Aurélien Stolzenbach

Near‐ultraviolet imaging of the planet Venus reveals inhomogeneous absorption features in the otherwise bright clouds. This absorption has been studied for nearly 100 years, but identification of the near‐UV absorber remains one of the largest open questions in Venusian research. Based on a multiple scattering radiative transfer model, the observed absorption between 300 and 600 nm can be produced by 1.0–1.2 wt% ferric chloride (FeCl ₃ ) in the mode 1 (∼0.2 μm radius) sulfuric acid cloud droplets—less than the in‐cloud iron mass loading measured during the Venera‐12 mission. We present a novel chemical network within a global chemistry‐climate model, which provides the required source of FeCl ₃ in the upper cloud region from the reaction of Fe‐containing molecules, produced by the ablation of cosmic dust particles, with atmospheric HCl. A residence lifetime of ∼3,000 years is required to provide sufficient optical absorption from a purely meteoric source of FeCl ₃ .

IntroductionThe Martian atmospheric aerosols are mainly composed of mineral dust and/or water ice. Water ice clouds affect both the radiative balance [1] and the water cycle [2], while mineral dust is globally the most abundant component, modifying the thermal structure and atmospheric dynamics [3,4]. It can affect observations from both orbiting satellites and rovers on ground, especially at the dusty season around perihelion, where many storms are initiated at different temporal and spatial scales [5].Both aerosol types are related, as dust can serve as a base for water ice cloud formation [5]. Therefore, a comprehensive understanding of the Martian atmosphere requires a simultaneous climatology of both components, such as the one presented in this work.Dataset and methodologyNOMAD is a three spectrometers suite on board the Trace Gas Orbiter (TGO). It has been routinely observing the Martian atmosphere since April 2018 [6], providing detailed atmospheric transmission vertical profiles. Data acquired from its solar occultation (SO) channel are used to retrieve aerosol properties and their vertical distributions in the infrared at high vertical resolution. This is achieved by combining multiple diffraction orders (i.e., wavelength intervals), selected to maximize coverage of the SO spectral range while consistently including an order near 3 μm. This spectral region is key to distinguish between dust and water ice aerosols [6].Our retrieval strategy builds on [7] and is thoroughly described there. The inferred aerosol characteristics using this methodology are size (effective radius and variance) and composition (proportions of mineral dust and water ice). These magnitudes are used to derive other aerosol magnitudes such as number density, mass loading and mass mixing ratio.We processed ~3000 SO observations from Ls = 194° (MY34) to the end of MY37, significantly extending the dataset in [7], focused only on the first year of NOMAD observations. We have also developed a scheme to identify cases with mixed aerosol components (dust and ice) from those where there is a single dominant component, and the secondary one is only adding noise to the results. ResultsIn contrast to [7], due to our extended dataset we can study latitudinal and interannual comparisons between the different Martian Years, being able to detect the global patterns that govern the distribution of dust and water ice clouds.We report an average maximum altitude of aerosol detection around 60 km near perihelion (Ls ~ 270°) and 40 km near aphelion (Ls ~ 90°). Variations within these values are sometimes significant, and linked to the latitude of the observations. Our results reveal features such as water ice clouds and signatures of both regional and global dust storms. All four Martian Years show systematic differences of their aerosol properties between aphelion and perihelion, which is consistent with the previous work by [7].Lastly, a comparison of our results with similar datasets from NOMAD-UVIS [8] and ACS [9] has been performed. This comparison focuses on comparing the extinctions and effective radius retrieved, as well as their seasonal and latitudinal distribution. We have found that NOMAD-UVIS retrieves in average smaller particle sizes than SO, which is more sensitive lo larger sizes. Therefore, both wavelength ranges, ultraviolet and infrared, are complementary to produce a complete climatology of Martian aerosols. This combined information is important for improving climate models, which rely on a priori assumptions about aerosol microphysics, composition, and optical properties. References: Wilson, R.J, et. al., 2008.Montmessin, F. et. al, 2004.Michael Battalio and Huiqun Wang, 2021.Smith, M.D. et. al, 2019.Määttänen, A. and Montmessin, F., 2021Vandaele, A, C. et. al, 2018Stolzenbach, A. et. al, 2023.Flimon, Z., et. al, 2025.Schterbinine, A. et. al, 2022

The depletion of SO2 in Venus's middle atmosphere, observed from below and above the cloud deck, remains unexplained. SO2 abundance decreases from 130 ppmv below the clouds [1] to a few hundred ppbv at the cloud top [2,3,4,5]. The VenSpec suite onboard EnVision aims to provide new observations of key gas species (SO2, H2O, CO, etc.) in the middle atmosphere to help constrain its chemical landscape.Modeling efforts have explored different approaches to Venusian sulfur chemistry. [6] proposed that increased SO2 dissolution, facilitated by salts in cloud droplets, may account for the observed depletion. However, most chemical models instead simplify the issue by reducing the bulk SO2 reservoir [7, 8, 9, 10, among others]. Despite this simplification, sulfur photochemistry remains intricate, governed by a balance between SO2 photolysis and the condensation of sulfuric acid against vertical transport. To further investigate the modeled sulfur chemistry in the Venusian middle atmosphere, we employed the chemical pathways analysis tool PaPy [11], which builds on [12]. PaPy has been successfully applied to Early Mars simulations [13] and has now been adapted for use with the Venus Planetary Climate Model (Venus PCM) results.We will detail the key chemical pathways governing SO2 behavior in the Venusian cloud deck using 3D outputs from the Venus PCM. We will examine how sulfur chemistry varies with local time and latitude throughout the Venusian day. On the dayside, photolysis dominates SO2 chemistry and the production of H2SO4 is crucial. Nonetheles, we will present how the SO2 chemistry remains more complicated. On the nightside, the SO2 chemistry involves significant contributions from NOx, ClOx, and Sx species while keeping interesting and unexpected behavior as some altitude ranges still are being dominated by a NET chemical loss of SO2. At the morning and evening terminators, we will identify the specific chemical pathways responsible for observed variations in SO2 abundances. These pathways will be analyzed within their local transport context, considering both advective and diffusive dynamics as evaluated by the Venus PCM.Acknowledgements: Authors FDS, AS, LML and AM acknowledges financial support from the Severo Ochoa grant CEX2021-001131-S funded by MCIN/AEI/ 10.13039/501100011033. Author AS acknowledges financial support from project PID2021-126365NB-C21 funded by MCIN/AEI/10.13039/501100011033/ and FEDER.[1] Marcq, E., et al. JGR: Planets 113.E5 (2008).[2] Marcq, E., et al. Icarus 335 (2020): 113368.[3] Encrenaz, T., et al. A & A 639 (2020): A69.[4] Encrenaz, T., et al. A & A 703 (2025): A219.[5] Iwanaka, T., et al. JGR: Planets 130.7 (2025): e2024JE008775.[6] Rimmer, P., et al. The Planetary Science Journal 2.4 (2021): 133.[7] Stolzenbach, A., et al. Icarus 395 (2023): 115447.[8] Shao, W. D., et al. JGR: Planets 125.8 (2020): e2019JE006195.[9] Shao, W. D., et al. The Planetary Science Journal 3.1 (2022): 3.[10] Bierson, C. J., and Zhang, X. JGR: Planets 125.7 (2020): e2019JE006159.[11] Stolzenbach, A., PaPy (Pathway analysis for Python) (1.0.0). Zenodo. (2025)[12] Lehmann, R. Journal of atmospheric chemistry 47.1 (2004): 45-78.[13] Maurice, M., et al. Icarus (2025): 116914.

IntroductionUnlike the other terrestrial bodies in the Solar System, Venus is shrouded in a thick, multi-layered cloud at an altitude ranging from 48 to 70 kilometers[1]. These clouds consist of droplets of liquid SO₂ (70–95% by mass) and H₂O, divided into three modes with radii of approximately 0.3, 1, and 2 micrometers, respectively[2].The lack of in situ observations (only one in-situ profil of the cloud droplets characteristics[2]) means we have to actively try to model these clouds to understand them. The models developed so far are primarily 1D[3-7], and the few 2D and 3D models are restricted to simplified equilibrium schemes[8-11]. Here, we present the first 3D simulation of Venus's clouds using the Venus PCM[12-13], a global climate that includes chemistry, radiative transfer and dynamics, coupled with the microphysical scheme MAD-VenLA[14-16]. We use this opportunity to make comparison with Akatsuki UV observations. MethodWe coupled the MAD-VenLA model, developed by Guilbon[14], Määttänen[15] and Streel[16], with the Venus PCM. MAD-VenLA is a modal model that describes two particle modes with a lognormal shape and fixed standard deviation. It includes a homogeneous nucleation scheme[17], a simplified parametrization of heterogeneous nucleation, and Brownian coagulation, condensation, and evaporation[14-15]. Additionally, MAD-VenLA incorporates mode merging[18], which allows particles to be transferred from one mode to another. In its latest version, a sedimentation scheme has also been added[16].ResultsWe will present the first 3D comparisons between and observations from previous missions.For example, an ongoing study[19] of the Japanese Akatsuki mission's ultraviolet (UV) observations revealed different reflectivity patterns between sulfur dioxide (SO₂) at 283 nm and the unknown UV absorber at 365 nm at low latitudes. The averaged minimum reflectivity at 365 nm occurred ~2 hours earlier in local time than at 283 nm. Our simulations using the Venus PCM (Fig. 1) reveal the presence of a droplet layer situated in the afternoon at low latitude at 70 km, which could mask the unknown UV absorber below and alter the afternoon albedo, explaining the observations made. These droplets are the result of homogeneous nucleation episodes, which are allowed by the chemistry and vertical transport. This highlights the need for 3D simulations in order to understand such processes.Figure 1: Horizontral map of the integrated number of droplets from the top of the atmosphere to a specific altitude (resp. 80,75,70 and 65 km) after 2 venusian days.ReferencesP. Mills, L. W. Esposito, Y. L. Yung, Geophys. Monogr. Ser. 176, 73–100 (2007).R. G. Knollenberg, D. M. Hunten, J. Geophys. Res. 85, 8039–8058 (1980).P. James, O. B. Toon, G. Schubert, Icarus. 129, 147–171 (1997).Imamura, G. L. Hashimoto, J. Geophys. Res. 103, 31349–31366 (1998).Yamamoto, M. Takahashi, J. Geophys. Res. 111, 2006JE002688 (2006).McGouldrick, E. L. Barth, Planet. Sci. J. 4, 50 (2023).Karyu et al., Planet. Sci. J. 5, 57 (2024).Lee, S. R. Lewis, P. L. Read, Icarus. 206, 662–668 (2010).Ando, M. Takagi, N. Sugimoto, H. Sagawa, Y. Matsuda, Journal of Geophysical Research: Planets, in press, doi:10.1029/2019JE006208.Karyu et al., Journal of Geophysical Research: Planets, in press, doi:10.1029/2022JE007595.Stolzenbach, F. Lefèvre, S. Lebonnois, A. Määttänen, Icarus. 395, 115447 (2023).Lebonnois, N. Sugimoto, G. Gilli, Icarus. 278, 38–51 (2016).Gilli et al., Icarus. 281, 55–72 (2017).Guilbon, thesis, Université Paris Saclay (COmUE) (2018).Määttänen, S. Guilbon, J. Burgalat, F. Montmessin, Advances in Space Research. 71,1116–1136 (2023).Streel, thesis, Sorbonne-Université (2025).Määttänen et al., JGR Atmospheres. 123, 1269–1296 (2018).Whitby, F. Stratmann, M. Wilck, Journal of Aerosol Science. 33, 623–645 (2002).R. L. S. E. Aliste Castillo et al., BACO-25 conference abstract (2025).

Venus is home to vivid sulfur chemistry, with as the major sulfur gas species and a global cloud layer between 47 and 70 km composed of and O. The chemistry in the clouds has been extensively studied with 1D models, but none is able to reproduce the three orders of magnitude decrease of inside the clouds. Sulfur allotrope chemistry could be a candidate for buffering sulfur atoms, as it can grow into long chains. Sulfur hydrides such as S is considered as one of the main sulfur‐bearing compounds, possibly linked to surface reactions or have a volcanic origin like on Earth. In this study, we use for the first time a 3D model to study the climatology of the sulfur allotrope in gas and condensed phase and hydride chemistry in the clouds with an explicit photolysis calculation and updated UV cross‐section. S is close to ppmv levels below the cloud base, the third most abundant sulfur‐bearing species in this region after and OCS. Below the clouds, gas phase is the most abundant sulfur allotrope. Above, the photolysis and condensation are preponderant, and the gas phase decreases to extremely low values. Polysulfur can therefore not be the unknown absorber at cloud‐top altitudes. The condensed phase polysulfurs are present from 30 to above 100 km. condensed phase is the most abundant due to its low saturation mixing ratios. Polysulfur species represent a substantial sulfur reservoir in the upper cloud, equivalent to a few ppmv.

The ultraviolet nightglow of nitric oxide (NO) on Venus offers a unique window into the dynamics and chemistry of its upper atmosphere. We present three‐dimensional simulations of Venus' NO nightglow using a ground‐to‐thermosphere model, revealing a strong, short‐timescale variability consistent with observations. The Venus Planetary Climate Model accurately reproduces the altitude of peak emission at 115 km, matching SPICAV data, and shows an average peak brightness of 53 ± 33 kR, only 5% below the observed values. Crucially, the observed variability and morphology of the NO emission are tightly linked to atmospheric dynamics; its maxima strongly correlate with horizontal wind convergence, leading to localized subsidence. This downward transport is essential for delivering nitrogen atoms to fuel the nightglow, making it a critical tracer for understanding Venus's complex atmospheric circulation. Our findings underscore the importance of the NO nightglow as a powerful diagnostic for the solar‐to‐antisolar circulation on Venus.

L'étude de la planète Vénus est particulièrement intéressante car elle possède de nombreuses caractéristiques avec la Terre, des dimensions et une composition semblables. Malgré ces éléments en commun, ces planètes sont foncièrement différentes à notre époque. Vénus possède une atmosphère cent fois plus dense à sa surface que celle de la Terre. Cette atmosphère est composée essentiellement de dioxyde de carbone là où l'atmosphère terrestre est composée d'un mélange de diazote et d'oxygène moléculaire. La composition chimique de l'atmosphère vénusienne pose de nombreux problèmes aux chimistes. Vénus fait l'objet d'observations modernes depuis la Terre, de missions spatiales depuis les années 70 et l'ESA a lancé en 2005 la mission Venus Express qui est restée en orbite autour de Vénus jusqu'en décembre 2014. L'explication de la distribution et des concentrations des espèces chimiques mesurées est étudiée par une communauté de modélisateurs en parallèle des observations. Cependant, ces modélisations étaient contraintes à utiliser des modèles 1D qui contenaient de nombreuses paramétrisations. Durant ma thèse j'ai mis à jour un modèle chimique, développé au LATMOS, et développé un modèle des nuages de Vénus qui ont tout deux été introduit à un GCM 3D de Vénus développé au LMD. Ce nouveau modèle permet d'appliquer en 3D les hypothèses les plus récentes sur les cycles chimiques vénusiens, tels que la décroissance du dioxyde de soufre et de la vapeur d'eau dans la couche nuageuse, les variations en latitudes du monoxyde de carbone, la stabilité du dioxyde de carbone, la composition des gouttelettes des nuages de Vénus et la masse condensée dans les nuages.

Our current understanding of Venusian sulfur chemistry remains one of the major open questions regarding the planet's atmospheric composition. The depletion of SO2 abundance in the mesosphere is evidenced by merging observations from both below and above the cloud deck. Values decrease from 130 ppmv at 30–40 km (Marcq et al., 2008) to a maximum of a few hundred ppbv at the cloud top, around 70 km (Marcq et al., 2020; Encrenaz et al., 2020, 2025; Iwanaka et al., 2025). This depletion is still not theoretically explained. The VenSpec suite of spectrometers onboard EnVision is designed to provide new observations of key gas tracers (SO2, H2O, CO etc.) in the Venusian mesosphere to help constrain its chemical landscape.Regarding modeling, while Rimmer et al. (2021) proposed increased SO2 dissolution aided by salts present in cloud droplets, most other chemical models simplify the issue by reducing the bulk reservoir of SO2 (Stolzenbach et al., 2023; Shao et al., 2020; Bierson et al., 2020). Despite this assumption, sulfur photochemistry remains complex and is linked to a delicate equilibrium between SO2 photolysis and the condensation of one of its products, sulfuric acid. To better decipher sulfur chemistry in the Venusian mesosphere, we used a chemical pathways analysis program named PaPy (Stolzenbach, 2025), based on the work of Lehmann (2004). PaPy was successfully applied to Early Mars simulations (Maurice et al., 2025), and we have extended its use to the Venus Planetary Climate Model (PCM) results.We propose to detail the most prominent chemical pathways affecting the state of SO2 in and above the cloud deck. We will focus specifically on the vertical evolution of these pathways; some are locally significant, such as the cycles involving H2SO4 production at 62–64 km, while others span a wider range of altitudes above the clouds. Finally, we will interpret these results in light of previous versions of the Venus PCM chemical system to evaluate the impact of the added nitrogen and sulfur allotrope chemistry on SO2 pathways.Marcq, E., et al. Journal of Geophysical Research: Planets 113.E5 (2008).Marcq, E., et al. Icarus 335 (2020): 113368.Encrenaz, T., et al. Astronomy & Astrophysics 639 (2020): A69.Encrenaz, T.,, et al. Astronomy & Astrophysics 703 (2025): A219.Iwanaka, T., et al. Journal of Geophysical Research: Planets 130.7 (2025): e2024JE008775.Rimmer, P. B., et al. The Planetary Science Journal 2.4 (2021): 133.Stolzenbach, A., et al. Icarus 395 (2023): 115447.Shao, W. D., et al. Journal of Geophysical Research: Planets 125.8 (2020): e2019JE006195.Bierson, C. J., and X. Zhang. Journal of Geophysical Research: Planets 125.7 (2020): e2019JE006159.Stolzenbach, A., "PaPy, a Python module for chemical pathways analysis", (2025), https://doi.org/10.5281/zenodo.17223321Lehmann, R. Journal of atmospheric chemistry 47.1 (2004): 45-78.Maurice, M., et al. Icarus (2025): 116914.

The rate of injection of cosmic dust into the Earth’s atmosphere is estimated to be 28 ± 16 tonnes per day. As this dust ablates between altitudes of 80 and 105 km, it generates neutral and ionized metal layers. These layers have been characterized through space-based spectroscopy, ground-based lidar observations, and rocket-borne mass spectrometry. A cosmic dust input rate of 31 ± 18 t d−1 is estimated for Venus [Carrillo-Sanchez et al., 2020]; this material should ablate and form metal layers between 105 and 120 km. However, these layers have not yet been observed on Venus. In this study, we model the latitudinal and diurnal variability of the Venusian metal layers (mainly Na, Mg, Fe and SiO), and assess the probability of their spectroscopic detection. For the simulations, the whole atmosphere Venus Planetary Climate (PCM) Model was used, with the metals being injected into the upper atmosphere through an orbitally varying Meteoric Input Function (MIF). The model also incorporates detailed neural and ion-molecule atmospheric chemistry networks for Fe, Mg, Na and Si, and was run for over two Venus years. A pronounced diurnal variability in the Venusian metal layers is predicted: the neutral metal atom layers peak on the night side, with maxima occurring around the morning terminator, when neutral metals have had the longest time to build up through the neutralization of their corresponding metal ions. Latitudinal variability in metal column density is highly correlated with Venusian circulation, driven by strong meridional and zonal winds. The Fe, Na, Mg and SiO layers peak at different altitudes, which results in species-dependant latitudinal distributions shaped by the horizontal winds. The metal layers are potentially observable in the atmosphere of Venus. In particular, the predicted Na layer should be detectable from terrestrial telescopes, either via solar-pumped resonance fluorescence or via occultation at 589 nm. Venusian Na should produce a particularly strong signal at the morning terminator in the northern hemisphere, consistent with the variability highlighted above. Different observational techniques will be discussed. Finally, we emphasize the significance of detecting metal layers in Venus’s CO2-rich atmosphere, in contrast to Earth’s O2–rich atmosphere. Knowledge of the metal layers on Venus should provide a useful framework for probing the atmospheres of Venus-like exoplanets. In general, the distribution and behaviour of meteoric sodium in exoplanetary atmospheres merits further exploration, as indicated by the growing number of sodium detections in the atmospheres of Hot Jupiters and Hot Neptunes.

Venus' atmosphere layers between 80 and 130 km mark the transition between the superrotation and the day‐to‐night circulation regimes. Accurately modeling this layer is essential to better understand the planet's atmospheric dynamics. However, this region remains poorly constrained by observations, and its variability is not yet fully captured by current 3D models. Here we use the latest version of the Venus Planetary Climate Model (V‐PCM), a ground‐to‐thermosphere global circulation model, to investigate possible scenarios relevant to future EnVision observations above the cloud tops. We focus on current data‐model biases and provide a tentative interpretation of their origin. Benchmark simulations by Martinez et al. (2024, https://doi.org/10.1016/j.icarus.2024.116035 ) overestimate the nightside airglow emission by a factor of two and place the emission peak 5–7 km higher than observed. Furthermore, the emission distribution is not centered around midnight, but shifted to LT = 4h, likely due to a strong (100 m/s) zonal wind component below 105 km. We performed sensitivity tests on unconstrained parameters (e.g., gravity wave drag and eddy diffusion) to evaluate their impact on the dynamical structures. Results show that reducing non‐orographic gravity wave forcing below 105 km weakens that superrotation wind component, and recenter the emission around midnight. However, the altitude bias appears linked to insufficient vertical transport in the model. These findings underline the need for future space missions capable of continuously monitoring mesospheric gravity waves and nightglow to better constrain their spatial and temporal variability and improve the representation of key dynamical processes in Venus' upper atmosphere.