEN [5] over one month. Especially, the third peak shows a significant enhancement of the flux at 5 MeV ions, lasting for 4 days. Our Monte Carlo precipitation model, driven by SEP spectra measured by MAVEN/SEP, predicted that the SEP energy up to 5 MeV was sufficient to penetrate the Martian atmosphere down to ~50 km.

The ozone column density observed by NOMAD showed multiple decreases of approximately 10-20% at which timings coincided with the peak flux increase of the SEP event. However, regardless of the SEP, Martian ozone levels can naturally fluctuate due to factors such as clouds, atmospheric circulation, and dust. The remarkable point is that this global decline in ozone was observed simultaneously in both the northern and southern hemispheres. Such global ozone fluctuations are consistent with the predicted impact of the SEP event, rather than localized meteorological factors. This decrease is compared to our model prediction of ~10% decrease. Our result supports the model hypothesis that suggests water cluster ions (H⁺(H₂O)ₙ), generated by SEP, produce HOx compounds which destroy ozone in the CO2-dominated atmosphere. NOMAD observations demonstrate a global-scale ozone depletion coinciding with SEP penetration on Mars. This study presents remarkable observational results indicating that ozone depletion can occur due to SEPs even on planets with CO₂-dominated atmospheres, such as Mars. Understanding the impact of SEPs on Mars is crucial for elucidating the evolution of the Martian atmosphere from its early stages - when solar activity was more intense than it is today - to its current state.

References:

[1] Jackman et al. (2005) J. Geophys. Res., 110 A09S27.

[2] Schneider et al. (2018) Geophysical Research Letters, 45, 7391–7398.

[3] Nakamura et al. (2023) Earth, Plan. and Space, 75, 140.

[4] Vandaele et al. (2018) Space Sci. Rev., 214, 80.

[5] Larson et al. (2015 Space Sci. Rev., 195, 153.

How to cite: Ao, T., Nakagawa, H., Willame, Y., Nakamura, Y., Thomas, I., Aoki, S., Daerden, F., Neary, L., Piccialli, A., Jolitz, R., Rahmati, A., and Terada, N.: Global ozone depletion on Mars during major SEP events observed by TGO/NOMAD, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-307, https://doi.org/10.5194/epsc2026-307, 2026.

Ozone is a key trace gas of the Martian atmosphere. It is produced through the photolysis of carbon dioxide followed by a three-body reaction between atomic and molecular oxygen. Its primary sinks are photodissociation, which recycles lighter odd oxygen (O_x) species, and gas-phase reactions with odd-hydrogen (HO_x) species, which are produced by water vapour photolysis.

Many previous studies have shown that current Mars atmospheric general circulation models (GCMs) fail to reproduce the ozone columns observed by Mars Express and Mars Reconnaissance Orbiter studies (e.g. [1,2]), typically underestimating them by a factor of two [3].To help bridge the gap between models and observations, some studies examined the influence of various factors on the modelled ozone, including heterogeneous chemistry on water ice and dust aerosols, reaction rates, or radiative transfer [1,2]. These adjustments led to some local and temporally limited improvements, but often increased discrepancies elsewhere, suggesting that key processes may still be missing from Martian GCMs.

In this study, we adopt a different approach by investigating the propagation of chemical uncertainties using the 1-D sub-model of the Mars Planetary Climate Model (MPCM). We conducted a global sensitivity study using Monte Carlo simulations in which all reaction rates were randomly varied within their measured or estimated uncertainty ranges. Then, we identified correlations between the volume mixing ratios of key species (O_x and HO_x) and reaction rates variations, which allowed us to determine which reactions most strongly influenced model outputs. We then adjusted the rates of these key reactions within the full GCM and found that doing so significantly improves simulated ozone VMRs relative to observations from the Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). Based on our analysis, we recommend an assessment of these key reaction rates under relevant conditions, along with their implementation in GCMs, to better constrain the remaining discrepancies between models and observations.

References:

[1] Lefèvre, F., A. Trokhimovskiy, A. Fedorova, L. Baggio, G. Lacombe, A. Määttänen, J.‐L. Bertaux, et al., JGR: Planets 126, no. 4 (2021)

[2] Daerden, F., Crowley, J. N., Neary, L., Smith, M. D., Loeffler, M. J., Clancy, R. T., et al., JGR: Planets, 128, (2023)

[3] Olsen, K. S., A. A. Fedorova, A. Trokhimovskiy, F. Montmessin, F. Lefèvre, O. Korablev, L. Baggio, et al., JGR: Planets 127, no. 10 (2022)

How to cite: Benne, B., Palmer, P., Lefèvre, F., and Millour, E.: Reconciling Martian ozone with model photochemistry: new insights from an uncertainty propagation study, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-135, https://doi.org/10.5194/epsc2026-135, 2026.

Introduction: Ozone is an important trace gas in the martian climate system. It is intimately linked to the odd-hydrogen (HO_x) species (H, OH, HO₂), low-abundance but highly reactive chemicals essential for maintaining the stability of Mars’ overwhelmingly CO₂ atmosphere [1]. These HO_x species are difficult to observe directly, but their presence can be inferred via their destruction of (and therefore anti-correlation with) ozone.

Ozone has a short photochemical lifetime on the dayside of a few hours. In the polar night, however, it is able to persist for significantly longer, allowing it to serve as a tracer for large-scale atmospheric dynamics, such as those associated with the polar vortices. Indeed, observed ozone has been shown to be closely correlated with modelled potential vorticity (PV, a diagnostic thermal-dynamical variable) in the polar winter [2].

Ozone observations can serve as valuable constraints for our understanding of both chemical and dynamical processes in the martian atmosphere. However, numerical models have long had issues in representing martian ozone, tending to underestimate total ozone abundance [e.g. 3]. Several tacks have been taken in trying to resolve this discrepancy, including incorporating heterogeneous chemical reactions (gas-solid; e.g. uptake of HO_x species onto water ice clouds) and modifying existing reaction rates [e.g. 3,4]. While some improvement has resulted, model-observation disagreements remain.

In this work, we explore the impacts of data assimilation (see below) on the modelled ozone representation, and compare to retrieved ozone observations, in order to better characterise the model biases and the causal mechanisms behind them.

Approach: We employ a statistical technique called data assimilation. Data assimilation is a means for combining a numerical model of a system with discrete observations of that system; for example, a global climate model (GCM) of Mars with satellite observations of the martian atmosphere. By doing this, one can get the best of both worlds: the full spatio-temporal coverage and deterministic causal chain of the model, and the accuracy of actual observational data.

We use the Open University’s Mars GCM, also known as the Mars Planetary Climate Model – UK (PCM-UK) [5,6,7]. This shares model physics with the Mars PCM, but possesses a different dynamical core and data assimilation capability using the analysis correction scheme adapted from the UK Met Office [8].

We assimilate temperature profiles from the Mars Climate Sounder (MCS) and Atmospheric Chemistry Suite (ACS), together with water column and profile retrievals from ACS and the Nadir and Occultation for MArs Discovery (NOMAD) instrument. We do not assimilate ozone, but compare the model ozone to column retrievals from NOMAD-UVIS [9].

Results & Discussion: We present results comparing the baseline model simulation (“Control”), assimilated model simulation (“Assimilation”), and NOMAD-UVIS ozone retrievals (“Observations”) for the first half of Mars Year (MY) 36.

Preliminary results indicate that the Assimilation improves the fit with the Observations in some areas but worsens it in others, likely due to a wetter atmosphere in the Assimilation. At higher latitudes nearer the poles, both Control and Assimilation significantly underpredict total ozone column.

Over the southern winter pole, where Observations are unfortunately lacking, we find that the Assimilation and Control show radically different ozone distributions within the polar vortex. This appears to be due to the improved representation of the characteristic annular vortex structure in the Assimilation, where the PV maximum is located at least 10 degrees off-pole (see Fig. 1).

We discuss these and other results in the context of existing work. By comparing the ozone distribution in the two model simulations and the retrieved ozone, we can identify where model biases are due to missing chemistry/physics, and where they may be attributable to incorrect representation of other key atmospheric variables (such as temperatures and the water cycle).

Figure 1. Comparison of modelled ozone total abundance over the south pole between the Control (top) and Assimilation (bottom) for same period in MY 36.

References: [1] McElroy & Donahue (1972). Science. 177 (4053). [2] Holmes et al. (2017). Icarus. 282. [3] Lefèvre et al. (2021). JGR Planets. 126 (4). [4] Brown et al. (2022). JGR Planets. 127 (11). [5] Forget et al. (1999). JGR Planets. 104 (E10). [6] Lewis et al. (2007). Icarus. 192 (2). [7] Holmes et al. (2020). Planet. & Space Sci. [8] Lorenc et al. (1991). QJRMS. [9] Mason et al. (2024). JGR Planets.

How to cite: Streeter, P., Rajendran, K., Holmes, J., Lewis, S., Mason, J., and Patel, M.: Chemico-dynamical impacts of data assimilation on martian atmospheric ozone, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-519, https://doi.org/10.5194/epsc2026-519, 2026.

Investigations into martian paleoclimatology have shown Mars in the past (~ 4 billion years ago) used to contain significant amounts of surface liquid water in contrast to present day conditions [1,2]. Given the importance of liquid water for life on Earth, understanding the evolution of the martian climate from a planet containing surface liquid water to a cold and dry planet today will improve our understanding of the conditions that influence terrestrial planetary habitable environments. Hydrogen escape is proposed as a major process in the depletion of water from Mars. This is where water molecules disassociated into hydrogen and oxygen after which hydrogen escapes the martian atmosphere through atmospheric escape processes such as Jeans escape and sputtering [3].

Previous investigations of martian hydrogen escape using a 1-D upper atmospheric model have shown that water loss rates on Mars are driven by the availability of water vapour near the boundary between the middle and the upper atmosphere (~ 80 km altitude), with water loss on Mars seasonally driven during perihelion and enhanced atmospheric dust activity such as regional and global dust storms (GDS) [4,5]. GDS events on Mars strongly influence interannual variations in the martian climate, occurring approximately every 5–10 Earth years, with the most recent GDS occurring in 2018 (hereafter referred to as martian Year 34 or MY34) [6]. The MY34 GDS also coincided with the start of the ExoMars Trace Gas Orbiter (TGO) observations, with TGO providing the greatest level of spatial coverage of water vapour profiles to date. These observations indicate enhanced transport of water vapour to the upper atmosphere [7,8,9].

Constraining the seasonal and interannual availability of water vapour in the upper atmosphere is therefore integral to determine the amount of water escaping from Mars. One such method of constraining is through data assimilation. This methodology combines the data of accurate but incomplete spatiotemporal observations of the martian atmosphere by orbiters with the complete spatiotemporal coverage albeit parameterised simulation of the martian atmosphere provided by models to reconstruct the martian atmosphere most in line with observations ensuring physical consistency. Recent work on data assimilation of TGO water vapour column and profiles into the Mars Planetary Climate Model UK-Spectral version (MPCM) has shown there is improved representation in the MPCM of water vapour profiles in line with orbiter observations around the upper atmosphere in contrast to free run simulations [10]. Therefore in this presentation, we explore the hydrogen escape flux using MPCM water cycle simulations based on data assimilation of TGO data.

The martian water cycle is simulated from L_S = 160˚ in MY34 to the end of MY35 using the MPCM with data assimilation of ExoMars Trace Gas Orbiter water vapour column and profile measurements, alongside temperature profiles from Mars Reconnaissance Orbiter and dust optical depths from Mars Reconnaissance Orbiter and Mars Odyssey, to constrain the model’s atmospheric state and reproduce the martian water cycle in the most representative state. The outputs from the MPCM water cycle are coupled to the 1D upper atmosphere photochemical model from which the hydrogen escape is calculated from L_S = 160˚ in MY34 to the end of MY35. The hydrogen flux during the MY34 GDS is compared to MY35 which is a martian year with nominal atmospheric dust levels.

Figure 1: MY34/MY35 Hydrogen escape flux ratio vs L_S from MPCM simulations using data assimilation of ExoMars Trace Gas Orbiter water vapour profiles and columns along with Mars Reconnaissance Orbiter temperature profiles

We find that the hydrogen escape during the MY34 GDS is larger by a factor of 6 with respect to MY35 around L_S = 195 – 215˚. There is also enhanced hydrogen escape by around 4 times in magnitude with respect to MY35 between L_S = 330 – 345˚ in MY34 which correlates with the annual C – Type [6] regional dust storm activity which was considerably stronger in MY34 compared to MY35 (Figure 1). These results show there is enhanced water loss during increased atmospheric dust activity such as GDS and C-Type regional dust events, suggesting that interannual dust activity needs to be considered for long term martian water loss calculations. We present the reanalysis of the water cycle during the MY34 GDS and findings of the atmospheric processes influencing variations in the hydrogen escape flux. We will also present estimates for the interannual water loss from Mars based on MPCM water cycle simulations constrained by multi-orbiter assimilation.

Reference: [1] Ramirez, R.M. & Craddock, R.A. (2018) Nat. Geosci., 11, 230–237. [2] Pollack, J. et al. (1987), Icarus, 71(2), pp. 203–224. [3] Kasting, J.F. and Catling, D. (2003), Annu. Rev. Astron. Astrophys., 41(1), pp. 429–463. [4] Chaffin, M., Deighan, J., Schneider, N. et al. (2017), Nature Geosci 10, 174–178 [5] Holmes, J.A. et al. (2021) Earth Planet. Sci. Lett., 571, p. 117109. [6] Kass, D. et al. (2016), GRL, 43 (12), 6111-6118. [7] Vandaele, A. C. et al. (2018) Space Sci. Rev. 214(5). [8] Korablev, O. et al. (2018), Space Sci. Rev. 214(1), 7 (2018). [9] Aoki, S. et al. (2019), JGR (Planets) 124(12), 3482–3497. [10] Holmes, J.A. et al. (2022) JGR (Planets), 127(10).

How to cite: Shastri, V., Holmes, J., Patel, M., and Lewis, S.: Interannual variability in hydrogen escape from Mars and implications for water loss: Insights from ExoMars Trace Gas Orbiter data assimilation into a global climate model, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-869, https://doi.org/10.5194/epsc2026-869, 2026.

Argon is an inhert gas that can be used to measure different areas of thermal populations and non-thermal populations in the upper atmosphere of Mars. In this work, a statistical survey over 7 years' worth (2018-2024) of MAVEN/NGIMS measurements during Mars Year 35 – 37 (25,000+ standard survey orbits + 268 dedicated high-cadence argon orbits (1,100+) has been produced that provide reliable measurements from 150 km periapsis altitudes and up to 1200 km in the extended exosphere.

The thermal populations of argon in the thermosphere (150 – 300 km) have a pronounced diurnal variation with respect to solar-driven expansion. Temperatures derived from the scale height technique show a range of 150 K (dawn minimum around LST 3 - 6) to 300 K (afternoon maximum around LST 14 - 16) which results in a factor of 2 diurnal amplitude. Dayside temperatures in the thermosphere correlate strongly with EUVM Lyman-A irradiance as observed by EUVM indicating that the Ar thermosphere reacts to changing heliocentric as well as the Solar Cycles 24 – 25 transition with variations of 100 - 200 K, which indicates that dynamical effects (tides and gravity waves) also occurs.

At higher altitudes (above 400 km), Ar exhibits a coherent response to variations in UV flux  but experience reduced effective scale heights which is likely due to collisional damping. Increased EUV results in downward displacement of the O₂⁺ production layer into denser portions of the thermosphere, thereby increasing the column density above it and thermally stimulating the upward-moving suprathermal Ar resulting in more frequent collisions of the hot O and thus a partial thermalization.

On the night side,  high-altitude argon approaches NGIMS detection limits (~10 cm⁻³), while terminator passes reveal that the scale height temperature drastically inreases, which could be an indicator of a different suprathermal process than recombinative dissociation. Organized by SZA, altitude regime (thermal/transition/suprathermal), local time, season (Ls), latitude, and solar forcing, these results establish argon as a robust diagnostic of thermospheric dynamics, suprathermal production physics, terminator transport asymmetries, and baseline conditions for quantifying non-thermal neutral escape pathways at Mars.

Figure: Averaged distribution of the Ar corona at Mars

How to cite: Steichen, V., Leblanc, F., Chaufray, J.-Y., Benna, M., Nenon, Q., Modolo, R., and Curry, S.: A Statistical Survey of the Martian Argon Exosphere Using MAVEN  Data, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-417, https://doi.org/10.5194/epsc2026-417, 2026.

Carbon monoxide (CO) is one of the most important tracers of atmospheric circulation and photochemical activity in the Martian atmosphere. Because of its relatively long chemical lifetime, its spatial and vertical distribution is strongly linked to the large-scale transport processes that shape the Martian climate. At the same time, CO is directly connected to the CO₂ photochemical cycle, making it a valuable probe of the coupling between atmospheric dynamics and chemistry. Since its first detections through ground-based observations several decades ago [1,2], CO has been extensively investigated by orbiters and space missions dedicated to the exploration of Mars. These observations have revealed strong seasonal, latitudinal, and vertical variations, highlighting the complexity of the Martian atmosphere and the need for long-term, high-resolution measurements to better constrain atmospheric circulation and photochemical models.

The ExoMars Trace Gas Orbiter (TGO), launched in 2016, has significantly improved our capability to study trace species in the Martian atmosphere thanks to its high spectral resolution and global coverage. Among its scientific payload, two instruments are capable of observing CO: NOMAD and the Atmospheric Chemistry Suite (ACS) [3,4]. In particular, the ACS-MIR channel provides high-quality solar occultation measurements with exceptional sensitivity to the vertical structure of the atmosphere, allowing the retrieval of atmospheric profiles over a broad altitude range.

In this work, we analyze solar occultation spectra acquired by the ACS-MIR channel, focusing on the CO (2–0) overtone absorption band between 4150 and 4350 cm⁻¹. We describe in detail the preprocessing steps applied prior to the atmospheric retrievals, including corrections for spectral bending and wavelength shifts, as well as improvements in the characterization of the instrumental line shape [5]. Particular attention has been devoted to the refinement of the error budget in order to properly account for instrumental and spectroscopic uncertainties affecting the measurements. These improvements are essential to maximize the accuracy and stability of the retrievals, especially when combining information from lines with very different sensitivities throughout the atmosphere.

The spectra are analyzed using the KOPRA radiative transfer model coupled with the RCP inversion code [6] to retrieve vertical profiles of CO volume mixing ratio (VMR) and temperature. A detailed sensitivity analysis of the CO absorption lines at different atmospheric altitudes is also presented. Because the strongest CO lines become saturated below ~50 km, retrieving reliable information across the full atmospheric column requires a careful selection and combination of spectral lines probing different altitude regions. While saturated lines reduce sensitivity in the lower atmosphere, they remain crucial for constraining CO abundances in the upper atmosphere, particularly between ~80 and 120 km. To address this challenge, we developed a dedicated retrieval strategy that allows us to derive robust and continuous CO vertical profiles from the lower atmosphere up to the thermosphere.

In addition, we demonstrate for the first time the simultaneous retrieval of temperature and CO VMR from a dataset containing only CO absorption lines. This is possible because the full CO absorption band is observed: the overall shape of the band contains information about the atmospheric thermal structure, while the depth and relative intensity of the individual absorption lines constrain the CO abundance. The methodology developed for this simultaneous retrieval represents a significant step forward for the analysis of ACS-MIR observations, and a more detailed explanation of this approach will be presented in this work.

Using this method, we retrieve CO vertical profiles between ~7 and 120 km altitude and temperature profiles between ~7 and 100 km. We present temperature profiles for the first half of Mars Year (MY) 34 and compare them with other available datasets [8]. In addition, we show CO abundance maps covering MY 34 to MY 37, providing a multi-year overview of the seasonal and latitudinal variability of CO in the Martian atmosphere. The retrieved distributions are broadly consistent with previous studies [7,8], showing an approximately constant CO abundance below ~50 km and a marked increase at higher altitudes driven by photochemical production. Clear seasonal and latitudinal variations are also observed throughout MY 34–37, reflecting the influence of atmospheric transport and circulation processes. The implications of these results for our understanding of Martian atmospheric dynamics and photochemistry are discussed.

[1] Krasnopolsky, V. A. (2007). Icarus, 190(1), 93–102. https://doi.org/10.1016/j.icarus.2007.02.015

[2] Smith, M. D. (2004). Icarus, 167(1), 148–165. https://doi.org/10.1016/j.icarus.2003.09.010

[3] Vandaele, A. C. et al. (2018). Space Science Reviews, 214(5), 80. https://doi.org/10.1007/s11214-018-0517-2

[4] Korablev, O. et al. (2018). Space Science Reviews, 214(1), 7. https://doi.org/10.1007/s11214-017-0437-6

[5] López-Valverde, M. A. et al. (2023). J. Geophys. Res. Planets, 128(2), e2022JE007278. https://doi.org/10.1029/2022JE007278

[6] Stiller, G. P. (2000). FZKA Report, 6512, Forschungszentrum Karlsruhe.

[7] Modak, A. et al. (2023). J. Geophys. Res. Planets, 128(3), e2022JE007282. https://doi.org/10.1029/2022JE007282

[8] Fedorova, A. A. et al. (2022). J. Geophys. Res. Planets, 127(9), e2022JE007195. https://doi.org/10.1029/2022JE007195

How to cite: Rodriguez-Ovalle, P., Lopez-Valverde, M. A., Modak, A., Gonzalez-Galindo, F., Brines, A., Gamonal Gracía-Galán, M. Á., Alday, J., Trokhimoskiy, A., Belyaev, D. A., Olsen, K. S., Montmessin, F., Baggio, L., Fedorova, A. A., Korablev, O. I., Daerden, F., Thomas, I. R., Vandaele, A. C., Patel, M. R., and Bellucci, G.: Martian CO vertical distribution combining 4 Martian Years of TGO/ACS MIR solar occultation data, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-868, https://doi.org/10.5194/epsc2026-868, 2026.

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.

How to cite: Brines, A., Aoki, S., Liuzzi, G., Villanueva, G. L., Faggi, S., Koyama, S., Trompet, L., Thomas, I. R., Lopez-Valverde, M. A., Daerden, F., Neary, L., Viscardy, S., Palmer, P., Benne, B., Patel, M. R., Vandaele, A. C., Ristic, B., and Bellucci, G.: Vertical Structure and Variability of HCl in the Martian Atmosphere Observed by TGO-NOMAD., Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-722, https://doi.org/10.5194/epsc2026-722, 2026.

Despite constituting a tiny fraction of the Mars atmosphere, trace gases can play an important role in controlling atmospheric chemical cycling. However, some discrepancies between observed distributions of trace gases and modelled values indicate that there are ongoing processes in Mars’ atmosphere that are not fully understood.

Hydrogen chloride (HCl) was the first new gas detected by the ExoMars Trace Gas Orbiter (TGO)^[1,2], with observations from the Atmospheric Chemistry Suite (ACS) and Nadir and Occultation for Mars Discovery (NOMAD) instruments showing a strong seasonal variation over the last several Mars years. With a few exceptions, detections almost exclusively occur during the second half of the year (at solar longitudes between 180° and 360°). This is during southern hemisphere spring and summer, when temperatures, atmospheric dust content, and water vapour concentrations are higher, and ozone concentrations are low. Previous modelling has shown that heterogeneous chemical reactions involving dust or ice aerosols play a key role in controlling this seasonal pattern^[3,4,5,6].

Here we use the Mars Planetary Climate Model^[7,8], a 3-D global circulation model with photochemistry, to investigate some potential sources and sinks of HCl in the Martian atmosphere, which could account for the seasonal variation in measurements and the observed correlations and anticorrelations with other atmospheric factors. Firstly, we examine the indirect effect of heterogeneous chemistry of OH and HO₂ interacting with ice. We compare the effect of two different heterogeneous chemical schemes^[9,10] on HCl distributions via their effect on other oxidative species such as O and O₃. Secondly, we investigate the isolated effects of two further heterogeneous pathways involving water ice—one HCl source and one sink. Specifically, we model the uptake of HCl onto water ice, which has been studied before, and its subsequent release back to the atmosphere during ice sublimation, which has not been included in previous models.

Our preliminary results (e.g., Figure 1) show that the latter cycling could account for some of the seasonality of the HCl observations. HCl concentrations remain close to the ground during the first half of the year, and then increase at higher altitudes during the second half of the year, where they could be detected by TGO's instruments. Even without the addition of other HCl sources and sinks in the model, we expect this pattern to be repeated over multiple Mars years, reproducing at least part of the annual appearance and disappearance of HCl through the recycling of chlorine.

Continuing to reconcile models and observations of the cycling of HCl and other trace gases is important for achieving a more complete understanding of atmospheric processes operating on Mars today, as well as those that have played a key role over Mars’ history.

Figure 1: Preliminary model results showing seasonal distributions of HCl over more than one Mars year. Each panel shows zonally-averaged HCl volume mixing ratios with altitude and latitude, and there are 30° of solar longitude between each panel. The black contour indicates a mixing ratio of 0.5 ppbv, which is the detection limit for TGO ACS.

[1] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [2] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [3] Benne, B., et al. (2025). Astron. Astrophys., 699, A362. [4] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [5] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059. [6] Taysum, B. M. et al. (2024). Astron. Astrophys., 687, A191. [7] Forget, F., et al. (1999). JGR: Planets 104, E10. [8] Lefèvre, F., et al. (2004). JGR: Planets 109, E7. [9] Brown M. A. J. et al. (2022). JGR: Planets, 127, p.e2022JE007346. [10] Lefèvre, F., et al. (2021) JGR: Planets, 126, p.e2021JE006838.

How to cite: Gregory, B., Olsen, K., Millour, E., Brown, M., Rajendran, K., Streeter, P., Patel, M., and Lefèvre, F.: Modelling interactions with ice to understand the seasonal variation of HCl, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-879, https://doi.org/10.5194/epsc2026-879, 2026.

Atmospheric gravity waves (AGWs) are dynamically relevant in the Martian atmosphere [1], but orbital observations still provide an incomplete view of their morphology, altitude, and propagation conditions. Individual orbital images of clouds can identify wave packets and measure horizontal wavelengths, yet altitude, background wind, intrinsic phase speed, and vertical wavelength are only available when the observing geometry allows additional retrievals. This limits how far morphology alone can be interpreted dynamically. We compare three orbital datasets, that include clouds, each with different spatial resolution, coverage, and observing modes/geometry. They are not treated as a single climatology, because they cover different observing periods and have different sampling biases. Instead, we use them to test what part of the Martian AGW population each instrument can detect and characterize.

OMEGA/Mars Express nadir observations [2] during MY26–29 provide the widest occurrence baseline (Figure 1- a), although with a limited total number of observations. In this dataset, 263 wave packets were detected and 125 were characterized, with horizontal wavelengths from 6 to 83 km and a mean of 21 km [3]. Detections are concentrated in the northern hemisphere, especially at mid-to-high latitudes, with seasonal peaks during northern spring and autumn and southern winter. Their spatial distribution is consistent with topographic forcing near Tharsis–Alba and with convective activity over the northern plains. HRSC/Mars Express stereo-temporal imaging [4] during MY34–37 adds altitude and motion information (Figure 1- b). Wave packets identified in the HRSC Cloud Atlas [5] were characterized using blue-green inter-channel parallax to retrieve cloud-top altitude and approximately 30-minute repeat-track pairs to estimate horizontal motion. Retrieved altitudes are mostly 15–40 km, with uncertainties of about 3–10 km and a smaller number of cases extending to 60–100 km. For 11 packets with altitude, wind, and wave-geometry constraints, the analysis gives intrinsic phase speeds of 0.4–6.2 m/s and implied vertical wavelengths of 0.2–3.4 km under linear gravity wave assumptions [6]. Comparison with MCD static stability and background winds [7,8] indicates that some packets lie close to critical or turning levels, which may affect whether they remain visible at cloud level.

CaSSIS/ExoMars TGO imaging at 4 m/pixel [9] provides a substantially larger image dataset than HRSC, by roughly an order of magnitude, and samples smaller structures (Figure 1- c, Eira et al, in prep[10]). This part of the analysis remains preliminary, but it reaches sub-kilometer and few-kilometer horizontal wavelengths, below the OMEGA and HRSC detection ranges. A first comparison of spatiotemporal distribution, morphology, spatial scale, and observing geometry is used to assess instrument-dependent detection biases [10]. Whether the CaSSIS detections form the short-wavelength end of the OMEGA–HRSC population, or represent a separate subset controlled by different sources or visibility conditions, remains open.

Taken together, the datasets connect measurements that are usually obtained separately: morphology from cloud images, altitude from stereo geometry, and propagation constraints from repeat imaging and background winds. The comparison separates morphology-only detections from cases where altitude and wind information allow dynamical interpretation. These are the quantities needed to test how gravity wave drag is represented in Mars general circulation models.

Figure 1. Examples of Martian atmospheric gravity wave packets observed with (a) OMEGA/Mars Express, (b) HRSC/Mars Express, and (c) CaSSIS/ExoMars TGO, illustrating the different spatial scales and observing geometries used in the multi-instrument comparison.

Acknowledgments: This work was supported by the Portuguese Fundação para a Ciência e a Tecnologia through the research grant UID/04434/2025, and through the grants of reference 2021.05455.BD, and 2020.06389.BD. GG acknowledges financial support from Junta de Andalucía through the program EMERGIA 2021 (EMC21_00249) and from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/10.13039/501100011033. IAA is also supported by grant ID2022-137579NB-I00 funded by MCIN/AEI/10.13039/501100011033 and by "ERDF A way of making Europe", funded by the ESA Faculty Research Contract and Science Exchange Programme, which is in the frame of the MWWM Mars Wind and Wave Mapping project of reference ESA RFP/3-17570/22/ES/CM. We also thank Lucie Riu, Aurélien Stcherbinine, and the Mars Express and ExoMars TGO Teams for their support and encouragement in this work.

References: [1] Fritts & Alexander, 2003; [2] Bibring et al., 2007; [3] Brasil et al., 2025; [4] Jaumann et al., 2007; [5] Tirsch et al., 2024; [6] Brasil et al. (under review); [7] Forget et al., 1999; [8] Millour et al., 2018; [9] Thomas et al., 2017; [10] Eira et al. (in prep.). 

How to cite: Brasil, F., Machado, P., Gilli, G., Eira, H., Cardesin-Moinelo, A., T. Bickel, V., Tirsch, D., Carter, J., Thomas, N., E. Silva, J., Espadinha, D., Martin, P., and Wilson, C.: Martian atmospheric gravity waves across scales: morphology, altitudes, and dynamics from MEx/OMEGA, HRSC, and ExoMars TGO/CaSSIS, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-106, https://doi.org/10.5194/epsc2026-106, 2026.

Introduction

The SuperCam instrument onboard the Mars2020 Perseverance rover is a suite of remote sensing instruments that is operating on the Martian surface since February 2021 [1, 2, 3]. It notably includes a Visible-InfraRed (VISIR) spectrometer covering the 385–465 nm, 536–853 nm, and 1.3–2.6 µm spectral ranges [4], which regularly performs observations of the Martian atmosphere using the passive sky geometry to retrieve information on the composition of the atmosphere [5, 6]. At these wavelengths, scattering by aerosols is strongly sensitive to the particle size. The ability of the passive sky technique to retrieve the atmospheric dust content has been demonstrated in the VIS spectral range with MSL/ChemCam [7], and SuperCam is now able to probe for the first time the Martian atmosphere from the ground for both the VIS and near-IR domains, which provides further information on the aerosol properties.

Dust and water ice aerosols play an important role in the current Martian climate: they affect the thermal structure of the atmosphere as they absorb and scatter the incoming sunlight, and play an important role in the global water cycle of the planet [8]. Thus, monitoring the properties of these aerosols is of importance to better understand and model the current Martian climate. On Perseverance, the optical depth of the aerosols above the rover is monitored on a seasonal and local time basis by the MEDA and MastCam-Z (ZCAM) instruments [9-11].

Data & Methods

Regular Passive Sky

By measuring the spectra of the sky luminosity at two different elevation angles, and by comparing the measurement with the results of a multiple scattering radiative transfer model, we are able to retrieve the aerosol properties for both the dust and water ice over more than two Martian years. Here we use the DIScrete Ordinate Radiative Transfer (DISORT) code in version 4 [12] through the pyRT_DISORT Python module [13] to retrieve the respective optical depth of dust and water ice from the VISIR passive sky measurements of SuperCam performed since the beginning of the mission in 2021, and constrain their particle size. We assume asymmetric hexahydra dust particles and droxtals shapes for the water ice crystals, and we use vertical atmospheric profile from the Mars Climate Database version 6.1 [14, 15].

These retrievals complement the ones performed by the rover’s other instruments, notably ZCAM. While it is highly challenging with their measurements to distinguish between dust and water ice contributions in the total optical depth, their results can be directly compared with those from SuperCam, as the wavelength ranges of the two instruments overlap in the visible.

Thus, we assume the total column-integrated optical depth of the aerosols to be the same as the one derived by ZCAM at λ=880 nm, then we perform the retrievals on the relative amount of dust and ice, and on their particle size. In order to address the issue of local minima of the χ² when performing the retrievals, we run the solver on a range of values for the particle sizes, then compare the χ² values of all the runs to select the best fit.

Aerosols-specific Passive Sky

In addition to the regular passive sky performed at two altitudes, an other observation sequence has been implemented and run a few times on Mars with a series of measurements at the same altitude but for different azimuths. This allows to scan the lobe of the phase function of the scattering of the aerosols.

Comparison with MastCam-Z

On sols 1523 and 1563, two coordinated observations between SuperCam and MastCam-Z have been performed to compared the radiances measured by both instruments in the same regions of the sky, using the ZCAM filters whose wavelengths overlap with the SuperCam spectra range. These observations have been designed to investigate a potential straylight effect in the SuperCam passive sky data, but also provide an opportunity to compare and cross-calibrate the atmospheric retrievals between the two instruments.

Conclusion & Perspectives

For more than two Martian years now, the SuperCam instrument has been performing passive sky observations of the Martian atmosphere in the VISIR. Despite being challenging to perform, the aerosols retrievals from SuperCam provide information on the nature and particle size of the Martian aerosols in the lower layers of the atmosphere, and complement the measurements from the other instruments of the rover.

References

[1] Maurice et al. (2021) Nature, 605, 653-658. [2] Wiens et al. (2020) SSR, 217, 4. [3] Cousin et al. (2026) This conference. [4] Fouchet et al. (2022) Icarus, 373, 114773. [5] Bertrand et al. (2022) 7^th MAMO, #1549. [6] McConnochie et al. (2026) This conference. [7] McConnochie et al. (2018) Icarus, 307, 294-326. [8] Haberle et al. (2017) The Atmosphere and Climate of Mars. [9] Toledo et al. (2024) Comm Earth & Env, 5, 717. [10] Smith et al. (2025) Icarus, 425, 116313. [11] Moya-Blanco et al. (2026), EGU 2026, EGU26-8137. [12] Stamnes et al. (2017), Astrophysics Source Code Library. 1708.006. [13] Connour & Wolff (2024) GitHub Repository, v1.2.0. [14] Forget et al. (1999) JGR, 104, 24155-24176. [15] Millour et al. (2024) EPSC 2024, EPSC2024-516.

How to cite: Stcherbinine, A., Bertrand, T., Wolff, M., Lasue, J., McConnochie, T., Montmessin, F., Fouchet, T., Knutsen, E., Lacombe, G., Cousin, A., Gasnault, O., Maurice, S., and Wiens, R.: Retrieving the Properties of Martian Aerosols at Jezero Crater over Two Martian Years using SuperCam Passive Sky Observations, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-871, https://doi.org/10.5194/epsc2026-871, 2026.

Following our report of new cases of extremely long and narrow mid-latitude clouds at Thaumasia Highlands (267°E, 39°S), Alba Patera (250°E, 40°N) and Lyot crater (29.3°E, 50.4°N) (Larsen et al., 2026), we analyze the Planetary Climate Model (PCM) (Forget et al., 1999)  predictions in order to understand the conditions under which they form. For this study, we take into account the predicted water vapor concentrations and temperatures, as well as the wind regime (velocity and direction).

We see that elongated clouds form when wind speeds close to the surface are higher (20 m/s at the origin point) compared to those during spring and summer. The same happens at higher heights, where horizontal wind speeds are far greater during fall and winter. In regards to wind direction and cloud orientation, cloud orientations at Thaumasia and Lyot agree with the predicted wind direction at 10 – 22 km and 5 – 16 km respectively, while at Alba Patera, there is a discrepancy. At the volcano, winds matching the initial cloud orientation are located higher up (∼40-50 km).

Orographic clouds, both on Earth and Mars, often reveal the presence of mountain waves. Since there are many types of mountain waves, and each produces a different type of cloud, identifying a particular type of wave can help understand and deduce certain atmospheric parameters, such as, wind speed and stability. Therefore, in order to see if the formation of these clouds is consistent with the model’s atmosphere, we have used a simple, single layered, two-dimensional model (Houze 2014) to understand the flow over arbitrary “bell shaped ridges” roughly the same shape of the aforementioned obstacles. In order to examine the fundamental properties of the mountain waves, we consider a steady-state, two-dimensional airflow, so that linear theory can be used. Linearazing and combining the equations for an inviscid Boussinesq fluid one can obtain a single equation for the vertical wind speed,  w_zz + w_xx + l²w = 0  (Durran 1986), where l is the Scorer parameter (Scorer 1949).  With the basic state wind profile and static stability, considered to be those upstream, and the aid of the continuity equation (u_x +w_z = 0),  we obtain the streamlines in the steady airflow over the ridges.

Figure 1. (a) Elongated cloud that forms at Warrego Rise (Thaumasia Highlands) on January 5^th 2025 (MY38, Ls=26°). The dot marks the head or beginning of the cloud, while the arrows point at the tail. (b) Streamlines in the steady airflow over an isolated bell-shape similar in size to Warrego Rise for a Scorer value of l=0.00018 m^-1 . The red solid line is the zonal topographic profile of Warrego Rise (x=0 at 267.3°E, 40.4°S).

The streamlines that describe the steady airflow are very dependant on the Scorer parameter and the size of the obstacle. Therefore, vertical displacement is seen to depend very much on the details of the airstream, such as, atmospheric stability and wind speed. Obtaining the vertical stability and wind speeds from the PCM we are able to calculate the vertical displacements and, with the vertical temperature and water abundance profiles, see if clouds similar to the ones observed are obtained.

References

Durran, D.R. (1986) Mountain Waves. In: Ray, P.S. (eds) Mesoscale Meteorology and Forecasting. American Meteorological Society, Boston, MA. https://doi.org/10.1007/978-1-935704-20-1_20

Forget, F. et al. (1999) Improved general circulation models of the Martian atmosphere from the surface to above 80 km. J. Geophys. Res. Planets 104, 24155–24175

Houze, R. A., Jr. (2014) Clouds and Precipitation Associated with Hills and Mountains. In International Geophysics (pp. 369–402). Elsevier. https://doi.org/10.1016/B978-0-12-374266-7.00012-3

Larsen, E. et al. (2026) Orographic elongated clouds in mid-temperate and subpolar latitudes of Mars. I-Observations. Icarus, 445, 116864. https://doi.org/10.1016/j.icarus.2025.116864

Scorer, R. S. (1949) Theory of waves in the lee of mountains. Quarterly Journal of the Royal Meteorological Society, 75(323), 41–56. https://doi.org/10.1002/qj.49707532308

How to cite: Larsen, E., Sánchez-Lavega, A., del Río-Gaztelurrutia, T., and Hernández-Bernal, J.: Interpretation of the extremely long and narrow mid-latitude orographic clouds on Mars, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-536, https://doi.org/10.5194/epsc2026-536, 2026.

The present-day Martian climate is governed by three major cycles: CO₂, water, and dust. Every winter, approximately 30% of the atmospheric condenses to form the seasonal CO₂ caps [1]. In spring, these caps disappear, triggering surface activity such as CO₂ geysers and contributing to the formation of gullies [2,3]. In late spring, the perennial North pole ice cap begins to sublimate, injecting water vapor into the dry atmosphere, forming thin clouds at mid and low latitudes, and millimetric night frost forms at the surface that vanishes by early morning [4].

This climate may have been more pronounced in the past due to the planet's orbital variations, and in particular its obliquity, which varied by as ±15° in the recent past [5]. Climate models show that during high-obliquity periods the polar night would be more extensive, intensifying the CO₂ cycle, and that the amount of water vapor injected from the North Polar cap would be greater due to higher summer insolation [6]. Recent studies [e.g, 7] have notably shown that water ice clouds forming following this summer injection play a key role in Martian paleoclimates: by absorbing solar radiation, they warm the atmosphere, which can then hold more water vapor, producing larger clouds and creating feedback mechanisms that make the Martian atmosphere far more humid than it is today. While most models suggest that the CO₂ cycle was more pronounced than today, a recent study by [8] shows that higher atmospheric humidity could promote the formation of pore-filling ground ice at mid- to low latitudes, thereby limiting the amplification of the CO₂ cycle.

A second effect of these clouds is the warming of the surface. These clouds emit infrared radiation toward the surface, which can be significant during periods of low solar insolation such as winter or nighttime. On present-day Mars, this warming can reach 10 to 15 K at night in equatorial regions [9] Given how significant this effect already is today, the question of its impact on Martian paleoclimates naturally arises given the greater cloud abundance expected under past conditions. Yet it has never been studied to date.

In this presentation, we will discuss the surface warming induced by the downwelling infrared flux emitted by these clouds. We will show in particular that for obliquities above 35°, this warming is large enough that surface temperatures can no longer reach the CO₂ condensation point over most of the Martian surface. As a result, the CO₂ cycle, a defining feature of the present Martian climate, nearly disappears entirely under past high-obliquity conditions. Surface activity driven by CO₂ condensation and sublimation is therefore not representative of recent surface history. In particular, we will show that if CO₂ is indeed the agent responsible for forming the enigmatic Martian gullies, then these features could only have formed during low-obliquity periods below 30°, which is consistent with their young age [10]. Finally, while previous studies suggest that water ice could form within the pore spaces of dusty equatorial regions during high-obliquity periods [e.g., 8,11], we will show that this is unlikely due to surface warming. The presence of water-ice deposits within the pores of the upper equatorial regolith is therefore unlikely, although remnants of past massive ice sheets may still persist today [12,13].

References :

[1] Titus et al., 2017: The CO₂ Cycle, in The Atmosphere and Climate of Mars, Cambridge University Press.

[2] Diniega et al., 2021: Modern Mars’ geomorphological activity, driven by wind, frost, and gravity, in Geomorphology, 380, 107627. DOI : 10.1016/j.geomorph.2021.107627 .

[3] Dundas et al., 2021: Active Mars: A Dynamic World, in Journal of Geophysical Research: Planets, 126(8). DOI: 10.1029/2021je006876.

[4] Montmessin et al., 2017: The water Cycle, in The Atmosphere and Climate of Mars, Cambridge University Press.

[5] Laskar et al., 2004: Long term evolution and chaotic diffusion of the insolation quantities of Mars, in Icarus, 170(2), 343–364. DOI: 10.1016/j.icarus.2004.04.005.

[6] Forget et al., 2017: Recent Climate Variations, in The Atmosphere and Climate of Mars, Cambridge University Press.

[7] Madeleine et al., 2014: Recent Ice Ages on Mars: The role of radiatively active clouds and cloud microphysics, in Geophysical Research Letters, 41(14), 4873–4879. DOI: 10.1002/2014gl059861.

[8] David et al., 2024: The Effect of Ground Ice Redistribution on the Martian Paleo‐CO2 Cycle., in Journal of Geophysical Research: Planets, 130(1). DOI: 10.1029/2024je008398

[9] Wilson et al., 2007: Diurnal variation and radiative influence of Martian water ice clouds, in Geophysical Research Letters, 34(2). DOI: 10.1029/2006gl027976

[10] de Haas et al, 2017: Time will tell: temporal evolution of Martian gullies and palaeoclimatic implications, in Geological Society, London, Special Publications, 467(1), 165–186. DOI: 10.1144/sp467.1

[11] Aharonson et al., 2026: Milankovitch forcing of equilibrium ground-ice on Mars, Icarus, 444, 116772. DOI: 10.1016/j.icarus.2025.116772

[12] Forget et al,  2006: Formation of Glaciers on Mars by Atmospheric Precipitation at High Obliquity, Science, 311(5759), 368–371. DOI: 10.1126/science.1120335

[13] Vos et al., 2026, The Martian mid-latitude subsurface ice is the remnant of a past ice sheet, Communications Earth & Environment. DOI: 10.1038/s43247-026-03418-x

How to cite: Lange, L., Forget, F., Clément, J. B., Naar, J., Vos, E., and David, E.: Surface Warming Induced by the Radiative Effect of Water Ice Clouds in the Late Amazonian: Implications for (Sub)-Surface Activity, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-119, https://doi.org/10.5194/epsc2026-119, 2026.

Introduction

The Martian atmospheric aerosols are mainly composed of mineral dust and/or water ice. Water ice clouds affect both the radiative balance [1] and the water cycle [2], while mineral dust is globally the most abundant component, modifying the thermal structure and atmospheric dynamics [3,4]. It can affect observations from both orbiting satellites and rovers on ground, especially at the dusty season around perihelion, where many storms are initiated at different temporal and spatial scales [5].

Both aerosol types are related, as dust can serve as a base for water ice cloud formation [5]. Therefore, a comprehensive understanding of the Martian atmosphere requires a simultaneous climatology of both components, such as the one presented in this work.

Dataset and methodology

NOMAD is a three spectrometers suite on board the Trace Gas Orbiter (TGO). It has been routinely observing the Martian atmosphere since April 2018 [6], providing detailed atmospheric transmission vertical profiles. Data acquired from its solar occultation (SO) channel are used to retrieve aerosol properties and their vertical distributions in the infrared at high vertical resolution. This is achieved by combining multiple diffraction orders (i.e., wavelength intervals), selected to maximize coverage of the SO spectral range while consistently including an order near 3 μm. This spectral region is key to distinguish between dust and water ice aerosols [6].

Our retrieval strategy builds on [7] and is thoroughly described there. The inferred aerosol characteristics using this methodology are size (effective radius and variance) and composition (proportions of mineral dust and water ice). These magnitudes are used to derive other aerosol magnitudes such as number density, mass loading and mass mixing ratio.

We processed ~3000 SO observations from Ls = 194° (MY34) to the end of MY37, significantly extending the dataset in [7], focused only on the first year of NOMAD observations. We have also developed a scheme to identify cases with mixed aerosol components (dust and ice) from those where there is a single dominant component, and the secondary one is only adding noise to the results.

Results

In contrast to [7], due to our extended dataset we can study latitudinal and interannual comparisons between the different Martian Years, being able to detect the global patterns that govern the distribution of dust and water ice clouds.

We report an average maximum altitude of aerosol detection around 60 km near perihelion (Ls ~ 270°) and 40 km near aphelion (Ls ~ 90°). Variations within these values are sometimes significant, and linked to the latitude of the observations. Our results reveal features such as water ice clouds and signatures of both regional and global dust storms. All four Martian Years show systematic differences of their aerosol properties between aphelion and perihelion, which is consistent with the previous work by [7].

Lastly, a comparison of our results with similar datasets from NOMAD-UVIS [8] and ACS [9] has been performed. This comparison focuses on comparing the extinctions and effective radius retrieved, as well as their seasonal and latitudinal distribution. We have found that NOMAD-UVIS retrieves in average smaller particle sizes than SO, which is more sensitive lo larger sizes. Therefore, both wavelength ranges, ultraviolet and infrared, are complementary to produce a complete climatology of Martian aerosols. This combined information is important for improving climate models, which rely on a priori assumptions about aerosol microphysics, composition, and optical properties.

References:

• Wilson, R.J, et. al., 2008.
• Montmessin, F. et. al, 2004.
• Michael Battalio and Huiqun Wang, 2021.
• Smith, M.D. et. al, 2019.
• Määttänen, A. and Montmessin, F., 2021
• Vandaele, A, C. et. al, 2018
• Stolzenbach, A. et. al, 2023.
• Flimon, Z., et. al, 2025.
• Schterbinine, A. et. al, 2022

How to cite: Gamonal García-Galán, M. Á., López Valverde, M. Á., Brines, A., Stolzenbach, A., Modak, A., Funke, B., Rodríguez Ovalle, P., Alday, J., Thomas, I., Patel, M., Bellucci, G., Daerden, F., Ristic, B., and Flimon, Z.: A 3.5 full Martian Years dust and water ice climatology from Nomad-SO/TGO observations, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-699, https://doi.org/10.5194/epsc2026-699, 2026.

In this work we discuss the dust properties (column densities n_col, effective radii r_eff and optical depths τ) obtained from the analysis of the nadir data of TGO/NOMAD’s UVIS (200 – 650 nm, [1]) and LNO (2200 – 3800 nm, [2]) channels, encompassing Mars Years from 34 to 38. The combination of the two datasets increases the information content related to airborne dust scattering and extinction, hence allowing a more robust constraining of physical properties. Indeed, the separated investigation of UVIS or LNO ranges alone is prone to non-negligible biases in the results, due to the limited spectral information that only partially covers the dynamical change of the dust scattering efficiency between visual and near infrared wavelengths [3]. We perform the retrieval with the MITRA radiative transfer tool [4,3] and take advantage of surface albedo spectra retrieved from MEx/OMEGA [5] spectrometer’s data, processed with a modification of the Surface-Atmosphere-Separation (SAS) method [6,7]. Since the dust grains’ irregular shape is still largely unconstrained, we assume spherical scatterers. Moreover, we exploit the optical constants by [8]. The pipeline behind the coupling of spatially and temporally simultaneous UVIS and LNO observations involves footprint matching, ice filtering (exploiting the conditions from [9]), UVIS spectral range selection (based on the available spectral albedo information and on the presence of calibration/instrumental issues) and LNO data radiometric ad-hoc calibration (accounting for instrumental interferences of electrical origin) and spectral handling for reliably balancing the information content among the two datasets [6].

The results provide information on the integrated atmospheric column and, even if the patchiness of UVIS+LNO dataset only allows a partial coverage of the dusty seasons (Ls > 180°), they give the possibility to discuss the order of magnitude of physical properties and their trends. Extreme dust opacities (τ_2.6 ~ 10, evaluated at 2.6 μm, n_col ~ 10^-9 cm^-2) are observed during both the main (180° < Ls < 240°) and the secondary (320° < Ls < 340°) MY34 dust storms. As expected, the retrieved r_eff appear anti-correlated with τ during the storm, as they provide information only down to those altitudes where the dust layer becomes optically thick (i.e. the lowest layers characterized by larger grains are not sounded). As a result, the storm peak is characterized by smaller particles lifted at higher altitudes (r_eff ~ 1.0 μm) while larger ones are detected at the storm onset and decay phases (r_eff > 1.5 μm). While the dust scenario in MY35 and 36 appears more quiet (n_col ~ 10^-7 cm^-2), MY37 shows enhanced dust activity at Ls > 200° (n_col ~ 10^-8 cm^-2). Finally, MY38 results indicate a return to quiet conditions at Ls < 100° (n_col < 10^-8 cm^-2) with the rest of the dataset currently under processing.

Acknowledgements

ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB- BIRA), assisted by 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), by the Spanish MICINN through its Plan Nacional and by European funds under grants PGC2018-101836-B-I00 and ESP2017-87143-R (MINECO/FEDER), as well as by UK Space Agency through grants ST/V002295/1, ST/V005332/1, ST/Y000234/1 and ST/X006549/1 and Italian Space Agency through grant 2026-6-HH.0. The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS under grant numbers 30442502 (ET_HOME) and T.0171.16 (CRAMIC) and BELSPO BrainBe SCOOP Project. US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canada Space Agency.

References

[1] Patel, M. R., et al., 2017. Appl. Opt., 56(10), 2771–2782

[2] Neefs, E., et al., 2015. Appl. Opt. 54, 28, 8494-8520.

[3] D'Aversa, E., Oliva, et al., 2022. Icarus, 371, 114702.

[4] Oliva, F., et al, 2018. Icarus 300, 1-11.

[5] Bibring, J.P., et al., 2004. ESA SP-1240.

[6] Oliva, F., et al., 2025. EPSC-DPS2025-1416.

[7] Geminale, A., et al, 2015. Icarus 253, 51-65.

[8] Wolff, M.J., et al, 2010. Icarus, 208.

[9] Wolff, M.J., et al., 2019. Icarus, 332, 24-29.

How to cite: Oliva, F., D'Aversa, E., Bellucci, G., Carrozzo, F. G., Thomas, I. R., Ruiz Lozano, L., Karatekin, O., Altieri, F., Daerden, F., Ristic, B., Patel, M. R., Willame, Y., López-Valverde, M. A., Vandaele, A. C., and Ammannito, E.: Mars Years 34-38 dust properties from TGO/NOMAD UVISand LNO channels’ nadir data analysis, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-880, https://doi.org/10.5194/epsc2026-880, 2026.

Dust is a crucial parameter for planetary atmospheres, particularly for Mars. These mineral micrometre-sized particles absorb and diffuse incoming sun rays, which heat the atmosphere locally. Dust also affects many atmospheric processes, such as the circulation, the water cycle, the atmospheric escape, and the CO₂ cycle. In this work, we will present a novel catalogue of local dust storms on Mars. Here, we consider the following dust storm definition: a large and optically thick dust cloud [1]. The dust storms reach different stages during their lifetime: formation, growth/evolution, and dissipation/sedimentation. These storms can be classified by their spatial extension, called local storms (< 1.6 10⁶ km²), regional storms (> 1.6 10⁶ km²) and planetary (global) storms [2]. Questions remain about the storm formation and growth mechanisms, which are relevant to storm forecasting.

While many studies dedicated to dust storms are focused on regional storms [3, 4, 5], studying dust activity at a local scale is key to better understanding the first stages of storm formation and growth. Here, we present our work on detecting dust storms in the OMEGA/Mars Express (near-IR imaging spectrometer) dataset, chosen for its high spatial resolution (0.36-4.8 km/pixel), allowing the detection of small storms. We developed a method to detect automatically storms from dust optical depth data (see Figure 1) computed at 0.9 µm [6] available from late Martian Year 26 to mid MY 30. The storm detections have been confirmed manually and compiled into the OMEGA Dust Storm Catalogue (ODSC) [7]. This catalogue is composed of 448 storm detections, from which ~81% are local storms and ~19% regional ones.


Figure 1: Automatic detections of two dust storms in an OMEGA/Mars Express observation. [Left] and [right] correspond respectively to the dust optical depth map of the observation derived by [6] and the storm detection mask number. Adapted from [8].

Using this catalogue, we studied the spatial, time and diurnal distribution of the detections [8], and we present here a summary of the results. The local storm activity is widespread on Mars (see Figure 2), with favoured areas such as southern polar cap edges, flushing dust storm channels, or Hellas Planitia and Valles Marineris [8]. Most of the storm detections have been made between 10:00 and 18:00, reaching a maximum at 14:00-16:00 [8]. The storm growth mechanisms seem to be less efficient during particular periods of the Martian year, as during the northern solstitial pause (Ls ~240-270°), when we are still detecting many local storms, while the regional storm activity is low. Regional storm detections are mostly concentrated during MY 29 and MY 28, more specifically during the early formation stage of the Global Dust Storm (Ls~265-270°) [8]. This suggests that the lifting and transport processes were stronger and more effective during this period.


Figure 2: Spatial distribution of the OMEGA Dust Storm Catalogue detections [top] and OMEGA/Mars Express observations [bottom] from MY 26 to 30 without the MY 28 Global Dust Storm period (Ls~265-305°). For [top], the circles and hexagons symbolised respectively local and regional storms. Extracted from [8].

This new local and regional storm catalogue will also be useful for preparing observations of future missions such as the Martian Moons eXploration (MMX, JAXA), which will also study the Martian atmosphere [9] from 2027. Due to its equatorial orbit, MMX and its instrumental payload, especially the MMX InfraRed Spectrometer (MIRS, [10]), will give the opportunity to study diurnal variations of the storms, and therefore to constrain the storm formation and growth mechanisms. The ODSC, among other catalogues [3, 4, 5], is notably used to schedule Martian dust storm observations.

References:
[1] Kahre M. A. et al. (2017), Cambridge Univ. Press, The Atmosphere and Climate of Mars, 295-337. [2] Cantor, B. A et al. (2001) JGR, 106(E10), 23653–23687. [3] Battalio M. & Wang H. (2021) Icarus, 354, 114059. [4] Guha B. K. et al. (2024) JGR Planets, 129, e2023JE008156. [5] Lombard T. & Montabone L. (2024), 10th International Conference on Mars, abstract #3041.  [6] Leseigneur Y. and Vincendon M. (2023) Icarus, 392, 115366. [7] Leseigneur Y. et al. (2025) Zenodo, 10.5281/zenodo.17380018. [8] Leseigneur Y. et al. (2026) JGR: Planets, 131, e2025JE009502. [9] Ogohara K. et al. (2021) Earth, Plan. and Space, 74, 1. [10] Barucci M. A. et al. (2021) Earth, Plan. and Space, 73, 211.

How to cite: Leseigneur, Y., Gautier, T., Bertrand, T., Spiga, A., Battalio, J. M., Lombard, T., and Montabone, L.: Chasing the Martian Dust Storms with the OMEGA Dust Storm Catalogue, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-989, https://doi.org/10.5194/epsc2026-989, 2026.

Introduction

Dust is the main radiatively active component of the Martian atmosphere and the key driver of its thermal structure, its dynamics and thus of the water vapor distribution. Modelling the water cycle, in particular its upper atmospheric distribution, therefore relies on a dust scheme that produces a realistic representation of the dust vertical distribution in the atmosphere of Mars.

In the current Mars Planetary Climate Model (Mars PCM) [1], the dust size distribution is assumed to follow a log-normal distribution, characterized by moments . The model scheme uses two of them [2] : M₀ (the number of particles) and M₃ (linked to the dust mass mixing ratio). From these two moments, the dust mean radius can be determined :

However, to permit this calculation, the variance σ₀² is fixed (σ₀= 0.64) everywhere in the model. This is a strong limitation, as sedimentation acts more efficiently on large particles than on small ones, so the size distribution naturally becomes narrower with altitude, and the variance is thus not constant. Since small particles dominate the radiative heating at high altitudes and act as condensation nuclei for water-ice clouds, a better representation of the size distribution, and especially of its variance, is critical for the simulated atmosphere as a whole.

Methods

We overcome this limitation by introducing a third moment, with the new tracer M₂, transported by the model alongside M₀ and M₃. M₂ is initialised from the two existing moments under the log-normal assumption and is then free to evolve. With three moments, the variance is now able to vary in space and time :

The mean radius and the effective radius are now obtained directly from the transported moments, without any prescribed variance :

The new tracer is included in every process that acts on the dust distribution. Sedimentation now redistributes M₀, M₂ and M₃ in a way that depends on the size of the particles, so that the variance naturally decreases with altitude as small particles remain high longer than large ones. The cloud microphysics scheme [3] also needs the implementation of a new condensation nuclei tracer to ensure the conservation. The dust injection scheme is also adapted to provide the three moments in a consistent way. The radiative transfer, which is sensitive to the effective variance, now sees a distribution that varies with altitude and season, rather than a fixed one.

Results

Simulations with the new three-moment scheme show several improvements compared to the two-moment reference. The simulated column dust opacity is slightly closer to the dust climatology of Montabone et al. (2015) [4], both in amplitude and in seasonal distribution. More importantly, the effective radius now decreases with altitude (Figure 1), which is the direct signature of the improved sedimentation process, while we observe the same effect on the effective variance, which is not fixed anymore. This new vertical evolution of the dust distribution impacts the radiative rates and, through it, the water cycle.

The most important result concerns water vapor at high altitudes during the dusty perihelion season. The two-moment scheme underestimates the amount of water vapor observed by ACS (Atmospheric Chemistry Suite) [5] above the lower atmosphere at this season, a long-known issue of the model. With the three-moment scheme, more water vapor reaches the upper atmosphere and the agreement with ACS profiles is significantly improved (Figure 2). The implementation doesn’t only affect the vertical distribution of water vapor, the results also feature the water vapor column-integrated distribution. First results on the D/H cycle are also presented, looking at how the new size distribution changes the fractionation between HDO and H₂O through cloud microphysics and through transport to the altitudes at which water can be photodissociated.

Conclusion

Adding a third moment to the Mars PCM dust scheme is a physically motivated modification of the model that lifts the fixed variance assumption and lets the dust distribution evolve more naturally with altitude and season. It brings the simulated dust opacity and the high-altitude water vapor closer to observations, and opens a new path to study the D/H cycle and water escape in a consistent framework.

Figure 1. Zonal-mean effective radius (top) and effective variance (bottom) as a function of altitude and Ls, simulated with the three-moment scheme during MY33.

Figure 2. Alt–Ls distributions for ACS (top), PCM (middle) and their difference PCM−ACS (bottom) for MY35 with the two-moment scheme (left) and the new three-moment scheme (right)

References

[1] Forget, F., et al. (1999), Improved general circulation models of the Martian atmosphere from the surface to above 80 km, J. Geophys. Res., 104(E10), 24155–24175, doi:10.1029/1999JE001025.

[2] Madeleine, J.‐B., et al. (2011), Revisiting the radiative impact of dust on Mars using the LMD Global Climate Model, J. Geophys. Res., 116, E11010, doi:10.1029/2011JE003855.

[3] Navarro, T., et al. (2014), Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds, J. Geophys. Res. Planets, 119, 1479–1495, doi:10.1002/2013JE004550

[4] Montabone, L., et al. (2015), Eight-year climatology of dust optical depth on Mars, Icarus, Volume 251, 2015, Pages 65-95, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2014.12.034.

[5] Fedorova, A., et al. (2023). A two-Martian years survey of the water vapor saturation state on Mars based on ACS NIR/TGO occultations. Journal of Geophysical Research: Planets, 128, e2022JE007348. https://doi.org/10.1029/2022JE007348

How to cite: Petzold, G., Montmessin, F., Alday, J., Forget, F., Millour, E., and Verdier, L.: Implementing a third moment in the dust scheme of the Mars PCM : impacts on water vapor and D/H, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-977, https://doi.org/10.5194/epsc2026-977, 2026.

Introduction:
The interannual variability of Martian global dust storms remains one of the major questions in Mars atmospheric science. While regional dust storms occur every year during southern spring and summer, only some Martian years exhibit planet-encircling dust storms. Several studies have proposed that this variability may be statistically correlated to Mars orbital angular momentum relative to the solar system barycenter. In particular, Shirley (2015, 2017) proposed that an inertial acceleration, referred to as the Coupling Term Acceleration (CTA), could couple the orbital motion of Mars to its proper rotation and act as a dynamical forcing on the atmosphere. Consequently, this term was used in both numerical and observational studies to investigate whether orbit-spin coupling could contribute to the timing and occurrence of Martian global dust storms (Shirley and Mischna, 2017; Mischna and Shirley, 2017; Newman et al., 2019; Shirley et al., 2020). Here, following Pierron and Forget (2026), we reassess the theoretical framework of this orbit-spin coupling hypothesis by rederiving the inertial forces acting on a parcel of air in a rotating and orbiting planetary frame.

The orbit-spin coupling hypothesis:
In Shirley (2017), the CTA was obtained from the derivation of the velocity of an atmospheric parcel expressed in an inertial frame and then transformed into rotating reference frames associated with the orbital and spin motions of the planet. The resulting acceleration was argued to contain a term proportional to (Ω × ω) × r, where Ω is the orbital angular velocity, ω is the planetary spin angular velocity, and r is the position of the air parcel relative to the planet center. Such a term would represent a direct inertial coupling between orbit and spin. Since it is proportional to the orbital angular velocity, and expressed in terms of the time variation of the orbital angular momentum, it has been hypothesized to provide a dynamical forcing depending on time, that could contribute to the initiation of Martian global dust storms.

As shown in Pierron and Forget (2026),  this term results from an inconsistent use of the transport theorem between rotating frames and should not appear in the correctly derived momentum equation.

Inertial forces and gravitational tides:
We start from the velocity of a parcel of air in the inertial frame,

V = dR/dtR0 + ω × r + v,

where R is the position of the planet center, ω is the planetary spin angular velocity, r is the parcel position relative to the planet center, and v is the parcel velocity in the planet-fixed frame. The standard transport theorem gives

dV/dtR0 = dv/dtR2 + ω × (ω × r) + 2ω × v + dω/dtR2 × r + d²R/dt²R0.

This expression contains only the classical inertial accelerations (centrifugal, Coriolis, and Euler terms). The last term is the translational acceleration of the planet along its orbit. It can be expressed as a function of the orbital angular momentum and its time derivative, but it does not involve the planetary spin and therefore is not an orbit-spin coupling term.

Moreover, this orbital inertial acceleration is not an atmospheric forcing. Applying Newton's second law to the whole planet-atmosphere system gives, to first order,

d²R/dt²R0 = g_ext(R),

therefore, for an atmospheric parcel we have

m g_ext(R + r) - m d²R/dt²R0 = m ∇g_ext(R) · r.

Thus, the orbital inertial acceleration is almost entirely compensated by the external gravitational field. The remaining term is the well known gravitational tide and not a direct orbit-spin coupling acceleration.

Implications for Martian global dust storms:
Our calculations show that the CTA should not be included in the atmospheric momentum budget of Mars general circulation models. Although true orbit-spin coupling exists through gravitational tidal torques, it acts slowly by modifying the spin rate of a planet over long timescales. It does not provide an instantaneous atmospheric forcing capable of explaining the interannual variability of Martian global dust storms. The occurrence of such storms is therefore more likely controlled by atmospheric dynamics, radiative-dust feedbacks, and the spatial and temporal variability of surface dust reservoirs (Mulholland et al., 2013; Newman and Richardson, 2015).

Figure 1: Schematic representation of the reference frames and vectors used in this study. R0 is the inertial barycentric frame, R1 rotates with the orbital angular velocity Ω, and R2 is the planet-fixed frame rotating with spin angular velocity ω. The vectors R, r, and v denote the planet position, the parcel position relative to the planet center, and the parcel velocity in R2, respectively.

References:
Shirley, J. H. (2015). Icarus, 251, 128-144.
Shirley, J. H. (2017). Planetary and Space Science, 141, 1-16.
Shirley, J. H., and Mischna, M. A. (2017). Planetary and Space Science, 139, 37-50.
Mischna, M. A., and Shirley, J. H. (2017). Planetary and Space Science, 141, 45-72.
Newman, C. E., Lee, C., Mischna, M. A., Richardson, M. I., and Shirley, J. H. (2019). Icarus, 317, 649-668.
Shirley, J. H., McKim, R. J., Battalio, J. M., and Kass, D. M. (2020). Journal of Geophysical Research: Planets, 125, e2019JE006077.
Mulholland, D. P., Read, P. L., and Lewis, S. R. (2013). Icarus, 223, 344-358.
Newman, C. E., and Richardson, M. I. (2015). Icarus, 257, 47-87.
Pierron, T., and Forget, F. (2026). Icarus, 450, 116984.

How to cite: Pierron, T. and Forget, F.: No theoretical basis for orbit–spin coupling as a forcing of Martian global dust storms, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-687, https://doi.org/10.5194/epsc2026-687, 2026.

Our solar system’s planetary variety stems from distinct evolutionary paths that also influenced the presence of atmospheres. They represent primary determinants of planetary environments and potential habitability, making their characterization essential in both planetary and exoplanetary science.

Central to this study is having detailed spectroscopic databases, such as HITRAN [1]. Specifically, the Collision-Induced Absorption represents a critical opacity source, most of all in high-density atmospheres. Despite its importance, significant gaps remain in both experimental and theoretical CIA data. To address this, we performed new experimental measurements of H₂+H₂ and H₂+CO₂ mixtures, important for jovian, early martian, and exoplanetary atmospheres, covering previously unexplored temperature and spectral ranges.

We used an experimental setup called PASSxS which can be visualized in Figure 1.

Figure 1: A picture of the experimental setup

It consists of an atmospheric simulation chamber containing the gas or mixture of gases under investigation, which can sustain pressures up to 70 bar. It can be heated up to 550 K and cooled down to 100 K. It also contains a Multi-Pass cell, characterized by an optical path of 3.27 m, coupled with a high-resolution FT-IR spectrometer through a series of transfer optics placed in a vacuum chamber. The maximum resolution achievable is 0.002 cm^-1.

For pure H₂ (99.9999% purity), we measured the fundamental band of the H₂ CIA between 4000 and 5500 cm^-1 at a resolution of 0.05 cm^-1, across seven temperatures (116- 498 K), and various densities. Fig. 2 shows the absorption coefficients measured at each considered temperature and corresponding pressures. The high resolution allowed us to resolve the so-called interference dips, visible on the blow-up of Fig. 2.

Figure 2: CIA absorption coefficients for a pure H₂ gas measured at seven different temperatures

They represent a lack of absorption observed at the same spectral position as the H₂ quadrupolar lines, coming from the interference phenomenon between the induced dipole moments acquired by the H₂ molecules in consecutive collisions [3]. They have been previously observed in other experimental works [4-8] but only at temperatures equal to and less than 300 K. Applying the theoretical profile developed by Kelley et al. [6], we fitted the resolved interference dips at each density to retrieve the intracollisional halfwidth parameter. It exhibited a linear density dependence across all temperatures, as illustrated in Fig. 3 for the Q(1) interference dip at 4155 cm^-1.

Furthermore, the integrated absorption coefficients within the spectral range of the interference dips followed a cubic trend with density.

These results provide new insights into the density-dependence of those features currently unaccounted for in existing theoretical models. A paper containing these new results is currently in preparation.

The next step will be to perform the same high-resolution measurements using a H₂-He mixture to investigate the effect of He on the behavior of those features.

Figure 3: Q(1) intracollisional halfwidth as a function of the density for each investigated temperature.

In the same spectral region, new measurements of the H₂+CO₂ CIA have been performed. We used a total pressure of 12 bar, with 17% of volume mixing ratio (VMR) of CO₂ for six temperatures, from 241 K to 498 K, at a resolution of 1 cm^-1.

While room-temperature measurements have been recently reported [9-10], there is a lack of experimental data for other temperatures.

Fig. 4 shows the measured CO₂-H₂ CIA binary absorption coefficients for all six temperatures explored.

Figure 4: CO₂-H₂ CIA binary absorption coefficients

At room temperature, our experimental data have been compared with the existing experimental data, showing a good agreement. We also compared our results with semi-empirical calculations based on the approach described in [11] at each investigated temperature.

Although the theoretical band shape showed significant discrepancies with the experimental results, the integrated band intensity showed reasonable agreement. Finally, following the method employed in [12], we fitted the experimental BACs with a temperature-dependent exponential profile. This procedure allowed us to obtain a set of wavenumber-dependent coefficients that can be used to calculate the BACs at every temperature inside the investigated range. The wavenumber-dependent coefficients, along with the measured BACs, are now available on the Zenodo platform (https://doi.org/10.5281/zenodo.18327685).

These results, obtained in collaboration with Dr. Tran (LMD, Paris) and published in [13], highlight the necessity for more refined theoretical models, even if they can be used to provide a reasonable estimate of the integrated band intensity.

Future work will focus on high-resolution measurements of H₂-CO₂ mixtures to determine reliable H₂-broadening coefficients for CO₂ lines, which is so far not available at different temperatures.

Acknowledgements:  This work has been developed under the ASI-INAF agreement n. 2023-6-HH.0, and supported by the EMM (Earth Moon Mars) project of PNRR (WP 1500-13)

References

[1] Gordon I. E. et al. (2026) JQSRT, Vol. 353

[2] Terragni J. et al. (2025), JQSRT, Vol. 347

 [3] Van Kranendonk J. (1968), Canadian Journal of Physics, Vol. 46 N.10

[4] J. D. Poll et al (1975), Can. J. Phys., 53, 954

[5] A. R. McKellar et al. (1975), Can. J. Phys., 53, 2060

[6] J. D. Kelley et al. (1984), Phys. Rev. A, 29, 1168

[7] J. P. Bouanich et al. (1990), JQSRT, 44, 4

[8] J. Westberg et al. (2025), Optics Express, 33, 5

[9] W. Fakhardji et al. (2022), JQSRT, Vol. 283

[10] D. Mondelain et al. (2021), JQSRT, Vol. 260

[11] Turbet M. et al. (2020), Icarus, Vol. 346

[12] Tran H. et al. (2024), Icarus, Vol. 422 

[13] Vitali F., et al. (2026), Icarus, Vol. 455

How to cite: Vitali, F., Stefani, S., Piccioni, G., Tran, H., Snels, M., Grassi, D., Boulet, C., Biondi, D., and Boccaccini, A.: Collision-induced absorption in planetary atmospheres: present data and future perspectives, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-889, https://doi.org/10.5194/epsc2026-889, 2026.

Owing to gravitational perturbations from the giant planets, the absence of a large stabilizing moon, and its non-spherical shape, Mars could have experienced large obliquity variations over its history. Numerical simulations suggest that over the past 10 Myr, Mars’s obliquity has spanned a range of ~30°, varying between ~15° and ~45°, with the long-term mean shifting from ~35° to ~25° around 5 Myr ago and superimposed rapid oscillations of up to ~20° on ~100-kyr timescales.

High obliquity increases polar insolation, accelerating the sublimation of surface ice and thereby raising atmospheric water vapor, whereas low obliquity favors cold trapping at the poles and a much drier atmosphere. Because the photolysis products of water vapor act as key catalysts in Martian photochemistry, variations in Mars’s obliquity can strongly influence atmospheric chemistry by modulating the atmospheric water content.

We use a fully coupled 3D photochemistry–radiation–dynamics model, the Mars Planetary Climate Model (PCM), to test this hypothesis and to quantify how Martian atmospheric composition and chemistry respond to obliquity variations over the recent past. A key strength of this class of models is its ability to self-consistently simulate the spatiotemporal distribution of atmospheric water vapor through polar sublimation–condensation and 3D atmospheric transport, as well as the atmospheric CO₂ abundance through the seasonal exchange of CO₂ with the polar caps.

We first evaluate the capability of the model to reproduce the present-day composition of the Martian atmosphere. One-dimensional photochemical models underestimated CO by up to ~85%, a discrepancy that has persisted for more than three decades. The Mars PCM reproduces a much more realistic CO abundance, yielding a global annual mean of ~750 ppmv, close to observed values of 800–960 ppmv. We find that tuning key reaction rates or including heterogeneous chemistry on airborne dust particles can further improve agreement with observations. However, the model simultaneously predicts H₂ abundances more than an order of magnitude higher than observed, transforming the long-standing CO deficit problem into an H₂ surplus problem.

We then simulate the Martian atmosphere across obliquities from 10° to 45°. The results confirm the expected obliquity control on atmospheric water vapor. Near the present-day obliquity, increasing obliquity—and hence atmospheric water vapor—enhances the production of OH, a photolytic product of water vapor and a key atmospheric oxidant, thereby increasing the oxidizing capacity of the atmosphere and reducing the abundance of reduced species such as CO.

At obliquities below ~16°, extremely low polar temperatures lead to the formation of a massive CO₂ polar ice cap, substantially reducing the atmospheric CO₂ column. The weakened UV shielding enhances H₂O photolysis, resulting in a further decline in CO as obliquity decreases.

At high obliquity, rapid H₂O photolysis increases odd-hydrogen radicals by orders of magnitude, but the abundance of H₂O₂, which is derived from odd-hydrogen radicals, remains relatively stable, only modestly higher than present-day levels. This limits the likelihood that extremely elevated H₂O₂ concentrations at high obliquity would have sterilized organic matter produced by ancient life at the surface or in the shallow subsurface.

How to cite: Luo, Y., Lefèvre, F., and Forget, F.: Oscillations in the Composition and Oxidizing Capacity of the Martian Atmosphere Driven by Obliquity Variations, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1203, https://doi.org/10.5194/epsc2026-1203, 2026.

The Faint Young Sun Paradox posits that a modern Earth atmosphere placed on Earth in the Hadea/Archean would result in a snowball world. However, zircon data indicate that there was liquid water present at least 4.4 gigayears ago (Wilde et al. 2001). It is generally accepted that CO2 partial pressures (PP) were higher, but there are few consistent contraints on what that PP was. We use a full-complexity 3-D GCM (ROCKE-3D; Way et al. 2017) with ocean and sea ice dynamics to demonstrate that there are a wide variety of climate states possible in the Eoarchean. We see ice-free states with high CO2 PP, a variety of water belt states for medium CO2 PP, and snowball states with low CO2 PP. We also examine how the faster rotation rate of earth (shorter Length of Day) impact these climate states as well as what lower atmospheric pressures reveal given proxy data indicating pressures as low as 0.23 bar were possible in the Archean (Som et al. 2016). Studies like these not only have implications for characterizing ancient Earth's climate, but also contraining the climates of exoplanetary worlds, which we will touch upon.

Som et al. (2016) Nature Geoscience, 9, 448 http://dx.doi.org/10.1038/ngeo2713
Way et al. (2017) Astrophysical Journal Supplement Series 231:12 https://doi.org/10.3847/1538-4365/aa7a06
Wilde et al. (2001) Nature 409, 175 https://doi.org/10.1038/35051550

How to cite: Way, M., Wolf, E., and Green, M.: Exploring Earth's Eoarchean Climate in the context of the Faint Young Sun Paradox, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-11, https://doi.org/10.5194/epsc2026-11, 2026.

The atmosphere of the planet Venus contains an extensive sulfur cycle, with permanent sulfuric acid (H₂SO₄) clouds and haze layers spanning altitudes from 48 km to ~90 km. H₂SO₄ is formed from SO₂ near the cloud top. Observations of SO₂ in the Venusian atmosphere show concentrations of ~100 ppm below the sulfuric acid clouds and 0.1 – 1 ppb at the cloud top. Known chemistry is insufficient to produce the observed decrease in SO₂ and no chemical models have successfully reproduced the three orders-of-magnitude decrease. The precise cause of the SO₂ decrease remains unknown. Many chemical processes have been proposed that could contribute to the removal of SO₂, including conversion to other sulfur reservoir species such as polysulfur (S₂, S₃, S₄, S₈, etc.) or polysulfur oxides (S_nO); oxidative uptake to droplets by H₂O₂ or O₃; increased uptake due to buffering of the pH of the cloud droplets by dissolved salts, and reaction with NO_x species.

Two possible routes for SO₂ + NO_x chemistry have been postulated: reactive uptake [1] and the lead chamber process [2]. [1] reported single droplet levitation experiments which showed droplet growth only when SO₂ and NO₂ were both present. [2] proposed that NO and NO₂ could catalyse SO₂ conversion to SO₃, leading to conversion to sulfuric acid by reaction with water, in a process analogous to the Lead Chamber process previously used to produce sulfuric acid for industrial use on Earth.

We present a newly developed droplet chemistry module in the Venus Planetary Climate Model, which extends the gas phase chemistry scheme to permit aqueous and droplet surface reactions. Gases condense to the cloud droplets according to their Henry’s law solubilities, and react with other species, which are then released from the droplets when they exceed their solubility limits. We use this new scheme to test both reactive uptake of SO₂ and NO₂ and the “lead chamber” process. In both of these cases, we predict the increases required to the reaction rates or the NO_x reactant concentrations to fully explain the observed SO₂ decrease through the clouds.

References:

• Ubukata, S., et al., Uptake of SO₂ into sulfuric acid droplets through the oxidation by NO₂ under Venus-analogous conditions. ACS Earth and Space Chemistry, 2025. 9(6): p. 1525–1533.
• Sill, G.T., The clouds of Venus: Sulfuric acid by the lead chamber process. Icarus, 1983. 53(1): p. 10–17.

How to cite: Egan, J., Ceragioli, B., Cohen, M., Holmes, J., Plane, J., and Patel, M.: Depletion of Venusian SO2 by liquid phase NOx chemistry, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-491, https://doi.org/10.5194/epsc2026-491, 2026.

Venus provides a unique natural laboratory for the study of rocky exoplanets close to their host stars, where intense irradiation may drive atmospheric conditions analogous to those of Venus. The population of rocky planets has grown significantly with technological advances in the detection of such bodies, and most of them have been discovered orbiting M-type stars. However, the atmospheric characteristics of this population remain an open question. In addition, the detection of Earth-like planets is a major challenge in the search for extraterrestrial life because the main characteristics of Earth-like planets are similar to those found on other terrestrial planets like Venus.

Venus and Earth are regarded as twin sisters due to the many physical characteristics they share (size, mass, density, etc.). However, the main difference lies in their atmospheres. Possible atmospheric constituents such as CO2 are now detectable with JWST [1] but most of species will be corroborated only with new generation of instruments on board the Extremely Large Telescope (ELT) (e.g. ANDES [2] or with the Large Interferometer for Exoplanets (LIFE) [3]). The study of rocky exoplanet atmospheres is strongly complicated by clouds and aerosols. Cloud layers can mask the deeper atmosphere in transmission and emission spectroscopy [4], [5], while cloudy CO₂–N₂ atmospheres may produce similar spectra for both CO₂- and N₂-dominated cases [6]. As a result, observations cannot easily distinguish between Venus-like and Earth-like planets based only on CO₂ and N₂ spectral signatures.

Thus, studying photochemistry is essential because it occurs primarily above the cloud layer, where atmospheric species remain observable. Photochemical processes can produce distinctive spectral signatures that may be detectable remotely and could therefore help distinguish between different types of planetary atmospheres, such as Venus-like and Earth-like worlds. The photochemistry of planetary atmospheres around M-type stars differs from that around the Sun due to a different stellar spectrum and thus different photolysis and photochemical reactions. [7] used 1D photochemical models to study the atmospheres of exo-Venuses orbiting K- and M-type stars. While on Venus, SO2 is heavily depleted in the cloud, on Venus-like planets receiving low UV flux (e.g. from M-dwarf colder stars) the photolysis would be inefficient to convert SO2 into H2SO4 clouds, allowing sulphur species to survive in the upper atmosphere. To date, no 3D studies have been carried out. Tidal-locked rocky planets could exhibit different spectral signatures between the solar and antisolar sides. However, the resulting atmospheric circulation on such planets could also redistribute these species and alter their abundances. Studying 3D photochemistry on Venus analogues around M-dwarf stars will enable us to narrow down the species that may be detected in the future by the JWST or the ELT.

To simulate the photochemistry of planets with physical characteristics similar to those of Venus orbiting M-dwarf stars, we used the Generic-PCM coupled with the photochemical module developed by [8]. This photochemical module is a chemical solver that tracks the evolution of chemical species linked through a network of chemical reactions and photodissociation processes. We included sulfur-bearing species as well as the main reactions involved in the production and loss of H2SO4, following the work of [9]. We adopted synthetic M-dwarf spectra from [10,11] to reproduce the photochemistry of Venus-like planets orbiting this type of star. In addition, we developed a simplified H2SO4-H2O condensation scheme to simulate the condensation of H2SO4-H2O droplets without specific microphysical processes similarly as [12]. This module includes condensation, re-evaporation and sedimentation processes with a fixed particle distribution.

We performed simulations from the surface up to 100 km altitude for Venus-like planets subjected to different M-dwarf stellar spectra. In this work, we investigated how sulfur-bearing species survive above the cloud deck at both the substellar and antistellar points, and how atmospheric circulation influences photochemistry and the spatial distribution of chemical species across the planet. We also evaluated the changes in H2O and H2SO4 production and loss between a Venus analogue orbiting an M-dwarf star and one orbiting the Sun, and finally, identified the chemical species above the cloud layer that could be detected on potential Venus-like planets.

References :

[1] Greene et al. (2023), Nature, doi:10.1038/s41586-023-05951-7

[2] Palle et al. (2023), Experimental Astronomy, doi:10.1007/s10686-025-10000-4

[3] Konrad et al. (2022), Astronomy & Astrophysics, doi:10.1051/0004-6361/202141964

[4] Ehrenreich et al. (2012), Astronomy & Astrophysics, doi:10.1051/0004-6361/201118400

[5] Lustig-Yaeger et al. (2019), The Astrophysical Journal Letters, doi:10.3847/2041-8213/ab5965

[6] Benneke et al. (2012), The Astrophysical Journal, doi :10.1088/0004-637X/753/2/100

[7] Jordan et al. (2021), The Astrophysical Journal, doi:10.3847/1538-4357/ac1d46

[8] Jaziri (2021), doi:10.70675/e18a9ce7z543dz4540zba04zea34b71ab894

[9] Stolzenbach et al. (2023), Icarus, 10.1016/j.icarus.2023.115447

[10] Rugheimer et al. (2013), Astrobiology, doi:10.1089/ast.2012.0888

[11] Rugheimer et al. (2015), The Astrophysical Journal, doi:10.1088/0004-637X/809/1/57

[12] Dai et al. (2022), Journal of Geophysical Research: Planets, doi:10.1029/2021JE007060

Acknowledgments: This work is funded by the Project CNS2024-154576 funded by MCIU/AEI, PN2024 -Research Consolidation-State Subprogram for Training, Attraction and Retention of Research and Innovation Talent - State Program of Human Resources-PEICTI 2024-2027.”

How to cite: Milcareck, G., Gilli, G., Stolzenbach, A., Martinez, A., Mendi, A., and Kozakis, T.: 3D photochemistry of Venus-like planets orbiting M-dwarf stars, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-992, https://doi.org/10.5194/epsc2026-992, 2026.

Exoplanet atmosphere characterization heavily relies on spectroscopic data (as molecular cross-sections or high-resolution line-by-line lists). Despite efforts to obtain comprehensive molecular spectral libraries for exoplanet atmospheres, large gaps remain, particularly for larger molecules and higher frequencies at high spectral resolution (1, 2). One such key example is the methane (CH₄) visible spectrum. CH₄, the simplest hydrocarbon, is a crucial species for exoplanet atmosphere characterization and a possible biosignature in telluric planets (3), but whose complexity has prevented the production of high-resolution linelists at optical wavelengths (4, 5). This is due to the complexity of methane’s visible spectrum (making it computationally impossible to extract its visible linelists through quantum ab-initio calculations) and due to the relatively weak absorption strength of its optical absorption bands (requiring very long pathlengths which are not attainable in experimental set-ups on Earth). This lack of spectroscopic data hampers the exploration of CH₄ atmospheric features at the optical wavelength regime, which will become increasingly relevant as upcoming instruments such as ELT-ANDES or VLT-RISTRETTO will start probing the optical reflection spectra of the atmospheres of close, temperate telluric exoplanets, in search of possible biosignatures (6, 7).

To address this spectroscopic data limitation, we aimed to use Titan as a natural spectroscopy laboratory, using its visible methane absorption bands to extract an empirical high-resolution spectrum of CH₄ optical absorption (8, 9). For this, we observed Titan’s visible spectrum – dominated by CH₄ absorption – at the highest spectral resolution ever with VLT-ESPRESSO (10). From this spectrum we extract a list of all present spectral lines and compare it with a stellar calibration spectrum obtained in the same night with the same instrument (to remove contaminating telluric lines) and with a solar high-resolution spectrum (to remove contaminating solar lines from Titan’s reflection spectrum). Following this analysis we are left with a set of spectral lines originating in absorption occurring on Titan’s atmosphere that is attributable to CH₄ absorption.

Thus, we present here for the first time RRS-2026, an empirical, low-temperature high-resolution (R ∼ 190000) linelist of CH₄ in optical wavelengths, with several thousands of previously unidentified lines. We employ this retrieved CH₄ linelist to model spectroscopic products, such as templates suitable for high-resolution cross-correlation spectroscopy (HRCCS) studies - a first for CH₄ in optical wavelengths. This HRCCS technique is a powerful tool to search for minor chemical compounds in exoplanet spectra, requiring accurate, high-resolution spectral linelists of the chemical species of interest in the observed wavelength range (11).

Our extracted linelist enabled us to perform the first HRCCS detection of CH4 in visible High-Resolution observations of Titan and Jupiter – which demonstrates the applicability of our CH₄ linelist to a diverse set of planetary atmospheres (12). We also use the new retrieved methane linelist to explain past measurements of CH₄ visible bands done in lower spectral resolutions (9, 13). This work sets the stage for the search for CH₄ in exoplanet atmospheres through visible HRCCS using our empirical CH₄ linelist, opening a new window to probe for this crucial chemical species in exoplanet atmospheres. This work also further showcases how Solar System observations provide useful products for exoplanet atmospheres research.

References:

1) Chubb, K., Robert, S., Sousa-Silva, C., et al. 2024, RAS Techniques and Instruments, 3, 636–690, doi: https://doi.org/10.1093/rasti/rzae039

2) Yurchenko, S., Tennyson, J., & Brogi, M. 2025, Nature Reviews Physics, 7, 645–659, doi: 10.1038/s42254-025-00839-z

3) Thompson M., et al, 2022; PNAS, doi.org/10.1073/pnas.2117933119;

4) Boudon, V., Champion, J., Gabard, T., et al. 2009, EPN, 40, 17, doi: DOI:10.1051/epn/2009601

5) Yurchenko, S., Owens, A., Kefala, K., & Tennyson, J. 2024, MNRAS, 528, 3719–3729, doi: https://doi.org/10.1093/mnras/stae148

6) Palle, E., Biazzo, K., Bolmont, E., et al. 2025, Experimental Astronomy, 59, 29, doi: https://doi.org/10.1007/s10686-025-10000-4

7) Martins, J., Santos, N., Figueira, P., & Melo, C. 2016, Orig. Life Evol. Biosph., 46, 487–498, doi: https://doi.org/10.1007/s11084-016-9493-2

8) Rianço-Silva R., et al, 2024, Planetary and Space Sciences, 240, 105836, https://doi.org/10.1016/j.pss.2023.105836

9) Karkoschka, E., & Tomasko, M. 2010, Icarus, 205, 674–694, doi:10.1016/j.icarus.2009.07.044

10) Pepe, F., Cristiani, S., Rebolo, R., et al. 2021, AA, 645, A96, doi: https://doi.org/10.1051/0004-6361/202038306

11) Snellen, I. 2025, ARAA, 63, 83, doi: https://doi.org/10.1146/annurev-astro-052622-031342

12) Irwin, P., Bowles, N., Braude, A., et al. 2019, Icarus, 321, 572–582, doi: https://doi.org/10.1016/j.icarus.2018.12.008

13) P.Giver. 1978, Journal of Quantitative Spectroscopy and Radiative Transfer, 19, 311, doi: https://doi.org/10.1016/0022-4073(78)90064-X

How to cite: Rianço-Silva, R., Machado, P., Sousa Silva, C., Yurchenko, S., Rannou, P., and Tinetti, G.: A high-resolution optical spectrum of methane (CH4) for exoplanet atmospheric science from VLT-ESPRESSO observations of Titan’s atmosphere, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-145, https://doi.org/10.5194/epsc2026-145, 2026.

How to cite: Rengel, M., Rezac, L., Hartogh, P., Jarchow, C., Cavalié, T., and Moreno, R.: JUICE/SWI measurements of HF and HCl in Earth’s Atmosphere during the Lunar Earth Gravity Assist, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-654, https://doi.org/10.5194/epsc2026-654, 2026.