Fall and winter on Titan are marked by the formation and persistence of a vast polar “hood” cloud, covering the cold pole and extending equatorward as far as 40° latitude [e.g., 1]. Extending from the upper troposphere into the lower stratosphere, a polar hood cloud was present in the northern hemisphere during the Voyager I flyby and then again when Cassini arrived during the next northern winter in 2004 (L_s ~ 290°) [2]. Although the full lifecycle of Titan’s polar hood cloud has not been observed, the Cassini mission provided observations of the north polar hood from mid-winter through northern spring, as well as observations of south polar clouds that started forming early in southern fall. In 2012 (L_s ~ 32°), near-infrared imagery from early southern fall revealed the formation of a vast south polar cloud near the top of the stratosphere, over 150 km higher than any previously observed cloud [3,4]. By combining observations of south polar fall with observations of north polar winter and spring, others have started to construct a story of the formation, evolution, and dissipation of Titan’s polar hood [1]. Despite the polar hood cloud’s optical thickness, details of the cloud composition remain vague. While signs of HCN, benzene, and cyanoacetylene ices have been identified in this fall south polar cloud, and radiative transfer retrievals indicate the presence of mixed ices, the abundances of less radiatively active species like methane or ethane is not as well constrained [2]. We contribute to the story of the polar hood lifecycle using a combination of cloud microphysical modeling and image analysis [5,6]. Through careful analysis of imagery, we track south polar cloud’s spatial evolution and find that it descends from an altitude of about 300 km in 2012 to below 230 km by 2016 (L_s = 79°), while expanding equatorward following the terminator [6]. We show using microphysical modeling that a pure HCN cloud with the observed characteristics [3,4] requires temperatures below 110 K at an altitude of 300 km over the pole [5]. Finally, using simulations of several additional volatile species, we trace the evolving chemical composition, and estimate the impact of condensation and precipitation on the stratospheric volatile budget.

[1] Le Mouelic et al. 2018, Icarus, 311, 371–383. [2] Anderson et al. 2018, Space Sci Rev, 214, 125. [3] West et al. 2016, Icarus, 270, 399. [4] de Kok et al. 2014, Nature, 514, 65. [5] Hanson et al. 2023, Planetary Sci J, 4, 237. [6] Hanson et al. 2025, Geophys Res Lett, 52, e2024GL113415.

How to cite: Hanson, L., Waugh, D., Anderson, C., Barth, E., and French, R.: Evolution of Titan’s Fall and Winter Polar Hood Cloud, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-221, https://doi.org/10.5194/epsc-dps2025-221, 2025.

Aerosols, the main product of Titan’s atmosphere, significantly influence the moon’s global climate, and photochemical processes [2, 7, 6, 3]. Characterizing this haze is crucial to constrain and validate theoretical microphysics models [8], and understand the complex prebiotic processes occurring in what is considered as a primitive Earth-like environment [10, 4].

The Cassini spacecraft’s instruments can help characterize aerosols by the extinction they provoke, through stellar occultation measurements performed by VIMS [1] or UVIS [5], retrievals from thermal emission observed by CIRS [14], or by direct imaging with ISS [12]. Extinction, however, depends on both the density and size of aerosol particles, and determining these two quantities remains a challenge as well as a necessity. Huygens measurements analyzed by Tomasko et al. [13] constrained these physical properties, but only for a narrow range of altitude on a specific Titan location. Some studies [1, 14] derived density profiles assuming these size properties. Scattered light observations with ISS allowed to constrain the haze radius and density at the detached haze layer near 500 km [11, 15]. However, at higher altitudes where the haze formation takes place, there are no constraints for the haze microphysical properties.

We present a new analysis of Cassini’s Ultraviolet Imaging Spectrograph (UVIS) data allowing to retrieve both haze particle size and density as altitude profiles at different latitudes across time during the mission. UV light is suitable to derive haze properties at high altitudes where the early stages of photochemistry and haze formation and growth take place [8, 9]. We use FUV channel airglow spectra, where the strong effect of haze scattering can be seen as a continuous radiance feature between 1600 and 1900 Å. We integrate this wavelength range to obtain a single radiance value and we use limb observations to derive the radiance profile as a function of altitude.

To reproduce these profiles, we forward model the radiative transfer processes (from gases and aerosols) and the N2 airglow based on each observation geometry. We model the atmosphere as spherical layers, considering a log-normal distribution of haze particle radius, with mean radius and particle density set as free parameters in each layer. With sufficient observations at different phase angles, we fit the aerosol phase function curve, which directly depends on the aerosol bulk radius. Thus we retrieve the particle radius and the particle density. We fit for this reason the radiance profiles of all observations in a given year at once, assuming the atmospheric structure remains stable throughout the year.

Observations for each fly-by are binned by altitude and latitude to increase signal-to-noise ratio while keeping a sufficient spatial resolution. Our radiance profiles cover altitudes from 1500 km to the surface. We retrieve haze properties between 200 km and about 800 km. The whole Cassini mission coverage is used to derive haze properties across different latitude locations, and across the years to derive time-dependant characteristics (on annual timescale) and seasonal changes. Simultaneous analysis of both limbs allow to differentiate morning and evening parts of Titan and derive differences on a Titanian day scale.

Several fly-bys performed limb observation with a very high phase angle (>150°), giving us unique opportunities to observe both sunrise and sunset limbs in similar illumination conditions. We report an unforeseen feature in the radiance profiles observed during such flybys. Peak radiance is produced at notably different altitudes and haze properties are not sufficient to explain such daily difference. We present the analysis conducted to confirm this feature and explore potential phenomena explaining its specific characteristics.

References

[1] Bellucci, A., et al. “Titan Solar Occultation Observed by Cassini/VIMS: Gas Absorption and Constraints on Aerosol Composition”. Icarus, (2009)

[2] Courtin, R. “Aerosols on the Giant Planets and Titan”. Space Science Reviews, (2005)

[3] De Batz De Trenquelléon, B., et al. “The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles”. The Planetary Science Journal, (2025)

[4] Hasenkopf, C., et al. “Potential Climatic Impact of Organic Haze on Early Earth”. Astrobiology, (2011)

[5] Koskinen, T., et al. “The Mesosphere and Lower Thermosphere of Titan Revealed by Cassini/UVIS Stellar Occultations”. Icarus, (2011)

[6] Larson, E., et al. “Simulating Titan’s Aerosols in a Three Dimensional General Circulation Model”. Icarus, (2014)

[7] Lavvas, P., et al. “Condensation in Titan’s Atmosphere at the Huygens Landing Site”. Icarus, (2011)

[8] Lavvas, P., et al. “Surface Chemistry and Particle Shape: Processes for the Evolution of Aerosols in Titan’s Atmosphere”. The Astrophysical Journal, (2011)

[9] Lavvas, P., et al. “Aerosol Growth in Titan’s Ionosphere”. Proceedings of the National Academy of Sciences, (2013)

[10] Sagan, C., and Chyba, C. “The Early Faint Sun Paradox: Organic Shielding of Ultraviolet-Labile Greenhouse Gases”. Science, (1997)

[11] Seignovert, B., et al. “Aerosols Optical Properties in Titan’s Detached Haze Layer before the Equinox”. Icarus, (2017)

[12] Seignovert, B., et al. “Haze Seasonal Variations of Titan’s Upper Atmosphere during the Cassini Mission”. The Astrophysical Journal, (2021)

[13] Tomasko, M., et al. “A Model of Titan’s Aerosols Based on Measurements Made inside the Atmosphere”. Planetary and Space Science, (2008)

[14] Vinatier, S., et al. “Analysis of Cassini/CIRS Limb Spectra of Titan Acquired during the Nominal Mission II: Aerosol Extinction Profiles in the 600–1420 cm^-1 Spectral Range”. Icarus, (2010)

[15] West, R., et al. “The Seasonal Cycle of Titan’s Detached Haze”. Nature Astronomy, (2018)

How to cite: Le Guennic, N., Lavvas, P., Koskinen, T., and Hoover, D.: Aerosol properties and evolution at different timescales in Titan's atmosphere from Cassini UVIS observations throughout the mission, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-288, https://doi.org/10.5194/epsc-dps2025-288, 2025.

The Cassini-Huygens mission revealed that energetic (1 – 1000 keV) water ions (OH_x⁺, x = 0,1,2), originating from Enceladus' geysers, precipitate in Titan's upper atmosphere where molecules reaching a mass-to-charge (m/z) of several thousand atomic mass units have been detected. These aerosol embryos, that were not expected at such high altitudes, have been attributed to polycyclic aromatic (nitrogen bearing) hydrocarbons (PANHs) that would result from the ionization and dissociation of the major atmospheric compounds, N₂ and CH₄ by solar photons [1].

A fraction of the Enceladus’ water ions must collide with Titan’s macromolecules but the impact on the atmospheric chemistry is unclear. Do the ions trigger the formation of more complex organic species, maybe including oxygen? Do they sputter some organic material back into the gas phase? Since aerosols sediment towards the surface, the formation of complex oxygenated molecules in the upper atmosphere would add a new dimension with strong exobiological implications to the carbon / nitrogen / hydrogen chemistry endogenous to Titan.

Titan’s photochemistry has inspired many to simulate these conditions in the laboratory [2]. The complex organic compounds resulting from photolysis or radiolysis of N₂/CH₄ mixtures have been deemed “tholins” and numerous studies have attempted to characterize their properties. Infrared spectroscopy has shown that the tholins chemical functions include primary and secondary amines, (iso)cyanides, carbodiimides, aliphatic and heteroaromatic groups. Exact mass Fourier transform mass spectrometry (FT-MS) has demonstrated that their constituent molecules extent to ~500 Da and have unsaturation levels consistent with high degrees of both cyclization and aromaticity, favoring PANH-type structures.

Despite the important literature on tholins, the chemical evolution of nitrogen-rich organics upon interaction with energetic particles has been largely unexplored. One study simulated in the laboratory how vacuum ultraviolet irradiation affects the tholins optical properties as probed by infrared spectroscopy [3]. This work provided evidence that photochemistry could deplete the sensitive primary and secondary amine functions while preserving nitrogen-bearing functionalities that are more strongly bound, such as tertiary amines, imines and nitriles. Adenine (C₅H₅N₅) is a simple heterocyclic aromatic molecule that has been identified in Titan’s tholins [4]. Infrared spectroscopy of adenine samples irradiated by photons, electrons or ions over a wide energy range shows its destruction as well as the formation a solid residue probably of macromolecular nature [5,6]. However, neither the molecular content of the organic residue or the sputtering into the gas phase have been investigated so far.

In this work, we have irradiated under ultra-high vacuum thin film samples of adenine. Adenine was deposited homogeneously in several hundred nm thin layers onto MgF₂ or ZnSe windows [10]. Irradiation experiments were performed either at the ARIBE beam line at GANIL (Caen, France), at the SIDONIE isotope separator (Orsay, France) or at the HUN-REN Institute for Nuclear research (Atomki) in Debrecen, Hungary [7,8,9]. Adenine samples were irradiated with either Ne^𝑞+, O^𝑞+, OH^𝑞+ or H₂O^𝑞+ at various energies from 10 to 70 keV, temperatures (150, 300 K), and fluences (up to 2x10¹⁵ ions.cm⁻²). Infrared absorption spectra of the adenine samples were obtained in situ during the irradiation with a FTIR spectrometer while the residual gas in the experimental chamber was measured with in-situ quadrupole mass spectrometry, analyzing the material that was sputtered into the gas phase during irradiation. The molecular content of the soluble phase of the irradiated samples was obtained with a very high-resolution mass spectrometer at the Institut de Planétologie et d’Astrophysique de Grenoble (France).

We show that the energetic ion irradiation of the samples leads to their destruction, through both chemical processes forming new species in the solid phase and sputtering into the gas phase. The macromolecules detected in the solid phase far exceed the molecular mass of adenine, reaching up to m/z 500. They show great chemical diversity and can be expressed as (HCN)_z-R families, where z can reach 17 and R can be C, H, N, NH, CH or CN. In total, nearly 100 individual families have been identified, 28 of which can be found in every irradiated sample. Their aromaticity equivalent is higher than that in other N rich samples such as Titan tholins and HCN-polymers, corresponding to polycyclic aromatic nitrogen-bearing hydrocarbons. A number of small adenine fragments, including N₂, nitriles and hydrocarbons, are sputtered into the gas phase, throughout the irradiations. Larger species like intact adenine are only detected at the on-set of the irradiations.

These experimental results suggest that ion deposition in Titan’s atmosphere may play a role in the rapid molecular growth occurring at high altitudes and should be considered in photochemical-microphysical models.

Acknowledgments

This work is supported by the Programme National de Planétologie (PNP), the French National Research Agency in the framework of the "Investissements d’avenir” program (ANR-15-IDEX-02) and the generic call for proposals (ANR-22-CE49-0017). The experiments were performed at the Grand Accélérateur National d’Ions Lourds (GANIL) by means of the CIRIL Interdisciplinary Platform, part of CIMAP laboratory, Caen, France. We acknowledge the fundings from ANR IGLIAS grant ANR-13-BS05-0004 of the French Agence Nationale de la Recherche. INGMAR is a IAS-IJCLab facility funded by the French Programme National de Planétologie (PNP), Faculté des Sciences d’Orsay, Université Paris-Sud (Attractivité 2012), P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d’Avenir (ANR-11-IDEX-0003-01). We acknowledge the funding from Europlanet 2024 RI (under the grant agreement No 871149).

References

[1] V. Vuitton et al., In : Titan After Cassini-Huygens, 157 (2024)

[2] M. L. Cable et al., Chem Rev, 112, 1882 (2012)

[3]   N. Carrasco et al., Nature Astronomy, 2, 489 (2018)

[4] J. A. Sebree et al., Astrophys. J., 865, 133 (2018)

[5] O. Poch et al., Icarus, 242, 50 (2014)

[6] G. S. Vignoli Muniz et al., Astrobiology, 17, 298 (2017)

[7] B. Augé et al., Rev. Sci. Instrum., 89, 075105 (2018)

[8] N. Chauvin et al., Nucl. Instrum. Meth. A, 521, 149 (2004)

[9] S. Biri et al., Eur. Phys. J. Plus, 136, 247 (2021)

[10] K. Saïagh et al., Planet. Space Sci., 90, 90 (2014)

How to cite: Vuitton, V., Matuszewski, F., Hénault, E., Shouse, J., Chaouche, N., Rácz, R., Flandinet, L., Gautier, T., Quirico, E., Orthous-Daunay, F.-R., Brunetto, R., Boduch, P., Domaracka, A., Rothard, H., Juhász, Z., Sulik, B., Stalport, F., and Cottin, H.: Ion irradiation of adenine: Implications for molecular growth in Titan’s atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1384, https://doi.org/10.5194/epsc-dps2025-1384, 2025.

Figure 1: Schematics of the four independent tholin production laboratory facilities used in this work: (a) PHAZER/JHU; (b) PAC/UNI; (c) COSmIC/ARC; (d) Tholinator/SwRI.

Figure 2: The ellipsometry system at PMCHEF/UTSA for optical constant characterization.

Figure 4: Tholin thickness (nm) vs position (cm) for JHU/PLA 5% CH₄ in N₂ sample.

How to cite: Montes Bojorquez, J. R., Yu, X., Psciotta, C., Sciamma-O'Brien, E., Blase, R., Sebree, J., Hörst, S., Salama, F., and Patrick, E.: The Optical Properties of Titan’s Haze Analogs: A Cross-Laboratory Study , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1180, https://doi.org/10.5194/epsc-dps2025-1180, 2025.

The observations of planetary atmospheres and surfaces strongly rely on the use of experimental data to understand the interaction between light and particles. The intrinsic optical properties of these particles, also known as the refractive indices, describing light dispersion (i.e., n) and absorption (i.e., k), are required to consider the influence of their chemical composition. Only with these data can we interpret observations and avoid large degeneracy in the retrieval data analyses. Climate modeling is also strongly sensitive to these experimental data as atmospheric particles absorb and scatter radiations, and thus modify the temperature profile.

Photochemical hazes are observed in the atmospheres of the different objects in the outer Solar System (Titan, Pluto, Triton, Jupiter, Saturn) as well as in exoplanet atmospheres (e.g., GJ1214 b). The observations of these different objects as well as the modeling suggest that the composition of photochemical hazes varies from one object to the next following changes in the irradiation efficiency, temperature, pressure and gas composition. These differences in composition suggest that the refractive indices of photochemical hazes are also function of pressure, temperature, irradiation efficiency and gas composition. In the present work, we produced laboratory analogs of these hazes from various gas mixtures with controlled abundances. We used 6 different gas compositions to mimic the atmospheric compositions of Titan, Pluto and exoplanets. Among the 6 conditions, we modified the N2/CH4 and CO/CH4 abundance ratios in the gas mixture to assess the influence of N2 and CO on the refractive indices of the haze material. In addition, we compared the influence of the experimental setup by using haze analogs produced with the PAMPRE (LATMOS, France) and COSmIC (NASA Ames, USA) experimental setups. This cross-laboratory comparison allows us to assess the influence of temperature, pressure, gas residence time and irradiation which are changing between PAMPRE and COSmIC. Using several optical measurements, we covered a broad spectral range from UV to far-IR (up to 200 microns) which is essential for climate calculations and to interpret the various remote-sensing observations of these planetary bodies.

Our results revealed a significant difference between the refractive indices obtained on PAMPRE and COSmIC analogs, even for similar gas compositions. Based on previous elemental analyses of the haze analogs, we know that the COSmIC analogs are richer in nitrogen relative to carbon compared to the PAMPRE analogs.  Here, we show that this difference in composition leads to higher n and k values for the COSmlC analogs in the entire spectral range. We found that changes in the CO/CH4 gas ratio have a rather poor influence on the refractive indices compared to the effect of the N2/CH4 ratio. We also found that hazes produced without nitrogen are more transparent in the entire spectral range from UV to IR with very different mid-IR absorption features that could help distinguish between N-rich and N-poor exoplanet atmospheres. These data should be used for future observational analyses and modeling simulations of sub-Neptune atmospheres and Solar System gas giants.

How to cite: Drant, T., Sciamma-O'Brien, E., Jovanovic, L., Perrin, Z., Maratrat, L., Vettier, L., Garcia-Caurel, E., Wooden, D., and Rannou, P.: Refractive indices of Titan, Pluto and Exoplanet photochemical haze analogs from UV to far-IR : a comparative study between the PAMPRE and COSmIC experimental setups, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1740, https://doi.org/10.5194/epsc-dps2025-1740, 2025.

Deciphering the origin of organo-sulphur hazes: an exploration of the actual limits of sulphur chemistry with potential implications for prebiotic chemistry, the Early-Earth, and habitability of sulphur exoplanets

Introduction

 Sulphur volatiles are components currently observed in the atmospheres of planetary bodies. This is true for the solar system with the examples of Venus and Io, but also for extrasolar systems as suggested by the recent findings of the James Webb Space Telescope (JWST). Infrared Transit spectroscopy performed with this instrument revealed SO₂ signatures in several different types of exoplanets such as hot-Jupiter, warm-Neptunes, temperate Sub-Neptune , or Super-Earth. This highlights the extended diversity of objects in term of atmospheric conditions which host sulphur species. In many of these very diversified contexts, photochemical processes implying sulphur volatiles are known to play a key role in the chemistry of the global atmosphere. However, despite the recurrence of sulphur in planetary/exoplanetary atmospheres and the importance of the photochemistry induced by such species, sulphur atmospheric reactivity remains in some respects poorly understood. One of the most relevant examples to illustrate this lack of knowledge concerns sulphur aerosol formation processes. Several laboratory experiments revealed the existence of organo-sulphur hazes which are currently not predicted by atmospheric simulation models . To overcome this discrepancy between experimental observations and model, the chemical mechanisms leading to the formation of such aerosols need to be identified. The understanding of such mechanism could notably have great implications for the Early-Earth at the Archean era. During this period, the atmospheric composition, containing significative amount of CO₂, and CH_4, coupled with emission of sulphur volatiles from volcanic activity could have favoured the formation of such hazes. In this context, organo-sulphur hazes represent a new sink in the sulphur atmospheric budget which could considerably change the description of sulphur chemistry. This unexplored reactivity could change our current understanding of the origin of sulphur isotopic fractionation (Sulphur-Mass Independent Fractionation S-MIF) observed in ancient rocks which is one of the most relevant paleo-climate indicator.

Methods

In this work we adopt an experimental approach to better constrain the mechanisms of formation of organo-sulphur aerosols. The objectives were more precisely to characterise the molecular signatures of the organo-sulphur molecules contained in such hazes with their chemical functions, and to identify potential precursors of these aerosols in gas phase. To do so, we synthetise organo-sulphur aerosols in a N₂/CH₄/S (where S represents either SO₂ either H₂S) plasma mixtures. We deliberately considered more concentrated conditions in sulphur volatiles than the previous experiments (from 0.1% up to 10%) to enhance the sulphur signatures in the synthetised hazes. Such high sulphur proportions are also necessary to identify the sulphur volatiles precursors in the gas phase, which constrain the first elementary steps leading to the formation of hazes.

Results

 The analyses have revealed several sulphur volatiles (such as CS₂, H₂S, OCS, CH₃SH) but none of them have been observed specifically in presence of organo-sulphur hazes. This suggests that the incorporation of sulphur into the organic matter is mainly done by direct addition of sulphur radicals/intermediates on existing organic chains and not by an organic growth implying organo-sulphur volatile precursors. Analyses were also performed on the solids produced. In the figure below, spectra of two of the organo-sulphur aerosol samples show a strong organo-sulphur band around 2060cm^-1attributed to isothiocyanate function (-N=C=S). This could result from an addition of sulphur into an isonitrile group of the organic chain. This remark is consistent with the anti-correlation observed between the nitrile/isonitrile band and the isothiocyanate one. Finally, GC-MS measurements performed on the solid particles produced allow to identify the corresponding isothiocyanate organics with the detection of methyl-isothiocyanate, ethyl-isothiocyanate and butyl-isothiocyanate (see Fig.2). Thiocyanide ethers (R-SCN) were also observed. It is the first time that such functions are observed in organo-sulphur hazes matrix. This finding provides new elements in the understanding of the formation of such hazes by highlighting the strong coupling existing between nitriles/isonitriles and sulphur chemistry. These results prompt us to consider more deeply the heterogenous reactivity of sulphur on organic matrix which appears to be the dominant source of organo-sulphur hazes in the conditions studied.

Chemical characterisation of organo-sulphur aerosol as well as their formation route have potentially great impacts for the habitability of sulphur exoplanets. Indeed, their organic composition may include complex molecules composed of C,H,N,O,S which are five of the six indispensable elements to life on Earth: the CHNOPS. These hazes strongly differ from the inorganic S_x/H₂SO₄ clouds which have been the main sulphur condensed forms considered so far for this type of atmosphere. These remarks also highlight the prebiotic potential of such material which can give clues about the origin of sulphur in the organic matter of life.

Fig. 1. Comparison between the Infrared spectrums of an organic aerosol produced without sulphur in green, and two organo-sulphur samples produced with an input of H₂S in blue and an input of SO₂ in red. The details of the composition used to form these deposits are indicated in the legend. An intense band at 2060cm^-1 is specific to sulphur samples. This suggests the presence of isothiocyanate -N=C=S which gives indication about the sulphur functions present in the sample. Moreover, the blue spectrum (produced with H₂S) present strong differences with the two others. The analysis of its differences gives clues to the mechanism associated in the formation of these organo-sulphur samples.

Fig. 2. Example of chromatogram of one organo-sulphur aerosol sample produced (N2/CH4/Ar/H2S 85.5/4.5/9/1) got after a pyrolysis between 200 and 400°C with a ramp of 35°C/min.  Several organic isothiocyanate (2=Methyl, 4=Ethyle, 5=Isopropyle, 6=Butyle) and thiocyanide ethers (1=Methyl, 3=Ethyl) are observed in this chromatogram. The different peaks are associated to their corresponding species with the arrows. 

How to cite: Maratrat, L., Carrasco, N., Jaziri, Y. A., Vettier, L., and Millan, M.: Deciphering the origin of organo-sulphur hazes: an exploration of the actual limits of sulphur chemistry with potential implications for prebiotic chemistry, the Early-Earth, and habitability of sulphur exoplanets, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-326, https://doi.org/10.5194/epsc-dps2025-326, 2025.

We introduce new functionality to treat fractal aggregate aerosol particles for planetary, exoplanetary, and substellar atmospheres within the Virga cloud modeling framework. Previously, the open source cloud modeling code Virga assumed spherical particles to compute particle mass and size distributions throughout the atmosphere. The initial release of Virga also assumed spherical particles to compute Mie scattering properties. However, extensive evidence from solar system aerosols, astrophysical disks and dust, and Earth climate studies suggests that aggregate particles are common compared to idealized compact spherical particles. Following recent advances in microphysical and opacity modeling, we implement a simple parametrization for dynamical and optical effects of fractal aggregate particles into Virga. We then use this new functionality to perform case studies of canonical cloudy exoplanets. We compare previous fractal aggregate particle treatments to our methods and show how our new fractal treatment affects theoretical spectra of cloudy atmospheres, which has important implications for observations from Hubble, JWST, and eventually Roman and Ariel. Overall, our model is faster and more flexible for a wider range of parameter space than previous studies. We explore the limitations of our modeling set-up and offer guidance for future investigations using our framework.

How to cite: Moran, S., Lodge, M., Batalha, N., Vahidinia, S., Marley, M., Ohno, K., and Wakeford, H.: Aggregate Aerosols in the Virga Cloud Code: First Model Results, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-333, https://doi.org/10.5194/epsc-dps2025-333, 2025.

Recent JWST transmission and emission spectroscopic observation of hot Jupiters have demonstrated that mineral clouds are likely common in hot Jupiter atmospheres. These mineral clouds have long been predicted to form and persist on the nightside and western dayside of hot Jupiters by cloud microphysical models and 3D General Circulation Models. Given the capability of JWST and recent advancements in modeling techniques, the time is right to determine the prevalence and distribution of mineral clouds across the parameter regime of hot Jupiters in order to provide a detailed test of our theoretical understanding of cloud nucleation, transport and growth processes, and the radiative feedback of clouds on the atmospheric circulation and climate of hot Jupiters. In this work, we develop an indirectly coupled cloud microphysics and atmospheric dynamics framework in order to present theoretical expectations for the 3D distribution of mineral clouds across hot Jupiter planetary parameter space. To do so, we develop a fundamental understanding of the cloud speciation alongside particle size and spatial distribution by feeding the results of 3D cloud-free GCMs into 1D CARMA cloud microphysics simulations. We then use these results to drive 3D MITgcm simulations of hot Jupiters with cloud-radiative feedback. We use a grid of GCM simulations to assess the radiative impact on clouds of the climates of hot and ultra-hot Jupiters. Notably, we find that mineral cloud particle size distributions are not ubiquitously unimodal and log-normal, leading to potentially stark differences between the 3D cloud distributions in our work compared to previous work that assumed a single log-normal cloud particle size distribution. Finally, we will discuss paths forward toward coupling the cloud microphysics and atmospheric dynamics of hot Jupiters using a modeling hierarchy encompassing multi-dimensional cloud microphysics and atmospheric dynamics. 

How to cite: Komacek, T., Cottingham, J., Fromont, E., Gao, P., Powell, D., Kempton, E., and Tan, X.: The impact of cloud microphysics on the atmospheric dynamics of hot Jupiters, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-190, https://doi.org/10.5194/epsc-dps2025-190, 2025.

Clouds play a crucial role in shaping the atmospheres of exoplanets, influencing their albedo, heat distribution, and spectrum. While most past studies focused on hot and ultra-hot planets, JWST now allows an in-depth characterisation of warm objects, where the interactions between cloud, circulation, and radiative transfer are yet to be studied in detail.

In this study, we integrate physically motivated, radiatively active, tracer-based clouds in General Circulation Models (GCM) (ADAM, former SPARC) to analyze the atmosphere of WASP-80b, a warm Jupiter orbiting an M-dwarf star. This planet, with an equilibrium temperature of ~800 K, is analogous to warm sub-Neptunes, making it a crucial target for understanding sub-Neptune atmospheres. We take advantage of the high-quality dataset from JWST obtained through the MANATEE GTO collaboration, allowing us to jointly interpret both high-quality panchromatic emission and absorption spectra ranging from 2.5 to 12 microns with the outputs from a GCM. We provide an in-depth characterisation of the planet’s atmospheric dynamics and possible cloud distribution. By comparing our models to the data, we constrain the likelihood of different expected cloud species, such as silicate, sulfide, or chloride clouds, on this planet. 

Figure 1 shows the zonal mean averaged over latitudes and longitudes of the mass mixing ratio distribution of the cloud particles in the atmosphere. The region around 0° latitude (Row: 1, 3, 5) is the region around the equator averaged over all longitudes. The region around 0° longitude (Row: 2, 4, 6) is the dayside region averaged over all latitudes. 

Figure 2: Emission Spectrum from ADAM-GCM plotted over the JWST observations from the MANATEE-GTO program, with the colored regions indicating the instruments. Figure 3: Transmission Spectrum from ADAM-GCM plotted over the JWST observations from the MANATEE-GTO program, with the colored regions indicating the instruments.

While both emission and transmission spectra are very well fitted by cloudless GCMs (Figures 2 and 3), the data also appear compatible with small KCl cloud particles, but Na2S condensates can be ruled out due to the strength of their radiative feedback.

Figure 4 shows the dayside average temperature-pressure profiles (Row 1) and the corresponding emission (Row 2) and transmission spectra (Row 3) for different clouds and particle sizes.

This showcases the unique insights that can be obtained from global modeling of exoplanet atmospheres. Our results point towards a homogeneous atmosphere with minimal temperature contrast between the day and night sides, suggesting efficient heat redistribution. Additionally, the relatively low abundance of CH4 points to active atmospheric chemistry and the possibility of a high internal heat flux, which can lead to quenching of CH4 in the atmosphere.

This work not only provides a comprehensive framework for interpreting JWST observations but also enhances the capabilities of GCMs in characterizing the global atmospheres of exoplanets in the JWST era.

How to cite: Mehta, N. and Parmentier, V.: Combining JWST data and General Circulation Models for a 3D study of the clouds on warm Jupiter WASP-80b, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1597, https://doi.org/10.5194/epsc-dps2025-1597, 2025.

The New Horizons flyby of Pluto mapped temperature profiles, vertical profiles of N₂, CH₄, C₂H₂, C₂H₄, C₂H₆, and the vertical profile of hazes in Pluto’s atmosphere [1]. Zhang et al. 2017 showed that Pluto’s hazes can explain the colder-than-predicted upper atmosphere [2]. Previous stellar occultations reported large changes in haze optical depths on timescales of a few years. The observed presence and variability of hazes in Pluto’s atmosphere and its impact on the atmosphere motivates this work. We used KINETICS, a 1-D photochemical model and PlutoCARMA, a microphysical transport model, to study how Pluto's atmosphere responds to changes in solar UV flux. We simulated two scenarios (a sudden turn-on of a solar flare and a sudden transition from a solar minimum to solar maximum) with KINETICS to investigate formation and destruction timescales and abundances for species expected to be present in Pluto's atmosphere. Cation reactions were added to KINETICS, resulting in a substantial decrease (up to 5x) of certain C₂H_x production rates at high altitudes compared to a neutrals-only case.

The main means of formation for all haze precursor species originates with photodestruction of and reactions involving CH₄. These form CH₃, 1CH₂, and C₂H₄ in roughly equal amounts. CH₃ forms HCN and C₂H₆, C₂H₄ forms C₂H₂, and the majority (> 90%) of 1CH₂ eventually forms the methylidyne radical (CH) which is free to react with anything in the atmosphere. Reaction of the methylidyne radical with methane is responsible for > 90% of C₂H₄ production. C₂H₄ either reacts with CH or photodissociates to form C₂H₂, which is the major chemical bottleneck for production of the other species. C₂H₂ is the predecessor of several other haze precursor reactions, notably C₈H₂ (for which we observe one of the greatest increases in mixing ratio in response to a change in solar flux), HCN, C₄H₂, and HC₃N. C₂N₂, another species which observes a dramatic increase in mixing ratio, forms mainly (>90% of production) from the reaction CN + HNC. 

KINETICS simulations show that photochemical formation of haze precursors occurs on timescales of hours during a solar flare and months during a solar maximum. PlutoCARMA simulations demonstrate settling timescales of several days for micron-size particles. The mixing ratio of key haze precursors at certain times is shown in Fig. 1. These results suggest that photochemical changes to haze opacity will not be detected on solar flare timescales but could be detectable over an 11-year solar cycle. The high haze optical depths were measured in 2002 by ground-based observation and in 2015 during New Horizons’ flyby, i.e., at a time close to the peaks of the 23 and 24 solar cycles. This correlation between haze opacity and solar cycle was one of the motivations for this work. Results from KINETICS simulations show that some haze precursor species are created or destroyed in significant amounts on timescales that are shorter than a solar cycle, but the simulations do not match the amplitude of τ_HAZE change measured between 2002 and 2007 (at least a 30x decrease) or between 2007 and 2015 (at least a 2.8x increase) [3]. In other words, photochemistry due to changing solar UV flux cannot be the sole explanation for the changes in haze optical depths that have been observed from occultations. 

Fig. 1a: mixing ratio of key haze precursors for the version of the model including neutrals and cations at key time steps run with flux associated with solar flare

Fig. 1b: mixing ratio of key haze precursors for the version of the model including neutrals and cations at key time steps run with flux associated with solar maximum

References:

[1] Young, L. A. et al. (2018) Icarus, 300, 174–199. [2] Zhang, X., Strobel, D. F., Imanaka, H. (2017). 551(7680), 352–355. [3] Young, E., Young, L., & Buie, M. (2023, October). Abstract presented at the 55th Annual Meeting of the Division for Planetary Sciences, Bulletin of the American Astronomical Society, 55(8), 308.03.

How to cite: Shawcross, J., Young, E., Adams, D., Yung, Y., Thiemann, E., Barth, E., Sciamma-O'Brien, E., and Dubois, D.: Response of Haze Precursors on Pluto to Solar Variability , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1100, https://doi.org/10.5194/epsc-dps2025-1100, 2025.

Jupiter, the largest planet in our solar system, is a vital reference point for understanding gaseous exoplanets and their atmospheres. While we know its upper tropospheric chemical composition well, the nature and structure of its clouds remain puzzling. We, therefore, rely on theoretical models and remote sensing data to address this.

While traditional equilibrium chemistry condensation models (ECCM) are sensitive to input parameters, advanced models [1] offer more realistic cloud property predictions. Remote sensing data can help determine cloud properties and test theoretical predictions thanks to the application of multiple scattering atmospheric retrieval. Still, the process is highly degenerate and, therefore, computationally demanding. The predicted tropospheric layers are upper ammonia ice (∼0.7 bar) and ammonium hydrosulfide (∼2 bar) clouds [2], but their spectral detection has been limited to small, dynamically active regions (<2% of the disk) [3, 4].

This study analyzes JIRAM/Juno data [5] to investigate Jovian clouds and aerosols, focusing on viewing conditions relevant to future exoplanet missions. Preliminary radiative transfer simulations (using PSG [6]) indicated aerosol signatures are most prominent in the 2.6-2.8 μm and 4.5-5 μm high gas transmittance windows. The low gas opacity makes the aerosol contributions observable by JIRAM in terms of reflection of the incident solar radiation (2.6-2.8 μm) and attenuation of thermal black body radiation from the planet (4.5-5 μm). In particular, the observed solar reflection necessitates a vertically extended tropospheric haze layer consistent with previous observations [7] and Titan-adapted microphysical models [8, 9], suggesting stratospheric formation via methane photochemistry [10].

Retrievals across the full 2-5 μm JIRAM range were performed using PSG [6], varying cloud/haze parameters and gas mixing ratios, employing a single haze layer and two cloud layers (main and deep). Three compositions were tested for the main cloud: pure ammonia ice, tholins [11], and “Jupiter-adapted” tholins ([11] data removed of the 4.7 μm feature). Haze and deep cloud were modeled as reflecting and grey absorbers, respectively.

Results confirm the necessity of a haze and constrain its size and density. “Adapted” tholins provided the best spectral fit. Retrieved main cloud densities were significantly lower than ECCM and [1] predictions, with particle radii around 2-3 μm. Deep cloud densities aligned with the predicted ammonium hydrosulfide cloud top. These results may suggest that photochemistry may play a huge role in shaping the aerosol layers of Jupiter, combined with classic condensation. The observation of photochemistry in cold gaseous exoplanets such as GJ-1214b [12] makes Jupiter's troposphere a remarkably accessible laboratory for investigating these processes up close.

We formulate at least two options regarding the creation of these “adapted” tholins. The first is that they are the result of successful coagulation of some haze hydrocarbon small particles and the products of (gaseous) ammonia photolysis at pressures between 0.2 and 0.7 bar. However, we have to test if this process can efficiently create large particles around 3 microns. Alternatively, the tholins can be produced at the top of freshly produced ammonia ice clouds. Here, UV radiation can easily dissociate larger ice particles that successively can react with hydrocarbons and the gaseous ammonia photolysis products. This scenario is particularly intriguing as it should be consistent with the so-called “chromophore model” [13].  More accurate chemical models and laboratory measurements are, however, needed to verify our hypothesis.

References:

[1] Ackerman A. S., Marley M. S., 2001, ApJ, 556, 872

[2] Atreya S. K., Wong M. H., Owen T. C., Mahaffy P. R., Niemann H. B., dePater I., Drossart P., Encrenaz T., 1999, Planet. Space Sci., 47, 1243

[3] Baines K. H., Carlson R. W., Kamp L. W., 2002, Icarus, 159, 74

[4] Biagiotti, F., Grassi, D., Liuzzi, G., et al. 2025, Monthly Notices of the Royal Astronomical Society, staf38

[5] Adriani, A., Filacchione, G., Di Iorio, T., et al. 2017, Space Sci. Rev., 213, 393

[6] Villanueva G. L., Smith M. D., Protopapa S., Faggi S., Mandell A. M., 2018, Quant. Spec. Radiat. Transfer, 217, 86

[7] Dahl E. K. et al., 2021, Planet. Sci. J., 2, 16

[8] McKay, C. P., Pollack, J. B., & Courtin, R. (1989). Icarus, 80(1), 23–53.

[9] Rannou, P., McKay, C. P., & Lorenz, R. D. (2003). Planetary and Space Science, 51(14–15), 963–976.

[10] Moses, J. I., Fouchet, T., Bézard, B., et al. 2005, JGRE, 110, E08001

[11] Imanaka H., Cruikshank D. P., Khare B. N., McKay C. P., 2012, Icarus, 218, 247

[12] Kreidberg L. et al., 2014, Nature, 505, 69

[13] Baines K. H., Sromovsky L. A., Carlson R. W., Momary T. W., Fry P. M., 2019, Icarus, 330, 217

How to cite: Biagiotti, F., Grassi, D., Guillot, T., Fletcher, L. N., Atreya, S., Liuzzi, G., Villanueva, G., Rannou, P., Irwin, P., Piccioni, G., Mura, A., Tosi, F., Adriani, A., Sordini, R., Noschese, R., Cicchetti, A., Sindoni, G., Plainaki, C., Li, C., and Bolton, S.: A comprehensive picture about Jovian clouds and hazes from Juno/JIRAM infrared spectral data, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1656, https://doi.org/10.5194/epsc-dps2025-1656, 2025.

The identity of the coloring agent, or chromophore, in Jupiter’s atmosphere that causes the planet’s striking red clouds and storms is an area of active research. Despite being studied for more than 50 years across multiple observational missions, including the Pioneer and Voyager flybys, Galileo and Cassini missions, Hubble Space Telescope (HST) observations, and recent data from the Juno spacecraft and the James Webb Space Telescope, the precise mechanisms behind the origins of Jupiter’s red color remain unknown (e.g., West et al., 2004, Sánchez-Lavega et al., 2024). Simon-Miller et al. (2001) using HST images of Jupiter identified three components contributing to Jupiter’s brightness variations. They found that gray spectral brightness variations accounted for ~91% of variation within the images, a strongly blue-absorbing chromophore was responsible for ~8% of variation in or around the tropospheric cloud deck, and a second blue/green coloring agent was necessary to explain the remaining ~1% of variation in upper tropospheric clouds or hazes, including the Great Red Spot (GRS), an anticyclonic storm located 22° south of the planet’s equator.

Various hypotheses have been proposed regarding the chemical composition of the coloring agents or chromophores responsible for the red color of Jupiter’s atmosphere. These chromophores can be divided into three main hypothesized categories:

• Compounds resulting from sulfur chemistry (e.g., Lewis and Prinn, 1970, Sill, 1976, Loeffler et al., 2016, Loeffler and Hudson, 2018);
• Compounds resulting from phosphorus chemistry (e.g., Prinn and Lewis, 1975, Prinn and Owen, 1976, Noy et al., 1981);
• Organic compounds resulting from methane and ammonia photochemistry (e.g., Ferris and Ishikawa, 1988, Carlson et al., 2016).

Different studies have shown that the most promising chromophore analog for fitting spectra of various Jovian atmospheric features, including the GRS and major atmospheric cloud bands such as the Equatorial Zone and Northern Equatorial Belt is the refractory organic material synthesized by Carlson et al. (2016) from the ultraviolet photolysis of a mixture of ammonia and acetylene (Sromovsky et al., 2017, Baines et al., 2019, Braude et al., 2020, Dahl et al., 2021). While the Carlson et al. (2016) chromophore analog shows promise as the likeliest candidate, it remains uncertain due to both a) the complexities and degeneracies with aerosol properties present in radiative transfer models at wavelengths dominated by scattered sunlight, and b) the fact that Carlson et al. (2016) measured the transmission spectrum of the chromophore analog over a limited wavelength range at intermittent times during the sample production and did not determine the chemical composition or the complex refractive index directly over time.

Motivated by the need to understand which gas-phase precursor molecules enables the ultimate formation of solid coloring agents, and to characterize the chemical, physical and morphological properties of these solid chromophores, we therefore propose to build on the work of Carlson et al. (2016) and synthesize new chromophore analogs in the laboratory using the NASA Ames’ COsmic SImulation Chamber (COSmIC, see Figure 1).

Figure 1. NASA Ames’ COSmIC experimental setup.

COSmIC is composed of a vacuum chamber coupled with a pulsed discharge nozzle (PDN). In the PDN, a plasma discharge is generated in the stream of a pulsed supersonic jet-cooled gas expansion (Salama et al., 2017). In the study presented here, we have used COSmIC to produce two different Jupiter chromophore analogs (or Jupiter tholins) from plasma chemistry in Ar:NH₃:CH₄ (95.5:1:3.5) and Ar:NH₃:C₂H₂ (98:1:1) gas mixtures. In COSmIC, solid particles are produced in the form of grains and carried in the accelerated gas expansion before being jet-deposited onto substrates placed 5 cm downstream of the electrodes. During deposition, the grains stack up and produce a deposit hundreds of nanometers thick.

In this presentation, we will show the morphology and size distribution of the grains that have been analyzed by scanning electron microscopy. We will also present the real and imaginary parts of the complex refractive index (respectively, n and k) of the two Jupiter tholin samples, which have been determined using the Optical Constants Facility (OCF) consisting of a reflectance microscope (0.2-1.7 µm) and a FTIR spectrometer (0.6-200 µm). Additionally, we will present initial radiative transfer model results applying the new chromophore analogs’ optical constants to models of Jupiter’s atmosphere and compare them to models using the Carlson et al. (2016) chromophore.

Acknowledgements: L.J., E.S.O., C.L.R., D.H.W and F.S. acknowledge the NASA SMD PSD ISFM program.

References

West, R. A., et al. (2004) Jupiter: The Planet, Satellites and Magnetospheres, Cambridge Planetary Science, pp. 79-104.

Sánchez-Lavega, A., et al. (2024) Geophysical Research Letters 51:12.

Simon-Miller, A. A., et al. (2001) Icarus 149.1:94-106.

Lewis, J. S. and Prinn, R. G. (1970) Science 169:472-473.

Sill, G. T. (1976) IAU Colloq. 30: Jupiter: Studies of the Interior, Atmosphere, Magnetosphere and Satellites, pp. 372-383.

Loeffler, M. J., et al. (2016) Icarus 271:265-268.

Loeffler, M. J. and Hudson, R. L. (2018) Icarus 302:418-425 .

Prinn, R. G. and Lewis, J. S. (1975) Science 190:4211, pp. 274-276.

Prinn, R. G. and Owen, T. (1976) IAU Colloq. 30: Jupiter: Studies of the Interior, Atmosp here, Magnetosphere and Satellites, pp. 319-371.

Noy, N., et al. (1981) Journal of Geophysical Research:Oceans 86:C12, pp. 11985-11988.

Ferris, J. P. and Ishikawa, Y. (1988) Journal of the American Chemical Society 110:13, pp. 4306-4312.

Carlson, R. W., et al. (2016) Icarus 274:106-115.

Sromovsky, L. A., et al. (2017) Icarus 291:232-244.

Baines, K. H., et al. (2019) Icarus 330:217-229.

Braude, A. S., et al. (2020) Icarus 338:113589.

Dahl, E. K., et al. (2021) The Planetary Science Journal 2.1:16.

Salama, F., et al. (2017) Proceedings of the International Astronomical Union 13.S332:364-369.

How to cite: Jovanovic, L., Sciamma-O'Brien, E., Drant, T., Dahl, E., Braude, A., Ricketts, C., Wooden, D., Baines, K., and Salama, F.: Optical constants of laboratory-generated analogs of the red chromophores in Jupiter’s atmosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1347, https://doi.org/10.5194/epsc-dps2025-1347, 2025.

The upper haze region of Venus’ atmosphere (~70-90 km) has been shown to experience cold pockets that may give rise to ice clouds that form from the freezing of sulphuric acid droplets (Turco et al., 1983) On Earth cirrus cloud often form through the hygroscopic growth of aquoue aerosol, but temperatures in Venus’ upper haze region regularly drop well below those of the coldest cirrus clouds on earth (~185 K). However, freezing of sulphuric acid droplets under conditions relevant for Venus’ upper haze region have not been experimentally investigated.  Here, we describe our work using new laboratory freezing measurements in combination with the Solar Occultation in the InfraRed SOIR retrievals to better understand the atmospheric conditions around 80 km and determine if these conditions are suitable for the crystallisation of water and CO₂ ice.

First, we experimentally explored the homogeneous nucleation of H₂O ice in sulphuric acid solutions using a liquid nitrogen-cooled cryo-microscope setup, where droplet emulsions (oil/surfactant mix and H₂SO₄ solution with droplets of around 10 to 100 µm) are created using a microfluidic device. With this setup, we were able to extend the results from previous studies to lower temperatures and higher sulphuric acid concentrations (Bertram et al., 1996; Koop et al., 1998). We observed crystallisation down to 154 K, but this crystallisation was increasingly restricted by slow crystal nucleation and growth rates at lower temperatures. Crystallisation was not observed below 154 K, consistent with the formation of ultra-viscous or glassy solutions. To further explore the possibility of water ice cloud formation on Venus, we also examined the retrievals of temperature, water vapour mixing ratio and pressure from the Solar Occultation in the InfraRed (SOIR) instrument onboard the Venus Express orbiter (Mahieux et al., 2023). Using this data, we were able to calculate the expected sulphuric acid concentrations using the parameterisation from Tabazadeh et al. (1997), indicating that, in about 30% of the SOIR profiles, H₂O ice nucleates homogeneously in liquid aqueous H₂SO₄ droplets or heterogeneously on glassy aqueous H₂SO₄ droplets. Using an average particle number density of 0.5 cm^-3, which was previously reported by Luginin et al. (2018), we would expect an average size of ~1 µm ice crystals to form in the upper haze layer of Venus. 

Around 36% of the SOIR profiles reveal that these altitudes experience temperature extremes which low enough (< 140 K) that the atmosphere is stable with respect to crystalline CO₂ particles. A 1D model was developed to investigate the influence of gravity waves, the formation of crystalline CO₂ and the impact on the upper haze region of the atmosphere. Our 1D model showed that under these conditions, crystals will grow rapidly in the cold phase of a wave to sizes large enough for precipitation downwards to the underlying warm phase where the CO₂ sublimates. Therefore, the formation of crystalline CO₂ particles creates a mechanism for the redistribution of sulphuric acid particles and water to lower levels in the atmosphere.

Overall, we conclude that cirrus-like water ice clouds likely form and persist in large parts of the upper haze layer on Venus. Due to the variable nature of conditions in this region of the Venusian atmosphere, temperatures will sometimes fall well below 140 K, resulting in the rapid precipitation of CO₂ particles. This will likely create a mechanism for the downward transfer of sulphuric acid, water and other materials to the warmer regions of the atmosphere.

Figure 1. The hypothesised mechanism of how temperature variations impact cloud formation and the redistribution of atmospheric constituents. We propose that persistent water ice clouds may be a common feature of the upper haze layer, and these ice crystals may provide the surface on which CO₂ ice nucleates in temperature minima driven by gravity waves. The rapid growth and sedimentation of cubic CO₂ ice particles would then redistribute H₂SO₄ particles downwards to the warmer regions of the Venusian atmosphere.

References:

Bertram, A. K., Patterson, D. D., & Sloan, J. J. (1996). Mechanisms and temperatures for the freezing of sulfuric acid aerosols measured by FTIR extinction spectroscopy. The Journal of Physical Chemistry, 100(6), 2376–2383. https://doi.org/10.1021/jp952551v

Koop, T., Ng, H. P., Molina, L. T., & Molina, M. J. (1998). A new optical technique to study aerosol phase transitions: The nucleation of ice from H₂SO₄ aerosols. The Journal of Physical Chemistry A, 102(45), 8924–8931. https://doi.org/10.1021/jp9828078

Luginin, M., Fedorova, A., Belyaev, D., Montmessin, F., Korablev, O., & Bertaux, J.-L. (2018). Scale heights and detached haze layers in the mesosphere of Venus from SPICAV IR data. Icarus, 311, 87–104. https://doi.org/10.1016/j.icarus.2018.03.018

Mahieux, A., Robert, S., Piccialli, A., Trompet, L., & Vandaele, A. C. (2023). The SOIR/Venus Express species concentration and temperature database: CO₂, CO, H₂O, HDO, H³⁵Cl, H³⁷Cl, HF individual and mean profiles. Icarus, 405, 115713. https://doi.org/10.1016/j.icarus.2023.115713

Tabazadeh, A., Toon, O. B., Clegg, S. L., & Hamill, P. (1997). A new parameterization of H₂SO₄/H₂O aerosol composition: Atmospheric implications. Geophysical Research Letters, 24(15), 1931–1934. https://doi.org/10.1029/97GL01879

Turco, R. P., Toon, O. B., Whitten, R. C., & Keesee, R. G. (1983). Venus: Mesospheric hazes of ice, dust, and acid aerosols. Icarus, 53(1), 18–25. https://doi.org/10.1016/0019-1035(83)90017-9

How to cite: Alden, K. A., Egan, J. V., James, A. D., Tarn, M. D., Plane, J. M. C., and Murray, B. J.: The Formation of Cirrus-Like Water and Carbon Dioxide Ice Clouds in Venus’ Upper Haze Layer, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1798, https://doi.org/10.5194/epsc-dps2025-1798, 2025.

Introduction: Aerosols in the Martian mesosphere (40 – 100 km altitude), consisting of dust, water ice and CO₂ ice, act as tracers of both the water cycle and atmospheric dynamics [1,2,3], and are generated by the complex interplay of dust lofting with local perturbations in temperature and atmospheric circulation driven by zonal planetary waves (e.g. [4,5,6,7]). Although well-studied by various instruments on board Mars Express (e.g. [8,9,10,11]) and the ExoMars Trace Gas Orbiter (e.g. [12,13,14,15]), these instruments are unable to simultaneously probe the zonal and vertical structure of these layers for a given season and local time. This makes it difficult to distinguish mesospheric aerosol features driven by transient atmospheric instabilities from more persistent and predictable climatological phenomena.

 Since 2015, the Mars Atmosphere and Volatile EvolutioN (MAVEN) probe’s Imaging UltraViolet Spectrometer (IUVS) has conducted multiple targeted stellar occultations in close geographical and temporal proximity to limb observations made by the Mars Climate Sounder (MCS) instrument onboard the Mars Reconnaissance Orbiter (MRO). In this talk, we will present a climatology of these joint observations, identifying a) to what extent we detect mesospheric aerosol features that are consistent across both datasets and b) the lifetimes of these features. Our results will then be used to identify regional climatologies in aerosol features that can be distinguished from aerosol features generated by transient weather events.

Data: MCS joint observations began in March of 2015 and continued through January of 2022, corresponding to ~40 campaigns that targeted at least 1 IUVS occultation set each. We have available 140 MCS profiles that occurred within 60 minutes, and are within 10° of latitude, 15° of longitude, and 30 minutes of local time, of a single IUVS occultation.

IUVS and MCS are both sensitive to the vertical structure of both aerosols and temperature, while the longitudinal sampling of these observations at fixed latitudes and local times allows for zonal variations in aerosol structure to be extracted. From IUVS stellar occultation measurements [16], we can retrieve vertical profiles of two independent quantities: aerosol opacity up to ~110 km altitude, and an ’Ångström coefficient’ [8,17] loosely correlated with particle size (with non-zero values indicating a submicron particle population). However, aerosol particle composition is difficult to determine with great accuracy. Conversely, MCS limb observations can easily distinguish vertical profiles of dust and water ice (in units of extinction coefficient, km^-1), but have little sensitivity to particle size or to aerosol opacity above approximately 60 km altitude. These measurements therefore complement each other.

Preliminary Results: Detections of detached aerosol layers below 60 km in IUVS data generally map well onto water ice cloud layers in joint MCS observations, with large densities of joint water ice cloud detections at equatorial and northern latitudes during the dusty season. Conversely, aerosol features at the lowest altitudes to which IUVS is sensitive mostly correlate with increases in dust opacity detected by MCS. The main exception is during winter and early spring at southern high latitudes, where detached aerosol layers are often retrieved by IUVS at approximately 50 km altitude but entirely absent from corresponding MCS data (with an example shown in Figure 1), even though they should be easily resolvable by both datasets.

Figure 1: Aerosol opacity and Ångström coefficient profiles retrieved from IUVS stellar occultations compared with dust and water ice extinction profiles retrieved from the closest three MCS observations. (top) in this observation, obtained in Southern winter at 63S latitude, IUVS observes a detached aerosol layer centred around 45 km altitude for which MCS failed to detect either a dust or a water ice layer, (bottom) in this observation, obtained in the northern tropics close to the autumn equinox, two detached layers are observed by IUVS at 40 km and 52 km altitude that are clearly determined to be water ice clouds by MCS.

As with [8,18] we also often observe regions of Ångström coefficient greater than 0 directly above either the level of detectable dust in MCS observations or just above water ice cloud layers, indicating layers of smaller aerosol particles – most likely dust - located just above the haze-tops and water ice clouds detected by MCS, and often undetected by MCS. However, we find as with [11] that this pattern is neither consistent nor usually correlated with the presence or absence of water ice clouds, and appears to be highly longitude-dependent.

Acknowledgments: This work was carried out at the Jet Propulsion Laboratory California Institute of Technology under a contract with NASA. We recognize support for this project from NASA grant 23-MDAP23-0060.

References: [1] Bony et al. (2015), Nat. Geosci., 8, 261–268. [2] Guzewich et al. (2016), Icarus, 278, 100–118. [3] Gronoff et al. (2020), J. Geophys. Res. Space Phys., 125, e27639. [4] Banfield et al. (2000), J. Geophys. Res., 105, 9521–9538. [5] Banfield et al. (2003), Icarus, 161, 319–345. [6] Lee et al. (2009), J. Geophys. Res. Planets, 114, E03005. [7] Guzewich et al. (2012), J. Geophys. Res. Planets, 117. [8] Montmessin et al. (2006), J. Geophys. Res., 111, e09S09. [9] Montmessin et al. (2007), J. Geophys. Res., 112, e11S90. [10] Määttänen et al. (2010), Icarus, 209, 452–469. [11] Määttänen et al. (2013), Icarus, 223, 892–941. [12] Liuzzi et al. (2020), J. Geophys Res, Planets, 125, e2019JE006250. [13] Luginin et al. (2020), J. Geophys. Res., 125, e2020JE006419. [14] Stcherbinine et al. (2020), J. Geophys. Res. Planets, 125, e2019JE006300. [15] Stcherbinine et al. (2022), J. Geophys. Res., 127, e2022JE007502. [16] Gröller et al (2018), J. Geophys. Res. Planets, 123, pp. 1449–1483. [17] O’Neill and Royer (1993), Appl. Opt., 32, 1642–1645. [18] Rannou et al. (2006), J. Geophys. Res. Planets, 111, e09S10.

How to cite: Braude, A., Slipski, M., Gupta, S., Schneider, N., Kleinboehl, A., Montmessin, F., Jain, S., Yelle, R., and Deighan, J.: Joint Observations of Mesospheric Aerosols on Mars from MAVEN/IUVS and MRO/MCS, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1113, https://doi.org/10.5194/epsc-dps2025-1113, 2025.

On Mars, the most common types of atmospheric aerosols are composed of mineral dust and/or water ice. They have large effects on Martian climate, such as the absorption of solar radiation, altering the radiative balance of the planet, and they are key to atmospheric dynamics and circulation. In the case of dust, it can act as a base to form water ice or CO₂ ice clouds, and it can affect observations from both orbiting satellites and rovers on ground, especially at the dusty season around perihelion.

The instrument Nadir and Occultation for Mars Discovery (NOMAD) onboard the ExoMars/Trace Gas Orbiter (ExoMars/TGO) is a suite of three spectrometers which has been observing the Martian atmosphere routinely since April 2018. Using data from its solar occultation channel (SO), we combine several sets of diffraction orders, or wavelengths to retrieve the aerosol properties and distribution during that period with a very fine resolution in the vertical from the ground up to the thermosphere. Our retrieval approach consists in three main steps. First, we perform a "cleaning" of the NOMAD observations, provided as transmittance spectra at the tangent altitudes, using an in-house pre-processing algorithm developed at IAA/CSIC. This is intended to eliminate residual imperfections in the calibrated transmittances, such as spectral shifts and continuum curvatures. Second, the cleaned spectra are used to retrieve the aerosol extinction vertical profiles following a global fit approach. This is performed with a retrieval program (RCP) together with a radiative transfer model (KOPRA) which has been well tested in Earth's atmospheric remote sounding. Finally, we implement a fitting algorithm to compare the retrieved extinctions, coming as spectral ratios of the retrieved extinctions, with the extinction ratios simulated using a Lorenz-Mie code by Mishchenko et. al., 2002. The aerosol properties inferred are the size distribution, which is described by an effective radius and its effective variance, nature (mineral dust and water ice proportions), mass of the particles and number density, as well as their vertical distribution and time variability.

In this talk we will discuss the results obtained by analyzing more than three full Martian Years. This is a significant extension of a previous first analysis by our team (Stolzenbach et. al, 2023 a,b), focused on the 1st year of NOMAD data. We have also improved a couple of aspects from the previous work. First, the wavelength coverage has been extended so that we are able to retrieve the aerosol information using any order combination, making us able to cover the SO spectral range as widely as possible, as well as exploiting the wavelengths where the aerosol nature can be better determined. Second, we have developed a new methodology to describe the uncertainties of our retrievals by computing the mean of the transmittances from two distinct regions inside the diffraction order.

We will describe the major results obtained on the global distribution and properties of the aerosols, analyzing latitudinal and seasonal trends during the time range studied.

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). US investigators were supported by the National Aeronautics and Space Administration. Canadian investigators were supported by the Canadian Space Agency."

How to cite: Gamonal García-Galán, M. Á., López-Valverde, M. Á., Brines, A., Stolzenbach, A., Modak, A., González-Galindo, F., Funke, B., López-Moreno, J. J., Rodríguez-Gómez, J., Sanz-Mesa, R., Daerden, F., Ristic, B., Belucci, G., Patel, M., and Thomas, I.: Martian atmospheric aerosol latitudinal and seasonal analysis over 3 full MYs from Nomad/TGO solar occultation observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1551, https://doi.org/10.5194/epsc-dps2025-1551, 2025.