Patrick Gerard Joseph Irwin

Jupiter is a hugely varying and diverse planet, demonstrated through spectacular changes in the cloud top appearance. However, the cloud structures that contribute to this appearance are hard to determine and are not known with certainty. A lack of in situ observations and many unknown parameter values and numerous near-degenerate pairings within the parameter space for visible/near-infrared observations of the aerosol profiles makes the problem underconstrained. This work utilizes several methods to alleviate these issues. Fitting simultaneously to dual viewing angle VLT/MUSE observations, using a radiative transfer algorithm, provides greater constraint to the aerosol profile than fitting to a single viewing geometry alone. To reduce the degeneracy between parameter pairings in the model, we utilize an aerosol profile with a minimized number of variable parameters by predetermining these values wherever possible. We find two clouds are needed, with a main cloud and haze layer fixed at 1.4 and 0.2 bars, respectively, to successfully reproduce 6 yr of VLT/MUSE observations (2018–2023), including dual-angle observations of the Great Red Spot (GRS). We also retrieve an ammonia gas distribution across the disc similar to previously derived profiles, including the peak in deep volume mixing ratio at 5$^{\circ }$N before a rapid depletion at 10$^{\circ }$N. This cloud parametrization successfully reproduces spectra extracted from 40$^{\circ }$N to 40$^{\circ }$S across the disc of Jupiter. For all locations (including the GRS) we find the material responsible for colouring Jupiter to be located in the main cloud layer (1.4 bar).

Titan's middle atmosphere has a well-observed north-south haze asymmetry (NSA), with a mysteriously sharp boundary that migrates seasonally across the equator. Several stratospheric trace species display similarly steep latitudinal gradients near the equator, yet the origin of these abrupt transitions is not well understood. Here we map the distributions of multiple trace species between 40°S and 40°N using observations from Cassini's Composite Infrared Spectrometer spanning almost half a Titan year, providing the highest-latitudinal-resolution view of Titan’s equatorial stratosphere to date. The measurements provide unique insight into the dynamics of Titan's equatorial region, which cannot be constrained using conventional thermal wind balance techniques. We show that composition boundaries near the equator follow a strikingly similar migration to Titan's haze NSA boundary, suggesting the NSA boundary is driven primarily by dynamics rather than chemistry or haze microphysics. Comparison with the Titan Atmospheric Model (TAM) general circulation model shows that the steep equatorial gradients are established primarily by vertical transport associated with the meridional overturning circulation. The observed offset of the boundary from the equator is explained by horizontal transport of enriched air, driven predominantly by horizontal eddies rather than mean-flow advection. These results reveal the dynamical origin of Titan’s enigmatic equatorial boundary and constrain the seasonal evolution of its global circulation.

The Hubble Space Telescope collected imaging data spanning the full lifetime of Neptune's dark spot NDS‐2018, which is the sixth large, persistent dark spot seen in the planet's atmosphere. Neptune's dark spots are thought to be anticyclonic vortices, although internal flows have never been directly measured to confirm their rotation. Previous reports covered the formation and evolution of the mature NDS‐2018, while here we report that the contrast weakened over the 2021 to 2022 period, while the feature rapidly drifted equatorward from 14 deg N to 6–7 deg N planetographic latitude. The persistence of the spot so close to the equator is surprising, and represents a challenge for numerical models which find anticyclones to be disrupted within 15 deg of the equator. The changing contrast of the dark spot constrains changes in the aerosols over time, but the link between dynamical and microphysical properties of dark vortices is not known.

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.

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...

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.

JWST-NIRSpec observations are providing our first detailed view of the Ice Giants in the 5-micron spectral window [1]. Observations of Jupiter and Saturn (see Figure) at 5 microns have historically revealed important insights into their tropospheric composition, along with striking images of clouds silhouetted against the glowing thermal emission from deeper atmospheric layers [2]. Comparable observations of the colder Ice Giants, however, have long remained beyond observational reach—until now.In this talk, we present JWST-NIRSpec 5-micron observations of the Ice Giants and discuss their implications. We examine what these observations reveal about the tropospheric temperatures, clouds, and composition of Uranus and Neptune, as well as what they may imply for current radiative transfer models at these wavelengths.In particular, we find that the JWST data may suggest sub-adiabatic temperature gradients near the cloud layers, resulting in colder temperatures in the deeper atmosphere. However, these conclusions depend sensitively on the scattering properties of Ice Giant clouds and on accurate modelling of the gaseous opacity, both of which, we argue, remain uncertain under these conditions.Figure: Saturn at 5 microns (left, in red) from Cassini, with hazes shown in green (Image credits: NASA/JPL-Caltech/University of Arizona), compared to Uranus at 5 microns from JWST-NIRSpec [1].[1] Roman, M.T., et al., The Infrared Spectrum of Uranus Revealed with JWST (submitted)[2] Wong, M.H., Bjoraker, G.L., Goullaud, C., Stephens, A.W., Luszcz-Cook, S.H.,1047Atreya, S.K., Pater, I., Brown, S.T.: Deep clouds on jupiter. Remote Sensing104815(3), 702 (2023)

Analysing observations of Jupiter made by VLT/MUSE (0.475 – 0.933 μm), Cassini/VIMS (0.40 – 5.15 μm), and Juno/JIRAM (2 – 5 μm), we present early results of a new combined cloud-ammonia profile model for Jupiter's equatorial atmosphere. We find this model to be consistent with all observations considered, at a range of observation geometries, within the Equatorial Zone (EZ), the North Equatorial Belt (NEB) and a North Equatorial Dark Feature (NEDF), also known as a '5-micron-hotspot'. Preliminary results suggest the presence of three main layers: 1) a deep 'Cloud-1' at 1-2 bar; 2) an upper 'Cloud-2' in the upper troposphere based at ~0.55 bar; and 3) a layer of chromophore particles situated within the Cloud-1 layer, responsible for the blue-absorption at visible wavelengths. Our best-fit ammonia profile is closely linked with our cloud profile, with Cloud-1 coinciding with a sharp drop in ammonia abundance, perhaps associated with the formation of a H2O-NH3 'mushball' cloud, or an ammonium hydrosulphide (NH4SH) cloud, or both, and Cloud-2 coinciding with the ammonia condensation level.We find the bulk of the cloud opacity in Jupiter’s atmosphere to be in the Cloud-1 layer, based at 1-2 bar and composed of relatively large particles (r ~ 10 μm), which are highly scattering at visible wavelengths to allow sunlight to penetrate and be Rayleigh-scattered from the deeper atmosphere, but more absorbing at 5 μm. The belt/zone difference at 5 μm is accounted for by changes in the opacity of Cloud-1 and also the single-scattering albedo of these particles. We find the Cloud-2 layer, based at the ammonia condensation level, to be approximately 10 times less opaque than Cloud-1 and have an absorption band near 3 μm, which is consistent with this layer having a significant opacity of large (r ~ 10 μm) ammonia ice particles.We will present preliminary insights into the spatial distribution of these clouds over the regions considered and their implications for our understanding of Jupiter’s chromophores and upper-level hazes.

During its flyby of Titan in 1980, Voyager 1 unveiled an atmosphere thick with an opaque, orange haze that completely obscured the surface, and provide the first close-up images of its vertical layering (Smith et al. 1981, Hanel et al 1981). Subsequent observations from the Cassini-Huygens mission (Flasar et al. 2004) expanded this view, revealing a world dominated by photochemical hydrocarbons and nitriles with lakes of methane (Stofan et al. 2007, Mastrogiuseppe et al. 2019), tholin-like organic sand (Lorenz et al. 2006), and seasonally evolving ice clouds (Jennings et al. 2012, West et al. 2016) as seen in Figure 1. While recent observations and laboratory studies (Anderson et al. 2011, Chatain et al. 2020) have provided greater insights into the physical properties of these aerosols and how they form, their composition and complexity remain a mystery.Figure 1: Image taken by Cassini Imaging Science Subsystem (Credit: NASA/JPL-Caltech/Space Science Institute) showing a large ice cloud possibly made of HCN at 300km in Titan’s south pole, 2012 (West et. 2016, Vinatier et al. 2018)In the upper atmosphere, nitrogen and methane are photodissociated by UV radiation and energetic particles to produce ions which act as embryos for the growth of large organic molecules (Vuitton et al. 2024). As these molecules sink through the atmosphere, they recombine, coagulate, and accumulate into larger aerosol particles, onto which trace species can condense, forming both photochemical hazes and stratospheric ice clouds (Vuitton et al. 2024). Cassini CIRS (Composite Infrared Spectrometer) far infra-red spectra (FP1) exhibit four types of hazes: Haze 0, Haze A, Haze B (otherwise known as the “Haystack”) and Haze C (de Kok et al. 2007). While Haze 0 is present throughout the CIRS spectral range and affects the spectral continuum, Haze A, B and C appear as broad features in the far-infrared range. Relatively little is known about these hazes, making their features difficult to fit and estimate. In addition, their signatures overlap with H2O, C4H2, C2N2 and CH4 rotational lines, complicating retrievals of these gases (Sylvestre et al.2017). Among these hazes, Haze B exhibits the strongest spectral signature. Anderson et al. 2018 suggested that the composition must be a mixture of more than one chemical compound due to its magnitude, breadth and opacity. Haze B is observed to only be present at the winter poles, forming and dissipating with the changing seasons, most likely caused by reduced sunlight and temperature (Jennings et al. 2012a,b , Anderson et al 2018).To enable accurate fitting of water features, Ford et al. (in review) retrieved an effective spectral cross-section of Cassini CIRS FP1 spectra between 147-257cm-1 of 156 FIRNADCMP 0.5cm-1 observations (Figure 2) across different latitudes and times (see also Ford et al. 2025). This was achieved by scaling gaussian basis functions to fit Haze A, B and C (and any other unknown aerosols) in the spectra baselines using the NEMESIS radiative transfer code (Irwin et al. 2008). Previous aerosol cross-sections did not account for latitude or time variation and therefore this technique empirically and agnostically modelled the changing baseline. In this study, we decomposed those 156 effective spectral cross-sections and extrapolated unique Haze B cross-sections for latitudes of -89° to 88° from June 2004 to April 2017. We used principal component analysis to determine the number of complete hazes present in the spectral range, finding that Haze B accounts for 98% of the observed variation. We then applied non-negative matrix factorisation to seperate the spectra into two unique and stable components: one representing Haze B, and a second representing a mixture of other haze contributions, along with numerical and spectral noise.Figure 2: Effective spectral cross-section of all 156 Cassini CIRS FIRNADCMP 0.5cm-1 observations before decomposition. Each colour represents a different observation. The plot shows the variation extent of the Haze B feature at ~220cm-1. Using the maximum Haze B cross-section of each observation as proxy, we find a large increase at the winter poles with the increase during northern spring being nearly twice that of northern winter, consistent with previous investigations. We also find Haze B extends to 60°, different to 70° proposed by Anderson et al. 2018. The results also give an insight into the time of formation/dispersion of Haze B at the poles. We see that Haze B at the south pole forms around 2013, reaching its peak during the summer solstice. The final dissipation of Haze B at the north pole cannot be determined due to limited data. ReferencesAnderson, C.M. et al. (2011), Icarus, 212.2, 762-778. DOI: 10.1016/j.pss.2010.10.009Anderson, C.M. et al. (2018), Organic Ices in Titan’s Stratosphere in Space Science Reviews, 214.8, 125 DOI: 10.1007/s11214-018-0559-5Chatain, A. et al. (2020), Icarus, 345, 113741. DOI: 10.1016/j.icarus/2020.113741Coustenis, A. et al. (1999), Planet Space Science, 47, 1305-1329. DOI: 10.1016/S0032-0633(99)00053-7de Kok, R. et al. (2007), Icarus, 191, 223. DOI:10.1016/j.icarus.2007.04.003Flasar, F.M. et al. (2004), Space Science Reviews, 115, 169–297. DOI: 10.1007/s11214-004-1454-9Ford, J.S. et al. (2025), EGU General Assembly 2025, Vienna, Austria, EGU25-3741. DOI: 10.5194/egusphere-egu25-3741Ford, J.S. et al. In review. Titan’s Stratospheric Water: Latitudinal and Seasonal Variation from Cassini CIRS Data and Implications for External Oxygen Sources. PSJHanel, R.A. et al. (1981), Science, 212, 192–200. DOI: 10.1126/science.212.4491.192Irwin, P.G.J et al. (2008), JQSRT, 109, 1136. DOI: 10.1016/j.jqsrt.2007.11.006Jennings, D.E. et al. (2012), The Astrophysical Journal Letters, 754, L3. DOI: 10.1088/2041-8205/754/1/L3Khanna, R.K. (2005), Icarus, 178,165-170. doi:10.1016/j.icarus.2005.03.011Lorenz, R. et al. (2006), Science, 312,...

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.