Solar system

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.

Abstract:

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.

Abstract:

We present a calibrated catalog of Cassini Visible and Infrared Mapping Spectrometer (VIMS) observations of Jupiter, together with the processing workflow used to generate the final publicly available products. Starting from the raw VIMS cubes, the workflow produces radiometrically consistent multiextension Flexible Image Transport System files and includes a revised visible-channel calibration, a revised infrared-channel calibration that resolves a subset of problematic cases not satisfactorily treated by the standard Integrated Software for Imagers and Spectrometers pipeline, corrections for pointing-related misalignments between spectral cubes and geometric backplanes, and customized dark signal correction strategies. The final products include calibrated spectral cubes together with geometry backplanes and wavelength information for subsequent scientific analysis. We assess the consistency of the calibrated products through internal validation tests and comparisons with independent reference spectra from the literature. The resulting products provide a uniform and validated data set of Cassini VIMS Jupiter observations for community use. The full catalog is available as a public dataset at doi:10.5281/zenodo.19223781.