Solar system

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:

Accurately characterizing the endogenic thermal signatures of icy moons requires a detailed understanding of surface heating driven by insolation, surface properties and local terrain. Due to the lack of high-resolution thermal observations, Europa’s surface temperatures have historically been sufficiently modelled using smooth surface or one-dimensional approximations [1]. However, Europa’s complex terrains alter local illumination conditions and surface temperatures. Here, we use TEMPEST, an open-source, python based thermophysical model [2, 3] to investigate the significance of radiative self-heating and light scattering on interpretation of Europa’s thermal IR emission, with a particular focus on detectability of near-surface liquid-water reservoirs by high spatial resolution (≤100 m/pixel) observations soon to be taken by Europa Clipper’s E-THEMIS instrument [4].Passive heating by solar illumination was modelled under Europa-like conditions using TEMPEST, which solves a surface energy balance that includes solar flux, thermal emission, vertical heat conduction, and (optionally) scattering, radiative self-heating and macroscopic surface roughness. The 1D periodic conduction solver at its core is based on thermprojrs [5]. Comparing a smooth sphere (fig. 1a, c) to the topographic digital elevation model (DEM) (fig. 1b, d) reveals that while mean temperatures remain comparable, topography introduces substantial heterogeneity. For example, the DEM has a wider distribution of surface temperatures due to local slope and shadowing effects. Thermal radiance scales with Temperature4 (Stefan-Boltzmann law), so the small minority of hotter regions can dominate the thermal IR signal (fig. 2). We show that rough terrain emits significantly higher total radiance than a smooth sphere despite the similar mean kinetic temperature.Simulated topographic temperature maps show that light scattering and radiative exchange with local terrain cause increased surface temperature heterogeneity, with local temperature increases in deep fractures as high as 35 K (fig. 3). Temperature heterogeneity can systematically bias mean temperature measurements, and localised mutual radiative heating can lead to apparently anomalous warm regions, potentially mimicking endogenic heating, or obscuring indications of conductive heating from near-surface water reservoirs. To place these effects in the context of E-THEMIS detectability, we will use first-order conductive heat-flux calculations to estimate the surface expressions of idealised subsurface liquid-water reservoirs at different depths and spatial scales. These calculations will provide test cases for distinguishing plausible endogenic thermal anomalies from topographically induced radiance variations. This work shows that if topography is ignored, the excess radiance caused by it could be misinterpreted as a region of lower thermal inertia (in daytime), or even an endogenic heat source (hotspot). Accurately modelling terrain reduces the risk of false positives when searching for plume sources or active regions.Figure 1: Surface temperature maps for a section of an icy moon modelled as a smooth sphere (a) and using a digital elevation model (DEM) of Enceladus (b) using Europa thermal parameters. Histograms (c) and (d) show the distribution of surface temperatures within the above terrain samples.   Figure 2: The integrated surface radiance from each terrain sample. The T4 dependence of radiance leads to significantly higher emission from the DEM terrain.Figure 3: Simulated Europa surface temperatures without (a) and with (b) multiple scattering and radiative self-heating. Panel (c) shows the resulting temperature difference, with local increases up to 35 K in shadowed terrain. Enceladus DEM [6] provides an icy-terrain analogue for topographic heating effects.  Future work will repeat this analysis using Europa-specific topography where available. References:[1] Rathbun, J. A., Rodriguez, N. J., & Spencer, J. R. (2010). Galileo PPR observations of Europa: Hotspot detection limits and surface thermal properties. Icarus, 210(2), 763-769.[2] Lyster, D., Howett, C., & Penn, J. (2025). TEMPEST: A Modular Thermophysical Model for Airless Bodies with Support for Surface Roughness and Non-Periodic Heating. EPSC-DPS 2025, 1479.[3] Chivers, C.J., Hayne, P.O. and Schmidt, B.E. (2025). Prospects For Detecting Shallow Liquid Water Bodies At Europa Using E-Themis. LPSC 2025, 1226.[4] Christensen, P.R., Spencer, J.R., Mehall, G.L., Patel, M., Anwar, S., Brick, M., Bowles, H., Farkas, Z., Fisher, T., Gjellum, D. and Holmes, A. (2024). The Europa thermal emission imaging system (E-THEMIS) investigation for the Europa clipper mission. Space Science Reviews, 220(4), 38.[5] Spencer, J.R., Lebofsky, L.A., and Sykes, M.V. (1989). Systematic biases in radiometric diameter determinations. Icarus, 78(2), 337-354. [6] Park, R.S., Mastrodemos, N., Jacobson, R.A., Berne, A., Vaughan, A.T., Hemingway, D.J., Leonard, E.J., Castillo-Rogez, J.C., Cockell, C.S., Keane, J.T. and Konopliv, A.S. (2024). The global shape, gravity field, and libration of Enceladus. Journal of Geophysical Research: Planets, 129(1), e2023JE008054.

Abstract:

Thermal measurements provide key constraints on the physical properties of icy satellite surfaces, including grain size, porosity, and regolith structure. On the icy moon of Jupiter Europa, previous analyses [e.g., 1,2] of the Galileo Photopolarimeter–Radiometer (PPR) dataset revealed heterogeneities in thermal inertia, but the limited spatial resolution and coverage prevented a detailed characterization of the thermophysical properties of the surface. Yet, the determination of these thermophysical properties is crucial to predict the surface temperatures of Europa that can be used to search for endogenic activity as hot spots, one of the main scientific targets of the upcoming NASA’s Europa Clipper mission [3].We derived high-resolution maps of Europa’s surface albedo and thermal inertia, and inferred the microphysical properties of its icy regolith, through a reanalysis of the Galileo PPR dataset. We specifically investigated the spatial variability of these properties to discuss the processes controlling the thermophysical evolution of Europa’s surface. To do so, we used the KRC thermal model [4] to analyze the PPR brightness temperatures and retrieve the albedo and thermal inertia that best fit the observations. These values were then interpreted using theoretical conductivity models of porous ice [5] to constrain grain size and porosity and to investigate possible sintering processes affecting the surface.We will present at the conference our main results: we derived a mean Bond albedo of 0.64 ± 0.06 (standard deviation of 1σ) and a mean thermal inertia of 56 ± 17 J m−2 K−1 s−1/2 (1σ). The thermal inertia shows significant spatial variations, including a band of low thermal inertia at the equator (39 ± 7 J m−2 K−1 s−1/2, 1σ) and higher values (56 ± 11 J m−2 K−1 s−1/2, 1σ) at mid-latitudes on the leading hemisphere (0°–180° W). The equatorial region of the trailing hemisphere (180° W–360° W) also exhibits higher thermal inertia (63 ± 17 J m−2 K−1 s−1/2, 1σ) than the leading hemisphere, likely related to compositional differences. Interpreting the thermal inertia with conductivity models indicates a porous icy regolith with grain sizes ranging from a few micrometers to a few centimeters and an average porosity of 0.61 ± 0.1 (1σ).Interestingly, the thermal inertia distribution shows little correlation with geological units, the Pwyll ejecta being a notable exception, with markedly higher values than the surrounding terrain. In contrast, the good agreement between the thermal inertia distribution and modeled sputtering rates suggests that sputtering-driven sintering may play a fundamental role in controlling the thermophysical properties of Europa’s surface. The absence of a high thermal inertia equatorial band analogous to the PacMan anomaly observed on Saturn’s icy moons [e.g., 6] indicates that electron-driven sintering is inefficient on Europa, while temperature-gradient metamorphism may instead enhance grain growth at depth, potentially explaining the absence of large grains at the surface. In addition, modeled surface temperatures range from ~67 to 148 K at mid to low latitudes, with peak daytime temperatures counteracting radiolytic amorphization, while limiting the stability of volatile species. During the Europa Clipper mission (2031–2034), temperatures are expected to be slightly lower, ranging between 67.6–141.2 K. Our predictions provide a framework for interpreting future observations by the Europa Thermal Emission Imaging System (E-THEMIS) onboard Europa Clipper and the Submillimetre Wave Instrument (SWI) on JUICE. These future thermal measurements will provide key constraints to test these hypotheses and refine our understanding of the evolution of Europa’s icy regolith as well as search for active hot spots.AcknowledgementsLL’s research was supported by an appointment to the NASA Postdoctoral Program administered by Oak Ridge Associated Universities at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Part of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Some of the computational analyses were run on Northern Arizona University’s Monsoon computing cluster, funded by Arizona’s Technology and Research Initiative Fund. © 2026. All rights reserved.References[1] Rathbun et al., 2010, Icarus, 210, 763–769[2] Rathbun & Spencer, 2020, Icarus, 338, 11350[3] Pappalardo, R. T., Buratti, B. J., Korth, H., et al. 2024, SSR, 220[4] Kieffer, H. H. 2013, JGR: Planets, 118, 451–470[5] Ferrari & Lucas, 2016, A&A, 588, A133[6] Howett et al., 2011, Icarus, 216, 221–22

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.  

Abstract:

The Great Red Spot (GRS) is one of the most prominent features observed on Jupiter due to its size and distinctive colour, yet many aspects of this vortex remain poorly understood. Fundamental questions regarding the vertical structure, the chromophores responsible for its colour, the distribution of such chromophores and other aerosols remain unanswered. Laboratory experiments in which ammonia (NH3) and acetylene (C2H2) are photolyzed by ultraviolet radiation (Carlson et al. 2016) may provide a solution for the identity of the chromophore. One study suggested that this chromophore could be responsible for the red colour across the whole disk of Jupiter, giving rise to the idea of a “universal chromophore” (Sromovsky et al. 2017), which may be located at the top of the main tropospheric cloud, resembling a “crème brûlée” structure (Baines et al. 2019). Other studies suggest that, while the universal chromophore could be plausible, it may not be the one caused by the NH3‒C2H2 reaction, instead it would be one with a steeper blue absorption gradient (Braude et al. 2020). Alternatively, some studies propose the presence of two chromophores located in both an upper haze and lower haze layer, with the upper haze chromophore possibly corresponding to that produced by the NH3‒C2H2 reaction (Anguiano‐Arteaga et al. 2021; Anguiano‐Arteaga et al. 2026).More generally, observations indicate that the top of the GRS is located at a higher altitude than the surrounding cloud deck. Consequently, vertical structure models developed for the rest of Jupiter’s disk may not provide accurate solutions for the vortex itself, where each atmospheric layer may be shifted to higher altitudes. We are analysing observations of the GRS obtained by VLT/MUSE (0.480 – 0.930 µm), Cassini/VIMS (0.884 – 5.122 µm), Juno/JIRAM (2.002 – 5.014 µm), and JWST/NIRSpec (1.660 – 3.170 µm, 2.870 – 5.270 µm), using the radiative transfer model ArchNEMESIS (Alday et al. 2025). The usage of multiple instruments, different geometries and a wide wavelength range aims to better constrain the vertical structure and chromophore distribution of the GRS, as well as those of other vortices of interest on Jupiter.