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Abstract:

The origin and vertical distribution of Jupiter’s red coloration remain uncertain, despite multiple proposed aerosol models. Laboratory work (Carlson et al., 2016) showed that photolyzed ammonia and acetylene can form a red compound consistent with Jupiter’s colours, motivating the “universal chromophore” hypothesis (Sromovsky et al., 2017), and the “Crème Brûlée” model (Baines et al., 2019), which places a thin absorber above the ammonia clouds. Later HST and VLT studies (Pérez‑Hoyos et al., 2020; Braude et al., 2020) suggested a more vertically extended, less blue‑absorbing material, while recent analyses of the Great Red Spot and Oval BA indicate the presence of two distinct colouring agents: a universal‑chromophore absorber and a deeper UV‑absorbing aerosol (Anguiano‑Arteaga et al., 2021, 2023). These findings highlight persistent ambiguity in Jovian aerosol composition and structure.To investigate this, we analysed 2019 Jupiter observations from CARMENES (The Calar Alto High-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs), (0.52–1.71 μm). Since no calibration star was available, we calibrated the spectra using Saturn’s B ring and Cassini/VIMS reflectivity (Cuzzi et al., 2009), achieving agreement with published Jupiter spectra to within 10% (Clark, R.N., McCord, T.B., 1979; Mendikoa, I., et al., 2017; Irwin, P.G., et al., 2018).Using 64 VIS–NIR observation pairs, we performed a Minnaert limb‑darkening analysis and generated synthetic spectra for five regions. These were used in NEMESIS retrievals with three aerosol models. Across all models, the highest‑altitude aerosol layer dominated the spectral behaviour, with particle size, cloud‑base abundance, and pressure level strongly influencing the fits. Model B (Braude et al., 2020) produced the lowest χ²/Nfree values, but no model fully reproduced the observations, likely due to the limited wavelength range, which lacks constraints on deeper clouds.The models diverged in retrieved particle sizes and cloud‑base pressures, with several results, such as extremely small tropospheric particles or overly large stratospheric particles, indicating physical inconsistencies. Model A’s tropospheric haze base aligns with Galileo probe measurements (Sromovsky and Fry, 2002); Model C retrieves a cloud base level near the NH₄SH level predicted by Atreya (1998), deeper than CIRS detections (Matcheva et al.,2005) but within the range of Baines et al. (2019), with implausible particle sizes.Overall, the study shows that CARMENES can deliver high‑quality, flux‑calibrated planetary spectra, but also that broader spectral coverage is essential to resolve Jupiter’s chromophore composition and aerosol vertical structure. Figure 1: Location of the spectra used to perform the Minnaert limb-darkening approximation for each region considered in this study. Red for EZ, yellow for NEB, green for SEB, pink for SEB transition and blue for NEB transition. The Jupiter AGC image represented corresponds only to the spectra of the EZ whose longitude was closest to 0º. Figure 2: Comparison between observed and modelled spectra and residuals for EZ using model B]{Comparison between observed (blue) and modelled (red) spectra (left column) and comparison between differences (red) and a priori errors (black) (right column) for the EZ using model B, with the grey shaded areas corresponding to telluric absorption. The top row corresponds to nadir (incidence and emission angle = 0º) and the bottom row to limb (incidence and emission angle = 61.45º). Figure 3: Comparison between the a priori aerosol vertical profiles and the retrieved profiles for every region for models A and B. We compare the optical depth/atm at 0.90 μm of model B with all three aerosol populations considered and model A's stratospheric and tropospheric hazes. The horizontal dashed line corresponds to 0.15 atm, separating model B's deep cloud layer from the haze. References:Carlson, R. W., et al. (2016). Chromophores from photolyzed ammonia reacting with acetylene: Application to Jupiter's Great Red Spot. Icarus, 274, 106–115.Sromovsky, L. A., et al. (2017). A possibly universal red chromophore for modeling color variations on Jupiter. Icarus, 291, 232–244.Baines, K. H., et al. (2019). The visual spectrum of Jupiter's Great Red Spot accurately modelled with aerosols produced by photolyzed ammonia reacting with acetylene. Icarus, 330, 217–229.Pérez-Hoyos, S., et al. (2020). Color and aerosol changes in Jupiter after a North temperate belt disturbance. Icarus, 132, 114021.Braude, A. S., et al. (2020). Colour and tropospheric cloud structure of Jupiter from MUSE/VLT: Retrieving a universal chromophore. Icarus, 338, 113589.Anguiano-Arteaga, A., et al. (2021). Vertical Distribution of Aerosols and Hazes Over Jupiter's Great Red Spot and Its Surroundings in 2016 From HST/WFC3 Imaging. Journal of Geophysical Research: Planets, 126, e2021JE006996.Anguiano-Arteaga, A., et al. (2023). Temporal variations in vertical cloud structure of Jupiter's Great Red Spot, its surroundings and Oval BA from HST/WFC3 imaging. Journal of Geophysical Research: Planets, 128, e2022JE007427.Irwin, P., et al. (2008). The NEMESIS planetary atmosphere radiative transfer and retrieval tool. J. Quant. Spectrosc. Radiat. Transf., 109, 1136–1150.Rodgers CD. (2000). Inverse methods for atmospheric sounding: theory and practice. Singapore: World Scientific.Cuzzi, J., et al., 2009. Ring Particle Composition and Size Distribution. Springer Netherlands, Dordrecht. pp. 459–509.Clark, R.N., McCord, T.B., 1979. Jupiter and Saturn: Near-infrared spectral albedos. Icarus 40, 180–188.Mendikoa, I., et al., 2017. Temporal and spatial variations of the absolute reflectivity of Jupiter and Saturn from 0.38 to 1.7 𝜇m with planetcam-upv/ehu. A&A 607, A72.Irwin, P.G., et al., 2018. Analysis of gaseous ammonia (NH3) absorption in the visible spectrum of Jupiter. Icarus 302, 426–436.Matcheva, K.I., Conrath, B.J., Gierasch, P.J., Flasar, F.M., 2005. The cloud structure of the jovian atmosphere as seen by the Cassini/CIRS experiment. Icarus 179(2), 432–448.Sromovsky, L., Fry, P., 2002. Jupiter’s cloud structure as constrained by Galileo probe and HST observations. Icarus 157 (2), 373–400.

Abstract:

Radiative transfer analyses of Uranus and Neptune spectra have revealed a cloud layer at pressures greater than ~2 bar (1,2), with H₂S gas detected above it on both planets (3,4), suggesting H₂S ice as its main constituent. However, the properties of these clouds and their dependence on the deep atmospheric composition remain poorly constrained.We present an extended version of a one-dimensional cloud microphysics model [5,6] previously applied to simulate CH₄ and H₂S clouds in the Ice Giants (7,8). The model now incorporates NH₄SH chemistry, extending the simulation domain to ~50 bar. Since NH₃ reacts with H₂S to form NH₄SH at depth, the deep N/S ratio controls how much H₂S is available to condense at higher altitudes. We explore how the deep NH₃ and H₂S abundances, together with the vertical mixing profile, determine the properties of the H₂S cloud layer, including its base pressure, total opacity, and particle size distribution.Preliminary results and the implications of this work for the interpretation of current and future observations of Uranus and Neptune will be discussed.References: [1] P. G. Irwin, et al., JGR: Planets, 127, e2022JE007189. [2] L. Sromovsky, et al., Icarus,Volume 317, (2019) [3] P. G. Irwin, et al., Nature Astronomy 2, 420 (2018). [4] P. G. Irwin, et al., Icarus 321, 550 (2019). ). [5] P. Rannou, et al., Science 311, 201 (2006). [6] F. Montmessin, et al., JGR: Planets 107, 4 (2002). [7] D. Toledo, et al., A&A, 694, A81 (2025). [8] D. Toledo, et al.,: Microphysical Modeling of Hydrogen Sulfide Clouds in the Atmospheres of the Ice Giants, EPSC-DPS Joint Meeting 2025,  https://doi.org/10.5194/epsc-dps2025-1456, 2025.

Abstract:

Introduction: Titan’s atmosphere has a north-south haze dichotomy (Fig.1), with an unexpectedly sharp boundary near the equator (e.g., Lorenz et al. 1997; Roos-Serote 2005). The boundary does not sit exactly at the equator, nor does it remain stationary throughout Titan’s year (R. Lorenz 1999; Roman et al. 2009; Kutsop et al. 2022; Vashist et al. 2023; Snell and Banfield 2024). Instead, the boundary migrates in latitude seasonally and is seen to disappear post-equinox then reappear with the dichotomy reversed shortly after. The sharpness of the boundary suggests that there is limited horizontal mixing over the equator. This, in addition to the boundary’s seasonal migration, makes Titan’s equator a dynamically intriguing region. Previously, dynamics in Titan’s stratosphere have been constrained observationally using the thermal wind relation (Sharkey et al. 2021;  Achterberg 2023;  Wright et al. 2025), but this equation breaks down at low latitudes. We instead map infrared-active trace species in Titan’s stratosphere to inspect the dynamics in Titan’s equatorial region.Data & Method: We use infrared spectra acquired by Cassini’s Composite Infrared Spectrometer (CIRS) instrument from 70 fly-bys of Titan spanning the entire 13-year mission. CIRS had an adjustable spectral resolution, typically observing at FWHM~0.5, 2.5, or 14.5 cm-1. We use CIRS FP3/4 observations acquired at a low spectral resolution (FWHM~14.5 cm-1), which achieved the best combination of seasonal and spatial coverage with high spatial resolution, allowing us to discern compositional variations over finer length-scales than in previous studies (Teanby et al. 2006; Teanby et al. 2010). Wright et al. 2024 showed that these data can be reliably forward-modelled despite having subtle and often blended spectral peaks. We use archNEMESIS (Alday et al. 2025) – an open-source Python package based on the NEMESIS (Irwin et al. 2008) radiative transfer and retrieval code – to fit CIRS FP3/4 spectra. We fit CIRS FP4 spectra by retrieving continuous temperature profiles and fit mid-IR spectra from 600-1100 cm-1 by scaling vertical profiles of gas volume mixing ratio.Results: We present maps of the variation in abundances of HCN, C2H2, C2H6, C3H4, C4H2, CO2 in Titan’s stratosphere (~5 mbar pressure) with the highest resolution mapping achieved to date, covering 40oS to 40oN throughout 2004—2017. Many species are seen to have a rapid change in abundance over the equator, with HCN exhibiting the steepest latitudinal gradient (e.g., Fig.2). The improved spatial resolution achieved here allows us to track the migration of the compositional gradient over time. We find that it follows a similar migration to Titan’s north-south haze boundary during the Cassini mission. Haze and HCN distributions appear to behave similarly at the equator, suggesting that the boundary is induced by dynamics, rather than by chemistry or microphysical processes.In addition, we use the meridional composition gradient to predict the tilt offset of Titan’s stratosphere, following the method of (Teanby et al. 2010). We do this over the full 13-year Cassini mission to inspect the seasonal evolution of Titan’s tilted stratosphere. This is compared to the tilt evolution inferred from temperature (Wright et al. 2025) and from images (Snell and Banfield 2024).Fig 1. Titan’s north-south albedo asymmetry. Infrared image taken in 2007 by Cassini’s Imaging Science Subsystem (ISS) Narrow-Angled Camera (NAC) using a 890 nm filter.Fig 2. Retrieved HCN volume mixing ratio (VMR) in Titan’s equatorial region, at 5 mbar. Example from observations taken during 2008. Different colours identify different observation sequences. The steepest gradient is seen to be ~5oS at this time (dashed line, shaded region is the uncertainty).ReferencesAchterberg, R. K. 2023. The Planetary Science Journal 4 (8): 140. https://doi.org/10.3847/PSJ/acebea.Alday, J., J. Penn, P. Irwin, J. Mason, J. Yang, and J. Dobinson. 2025. Journal of Open Research Software  13: 10. https://doi.org/10.5334/jors.554.Irwin, P. G. J., N. A. Teanby, R. de Kok, et al. 2008. Journal of Quantitative Spectroscopy and Radiative Transfer 109 (6): 1136–50. https://doi.org/10.1016/j.jqsrt.2007.11.006.Kutsop, N. W., A. G. Hayes, P. M. Corlies, et al. 2022. The Planetary Science Journal 3 (5): 114. https://doi.org/10.3847/PSJ/ac582d.Lorenz, R. 1999. Icarus 142 (2): 391–401. https://doi.org/10.1006/icar.1999.6225.Lorenz, R. D., P. H. Smith, M. T. Lemmon, E. Karkoschka, G. W. Lockwood, and J. Caldwell. 1997. Icarus 127 (1): 173–89. https://doi.org/10.1006/icar.1997.5687.Roman, M. T., R. A. West, D. J. Banfield, et al. 2009. Icarus 203 (1): 242–49. https://doi.org/10.1016/j.icarus.2009.04.021.Roos-Serote, M. 2005. Space Science Reviews 116 (1–2): 201–10. https://doi.org/10.1007/s11214-005-1956-0.Sharkey, J., N. A. Teanby, Melody Sylvestre, et al. 2021. Icarus 354 (January): 114030. https://doi.org/10.1016/j.icarus.2020.114030.Snell, C., and D. Banfield. 2024. The Planetary Science Journal 5 (1): 12. https://doi.org/10.3847/PSJ/ad0bec.Teanby, N. A., P. G. J. Irwin, and R. de Kok. 2010. Planetary and Space Science 58 (5): 792–800. https://doi.org/10.1016/j.pss.2009.12.005.Teanby, N., P. Irwin, R. Dekok, et al. 2006. Icarus 181 (1): 243–55. https://doi.org/10.1016/j.icarus.2005.11.008.Vashist, A. S., M. F. Heslar, J. W. Barnes, C. Hennen, and R. D. Lorenz. 2023. The Planetary Science Journal 4 (6): 118. https://doi.org/10.3847/PSJ/acdd05.Wright, L., N. A. Teanby, P. G. J. Irwin, and C. A. Nixon. 2024. Experimental Astronomy 57 (2): 15. https://doi.org/10.1007/s10686-024-09934-y.Wright, L, N. A. Teanby, P. G. J. Irwin, et al. 2025. The Planetary Science Journal 6 (5): 114. https://doi.org/10.3847/PSJ/adcab3.

Abstract:

The spectral signature of hydrogen sulphide (H2S) above the cloud tops in Uranus’ atmosphere was detected in 2018 [1]. The H2S humidity can be used as a tracer of Uranus’ overturning circulation [2] - peaks and troughs in the latitudinal humidity distribution may correspond to regions of local upwelling and downwelling near the H2S condensation level. We analysed observations from Gemini-NIFS and VLT-SINFONI, taken between 2009 and 2014, to study the H2S humidity distribution of Uranus. In our previous analysis of H2S on Neptune [3], we found a significant degeneracy between the methane (CH4) and H2S distributions, so we prescribe a latitudinally varying deep methane abundance previously derived from HST-STIS spectra [4]. We deconvolve the observations and extract spectra using the Minnaert limb-darkening approximation, which has been applied in several analyses of Ice Giant observations [3,4,5]. We fit a parameterised aerosol model and the H2S humidity to our extracted spectra with nested sampling using our open-source radiative transfer code, archNEMESIS [6]. Our atmospheric model has a large number of parameters, and to make nested sampling computationally feasible we utilise a trained neural network for early exploration of the parameter space during our retrievals. Since our observations span several years, we search for temporal changes. We find changes in the aerosol structure and aerosol spectral properties corresponding to the development of Uranus' north polar hood, in agreement with previous work [4,8]. If we assume that the CH4 distribution is stable over time, then our results show no significant changes in the H2S distribution.  Our results show a general equator-to-pole decrease in the H2S humidity, similar to what has been found in microwave analyses that are sensitive to the deep H2S distribution [7]. Superimposed on this are local increases, which are fairly evenly spaced in latitude. We found a somewhat similar pattern in our analysis of H2S on Neptune [3], and an analysis of Neptune with VLT/MUSE also found peaks in reflectivity with a similar spacing [5]. These results are suggestive of a complex circulation pattern near the deep H2S aerosol layer. [1] Irwin, P. G. J., et al. (2018). Detection of hydrogen sulfide above the clouds in Uranus's atmosphere. Nature Astronomy, 2(5), 420-427. [2] Fletcher, L.N., et al. (2020). Ice Giant Circulation Patterns: Implications for Atmospheric Probes. Space Sci Rev 216, 21[3] Penn, J., et al. (2026). Reconciling Near-Infrared and Microwave Analyses of Neptune’s Hydrogen Sulphide Distribution. Monthly Notices of the Royal Astronomical Society, 548, 2, [4] James, A., et al. (2023). The Temporal Brightening of Uranus' Northern Polar Hood From HST/WFC3 and HST/STIS Observations. Journal of Geophysical Research: Planets, 128(10), e2023JE007904.[5] Irwin, P. G. J., et al. (2023). Latitudinal Variations in Methane Abundance, Aerosol Opacity and Aerosol Scattering Efficiency in Neptune's atmosphere determined from VLT/MUSE. Journal of Geophysical Research: Planets, 128(11), e2023JE007980.[6] Alday, J., et al. (2025). archNEMESIS: An Open-Source Python Package for Analysis of Planetary Atmospheric Spectra. Journal of Open Research Software, doi:10.5334/jors.554.[7] Molter, E. M., et al. (2021). Tropospheric Composition and Circulation of Uranus with ALMA and the VLA. The Planetary Science Journal, 2(1), 3.[8] Sromovsky, L. A., et al. (2024). The puzzling north polar region of Uranus: Continued zero-shear winds and increasing brightness from 2015 through 2022 according to 7 years of Keck AO imaging. Icarus, 420, 116186.

Abstract:

IntroductionOxygen is the universes third most abundant element (Bergman et al. 2021) and is extremely rich in the Saturnian system (Feuchtgruber et al. 1997). Studies have found oxygen-bearing molecules and ions in Saturn’s atmosphere (Esposito et al. 2005) , in its plasma environment (Wilson et al. 2016) and on its moon Enceladus (Thomas et al. 2016). In contrast, Titan is scare in oxygen species, boasting a rich atmosphere of hydrocarbons and nitriles (Vuitton et al.2024) that interact uniquely, often consuming free oxygen or locking it away as water ice. This results in an anoxic, organic and diverse environment with little oxygen to terminate reactions (Nixon et al. 2024).To date, only three oxygen-bearing molecules have been detected in Titan’s atmosphere: CO (Lutz et al. 1983), CO2 (Sameulson et al 1983) and H2O (Coustenis et al. 1997). These molecules form from externally delivered OH, O+  and/or H2O being photodissociated by solar UV and energetic particles in the upper atmosphere, before recombining and being transported downwards via atmospheric mixing (Vuitton et al. 2019).  Of the detected species, the least well-understood is water vapour. CO and CO2 have been studied extensively and exhibit little variation in latitude, time or altitude (Teanby et al. 2019). Yet, investigations into H2O have been limited to single measurements or large averages (Vuitton et al 2007,  Cui et al 2009, Cottini et al. 2012, Bauduin et al 2018). Its weak infrared emission lines and low atmospheric abundances make it difficult to model. Water plays a vital role in Titan’s atmosphere, distributing oxygen and acting as a tracer for atmospheric dynamics. Its chemical pathways may produce species important for astrobiology like formaldehyde (Nixon 2024). Figure 1: Schematic showing the potential pathways of H2O and its transport through the atmosphere until condensation near the tropopause. Not all reactions have been included.. The chemistry shown is based on Vuitton et al. 2019 and Nixon 2024. Molecules in green denote those predicted by photochemical models but not yet been detected.MethodHere, we present the first reported latitudinal and temporal variability of H2O in Titan’s atmosphere. Using the NEMESIS radiative transfer code (Irwin et al. 2008) with temperature a priori profiles from Teanby et al. 2019 and a vertical water a priori profile from Vuitton et al.2019, we retrieve water abundance in Titan’s stratosphere from 157 far-infrared high-resolution Cassini CIRS FIRNADCMP observations (Flasar et al. 2004) across the entire mission and moon. To improve the fit and account for variations in the baseline caused by aerosols, we fit scaled gaussian basis function (see associated EPSC 2026 poster for more information). Due to low signal-to-noise in some spectra and low abundances, 51 observations were averaged in 13 bins. Our derived column abundances are consistent with all previous measurements of water in Titan’s middle atmosphere, and the retrieved profiles show consistency with upper atmosphere upper limits derived from INMS (Vuitton et al. 2007, Cui et al 2009). These results provide important constraints for future photochemical models and GCM's, particularly those focused on oxygen chemistry and organic molecule formation. ResultsAt the poles, lower column abundances are observed due to reduced temperatures, which decrease the saturation vapour pressure. We also find high retrieved scale factors (applied to the a priori) at the north pole indicating that water is mildly enhanced by a factor of 3 relative to mid-latitudes similar to trace gases like HCN (Teanby et al. 2012), although much weaker. This is likely caused by water-rich air subsiding into the polar vortex, concentrating and adiabatically heating within the confines of the polar mixing boundary (Teanby et al. 2017). We find no evidence of seasonality, however we do find statistically significant time-variability at low-to-mid latitudes. Analysis of atmospheric residence times and comparison with the TAM GCM (Lombardo et al. 2023) shows the variability is not explained by photochemistry or atmospheric dynamics and may indicate a time-varying source. This idea was also previously suggested by Moreno et al. 2012 and Bauduin et al. 2018. We explore potential drivers of time-variability and conclude that short-term month-scale variations in Enceladus’ neutral torus, and Saturn’s dynamic magnetospheric environment could be responsible. The derived incoming OH flux needed to explain our results matches the calculated OH flux from the neutral torus, implying Enceladus could be the dominant source of Titan's water.  Figure 2:  Step-by-step schematic showing the potential path of H2O (OH) molecules from Enceladus to Titan, highlighting consistency or variability at each step. Each band represents a different type of neutral torus and the dotted lines originating from Saturn represent the magnetic field.ReferencesBauduin. A. et al. 2018. Icarus 301, 136–151. doi: 10.1016/j.icarus.2017.09.039. Bergman. M.  et al. 2021. Monthly Notices of the Royal Astronomical Society 508 (2). Doi: 10.1093/mnras/stab2160Cottini. V. et al. 2012. Icarus 220 (2), 855–862. doi: 10.1016/j.icarus.2012.06.009. Coustenis. A. et al. 1998. A&A 336, 85-89.Cui. J. et al. 2009. Icarus 200, 581–615. doi: 10.1016/j.icarus.2008.11.005. Esposity. L. et al. 2005. Science, 307 (5713). Doi: 10.1126/science.1105606Feuchtgruber. H. et al. 1997. Nature 389, 159-162. Doi: 10.1038/38236Flasar. F. M. et al. 2004. Space Science Reviews 115 (1–4), 169–297. doi: 10.1007/s11214-004-1454-9. Irwin, P. G. J. et al. 2008. Journal of Quantitative Spectroscopy and Radiative Transfer 109 (6), 1136–50. Doi: 10.1016/j.jqsrt.2007.11.006.Lombardo. N.A. et al. 2023. JGR: Planets, 123. Doi: 10.1029/2023JE008061.Lutz. B.L. et al. 1983. Science, 220 (4604), 1374-1375. Doi:10.1126/science.220.4604.1374.Moreno. R. et al. 2012. Icarus 221, 753–767. 10.1016/j.icarus.2012.09.006Nixon. C.A. 2024. ACS Earth Space Chem 29, 8(3), 406-456. Doi: 10.1021/acsearthspacechem.2c00041.Samuelson. R. E. et al. 1983. JGR: Space Physics, 88(A11), 8709-8715. Doi: 10.1029/JA088iA11p08709.Teanby. N.A. et al. 2012. Nature, 491, 732. Doi: 10.1038/nature11611Teanby. N.A. et la. 2017. Nature Communications, 8, 1586.Doi:10.1038/s41467-017-01839-zTeanby. N.A. et al. 2019. Geophysical Research Letters, 46, 3079-3089. Doi: 10.1029/2018GL081401.Thomas. P.C. et al. 2016. Icarus, 264, 37-47. DOI: 10.1016/j.icarus.2015.08.037.Wilson, R. et al. 2015. JGR Space Physics 120 (8). DOI:10.1002/2014JA020557.Vuitton. V. et al. 2007. Icarus 191. doi: 10.1016/j.icarus.2007.01.028.Vuitton.V. et al. 2019. Icarus, 324, 120-197. Doi: 10.1016/j.icarus.2018.06.013Vuitton. V. et al. (2024), ‘Chapter 6 : Titan’s Atmospheric Structure, Composition, Haze, and Dynamics’ In Titan after Cassini–Huygens, COSPAR Scientific Symposium Series.