The UVI camera onboard the JAXA Akatsuki mission provided long-term series of images of the Venus cloud tops at 283 nm and 365 nm – two wavelength that correspond to the spectral bands of gaseous sulfur dioxide and unknown UV absorber. We used the automated correlation method to track motions of the cloud features and derive wind speed and its variations for two observation intervals one Venusian year each:  October 2019 – April 2020 (S07) and April 2022 – September 2022 (S11). The mean zonal velocity derived from 283 nm images at noon is by up to 5 m/s higher than the speed measured at 365 nm. Also the afternoon peak of zonal velocity at 283 nm is shifted towards evening terminator with respect to that measured at 365 nm. Zonal velocity increases with phase angle that implies positive altitude gradient of the wind velocity. This might suggest that the radiation at 283 nm forms in slightly higher layers than that at 365 nm that can explain the difference in velocity measured in two spectral bands.    

How to cite: Patsaeva, M., Khatuntsev, I., Ignatiev, N., and Titov, D.: Variability of the cloud top wind speed from the UVI/ Akatsuki imaging at 283 and 365 nm, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-280, https://doi.org/10.5194/epsc-dps2025-280, 2025.

Atomic oxygen is important for the photochemistry in the mesosphere and thermosphere of Venus and can be used as tracer for atmospheric dynamics. The altitude range where it predominantly occurs is between 90 km and 130 km with a peak at 100 km – 110 km. Atomic oxygen is mainly generated through photolysis of CO₂ on the dayside. From there it is transported to the nightside by the subsolar to antisolar circulation. It accumulates near the antisolar point and recombines to molecular oxygen [1, 2 ,3, 4]. The region between 90 km and 120 km altitude is the transition region from superrotation to subsolar-antisolar flow and is not yet well understood. This is the altitude range probed in this work.

We have detected atomic oxygen on the dayside as well as on the nightside of Venus by measuring its ground-state transition at 4.74 THz (63.2 µm) with the upGREAT (German Receiver for Astronomy at Terahertz Frequencies) heterodyne spectrometer on board SOFIA (Stratospheric Observatory for Infrared Astronomy) [5]. This is a direct detection in contrast to most past and current detection methods, which are indirect and rely on photochemical models to obtain atomic oxygen concentrations [1, 2]. We have used this transition to determine Doppler shifts and the corresponding wind velocities. Due to the high spectral resolution of the upGREAT heterodyne spectrometer of 0.2 MHz it is possible to measure the speed at which the atomic oxygen is moving towards the observer [6]. The observations were made on Nov. 10, Nov. 11 and Nov. 13 2021. The 2.5-m diameter telescope of SOFIA was pointing at Venus. The telescope provides a diffraction-limited beam with 6.3 arcsec, which is about five times smaller than the apparent diameter of Venus (29 arcsec). The phase of Venus was 42%.

The 4.7-THz channel of upGREAT has seven pixels in a hexagonal pattern separated by 13.6 arcsec. While most of the pixels were on Venus, three pixels were pointing at its limb. At these positions the component of the wind vector which points towards the observer is sufficiently large to be measured with upGREAT. As a reference we take the transition frequency measured by a pixel which points towards the center of the disk of Venus where the wind vector component towards the observer is negligible. The measurements at 45° and 15° north don’t show a wind speed component which is significantly different from zero (13±38 m/s and 2±31 m/s, respectively) while the measurement close to the south pole exhibits a wind speed component of 120±75 m/s. These values are in agreement with the global circulation model (GCM) of Navarro et al. [6].

For those spectra which are not close to the Venus limb, the wind component towards the observer is too small to be measured by upGREAT. However, the variation of column density and temperature of atomic oxygen may serve as an indicator for the dynamics. When comparing the column density determined by upGREAT with the wind velocity provided by the GCM of Navarro et al. [7] it stands out that on the night side the column density peaks at the time between 19 and 20 hour local time where the gradient of the wind speed is strongest (Fig. 1). This might be an indication of an adiabatic flow of an air parcel in the atmosphere of Venus which leads to an increased density when the wind speed drops sharply.

Fig. 1: Column density of atomic oxygen measured by upGREAT (dots, data from [5]) and zonal wind speeds in the region between 90 km and 130 km (from [6]). The dashed line marks the terminator at the surface of Venus. The grey lines are streamlines showing the circulation (from [7]).

References

[1] A. S. Brecht et al., Atomic oxygen distributions in the Venus thermosphere: Comparisons between Venus Express observations and global model simulations. Icarus 217, 759–766 (2012).

[2] L. Soret et al., Atomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping. Icarus 217, 849–855 (2012).

[3] J.-C. Gérard, Aeronomy of the Venus upper atmosphere. in: Space Sci Rev. Venus III edited by B. Bézard et al., Springer Dordrecht (2017).

[4] G. Gilli, et al., Venus upper atmosphere revealed by a GCM: II. Model validation with temperature and density measurements. Icarus 366, 114432 (2021).

[5] H.-W. Hübers et al., Direct detection of atomic oxygen on the dayside and nightside of Venus, Nature Communications, 14:6812 (2023).

[6] C. Risacher et al., The upGREAT dual frequency heterodyne arrays for SOFIA, J. Astron. Instrum. 7, 1840014 (2018).

[7] T. Navarro et al., Venus´ upper atmosphere revealed by a GCM: I. Structure and variability of the circulation, Icarus 366, 114400 (2021).

How to cite: Hübers, H.-W., Graf, U., Güsten, R., Klein, B., Kührt, E., Stutzki, J., and Wiesemeyer, H.: Wind measurements and dynamics in Venus’ upper atmosphere measured by high-resolution terahertz spectroscopy of atomic oxygen, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-918, https://doi.org/10.5194/epsc-dps2025-918, 2025.

Introduction

In this study which is a follow-up of [1], we use the most recent version of the ground-to-thermosphere VPCM [2] that includes an ionosphere model, ambipolar diffusion and nitrogen chemistry [3]. This tool allows us to investigate and identify in 3D potential mechanisms responsible for the observed variability in Venus’s atmosphere, in the region between 80 km and 130 km altitude.  NO and O₂(Δg) airglows, are commonly used to shed light on the global dynamics and circulation patterns above 90 km. Their characteristics are a combination of horizontal, vertical transport and chemical net reactions [1,4].

We performed sensitivity tests of unconstrained parameters (e.g. gravity waves drag and eddy diffusion vertical profile) to evaluate the impact on the dynamical structures, O number density, temperature and O₂(Δg) nightglow characteristics. These results depict possible scenarios useful to interpret future EnVision observations of trace compounds in those mesosphere layers currently poorly constrained, and not fully explained by current 3D models.

Motivation of this study

Modeling the region between 80 and 130 km, that marks the transition between superrotation regime in the deep atmosphere and the day-to-night circulation in the thermosphere, is a key step towards understanding the processes governing Venusian dynamics. Recent V-PCM improvements presented in [2] and [3] focused on Venus’s atmosphere above 130 km, and performed a comprehensive validation of model results with PVO, Magellan and VEX observations in the thermosphere and ionosphere. However, those model developments, together with ad-hoc tuning of parameters (see [2] for details) to fit observational data, changed dramatically the global circulation in the transition region in comparison to [1], as illustrated in Fig. 1. It was therefore necessary to carry out a follow-up to the study by [1].

Figure 1: Zonal wind around the equator (latitude 20ºS-20ºN) in local time and altitude predicted by the V-PCM in [1] (leftside) and [2,3] (rightside).

Sensitivity tests of unconstrained parameters of gravity waves

Among the processes studied here, gravity waves are an important source of variability, but they remain extremely poorly constrained by observations. Non-orographic gravity waves are generated in the convective layer of clouds. They will propagate upwards where they will eventually break above 90 km, injecting their momentum into the mesosphere/thermosphere, which will affect wind circulation. In this abstract, we present the cases of two key parameters: the initial amplitude of the GW and the dissipation parameter. In our scheme, the dissipation parameter ensures that the waves are dissipated before reaching the top of the model, and is intended to mimic the dissipation of GW at high altitude.

We found that this region (80-130 km) is very sensitive to the unconstrained parameters used in the non-orographic gravity wave parameterization implemented in the V-PCM [2,4]. For instance, the remnant superrotation (~100 m/s) simulated by the V-PCM peaks between 100 and 125 km, depending on the amplitude of non-orographic GW (see Figure 2). Decreasing the initial GW amplitude seems to reduce the retrograde supperrotative component between 90 and 110 km altitude.

Figure 2:  Zonal wind profiles around the anti-solar point simulated with the reference V-PCM [2,3] (in black) and varying the maximum EP-flux amplitude in the GW parameterization by a factor x5 (EPmul5) and /5 (EPdiv5).

Figure 3 shows simulated zonal wind between 80 and 160 km in the equatorial region as function of local time for several GW configurations. By varying this diffusion parameter by a factor of 0.1 to 100 around our reference value, we see that the larger this parameter, the lower the altitude at which gravity waves begin to dissipate, as expected. However, this explored range of Rdiss values had no effect on the horizontal and vertical positioning of the O₂(Δg) emission peak around 100 km. We found, however, a reduction in RSZ remanent around 110 km and larger zonal winds at the terminators with increasing Rdiss (see Fig. 3). This is also associated with an increase in temperature around the antisolar area, linked to an increase in adiabatic heating caused by a stronger downward vertical wind.

Figure 3: Zonal wind in local time and altitude predicted by VPCM tuning GW parameters (Rdiss is the diffusion parameter and EPflux is the amplitude of the non-orographic GW).

Acknowledgements

G.G. and A.M. acknowledge financial support from Junta de Andalucía through the program EMERGIA 2021 (EMC21 00249). AS and LML are funded by the Spanish MCIU, the AEI and EC-FEDER funds under project PID2021-126365NB-C21. IAA-team also acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033. This work was partly funded by project AST22_00001_23 from the European Union- NextGenerationEU, the Spanish Ministry of Science, Innovation and Universities, the Plan de Recuperación y Resiliencia, the Agencia Estatal Consejo Superior de Investigaciones Científicas and the Consejería de Universidad, Investigación e Innovación de la Junta de Andalucía. 

References:

[1] Navarro et al. Icarus, 366:114400, https://doi.org/10.1016/j.icarus.2021.114400

[2] Martinez et al. 2023 Icarus, 389, 115272, https://doi.org/10.1016/j.icarus.2022.115272

[3] Martinez et al. 2024, Icarus 415, 116035, https://doi.org/doi:10.1016/j.icarus.2024.116035.

[4] Gilli et al. 2021, Icarus, 366:114432, https://doi.org/10.1016/j.icarus.2021.114432

How to cite: Martinez, A., Gilli, G., Stolzenbach, A., Navarro, T., Lebonnois, S., González-Galindo, F., Lefevre, F., Streel, N., and Lara, L. M.: Impact of unconstrained parameters on the global dynamics in the “transition region” of Venus atmosphere with the Venus PCM, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1424, https://doi.org/10.5194/epsc-dps2025-1424, 2025.

Introduction

One of the outstanding questions regarding Venus is whether the planet once retained a significant amount of water. Observations of hydrogen atoms provide critical insights into atmospheric escape processes. Previous studies using Venus Express/SPICAV indicate that the Venusian hydrogen atmosphere consists of two distinct components characterized by different scale heights: a hot component and a cold component [1]. The hot hydrogen component primarily arises from charge exchange reactions and momentum transfer between cold hydrogen atoms and ionospheric ions [2]. Conversely, the cold component originates from the dissociation of sulfuric acid in the lower atmosphere. It is well known that, due to the absence of an intrinsic magnetic field, Venusian atmosphere interacts directly with the solar wind. However, it remains unclear whether the Venusian hydrogen corona dynamically responds to variations in solar wind conditions.

Observation

To address this question, we analyzed variations in global hydrogen column densities derived from the brightness of resonantly scattered Ly-α (121.6 nm) and Ly-β (102.6 nm) emissions observed by Hisaki[3-5], solar wind velocities and densities measured by ASPERA-4 on Venus Express[6], and solar UV irradiance at Ly-α and Ly-β wavelengths obtained from the Flare Irradiance Spectral Model (FISM) for Planets[7]. The analysis periods spanned March 9 to April 3, 2014 (Period1), and April 25 to May 23, 2014 (Period2). High-speed solar wind events were confirmed during Period1 but not during Period2.

Result

We derived variations in hydrogen column density at altitudes above approximately 310 km and 90 km from the observed Ly-α and Ly-β airglow brightness. Figure 1 shows that after the arrival of high-speed solar wind originating from a corotating interaction region (CIR) in Period1, the hydrogen column density derived from Ly-α increased by approximately 18% within a few days and subsequently remained nearly constant for several weeks. In contrast, the hydrogen column density derived from Ly-β remained relatively stable throughout the same period. Differences between Ly-α and Ly-β brightness suggest an increase in hydrogen atom abundance at higher altitudes during high-speed solar wind events. In Period 2, when no significant increase in both solar wind velocity and density was observed, there was no clear indication of the arrival of a corotating interaction region. During this period, the hydrogen column density remained nearly constant for both Ly-α and Ly-β.

Figure1 (a and b)Times series of column densities of Venusian hydrogen atoms derived from Ly-α and Ly-β observed by Hisaki respectively. The red line indicates the 1-day moving average. (c and d) Solar wind velocity and density respectively observed by Venus Express.

Discussion

A possible explanation for the observed ~18% variation in Ly-α emission is an increase in high altitude hot hydrogen abundance due to charge exchange reactions and momentum transfer between neutral hydrogen and ionospheric ions. By considering charge exchange between cold hydrogen and ionospheric ions as a production process, and charge exchange between hot hydrogen and the solar wind as a loss process, we estimated the reaction timescales and found consistency with the observed variation. Alternative explanations include an increase in low-altitude cold hydrogen abundance or a rise in hydrogen temperature. These findings provide important implications for understanding non-thermal hydrogen escape mechanisms, thus contributing significantly to our knowledge of the atmospheric evolution of Venus.

[1] Chaufray, J. Y., et al., Icarus, 217, 2, 767, 2012

[2] Hodges, R. R., and E. L. Breig, Journal of Geophysical Research: Space Physics, 96, 7697, 1991

[3] Yoshikawa, I., et al., Space Science Reviews, 184, 237, 2014

[4] Yoshioka, K., et al., Planetary and Space Science, 85, 250, 2013

[5] Yamazaki, A., et al., Space Science Reviews, 184, 259, 2014

[6] Barabash, S., et al., Planetary and Space Science, 55, 12, 1772, 2007

[7] Chamberlin, P. C., et al., Space Weather, 6., S05001, 2008

How to cite: Nose, C., Masunaga, K., Tsuchiya, F., Sakai, S., Kasaba, Y., Yoshikawa, I., Yamazaki, A., Murakami, G., Kimura, T., Kita, H., Chaufray, J.-Y., and Leblance, F.: Influence of the solar wind on the Venusian hydrogen in upper atmosphere observed by Hisaki, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1791, https://doi.org/10.5194/epsc-dps2025-1791, 2025.

Abstract

We present a new derivation of the hot oxygen density in the Venusian upper exosphere from Pioneer Venus Orbiter ultraviolet spectrometer, considering the effect of the solar photons backscattered by the oxygen atoms below ~ 300 km (Venus shine), not included in past analysis. The Venus shine increases the volume emissivity of the hot oxygen corona by ~60% compared to the direct illumination and the O I 130.4 nm brightness measured by PVO with a hot oxygen is reproduced with adensity reduced by 1.6 compared to the density derived without the Venus shine.

Introduction

At the surface of Venus, the atmosphere is predominantly composed of CO₂ but, above ~150 km,  O, generated through the photodissociation of CO₂ [1] and dissociative recombination of O₂⁺ becomes dominant [2]. Solar EUV photons not only dissociate neutral molecules in the thermosphere but also ionize them, driving the formation of Venus’ ionosphere. The dominant ion in the Venusian ionosphere is O₂⁺ produced from reactions between CO₂⁺ and O [3]. The major sink of O₂⁺ is dissociative recombination, which produced two energetic atoms. These energetic atoms were first observed in the exosphere of Venus by the PVO ultraviolet spectrometer (UVS) between ~400 – 1600 km [4]. However, these values were derived considering only the direct solar illumination of the oxygen corona above 400 km as the source of O I 1304 emission not the photons resonantly backscattered by the cold oxygen atoms (Venus shine) [5]. In this work, we include the Venus shine.

Model

We use a radiative transfer model, used in the past to study the Martian hot oxygen corona [6]. This model considers spherically symmetric cold and hot oxygen densities and use a Monte Carlo approach to compute the O I 1304 resonant scattering. Test particles, representative of the solar photons in the 130.4 nm triplet [7] are followed inside the Venusian thermosphere and exosphere. The spectral emission volume rate ε(r,λ,i) at the position r and wavelength λ of each line i of the triplet is the sum of five terms:

ε(r,λ,i)=ε_0,c(r,λ,i)+ε_m,c(r,λ,i)+ε_0,h(r,λ,i)+ε_s,h(r,λ,i)+ε_m,h(r,λ,i)

The two first terms are the single scattering and multiple scattering terms of the cold oxygen. The three last terms are the single scattering (photons scattered for the first time by a hot oxygen atom and not before), “shine term” (photons scattered for the first time by a hot oxygen atom but scattered one or several times by a cold oxygen before) and multiple scattering terms (photons scattered several times by a hot oxygen atom) for the hot oxygen.

Results and discussion

The shine term of the hot population, using the density profiles from [4] and normalized by the g-factor is not negligible compared to the single scattering term on the dayside below 2000 km but decreases faster with altitude (Fig. 1)

Fig. 1 (left) Altitude profile for the single-scattering (blue), multiple-scattering (red), and shine from the cold oxygen (green) volume emission rate (normalized by the g-factor) at SZA=0° of the hot oxygen. The hot oxygen density is indicated by the black line. (right) Variations of the same volume emission rates with the solar zenith angle for an altitude near 600 km.

Above ~400 km, the volume emission rate of the cold population is negligible and the brightness I(z) along the line of sight can be simplified by

I(z) ≈∑∫∫[ε_0,h(r,λ,i)+ε_s,h(r,λ,i)]dsdλ ≈g_exc∫[1+G_c(s)]n_hot(s)ds,

where G_c is the ratio between the “shine” term and the single scattering term, and n_hot(s) the density of hot oxygen.

The simulated brightness using the hot oxygen density derived by [4] with and without the shine term are compared to the observed brightness, showing that when the shine term is considered, the hot oxygen density derived by [4] is not in agreement with the observations. This hot density must be reduced by ~ 1.6 to agree with the observations when the shine term is considered (Fig. 2)

Figure 2. Simulated brightness vertical profile, when the shine term is neglected (solid green line) and included (solid red line) obtained with the hot oxygen density from [3]. The simulated profile including the shine term but with a hot oxygen density divided by 1.6 is also represented (red dashed line). The PVO-UVS data fit from [4] is represented by the black diamonds and the fit from [5] is represented by the black triangles.

This reduced density underscores the need to revisit previous models of hot oxygen corona [8, 9, 10]. Indeed, as shown by [8], the inclusion of non-elastic collisions can lead to a more efficient thermalization. These authors found that the simulated hot oxygen was underestimated compared to the PVO observations [3]. The new inferred density, including the Venus shine, could help to reconcile this simulation with the observation.

Conclusion

We revised the hot oxygen density in the Venusian upper atmosphere derived from Pioneer Venus Orbiter by using an updated radiative transfer model including the Venusian shine emission. This correction increases the excitation frequency of the hot atoms. Then, the oxygen density needed to reproduce the observations is reduced by 1.6 compared to the past derived density. This reduction requires reassessing key assumptions in exospheric models, particularly the roles of inelastic collisions of the hot oxygen with the atmosphere. Indeed, these collisions would increase the thermalization of the hot oxygen and then reduce their density that could match better the new derived density.

Acknowledgements

JYC is supported by the Programme National de Planétologie (PNP, France) of CNRS-INSU co-funded by CNES and Programme National Soleil Terre (PNST, France) of CNRS-INSU co-funded by CNES and CEA

References

[1] Martinez, A., et al. (2023), Icarus, 389, 115272, doi:10.1016/j.icarus.2022.115272

[2] Martinez, A., et al. (2024), Icarus, 415, 116035, doi :10.1016/j.icarus/2024.116035

[3] Fox, J., and Sung, K.Y., (2001) JGR, 106, 21,305-21,336, doi:10.1029/2001JA000069

[4] Nagy, A..F., et al., (1981), Geophys. Res. Lett., 8, 629-632

[5] Paxton, L.J., and Anderson, D.E., (1992), Geophysical Monograph, 66

[6] Chaufray, J-Y et al. (2016), JGR, 121, 11,413-11,421, doi:10.1002/2016JA023273

[7] Gladstone, G.R., (1992), J. Geophys. Res., 97, 19,519-19,525

[8] Gröller, H., et al. (2010), J. Geophys. Res., 115, E12017, doi:10.1029/2010JE003697

[9] Tenishev, V. et al. (2022), JGR : Space Phys., 127 ,doi:10.1029/2021JA030168

[10] Hodges Jr., R.R, (2000), J. Geophys. Res., 105, 6971-6981

How to cite: Chaufray, J.-Y., Carberry Mogan, S., and Deighan, J.: Effect of the planet shine on the corona: Application to the Venusian hot oxygen, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-768, https://doi.org/10.5194/epsc-dps2025-768, 2025.

The existence of carbon dioxide (CO₂) clouds in the Martian atmosphere necessitates extremely low temperatures for formation and was initially observed during polar night at low altitudes. Later observations revealed similar clouds at higher altitudes near the equator, especially during spring and summer [1]. Further evidence has shown their occurrence at northern mid-latitudes and in the southern hemisphere during late autumn. Unlike water vapour clouds, which form from a minor atmospheric component, CO₂ clouds are composed of a major atmospheric constituent. The polar CO₂ clouds are convective in nature. Data from multiple missions indicate that the temperature profiles in the polar regions often align with the CO₂ saturation curve up to 30 km, implying that CO₂ condensation helps regulate these temperatures. Significant cloud opacity between 0 and 25 km altitude also supports the presence of CO₂ clouds.

Figure 1: Formation of CO₂ clouds in the Martian atmosphere [2].

Data from the Pathfinder mission indicate that CO₂ exceeded saturation levels during equatorial descent phases at altitudes near 80 km, implying that CO₂ cloud formation in equatorial regions may occur at significantly higher altitudes compared to polar regions [3]. The genesis of these high-altitude equatorial CO₂ clouds is modulated by conditions in the Martian mesosphere. Notably, mesospheric temperatures can drop well below the CO₂ condensation threshold, particularly near aphelion, when diurnal atmospheric tides promote additional cooling conducive to cloud formation. Furthermore, high-altitude CO₂ cloud formations were detected at solar longitudes between 264° and 330°, located above 90 km in altitude [4]. These clouds exhibit limited horizontal extent, spanning approximately 500 to 700 km.

This study investigates Martian CO₂ cloud formations and their duration during the Northern Hemisphere winter and dust season. For this, we use the open-access data from the Mars Climate Sounder (MCS) on board the Mars Reconnaissance Orbiter (MRO) as well as Mars Express (MEX) and Mars Atmosphere and Volatile EvolutioN (MA VEN) radio occultation (RO) to detect clouds in the atmosphere. We also explore the inter-annual variations to see the impact of dust storms on CO₂ cloud formation.

Figure 2: Examples of MCS temperature profiles (blue) with the CO₂ saturation curve [5].

References:

[1] Määttänen A. et al. (2010), Icarus, 209(2) :452–469.

[2] Mars Climate Modelling Centre. GCM overview: Lecture, November 2021.

[3] Schofield J. T. et al. (1997), Science, 278(5344) :1752–1758.

[4] Jiang F. Y. et al. (2019), GRL, 46(14) :7962–7971.

[5] Mathilde V. (2024), Master Thesis, Université Catholique de Louvain, Belgium.

How to cite: Krishnan, A. and Karatekin, Ö.: Martian CO2 cloud formation as observed by MCS and radio occultation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1352, https://doi.org/10.5194/epsc-dps2025-1352, 2025.

Water vapor has long been a key target in Martian exploration, as its detection confirmed the existence of an active water cycle driven by dynamic exchanges between surface ice reservoirs and the atmosphere. Since its first spectroscopic identification in 1963, ongoing observations—most notably from orbiting spacecraft—have significantly advanced our understanding of the spatial and temporal behavior of water on Mars.

In this study, we present a comprehensive climatology of water vapor column abundances spanning 11 Martian years (MY), derived from observations by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument aboard the European Space Agency’s Mars Express mission [1]. Operating in nadir, SPICAM measures the near-infrared sunlight reflected from the Martian surface and atmosphere, providing daytime water vapor data with broad seasonal and latitudinal coverage. However, due to its reliance on solar illumination, SPICAM is unable to probe the polar night, where water vapor is predicted to be extremely scarce and mostly below the instrument’s detection threshold.

Despite the limitations imposed by the orbital configuration of Mars Express—which results in uneven spatial and temporal coverage—SPICAM has successfully monitored the Martian atmosphere across nearly all seasons and latitudes during local daytime conditions. This long-term dataset offers a unique opportunity to study interannual variability in the water cycle, including responses to major atmospheric perturbations.

Our climatology includes two Martian years that experienced Global Dust Events (GDEs), allowing us to conduct a preliminary assessment of how such planet-encircling storms impact water vapor distribution. We also perform cross-comparisons with water vapor datasets from other past and ongoing missions, addressing a long-standing challenge in reconciling inter-mission measurements.

To enhance the completeness of the climatology, we apply the kriging method—a well-established geostatistical interpolation method based on Gaussian process regression—to estimate water vapor values in regions and seasons with sparse coverage. This gap-filling enables a more continuous picture of the Martian water cycle and facilitates the analysis of year-to-year variability.

Finally, by averaging over the full 11-MY dataset, we construct a reference annual cycle of water vapor on Mars, which serves as a baseline for future comparisons, model validation, and the identification of anomalous behavior.

To further improve the accuracy and vertical sensitivity of water vapor retrievals, we also incorporate results from a synergistic retrieval approach developed by [2] and [3] This method combines simultaneous nadir-pointing observations from SPICAM (in the near-infrared) and the Planetary Fourier Spectrometer (PFS, in the thermal infrared), both aboard Mars Express. Individually, each instrument is sensitive to different portions of the atmospheric column—SPICAM to the lower atmosphere under illuminated conditions, and PFS to higher altitudes through thermal emission. When used together in a joint retrieval framework, they offer a more complete and vertically constrained view of water vapor distribution than either instrument alone.

The synergy method thus yields more accurate water vapor column abundances and enables the first nadir-based estimates of vertical partitioning of water vapor—an aspect traditionally inaccessible to single-instrument nadir retrievals. The resulting composite dataset, which spans almost the entire SPICAM survey, has proven to be highly robust and serves as an important reference for climatological studies. Notably, the synergy also reveals significant discrepancies with predictions from the Mars Climate Database, especially in the northern hemisphere during summer, highlighting potential limitations in current models of water vapor transport and vertical confinement.

Finally, in addition to nadir observations, SPICAM also conducted measurements in solar occultation mode [4], which allowed the retrieval of vertical profiles of water vapor at high vertical resolution (~1–2 km), primarily during the twilight terminator. These observations complement the nadir dataset by providing a window into the vertical structure of water vapor in the lower and middle atmosphere (typically from 10 to 70 km), including its diurnal variations and seasonal evolution. Solar occultation data are especially valuable in characterizing the hygropause altitude, tracking the seasonal ascent and descent of water vapor, and capturing sharp vertical gradients during northern summer, when water transport to high altitudes is most active.

This work not only contributes to a more detailed understanding of Mars’ water cycle dynamics but also provides critical observational constraints for atmospheric models and climate evolution studies.

[1] Montmessin, F., Korablev, O., Lefèvre, F., Bertaux, J.-L., Fedorova, A., Trokhimovskiy, A., et al. (2017). SPICAM on Mars Express: A 10 year in-depth survey of the Martian atmosphere. Icarus, 297, 195–216. https://doi.org/10.1016/j.icarus.2017.06.022 

[2] Montmessin, F., & Ferron, S. (2019). A spectral synergy method to retrieve martian water vapor column-abundance and vertical distribution applied to Mars Express SPICAM and PFS nadir measurements. Icarus, 317, 549–569. https://doi.org/10.1016/j.icarus.2018.07.022

[3] Knutsen, E. W., Montmessin, F., Verdier, L., Lacombe, G., Lefèvre, F., Ferron, S., et al. (2022). Water Vapor on Mars: A Refined Climatology and Constraints on the Near‐Surface Concentration Enabled by Synergistic Retrievals. Journal of Geophysical Research: Planets, 127(5). https://doi.org/10.1029/2022JE007252

[4] Fedorova, A., Montmessin, F., Korablev, O., Lefèvre, F., Trokhimovskiy, A., & Bertaux, J. (2021). Multi‐Annual Monitoring of the Water Vapor Vertical Distribution on Mars by SPICAM on Mars Express. Journal of Geophysical Research: Planets, 126(1). https://doi.org/10.1029/2020JE006616

How to cite: Montmessin, F., Fedorova, A., Verdier, L., Korablev, O., Lefèvre, F., Trokhimovskiy, A., Knutsen, E. W., Lacombe, G., Baggio, L., Giuranna, M., and Wolkenberg, P.: Mars water cycle: an 11 Mars year climatology of water vapor by SPICAM on Mars Express, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-932, https://doi.org/10.5194/epsc-dps2025-932, 2025.

Water is an important component of the atmosphere. Most of the water vapor of Mars exists in its lower atmosphere, including the planetary boundary layer, where the water vapor exchanges between the surface and the atmosphere. The regolith, which is a loose, unconsolidated material consisting of fine dust, sand, and fragmented rock at the planetary surface, has a water-adsorbing property (Fanale & Cannon, 1971; Zent & Quinn, 1995). 1-D model simulations suggest that the water vapor exchange between the regolith and the atmosphere through adsorption affects the diurnal variations of water vapor near the surface and obtain good agreement with lander observations (Zent et al., 1993; Savijärvi et al., 2016, 2019; Savijärvi & Harri, 2021). However, the observation of the daytime water vapor column map suggests that water is not well mixed in the atmosphere, with a significant part of the water vapor column apparently not influenced by the topography (Smith, 2002, Fouchet et al., 2007) as it should be if it were well mixed. This could be explained by an enrichment near the surface, possibly related to water exchanges with the regolith, or by an enrichment well above the surface, for instance, below the cloud level where ice sublimes (Fouchet et al., 2007). This study investigates the effects of the water vapor exchange between the regolith and the atmosphere to explain the observed spatial distribution of the atmospheric water vapor column.

This study uses a Mars Global Climate Model (MGCM) fully coupled with a regolith model. Our MGCM traces the Martian seasonal water cycle, including seasonal water ice caps and frost formation, turbulent flux in the atmospheric boundary layer (Kuroda et al, 2005, 2013), and simple cloud formation based on the large-scale condensation (Montmessin et al., 2004). The regolith model calculates water vapor diffusion, adsorption, and condensation in the regolith, using an adsorption coefficient as a free parameter (Kobayashi et al., 2025). The adsorption isotherm of Jakosky et al. (1997) is used to obtain the adsorbed water amount in the regolith, and the isotherm assumes palagonite on Mars. We examine several adsorption coefficients including zero (only considering pore ice) and the inhomogeneous adsorption coefficient using the regolith property model (Kobayashi et al., 2025). The annual mean water flux in high latitudes, where stable ice tables thermodynamically exist, is less than 10^-10 kg m^-2 s^-1 because pore ice fills pores and prevents water transport. We use the atmospheric water vapor column abundance normalized to a fixed pressure of 610 Pa to remove the effect of topography (Smith, 2002).

The subsurface layers are initialized with the subsurface water amount obtained from a spin-up run without the regolith-atmosphere interaction for approximately tens of thousands of Martian years. With the globally homogeneous adsorption coefficient, the subsurface adsorbed water amount increases with latitudes up to ±60° (corresponding to the annual mean surface temperature of 195 K) and rapidly decreases in higher latitudes. With the inhomogeneous adsorption coefficient, the subsurface adsorbed water amount is strongly controlled by the adsorption coefficient, leading to higher adsorbed water in areas of higher adsorption coefficient. We used those results as initial conditions of the subsurface water amount for the following simulations of the regolith-atmosphere interaction.

Our results show that a larger source of adsorbed water in the regolith supplies more water vapor into the atmosphere, with a small adsorption coefficient dependence. The daytime water vapor normalized to 610 Pa anti-correlates with the surface pressure and with thermal inertia in some seasons. The anti-correlations with the surface pressure and with thermal inertia are very slightly strengthened by the regolith-atmosphere interaction. There results indicate that the regolith adsorption contributes to the not-well mixed condition in the Martian lower atmosphere, and the effect is smaller than expected. The exchanged water flux at the surface in our simulation is approximately 10^-10-10^-9 kg m^-2 s^-1, corresponding to 10^-2-10^-1 pr-µm per 1 sol and increasing polewards, for any adsorption coefficient. Thus, we estimate that the regolith-atmosphere interaction integrated over hundreds of sols affects the change of up to several precipitable microns, and the amount becomes smaller due to transport. The magnitude is small or comparable to the sensitivity of the orbital observations nowadays (Fouchet et al., 2007). We conclude that the anti-correlation of water vapor with the surface pressure and with thermal inertia would not be fully explained by the regolith-atmosphere interaction alone, and it would be necessary to focus on transport near the ground surface.

How to cite: Kobayashi, M., Kuroda, T., Forget, F., Kamada, A., Kurokawa, H., Aoki, S., Kazama, A., Nakagawa, H., and Terada, N.: Effects of regolith-adsorption on the spatial distribution of the atmospheric water vapor simulated with a Mars GCM, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-834, https://doi.org/10.5194/epsc-dps2025-834, 2025.

Homogeneous nucleation has not been considered a possibility in cloud formation processes in the atmosphere of Mars (e.g. Clancy et al., 2017), since Määttänen et al. (2005) made a careful analysis that indicated that extreme supersaturations in the order of 10⁵ were required. Such extreme supersaturations were considered unlikely, especially because the abundant dust in the atmosphere of Mars was expected to deplete water in excess of saturation very quickly by heterogeneous nucleation.

The Arsia Mons Elongated Cloud (AMEC) is an eye-catching and mysterious cloud occurring recurrently every morning during the perihelion season over the Arsia Mons volcano on Mars (Hernández-Bernal et al., 2021). It shows a peculiar elongated shape that in only 3 hours expands up to 1800 km from its origin point. Hernández-Bernal et al. (2022) investigated this cloud based on the LMD Mars Mesoscale model (Spiga and Forget, 2009). The tail of the cloud was not reproduced in the model, but a cold pocket with temperatures down to 30K below the environment and supersaturation up to 105 appeared next to Arsia Mons, in a position, altitude, and local time and season coincident with the origin point of the AMEC in observations. 

In this work we show that these are conditions conducive to homogeneous nucleation, and when we introduce this process as a new cloud formation process in the LMD Mars Mesoscale model, we obtain a good representation of the AMEC, and its long tail.

This provides an excellent explanation for this mysterious cloud and shows that homogeneous nucleation is possible and can have significant effects in the atmosphere of Mars, contrary to the widespread assumptions during the last twenty years of Mars exploration. The finding of supersaturations up to 108 in the surveys performed by Fedorova et al. (2020; 2023) observationally supports that these extreme supersaturations can indeed happen in the atmosphere of Mars and homogeneous nucleation could be happening in other clouds. As a first example, we find that the Perihelion Cloud Trails (Clancy et al., 2009; 2021) could be the result of homogeneous nucleation, as our mesoscale model also predicts cold pockets spatially coincident with locations where Clancy et al. observed cloud trails. We intend to explore these and other clouds on Mars possibly involving homogeneous nucleation.

References:

• Clancy, R. T., Wolff, M. J., Cantor, B. A., Malin, M. C., & Michaels, T. I. (2009). Valles Marineris cloud trails. Journal of Geophysical Research: Planets, 114(E11). https://doi.org/10.1029/2008JE003323 
• Clancy, R., Montmessin, F., Benson, J., Daerden, F., Colaprete, A., & Wolff, M. (2017). Mars Clouds. In R. Haberle, R. Clancy, F. Forget, M. Smith, & R. Zurek (Eds.), The Atmosphere and Climate of Mars (Cambridge planetary science (pp. 76–105). Cambridge: Cambridge University Press. https://doi.org/10.1017/9781139060172.005 
• Clancy, R. T., Wolff, M. J., Heavens, N. G., James, P. B., Lee, S. W., Sandor, B. J., ... & Spiga, A. (2021). Mars perihelion cloud trails as revealed by MARCI: Mesoscale topographically focused updrafts and gravity wave forcing of high altitude clouds. Icarus, 362, 114411. https://doi.org/10.1016/j.icarus.2021.114411 
• Määttänen, A., Vehkamäki, H., Lauri, A., Merikallio, S., Kauhanen, J., Savijärvi, H., & Kulmala, M. (2005). Nucleation studies in the Martian atmosphere. Journal of Geophysical Research: Planets, 110(E2). https://doi.org/10.1029/2004JE002308 
• Fedorova, A. A., Montmessin, F., Korablev, O., Luginin, M., Trokhimovskiy, A., Belyaev, D. A., ... & Wilson, C. F. (2020). Stormy water on Mars: The distribution and saturation of atmospheric water during the dusty season. Science, 367(6475), 297-300. https://doi.org/10.1126/science.aay9522 
• Fedorova, A., Montmessin, F., Trokhimovskiy, A., Luginin, M., Korablev, O., Alday, J., ... & Shakun, A. (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(1), e2022JE007348. https://doi.org/10.1029/2022JE007348 
• Hernández‐Bernal, J., Sánchez‐Lavega, A., del Río‐Gaztelurrutia, T., Ravanis, E., Cardesín‐Moinelo, A., Connour, K., ... & Hauber, E. (2021). An extremely elongated cloud over Arsia Mons volcano on Mars: I. Life cycle. Journal of Geophysical Research: Planets, 126(3), e2020JE006517. https://doi.org/10.1029/2020JE006517 
• Hernández‐Bernal, J., Spiga, A., Sánchez‐Lavega, A., del Río‐Gaztelurrutia, T., Forget, F., & Millour, E. (2022). An extremely elongated cloud over Arsia Mons volcano on Mars: 2. Mesoscale modeling. Journal of Geophysical Research: Planets, 127(10), e2022JE007352. https://doi.org/10.1029/2022JE007352 
• Spiga, A., & Forget, F. (2009). A new model to simulate the Martian mesoscale and microscale atmospheric circulation: Validation and first results. Journal of Geophysical Research: Planets, 114(E2). https://doi.org/10.1029/2008JE003242 

How to cite: Hernandez-Bernal, J., Määttänen, A., Spiga, A., and Forget, F.: Homogeneous nucleation on Mars. An unexpected process that deciphers mysterious elongated clouds, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1600, https://doi.org/10.5194/epsc-dps2025-1600, 2025.

Introduction

Water ice clouds play an important role in the Martian water cycle and climate as they are a major actor in the inter-hemispheric water exchange, and impact the atmospheric structure and temperature by absorbing and scattering the incoming solar radiation [1,2 and references contained within]. Thus, monitoring the spatial and temporal evolution of the Martian water ice clouds, along with their physical properties (crystal effective radius, opacity, and altitude) is of importance to improve our understanding and modeling of the current Martian climate.

Plus, we showed in [3] that the vertical structure of the clouds has a non-negligible impact on cloud optical depth retrievals performed from nadir measurements; and in [4] that the current climate models such as the Planetary Climate Model (PCM) [5] still tends to slightly underestimate the altitude of the water ice clouds. Thus, there is a need for a climatology of the vertical structure of the clouds to improve both the atmospheric models, and the nadir clouds surveys that are the main way to perform spatial and temporal surveys of water ice clouds on Mars [e.g., 6,7,8].

Data & Methods

The Atmospheric Chemistry Suite (ACS) Mid-InfraRed (MIR) channel is a high-resolution spectrometer dedicated to Solar Occultation geometry onboard the ExoMars Trace Gas Orbiter (TGO) ESA-Roscosmos spacecraft [9,10]. This observing geometry provides detailed vertical profiles of the atmospheric transmission. In this study, we use ACS-MIR observations acquired in the so-called position 12 of the secondary grating, which covers the 3.1–3.4 μm spectral range to retrieve the properties of the Martian water ice clouds from their 3 μm absorption band. Following the method described in [11,4], we extract the spectral continuum of the atmospheric transmission at each altitude observed by ACS-MIR (typical vertical resolution of ~2.5 km), then we perform an onion-peeling vertical inversion to retrieve the spectral extinction of each atmospheric layer, and we compare these spectra with models for water ice and dust particles of various sizes to identify the layers where water ice crystals are present, and get constraints on their effective radii.

At the time of the publication of [4] in 2022, we had a dataset containing 514 observations acquired between L_s=163° (MY 34) and L_s=181° (MY 36). Now, with two more years of data, we processed an extended dataset of ~1500 observations running until L_s=72° (MY 38). In addition, we use new data for ACS-MIR observations processed at LATMOS that provide better corrections from instrumental effects, which will help to strengthen again our results.

Results

The ACS-MIR clouds dataset now encompasses four Martian Years (MY 34 to 38), including the Global Dust Storm (GDS) event in the end of MY 34, and three "regular" years (i.e., without GDS) with several local dust storms. This allows us to monitor the behavior of the clouds as a function of season and latitude for several MY, and conduct inter-annual comparisons between the regular MY and the GDS of MY 34.

Figure 1 shows the vertical profiles of the water ice clouds obtained in the equatorial regions from L_s=163° (MY 34) to L_s=72° (MY 38). We can see that the altitude of the clouds typically varies by about 30–40 km over the year: they do not extend over 45 to 50 km around aphelion (Ls ~ 90°) but they can easily reach 80 km around perihelion (Ls ~ 270°). This pattern and altitudes are observed for MY 35 to 38, which highlights the unusual altitudes of the clouds during the GDS, where water ice crystals have been detected up to 100 km at Ls ~ 180°.

Figure 1 – Vertical profiles of water ice clouds in the Martian atmosphere as observed by ACS-MIR over mid-MY 34 (L_s=163°) to the beginnirg of MY 38 (L_s=72°) in the equatorial regions (latitudes between 45°S and 45°N), with their crystal size determined using the method described in [4]. Observations without water ice detections are in gray.

Conclusion & Perspectives

To conclude, we present here the results of the monitoring of the vertical distribution and properties of the Martian water ice clouds over more than three Martian Years by ACS-MIR, since the beginning of the science phase of TGO in April 2018. We now have access to a unique and rich dataset for the Martian clouds and climate science, which allows us to discuss the multi-annual evolution of the Martian water ice clouds, and to derive a new climatology of the vertical structure of the clouds in the atmosphere that will be of significant interest for helping to improve both the climate models and the retrievals performed using radiative transfer algorithm that currently relies on assumptions on the vertical distribution of the ice and aerosols in the atmosphere.

Acknowledgments

ExoMars is a space mission of ESA and Roscosmos. The ACS experiment is led by IKI Space Research Institute in Moscow. The project acknowledges funding by Roscosmos and CNES. Science operations of ACS are funded by Roscosmos and ESA. Science support in IKI is funded by Federal agency of science organization (FANO). Raw ACS data are available on the ESA PSA at https://archives.esac.esa.int/psa/#!Table%20View/ACS=instrument. ACS-MIR level 2B data are available on the LATMOS servers, as described at https://acs.projet.latmos.ipsl.fr/en/data. A. S. also acknowledges funding by CNES.

References

[1] Clancy et al. (2017) The Atmosphere and Climate of Mars, 76–105. [2] Montmessin et al. (2017) The Atmosphere and Climate of Mars, 338–373. [3] Stcherbinine et al. (2025) Icarus, 425, 116335. [4] Stcherbinine et al. (2022) JGR: Planets, 127, e2022JE007502. [5] Forget et al. (2022) 7^th MAMO workshop. [6] Wolff et al. (2022) GRL, 49, e2022GL100477. [7] Smith et al. (2022) GRL, 49, e2022GL099636. [8] Atwood et al. (2024) Icarus, 418, 116148. [9] Korablev et al. (2018) SSR, 214(1), 7. [10] Trokhimovskiy et al. (2015) SPIE, 960808. [11] Stcherbinine et al. (2020) JGR: Planets, 125, e2019JE006300.

How to cite: Stcherbinine, A., Montmessin, F., Baggio, L., Vincendon, M., Wolff, M., Korablev, O., Fedorova, A., Trokhimovskiy, A., and Lacombe, G.: A three Martian years climatology of the vertical properties of Martian water ice clouds TGO/ACS-MIR, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1495, https://doi.org/10.5194/epsc-dps2025-1495, 2025.

Introduction: Isotope ratios in water vapour provide key insights about the history of water on Mars and can help us unravel the fate of the large amounts of liquid water that once existed on the surface of early Mars [1]. A five-fold enrichment of the deuterium-to-hydrogen (D/H) ratio in Martian water vapour with respect to Earth suggests that a substantial amount of the water inventory escaped to space, but more quantitative estimates rely on a rigorous understanding of the relative escape between the light and heavy isotopes (e.g., [2]).

Atmospheric processes such as condensation or photolysis shape the vertical distribution of the water vapour isotopes [3,4] and in turn impact the relative supply of isotopes to the upper atmosphere, where they can escape through thermal and non-thermal processes [5]. Therefore, an in-depth understanding of the vertical distribution of the water vapour abundance and its isotopic fractionation is crucial for reconstructing the escape history of water on Mars.

In this study, we measure and model the vertical distribution of D/H and ¹⁸O/¹⁶O on Martian water vapour using infrared solar occultation observations from the Atmospheric Chemistry Suite (ACS) aboard the ExoMars Trace Gas Orbiter (TGO), together with simulations of the D/H and ¹⁸O/¹⁶O cycles on Mars from the Mars Planetary Climate Model (PCM).

TGO/ACS solar occultation observations: The mid-infrared (MIR) channel of ACS monitors the Martian atmosphere at high spectral resolution (λ/Δλ ≈ 30,000) within the spectral range 2.3-4.2 μm (~2400-4300 cm^-1) in solar occultation mode [6]. To achieve high spectral resolution within the whole range, ACS MIR incorporates a secondary movable grating that allows the simultaneous selection of 7-25 diffraction orders.

In this work, we analyse observations made with secondary grating positions #5 and #11, which allow the selection of diffraction orders covering a spectral range of 3780-3990 cm^-1 and 2650-2950 cm^-1, respectively (see Figure 1). These spectral ranges include the most suitable absorption features to measure the vertical distribution of D/H and ¹⁸O/¹⁶O from the ACS spectra.

We will present the retrieved vertical profiles of D/H and ¹⁸O/¹⁶O using this experimental setup and following a retrieval methodology previously validated for the derivation of other isotopic ratios with TGO/ACS [7,8]. In particular, we will focus our analysis on the observations made close to aphelion (L_S ~ 70˚) and perihelion (L_S ~ 250˚), when the water vapour vertical distribution is particularly different, and discuss the vertical variability of the D/H and ¹⁸O/¹⁶O isotopic ratios during these two distinct seasons. 

Figure 1: Synthetic transmission spectrum of the Martian atmosphere within the spectral range of ACS MIR. The figures shows the instantaneous spectral range covered by ACS MIR when using different secondary grating positions (black dashed lines), as well as the contribution by CO₂ and different H₂O isotopes to the spectrum (coloured lines) [7].

Simulations with the Mars PCM:  Several studies have been conducted to model the variations of the D/H ratio in the atmosphere of Mars (e.g., [3,4,9]), but the variability of the ¹⁸O/¹⁶O isotopic ratio remains largely unexplored.

In this work, we include the condensation-induced fractionation effect of ¹⁸O/¹⁶O to the most recent version of the HDO scheme on the Mars Planetary Climate Model [4,10], aiming to simulate the simultaneous fractionation of both water isotopic ratios. This scheme includes the implementation of equilibrium fractionation due to the different saturation vapour pressures of the water isotopes [11,12], as well as the implementation of kinetic effects due to their different diffusivities [13].

Figure 2 shows the vertical distribution of the D/H and ¹⁸O/¹⁶O isotopic ratios modelled with the Mars PCM at the locations and times of ACS MIR measurements made during the aphelion and perihelion seasons of Martian Year 35. The altitudinal profiles of these isotopic ratios are driven by the condensation of water, either on the ground in the winter hemispheres, or through the formation of water ice clouds, which form at much higher altitudes during the warmer perihelion season than close to aphelion. While the distribution of both isotopic ratios from the model is very similar, the amplitude of the variations in the ¹⁸O/¹⁶O isotopic ratio are much smaller than those in D/H.

We will compare the simulations of the D/H and ¹⁸O/¹⁶O ratios from the Mars PCM with the measured profiles from TGO/ACS, aiming to investigate the relative supply of water isotopes to the Martian upper atmosphere.

Figure 2: Vertical distribution of the water vapour volume mixing ratio, D/H and ¹⁸O/¹⁶O modelled with the Mars PCM at the locations and times of the ACS MIR during the aphelion (top) and perihelion (bottom) seasons in Martian Year 35. The lines in the different panels are coloured based on the latitude of the locations.

 References:

[1] Scheller et al., Science, 2021, eabc7717. [2] Villanueva et al., Science, 2015, 348, 218. [3] Montmessin et al., J. Geophys. Res., 2005, 110, E03006. [4] Vals et al., JGR Planets, 2022, 127. [5] Cangi et al., JGR Planets, 2023, 128, e2022JE007713. [6] Korablev et al., Space Sci. Rev., 2018, 214, 7. [7] Alday et al., Nat. Astron., 2021, 5, 943. [8] Alday et al., Nat. Astron., 2023, 7, 867. [9] Daerden et al., JGR Planets, 2022, 127. [10] Rossi et al., JGR Planets, 2022, 127. [11] Lamb et al., PNAS, 2017, 114, 5612. [12] Majoube, Nature, 1970, 226, 1242. [13] Hellmann & Harvey, JGR Planets, 2021, 126.

How to cite: Alday, J., Patel, M. R., Montmessin, F., Fedorova, A. A., Holmes, J., Petzold, G., Baggio, L., Trokhimovskiy, A., Olsen, K. S., Belyaev, D., Mason, J. P., and Korablev, O.: The vertical distribution of water vapour isotopes on Mars from the Atmospheric Chemistry Suite aboard the ExoMars Trace Gas Orbiter, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-626, https://doi.org/10.5194/epsc-dps2025-626, 2025.

Context : The deuterium/hydrogen isotopic ratio, D/H, is one of the keys to understand the origin of water on terrestrial bodies within the Solar System and its evolution over time in their atmospheres.

In the atmosphere of Mars, this D/H ratio is on average 5 to 6 times higher than the Vienna Standard Mean Ocean Water (VSMOW), the Earth’s oceans reference. Although water vapor is present only in very low quantities on Mars (100 ppmv on average), it is rich in deuterium, which points to a wetter past for the Red Planet, a fact corroborated by various geological indicators (valleys, ancient lakes, shorelines). This enrichment is understood as a cumulative effect of differential escape between H and D atoms, the latter being more prone to gravity than its lighter isotopologue. This differential escape is deeply rooted into the seasonal behavior of HDO and H₂O, sole precursors of D and H on Mars, which release H and D at high altitude, when climatic conditions allow photochemistry. [1, 2]

Model : The Mars PCM (Planetary Climate Model) simulates the physical, chemical and dynamical processes in the Martian atmosphere, including water ice cloud-related phenomena [3], such as condensation, that fully control the relative behavior of HDO [4, 5]. This model, coupled with observations and data from ACS (Atmospheric Chemistry Suite), has shed light on the HDO cycle recently. However, differences still exist between the model and the observations. This is particularly the case for the vertical distribution of water vapor in the upper atmosphere [5, 6]. Some improvements to the Mars PCM, namely new dust injection scheme and non-orographic gravity waves [7] have been implemented.

Results : This study presents a 12-Martian-year simulation, covering Martian Years 26 to 37, with a focus on interannual variations in the HDO and H₂O cycles. The results from the model are compared with multiple observations from instruments such as ACS and SPICAM, capturing a wide range of dust events and revealing key processes to which the HDO and H₂O cycles are particularly sensitive and predicted by the Mars PCM. A third moment is being implemented in the dust particles size distribution to improve its vertical distribution with respect to observations. The first outcomes of this modification will also be addressed.

This study is part of a broader effort to better understand the origin and the long-term evolution of the water on Mars. By investigating the processes behind the Mars’ deuterium enrichment, it contributes to unraveling the history of atmospheric escape and climate change on the Red Planet.

References

[1] Villanueva, G. et al. (2015), Strong water isotopic anomalies in the martian atmosphere: probing current and ancient reservoirs, Science

[2] Owen, T. et al. (1988), Deuterium on Mars: The Abundance of HDO and the Value of D/H, Science

[3] Navarro, T. et al. (2014), Global climate modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds, JGR Planets

[4] Bertaux, J-L. & Montmessin, F. (2001), Isotopic fractionation through water vapor condensation: The Deuteropause, a cold trap for deuterium in the atmosphere of Mars, JGR Planets

[5] Vals, M. et al. (2022), Improved Modeling of Mars' HDO Cycle Using a Mars' Global Climate Model, JGR Planets

[6] Rossi, L. et al. (2022), The HDO cycle on Mars : Comparison of ACS observations with GCM simulations, JGR Planets

[7] Gilli, G. et al. (2020), Impact of Gravity Waves on the Middle Atmosphere of Mars: A Non-Orographic Gravity Wave Parameterization Based on Global Climate Modeling an

How to cite: Petzold, G., Montmessin, F., Alday, J., Verdier, L., Millour, E., Robinson, T., and Robinthal, L.: A twelve-year survey of the HDO and H2O cycles using the Mars Planetary Climate Model from MY26 to MY37, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-957, https://doi.org/10.5194/epsc-dps2025-957, 2025.

Mars is known to host a variety of auroral processes (Bertaux et al. 2005, Schneider et al. 2015, Lillis et al. 2022) despite the planet’s tenuous atmosphere and lack of a global magnetic field. The first detection of visible-wavelength aurora at 557.7 nm was made in 2024 by the SuperCam and Mastcam-Z instruments on the Mars 2020 Perseverance rover (Knutsen et al. 2025 (in press)), which represented the first observation of aurora from any planetary surface other than Earth, the first detection of visible-wavelength aurora at Mars, and demonstrates that auroral forecasting at Mars is possible. During events with higher particle precipitation, or under less dusty atmospheric conditions, green aurorae will be visible to future astronauts. 

 Here we present the results of all detection attempts made to date using the Mars 2020 Perseverance rover. We describe the selected solar storms as they were forecasted, and compare with orbital particle and plasma measurements from MAVEN and Mars Express leading up to and during each event, along with the resulting surface aurora detection attempts.

 A total of eight detection attempts have been made between May 2023 and August 2024. Two of the targeted solar storms led to successful identification of green aurora at Mars with the SuperCam and Mastcam-Z instruments on the Mars 2020 Perseverance rover.

 References

Bertaux, J.-L., et al. "Discovery of an aurora on Mars." Nature 435.7043 (2005): 790-794.

Schneider, N. M., et al. "Discovery of diffuse aurora on Mars." Science 350.6261 (2015): aad0313.

Lillis, R. J., et al. "First synoptic images of FUV discrete aurora and discovery of sinuous aurora at Mars by EMM EMUS." Geophysical Research Letters 49.16 (2022): e2022GL099820.

Knutsen, E. W., et al. (in press), “First detection of visible-wavelength aurora on Mars”, Science Advances (2025).

How to cite: Knutsen, E. W., McConnochie, T. H., Lemmon, M., Viet, S., Cousin, A., Wiens, R. C., and Bell, J. F.:  Green-line aurora detection attempts from the surface of Mars , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1314, https://doi.org/10.5194/epsc-dps2025-1314, 2025.

Argon (Ar) is a chemically inert noble gas used as a tracer to investigate transport processes and energy deposition in the Martian upper atmosphere. Its high atomic mass makes it sensitive to both thermal and non-thermal mechanisms, while its chemical stability ensures that its distribution reflects physical processes occurring in the thermosphere. NGIMS, the Neutral Gas and Ion Mass Spectrometer aboard NASA’s MAVEN spacecraft, has been measuring neutral species including Ar since late 2014, enabling systematic monitoring of the Martian atmosphere from ~150 to over 1000 km in altitude. First studies of Ar densities reflect that below 350 km the population is dominated by thermal processes, whereas above this altitude, detected Ar is primarily attributed to suprathermal components (Mahaffy et al., 2014; Leblanc et al., 2019).

Over its seven years of operation, MAVEN has collected more than 25,000 orbits with NGIMS in neutral mode, including over 1,100 orbits dedicated to high-cadence observations of argon. These targeted campaigns increase vertical resolution and sampling near periapsis and provide enhanced sensitivity up to 1200km. The compiled dataset spans a broad range of local times, solar zenith angles (SZA), seasons (solar longitudes), and latitudes, offering extensive spatial and temporal coverage.

This study investigates the dependence of Ar density on SZA using both standard and high-cadence NGIMS measurements. The resulting profiles indicate systematic variations with SZA, particularly in the 300–1000 km altitude range. Ar densities tend to decrease with increasing SZA, with the strongesat gradients observed above the nominal exobase.

A subset of high-cadence profiles is also examined in greater detail to contextualize vertical structure in terms of MAVEN's orbital geometry, local time, latitude, and season. Particular attention is given to trajectories passes occurring near the morning and evening terminators. These profiles are analyzed individually to assess variability in Ar density structure and its possible relationship to viewing geometry and seasonal conditions. In several cases, localized enhancements in Ar density are detected at altitudes above 700 km. The origin of these features remains under investigation but may be consistent with the influence of waves or perturbations propagating upward from lower atmospheric layers. The apparent asymmetry between dusk and dawn sectors, as previously noted in global maps, is also investigated through this targeted approach.

How to cite: Steichen, V., Leblanc, F. L., and Benna, M.:  Vertical Structure and Variability of Argon in Mars’ Exosphere from High-Resolution NGIMS Data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-800, https://doi.org/10.5194/epsc-dps2025-800, 2025.

Nightglow emissions in the Mars atmosphere provide critical insights into its composition and dynamics, allowing remote evaluation of atmospheric constituents and photochemical processes. The UVIS spectrometer aboard ESA’s ExoMars Trace Gas Orbiter (TGO) recently detected a new emission in Mars' atmosphere—the Herzberg II bands of molecular O₂—particularly intense around the winter poles. Additionally, NASA’s CRISM instrument aboard the Mars Reconnaissance Orbiter (MRO) has recorded the infrared counterpart of this emission: the O₂ (a¹Δ) emission at 1.27 µm.

This study aims to merge these two datasets (UVIS and CRISM) to create the most comprehensive map of nocturnal oxygen emissions on Mars to date. By integrating observations in both the visible and infrared domains, this novel approach will enhance our understanding of oxygen transport and distribution in the Martian atmosphere.

The final dataset will be compared to 3D global circulation models to provide unprecedented constraints on atmospheric dynamics, improving our knowledge of photochemical and transport processes on Mars.

How to cite: Soret, L., Lejeune, S., and Gérard, J.-C.: Mapping of the Atomic Oxygen Nightglow Emissions on Mars in the Visible and Infrared Domains, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-32, https://doi.org/10.5194/epsc-dps2025-32, 2025.

The ExoMars Trace Gas Orbiter has been operating around Mars since April 2018, amassing a wealth of knowledge about the atmosphere and surface of the Red Planet. Onboard are two spectrometer suites, NOMAD and ACS, a surface camera, CASSIS, and a neutron detector, FREND.

NOMAD consists of three spectrometers: two observe in the infrared, in the 2-4 µm region, and one observes the 200-650 nm region. The two infrared spectrometers [1] are named “SO”, which is designed for solar occultation observations, and “LNO”, which is primarily designed for nadir observations, but also can measure solar occultations and occasionally observes Phobos. The ultraviolet-visible spectrometer can do all the above: solar occultation, limb, nadir, and Phobos and Deimos observations [2]. At the time of writing all three spectrometers continue to operate nominally.

In this presentation we will give a summary of recent results and ongoing projects within the NOMAD team: results have recently been published looking at dust and aerosol particle sizes [3], water vapour pumping to high altitudes [4], ultraviolet and visible dayglow [5] and nightglow [6]; plus there are upcoming joint observation campaigns with the iSHELL instrument on IRTF, EUVM on MAVEN, and ACS (also on TGO). Also, the climatologies of the principal atmospheric constituents measured by NOMAD are regularly updated - for example ozone, water vapour, carbon monoxide, carbon dioxide (for temperature and pressure retrievals), hydrogen chloride, water ice and CO2 ice clouds.

Acknowledgements

The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA) with co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). This project acknowledges funding by: the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000140753, 4000140863); by the Spanish Ministry of Science and Innovation (MCIU) and European funds (grants PGC2018-101836-B-I00 and ESP2017-87143-R; MINECO/FEDER), from the Severo Ochoa (CEX2021-001131-S) and from MCIN/AEI/10.13039/501100011033 (grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00); by the UK Space Agency (grants ST/V002295/1, ST/V005332/1, ST/X006549/1, ST/Y000234/1 and ST/R003025/1); and by the Italian Space Agency (grant 2018-2-HH.0). This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS (grant 30442502; ET_HOME).

References

• E. Neefs,. A.C. Vandaele , R. Drummond, I. Thomas, S. Berkenbosch, R. Clairquin, S. Delanoye, B. Ristic, J. Maes, S. Bonnewijn, G. Pieck, E. Equeter, C. Depiesse, F. Daerden, E. Van Ransbeeck, D. Nevejans, J. Rodriguez, J.-J. Lopez-Moreno, R. Sanz, R. Morales, G.P. Candini, C. Pastor, B. Aparicio del Moral, J.M. Jeronimo, J. Gomez, I. Perez, F. Navarro, J. Cubas, G. Alonso, A. Gomez, T. Thibert, M.R. Patel, G. Belucci, L. De Vos, S. Lesschaeve, N. Van Vooren, W. Moelans, L. Aballea, S. Glorieux, A. Baeke, D. Kendall, J. De Neef, A. Soenen, P.Y. Puech, J. Ward, J.F. Jamoye, D. Diez, A. Vicario, and M. Jankowski; NOMAD spectrometers on the ExoMars trace gas orbiter mission: part 1 - design, manufacturing and testing of the infrared channels. (2015) Applied Optics 54 (28), 8494- 8520
• M. R. Patel, P. Antoine, J. Mason, M. Leese, B. Hathi, A. Stevens, D. Dawson, J. Gow, T. Ringrose, J. Holmes, S. Lewis, D. Beghuin, P. Van Donink, R. Ligot, J.-L. Dewandel, D. Hu, D. Bates, R. Cole, R. Drummond, I.R. Thomas, C. Depiesse, E. Neefs, E. Equeter, B. Ristic, S. Berkenbosch, D. Bolsée, Y. Willame, A.C. Vandaele , S. Lesschaeve, L. De Vos, N. Van Vooren, T. Thibert, E. Mazy, J. Rodriguez-Gomez, R. Morales, G.P. Candini, C. Pastor-Morales, R. Sanz, B. Aparicio del Moral, J.-M. Jeronimo-Zafra, J.M. Gomez-Lopez, G. Alonso-Rodrigo, I. Pérez-Grande, J. Cubas, A. Gomez-Sanjuan, F. Navarro-Medina, A. Ben Moussa, B. Giordanengo, S. Gissot, G. Bellucci, and J.J. Lopez-Moreno; The NOMAD spectrometer on the ExoMars Trace Gas Orbiter mission: part 2—design, manufacturing and testing of the ultraviolet and visible channel . (2017) Applied Optics 56(10), 2771-2782
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• L. Soret, H. Robin, J.-C. Gérard, L. Gkouvelis, I. Thomas, B. Ristic, Y. Willame, B. Hubert, A. C. Vandaele, J. P. Mason, F. Daerden, M. R. Patel; The Martian oxygen green line dayglow: response to solar activity. (submitted to Icarus 2025)
• L. Soret, F. González-Galindo, J.-C. Gérard, I. R. Thomas, B. Ristic, Y. Willame, A. C. Vandaele, B. Hubert, F. Lefèvre, F. Daerden, M. R. Patel; Ultraviolet NO and Visible O2 Nightglow in the Mars Southern Winter Polar Region: Statistical Study and Model Comparison. (2024) J. Geophys. Res. Planets

How to cite: Thomas, I., Vandaele, A. C., Trompet, L., Aoki, S., Brines, A., Willame, Y., Piccialli, A., Hendrick, F., Flimon, Z., Daerden, F., Neary, L., Ristic, B., Robert, S., Gérard, J.-C., Mason, J., Lopez Valverde, M. A., Patel, M., and Bellucci, G. and the The NOMAD Team: The ExoMars 2016 Trace Gas Orbiter: Recent Results from NOMAD, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-484, https://doi.org/10.5194/epsc-dps2025-484, 2025.

The observation of temporal and spatial variability in methane concentration on Mars is puzzling, as the current 300-year photochemical lifetime of methane should ensure that it is well mixed throughout the atmosphere (Atreya et al., 2007; Wong et al., 2003). For instance, the spectrometer onboard the Mars Science Laboratory (MSL) rover has observed a significantly higher methane concentration near the surface of Mars than has been observed above 5 km by the instruments aboard the Trace Gas Orbiter (TGO). Near the surface, MSL sees approximately a 0.4 ppbv concentration of background methane that varies on diurnal and seasonal timescales with periodic spikes of over 20 ppbv (Webster et al., 2018, 2021). By contrast, in the middle and upper atmosphere, no methane is observed by TGO to below the detection threshold of ~0.02 ppbv (Korablev et al., 2019; Montmessin et al., 2021).

Previously, these differences were explained by considering the different timing and airmasses being sampled by these measurements – at night and near the surface for MSL and at sunset or sunrise and at altitude for TGO (Moores, et al., 2019). This framework required that the methane decrease from its relatively high surface concentration to a much lower concentration at altitude. While this is possible purely through atmospheric mixing and dilution, if the amount of methane being injected into the atmosphere near the surface is sufficiently large this methane will eventually build up in the upper atmosphere.

In this work, we consider the effect of a recently quantified destruction mechanism described by Zhang et al., (2022). Their derived destruction rates using a simplified model of the atmosphere suggest that methane adsorbed onto a surface with UV-activated perchlorate can contribute to the destruction of methane on the order of hours to days. This model is appealing for several reasons: (1) it can rapidly destroy methane, (2) it operates only during the day when both MSL and TGO observe methane to be low and not at night when MSL observes methane concentration to be high, and (3) destruction occurs on the surfaces of dust grains which are in abundance in the lower atmosphere. To evaluate the effects of this destruction mechanism with a more realistic representation of the Martian atmosphere, we couple its effects into a martian vertical diffusion model (VDM) that was previously developed by Walters et al (2024).  

Walters et al. (2024) previously investigated methane flux from the surface of Mars in a 1-D VDM that incorporated eddy diffusivity coefficients derived from GEM-Mars, a global climate model, to model the effects of transport and diffusion of methane in the VDM (Neary & Daerden, 2018; Stroud et al., 2005). The methane is injected at the surface, and the methane mixing ratio is tracked throughout the vertical column using 1 m layers in half-hour time steps for select sols. While Walters et al. (2024) were able to closely replicate the required flux to replicate the SAM-TLS measurements, the diffusion and removal at the top layer was insufficient to decrease methane below 0.02 ppbv at the top of their model at 5 km above the surface.

As part of the inclusion of a destruction mechanism in the VDM simulation procedure (Fig. 1), we made the following changes to the model of Walters et al. (2024): (1) we incorporated a Conrath profile to describe the dust distribution in the atmosphere (Conrath, 1975), (2) we added in a radiative transfer model to estimate the available UVC for the destruction mechanism (Smith & Moores, 2020) at each step in the VDM, and (3) we also added a simple box model on top of the VDM to track the methane concentration in the portion of the atmosphere that is visible to TGO. Diffusion into this upper box is dependent on the relative concentration of methane in the top layer in the VDM and the box.

Figure 1. Plots showing the concentration of methane in the atmosphere repeated over L_s=180^o in MY 35 between 0 and 6000 m above the surface. The y-axis shows the height in the simulation, and the x-axis shows the time during the sol after a 1-day spinup. On the left, we show the simulation using the method produced by Walters et al. (2024) where methane can build up in the atmosphere if methane is continuously provided into the system. This model demonstrates the effect of diffusion alone, where there is a build up at night, and diffusion throughout the atmosphere. On the right, we show the same simulation with the inclusion of the destruction mechanism from Zhang et al. (2022).

We report our preliminary results incorporating the effects of oxidation of activated perchlorate surfaces as our methane destruction mechanism for select sols and compare them to the available methane measurements.

References

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How to cite: Dong, E., Moores, J., Walters, M., Bischof, G., Martínez, G., and Gordon, M.: Diffusion of Martian Methane in Concert with Destruction Via Adsorption Onto UV-Activated Perchlorate in Martian Dust, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-370, https://doi.org/10.5194/epsc-dps2025-370, 2025.

Introduction: Atmospheric hydrogen chloride (HCl) was first detected in the martian atmosphere in 2021 by the ExoMars Trace Gas Orbiter (TGO) [1], and more recently also studied by ground-based telescopes [2]. Observed HCl occurs in abundances of parts per billion by volume (ppbv) and shows strong seasonal and spatial variation. The vast majority of confirmed detections have been during Mars’ dustier perihelion season and there is a notable paucity of detections during the clearer aphelion season; HCl abundances show steep increases/decreases at the corresponding equinoxes [3,4].

When confirmed to be present, observed atmospheric HCl shows a bias towards higher abundances in the southern (summer) hemisphere. Measured HCl profiles also display vertical structure across their retrieved altitudes (generally 10-50 km above the surface), including sharp drops in abundance coinciding with the presence of water ice clouds [5].

Analysis of observed profiles has indicated a weak positive correlation between HCl abundance and dust loading and a stronger positive correlation between HCl and water vapour abundance [6]; similar correlations are apparent from analysis of ground telescope measurements [2].

Consideration of the observed sharp temporal and spatial gradients in HCl, together with its apparent relationship to aerosol and vapour abundances, has led to the idea that heterogeneous chlorine chemistry might explain these strong variations. This is supported by modelling of gas-phase chlorine chemistry in a Mars global climate model (GCM), which shows that in the absence of heterogeneous reactions the HCl distribution remains more uniformly mixed in time, latitude, and height [7].

Candidate heterogeneous chlorine chemical reactions, involving dust and water ice aerosol, have recently been incorporated into both 1D models [8,9] and a GCM [10]. The resulting modelled HCl profiles and global distributions show a marked improvement relative to observations and are able to reproduce important aspects of the observed distribution including the southern hemisphere bias, enhanced abundances during the perihelion season, and strong vertical structure. However, modelling also reveals features which either conflict with observations or have not yet been observed, and the exact nature of the significant reactions involved remains an open question.

Results & Discussion: We discuss results of our recently published GCM modelling work [7,10] and the gas-phase and heterogeneous reactions involved. We also discuss results from ongoing GCM modelling to better constrain the nature of proposed heterogeneous chlorine reactions.

Figure 1. Comparison of atmospheric HCl profiles for TGO/ACS observations, gas-phase only model outputs, and heterogeneous model outputs. All model data is masked to best match the observation times and locations. Subplot (a) shows mean (solid lines) and population standard deviation (shaded area) for all observations/data within the perihelion seasons of MY 34–36. Subplots (b–d) show comparisons for individual observed profiles in MY 34–36, with error bars representing standard errors. Adapted from [10].

Our results show that gas-phase chlorine chemistry alone cannot reproduce the observed global HCl distribution in a GCM [7]. Inclusion of idealised heterogeneous chlorine chemistry improves representation of HCl abundance and is able to reproduce key features of the observed distribution, suggesting a crucial role for heterogeneous chemistry in the contemporary chlorine cycle on Mars [10]. Figure 1 displays selected model profiles (with and without the idealised heterogeneous reactions) compared to TGO observations, showing the more realistic vertical structure obtained when heterogeneous chemistry is considered.

Further investigation of more specific and realistic potential heterogeneous reactions, adapted from [8] and [9] for our GCM, indicates that some reactions are more plausible than others as significant drivers in the contemporary Mars chlorine cycle. In the wait for further laboratory and observational work to better characterise these reactions under martian conditions, we discuss which reactions are more likely to be prominent based on analysis of their structural effects.

References: [1] Korablev et al. (2021). Science. 7(7). [2] Aoki et al. (2024). Planet. Sci. J. 5 (158). [3] Olsen et al. (2021). Astronom. & Astrophys. 647 (161). [4] Aoki et al. (2021). Geophys. Res. Lett. 48 (11). [5] Luginin et al. (2024). Icarus. 411 (115960). [6] Olsen et al. (2024). JGR Planets. 129 (8). [7] Rajendran et al. (2025). JGR Planets. 130 (3). [8] Krasnopolsky (2022). Icarus. 374 (114807). [9] Taysum et al. (2024). Astronom. & Astrophys. 687 (191). [10] Streeter et al. (2025). Geophys. Res. Lett. 52 (6).

How to cite: Streeter, P., Lewis, S., Patel, M., Rajendran, K., and Olsen, K.: Gas-phase and heterogeneous chlorine chemistry in the contemporary martian atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-571, https://doi.org/10.5194/epsc-dps2025-571, 2025.

One of the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission was to search for previously undetected trace gases that could be diagnostic of active geology or a biosphere [1]. The first such gas was hydrogen chloride (HCl) [2], detected with the mid infrared channel of the Atmospheric Chemistry Suite (ACS MIR) [3]. The presence of HCl on Mars was expected to be an indication of active magmatic processes. However, HCl was found to be widespread and we quickly identified a pronounced seasonal cycle in HCl [4-7]. These aspects indicated that its behaviour was mainly governed by strong photochemical interactions linked to water vapour. The original source of HCl, its sinks, and how its abundance is regulated over time remain a mystery.

ACS is a suite of three spectrometers used to study the composition and chemistry of the Martian atmosphere in detail. ACS MIR is a cross-dispersion spectrometer operating in solar occultation geometry, which provides excellent sensitivity to weak absorption signatures and vertical structure. HCl was discovered in data from shortly after the 2018 Mars Global Dust Storm (solar longitude 220° in Mars year 34) and its signal disappeared shortly after the late season storm that occurred around solar longitude (Ls) 220°. A similar trend has since been observed in the following Mars years (MYs), with HCl returning alongside warm atmospheric temperatures, increasing water vapour content, and dust activity - all driven by southern summer occurring at perihelion. Modeling work to define HCl behaviour [8-10] leave the unanswered question: if HCl has a limited photochemical lifetime, what sources are replenishing atmospheric chlorine?

In MY35, two exceptional observations were made in northern summer, near aphelion, to the north of Alba Mons, on the northern side of the elevated Tharsis plateau [4,6,7]. No other HCl detections have been made during the aphelion period, during southern fall and winter, during which the entire Mars atmosphere is characterised as cold and dry, and has limited dust activity. We performed a dedicated search for HCl using ACS in MY 37. As presented here, several observations detected HCl again in the Alba Mons area. The atmospheric conditions below 10 km are not inconsistent with those during perihelion, with the northern summer raising atmospheric temperatures, introducing water vapour, and limited dust aerosols. However, HCl detections remain rare, it is not well-mixed, and its residence time is short. Many ACS observations made at other latitudes and other times exhibit very good viewing conditions and similar atmospheric states, but no HCl absorption features.

This leads to the idea that perhaps there is a localized source of HCl over the region. We have used high resolution imagery recorded by TGO's Colour and Stereo Imaging System (CaSSIS), Mars Reconnaissance Orbiter's (MRO's) High Resolution Imaging Science Experiment (HiRISE), and MRO's Context Camera (CTX) to characterise the terrain beneath the ACS HCl detections over the Alba Mons region. Below some of the detection sites, we identified linear swarms of tens to hundreds of fissures less than 1 m wide and up to hundreds of meters long, unreported in previous works, which affect the ice-rich latitude-dependent mantle and the periglacial polygons east and north of Alba Mons. They are therefore thought to be younger than 1 million years [11]. Some are located on top of linear uplifts marked by bright and dark deposits, denoting possible diffusion of chemically modified subsurface fluids to the surface. The swarms are aligned with Tantalus Fossae, and in particular, Phlegeton Catena, above which similar fissures are observed and the other HCl detections were made.

1. Vago, J. et al. (2015). ESA ExoMars program: The next step in exploring Mars. Solar Syst. Res., 49(7), 518–528.

2. Korablev, O. et al. (2021). Transient HCl in the atmosphere of Mars. Sci. Adv., 7(7), eabe4386.

3. Korablev, O. et al. (2018). The atmospheric chemistry suite (ACS) of three spectrometers for the ExoMars 2016 trace gas orbiter. Space Sci. Rev., 214(1), 7.

4. Olsen, K. S. et al. (2021). Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season. Astron. Astrophys., 647, A161.

5. Aoki, S. et al. (2021). Annual appearance of hydrogen chloride on Mars and a striking similarity with the water vapor vertical distribution observed by TGO/NOMAD. Geophys. Res. Lett., 48(11).

6.Olsen, K. S. et al. (2024b). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 1. Climatological outlook. J. Geophys. Res., 129(8).

7. Olsen, K. S. et al. (2024b). Relationships between HCl, H2O, aerosols, and temperature in the Martian atmosphere: 2. Quantitative correlations. J. Geophys. Res., 129(8).

8. Krasnopolsky, V. A. (2022). Photochemistry of HCl in the Martian atmosphere. Icarus, 374, 114807.

9. Taysum, B. M. et al. (2024). Observed seasonal changes in Martian hydrogen chloride explained by heterogeneous chemistry on atmospheric dust and ice. Astron. Astrophys., 687, A191.

10. Streeter, P. M., et al. (2024). Global distribution and seasonality of Martian atmospheric HCl explained through heterogeneous chemistry. Geophys. Res. Lett., 52(6).

11. Schon, S. C. et al. (2012). Recent high-latitude resurfacing by a climate-related latitude-dependent mantle: Constraining age of emplacement from counts of small craters. Planet. Space Sci., 69, 49–61.

How to cite: Mège, D., Trokhimovskiy, A., Korablev, O., Olsen, K., Fedorova, A., Guerlet, S., Taysum, B., Tesson, P.-A., Luginin, M., and Montmessin, F. and the HCl aphelion detection team: On HCl in the Martian atmosphere during northern summers, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-985, https://doi.org/10.5194/epsc-dps2025-985, 2025.

In this presentation, we will show efforts made to include accurate photochemical modelling of hydrogen chloride (HCl) and ozone (O₃) in the Mars Planetary Climate Model in order to reconcile recent observations.

The ExoMars Trace Gas Orbiter (TGO) has detected and characterised trace gases in the Martian atmosphere over several Mars years. With its data, upper limits of potential constituents have been constrained, the accuracy of species’ concentration measurements has been improved, and seasonal and spatial variations in the atmosphere have been observed. The wealth of data obtained has addressed several open questions about the nature of Mars’ atmosphere, while other measurements have revealed much that remains poorly understood. For example, models continue to struggle to reproduce ozone distributions, both spatially and temporally, as well as seasonal variations in atmospheric oxygen (O₂), suggesting that some key photochemical interactions may be being overlooked. As another example, despite seven years of dedicated observations producing very low upper limits on atmospheric methane levels, there remains no unifying hypothesis that simultaneously explains the detections reported by other Mars assets at Gale Crater [e.g., 1-4].

Hydrogen chloride—the first new gas detected by TGO [5,6]—has been investigated recently using the mid-infrared channel on TGO’s Atmospheric Chemistry Suite (ACS MIR) [7,8]. Observations show a strong seasonal dependence of HCl in the atmosphere, with almost all detections occurring during the latter half of the year between the start of dust activity and the southern hemisphere autumnal equinox. There are also unusual measurements of HCl, localised in both time and space, during the aphelion season. Chlorine-bearing species such as HCl are important to understand in the Mars atmosphere because on Earth they are involved in numerous processes throughout the planetary system, including volcanism, from which HCl on Earth ultimately originates. Further, chlorine species play a key role in atmospheric chemistry: they influence oxidative chemistry and variations in the aforementioned O₂ and O₃ concentrations (e.g., by catalysing the destruction of ozone), and by extension, potential CH₄ in the Martian atmosphere [9]. However, much remains unknown about original source and sinks of HCl, as well as the factors controlling its distribution and variation.

Here, we use the Mars Planetary Climate Model—a 3-D global climate model that includes a photochemical network—to investigate potential mechanisms accounting for patterns in ozone and HCl detections and interactions between them. We begin with the role of heterogeneous chemistry involving ice and dust aerosols, by implementing modelling developed for the Open University Mars Global Climate Model [10] and building on existing chlorine photochemical model networks [11,12,13]. Heterogeneous chemistry affects the abundances of oxidative species such as OH and HO₂, and by extension, O and O₃. In addition, we investigate how such processes can potentially serve as a mechanism for direct release and sequestration of HCl from the atmosphere. We also explore potential mechanisms behind the annual occurrence of spatially-constrained aphelion HCl, including volcanic sources, and we investigate the interplay between chlorine-bearing species and OH, HO₂,O, and O₃. Figure 1 shows the way that HCl appears during spring and summer in the southern hemisphere (solar longitudes 180-360°) when water vapour is present in the Martian atmosphere. Ozone behaves in the opposite manner and is present when water vapour abundances are low. As shown, these species are anti-correlated; we explore the important chemical pathways connecting them.

Understanding the role of oxidative chemistry on HCl and other trace gases is key to achieving a more complete picture of processes occurring in the present-day Mars atmosphere, as well as processes that have shaped its evolution and habitability.

Figure 1: Observations of CO, O₂, O₃ and HCl seasonally and across multiple Mars Years. Upper panel: CO and O₂ observations from Curiosity’s Sample Analysis at Mars (SAM) instrument (stars; [14]) and the Mars Climate Database (lines; [15]). Lower panel: O₃ and HCl observations from TGO’s ACS instrument [8]. MY=Mars Year; NH/SH=northern/southern hemisphere. Figure from Kevin Olsen.

References:

[1] Giuranna, M., et al. (2019). Nat. Geosci. 12, 326–332. [2] Korablev, O. et al. (2019). Nature 568, 517–520. [3] Montmessin, F. et al. (2021). Astron. Astrophys. 650, A140. [4] Webster, C. R. et al. (2015). Science 347, 415-417. [5] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [6] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [7] Olsen K. S. et al. (2024a). JGR, 129, e2024JE008350. [8] Olsen K. S. et al. (2024b). JGR, 129, e2024JE008351. [9] Taysum, B. M. et al. (2024). Astron. Astrophys., 687, A191. [10] Brown M. A. J. et al. (2022). JGR, 127, e2022JE007346. [11] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [12] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059. [13] Benne, B. et al. (2024). EPSC, pEPSC2024-1037. [14] Trainer, M. G. et al. (2019). JGR 124, 3000. [15] Millour, E. et al. (2022). Mars Atmosphere: Modelling and Observations, p. 1103.

How to cite: Gregory, B., Olsen, K., Millour, E., and Brown, M.: Modelling the Influence of Oxidative Chemistry on Trace Gases in Mars' Atmosphere., EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1687, https://doi.org/10.5194/epsc-dps2025-1687, 2025.

The photochemistry of ozone in the Martian atmosphere is generally considered to be well understood. Ozone forms through a three-body reaction involving O and O₂, both products of CO₂ photolysis, and it is destroyed by odd-hydrogen species (HOₓ) generated from water vapour photolysis, which helps to explain the observed anticorrelation between ozone and water vapour [1,2]. However, current photochemical models cannot reproduce ozone observations from various missions (e.g., Trace Gas Orbiter (TGO), Mars Express), with models generally suffering from a negative bias [2,3]. This discrepancy highlights gaps in our knowledge of the photochemical links between odd-oxygen (Oₓ), odd-hydrogen, and water vapour in the Martian atmosphere. A recent study investigated different factors that could influence the ozone content and concluded that the underestimation of ozone in the MPCM might be due to heterogeneous uptake of HOₓ species on water ice clouds or an overestimation of HOₓ photochemistry efficiency in the model [2].

We build on that explorative study and use the latest configuration of the Mars Planetary Climate Model (MPCM) with initial conditions from the Mars Climate Database (MCD) v6.1 [4] to investigate how different parameters could influence the ozone vertical profiles. We study data collected in MYs 34 and 35, including Ox, HOx, CO, and water vapour retrievals from the ACS (Atmospheric Chemistry Suite) and NOMAD (Nadir and Occultation for MArs Discovery) instruments aboard TGO. This approach allows us to examine any altitude-dependent changes in chemistry. We will present the results from a systematic investigation into the impact of various assumed model parameters, e.g., absorption cross sections, reaction rates, and heterogeneous chemistry, on these species. We will also consider the impact of introducing new chemistry into the model, e.g., chlorine photochemistry that was recently implemented in the MPCM by Benne et al. (2025) (in review). We will conclude our presentation by highlighting the parameters with the greatest impact on model ozone, the interactions and variations of ozone and its precursors across the two MYs, and prioritising the future research required to bridge the gap between model and observed Martian ozone.

References:

[1] Lefèvre, F., and Krasnopolsky V. “Atmospheric Photochemistry.” In The Atmosphere and Climate of Mars, 1st ed., 405–32. Cambridge University Press, (2017).

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

[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)

[4] Millour, E, Forget F., Spiga A., Pierron T., Bierjon A., Montabone L., Vals M., et al., Copernicus Meetings, (2022).

How to cite: Benne, B., Palmer, P., Olsen, K., and Lefèvre, F.: Investigating the discrepancy between observed and modelled ozone on Mars using ACS and NOMAD data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1341, https://doi.org/10.5194/epsc-dps2025-1341, 2025.

Carbon monoxide (CO) has extensively been monitored in the martian atmosphere due to its dual significance: it provides insights into the dymanics and into photochemical processes between this molecule with others in the Martian atmosphere. CO is primarily produced through the CO₂ photolysis in the upper atmosphere; it is then transported downward to the lower atmosphere where is destroyed by OH radicals - which are more abundant in water-rich regions - completing a recycling loop back to CO₂. Initial monitoring of CO was carried out using ground-based observations [1,2], followed by space-based measurements with orbiters [3,4]. The ExoMars Trace Gas Orbiter, a joint mission by ESA and ROSCOSMOS was launched in 2016, and it carries two instruments capable of detecting CO via solar occultation observations: NOMAD [5] and ACS [6]. These multi-channel spectrometers began scientific operations in 2018, and their solar occultation modes (SO and MIR, respectively) observe the Infrared (2325-4348 cm^-1), with a spectral resolution of 0.08 cm^-1, and an SNR of ~4000.

In this work, we focus on ACS MIR observations targeting the overtone CO (2-0) absorption band located between 4150 and 4350 cm^-1 (see figure 1). Before performing retrievals, the dataset provided by ACS MIR was first processed through a pipeline made at IAA-CSIC that include corrections in the spectral shift, continuum spectral bending, and a separation of random and systematic components of the measurement noise. [7,8] A critical part of this preprocessing involved characterizing the Instrumental Line Shape (ILS). For ACS MIR, the ILS presents a double peak response. The resulting lines can be modeled as a double gaussian, whose parameters can vary accross diffraction orders. We studied this evolution for the ACS MIR diffraction orders where the CO band is located and the resulting ILS chracterization will be presented in this work (see figure 2).

After the preprocessing, the spectra were analyzed using the KOPRA radiative transfer model coupled with the RCP inversion code [9], enabling the retrieval of CO VMR vertical profiles from ~8 to 90 km. This study presents the results of these inversions for CO using the detector position 7, which contains information for part of the Martian Years 34, 36 and the complete MY 37. The varying line intensities within the CO band facilitate profiling of both the upper and lower atmosphere while avoiding spectral saturation. Our results using ACS MIR will be finally compared with those previously retrieved using NOMAD [10], with previous ACS MIR and NIR retrievals [11,12], and we also provide a detailed analysis of the seasonal evolution of CO during the more recent MY 37.

Figure 1. ACS MIR position 7 transmittance spectrum of CO for a tangent height of 64 km, located at 56.9 ^oN, 56.7 ^oW, Ls = 212.6^o. This spectrum corresponds to the TGO orbit “019665”. The error is presented as grey vertical lines.

Figure 2. ILS parametrization for a subset of 20 orbits for order 251. The variation of the different parameters used for the double gaussian fitting are presented in red. The mean value and standard deviation are shown in black.

[1] Encrenaz, T. et al. (2006). Seasonal variations of the Martian CO over Hellas as observed by ground-based infrared spectroscopy. Astronomy & Astrophysics, 459, 265–270. https://doi.org/10.1051/0004-6361:20065984

[2] Krasnopolsky, V. A. (2007). Long-term spectroscopic observations of Mars using ground-based telescopes: Detection of CO and its seasonal variations. Icarus, 190(1), 93–102. https://doi.org/10.1016/j.icarus.2007.02.015

[3] Smith, M. D. (2004). Interannual variability in TES atmospheric observations of Mars during 1999–2003. Icarus, 167(1), 148–165. https://doi.org/10.1016/j.icarus.2003.09.010

[4] Bertaux, J.-L., et al. (2006). SPICAM on Mars Express: Observing modes and overview of UV spectrometer data and scientific results. Journal of Geophysical Research: Planets, 111, E10S90. https://doi.org/10.1029/2006JE002690

[5] Vandaele, A. C., 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, 80. https://doi.org/10.1007/s11214-018-0517-2

[6] Korablev, O., et al. (2018). The Atmospheric Chemistry Suite (ACS) of three spectrometers for the ExoMars 2016 Trace Gas Orbiter. Space Science Reviews, 214, 7. https://doi.org/10.1007/s11214-017-0437-6

[7] López‐Valverde et al. (2023). Martian atmospheric temperature and density profiles during the first year of NOMAD/TGO solar occultation measurements. Journal of Geophysical Research: Planets, 128 (2), https://doi.org/10.1029/2022JE007278

[8] Brines et al. (2023). Water vapor vertical distribution on Mars during perihelion season of MY 34 and MY 35 with ExoMars‐TGO/NOMAD observations. Journal of Geophysical Research: Planets, 128 (11), https://doi.org/10.1029/2022JE007273

[9] Stiller, G. P. (2000). The Karlsruhe Optimized and Precise Radiative Transfer Algorithm (KOPRA), Vol. FZKA 6512, Forschungszentrum Karlsruhe.

[10] Modak A. et al. (2022). Retrieval of Martian Atmospheric CO Vertical Profiles From NOMAD Observations During the First Year of TGO Operations. J. Geophys. Res. Planets 128, 3. https://doi.org/10.1029/2022JE007282

[11] Olsen, K. et al. (2021). The vertical structure of CO in the Martian atmosphere from the exoMars trace gas orbiter. Nature Geoscience, 14(2), 67–71. https://doi.org/10.1038/s41561-020-00678-w

[12] Fedorova, A. et al. (2022). Climatology of the CO vertical distribution on Mars based on ACS TGO measurements. Journal of Geophysical Research: Planets, 127(9), e2022JE007195. https://doi.org/10.1029/2022je007195

Acknowledgements

The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA) with co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS) and the United Kingdom (Open University). This project acknowledges funding by: the Belgian Science Policy Office (BELSPO) with the financial and contractual coordination by the ESA Prodex Office (PEA 4000103401, 4000121493, 4000140753, 4000140863); by the Spanish Ministry of Science and Innovation (MCIU) and European funds “ERDF A way of making Europe”, from the Severo Ochoa (CEX2021-001131-S) and from MCIN/AEI/10.13039/501100011033 (grants PID2022-137579NB-I00, RTI2018-100920-J-I00 and PID2022-141216NB-I00); by the UK Space Agency (grants ST/V002295/1, ST/V005332/1, ST/X006549/1, ST/Y000234/1 and ST/R003025/1); and by the Italian Space Agency (grant 2018-2-HH.0). This work was supported by the Belgian Fonds de la Recherche Scientifique – FNRS (grant 30442502; ET_HOME). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency

How to cite: Rodriguez-Ovalle, P., Lopez-Valverde, M. A., Modak, A., Gonzalez-Galindo, F., Brines, A., Trokhimoskiy, A., Belyaev, D., Olsen, K., Montmessin, F., Baggio, L., Fedorova, A., Korablev, O., Daerden, F., Thomas, I. R., Vandaele, A. C., Patel, M., and Bellucci, G. and the ACS and NOMAD teams: Martian CO vertical distribution combining 3 Martian Years of TGO/ACS MIR solar occultation data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1177, https://doi.org/10.5194/epsc-dps2025-1177, 2025.

Recent observations by NASA’s Curiosity Rover have revealed unexpected seasonal changes in molecular oxygen (O₂) levels in the Martian atmosphere at Gale Crater, its landing site [1]. This finding is puzzling, as the atmospheric lifetime of O₂ is expected to be much longer—at least 10 years [2]. Although O₂ is generally stable, its volume mixing ratio is influenced by the seasonal sublimation and condensation of CO₂ between the polar ice caps and the atmosphere. However, the observed O₂ variability is significantly greater than predicted, with increases of up to 30%. These changes peak in late spring and early summer and decline in mid-winter, exceeding the variability that can be explained by known chemical processes.

O₂ was first detected in the Martian atmosphere through ground-based observations in 1971–1972 [3,4] and again in 1982 [5]. Using high spectral resolution (R > 100,000) instruments in the visible range on 2–3 meter-class telescopes, faint O₂ spectral lines were detected near 762 nm. These measurements yielded a globally averaged O₂ mixing ratio of 1300 (±130) ppm. Later observations using the Herschel Space Telescope in the sub-millimeter range confirmed a similar average of 1400 (±200) ppm [6]. However, these earlier studies provided only disk-averaged values and did not resolve spatial distribution, and the measurement uncertainties remained relatively large.

In this study, we report new measurements of Martian oxygen using the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Subaru/HDS offers high spectral resolving power (R > 100,000) in the visible range and features a long slit (~14"), which enables spatially resolved measurements of O₂. This allows us to map the distribution of oxygen across latitude, longitude, and local time—helping to determine whether the seasonal O₂ variations observed by Curiosity are global or localized. An additional advantage of Subaru/HDS is its wide spectral coverage, enabling simultaneous observation of ~13 O₂ lines between 762–765 nm. This improves the accuracy of the retrieved O₂ mixing ratios and enables a more detailed analysis of seasonal variability. Furthermore, CO₂ lines at 869 nm—whose strengths are comparable to those of O₂—can be used to estimate the effective airmass and assess the influence of atmospheric aerosols.

Observations were conducted on 15 February and 11 April 2025, corresponding to solar longitudes (L_s) of 44° and 68° in Martian Year 38. The latter coincides with the O₂ enrichment period reported by Curiosity, while the former precedes it. The angular diameters of Mars during the observations were 12.3" and 7.6", respectively. The Doppler shifts between Mars and Earth were 11 km/s and 16 km/s, sufficient to separate Martian O₂ lines from strong telluric O₂ features. We present the resulting global O₂ distribution maps and compare them with predictions from a Mars climate model.

References:
[1] Trainer et al. 2019, Journal of Geophys. Res.: Planets, 124, 3000. 
[2] Lefèvre & Krasnopolsky, 2017, Cambridge University Press. 
[3] Barker, E. S. 1972, Nature, 238, 447.            
[4] Carleton, N. P., & Traub, W. A. 1972, Science, 177, 988. 
[5] Trauger & Lunine, 1983, Icarus, 55, 272. 
[6] Hartogh et al., 2010, Astron. & Astro., 521, L49. 

How to cite: Aoki, S., Sagawa, H., Arai, A., Kagitani, M., Villanueva, G., Faggi, S., Liuzzi, G., Daerden, F., Viscardy, S., Robert, S., and Vandaele, A.: Global mapping of molecular oxygen on Mars, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-430, https://doi.org/10.5194/epsc-dps2025-430, 2025.

The upper atmosphere of Mars, including the mesosphere (60-120 km) and thermosphere (120 up to the exobase, around 180-230 km), is no doubt the less explored region of the red planet. Two recent and on-going missions to Mars are investigating these altitudes with new instrumentation and unprecedented detail: (1) the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN), entirely devoted to the uppermost layers and the escape to space, started its science measurements in 2014; and (2) the ESA-Roscosmos Trace Gas Orbiter (TGO), which started its routine science operations in 2018 after a long period of aerobraking, and which contains two solar occultation instruments, NOMAD and ACS, which permit sounding from the ground up to upper thermosphere, about 180 km.

This work is devoted to the ambitious goal of combining datasets from diverse instruments on board these two missions, of exploiting the sinergy and complemetariety between them, and of obtaining a joint picture of the thermal structure of the whole Martian atmosphere, from the ground up to the exobase. This is part of an ISSI project entitled “A multi-mission approach to close the gaps in understanding of the structure and variability in the Mars upper atmosphere”. We will present the results of a broad team of scientists working for the past two years on this project, with focus on the global, planet encircling climatology of the Mars thermal structure (temperature and CO2 densities), and also its variability. The combination of MAVEN and TGO data permits to obtain the regular patterns of the Martian temperature profiles and how these vary with season, latitude, longitude and local time at different altitudes. Also we can combine the two missions’ datasets to explore year-to-year variations, dust storm impacts and solar-rotation effects at such a wide range of altitudes as those covered jointly by MAVEN and TGO, for the first time.

This work combining MAVEN and TGO is also very informative for the validation of Mars Global Climate Models, given the complementariety of the two missions from the observational point of view, with very different coverage and sampling in the key magnitudes like local time, longitude, latitude and season. A small number of other companion contributions to this conference tackle some of these specific aspects.

How to cite: Lopez-Valverde, M. A., Thiemann, E., Alday, J., Jones, N., Evans, S., Stone, S. W., Gonzalez-Galindo, F., Belyaev, D. A., Fedorova, A. A., Trompet, L., Jain, S., Gupta, S., Pillinski, M., Neary, L., MIllour, E., and Forget, F.: The Martian upper atmosphere’ thermal structure and variability as revealed by Maven & TGO combined datasets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1746, https://doi.org/10.5194/epsc-dps2025-1746, 2025.

Atmospheric gravity waves (AGWs) significantly influence Mars’ climate by facilitating energy and momentum exchanges, thus modulating atmospheric circulation, cloud formation, and dust distribution processes [1]. Despite recent advances in the characterization of Martian AGWs using observations from Mars Express/OMEGA [2], this study was limited by the narrow field of view and restricted spatial coverage. Here, we present an extended analysis using data from the High-Resolution Stereo Camera (HRSC) [3], likewise onboard Mars Express (MEx), benefiting from its higher spatial coverage, resolution (map scales between 200-800 m/px), and consistent imaging quality.

We examined HRSC high-altitude imagery covering Mars Years (MY) 34-37 [4], identifying and characterizing over 100 distinct wave packets, nearly doubling the number previously analyzed. Our refined cloud-altitude measurement technique significantly improved the accuracy and robustness of cloud height retrievals, resulting in estimated altitudes of 15-40 km for water-ice clouds and 60–100 km for CO2-ice clouds, with uncertainties around ±2 km. Wind speeds were determined through precise cloud displacement analyses from HRSC double-broom imagery, taken with an interval of 30 minutes apart, consistently yielding values between 5-20 m/s and uncertainties of approximately ±1 m/s, though these uncertainties likely underestimate the total error due to unresolved spacecraft pointing inaccuracies. Additionally, by retrieving the wind speeds of wave packets and the background, we derived preliminary phase speed estimates for the detected wave packets. Comparison with local background winds suggests a range of intrinsic wave behaviours, which reflect differences in their generation mechanisms or propagation environments.

Our enhanced dataset allows us to establish seasonal and spatial patterns of gravity wave occurrence, highlighting a prominent hemispheric asymmetry with significantly more wave observations in the northern hemisphere. This raises new questions about observational biases potentially related to spacecraft observational strategies. Additionally, the measured wind speeds remain consistently lower than mesoscale model predictions from the Mars Climate Database [5,6], suggesting that current atmospheric models may inadequately represent mesoscale dynamics associated with gravity waves.

Collaborations with the OMEGA and TGO/CaSSIS teams are now extending this analysis further, incorporating spectral information for cloud composition (OMEGA) and complementary observational geometries (CaSSIS), crucial for addressing the limitations identified in this work. These multi-instrument synergies will allow better discrimination of physical processes involved in wave formation, propagation, and dissipation.

This comprehensive analysis using HRSC data provides critical insights into Martian atmospheric dynamics, which can be used to improve current atmospheric models using the parameterization of gravity waves characterized in this work.

Acknowledgments: This work was supported by the Portuguese Fundação Para a Ciência e a Tecnologia of reference PTDC/FIS-AST/29942/2017, through national funds and by FEDER through COMPETE 2020 of reference POCI-01-0145-FEDER-007672, and a grant of reference 2021.05455.BD. GG acknowledges financial support from Junta de Andalucia 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 Team for their support and encouragement in this work.

References

[1] Fritts et al., 2003. Reviews of Geophysics, 41(1).

[2] Brasil et al., 2025. JGR: Planets, 130(3), e2024JE008726.

[3] Jaumann et al., 2007. Planetary and Space Science 55, 928-952

[4] Tirsch, et al. 2024. EPSC2024-44, DOI:10.5194/epsc2024-44.

[5] Forget et al., 1999. JGR: Planets, 104(E10), 24155-24175.

[6] Millour et al., 2017, EGUGA, 12247

How to cite: Brasil, F., Machado, P., Gilli, G., Tirsch, D., Cardesin-Moinelo, A., E. Silva, J., Espadinha, D., Carter, J., Wilson, C., and Martin, P.: Morphological and dynamical analysis of Martian Gravity Waves using MEx/HRSC observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-619, https://doi.org/10.5194/epsc-dps2025-619, 2025.

Introduction : Dust storms strongly influence the current Martian climate, yet their seasonal cycle and the interannual variability of Global Dust Storms (GDS) remain poorly understood. General circulation models (GCMs) often fail to reproduce both solsticial and equinoctial GDS, as well as the recurring seasonal pattern of regional Z, A, B and C storms (Montabone et al. (2015), Kass et al. (2016)). We present an offline dust model based on Mars Planetary Climate Model (Mars PCM) (Forget et al. (1999)) simulations with prescribed opacity and subgrid-scale parameterizations informed by high-resolution models. This approach captures key features of the dust cycle, including both types of GDS, while enabling multi-decadal simulations at low computational cost.

Preliminary GCM simulations : We perform six simulations of one Martian year with the Mars PCM, varying visible dust opacity from τ=0.05 to τ=5, kept constant throughout the year. The surface wind stress in the offline model is computed by interpolating wind data from these GCM simulations, adjusted for the mean dust opacity in the offline model and weighted by large zonal dust gradients, as strong zonal dust contrasts can enhance surface winds.

Dust Lifting, Transport, and Deposition : Dust lifting on Mars is driven by two main mechanisms: saltation by surface wind stress, dominant during the storm season (late northern summer to winter), and dust devils, active in spring and early summer. The offline model uses two tracers (mass and number) to represent dust, allowing diagnosis of loading and particle size under a log-normal distribution. Saltation flux is computed from surface wind stress and a lifting threshold, with subgrid wind variability captured using a Weibull distribution, following Lorenz (1996). Dust devil lifting is also included, with the flux computed as a function of vertical wind variance, based on insights from LES simulations. Advection is handled using a mass-conserving Van Leer scheme, while sedimentation is computed by discretizing the dust size distribution into 12 bins and applying mean fall speeds from Rossow (1978).

Dust sinks and sources : A key challenge in dust cycle modeling is the formation of dust sinks, where dust accumulates in low-wind or high-threshold regions, leading to a gradual depletion of atmospheric dust. To counter this, erosion is introduced: empty reservoirs can still lift dust if the wind exceeds a fixed erosion threshold.

Baroclinic waves : Baroclinic waves are included in the offline model's wind fields, but the coarse resolution limits the proper resolution of wave fronts. High-resolution GCMs show stronger winds and sharper contrasts during high baroclinic activity. Parameterizing these features is crucial to improve the dust cycle simulation, especially for capturing A and C storms.

Storm edge lifting : Dust storms often evolve as fronts with sharp opacity contrasts over short distances, which can enhance winds and trigger additional lifting, creating a positive feedback loop (Wu et al., (2021); Spiga and Lewis, (2010)). Due to model resolution limitations, these strong winds at storm edges must be parameterized.

Cap Edge Lifting: Strong winds near the polar cap edge, driven by temperature contrasts between Martian soil and CO₂ ice caps, are better captured in mesoscale models (Smith and Spiga, (2018)). To simulate this in our model, we add a surface wind stress term proportional to the thermal contrast between adjacent grid cells, accurately representing the B storm near the South polar cap.

Wind stress thresholds : In the offline model, we use a distribution of wind stress thresholds instead of a single value per grid cell. Each cell contains subgrid reservoirs with unique lifting thresholds and limited dust supplies, which are replenished equally by sedimentation, allowing rapid recovery of low-threshold reservoirs. This setup introduces interannual variability in GDS activity by modulating local dust availability over time.

Results : The offline model reproduces both solstitial and equinoctial GDS with a realistic interannual variability, some years producing GDS, others not, as observed (see Figure 2). In the absence of GDS, the model captures the main features of the seasonal dust cycle, with recurring regional storms matching the observed pattern of the Z, A, B, and C storms, as illustrated in Figure 1. On average, the simulated dust cycle closely resembles the climatology derived from observations, supporting the model's capacity to represent the baseline seasonal behavior of Martian dust.

Figure 1: Effect of different parameterizations in the offline model compared to observations (last panel)

Figure 2: Planetary mean of visible dust column optical depth in the offline model as a function of season.

References :

Montabone, L., Forget, F., Millour, E., Wilson, R. J., Lewis, S. R., Cantor, B., Kass, D.Kleinböhl, A., Lemmon, M.T., Smith, M.D & Wolff, M. J. (2015). Icarus, 251, 65-95.

Kass, D. M., Kleinböhl, A., McCleese, D. J., Schofield, J. T., & Smith, M. D. (2016). Geophysical Research Letters, 43(12), 6111-6118

Forget, F., Hourdin, F., Fournier, R., Hourdin, C., Talagrand, O., Collins, M., Lewis, S.R., Read, P.L. & Huot, J. P. (1999). Journal of Geophysical Research: Planets, 104(E10), 24155-24175.

Lorenz, R. D. (1996). Journal of Spacecraft and Rockets, 33(5), 754-756

Rossow, W. B. (1978). icarus, 36(1), 1-50.

Wu et al. (2021). Journal of Geophysical Research: Planets, 126(9), e2020JE00675

Spiga, A., & Lewis, S. R. (2010). Mars, 5, 146-158.

Smith, I. B., & Spiga, A. (2018). Icarus, 308, 188-196.

How to cite: Pierron, T., Forget, F., and Bertrand, T.: An Offline Model for long-term Martian Dust Storm Simulations and Interannual Variability, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-724, https://doi.org/10.5194/epsc-dps2025-724, 2025.

Global dust events (GDEs), occurring every few Martian years, are certainly the most impressive wheather phenomena on Mars and have been investigated by numerous studies. The reasons why GDEs occur in some Mars years, and not in others, remain challenging to unveil and many hypotheses have been set up, including that dust reservoirs need to be replenished after a GDE before the next one can occur. However, to our knowledge, the question of whether surface dust redistribution on Mars follows such a cyclic behavior was not adressed by mutli-year modeling studies yet.

In order to follow the time evolution of dust emission and deposition fluxes at the surface of Mars, we use the Mars Planetary Climate Model in its newest version 6 (Mars PCM6) (Bierjon et al., 2022), where dust injection is prescribed by observations, while transport and sedimentation freely follow the model winds. For prescribing dust injection, the Mars PCM6 uses daily global maps from observations of the Column Dust Optical Depth (CDOD) (Montabone et al., 2015). We further complete these maps with daily dust storm obervations from the Mars Dust Storm Sequences Database (MDSSD) (Wang et al., 2023), to take into account the intense dust storm activity near the Martian polar caps.

We thus obtain maps of the surface dust redistribution in the time frames covered by these observations (MY24-35 for the CDOD maps and MY28-33 for the MDSSD). We validate our modeling results with surface dust redistribution patterns from the literature, derived from remote-sensing observations of the Martian surface. Finally, we will analyse whether we find a periodic redistribution pattern of surface dust, in order to test the hypothesis of replenishing dust reservoirs between GDEs. This will utlimately help improving our understanding of Martian GDEs, on the road towards reliable weather prediction on Mars.

References

A. Bierjon, E. Millour, F. Forget, Finalization of the GCM version 6 - Improving the Dust Cycle, Laboratoire de Météorologie Dynamique, CNRS, IPSL, Paris, France, 2022, https://www-mars.lmd.jussieu.fr/esa/contract2020_2022/deliverables/MS3/D2.1_dust_cycle.pdf

L. Montabone, F. Forget, E. Millour, R.J. Wilson, S.R. Lewis, B. Cantor, D. Kass, A. Kleinböhl, M.T. Lemmon, M.D. Smith, M.J. Wolff, Eight-year climatology of dust optical depth on Mars, Icarus, Volume 251, 2015, doi:10.1016/j.icarus.2014.12.034

H. Wang, M. Saidel, M.I. Richardson, A.D. Toigo, J.M. Battalio, Martian dust storm distribution and annual cycle from Mars daily global map observations, Icarus, Volume 394, 2023, doi:10.1016/j.icarus.2022.115416

How to cite: Ramette, D., Noack, L., and Schepanski, K.: Investigating inter-annual dust redistribution at the surface of Mars with planetary climate model simulations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-575, https://doi.org/10.5194/epsc-dps2025-575, 2025.

1.    Introduction and Motivation

Regional to large-scale dust storms are climatologically favored in the Hellas Basin region, especially during the northern fall equinox.  The dust storms, while often occurring in the same general season year after year, do not occur repeatedly sol after sol. It follows that the wind stress must cross an initial threshold above which dust lifting occurs and then fall below that threshold in two sols or less. Prior to and after that peak, the wind stress must be below the lifting threshold as the canonical scenario. The most common scenario is for an absence of storms.

The Hellas region exhibits numerous superimposed atmospheric circulations of different spatial and temporal scales: The seasonally-varying large-scale mean meridional (Hadley) circulation; large-scale stationary waves, the ubiquitous large-scale tidal circulations (primarily diurnal and semi-diurnal); traveling/transient eddies primarily associated with large-scale baroclinic instabilities; standing waves within Hellas itself (e.g., basin-wide gyres); and mesoscale direct thermal circulations associated with topographic slopes, ice/ice-free boundaries, and thermal inertia and albedo variations. All these individual circulations sum in non-linear ways to produce the total net circulation in the Hellas region. Except for traveling baroclinic waves, all the other circulations should have nearly repeatable amplitude and phase from sol to sol at a given season.  Therefore, these quasi-constant circulations likely are not the source of dust storm genesis since constant amplitude and constant phase circulations are unlikely to add together to exceed the dust lifting threshold. 

2.    Hypothesis

Prior work suggests that baroclinic storm systems circling along the ice cap edge are an important mechanism for the generation of dust events.  Although the baroclinic activity is ubiquitous around Ls 180, only a small number of dust events occur each year. Baroclinic waves typically have a period of several days, but dust lifting in the region is stochastic and not periodic. Therefore, if traveling baroclinic waves are responsible for the dust lifting, only a handful of these systems per year trigger dust lifting events while most of the waves do not. We hypothesize that only special and rare types of baroclinic systems can push the wind stress beyond the lifting threshold. These rare systems, perhaps only one or two per year, explain the stochastic and infrequent nature of dust storms. However, the storms depend on a climatologically strong non-baroclinic background circulation to generate a high surface wind stress condition that is, by itself, below the lifting threshold. 

3.    Hypothesis Testing Methodology

Statistical analysis is performed on 10 years of NASA Ames Mars General Circulation Model  output in the Hellas Basin region to identify the most intense surface wind stress events (Figure 1).  From this catalog of events, the highest stress events are analyzed to decompose the circulation into separate background and traveling (mostly baroclinic) components and to characterize and correlate baroclinic characteristics with high surface wind stress events.  

4.    Results

The highest stress events all occur around Ls 180 and are associated with extratropical (baroclinic) storm system. Further synoptic analysis shows that there are three types of systems that are conducive to producing the highest surface wind stress.  In each type, the strength and orientation of the high and low pressure systems align to constructively add to the background circulation. These three storm types are rare and are clearly distinguishable from the far more common baroclinic storm systems that regularly evolve and traverse the Hellas region.

There are shared characteristics of the maximum wind stress events in the model analysis. Maximum stress events consistently occur at 0000 – 0600 LT when downslope flows are strongest. Only one case occurs at 1500 LT. The strongest events also occur in a localized area in southwest Hellas from 45 to 55 S and 54 to 66 E. This region corresponds to the strongest non-baroclinic surface stresses, thus setting the stage for a transient baroclinic eddy to trigger a dust storm (Figure 2). Systems that do not fall into one of the three categories, even though they may be deep and intense baroclinic storms, do not produce the highest wind stresses.

An example of a baroclinic system responsible for the highest stress case in year 3 is shown in Figure 3.  This event was produced by a baroclinic low pressure in South Hellas and a strong anticyclone moving from the West into the Western Hellas ridge between latitudes -30 to -50 N. The result was an anomalous pressure gradient perturbation driving strong perturbation winds that aligned with the mean wind. Although large wind speeds are collocated with maximum stress, high speeds are also recorded west of Hellas Basin. However, the lower density at higher elevations reduces the wind stress compared to lower elevations.

The other two types of baroclinic systems have different high/low pressure configurations compared to Figure 3, but all result in an anomalous pressure gradient that drives strong perturbation winds that constructively add to the mean. The vast majority of baroclinic systems, even those with strong pressure gradients, do not align with the background circulation and do not contribute to high stress events.  The analysis of the GCM model output is consistent with the hypothesis invoking rare and special baroclinic events as the source of Hellas Basin dust storms near Ls 180.

Figure 1.  Histogram of the maximum surface stress events in 10 years of model data within the Hellas Basin region.  There are a very small number of high stress events (1 to 2 per year).

Figure 2.  The mean Hellas Basin circulation from Ls 180-193.  The averaging effectively removes the baroclinic eddy component of the circulation.  Surface stress is contoured in green showing a maximum on the southwest slope of Hellas.

Figure 3.  An example of a Type 2 event where the baroclinic system occurs at just the right time, in just the right place, at just the right time to produce a strong perturbation pressure gradient and strong winds aligned with the mean circulation.

How to cite: Rafkin, S., Dolski, S., and Kahre, M.: The Association of Hellas Basin Dust Storms to Specific Types of Baroclinic Storm Systems, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1051, https://doi.org/10.5194/epsc-dps2025-1051, 2025.

Annular modes represent some of the largest sources of climate variability for Earth and Mars.  Annular modes are zonally symmetric patterns of variability that represent the jet stream shifting north and south in time and are associated with anomalous occurrences of atmospheric eddies, clouds, and dust.  Mars's Northern Annular Mode (MNAM) develops as anomalies of zonal-mean zonal wind emerging near the subtropics and migrating towards the pole.  New anomalies appear in the subtropics and begin propagating with a period of ~150 sols, as diagnosed from the OpenMARS reanalysis.  The propagation is induced by the interaction of the two leading empirical orthogonal functions that define the MNAM, which can be predicted using a pair of simple prognostic equations similar to what is done for Earth.  The shifting jet stream coincides with bands of anomalous surface wind stress and encourages anomalous dust optical depth.  The MNAM's internally forced periodicity on the surface wind stress combined with the seasonally forced dust cycle may help explain the inter-annual variability of global dust events, as suggested by a Monte Carlo estimate that correctly approximates the observed incidence of global dust events.  Further quantification of the other modes of internal climate variability of Mars may be possible as the record of Mars's present-day climate surpasses a decade of measurement.

How to cite: Battalio, M., Lora, J., Lubis, S., and Hassanzadeh, P.: Periodicity of Mars's Northern Annular Mode May Help Explain Global Dust Storm Frequency, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1209, https://doi.org/10.5194/epsc-dps2025-1209, 2025.

1. Introduction

Aerosol dust particles are an important component in planetary atmospheres, influencing energy balance, thermal structure, weather systems, and remote observations. Despite their importance, radiative-transfer models and climate simulations for planets are often limited by assumptions of oversimplified particle shapes and fixed complex refractive indices for aerosol dust, which hinders the interpretation of observational data. To address this gap, we present an experimentally validated database of aerosol dust optical properties derived from Martian dust analogs. The work combines laboratory measurements with advanced light-scattering modeling and provides a transferable methodology for studying aerosols in a variety of planetary environments.

2. Sample Preparation and Characterization

We selected three spectrally and mineralogically distinctive Martian dust analogs representative of different Martian terrains: JSC Mars-1, MGS-1, and MMS-2 (see Figure 1). The samples were processed at Instituto de Cerámica y Vidrio (ICV) to produce narrow particle size distributions in the geometric optics domain for the retrieval of complex refractive indices, as well as narrow size distributions representative of airborne dust aerosols for the computation of optical properties. Detailed morphological and compositional characterization was performed using SEM imaging, laser diffraction particle sizing, and X-ray diffraction (XRD) (see Martikainen et al. 2023).

Figure 1: The JSC Mars-1, MGS-1, and MMS-2 analogs.

3. Experimental Data

Diffuse reflectance spectra were measured from 200 to 2000 nm using a Varian Cary 5000 UV-vis-NIR spectrophotometer for samples with narrow particle size distributions in the geometric optics domain (Martikainen et al. 2023). The scattering matrix measurements at 488 and 640 nm were carried out at the Cosmic Dust Laboratory (CODULAB, Muñoz et al. 2010) of the Instituto de Astrofísica de Andalucía (IAA), covering scattering angles from 3° to 177° using analogs with narrow size distributions representative of airborne dust (Martikainen et al. 2024). The measurements include the phase function (F₁₁), degree of linear polarization (−F₁₂/F₁₁), and, where possible, additional scattering matrix elements (F₂₂, F₃₃, F₃₄, F₄₄). Together, these datasets provide the experimental basis for retrieving the complex refractive indices and computing bulk optical properties for radiative-transfer modeling.

4. Light-scattering modeling

The complex refractive indices were retrieved using the SIRIS4 ray-tracing model (Muinonen et al. 2009), which accounts for the irregular shapes of real dust grains. The model was applied to the measured diffuse reflectance spectra and corresponding narrow particle size distributions in the geometric optics domain. In this regime, the scattering behaviour of large particles is highly sensitive to the complex refractive index, allowing it to be reliably constrained from the measurements.

Bulk optical properties were computed using the TAMUdust2020 database of irregular hexahedral particles (Saito et al. 2021), based on the retrieved complex refractive indices and narrow size distributions representative of airborne dust. The modeled phase matrices were compared with measurements at 488 and 640 nm to validate both the retrieved complex refractive indices and the hexahedra particle model. The final set of calculated properties includes single-scattering albedo, extinction efficiency, extinction cross-section, asymmetry factor, and phase matrix elements across the 200–2000 nm spectral range.

5. Broader Applications
While the database is based on Martian dust analogs, both the experimental approach and the modeling framework are designed to be transferable. Similar methods can be applied to characterize aerosols in other planetary atmospheres, such as those of Venus or exoplanets. Our work demonstrates the value of combining controlled laboratory measurements with advanced numerical scattering models to improve the realism of aerosol dust optical data. Moreover, our derived optical properties reproduce the key spectral features of observed Martian regolith. This confirms the physical relevance of the results and supports their use in forward modeling and data interpretation.

6. Conclusions
We provide a validated, multi-wavelength database of aerosol dust optical properties derived from Martian dust analogs (Martikainen et al. 2025), intended for use in radiative-transfer modeling and observational retrievals across the planetary sciences. The methodology is designed for adaptation to other planetary atmospheres and dust compositions. The full dataset is available at the Granada-Amsterdam Light Scattering Database (Muñoz et al. 2025).

References: Martikainen et al. (2023), ApJS 268; Martikainen et al. (2024), ApJS 273; Martikainen et al.(2025), MNRAS 537; Muñoz et al. 2011, JQSRT 111; Muñoz et al. (2025), JQSRT 331; Muinonen et al. (2009), JQSRT 110; Saito et al. (2021), JAS 78.

How to cite: Martikainen, J., Muñoz, O., Gómez Martín, J. C., Passas Varo, M., Jardiel, T., Peiteado, M., Willame, Y., Neary, L., Becker, T., and Wurm, G.: Aerosol Dust Optical Properties from Martian Analogs in the UV-vis-NIR, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-725, https://doi.org/10.5194/epsc-dps2025-725, 2025.

Introduction

Dust is an essential part of the Martian atmosphere, and the notable dynamic phenomena of dust storms are quite intriguing on the planet. These storms significantly modify the dynamics and thermodynamics of the Martian atmosphere. Due to the lifted dust in the atmosphere, a layer is created that blocks the sunlight from reaching the surface and consequently generates a cooling effect. At the same time, it heats the atmosphere at higher altitudes due to the absorption and scattering of solar radiation [1]. Dust storms can also alter the atmospheric circulation and vertical structure [2, 3]. Thus, studying Martian dust storms can help us understand the comprehensive behavior of the Martian atmosphere. Studies have used both observational and numerical modeling datasets to analyze the characteristics of dust storms [3, 4]. The dust availability, dust lifting centers, trajectories, and several other features of the dust storms are identified through such analysis. But they are a bit time-consuming and often require high computational resources. However, several characteristics of dust storms can be statistically interpreted through probability distributions or polynomial functions. This study uses the Landau distribution to analyze several dust storms and investigate the peak, dust availability, duration, etc.

Data and Methodology

During the growth phase of a dust storm, a rapid increase in dust is observed, which continues till the peak, followed by an elongated decay phase. So, a time series analysis of dust opacity will indicate a right-skewed variation. Thus, right-skewed probability distribution, like the Landau type, is used here to fit the time series plot and get a statistical interpretation of dust storms. The dust opacity (d) for a storm can be fitted using the distribution as:

                                                                                                                                                      d = τ + αL( μ, σ)                                                                                                                                   (1)

where L is the Landau distribution. τ, α, μ, and σ are constants that signify several physical characteristics of the storm. Column dust optical depth (CDOD) is a measure of the total amount of dust over the entire atmospheric column, and it is used in the current analysis as it incorporates all the pressure levels. CDOD data obtained from the Montabone dust scenario [5, 6] is used to fit the probability distribution.

Results

The dust availability is determined by the background dust (τ). The strength and maximum opacity during the storm are determined through the intensity parameter (α). The peak of the dust storm is specified by the location parameter (μ). The duration of the storm is signified by the scale parameter (σ). In this study, four storms are considered across different Martian years (MY) and seasons. The storms happened around L_S=180°-270° in MY 34 (DS1); L_S=310°-340° (DS2) in MY 35; L_S=150°-170° (DS3), and L_S=250°-280° (DS4) in MY 36. DS1 is a global dust storm, and DS2 is a large regional storm. Notably, DS3 is a small regional storm, and DS4 is a regional storm near the southern polar cap. The Landau distribution fit seems to capture the growth of the dust storm very well (Figure 1). However, there are slight deviations around the peak, and underestimation is observed in most instances. The exact peak for all the storms is not properly realized. The distribution usually overestimates in the early stage of the decay phase and underestimates in the later stages. However, if the decay phase is prolonged, like that for a global dust storm (DS1), it follows closely the Landau distribution (Figure 1a). However, if the dust storm has a slow growth phase (Figure 1d), it fails to capture the trend completely, as the slow-growth dust storms show a symmetrical distribution.

Figure 1. The Landau distribution fits (red line) with the actual data (black dots) for DS1 (a), DS2 (b), DS3 (c), and DS4 (d).

The background dust (τ) is strictly dependent on the season and provides an idea about the dust availability (Table 1). For the global storm (DS1), the τ value is greater, hinting at higher dust availability, which is the probable reason for the growth of the storm into a global one. A high τ value for DS4 signifies the greater dust availability due to the southern summer. As DS3 happens in the early part of the dust season, it shows the lowest τ. The larger value of α indicates a higher intensity of the storm, which is realized through the maximum CDOD values. DS1, being a global event, shows the highest α. And DS3, being an early-season storm with lower dust availability, shows the lowest α. However, any empirical relationship between the peak CDOD and α could not be established. The peak of the dust storms can be estimated as  , which is found to match with peaks of DS2 and DS3, but deviates a lot for DS1 and DS4. The Landau distribution fit captured the peak of DS1 much earlier (Figure 1a), and it did not show a good fit for DS4, indicating a mismatch. The higher σ value represents the longer duration of the storm. As DS1 is a global storm, it has the longest duration, and with DS4, a storm with a slow growth phase, an overall extended duration is realized. In terms of L_S, the approximate duration (initiation to decay) is estimated to be 4.338σ.

Table 1. The parameters for the dust storms obtained from the Landau distribution.

Conclusions

The Landau distribution greatly helps in understanding several characteristics of the Martian dust storms. The growth phase is captured well, and the decay phase is more or less followed, whereas the exact peak is not properly captured. The proposed approach can be useful to study the dust storms alongside the conventional analysis. Although this study has used CDOD values, the distribution is expected to work with dust opacity for different pressure levels. However, altitudes near the surface would not perform well due to non-uniform dust-lifting.

References

[1] Panda et al. (2025). New Astronomy Reviews, 101723. [2] Haberle (1986). Science, 234(4775). [3] Kass et al. (2016). GRL, 43(12). [4] Bertrand et al. (2020). JGR: Planets, 125(7). [5] Montabone et al. (2015). Icarus, 251, 65-95. [6] Montabone et al. (2020). JGR: Planets, 125(8).

How to cite: Mandal, A. and Panda, J.: A statistical analysis of Martian dust storms using the Landau probability distribution, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1632, https://doi.org/10.5194/epsc-dps2025-1632, 2025.

            Martian dust, composed of micrometer-sized mineral particles, is a key parameter of the Martian atmosphere because of its high mobility and capacity to form dust storms and alter the heat balance of the atmosphere. These dust storms are part of the Martian dust cycle and can occur over a wide range of spatial and time scales (from sub-km to planetary scale, and from the minute to months). The dust studies are mostly concentrated on regional storms (> 2000 km), which are detected using UV-VIS global imagers [1, 2] or thermal-IR spectrometers [3]. These observations provide information about the frequency and size of the storms. However, some characteristics of the dust cycle remain uncertain, such as the exact mechanisms of storm formation and growth processes, with implications for the predictability of dust storms and Global Dust Storms (GDS, storm at a planetary scale).

            Thus, it is important to study dust activity at a local scale, such as the local dust storms (< 2000 km) which have not been massively detected (some detections by [2] and a few individual storm characterisations as [4]). We focus our work on detecting dust storms in the OMEGA/Mars Express (near-IR imaging spectrometer) dataset, which has a high enough spatial resolution (0.36-4.8 km/px) to detect local dust storms. More precisely, we develop a method to detect automatically dust storms using dust optical depth computation (at 0.9 µm) made by [5] from late Martian Year 26 to early MY 30 (including the MY 28 GDS). The 1^st-level detection is to exclude OMEGA observations that are least likely to contain dust storms (dust optical depth < 1.25). Then the 2^nd-level detection principle is to identify pixel clusters of high optical depth within individual observations to identify storms, to confirm the detection using different criteria (e.g., decrease of surface reflectance) and to extract key characteristics of the storm (e.g., size, centroid, local time).


Figure 1: Spatial distribution of the dust storms (local and regional) detected with OMEGA/MEx from late MY 26 to early MY 30 (without MY 28 GDS period). The background corresponds to a topography map (MOLA/MGS).

            With this 2-level method, we detected 287 dust storms (excluding the MY 28 GDS period during which 114 detections have been made) composed of 283 local and 4 regional. Here we summarised our results that will be presented in detail during the conference (notably by separating the local and regional dust storms). From the spatial distribution of the detections (Figure 1), we notice some known preferential areas such as the flushing dust storm channels (e.g., Acidalia-Chryse, Utopia-Isidis, see [1]) or Hellas Planitia, as well as next to the southern polar cap. The seasonal distribution of the detected storms (Figure 2) shows many detections in the Ls=240-270° period, particularly in MY 27, and also during the Ls=330-360° period, notably in MY 29. Interestingly, only a few storms were reported during these periods using UV-VIS imagery [1], suggesting that our method may be capturing local-scale events that were previously undetected. Another strength of OMEGA data is the local time coverage during the Mars Express mission, allowing the study of the time dependence. We noticed many detections from 10:00 to 18:00 with an increase (compared to the OMEGA local time coverage) at the end of the afternoon (16:00-18:00), and also some detections early in the morning (04:00-06:00).


Figure 2: Seasonal distribution of the dust storms (local and regional) detected with OMEGA/MEx from late MY 26 to early MY 30 (without MY 28 GDS period).

            We also worked on the MY 28 GDS onset, for which one doubt remains about the formation area, specifically, whether it originated in Noachis (southern hemisphere) or Chryse (northern hemisphere). More precisely, the question is whether the precursory storm observed in Noachis initiated the GDS independently, or whether it was dependent on the one in Chryse. The consecutive OMEGA observations of these two areas (during the storms) show high dust optical depth values for Noachis and Chryse, but separated from each other by low optical depths, which could be interpreted as two separate clouds, and therefore a GDS onset in Noachis. We are currently working to evaluate the dust altitude by comparing dust optical depth retrievals at different wavelengths [6].

References:
[1] Battalio M. & Wang H. (2021) Icarus, 354, 114059.
[2] Guha B. K. et al. (2024) JGR Planets, 129, e2023JE008156.
[3] Lombard T. & Montabone L. (2024), 10th International Conference on Mars, abstract #3041.
[4] Määttänen A. et al. (2009) Icarus, 201 Issue 2, 504-516.
[5] Leseigneur Y. and Vincendon M. (2023) Icarus, 392, 115366.
[6] Kazama A. (2025) EPSC-DPS2025, abstract #547.

How to cite: Leseigneur, Y., Bertrand, T., Gautier, T., and Spiga, A.: OMEGA/Mars Express dust storm catalogue: a local dust storm survey, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1672, https://doi.org/10.5194/epsc-dps2025-1672, 2025.

Understanding the Martian dust cycle is essential for clarifying the atmospheric circulation and meteorological phenomena. Unlike Earth, Mars has a thin atmosphere primarily composed of CO₂, where atmospheric dust plays a dominant role in regulating the energy balance and driving atmospheric motion. Among various dust-related phenomena, Local Dust Storms (LDS), defined as storms that span less than 1.6 × 10⁶ km² or last fewer than three Martian days (Cantor et al., 2001), are particularly important for studying localized dust lifting and its potential connection to regional or global dust events (Martin and Zurek, 1993; Cantor et al., 2001; Hinson and Wang, 2010; Wang and Richardson, 2015). However, due to their limited spatial extent and short lifetimes, LDS have remained challenging to detect and characterize comprehensively.

In this study, we developed a method for identifying LDS using data from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité) imaging spectrometer onboard Mars Express (Bibring et al., 2004). Specifically, we used the strong CO₂ absorption band at 2.77 µm to retrieve dust optical depth. This band is sensitive to altitudes around 20–30 km and offers the critical advantage of being largely insensitive to surface reflectance properties. Applying this retrieval method to data from Martian Years (MY) 27 through 29, we detected 146 LDS events. Our statistical analysis revealed clear seasonal and diurnal patterns. LDS occurrences were most frequent during southern summer (Ls = 270°–360°), consistent with past findings that identify this season as conducive to dust activity (e.g., Smith, 2004; Montabone et al., 2015). However, we also observed an anomalously high frequency of LDS during northern summer in MY27 (Ls = 130°–150°), a period not typically associated with elevated dust activity. Furthermore, a noticeable increase in LDS activity was detected just before the onset of the Global Dust Storm (GDS) in MY28. Diurnally, LDS were most often observed near noon, implying that storm initiation may begin in the morning hours. Their spatial distribution varied significantly with season. During Ls = 0°–180°, LDS tended to be confined to specific regions such as Chryse Planitia and southern Acidalia. In contrast, during Ls = 180°–360°, LDS appeared more widely across mid-latitudes, with a notable absence in the northern high-latitude region (above 40°N). These results offer new insight into the role of LDS in the broader Martian dust cycle, particularly their potential influence on triggering regional or global events.

Building on these findings, we are working on two additional research directions. The first involves estimating the vertical structure of dust by multiple spectral bands with different CO₂ absorption strengths. For example, the 2.01 µm band is sensitive to lower altitudes than the 2.77 µm. By comparing dust optical depth values retrieved from the 2.01 µm (Leseigneur and Vincendon, 2023) and the 2.77 µm (Kazama et al., under review), we aim to characterize whether the dust is well-mixed throughout the column or exists as a detached layer. Preliminary results have demonstrated the feasibility of this technique in distinguishing between detached and uniform dust layers, suggesting the potential for more detailed three-dimensional analyses of dust transport.

The second one concerns the retrieval of surface pressure, which is typically retrieved from the 2.01 µm CO₂ absorption feature (Forget et al., 2007). However, this approach is strongly influenced by dust conditions, which introduces systematic uncertainties under dusty conditions. By incorporating independently retrieved dust optical depth from the 2.77 µm band as a fixed input, we can better decouple atmospheric and surface effects. This improvement may help us better explore potential interactions between dust storms and surface pressure variations, including possible pressure drops or wave-like features linked to thermal tides and gravity waves.

We plan to extend these techniques to data from upcoming Mars missions, including the Martian Moons eXploration (MMX) mission, set to launch in 2026. The MIRS (MMX InfraRed Spectrometer) instrument onboard MMX features high spatial resolution and broad coverage, making it well-suited for continuous monitoring of atmospheric dust and surface pressure at global scales (Barucci et al., 2021; Ogohara et al., 2022; Kuramoto et al., 2022). We aim to investigate seasonal and interannual variability in Martian dust activity from a long-term perspective using MIRS data.

In conclusion, this work integrates high-resolution imaging spectroscopy, retrieval techniques, and statistical analysis to provide a multi-scale understanding of Martian dust dynamics. 

How to cite: Kazama, A., Aoki, S., Leseigneur, Y., Vincendon, M., Kasaba, Y., Nakagawa, H., Gautier, T., Spiga, A., Bertrand, T., Montmessin, F., Ogohara, K., Imamura, T., Murata, I., and Carter, J.: Integrated Study of Martian Dust: Detection of Local Dust Storms, Estimation of Vertical Distribution, and Surface Pressure Analysis Using OMEGA Observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-574, https://doi.org/10.5194/epsc-dps2025-574, 2025.

The great diversity of detected exoplanets offers a wealth of opportunities for atmospheric characterization studies. Transmission spectroscopy has proven effective in probing the chemical composition and dynamics of hot gas giants, and is now beginning to deliver the first insights into the atmospheres of Super-Earths and terrestrial exoplanets. The extension of this technique to rocky worlds, however, has revealed fundamental challenges to atmospheric characterization, associated with their shallow atmospheres and the impact of stellar contamination on observational data [1,2].

The prospect of retrieving the atmospheric chemical composition of an Earth-sized exoplanet is currently beyond reach. Importantly, as these environments begin to be characterized, the most favourable targets for transmission spectroscopy are expected to host atmospheres more akin to Venus than to a truly Earth-like composition [3]. With the goal of informing future observation campaigns of a potential population of Venus-analogues in our galaxy (e.g., using the next-generation facility ELT-ANDES), this work analysed near-infrared transmission spectra from Venus, between 15,624-15,680 Å, as observed with DST-FIRS during the planet’s solar transit of 5–6 June 2012.

Benefiting from a high resolving power (R ~ 90,000), we could resolve distinct molecular absorption lines and disentangle between spectral features from the main isotopologues of carbon dioxide, ¹²C¹⁶O₂ and ¹³C¹⁶O₂, and carbon monoxide, ¹²C¹⁶O (Figure 1). We performed cross-correlation analyses with spectral templates from petitRADTRANS [4], enabling detections of both CO₂ isotopologues and CO, together with a tentative signal for atmospheric ozone on Venus (Figure 2 and Figure 3). Our results highlight potential spectral observables to be expected for Venus-like exoplanet atmospheres, and showcase how current line lists offer sufficient precision to perform such detections with the cross-correlation technique.

This study provides a clear motivation to explore the synergy between Solar System research and the rapidly growing field of exoplanetary science, by enabling direct comparison between ground-based high-resolution data and transmission spectroscopy datasets from space probes [5,6]. Such a comparison provides a valuable calibration template to strengthen the interpretation of transmission spectra from Earth-sized exoplanets near the habitable zone.

Figure 1: Average transmission spectrum of Venus extracted from the atmospheric aureole as observed during the solar transit of 2012 (in gray). Synthetic transmission spectra for CO₂ and CO isotopologues were generated with petitRADTRANS and are shown for comparison. The absorption lines identified upon visual inspection of the observed spectrum have been marked: ¹²C¹⁶O₂ (yellow), ¹³C¹⁶O₂ (red), ¹²C¹⁶O (dark blue).

Figure 2: Cross-correlation functions for (a) ¹²C¹⁶O₂, (b) ¹³C¹⁶O₂, (c) ¹²C¹⁶O (dark blue line). The best-fit Gaussian profiles are shown for each CCF (red dashed line). All panels show the CCFs resulting from the self cross-correlation of the templates (light orange area), which are scaled arbitrarily.

Figure 3: Cross-correlation function for ¹⁶O₃ (dark blue line) along with the best-fit Gaussian profile (red dashed line). The CCF resulting from the self cross-correlation of the template is shown as a light orange area, arbitrarily scaled for comparison.

References:

[1] Moran, S.; et al. High Tide or Riptide on the Cosmic Shoreline? A Water-rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations. Astrophys. J. Lett. 2023, 948, L11

[2] Lim, O.; et al. Atmospheric Reconnaissance of TRAPPIST-1 b with JWST/NIRISS: Evidence for Strong Stellar Contamination in the Transmission Spectra. Astrophys. J. Lett. 2023, 955, L22

[3] Kane, S.; et al. A potential super-Venus in the Kepler-69 System. Astrophys. J. Lett. 2013, 770, L20

[4] Mollière, P.; et al. petitRADTRANS. A Python radiative transfer package for exoplanet characterization and retrieval. Astron. Astrophys. 2019, 627, A67.

[5] Ehrenreich, D.; et al. Transmission spectrum of Venus as a transiting exoplanet. Astron. Astrophys. 2012, 537, L2

[6] Hedelt, P.; et al. Venus transit 2004: Illustrating the capability of exoplanet transmission spectroscopy. Astron. Astrophys. 2011, 533, A136

How to cite: Branco, A., Machado, P., Demangeon, O., Azevedo Silva, T., Sousa-Silva, C., A. Jaeggli, S., Widemann, T., Tanga, P., and Mendoza, L.: High-Resolution Transmission Spectroscopy of Venus: A Proxy for Atmospheric Characterization of Earth-Sized Exoplanets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-580, https://doi.org/10.5194/epsc-dps2025-580, 2025.

Introduction:

New Horizons revealed that complex processes of photochemistry and microphysics are taking place in Pluto’s N₂ atmosphere [1, 2]. Similarly to Titan, the presence of CH₄ in Pluto’s atmosphere leads to the UV photochemical production of more complex molecules such as C₂H₂, C₂H₄, and C₂H₆, as well as nitriles like HCN at high altitudes. These compounds can aggregate through collisions to form increasingly larger spherical particles, creating a haze of solid organic aerosols that encompasses Pluto [1]. The resulting haze particles can continue to grow through coagulation, forming fractal aggregates that lead to a bimodal distribution near the surface [3]. In addition, photochemical-microphysical modeling by [4] shows that condensation of HCN and various hydrocarbons occurs in Pluto’s atmosphere on the tholin-like particles.

Our objective is to investigate the thermal balance of Pluto’s atmosphere using the Pluto Planetary Climate Model (Pluto PCM). We focus on three aspects of the thermal profile:

• (1) The strong negative gradient between the stratosphere at 110 K and the upper atmosphere at 70 K.
• (2) The strong thermal gradient (7 ± 3.5 K) from equator to North Pole tentatively observed (2) with ALMA [5], and the differences in temperatures observed by New Horizons between the entry and exit profiles above 5 km altitude [6].
• (3) The 3 km-deep cold layer observed by New Horizons.

The thermal profile of Pluto’s atmosphere has been measured from ground-based observations and from the REX instrument on-board New Horizons [6, 7]. It has been proposed by [8] that the thermal structure is primarily regulated by the heating and cooling properties of tholin-type haze, whose heating/cooling rates could exceed that of gases by two orders of magnitude. However, in scenarios where the haze is dominated by icy components, its radiative impact on the atmosphere is expected to be more limited [4]. Therefore, the main cooling mechanism responsible for the observed vertical thermal gradient remains uncertain.

The Pluto PCM:

The Pluto PCM is an ideal tool for understanding the thermal balance of Pluto's atmosphere and the role of hazes in its heating and cooling properties. This model is an updated version of the Legacy Pluto PCM described in [9, 10]. It now includes a microphysical model for organic haze, accounting for its formation (through methane photolysis), evolution (via coagulation), and transport (by sedimentation). This microphysical model has been coupled with the radiative transfer scheme of the Pluto PCM in order to assess the impact of the haze on Pluto’s climate and to study their radiative effects on the atmosphere.

Preliminary results:

In this presentation we will present the results obtained with our model and compare them to observations (e.g. [5, 6]). Initially, we will focus on the spatial distribution of the haze and its physical properties predicted by the model. We will then examine the impact of the haze on the global climate, particularly on the heating and cooling rates of the atmosphere. Figure 1 illustrates the radiative, dynamic, and total heating/cooling rates of the atmosphere during the year 2015. Our model supports the findings of [8], showing haze heating/cooling rates between 10^-8 and 10^-6 W.m^-3 below 100 km — about 100 times higher than those of the gases. The switch to the 3D model allows us to observe that the Northern Hemisphere is heated by the absorption of solar flux by CH₄ and haze during the Northern summer, while the Southern Hemisphere, in winter, is cooled by the infrared emission of gas (CH₄ and CO) and haze. The dynamic trend acts in opposition to the radiative trend, redistributing energy throughout the atmosphere, so that the heating and cooling rates become nearly uniform across the entire atmosphere.

Figure 1: Radiative (left), dynamical (center), and total (right) heating/cooling rates predicted by the Pluto PCM for the year 2015 in Pluto’s atmosphere. The radiative component is related to the absorption and emission by atmospheric constituents, while the dynamic component is associated with general circulation and atmospheric transport.

Future work:

In a future study, we will couple the haze microphysical model with a cloud microphysical model, allowing atmospheric hydrocarbons (CH₄, C₂H₂, C₂H₄, and C₂H₆) and nitriles (HCN) to condense onto haze particles. Accounting for the effects of condensation and the resulting new optical properties of the particles in the Pluto PCM’s radiative transfer will provide us with a comprehensive understanding of the haze’s impact on Pluto’s global climate.

References: [1] Gladstone G. R. et al (2016) Science, 351, 6279. [2] Cheng A. F. et al (2017) Icarus, 290, 112-133. [3] Fan S. et al (2022) Nature Communications, 13, 240. [4] Lavvas P. et al (2021) Nature Astronomy, 5, 289-297. [5] Lellouch E. et al (2022) Icarus, 372, 114722. [6] Hinson D. P. et al (2017) Icarus, 290, 96-111. [7] Dias-Oliveira A. et al (2015) The Astrophysical Journal, 811, 1. [8] Zhang X. et al (2017) Nature, 511, 7680. [9] Forget F. et al (2017) Icarus, 287, 54-71. [10] Bertrand T. et al (2020) Journal of Geophysical Research (Planets), 125, 2.

How to cite: de Batz de Trenquelléon, B., Bertrand, T., Falco, A., Lellouch, E., Millour, E., and Forget, F.: Investigating the Radiative Balance of Pluto's Atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-638, https://doi.org/10.5194/epsc-dps2025-638, 2025.