OPS2
In preparation for the arrival of these missions to the Jupiter system, this session invites contributions from across the planetary science community, with the goal of fostering collaborations that will advance our understanding of topics relevant to the Galilean moons and maximize the scientific return of the missions. This session welcomes presentations concerning laboratory experiments, numerical modeling, terrestrial analog studies, and Earth-based observations (such as those from JWST, ALMA, or HISAKI), as well as analyses of past or ongoing mission data and comparative investigations of icy moons across systems. Topics of interest include the surface geology and composition of the icy Galilean moons, their interior structures and subsurface ocean dynamics, their interactions with Jupiter’s magnetosphere, surface weathering processes, and the formation, structure, and composition of their exospheres. The detection and characterization of potential Europa plumes is also highly relevant.
Additionally, we welcome discussions on the recent Juice Moon-Earth flyby and the Europa Clipper Mars flyby, examining how these events inform upcoming observations at Jupiter. The session will also provide a platform to explore mission objectives, instrumentation, recent developments, and the potential for Juice-Clipper synergistic science, as described by the recent Juice-Clipper Steering Committee report.
Session assets
Introduction:
The icy Galilean moons, Europa, Ganymede, and Callisto, have surfaces with visible to near-infrared (VNIR) reflectances that are darker than expected for ices composed of water, acids, and salts. (see Figure 1a, 1d). Past investigations of these moons suggest neutral darkening agents (low reflectance and spectrally featureless materials) may be present. Common darkening agents include Fe- or C-bearing minerals, such as magnetite and graphite, which can be delivered through micrometeorite bombardment, radiolysis of surface species, or endogenic activity [1-3]
Darkening agent abundances are reportedly <20%, ~50%, and ~90% for Europa [4], Ganymede [5,6] and Callisto [7], respectively; however, large uncertainties are present in these estimates. These moons’ surfaces are likely intimately mixed due to space weathering and potential endogenic activity. A few wt.% darkening agent when intimately mixed with bright materials can drastically reduce the overall mixture’s reflectance [8]. Previous darkening agent abundances [4-7] are influenced by more than physically present species. They are sensitive to particle size effects and act as free variables for unknown surface materials. Due to this, our understanding of these moons’ surfaces and therefore the processes happening on and within them may be inaccurate.
This study seeks to provide constraints for magnetite and graphite as darkening agents in mixtures of water and hydrated sulfuric acid ices. The VNIR reflectance spectra of these mixtures are compared to those collected by the Galileo’s Near Infrared Mapping Spectrometer (NIMS) to determine rough abundances needed to achieve sufficient darkening. We additionally compare laboratory measurements with modeled spectra computed via radiative transfer theory [9] to further investigate model limitations.
Results:
VNIR reflectance spectra for mixtures of sulfuric acid octahydrate ("SAO") ice, water ice, and graphite or magnetite are shown in Figure 1 (b, c, e, f). For each darkening agent, two particle size groups are investigated, fine (<45 μm) and coarse (180 – 250 μm), to probe effects due to particle size variations. Each mixture set investigates darkening agents at 1, 10, 50, and 90 wt.%. Water and SAO ices comprise the remainder of these mixtures equally.
The fine and coarse darkening agent mixtures suggest a few wt.% darkening agents may not cause drastic overall darkening. Darkening by graphite is more efficient than magnetite at similar abundances, with total saturation being achieved at ~50% graphite compared to 90% magnetite. These mixtures are compared to the NIMS reflectance spectra shown in Fig. 1a, 1d to determine rough abundance estimates for the amount needed to match NIMS spectra (see Table 1, which displays the best matching abundance value).
Hapke radiative transfer modeling [9] was used to produce spectra of each mixture to investigate model accuracy and limitations. Figure 2 shows a comparison of the 1 wt.% fine graphite mixture to a modeled spectrum produced by a nonlinear least square fitting algorithm to the Hapke reflectance equation. The best-fitting model spectrum used 0.3% graphite (45 μm), 49.37% water (36 μm), and 50.33% sulfuric acid octahydrate (45 μm).
Discussion:
Definitive statements about upper and lower limits of darkening agents through comparisons between NIMS data and lab mixtures (Table 1) are difficult to make. The surfaces of these bodies are undoubtedly more complex than simple three-component mixtures. However, we can still draw rough conclusions based on darkening agent abundances that give similar reflectance levels, despite not being able to match features perfectly.
Europa’s spectrum is closest to the 10 and 50 wt.% graphite and magnetite mixtures at fine and coarse particle sizes. For graphite, 10 wt.% is within previous darkening agent abundance estimates, but 50 wt.% of either graphite or magnetite is much higher than any reported darkening agent abundance. Ganymede’s spectra fall within expected ranges, suggesting that the darkening agent on Ganymede may be graphite, magnetite, both, or a similar material. At fine and coarse sizes, 50 wt.% magnetite is close to a lower limit of needed darkening agent abundance, as the Ganymede spectrum is darker by ~0.05. However, 90 wt.% graphite may serve as an upper limit, as the 90 wt.% mixture is darker than the Ganymede spectrum. An upper limit for magnetite or graphite of either particle size may be near 90 wt.% for Callisto.
Mixture results additionally highlight the overall plausibility of some darkening agents. For example, fine and coarse magnetite abundances need to be near 90 wt.% to match Callisto’s surface in some regions. Our mixtures may be spectrally similar to the Callisto spectrum; however, this is likely not representative of Callisto’s actual surface composition. The main avenue by which magnetite, or any Fe-rich material, can be delivered to Callisto’s surface is through (micro)meteorite impacts. A significant flux of Fe-rich impactors needed to have been present near Callisto to provide this amount. Graphite or C-bearing materials are more feasible. They can be delivered via (micro)meteorite bombardment, produced through radiolytic breakdown of surface C materials like CO2, or may also be sourced endogenically like on Europa, where CO2 signatures are linked to chaos terrain [10].
Laboratory mixtures were compared against Hapke modeled spectra to probe model limitations, especially in mixtures of very bright and very dark materials. More work needs to be done, but Figure 2 suggests the Hapke model struggles to reproduce visible wavelengths when the reflectance of bright materials is near unity. Figure 1 disagrees with the longstanding notion that a few wt.% dark materials can significantly darken bright materials when mixed. The agreement between Hapke-derived abundance with the actual abundances used in each mixture suggest this idea may not be true when the materials being mixed are at opposite reflectance extrema.
References:
[1] Strazzulla, G., et al., (2023). Earth Moon Planets 127, 2. [2] Carlson, R. W., et al., (1999) Science 286, 97-99. [3] Carlson, R. W., et al. (2002) Icarus 157, 456-463. [4] Dalton III, J. B., et al. (2013) Pl and Sp. Sci 77, 45-63. [5] Ligier, N., et al. (2019). Icarus 333, 496-515. [6] Tosi, F., et al. (2023) Nature Ast. 8, 82-93. [7] Ligier, N., et al. (2019). EPSC 13, 492-2. [8] Clark, C. (1982) Icarus 49, 244-257. [9] Hapke, B. (1981) JGR: Solid Earth 86, B4, 3039-3054. [10] Trumbo, S., et al., (2023) Science 381, 1308-1311.
How to cite: Hayes, T. and Li, S.: Constraining possible darkening agents on the surfaces of the icy Galilean moons, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1097, https://doi.org/10.5194/epsc-dps2025-1097, 2025.
Background
Sodium chloride (NaCl), the most common salt on Earth, has been detected at several icy worlds that could be habitable in the present day, including Europa [1], Enceladus [2], Ganymede [3] and Ceres [4], providing evidence that salty liquid water from their interiors has been delivered to their surfaces. Areas that have experienced the emplacement of subsurface fluids through mechanisms such as plumes could contain a record of recently exposed ocean material and thus provide information on ocean chemistry and potential habitability. Identifying such regions will be a major priority for upcoming missions such as ESA’s JUpiter ICy moons Explorer (JUICE) and NASA’s Europa Clipper.
Here, we report the discovery of a metastable NaCl dihydrate formed through rapid freezing of a NaCl solution at ambient pressure (Fig. 1) [5]. This new NaCl hydrate expands on the recently identified NaCl hydrates formed in high-pressure experiments [6], and together with these reveals a rich phase behaviour in the low temperature Na-Cl-H2O system that had been overlooked for over 200 years. Using synchrotron X-ray and neutron powder diffraction, we show that the metastable form transforms irreversibly to the stable hydrate hydrohalite above 190 K, exothermically releasing 3.47 kJ mol-1 of latent heat. Additionally, we used Raman and near-infrared (NIR) reflectance spectroscopy to show experimentally that the solid phase composition of NaCl-bearing ices varies as a function of fluid cooling rate, promising a means of reconstructing the formation history of NaCl-bearing icy world surface materials from remote measurements of their composition.
Methods
NaCl solutions were frozen from room temperature to liquid nitrogen (LN2) temperature (~77 K) using multiple techniques that allowed us to probe a wide range of cooling rates spanning < 1 K min-1 to > 107 K min-1. Neutron and X-ray diffraction (XRD) were carried out at ISIS Neutron and Muon Source and Diamond Light Source, UK, respectively. We exploited the diagnostic Raman signatures of the stable and metastable hydrates [5] to identify the cooling rates required for their formation, and how these varied with NaCl concentration. NIR spectra were recorded at wavelengths between 1.0 and 2.5 micron from powdered samples at 77 K.
Figure 1. (a) Updated phase diagram of the low-temperature NaCl-H2O system incorporating phase behavior of high-pressure hydrate 2NaCl·17H2O (SC8.5 [6]) and proposed metastable dihydrate (SC2-II, this study, [5]). Dashed lines indicate observed metastable transition temperatures to hydrohalite (SC2-I) and ice Ih. (b) Neutron diffraction patterns for a flash frozen sample that has been heated. Bragg peaks for the new NaCl hydrate are marked with arrows, while Bragg peaks for hydrohalite (SC2-I) and ice Ih are marked with dashed lines (reproduced from [5]).
Results and Discussion
Our findings contribute to a new recognition of overlooked structural diversity and phase behavior complexity in the low-temperature NaCl-H2O system. We found that the newly discovered hydrate is a dihydrate structurally related to hydrohalite, with a proposed crystal structure comprising a 3 × 1 × 3 supercell of the hydrohalite unit cell. Because the metastable hydrate forms from liquid solutions at low pressures, it could feasibly form directly through cooling of ocean water at the surface or within the shallow ice shells of icy worlds and remain stable unless warmed above ~190 K. At active icy worlds such as Enceladus and Europa, rapid cooling of fluids could be achieved in various geological scenarios including plumes and chaos formation.
We found that the phase composition of NaCl-bearing ice is dependent on the cooling rate, indicating that compositional properties of NaCl-rich ices can act as a record of thermal history. At rates below ~90 K min-1, only the stable hydrohalite was produced. The metastable phase formed in distinct cooling rate regimes either in combination with hydrohalite, or as the sole NaCl phase. In addition, we used XRD to confirm that vitreous glass forms at the fastest rates, a phenomenon which has been proposed by previous studies [7,8]. Finally, we show that the new hydrate possesses near-infrared spectral features that are distinct from hydrohalite and thus could be used to identify it with existing or future remote sensing observations.
Our data show that in such regions distinguishing between different NaCl phase assemblages holds great promise in reconstructing the formational cooling rate of salty surface materials. Connecting surface composition to ice shell processes is a next major frontier in understanding the geology of icy worlds, a challenge that can be addressed by combining laboratory insights with new observations from upcoming planetary missions including NASA’s Europa Clipper and ESA’s JUICE.
References
[1] S.K. Trumbo et al. (2019). Sci. Adv., 5, eaaw7123
[2] F. Postberg et al., (2009). Nature, 459, 1098–1101
[3] F. Tosi et al., (2024). Nat. Astron., 8, 82–93
[4] M.C. De Sanctis et al., (2020) Nat. Astron. 4, 786–793
[5] R.E. Hamp et al. (2024). J. Phys. Chem. Lett. 15(50), 12301–12308
[6] Journaux et al., (2023). PNAS, 120 (9) e2217125120
[7] M.G. Fox-Powell & C.R. Cousins, (2021). J. Geophys. Res. Planets, 126, e2020JE006628
[8] F. Klenner et al. (2025) Planet. Sci. J. 6 (65)
How to cite: Hamp, R., Salzmann, C., Fawdon, P., Amato, Z., Beaumont, M., Chinnery, H., Henry, P., Headen, T., Perera, L., Thompson, S., and Fox-Powell, M.: Metastable hydrate of sodium chloride: A new mineralogical indicator of rapid freezing of brines at icy worlds, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1413, https://doi.org/10.5194/epsc-dps2025-1413, 2025.
Introduction: There is a growing consensus that ocean-derived impurities, and particularly salts, play a key role in the geophysical evolution and habitability of planetary ice shells [1]. This is bolstered by several observations including 1) the association of endogenic material with geologically young surface features, 2) the ability of salts to depress the freezing temperature and extend the longevity of liquids within planetary ices, 3) the critical role salts play in governing the material properties, biogeochemistry, and habitability of salt-rich ice, and 4) the fundamental role small melt fractions and impurity levels play in the analogous terrestrial mantle-lithosphere system [2]. As the primary medium for the transport and expression of observable signatures from underlying oceans these impurity enriched ices provide a geological record of subsurface ocean properties and processes.
Given these geophysical and astrobiological implications, there has been a recent effort to constrain the material entrainment rates occurring at ice-ocean and ice-brine interfaces [3]. Bred from multiphase models of analogous terrestrial systems (sea ice, magma chamber dynamics, solidifying metal alloys), these investigations have produced parameterizations linking interface conditions to material entrainment rates and resultant ice properties [2-3]. Moreover, they have been validated against salinity profiles and material entrainment rates observed in natural and laboratory grown sea ice cores [3]. That said, it is likely that the ocean compositions of other ocean worlds in the solar system may differ from that of the Earth. If this is the case, it bodes the question, are all salts species entrained at an equal rate? Recent research suggests that ion fractionation, the preferential entrainment/rejection of salt species into/out of the forming ice, could be prevalent under ice-ocean world thermodynamic conditions [4]. If so, ionic speciation within planetary ices may not be directly representative of the progenitor fluid reservoir from whence they came.
While there exist extensive ionic composition measurements for ice cores derived from our own NaCl-dominated ocean, investigations of ion fractionation in natural ices have been inconclusive and even contradictory [4-5], and there currently exists a dearth of empirical data related to the ionic composition of ices formed from alternate ocean chemistries [2]. As such, there remains a large gap in our understanding of the entrainment rates of various salt species in ices formed from planetary relevant brines. Moreover, contemporary models of ice-brine systems that include the physics needed to describe ion fractionation (e.g., multispecies ion diffusion, salt precipitation) [6] are in desperate need of empirical measurements to assess their accuracy.
These are potentially critical processes operating within the ice shells of ocean worlds, controlling their geochemistry, geophysics, and habitability [4]. As such, a well constrained dataset of salt entrainment rates in compositionally diverse ices is needed to improve our understanding of the physics governing these high-priority systems and benchmark evolving predictive models of planetary ice-brine systems to guarantee their accuracy and optimize their utility for upcoming missions (e.g., Europa Clipper, Dragonfly).
Methods: To bridge this knowledge gap, we have carried out novel top-down ice growth experiments (Figure 1) and established a database of physical, thermal, chemical, and material properties of compositionally diverse saline ices grown from putative ice-ocean world ocean compositions (NaCl, MgSO4, and Na2CO3 dominated). Leveraging the ionic composition and temperature profiles of these ices alongside the equilibrium geochemistry software PHREEQC, we additionally simulate their mineralogical assemblages and interstitial brine properties (e.g., water activity, ionic concentration) – key characteristics when assessing aqueous environment habitability [7].
Figure 1 – Ice growth apparatus and vertical sectioning for ionic composition/fractionation analysis.
Results: Here we present ionic composition and fractionation profiles of these ices, as well as their associated hydrate minerology and liquid phase properties (e.g., Figure 2). We describe the novel physics that govern the diverse evolution of these complex multiphase systems, such as multispecies ion diffusion and thermochemically dependent precipitation pathways. We show that:
- Ion fractionation signals are present in all of our analog ice samples.
- Depletions and amplifications in relative ion abundance, compared to the source fluid, range from -40% to +77%.
- The level of fractionation is dependent on both the thermal and chemical conditions under which the ice forms.
- The simulated precipitate mineralogical assemblages throughout the ice columns are consistent with the fractionation signals.
- The observed amplifications and depletions of relative ion concentrations (compared to the parent underlying fluid) are consistent with the processes of hydrate precipitation and multispecies diffusion, respectively.
- The ionic composition of saline ices are not necessarily a reflection of the relative ion abundances of the fluid from whence they formed – i.e., all salts are not entrained at an equal rate.
We discuss the important implications these results have for our understanding of ice-ocean world geophysics, habitability, and mission science interpretation.
Figure 2 – Ion fractionation profiles in analog ice cores (A). Amplifications relative to Cl- correlate with precipitated minerals, while depletions are associated with amplified diffusion rates. Simulated mineralogical assemblages (B) using in situ temperatures (C) agree with observed fractionation signals.
Conclusions: Through novel laboratory measurements we demonstrate that ion fractionation in planetary ices is likely a prevalent process, capable of generating heterogeneous and compositionally diverse ices – even from the same parent fluid. The resultant geochemical complexity of these ices directly correlates to associated variations in ice material properties (e.g., melting points, strengths, viscosity, porosity, etc.) that will significantly impact the geophysical processes and habitability of planetary ice shells. As high-priority planetary science and astrobiology targets for current and upcoming missions constraining the chemical and thermophysical properties of planetary ices and their relationships to the characteristics of their underlying oceans will be imperative for constraining predictive models of these environments and maximizing the science return from observational datasets.
References: [1] Vance S. D. (2021) JGR: Planets, 126.1. [2] Buffo J. J. et al. (2023) JGR: Planets, 128.3. [3] Buffo J. J. et al. (2020) JGR: Planets, 125.10. [4] Wolfenbarger N. S. et al. (2022) Astrobiology, 22.8, 937-961. [5] Maus S. et la., (2011) AoG, 52.57. [6] Meyer et al., (2024), AbSciCon, 412.03. [7] Wolfenbarger N. S. et al., (2022) GRL, 49.22.
How to cite: Buffo, J., Fox-Powell, M., Murdza, A., Tomlinson, T., Schultz, A., Barton, T., Gurd, C., McEwen, A., Wolfenbarger, N., Chivers, C., Schmidt, B., and Meyer, C.: As Above, Not So Below: Ion Fractionation in Planetary Analog Ices, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-823, https://doi.org/10.5194/epsc-dps2025-823, 2025.
Background
Europa's icy surface is exposed to low pressures and subject to intense radiation, resulting in complex processes such as sublimation and radiolytic modification that influence its spectral properties. Interpreting reflectance spectra from future missions like Europa Clipper and JUICE requires understanding how salts in Europa’s surface ice evolve under such conditions. Specifically, the formation and stability of hydrated salts such as hydrohalite (NaCl·2H₂O) are of great interest.
Methods
Granular icy analogs containing NaCl, MgSO₄, and MgCl₂ were produced by flash-freezing brine droplets with 5 wt% salt content. These samples were put in a vacuum chamber, simulating the surface evolution at low pressures and temperatures over hundreds of hours. A novel application of a thermopile sensor, with a custom sensor mount and calibration algorithm, allowed direct surface temperature measurement (160–185 K), enabling us to scale laboratory sublimation timescales to equivalent durations under Europa conditions. Reflectance spectra were acquired in the 400–2500 nm range using a hyperspectral imaging system. In a separate set of experiments, NaCl analogs were irradiated with 2 keV electrons to simulate Europa’s radiation environment and study the stability of observed spectral features.
Results
Sublimation caused notable changes in the reflectance spectra of all analog samples over hundreds of hours, which can be scaled to Europa-equivalent timescales of less than 10’000 years. The broad water absorption bands around 1.5 µm and 2.0 µm became shallower over time, and the overall reflectance increased, indicating the formation of optically dominant salt crusts on the surface due to water loss. The equivalent geometric albedo of all samples increased by more than 10% during sublimation, implying a substantial change in the surface's thermal properties.
In the NaCl-containing samples, a distinct narrow absorption band emerged at 1.98 µm, consistent with the formation of hydrohalite (NaCl·2H₂O) during sublimation, as can be seen in Figure 1. In contrast, samples containing MgSO₄ and MgCl₂ did not show narrow hydration bands during sublimation. However, changes in the shape of the 2 µm band were observed, with a slight skew in the MgSO₄ sample and a strong asymmetry in the MgCl₂ sample, both toward shorter wavelengths.
To investigate the stability of the hydrohalite feature, the NaCl samples were exposed to 2 keV electron irradiation at a total dose of 3.4 × 10¹⁶ electrons / cm². The 1.98 µm feature was significantly diminished after irradiation, corresponding to just a few years of surface exposure on Europa (see Figure 2). This shows a rapid dehydration of hydrohalite under electron bombardment.
These spectral and physical changes highlight the importance of accounting for sublimation and radiation effects when interpreting Europa’s surface composition from remote sensing data.
The presented data is publicly available under the DOI’s 10.26302/SSHADE/EXPERIMENT_RO_20240312_001 and 10.26302/SSHADE/EXPERIMENT_RO_20240701_000.
Conclusions
Our results demonstrate that sublimation alters the reflectance spectra of salty ice on Europa over short geological timescales. Sublimation processes must be accounted for in spectral interpretations to avoid overestimating bulk salt abundances. The formation of hydrohalite during sublimation and its rapid destruction under irradiation implies it is unlikely to be stable under typical conditions on Europa’s surface. If hydrohalite is present on the surface, it is either freshly exposed material (<10 years) or sustained by thermal anomalies (>145 K). Thus, any detection of hydrohalite in future observations would strongly indicate recent surface activity.
Figure 1: Sublimation of a grainy ice analog containing 5 wt% NaCl. The relative laboratory time and the measured surface temperature for all spectra are shown in the legend. The estimated timescales in the unit of 1000 years (ky), when the sublimation kinetics are scaled to Europa’s equatorial conditions, are given after the dash.
Figure 2: The reflectance spectra of a grainy ice analog with 5 wt% NaCl. The two lines show the reflectance before and after irradiation with 2 keV for 10 min with a current of 10 µA. The irradiation leads to the formation of color centers in the Vis and reduces the depth of the absorption band at 1.98 µm.
How to cite: Ottersberg, R., Pommerol, A., Stöckli, L. L., Obersnel, L., Galli, A., Murk, A., Wurz, P., and Thomas, N.: Evolution of Granular Salty Ice Analogs for Europa: Sublimation and Irradiation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-529, https://doi.org/10.5194/epsc-dps2025-529, 2025.
Introduction: The non-ice materials on the surface of Europa provide insight into its geologic history, the exogenous processes that affect its surface, and by extension the composition of its subcrustal ocean. Europa has been known to be covered by water ice and other (non-ice) materials, both trace and in abundance. Here we report on the use of spectroscopy at visible wavelengths to help constrain the composition of the non-ice material(s) on Europa’s surface. The visible to near infrared brightness and color of salts are affected by energetic particle radiation [e.g. 1,2].
Methods: Fourteen different materials investigated potentially representative of Europa’s surface, frozen subsurface and ocean compositions were investigated. They include halides and sulfates of Na, Mg, and Fe as well sulfuric acid octahydrate and water ice (Table 1). Visible and infrared reflectance spectra from ~ 400 to ~ 2500 nm or 8000nm were obtained of the materials in pellet form before and after irradiation using the LabSPEC facility at the John Hopkins Applied Physics Laboratory. All samples were in the form of pellets, pressed from powdered samples. Cryogenic temperature (~ 100K) was maintained for any material not thermally stable at room temperature. However, grain size was unconstrained.
Results and Conclusions: We have found that the visible spectrum of each material was altered by electron irradiation while the infrared was largely not affected. The materials investigated included cryogenic brines, salts, and hydrates. For NaCl brines, the discoloration in visible and near infrared is sensitive to even small amounts of NaCl being present. We confirm the 460-nm absorption band observed on the leading hemisphere of Europa is indicative of desiccated NaCl, and is not representative of either hydrohalite nor its frozen cryogenic brine. The color of Europa’s leading hemisphere is more consistent with either or both hydrated sulfuric acid or magnesium sulfates (Figure 1a,b) . Other chlorides, such as variants of MgCl2 are not present in abundance. A small amount of brine may also be present to account for the ~ 15% of NaCl being necessary to produce the observed depth of the color center. The color of the trailing hemisphere is also consistent with magnesium sulfates but the extensive irradiation and effects on the spectra of these and other potential surface materials has not been adequately simulated in the laboratory at relevant fluxes to confirm this (Figure 1c,d). MgSO4 is likely a precipitate from the ocean and not a radiolytic product and it is possible that radiolytic hydrated sulfuric acid could have formed from the degradation of the sulfate. Thus, the interior ocean appears to contain sulfates as well as chlorides, with the magnesium sulfates potentially preferentially concentrating in the crust.
Additional laboratory work, as well as higher spatial resolution spectral mapping of Europa’s surface are needed to better constrain Europa’s surface composition. Further irradiation of cryogenic samples, especially with ions for altering the physical structure, will be necessary for refining a spectral match to Europa’s surface, especially in the infrared. We emphasize that the spectral identification of a component responsible for coloring the surface of Europa must also not be inconsistent with spectral features in the infrared and also that infrared spectra of relevant materials need to account for the spectral effects of damaging ion irradiation. For instance, subtle features, such as near 1350 nm may or may not persist under ion irradiation. Also, other types of materials need to be considered that also darken and redden in the visible, such as nano-phase metallic iron [e.g. 3], which may be a component of meteoritic contamination.
Figure 1. The VNIR spectrum of Europa’s leading hemisphere (a & b) are best matched by magnesium sulfate epsomite, magnesium undecahydrate, and hydrated sulfuric acid. The infrared is best matched by the NaCl brines, because that portion of the spectrum of the disk-integrated leading hemisphere is dominated by water-ice. It is also likely the match of sulfuric acid hydrate would improve with different sample preparations and smaller grain sizes. The trailing hemisphere (c & d) is best matched by hydrated magnesium sulfate but also a mixture of NaCl and partially hydrated MgCl2. However, MgCl2•nH2O significantly mismatches in the shortwave. An additional material absorbing at 0.6 mm continues to be needed.
Acknowledgements: This work was supported primarily by the Solar Systems Working Grant # 80NSSC20K1044 with some support also provided through the Europa Clipper MISE Contract to APL.
References: [1] Hand, K. P., & Carlson, R. W. 2015, GRL, 42; [2] Hibbitts, C. A., Stockstill-Cahill, K., Wing, B., & Paranicas, C. 2019, Icar, 326; [3] Clark, R. N., Cruikshank, D. P., Jaumann, R., et al. 2012, Icar, 218, 2.
How to cite: Hibbitts, C., Stockstill-Cahill, K., Lloyd, E., Gloesener, E., Choukroun, M., Paranicas, C., and Clark, R.: Inferring Europa Surface Composition through Visible – Infrared Spectra of keV Electron-Irradiated Cryogenic Salts and Hydrates, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-962, https://doi.org/10.5194/epsc-dps2025-962, 2025.
The Galileo mission first detected CO2 on Europa over 25 years ago [1, 2], and it remains the only known carbon-bearing species on the satellite. Since this discovery, the origin and physical state of the CO2 have been important questions key to understanding any potential relationship between the CO2 and Europa’s subsurface chemistry. Specifically, the source of the CO2 could be endogenic (possibly from the subsurface ocean), exogenic, or radiolytically produced from endogenic or exogenic carbon-bearing surface materials. In addition, pure CO2 ice is highly unstable at Europa’s surface temperatures, so the CO2 must be trapped within other, so-far-unidentified surface materials [1,2,3].
Recently, a single Cycle 1 JWST NIRSPec observation of Europa’s leading hemisphere provided new detail of Europa’s CO2 by 1) resolving the 2.7 μm ν1+ν3 combination band for the first time, 2) revealing that the ν3 fundamental asymmetric stretch band near 4.26 μm exhibits a double-peaked minimum, and 3) finding an association with the large-scale chaos region Tara Regio [4,5]. This observation provided new clues to uncovering the still-unknown trapping mechanism(s) and host material(s) of the CO2, and suggested that the CO2 may have an endogenic origin. However, the observation was limited to the leading hemisphere, which limited the ability to investigate relationships to the widespread geologic terrain and radiation patterns across Europa’s variegated surface.
We present our analysis of the first full surface observations of Europa’s CO2 with JWST NIRSpec. We investigate how the CO2 band strengths and positions correspond to Europa’s geology, compositional components, and surface bombardment patterns. We find that all three CO2 features are widespread (Figure 1) and that the CO2 is enhanced in chaos regions on the leading, sub-Jovian, and anti-Jovian hemispheres (Figure 2). Conversely, the CO2 is depleted on the trailing hemisphere, which may be related to particle bombardment from Jupiter’s magnetosphere. We will discuss how these global JWST observations constrain the origin of Europa’s CO2, as well as its physical state, correlation with other species, and relationship to radiolytic processes.
Figure 1: The total band area of the ν3 asymmetric stretch feature across Europa’s surface, with regions of disrupted surface terrain outlined in black [6]. The CO2 correlates with the disrupted terrain on all but the trailing hemisphere, where CO2 appears substantially depleted and could reflect the effects of particle bombardment from Jupiter’s magnetosphere. The correlation with the disrupted terrain on the rest of the surface suggests that the CO2 reflects an endogenic carbon source.
Figure 2: Example spectra of the 2.7 μm ν1+v3 fundamental band and double-peaked ν3 fundamental asymmetric stretch bands of disrupted surface terrain on each of Europa’s hemispheres. While all of the CO2 signatures are widespread, their depths and shapes vary across the surface, providing clues to the mechanisms underlying the stability of the CO2.
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[4] Trumbo, S. K., & Brown, M. E. 2023, Science, 381, 1308, doi: 10.1126/science.adg4155
[5] Villanueva, G. L., Hammel, H. B., Milam, S. N., et al. 2023, Science, 381, 1305, doi: 10.1126/science.adg4270
[6] Leonard, E. J., Patthoff, A. D, Senske, D. A. 2024, USGS, 18, 3513, doi: 10.3133/sim3513
How to cite: Goldberg, C., Trumbo, S., Brown, M., Davis, R., and Loeffler, M.: Investigating the Origin and State of Europa’s CO2 with Global Observations from JWST, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-434, https://doi.org/10.5194/epsc-dps2025-434, 2025.
Background:
As though it were frozen in time, Jupiter’s moon Callisto has seemingly done little more than collect and degrade impact craters.1 This contrasts with other big icy moons in the solar system (e.g., Ganymede, Titan, Europa), which bear evidence for either past or present endogenic geologic activity. As such, by virtue of its ancient surface (>4 gyr old2), Callisto serves as a template for understanding how an icy, airless body evolves under the near exclusive control of exogenic processes. At present, the study of H2O ice on Callisto and its sibling moons is motivated by the centrality of ice in feeding surface chemistry and atmospheric processes. For example, H2O is an important precursor molecule in the formation of molecular oxygen, a molecule found in their atmospheres and likely trapped in H2O ice on their surfaces3–8. Additionally, H2O may serve as a host for the highly volatile CO2. Studies of the surface chemistry of the Galilean moons are currently being greatly advanced by JWST. In this work, we present a new global map of Callisto’s 3.1 μm Fresnel peak, as well as updated maps of CO2 and the 4.56 μm feature using a new Valhalla-centered cube.
Methods:
We used the JWST NIRSpec instrument as part of General Observer (GO) program 2060, which observed Callisto on 2022 November 15 and 25 (previously reported9) and 2023 September 20 (reported here, Camarca et al in prep). The 2023 September 20 observation is centered on the Valhalla impact basin. The G395H grating was used in all three observations, spanning 2.85–5.35 μm with an average R ∼ 2700. In this work, we mapped the distribution of both solid-phase and gaseous-phase CO2 at 4.25 μm. We also report band areas of the 3.1 μm H2O ice Fresnel peak, and band depths of the 4.56 μm absorption feature.
Major Findings:
Water Ice: We find that there is a dichotomy between the leading and trailing hemispheres regarding how the strength of the Fresnel peak is distributed on Callisto (Fig 1). On the leading hemisphere, the Fresnel peak correlates well with geologic features (i.e., impacts). We find that Valhalla, Asgard, as well as the region near Lofn/Heimdall bear elevated Fresnel peak band areas, consistent with their higher albedos and ice content as measured by Galileo NIMS1,10. By contrast, the ability of the Fresnel peak to track geology is erased on the trailing hemisphere, and instead exhibits a bull’s eye pattern with lower band areas at low latitudes, similar to what is found on Ganymede 11. We speculate that the Fresnel peak band areas may be suppressed on Callisto’s trailing hemisphere by the exogenic Jovian particle environment, or possibly because H2O ice is feeding the creation of radiolytically produced CO2.
CO2: On the leading hemisphere of Callisto, we find that a region near the Lofn/Heimdall impact terrain is one of Callisto’s most CO2 rich regions (Fig 2). This region is geologically significant on Callisto as the Lofn impact potentially excavated materials from a subsurface slushy/liquid zone12. We also report a new detection of a patchy CO2 atmosphere on the Valhalla-centered observation in which the peak column densities do not match the location of deepest solid-phase band depths near Lofn/Heimdall.
4.56 μm Feature: We find that consistent with past work, the band depths of the 4.56 μm feature are stronger on Callisto’s leading hemisphere than the trailing, including the new Valhalla-centered observation. Additionally, there is evidence for depletion of this feature in center of the Valhalla impact basin.
[1] Moore, J. (2004) In Jupiter: The Planet, Satellites and Magnetosphere, pp. 397–427. Cambridge University Press, Cambridge. [2] Zahnle, K. (1998) Icarus 136, 202–222. [3] Johnson, R. E. (1997) ApJ 480, L79. [4] de Kleer, K. (2023) Planet. Sci. J. 4, 37. [5] Cunningham, N. J. (2015) Icarus 254, 178–189. [6] Spencer and Calvin (2002). AJ 124 [7] Trumbo, S. K. (2021) Planet. Sci. J. 2, 139. [8] Oza, A. V. (2024) Icarus 411, 115944. [9] Cartwright, R. J. (2024) Planet. Sci. J. 5, 60. [10] Hibbitts, C. A. (2000) J. Geophys. Res. 105, 22541–22557. [11] Bockelée-Morvan, D. (2024) A&A 681, A27. [12] Greeley, R. (2001) J. Geophys. Res. 106, 3261–3273.
How to cite: Camarca, M., de Kleer, K., Cartwright, R., Villanueva, G., Holler, B., Hand, K., Glein, C., and Roth, L.: CO2-rich terrain on Callisto’s leading hemisphere and a global dichotomy in Callisto’s H2O ice as seen with JWST, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1046, https://doi.org/10.5194/epsc-dps2025-1046, 2025.
JWST observations of Europa's leading hemisphere show excess CO2 over chaos terrains [1, 2] where the subsurface ocean is likely to have breached the ice shell. Analysis of the 3.5-µm hydrogen peroxide (H2O2) absorption band reveals elevated amounts of H2O2 in these chaos regions. Peroxide abundance scales with CO2 abundance, presenting a strong pixel-level linear correlation, particularly with the υ3 CO2 feature at 4.27 mm (Figure 1).
New laboratory experiments [3] motivated by these observations show that trace inclusions of CO2 can substantially inflate the radiolytic peroxide yield, more so than in pure water ice. We considered various mechanisms by which CO2 boosts H2O2 synthesis and developed an analytical model to quantify the dependence of peroxide enhancement on CO2 abundance [3]. These experiments support the hypothesis that endogenic CO2 may amplify H2O2 synthesis in the Tara and Powys Regiones when processed by Jupiter's magnetospheric particles. We combine the CO2-enhanced H2O2 yields with the energy dose delivered by the magnetospheric particles onto the leading hemisphere [4, 5] to generate a peroxide distribution map to compare with the observed distribution. Our results highlight the intricate interplay of Europa’s interior ocean, geologic activity, and precipitating radiation in shaping surface chemistry, boosting the synthesis of molecules vital for habitability.
Figure 1: Exploring correlations between Europa’s peroxide and CO2 absorption. Integrated H2O2 band area against each of the three CO2 band areas. Grey data points are individual pixel values, while red points are binned data. The red lines are linear fits to these binned datasets. The strongest linear correlation, quantified by Pearson’s correlation coefficient, occurs between the peroxide and the 4.27 µm CO₂ absorptions.
The rapid transport of peroxide to the subsurface ocean via brine-percolated conduits [6] has strong implications for Europa's habitability. The mixing of these oxidants with reduced seawater, derived from geochemical cycling through the porous seafloor, could generate ‘redox potential’, supplying chemical energy that putative life forms may utilize to sustain metabolism, cellular maintenance, and reproduction [7]. These JWST observations combined with laboratory measurements set the stage for detailed mapping of CO2 (via its 2.7 and ~ 4.2 - 4.3 µm absorptions), H2O2 (via its 3.5 µm absorption) and possibly CHO organics with MISE (Europa Clipper) and MAJIS (Juice) at finer spatial scales to advance our understanding of Europa’s surface and subsurface composition and chemistry.
References:
[1] Villanueva, G. L., et al. 2023, Science, 381, 1305. [2] Trumbo, S. K., & Brown, M. E., 2023, Science, 381, 1308. [3] Mamo, B. et al., 2025, Planetary Science Journal, submitted. [4] Nordheim, T. A., et al., 2022, Planetary Science Journal, 3, 5. [5] Nordheim, T. A., 2018, Nat. Astro., 2, 673-679. [6] Hesse, M.A., et al., 2022, Geophysical Research Letters, 49, 5. [7] Hand, K., et al., 2007, Astrobiology, 7, 1006-1022.
How to cite: Raut, U., Protopapa, S., Mamo, B. D., Teolis, B. D., Erwin, T., Cartwright, R. J., Nordheim, T. A., Kammer, J. A., Retherford, K. D., and Villanueva, G.: Ocean-sourced CO2 inflates radiolytic H2O2 at Europa’s Chaos, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-423, https://doi.org/10.5194/epsc-dps2025-423, 2025.
We have observed the Galilean moons to determine their surface composition, any differences with longitude, and whether there are any temporal changes. One of the primary components we are trying to understand is the presence of salts. Salts play an important role in habitability because they can affect the stability of liquid water. Chlorine salts lower the freezing point of water significantly, while sulfate salts create a milder environment. For Europa, most of the “non-icy” spectra from NIMS are very similar to each other and have been suggested to be composed of heavily hydrated sulfate salts (McCord et al, 1999). However, Ligier et al., 2016 and King et al., 2022 observed Europa with VLT/SINFONI and SPHERE, respectively, and utilized recent NIR laboratory spectra (Hanley et al., 2014) to model spectral features, finding that Mg-bearing chlorinated species provide better spectral fits than sulfates in some areas. Additionally, studies suggest that magnesium is originally brought to the surface as magnesium chloride (Brown and Hand, 2013), and NaCl has been detected on the surface (Trumbo et al., 2019). The correlation with lineae and darker units suggests an endogenic origin for these salts.
Any detection of chlorine salts at the surface would further constrain theories that the dark surface material might in fact be emplaced by movement of the ice sheet and possible subsurface ocean interaction and/or cryovolcanism, rather than implantation from Io’s torus, as could be the case for sulfates. This is especially strengthened by observations of plumes on Europa. Identifying the primary constituent of Europa’s ocean salts would lead to greater understanding of the ocean temperature and the thickness of the ice shell. The composition of the ocean puts limits on the habitability of ocean worlds.
We have observed Europa, Io, Ganymede, Callisto and Titan with Lowell’s 4.3 m Discovery Telescope (LDT) with the NIHTS and EXPRES instruments. The Near-Infrared High-Throughput Spectrograph (NIHTS) is a low-resolution (R ~ 200) near-infrared (NIR) spectrograph, covering 0.86 - 2.4 µm. The EXtreme PREcision Spectrometer (EXPRES) observations measure the visible spectrum from ~0.35 - 0.85 µm at a resolution of R ~ 137,500. A unique setup of EXPRES is that it is simultaneously connected to a Solar telescope which observes the Sun daily. Thus we are able to correct for actual Solar features in our spectra. The observations are disk-averaged, and those of Europa and Titan are centered around six different longitude bins to enable us to look for longitudinal differences. We also have access to telescope time every semester, so we will be able to monitor for any temporal variations. The wavelength regions covered by our instruments cover the necessary wavelengths for previous chlorine salt identifications. This work allows for monitoring of any temporal changes in Europa’s surface composition, especially if plumes are depositing new material.
Initial analysis of the NIHTS data for Europa shows water ice/hydration features in the NIR at 1.25, 1.5, 1.65, 2.0 and 2.4 µm. The 1.65 µm water ice band can be used to determine the temperature of the ice, while the others can be compared to literature spectra to determine composition. Preliminary analysis of the EXPRES data centered around 310° longitude do not show salt color centers, as expected, but do show a red slope at lower wavelengths (0.4 – 0.55 µm) followed by a slight blue slope from 0.55 – 0.8 µm. We do see transient molecular oxygen features on Ganymede as well (Figure 1). We will present these observations, as well as those to be collected in June 2025, along with analysis at various longitudes for both NIHTS and EXPRES, for all the Galilean Moons. These ongoing observations will be useful to the upcoming JUICE and Clipper missions to monitor the surface compositions over time.
Figure 1: Visible Spectra of Ganymede from EXPRES. All data are disk-averaged, centered on the labelled longitude. Spectra have been normalized and offset for clarity. Dashed line represents the 0.5773 µm molecular oxygen feature.
References: Brown, M. E., and K. P. Hand. The Astronomical Journal 145.4 (2013): 110. Hanley, J., et al. Journal of Geophysical Research: Planets 119.11 (2014): 2370-2377. Ligier, N., et al. The Astronomical Journal 151.6 (2016): 163. King, O, L. N. Fletcher, and N. Ligier. The Planetary Science Journal 3.3 (2022): 72. McCord, T. B., et al. Journal of Geophysical Research: Planets 104.E5 (1999): 11827-11851. Trumbo, S. K., M. E. Brown, K. P. Hand. Science advances 5.6 (2019): eaaw7123.
How to cite: Hanley, J., Thieberger, C., and Grundy, W.: Ground-based Observing of the Galilean Moons in the Vis-NIR: Long-Term Monitoring and Detection of Transient Features, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1246, https://doi.org/10.5194/epsc-dps2025-1246, 2025.
Water ice in the Solar System is predominantly observed in two forms: amorphous or crystalline [1,2]. Spectroscopic measurements, particularly the absorption bands from about 1.5 to 3 µm, enable the determination of water ice structure. On Jovian satellites such as Europa and Ganymede, a majority of crystalline water ice with a superficial amorphous layer has been observed [3]. While an amorphous water ice layer is detected across the entire leading hemisphere of Europa [4], amorphous layers on Ganymede are primarily found on the polar caps [5,6]. Ganymede's magnetic field shields its equatorial regions from charged particle bombardment, resulting in higher irradiation at the poles [7,8]. Therefore, these amorphous water ice layers are likely produced by the amorphization of crystalline water ice due to charged particles coming from the Jovian magnetosphere. To better model the distribution between amorphous and crystalline water ice on Jovian satellite surfaces, a thorough understanding of the water ice amorphization process is essential. However, the kinetics of amorphization is poorly constrained in the literature, with variations observed between low- and high-energy ion irradiation (0,1-100 MeV) [9,10]. Most previous experiments were conducted at temperatures below 90 K with light ions, temperatures too low to be applied to the surfaces of Galilean satellites (see the review [9]).
In this study, we investigate the kinetics of water ice amorphization at temperatures between 90 and 120 K, typical of Jovian satellites. New irradiation experiments were conducted at GANIL (Grand Accélérateur National d’Ions Lourds, Caen), where water ice films were irradiated with Mg, O and S ions at energies around 100 keV. These ions have significant nuclear stopping powers, allowing us to explore a different energy loss regime compared to previous studies [11,12,13]. During irradiation, the temporal evolution of the amorphous fraction was monitored using infrared spectra. The amorphization fraction follows an exponential behavior either as a function of the dose (controlled by the K parameter, i.e. the dose for which it reaches 63.2 % of its maximal value during the experiment) or as a function of the fluence (controlled by the compact amorphization cross-section σam). We show that the K parameter depends not only on temperature but also on flux and ion energy, and it is not correlated with stopping powers. However, we found that the compact amorphization cross-section σam is very well correlated with the electronic stopping power Se [Fig.1], but not with the nuclear stopping power Sn. Expressions linking σam and Se were determined at 90, 100 and 110 K, enabling the estimation of the amorphous fraction based solely on the electronic stopping power of the ions. The correlation between σam and Se is discussed using the thermal spike model [14], which estimates the temperature reached within the track radius induced by ion irradiation in materials. This model has been developed to simulate the effects of swift heavy ions on insulators and metals at room temperature. Here, it successfully reproduces the linear correlation between σam and Se observed in the experimental data, thereby confirming that amorphization results from water ice melting and quenching along the ion track. However, the model does not explain the temperature dependence of the amorphization cross-section by varying the energy transfer from excited electrons to phonons, suggesting that it is most likely controlled by recrystallization or even thermal agitation.
In parallel, we develop a numerical model to determine the distribution between crystalline and amorphous water ice on Ganymede. As part of this work, doses received at Ganymede’s surface have been estimated [Fig.2]. Our experiments have shown that the electronic stopping power of the incident energetic particles controls the amorphization of water ice, rather than the dose. Our model will be refined with the new experimental data to provide more accurate numerical simulations of the evolution of water ice on Ganymede.
Fig. 1: Water ice amorphization cross-section σam as a function of electronic stopping power Se, derived from fitting the amorphous fraction as a function of fluence. Power regressions with their associated uncertainties are included.
Fig. 2: Doses delivered by ions at Ganymede’s surface as a function of depth, computed for different regions. A dynamic surface was considered, taking sputtering into account.
[1] Jenniskens et al. (1998), Solar System Ices 227, 139-155
[2] Mastrapa et al. (2013), Astrophysics and Space Science Library 356, 371-408
[3] Hansen and McCord (2004), Journal of Geophysical Research 109, E01012
[4] Ligier et al. (2016), The Astronomical Journal 151, 163
[5] Ligier et al. (2019), Icarus 333, 496-515
[6] Bockelée-Morvan et al. (2024), Astronomy and Astrophysics 681, A27
[7] Kivelson et al. (1996), Nature 384, 537-541
[8] Poppe et a. (2018), Journal of Geophysical Research 123, 389-391
[9] Fama et al. (2010), Icarus 207, 314-319
[10] Dartois et al. (2015), Nuclear Inst. and Methods in Physics Research, B 365, 472-476
[11] Strazzulla et al. (1992), Europhysics Letters 18, 517-522
[12] Leto and Baratta (2003), Astronomy and Astrophysics 397, 7-13
[13] Leto et al. (2005), Memorie della Società Astronomica Italiana Supplement 6, 57
[14] Toulemonde et al. (1993), Radiation Effects and Defects in Solids 126, 201-206
How to cite: Moingeon, A., Quirico, E., Poch, O., Faure, M., Boduch, P., Domaracka, A., Rothard, H., Bockelée-Morvan, D., Fouchet, T., Leblanc, F., Lellouch, E., Schmitt, B., and Zakharov, V.: Ion irradiation of crystalline water ice: investigation of amorphization kinetics and application to Ganymede, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-168, https://doi.org/10.5194/epsc-dps2025-168, 2025.
The abundance of icy materials on or near the surface of planetary bodies in the outer solar system dictates the need for lab measurements of ice properties at relevant environmental conditions – to unravel their age, evolution, and surface physics/chemistry. Deposition of ice on the surface could occur due to a variety of surface-subsurface exchange processes, and once emplaced, it can undergo phase transitions, changes in grain size and other physical properties (e.g., 1-3). While water ice spectral properties have been extensively studied in the past (4), and absorption coefficients of relatively warm ice have been published, we need insights (at lab scale) into formation/transport mechanisms of surface icy materials, and evolution with time via active surface processes.
NIR Spectroscopy: New infrastructure was custom-built (Fig. 1) to produce “thick” (~2 mm) pristine water ice samples and collect reflectance spectra in the near infrared (NIR) wavelength range, with grain-size ranging from 25-212 μm (under ultrahigh vacuum) at temperatures of 10-170 K (with annealing) for direct comparison to spectra of Solar System objects. Samples display spectral signatures of water ice, both crystalline and amorphous based on temperature and formation protocols and demonstrate the theorized evolution of absorption band features due to variations in sample thickness, grain size, and thermal cycling (Fig. 2). To demonstrate the efficacy of this new dataset for surface characterization, we determine bulk crystallinity of Europa’s leading hemisphere, and the environmental conditions required to meet current age estimates.
Calculation of 1.65/1.5 μm Integrated Band Area Ratios (B): 𝐵 was calculated for our ice samples as the ratio of integrated area of the 1.65 μm and 1.5 μm region. New laboratory end members for pure amorphous (Bamorph = 0.0166) and crystalline ice (Bcryst = 0.0529) were calculated using transmission and reflection spectroscopy.
Crystallinity: The amount of crystalline water ice compared to total abundance of both crystalline and amorphous ice phases, is referred to here as ‘‘crystallinity’’ (C) of a surface, defined by equation:
C = (𝐵𝐺𝐵𝑂 - 𝐵𝑎𝑚𝑜𝑟𝑝ℎ) / (𝐵𝑐𝑟𝑦𝑠𝑡 - 𝐵𝑎𝑚𝑜𝑟𝑝ℎ)
where BGBO is calculated for ground-based observations (GBO). Crystallinity captures the balance between crystallization and amorphization due to several processes operating on timescales of a few hours to millions of years [5]. Thus, our understanding of the environmental history of icy bodies (with limited observations) can be enhanced by constraining this crystallinity. Current crystallinity estimates of Europa’s leading hemisphere range from ~27 to 95% [6]. Using our new end members with 3 full-disk, spectroscopic GBO’s of Europa’s leading hemisphere, allows us to re-interpret its crystallinity, increasing from previously estimated 27-36% (using transmission spectra) to equal proportions as a full disk average of 46-52% (reflection spectra) - similar to [7] and global estimates of Ganymede. The discrepancies in measured crystallinities can be assigned to differences in methods of acquiring reflectance spectra from a real planetary surface and previous lab transmission spectra from thin films of water ice, grain sizes and thermal history contributing B values. The reflection spectroscopy technique with “thick” ice samples employed in this work should offer a more accurate comparison to planetary surface observations, with thicknesses far beyond 50 μm, and consistency with spectral mixing results increases our confidence. Spatial variations in crystallinity are expected due to changes in particle flux and thermal history with location, and due to other temporal resurfacing processes.
Age Ruler: Over the past three decades, there have been several estimates of Europa’s surface age, with [8] estimating a best-fit average age of ~30-70 Ma and [9] estimating ~40-90 Ma. We can deploy an alternate approach here with the calculated bulk crystallinity, using an age equation established by [10], based on measured rate of ice phase transformation in environmental conditions:
1-C = Φ Amax (1- exp [-kFt / N])
Any determination of age requires tight constraints on surface conditions, which are captured in equation inputs, derived either from the methodology of [6], or from recent literature. [11] used this equation to determine the approximate ages of 2 craters on Saturn's moon Rhea. We have similarly computed an age ruler for Europa's leading hemisphere. The age range of ~ 40 Ma to 90 Ma can be obtained for our range of surface crystallinity values under a surface temperature of 110 K, and ~low particle flux (7.92 x 106 cm-2 s-1).
Ongoing Work on Jovian System: Bulk crystallinity and age estimate techniques presented here provide first order approximations and constraints on surface conditions and evolution of ice. These techniques are immediately applicable to other icy moons in the Jovian system and will enable new discoveries with increased science return of past missions and support ongoing missions to Jovian moons such as Europa. Building on this work, we are performing calculations of spatial variation in the rate of water ice amorphization and thermal annealing in Europa’s radiation environment. We compare the rates of these two competing processes and identify a steady state condition for (location dependent) crystallinity on Europa. Overall, a broad global pattern of amorphous versus crystalline ice on both hemispheres will be established for Europa, and reported on here.
Acknowledgments: A part of the work presented here was carried out at the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. VS and ARR acknowledge funding from NASA PSIE, and MSG acknowledges funding from NASA SSW and DDAP programs. We also thank Jodi Berdis for contributions to crystallinity calculations.
References: [1] Kouchi, A. et al. (1994) Astronomy and Astrophysics 290, 1009–1018. [2] Cooper, J.F. et al. (2001) Icarus 149, 133-159. [3] Porter, S. B., S. Desch, J. C. Cook (2010) Icarus 208, 492-498. [4] Clark, R. N. (1980a). JGR Solid Earth 86(B4): 3087-3096. [5] Singh, V. (2021) ProQuest. [6] Berdis, J. et al. (2020) Icarus 341, 113660. [7] Ligier, N. et al. (2016) The Astronomical Journal 151(6):163. [8] Zahnle et al. (2008) Icarus 194:660-674. [9] Bierhaus, E.B. et al. (2009) in: Europa, p. 161. [10] Fama, M. et al. (2010) Icarus 207(1), 314-219. [11] Dalle Ore, C.M. et al. (2015) Icarus 261, 80-90.
How to cite: Singh, V., Gudipati, M., and Rhoden, A.: Crystallinity and Age Ruler for the Jovian System: Using 1.65/1.5 μm bands as indicators of Surface Crystallinity and Evaluating the role of Sample Thickness, Grain Size and Thermal History , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-373, https://doi.org/10.5194/epsc-dps2025-373, 2025.
Intro:
Europa’s surface ice exists in a dynamic equilibrium between thermally driven crystallization and radiation-induced amorphization. While thermal crystallization transforms amorphous ice into crystalline phases, energetic particles and UV photons disrupt the lattice, amorphizing the ice. Spectroscopic studies (Hansen & McCord 2004; Ligier et al. 2016) suggest that there exist a vertical crystallinity gradient, with amorphous ice dominating the surface of Europa and crystalline ice emerging around the 1 mm depth. However, the interplay of these processes across Europa’s diverse thermal and radiation environments remains poorly quantified.
Numerical modeling offers a powerful tool to simulate the complex competition between thermal crystallization and radiation-induced amorphization on Europa. Berdis et al. (2020) initiated this approach by modeling the interaction between ion radiations and thermal crystallization, assuming a fixed average temperature for Europa's surface. Here, we present the first depth-resolved estimates of Europa’s crystallinity using a coupled multiphysics surface model (MSM), Lunalcy (Mergny & Schmidt 2024a) that integrates all temperature fluctuations, to map subsurface crystallinity with depth across Europa and identify its evolution over time.
Methods
- Thermal Crystallization
The crystallization process is modeled using the Johnson-Mehl-Avrami-Kolmogorov (Avrami) equation, which describes the kinetics of crystallization. The crystalline fraction, θ(t), after a relaxation time t, is given by:
θ(t)=1−exp(−(t/τ)^n)
where n is the Avrami constant, and τ is the characteristic crystallization time. The crystallization timescale, τ, is highly temperature-dependent and is computed using parameters from Kouchi et al. (1994) and Schmitt et al. (1989). To account for diurnal temperature variations, we discretize the Avrami equation, allowing for the computation of crystallization rates coupled with a thermal solver.
- Radiation-Induced Amorphization
Radiation-induced amorphization is modeled by considering the accumulation of radiation energy per molecule over time, known as the dose, D. The amorphization of the crystalline fraction is given by:
θ(t)=exp(−k(T)D(t))
where k(T) is the amorphization factor, dependent on temperature and radiation type. We derive k(T) for ions, electrons, and UV photons using experimental data and fits from various studies (Strazzulla et al. 1992; Loeffler et al. 2020).
The dose rates for different particles are derived from literature (Cooper 2001; Paranicas et al. 2009; Pavlov et al. 2018) and interpolated to obtain dose rates at all depths. The UV-induced dose is computed using the solar flux received at Europa's surface, considering the UV absorption cross-section of water ice. The dose at depth is obtained using Beer-Lambert's law, accounting for the absorption of UV radiation in the shallow layers. The dose rates for electrons and protons are shown in Figure 1.
Figure 1: Particle dose rates as a function of depth on Europa, derived from literature. The combined dose rate (line) is computed by taking the maximum of interpolated values from the three datasets.
- Coupled Simulations
The LunaIcy model integrates these competing processes into a uni-dimensional block of ice, iteratively computing the crystallinity changes corresponding to the current temperature and radiation flux. The model is run over a range of parameters, simulating the evolution of Europa's icy surface crystallinity under various conditions for 100,000 years.
Results
The simulations reveal a complex interplay between thermal crystallization and radiation-induced amorphization, resulting in depth-dependent crystallinity profiles.
At high latitudes and albedo, the surface remains predominantly amorphous due to low temperatures and high radiation doses. While at low latitudes and albedo, thermal crystallization dominates, leading to a mostly crystalline profile.
Regions of particular interest are those where a balanced competition between crystallization and amorphization occurs, typically at mid-latitudes. These regions show a crystallization increase near the 1 mm depth, aligning with spectroscopic observations (Hansen & McCord 2004).
By interpolating the simulation results, we generate a crystallinity map of Europa, shown in Figure 2. This map reveals a transition from amorphous ice at the poles to fully crystalline ice at the equator, with a mixture of both at mid-latitudes.
Figure 2: (Top Left) Average crystalline fraction heatmap for depths <1 mm computed on Europa for a uniform flux of particles as a function of albedo and latitude. (Bottom) Interpolation of the averaged crystalline fraction heatmap to the albedo map of Europa.
The simulations also uncover periodic variations in crystallinity profiles due to seasonal and geological fluctuations in solar flux. Seasonal variations are particularly pronounced at mid-latitudes, with crystallinity fluctuations reaching up to 35%. These variations could be observed by upcoming missions such as Europa Clipper and JUICE, providing valuable insights into Europa's surface dynamics.
Conclusion
This study presents a novel approach to understanding Europa's surface dynamics by using a coupled multiphysics surface model (MSM), LunaIcy. By simulating both thermal crystallization and radiation-induced amorphization, we have produced the first depth-resolved, latitude-dependent simulations of Europa's crystallinity.
Our results align with existing spectroscopic observations and highlight transitions from amorphous ice at the poles to crystalline ice at the equator, with mixed phases at mid-latitudes. The crystallinity map generated from our simulations serves as a valuable tool for guiding future observations and improving surface interpretation from remote-sensing.
Remarkably, the simulations of these competing processes have revealed periodic variations in the crystallinity profiles. Seasonal variations have the highest amplitudes in mid-latitude regions, reaching crystallinity fluctuations of up 35%. Upcoming missions like Europa Clipper and JUICE could potentially observe these seasonal variations during their operational lifetimes.
Finally, we proposed the observation of key regions on Europa during upcoming missions Europa Clipper and JUICE to validate or refine our model.
References
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Ligier, N., et al. (2016). The Astronomical Journal, 151, 163.
Strazzulla, G., et al. (1992). Europhysics Letters (EPL), 18, 517.
How to cite: Mergny, C. and Schmidt, F.: Crystalline or Amorphous Ice on Europa? Simulating the Competition Between Surface Processes, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-327, https://doi.org/10.5194/epsc-dps2025-327, 2025.
Introduction. Europa, i.e., one of Jupiter's Icy Moons is a fundamental target of interest in the search for life in the solar system. The presence of a tidally heated liquid ocean beneath the moon’s icy shell is the main characteristic that makes it a strong candidate for hosting life [1-2]. In addition to this, the presence of a liquid water ocean in contact with a geothermally active subsurface, e.g., hydrothermal vents, which allows for chemical reactions and thermodynamic disequilibria, is what makes this body of exobiological interest [3]. Other icy moons, such as Ganymede for the Jovian system and Saturn’s moon Enceladus have shown to harbour liquid oceans which could be of exobiological interest [4]. Europa is the primary target of the Europa Clipper mission (NASA – 5- 6) and will also be observed via the JUICE probe (ESA – 7), with the possibility of joint observations from the two spacecraft. Before arriving at the Jovian system, it is of paramount importance to investigate which forms of life could be expected to survive and thrive under Europa’s pressure and temperature conditions, both at the bottom of the ocean and on the moon’s surface, as well as to identify possible biosignatures and assess their detectability by remote-sensing instruments onboard the two probes. Previous studies have already worked on spectral characterisation of isolated bacterial strains such as sulphur-metabolising bacteria, radiation-resistant Deinococcus spp. and Echerichia coli exposed to Europa-like temperature and pressure[8]. Recent works have focused on investigating the adaptability and survivability of microbial communities at pressures compatible with Europa’s depth, below the icy shell [9]. In particular, this work will use desert cyanobacteria [10 and references within] focusing on extremotolerant strains capable of using infra-red light to drive oxygenic photosynthesis. Thanks to the specific characteristics of those microbes they can be exposed to extreme conditions similar to the ones of Europa’s surface in terms of cryogenic temperature, pressure, and mineralogical composition to investigate their possible survivability and adaptability.These studies cope with the end goal to produce biomolecule’s spectroscopic data to compare with remote observations.
Materials and Methods. Different desert cyanobacteria strains from the genus Chroococcidiopsis spp. will be used for this experiment. These photosynthetic bacteria are able to establish both hypolithic (under the rocks) and endolithic (within the rocks) communities, which can survive extreme conditions such as water scarcity and temperature fluctuations, making them prime candidates for exobiological investigations. Their resistance has already been tested under simulated Mars conditions in low Earth orbit [11-12] and in several ground-based laboratory experiments testing their different properties (e.g.: radioresistance, limits of photosynthetic behaviour - [13-14]). In the present study, the biological material, e.g.dried-up/isolated bacteria and rehydrated/mixed bacteria within icy brines, will be exposed to cryogenic temperatures and low pressure in order to assess their metabolic activity upon exposure to Europa-like conditions. The biological material and brine’s evolution will be then followed in-situ via infrared spectroscopy using a Bruker Hyperion FTIR microscope coupled to a cryo-cell (Figure 1).
Figure 1. Cryo-cell coupled to the FTIR microscope.
Perspective. This project is currently a work-in-progress. The in-situ spectroscopic monitoring of the system is expected to produce data on the metabolic activities of the microorganisms and identify possible indicators of their vitality. Such data shall be then compared with currently available remote sensed data of Europa’s surface and future data from the MAJIS instrument [15-16] onboard JUICE. A more general comparison with spectral data from other icy moons of exobiological interest would be considered. One of the last goals of the experiment is to evaluate which biomolecules, e.g., carotenoids-like molecules, and microbes are able to survive in Europa-like conditions to create a potential model able to explain the presence and the composition of the famous “Red Stripes” on Europa’s surface. Understanding the resistance of photosynthetic cyanobacteria on icy moons can also be fundamental for the identification of potential biosignature in extremely cold environments and more in general in ice covered celestial bodies of astrobiological interest.
Acknowledgement. This work is supported by the EU and Regione Campania with FESR 2007/2013 O.O.2.1. SR is supported by the ASI-INAF agreement n.2023-6-HH.0 (Resp.: G. Piccioni).
References. [1] Belton, M. J. S. et al. Science 274, 377–385 (1996). [2] Smith, B. A. et al. Science 206, 927–950 (1979). [3] Pappalardo, R. T. et al. J. Geophys. Res. 104, 24015–24055 (1999). [4] Sagan, C. Space Sci Rev 11, 827–866 (1971). [5] Pappalardo, R. T. et al. Space Sci. Rev. 220, (2024). [6] Phillips, C. B. & Pappalardo, R. T. Eos (Washington DC) 95, 165–167 (2014). [7] Grasset, O. et al. Planet. Space Sci. 78, 1–21 (2013). [8] Dalton, J. B. (2001). [9] del Moral Jiménez, 2023. [10] Billi, D., Baqué, M., Verseux, C., Rothschild, L. & de Vera, J.-P. Desert Cyanobacteria: Potential for space and earth applications. in Adaption of Microbial Life to Environmental Extremes 133–146 (Springer International Publishing, Cham, 2017). [11] de Vera, J.-P. et al. Planet. Space Sci. 74, 103–110 (2012). [12] Baqué, M. et al. Orig. Life Evol. Biosph. 43, 377–389 (2013). [13] Di Stefano, G. et al. Life (Basel) 15, 622 (2025). [14] Billi, D. et al. Applied and Environmental Microbiology 66, 1489–1492 (2000). [15] Poulet F. et al. 2024. Space Sci Rev 220. [16] Piccioni G. et al. 2019. IEEE. pp. 318–323.
How to cite: Rubino, S., Tonietti, L., Furnari, F., Stefani, S., Piccioni, G., Tosi, F., Di Stefano, G., Billi, D., and Rotundi, A.: Infrared spectra of cyanobacteria and brine mixtures: a planetary analogue for life on Europa, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-537, https://doi.org/10.5194/epsc-dps2025-537, 2025.
Ocean worlds have become a major focus of interest in exobiology and planetary science due to their internal structure and dynamics [1]. Among them, Europa is one of the most promising candidates for the search for life in the Solar System. Its very young surface suggests intense tectonic activity and the existence of a subsurface liquid water ocean in direct contact with a rocky core allows strong water-rock interactions and efficient heat and mass transfer, which is a key ingredient for habitability.
Europa’s subsurface ocean likely contains salts, mainly NaCl and/or MgSO4 [2,3], inherited either from its differentiation phase and/or from water-rock interactions. Recent observations from multiple space missions also confirm the presence of salts at its surface [2,3,4]. While they might also originate from the exterior environment, their coincidence with geological structures rather suggest an internal origin. This work investigates the hypothesis that these salts may originate from the ocean and be transported to the surface by tectonic extension of the ice shell. The thickness of Europa's ice layer may vary considerably in time (~3-70km) due to gravitational interaction with neighboring moons Io and Ganymede [5]. When the ice layer is thinner, the ocean-ice boundary is highly tidally deformed, leading to fractures. Salty water can infiltrate the ice and recrystallize, enriching the ice with salts. When the ice layer thickens, deformations of the interface are smaller, limiting the infiltration of salts from the underlying ocean. However, salts that have previously been trapped in the ice can be transported through the shell by convection or tectonic processes. The oscillating thickness of Europa’s ice shell leads to global contraction or extension of the surface. The extension process may cause rifting [6], which facilitates mass exchanges between the ocean and the surface. To investigate how the combination of extension process and convection affects the heat and mass transfer, we solve the thermo-chemical convection equations in a 2D Cartesian model of Europa’s ice shell, using a finite element method and the advection of Lagrangian tracers. Preliminary results from two scenarios are shown in Figs.1 and 2, where a thin layer of salty ice (containing 10% and 15% MgSO4, respectively) is set in the upper part of the ice shell and for an extension velocity of 10km/Myr. The figures show viscosity (top) and density (bottom) fields at three different time steps. In both cases the rift due to the extension process is clearly visible in the viscosity fields. For 10% MgSO4 (Fig.1), the salts are easily transported upwards and spread along the rift at the surface. The presence of the salts within the band at the end of this run is interestingly similar to the distribution of salts on Europa’s surface. For 15% MgSO4 (Fig.2), the higher density causes only a fraction of the salts to reach the surface, while the rest collapses downward.
Fig1. Snapshots of the viscosity (top) and density (bottom) fields. 1-km-thick layer of salty ice containing 10% MgSO4, in the upper part of the ice shell.
Fig2. Snapshots of the viscosity (top) and density (bottom) fields. 2-km-thick layer of salty ice containing 15% MgSO4, in the middle part of the ice shell.
In addition to salts, which have a major effect on the overall dynamics, a recent study also suggests the existence of clathrate reservoirs at the ice-ocean boundary [7], trapping volatiles such as CO2 or CH4. Due to their material properties, which are significantly different from those of ice, the presence of clathrates at the base of the ice shell has no less important effect on the ice dynamics. Following outgassing episodes of the clathrate reservoirs, volatiles may be transported by convection through the ice and concentrate near the surface. Fig.3 shows clathrates rising to the surface, creating localized topography (middle panel), before spreading around the rift. This could explain the CO2 detected on the surface of Europa, which seems to have an internal origin [8].
Fig3. Snapshots of the viscosity (top) and density (bottom) fields. 1-km-thick layer of methane clathrates at the base of the ice shell.
Taken together, the results highlight how salts and clathrates affect the overall dynamics of the ice shell. The aim of this work is to determine the efficiency of mass exchanges between the ocean and the surface. Characterizing these processes is also necessary to understand Europa's surface features better.
References:
[1] F. Nimmo and R. T. Pappalardo (2016), J. Geophys. Res. Planets, 121, 1378–1399
[2] M. Y. Zolotov and E. L. Shock (2001), J. Geophys. Res., 106, 32815-32827
[3] S. K. Trumbo et al. (2022), Planet. Sci. J., 3, 27
[4] T. B. McCord et al. (2002), J. Geophys. Res., 107, 112
[5] H. Hussmann and T. Spohn (2004), Icarus, 171, 391-410
[6] S. M. Howell and R. T. Pappalardo (2018), Geophys. Res. Lett., 45, 4701-4709
[7] G. Tobie et al. (2006), Nature, 440, 61-64
[8] S. K. Trumbo and M. E. Brown (2023), Science, 381, 1308-1311
How to cite: Lebec, L., Kihoulou, M., and Čadek, O.: Exchanges of salts and volatiles in Europa’s hydrosphere, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-262, https://doi.org/10.5194/epsc-dps2025-262, 2025.
Introduction: The differentiation state of a planetary body affects evolution and thermal structure, but the differentiation state of Callisto remains unclear. Our understanding of Callisto relies on data retrieved from NASA’s Galileo mission, which measured the moment of inertia (MOI) under an assumption of hydostaticity [1]. Models of Callisto’s internal structure with this inferred MOI were used to suggest a partially differentiated interior without full ice/rock separation [1]. Other works, however, argued that an undifferentiated Callisto is improbable because partial differentiation would lead to a runaway differentiation process [2].
Interior structures that could explain the inferred high MOI value include: (1) a partially differentiated interior, (2) nonhydrostatic components on a differentiated Callisto, and (3) an undifferentiated outer shell overlying a fully differentiated interior, which we focus on here. An undifferentiated shell could result from a differentiated interior with full ice/rock separation but a melting front that does not reach the shallow subsurface (Fig. 1) and has been proposed for Uranian moons [3]. The undifferentiated shell would be a dense mixture of ice and rock, which would lead to a high MOI value with a more differentiated interior. Although this undifferentiated outer shell would have a negative density gradient, foundering of the shell is unlikely to occur [4].
Here, we investigate the evolution of Callisto’s surface assuming an undifferentiated outer shell overlying a pure-ice mantle. The undifferentiated shell would behave as a high-density, high-viscosity layer, and the pure-ice mantle would act as a low-density, low-viscosity layer, analogous to ice tectonics on Ceres [5]. Topography and density differences would cause differential stresses that could drive upward deformation. Ice tectonic deformation would uplift the crater floor and may be another pathway to the shallow, anomalous craters observed by [6] that has been suggested to be the result of viscous relaxation [7].
Methods: We simulate the evolution of topography on Callisto assuming an undifferentiated shell using the finite element method (FEM). We simulate an undifferentiated ice-rock layer (50% ice, 50% silicates [2]) on top of a pure ice layer. The interfaces between layers are flat, and the initial topography of the crater is taken from [7], which extrapolated depth-diameter ratios of smaller complex craters (<26 km diameter) on Callisto. Both layers deform as viscoelastic materials with non-Newtonian rheologies (grain boundary sliding and dislocation creep) [8]. The thermal profile is calculated for surface temperatures of 125 K and 110 K, with a constant heat flux of 15 mW/m2 [7].
We vary the undifferentiated layer thickness (5–25 km) for multiple crater diameters (10–100 km). As ice tectonics will uplift crater floors, we measure the time it takes for craters to reach current depths from a deeper initial depth to test if an undifferentiated shell could cause the shallow, observed craters [6].
Results: We found that craters ≥40 km in diameter in all undifferentiated shell thickness values deform to their currently observed states in <108 yrs at equatorial surface temperatures (Fig. 2). Craters ≤20 km in diameter experience little deformation in 2 Gyrs. We also tested mid-latitude surface temperatures and found craters ≥60 km in diameter achieve their observed depths in <1 Gyr (Fig. 2).
In simulations where deformation is highly favored (large craters, thin shells), our simulations begin to produce broad topographic domes at the center of the craters (Fig. 3). These domes are shallow and comprise most of the crater floor.
Discussion: Our simulations predict shallow Callisto craters that are consistent with observed crater depths in some of the model parameter space. Our smallest craters (≤20 km in diameter) do not deform even under the most favorable conditions. The shallow, anomalous crater transition diameter is ~26 km on Callisto [6], so the smallest craters begin the simulation with depths consistent with observations. Our current results show only the 40 km diameter craters at colder surface temperatures are unable to reproduce observations after 2 Gyrs. Future work that implements porosity into the undifferentiated shell will increase thermal conductivity and may help drive further deformation.
The domes produced in some of our simulations are different than central pit/domes found in many craters across Ganymede and Callisto [6]. Such central pit/domes are laterally smaller with steeper slopes. The domes produced in our simulations are morphologically distinct from these and would be more difficult to observe with low resolution data due to the shallow slopes. High resolution images of Doh craters reveal domes that may be consistent with our model predictions and an undifferentiated outer shell (Fig. 3).
Conclusions: We use viscoelastic FEM simulations to investigate how topography on Callisto would evolve in an undifferentiated outer shell. We found that ice tectonics from a pure ice layer underneath an undifferentiated ice shell can reproduce observed crater depth-diameter ratios (Fig. 2). Scenarios that favor deformation may produce subtle, broad topographic domes in craters, which may require high resolution data to observe (e.g., Doh crater, Fig. 3). The abundance or lack of these broad domes on Callisto can provide a test for the JUICE mission to rule out or favor an undifferentiated shell on Callisto.
References: [1] Anderson et al., Nature 387, 264–266 (1997). [2] Mueller & McKinnon, Icarus 76, 437–464 (1988). [3] Castillo-Rogez et al., JGR: Planets 128 (2023). [4] Miyazaki & Stevenson, PSJ 5, 192 (2024). [5] Bland et al., Nat. Geo 12, 797–801 (2019). [6] Schenk, Nature 417, 419–421 (2002). [7] Bland & Bray, Icarus 408, 115811 (2024). [8] Durham et al., JGR Planets 102, 16293–16302 (1997).
Figure 1: Left, a fully differentiated interior with an undifferentiated shell. Right, a partially differentiated core without full separation of ice and rock.
Figure 2: Shallowing time vs undifferentiated shell thicknesses for different crater sizes. Left, surface temperature of 125 K. Right, surface temperature of 110 K.
Figure 3: a) Doh crater (~60 km diameter). b) arial view of 60 km diameter simulated crater (20 km thick undifferentiated shell) with the two rings denoting the rim and floor. Color shows upward deformation. c) profile of the simulated crater showing the undifferentiated and pure ice.
How to cite: Pamerleau, I. and Sori, M.: Insights into Callisto’s interior: shallow craters and broad domes as a test for JUICE. , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-933, https://doi.org/10.5194/epsc-dps2025-933, 2025.
Introduction
Understanding the internal structure and composition of icy bodies in the Solar System is a central objective in planetary science, offering insights into their formation, thermal evolution, and potential habitability. However, this understanding is limited by a lack of direct observations, due to their distance from Earth and the fact that most data come from orbit. While missions like Voyager, Galileo, and Juno have greatly advanced our knowledge of Jupiter and its moons, major questions—such as the detailed nature of subsurface oceans and internal structures—remain unsolved. Upcoming missions like JUICE and Europa Clipper aim to reduce these uncertainties by measuring the tidal deformation and refining models of interior composition and structure [1–3].
Our work models the internal structures of Europa and Ganymede using a Bayesian framework to explore the range of possible configurations. We map the posterior probability distribution based on observed satellite parameters and thermodynamic properties [4–6]. We analysed how structural outcomes are influenced by variations in factors such as salt composition and concentration, and we examined correlations between key internal parameters. Our analysis also includes expected measurements of the Love number, which express gravitational changes due to tidal deformation.
Model
The internal density, pressure, and temperature profiles of the satellites are calculated numerically based on known structural parameters. Each satellite is assumed to be fully differentiated into three layers: a hydrosphere, silicate mantle, and iron core. From this, we derive observable quantities such as total mass (M), moment of inertia (MOI), radius (Rsurf), and Love numbers k2 and h2, using a library based on [7, 8].
To explore interior structures that fit observed values (M, MOI, Rsurf) and their uncertainties, we use statistical inversion with the Markov chain Monte Carlo method using the emcee sampler [9]. We test various combinations of constraints, including known k2 values and ocean salinity ranges, and study how these affect parameter estimates and correlations.
Results
An illustration of results for Europa with fixed ocean salinity is shown in Figure 1. The surface heat flux is poorly constrained and exhibits an almost uniform probability distribution over the allowed range. As expected, surface heat flux influences the temperature at the ice Ih/water interface, and we observe an almost linear decrease of the ice shell thickness with increasing temperature at ice Ih/water interface. Similarly, Love numbers k2 and h2 are strongly correlated with the ice shell thickness. In the deeper interior, we observe a trade-off between mantle radius, mantle density, and core radius, reflecting the limited ability to independently constrain these parameters.
Interestingly, the h2/k2 ratio suppresses the influence of the hydrosphere and increases its correlation with the deeper interior structure. Furthermore, when k2 is included among the observed parameters, anticipating future measurements, the parameter ranges within the 3σ confidence interval decrease significantly for the ice shell thickness, temperature at the ice Ih/ocean interface, h2, and the h2/k2 ratio. The lower bound of the surface heat flux is also slightly more constrained. Moreover, we observe an even stronger correlation between the h2/k2 ratio and the deeper interior layers.
In contrast, when the salt concentration in the ocean is treated as a free parameter, the concentration unsurprisingly follows a nearly uniform distribution over the allowed range. However, we find a strong anti-correlation between the concentration and the h2/k2 ratio, while the correlation between h2/k2 and the deeper interior is reduced.
The results for Ganymede are illustrated in Figure 2. The parameter dependencies and trade-offs follow similar trends to those observed for Europa. When examining the probability distributions of Love numbers, two different solutions occur. Lower values correspond to the case without a subsurface ocean present, while higher values are associated with models that include an ocean within the hydrosphere.
Additionally, the correlation between the h2/k2 ratio and mantle/core properties is weakened, with increased sensitivity density of high-pressure ice layers.
Summary
We investigated the internal structure of icy bodies, focusing on Europa and Ganymede, by fitting constraints on mass, radius, and MOI while considering variations in ocean salt concentration and incorporating a prospective constraint on Love number k2. Our results reveal several parameter trade-offs in both moons. For Europa, including the value of k2 among observables strengthens the correlation between the h2/k2 ratio and parameters of mantle and core. In contrast, for Ganymede, the h2/k2 ratio appears to be more sensitive to the density of high-pressure ices. In the future, we aim to incorporate magnetic induction data into the analysis, particularly for Ganymede, to further constrain its interior.
References
[1] Cappuccio et al. Planetary and Space Science, 187 (2020), 104902.
[2] Cappuccio et al. The Planetary Science Journal 3.8 (2022), p. 199.
[3] Mazarico et al. Space Science Reviews 219.4, (2023), 30.
[4] Choukroun and Grasset. The Journal of Chemical Physics 133 (2010), p. 144502.
[5] McDougall and Barker. SCOR/IAPSO WG 127 (2011), pp. 1–28.
[6] Journaux et al. Journal of Geophysical Research: Planets 125 (2020). e2019JE006176.
[7] Sabadini and Vermeersen. Global Dynamics of the Earth: Applications of Normal Mode Relaxation Theory to Solid-Earth Geophysics. 2004.
[8] Walterová et al. Is there a semi-molten layer at the base of the lunar mantle? [software]. Version v1.0.0. 2023.
[9] Foreman-Mackey et al. Publications of the Astronomical Society of the Pacific 125 (2013), pp. 306–312.
How to cite: Košíková, T. and Běhounková, M.: Statistical Modelling of Europa and Ganymede structure: The Role of Love Numbers and Ocean Composition, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-889, https://doi.org/10.5194/epsc-dps2025-889, 2025.
Introduction
Together with the ice shell thickness, grain size due to its effect on viscosity is perhaps the most crucial parameter determining the heat transfer regime inside the outer shells of icy moons. Grain growth rates derived from laboratory experiments and terrestrial ice sheets rather differ and strongly depend on the concentration of impurities (1, 2, 3), therefore grain size in the environment of icy moons is still highly uncertain. Most of published models still consider the grain size as a free parameter, yet a few studies have investigated the thermal convection coupled with dynamic recrystallization (4, 5, 6). Here, using the latest description of this process (3), we explore two possible mechanisms for onset of convection. Starting from a conductive temperature profile, we impose either (i) variations of the heat flux coming from the ocean (without any artificial perturbation of the temperature field), or (ii) temperature and melt distribution mimicking an impact (7).
Method
We developed a numerical model of thermal convection in the ice shell with phase transition, melt transport and grain size evolution. In our model, implemented in the finite-element library FEniCS (8), the bottom boundary represents a phase transition between water and ice, and its shape is governed by the flow in the ice shell and by the jump in the heat flux across the ice-water interface. The melt, carried by Lagrangian markers, affects the density and viscosity of the ice, acting as a Rayleigh-Taylor instability (two-phase flow is neglected). The rheology is nonlinear due to the feedback between the stress, the viscosity and the grain size (3, 9), therefore the Stokes problem is solved iteratively. Due to the short time scales of grain size growth and reduction, we evaluate a steady-state value of a grain size in every iteration.
Results
To assess the role of oceanic heat flux in the onset of convection, we performed a parametric study combining the ice shell thickness with the oceanic heat flux anomaly amplitude and width relative to ice shell thickness, see Fig. 1. The ice shell melts above the heat flux peak and depending on the amplitude of the resulting ice-water topography, the stresses might be sufficient to reduce grain size enough to ignite convection. In absence of other mechanisms limiting the grain growth (impurities, tidal stress, global stress due to shell thickening), Fig. 1 shows that convection is triggered only for shells at least 100 km thick and only for rather high heat flux variations (200%). However, once the convection starts, it spreads laterally with speed ca 75 km/Myr. The convective stress keeps the grain size between 1.8 and 3.5 mm, preventing transition back into the conduction regime by grain growth, as similarly reported by ref. (4).
Figure 1. Onset of convection triggered by oceanic heat flux anomaly. Initial shell thickness 100 km, grain growth law from ref. (2) assuming ice with small bubbles. Left: Heat transfer regime diagram for various shell thicknesses and oceanic heat flux anomaly parameters. Right: Shapshots of temperature and grain size.
Due to the vastness of parameter space for the impact scenario, we only performed several simulations varying the shell thickness (30 – 100 km) and the impactor radius (6 – 10 km), identifying four possible regimes. First, for a thick ice shell, the convection does not start since the thermal anomaly rapidly vanishes already in the cold lid. Second, with larger thermal anomaly, but with a thin ice shell, only transient convection occurs. Finally, there is a regime in between, with a long-term convection (> tens of Myr) which might either spread (similarly to Fig. 1) or stay localized beneath the impact site (see Fig. 2). In the latter case, the convection is rather sluggish, generating stress large enough to sustain the grain size, yet insufficient to spread laterally.
Figure 2. Impact simulation resulting in an isolated, long-term convection. Initial shell thickness is 50 km, grain growth law from ref. (3) combining laboratory and GRIP data. Left: grain size. Right: temperature (contours) with melt fraction (field).
Conclusions
Oceanic heat flux anomaly can trigger convection only if it is strong (> 200% of average heat flux) and short-wavelength compared to the shell thickness. Such anomalies in the oceanic heat flux are rather unlikely for large moons such as Titan or Ganymede, but might be expected for Enceladus (10). For Enceladus, however, thin ice shell (< 35 km (11)) and lower gravity might be the main limiting factors. Impact heating can trigger convection only if the thermal anomaly reaches the ductile layer and the shell is sufficiently thick. Collapsing melt, if present, provides a further impulse favoring the onset of convection. Our results therefore suggest that global convection in the outer shells of icy moons might be scarcer than previously thought. To investigate the scenarios presented above more thoroughly, we will include pinning effect of impurities (1) and use iSALE 2D (12) to compute realistic post-impact state.
Acknowledgments
This research is supported by Agence Nationale de la Recherche ANR-2020-CE49-0010, Czech Science Foundation project 25-16801S, Charles University project SVV 260825 and CNES through the preparation of the Juice and Europa Clipper missions.
References
1. Durand et al. (2006), JGR: Sol. Ea., 111, F01015.
2. Azuma et al. (2012), JSG, 42, 184–193.
3. Behn et al. (2021), Cryosphere, 15, 4589–4605.
4. Barr and McKinnon (2007), JGR: Planets, 112, E02012.
5. Rozel et al. (2014), JGR: Planets, 119, 416–439.
6. Mitri (2023), Icarus, 403, 115648.
7. Monteux et al. (2014), Icarus, 237, 377–387.
8. Logg et al. (2012), The FEniCS Book, Springer–Verlag
9. Goldsby and Kohlstedt (2001), JGR: Sol. Ea. 106, 11017–11030.
10. Choblet et al. (2017), Nat Astron 1, 841–847.
11. Čadek et al. (2016), GRL, 43, 5653–5660.
12. Gareth et al. (2016), iSALE-Dellen manual, Figshare.
How to cite: Kihoulou, M., Choblet, G., Tobie, G., Kalousová, K., and Čadek, O.: Grain size evolution and heat transfer regime in the shells of icy moons , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1519, https://doi.org/10.5194/epsc-dps2025-1519, 2025.
Introduction
Understanding the internal structure of Ganymede and the ongoing processes in its interior is critical for assessing its formation, evolution and the potential habitability of its subsurface ocean. While the observations of Ganymede’s auroral oval oscillations confirmed the presence of a salty, electrically conductive ocean1, the structure of the hydrosphere remains unconstrained2. The Juice mission will provide unique constraints on Ganymede’s hydrosphere by determining the tidal Love numbers, which are sensitive to the ice shell thickness and its mechanical properties3, but also to the composition and dynamics of ocean4,5,6.
In this study, we present a new 3D numerical tool, Oceanus, developed for calculating the Love numbers of realistic planetary interiors. Oceanus considers previously neglected physical effects, namely the compressibility, inertial and the Coriolis forces in the ocean. Our aim is to predict the Love numbers of Ganymede that Juice is expected to measure. This study consists of two parts: we first quantify the effects of compressibility and the dynamical flow in the ocean due to eccentricity tides. We then investigate the tidal perturbations due to moon-moon interactions4 and the resulting ocean’s response.
3D modeling of the ice-ocean dynamic response using Oceanus
Oceanus computes the tidal response of a planetary body by solving the 3D linearized Navier-Stokes equations in time domain. Unlike previous models that are used to compute the tidal response, Oceanus accounts for compressibility and tidal flow in liquid layers. It provides complete information about the tidally induced gravity signals, predicting the degree 2 Love number separately for orders 0 and 2 (e.g., k20 and k22 )4,6. For eccentricity tides, Love numbers remain constant during a tidal cycle, while for moon–moon tides, they vary in time and are expressed as the sum of a time-averaged component and an oscillatory part. The Love numbers for moon-moon tides differ from those in frequency domain.
We describe the interior of Ganymede with five layers: a solid ice Ih shell, a viscous liquid ocean, a solid high-pressure ice, a solid mantle, and a liquid core. The interior profiles are defined by two parameters: the ice shell thickness b (in the range of 10—152 km), and the ocean composition C. We consider three compositions corresponding to the NaCl solution of 0 wt.%, 5.8 wt.% and 11.7 wt.% . For each case, the reference density, bulk modulus, and shear modulus profiles are computed consistently from the pressure-temperature conditions using the SeaFreeze thermodynamic representation7. The mantle and core densities are fixed to 3500 and 8000 kg/m3, respectively, and their radii are set to match Ganymede’s observed mass and moment of inertia. Both the silicate mantle and the iron core are assumed to be incompressible and the tidal flow in the core is neglected. The solid layers are assumed to behave as an elastic or Maxwell viscoelastic body and the ocean is treated as a Newtonian fluid with a viscosity of 100 Pa s.
Response to eccentricity tides
Our results show that the compressible Love numbers of Ganymede range from 0.41 to 0.58 depending on the hydrosphere's structure. The value of k2 increases with decreasing ice shell thickness and increasing salt concentration. The difference in the Love numbers between the incompressible and compressible models ranges from 2% to 6% (Figure 1). For a given Love number, the estimated ice shell thickness can differ by 20 to 30 km between compressible and incompressible models. For thick oceans (>10 km, b<145km), the tidal flow in the ocean leads to a decoupling between the order 0 and 2 Love numbers by up to 7%. For thin oceans (d<10 km, b>150km), this difference increases to 10%.
Response to moon-moon tides
We consider the tidal interaction with Io and Europa. The time-averaged Love number differs by 2% from the Love number obtained from the eccentricity tides for both thin and thick ocean cases (Figure 2b). While the average response is similar, the behavior of the oscillatory part depends on the ocean thickness. For thin oceans, k2 shows high-frequency oscillations and its instantaneous value can vary by nearly 10%, whereas for thick oceans, the oscillations are dampened, and the deviations are about 2-3% (Figure 2a). Our results indicate that the response to the moon-moon tides are smaller than what was previously anticipated8, but remains detectable by 3GM experiment.
Conclusions
Depending on the ocean concentration of NaCl and the ice shell thickness, we predict the Love numbers of Ganymede to vary from 0.41 to 0.58. Our computations show that using incompressible models can lead to errors of up to 30 km in the estimated ice shell thickness. While the dynamics of a thin ocean (d<10 km) significantly alters the gravitational signal, the effects of the tidal flow in thicker oceans are weaker, but larger than the expected accuracy of Juice/3GM experiment9. The expected accuracy is 0.01% on the time-averaged k29, while in most of our simulated cases k20 and k22 differ by a few percent, and up to 10% for thin ocean. In response to moon-moon tides, k2 can vary during a tidal cycle by 2% for thick oceans and up to 10% for thin oceans, while its time-averaged value remains close to the Love number for eccentricity tides.
Acknowledgments
This project is supported by ANR-2020-CE49-0010, from CNES for the preparation of the Juice mission, Charles University project SVV 260825, and the Czech Science Foundation project No. 25-16801S.
References
[1] Saur et al., 2015, JGR: Spa. Sci. 120, 1715–1737
[2] Vance et al., 2018, JGR.: Planets 123, 180-205
[3] Kamata et al., 2016, JGR.: Planets 121, 1362–1375
[4] Hay et al., 2022, JGR: Planets 127, e2021JE007064
[5] Aygün & Čadek, 2024, GRL 51, e2023GL107869
[6] Aygün & Čadek, 2025, Icarus 436, 116567
[7] Journaux et al., 2020, JGR: Planets 125, e2019JE006176
[8] De Marchi et al. 2022, Icarus 386, 115150
[9] Cappuccio et al. 2020, Plane. & Spa. Sci. 187, 104902
How to cite: Aygün, B., Tabie, G., Choblet, G., Čadek, O., and Hay, H.: The tidal response of Ganymede with a realistic compressible dynamic ocean, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1684, https://doi.org/10.5194/epsc-dps2025-1684, 2025.
Introduction: The main reservoirs of liquid water in the Solar System are hidden below the icy surfaces of several moons of the gaseous giants, namely Jupiter and Saturn. None of these Icy Moons (as they are called) are found in the astronomical habitable zone, the zone of a planetary system where liquid water is possible on the surface of a body. However, tidal heating produced by the gaseous giants and internal heating due to radioactive decay in the silicate mantle allow the presence of liquid water oceans in the subsurface that could host the necessary conditions for life to emerge and persist [1,2].
Although these moons are directly accessible by space missions, today’s technology only gives us access to their surfaces, leaving the oceans hidden beneath the icy crust. For several of these worlds, the icy crust appears to be active today (or has been in the past), connecting the ocean to the surface and making orbiter observations even more interesting for understanding the internal oceans. Among these ocean worlds, Jupiter’s icy moons will soon be visited by two orbiters, ESA/ASI JUICE and NASA Europa Clipper. From this perspective, the dynamics of the internal oceans of such satellites have been investigated to better understand water-ice interaction and the possible surface-subsurface connection.
Methods: Icy moons’ oceans are known to be heated from below, facilitating natural, or Rayleigh-Bénard, convection that could transport enough heat to let the overlaying icy crust undergo phase changes. In this perspective, we explored the (expectedly turbulent) convective dynamics of a portion of the hidden ocean. In this work, we considered a Newtonian fluid layer, set in a 3D box with thickness D and horizontal sides 2πD. Periodic boundary conditions were used along the horizontal axes at x=0, 2πD and y=0, 2πD, while boundary conditions on temperature, salinity, and velocity along the vertical were defined at z=0, z=D. Considering icy moons, at the top of the ocean, a fixed temperature at the ice melting value was considered to maintain the water-ice interface in equilibrium, while at the bottom of the ocean, a fixed heat flux was imposed. Gravity is aligned with the vertical direction, and it points opposite to the z-axis. The model includes rotation that can be set in any direction in the y-z plane. As a first step, for simplicity, the fluid was treated under the Boussinesq approximation, which means that only small density changes were considered so that the equation of state became independent of pressure. In our simulations, only large-scale flows were represented, without resolving the small-scale turbulent motions, which were thus represented in terms of constant eddy viscosity and diffusivity; that is, the kinematic viscosity and the thermal diffusivity were considered as “eddy” quantities. Simulations have been performed with a CFD code [3,4] that solves a system of coupled equations for momentum and temperature in the Boussinesq approximation.
Results: Water-ice interactions were investigated by solving a proper equation that describes the interface between the ocean and the overlaying icy crust, which in non-dimensional units is:
where is the averaged thickness of the ice shell, while H’ is a small perturbation to this average thickness. A and B are constants that depend on the parameters of each moon, such as the internal heat flux, the ocean depth, sea water density, etc. The typical values for these two constants are A~10−5 and B~ - 1, hence, the evolution of the ice is 10−5 times slower than that of the ocean.
The equation was numerically solved in two dimensions using the temperature gradient derived from the ocean convection simulations. For Ganymede, the main target of the ESA/ASI JUICE mission, the results indicate that in 10 million years, ocean convection could melt the ice shell (taken to be about 70 km thick, on average [5]) by about 6% (Figure 1). Such topographic variations could be validated in the future by the ESA/ASI-JUICE mission. To better understand the spatial variations of the water-ice interface topography, three-dimensional simulations were performed. In addition, by introducing rotation, it was possible to investigate the effects of convection at different latitudes. Taking horizontal sections of the water-ice interface at the poles, mid-latitudes, and the equator, it was seen that the greatest melting effect is registered at the equator. After 158 years, the maximum melting reaches 0.2 m clustered in a hot spot of about 100 km.
Figure 1: Vertical section of Ganymede’s water-ice interface after 10 million years of ongoing ocean convection.
Figure 2: Horizontal section of Ganymede’s water-ice interface after about 158 years of ongoing ocean convection.
These results could support upcoming missions looking for surface-subsurface connection “hot-spots” on icy satellites.
References:
[1] Vance S. D. et al. (2014). Ganymede’s internal structure including thermodynamics of magnesium sulfate oceans in contact with ice. Planetary and Space Science, Vol. 96, Iss. 06. https://doi.org/10.1016/j.pss.2014.03.011
[2] Vance S. D. et al. (2018). Geophysical investigations of habitability in ice-covered ocean worlds. J. Geophys. Res. Planets 123, 180–205. https://doi.org/10.1002/2017JE005341
[3] Von Hardenberg J. (2008). Large-scale patterns in Rayleigh–Bénard convection, Physics Letters A, Vol. 372, Iss. 13. https://doi.org/10.1016/j.physleta.2007.10.099
[4] Novi L. et al. (2019). Rapidly rotating Rayleigh-Bénard convection with a tilted axis, Physical Review Vol 99, Iss. 5. https://link.aps.org/doi/10.1103/PhysRevE.99.053116
[5] Soderlund, K. M. (2019). Ocean dynamics of outer solar system satellites. Geophysical Research Letters, 46, 8700–8710. https://doi.org/10.1029/2018GL081880
Additional Information: This work is part of the JUICE Phase E project, and it was funded by ASI under agreement n. 2023-6-HH.0, CUP F83C23000070005.
How to cite: Pagnoscin, S., von Hardenberg, J., Brucato, J. R., and Provenzale, A.: Water-Ice Interactions Driven by Ocean Convection in Jupiter’s Icy Moons, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-362, https://doi.org/10.5194/epsc-dps2025-362, 2025.
JUICE (JUpiter ICy moons Explorer) was launched in April 2023 and will arrive at the Jupiter system in July 2031 [1, 2] after a tour that includes various gravitational assist maneuvers with the Earth, the Moon, and Venus. The first of these maneuvers was a Lunar and Earth Gravitational Assist (LEGA) that was run on August 19-20 2024, and provided an opportunity to test JUICE instruments’ performance in a real operational scenario. JANUS (Jovis, Amorum ac Natorum Undique Scrutator) [3] is the camera system on board JUICE. The instrument obtains images in the 340-1080 nm wavelength range with 13 filters in a filter wheel, and it uses a CMOS detector with 2000x1504 pixels with an angular resolution of 15 microrad/pixel.
JANUS imaged the Earth over 30 min. at a closest distance of 8,408 km obtaining sequences of single and multi-filter images. The set of observations started with a night-side inbound over the north of the Mozambique Channel with an elongated strip of observations that ended over the day-side over the Pacific Ocean. JANUS obtained night-side observations of Earth’s airglow and city lights from low population areas with some illumination of Earth’s surface provided by the Moon (Fig. 1). Observations at the terminator included dawn observations of Luzon in the Philippines (Fig. 2), followed by multi-filter images of meteorological systems over the Pacific Ocean with tangent illumination. Later morning observations of tropical convective clouds show elongated shadows that provide a means to measure cloud altitudes from the geometry of the shadows over the ocean. Observations in the 940 nm wavelength are sensitive to water absorption in the atmosphere and are also indicative of cloud altitude (Fig. 3). Images of the ocean in the green filter showed features compatible with internal waves.
Many of these observations are analogs to the observations of Jupiter’s atmosphere that JANUS will acquire. Earth’s airglow observations are akin to observations of the auroral oval in Jupiter. City lights of different intensity can be compared with expected observations of lightning on Jupiter, and water and multi-filter observations of tropical clouds with their projected shadows serve us to prepare for studies of relative cloud altitude determination on Jupiter’s atmosphere. Consecutive observations of cloud fields separated by a few minutes will be examined to develop image stacking and image correlation comparisons to improve image quality. These observations also allow us to quantify their potential for wind retrievals from image correlation algorithms. Weakly contrasted features in the ocean will be presented and compared with wave features on Jupiter. Finally, a full portrait of the Earth and the Moon was obtained days later after LEGA at a distance of about 5.6 million km at a phase angle of 70º in all filters (Fig. 4). The sequence includes exotic views of the Earth-Moon system in Ha and Na filters. Spectral trends over JANUS’ filter bandwidths of different locations on Earth will be presented. The performance of the JANUS camera will be compared to hyperspectral data acquired by the EnMAP satellite mission [4, 5] close to the time of the JUICE flyby and to data from the PRISMA mission [6].
Figures
Figure 1: Limb and night-side images of the Earth. (a) Night-side observation of Earth’s limb with airglow, stars and city lights from Madagascar. Clouds are illuminated by moonlight. (b) Clouds and city lights from Nom Pen in Cambodia. (c) Day-side limb view of the Earth.
Figure 2: JANUS panchromatic observation of Luzon Island in the Philippines before dawn. (a) Full frame image of 2000x1500 pixels. North is up and East is to the right. (b) Incidence angle from 91.5º (right) to 93.5º (left). (c) Fuga island north of Luzon. The mean emission angle and incidence angles of this region is 91.8º and 1.8º respectively. Saturated pixels correspond to elevated clouds over the island. The image was obtained at a distance of 9,334km and the pixel size is about 140m.
Figure 3: Multi-filter images of the Pacific Ocean. (a) RGB color composite. The color of the ocean is affected by sunglint. (b) Observation in the CMT_Strong JANUS filter at 940 nm. This filter is highly sensitive to absorption from atmospheric water and only elevated clouds are visible in the image. (c) Observation in the violet filter with the lowest contrast between clouds and ocean. (d) Spectral trends from JANUS filters for the ocean (red circle in a). (e) Spectral trends from JANUS filters from a convective cloud (blue circle in a). (f) and (g) Zoom over elevated clouds and shadows from panel (c).
Figure 4: Multi-filter images of the Earth. (a) RGB color composite of the Earth from a distance of 5.645 million km. (b) Spectral trend over the Indian Ocean with effects from clouds and sunglint.
References: [1] Grasset et al. JUpiter ICy moons Explorer (JUICE): An ESA mission to orbit Ganymede and to characterise the Jupiter system. Planetary and Space Science, 78 (2013). [2] Witasse et al. The JUpiter ICy moons Explorer (JUICE) mission: status report, science objectives, plans for the cruise phase, collaboration with Europa Clipper. AGU Fall Meeting 2023, (2023). [3] Palumbo et al. The JANUS (Jovis Amorum ac Natorum Undique Scrutator) VIS-NIR Multi-Band Imager for the JUICE Mission. Space Science Reviews, 221 (2025). [4] Chabrillat, S. et al. (2024), The EnMAP spaceborne imaging spectroscopy mission: Initial scientific results two years after launch. Remote Sensing of Environment, 315, 114379. doi: 10.1016/j.rse.2024.114379. [5] Storch, T. et al. (2023) The EnMAP imaging spectroscopy mission towards operations. Remote Sensing of Environment, 294, 113632. doi: 10.1016/j.rse.2023.113632ISSN 0034-4257. [6] Vangi et al. (2021) The New Hyperspectral Satellite PRISMA: Imagery for Forest Types Discrimination, Sensors 2021, 21, 1182; https://doi.org/10.3390/s21041182.
Acknowledgments: JANUS has been funded by the respective Space Agencies: ASI (lead funding agency), DLR, Spanish Research Ministry and the UK Space Agency. Main hardware-provider Companies and Institutes are Leonardo SpA (Prime Industry), DLR-Berlin, CSIC-IAA and Sener. PI and Italian team members acknowledge ASI support in the frame of ASI-INAF agreement n. 2023-6-HH.0.
How to cite: Hueso, R., Palumbo, P., Portyankina, G., Lara, L. M., Tubiana, C., Lucchetti, A., Penasa, L., Stephan, K., Antuñano, A., Roatsch, T., Aboudan, A., Agostini, L., Kersten, E., Matz, K.-D., Politti, R., Trauthan, F., Zinzi, A., Patel, M., Junichi, H., and Simon, A. and the JANUS Team: From Earth to Jupiter: JANUS observations of Earth in preparation of the Jupiter’s atmosphere investigation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-553, https://doi.org/10.5194/epsc-dps2025-553, 2025.
Gravity data from the Galileo mission suggest that Callisto has a partially differentiated interior, in contrast to the globally molten state of Ganymede. This dichotomy poses unique challenges to theories of the formation and evolution of the Galilean moons. During their formation, both moons were exposed to multiple heating mechanisms, including tidal dissipation, radiogenic heating from short-lived isotopes, impact-driven accretional heating, and thermal input from the circumplanetary disk.
In this study, we investigate the range of accretion conditions that could produce Callisto's incomplete differentiation while allowing Ganymede to undergo global melting. Our analysis focuses on key parameters such as the timing of accretion onset, its duration, and the impactor size distribution.
We find that the divergent internal structures of Ganymede and Callisto can arise under similar formation conditions, assuming an identical impactor size distribution and composition in the Jovian circumplanetary disk. Our results indicate that both satellites accreted gradually over periods longer than 2 million years, with accretion stopping at least 5.5 million years after the formation of calcium-aluminum-rich inclusions in the protosolar nebula. Our model also shows that Callisto can remain undifferentiated despite the accretion of a substantial influx of kilometer-sized impactors, while still allowing for the full differentiation of Ganymede.
Figure 1. Final states of Callisto and Ganymede as functions of accretion parameters tstart, τacc, and α which are respectively the timing of accretion onset, the duration of accretion, and the impactor size distribution. The white region indicates where Ganymede undergoes melting, while Callisto remains undifferentiated. From left to right, the panels display increasing values of α, ranging from 3 to 5, with a total of 100 simulations per panel.
How to cite: Bennacer, Y., Mousis, O., Monnereau, M., Hue, V., and Schneeberger, A.: Formation Conditions Leading to an Unmelted Callisto and a Differentiated Ganymede , EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-777, https://doi.org/10.5194/epsc-dps2025-777, 2025.
Understanding the internal structure of icy moons is essential to assess their potential habitability. Ganymede, the largest moon in the Solar System, is the only one known to have its own magnetic field [1] and likely harbors a subsurface ocean [2, 3], making it a prime target for future exploration such as ESA’s Juice mission. Constraining its internal structure is essential to assess its potential habitability.
Different observables (e.g., gravity, magnetic induction, tidal response, and libration) provide insight into the moon’s interior structure. While each observable is sensitive to different interior parameters, previous studies have typically considered them independently [2, 4, 5, 6]. This often results in degeneracies between interior parameters. To tackle this, we propose a joint inversion of interior properties.
To this end, we perform a global sensitivity analysis to understand how different observations relate to interior parameters, followed by a Bayesian inversion to combine these datasets into a single probabilistic model. This framework is adaptable to other icy moons and can be expanded to include new data types.
Sensitivity Analysis
In this work, we consider a five-layer spherical model of Ganymede’s interior, consisting of a metallic core, a silicate mantle, a high-pressure ice, a liquid salty ocean and an ice shell, all consistent with the moon’s mass and moment of inertia. The magnetic induction response is calculated following [7], the tidal response is computed using LOVE3D [8], and the libration response is based on [9].
To identify which parameters most influence each observable, we first vary each parameter independently. This confirms well-known behaviors. For instance, magnetic induction is most sensitive to ocean thickness and conductivity, tidal displacement to the ice shell's thickness and rigidity, and libration amplitude to shell rigidity. While informative, this single-parameter approach does not capture the non-uniqueness of the problem. To address this, we conduct a Monte Carlo analysis with 150k samples in which we vary all parameters simultaneously. We analyze the results using correlation coefficients to identify key dependencies between interior parameters and observables, and two-dimensional histograms to visualize the most relevant trends (see Figure 1).
Figure 1: Matrix of correlation coefficients and major trends in Ganymede's responses.
The Monte Carlo analysis confirms the same trends as the previous one, but makes parameter degeneracies evident. In particular, the interplay between shell thickness, ocean density, and shear modulus makes it challenging to constrain all three parameters simultaneously using tidal observations alone. However, other observables may help resolve these ambiguities, such as libration data for the shell's shear modulus and magnetic measurements for ocean composition, highlighting the need for a joint inversion approach.
These insights help to prioritize the most informative measurements and to define the most influential model parameters, which will be the focus of the inversion process.
Bayesian Inversion
The sensitivity analysis evidences the advantage of using multiple observations to estimate interior parameters. In a second step, we plan to perform a Bayesian inversion to estimate the uncertainties to which relevant interior parameters can be recovered using Juice observations [10]. This approach builds on recent Bayesian methods used for Europa [11, 12, 13] and extends them to incorporate a broader range of observations.
With the Bayesian inversion, we hope to provide a unified framework that can be used to most efficiently exploit Juice data and reveal the structure of Ganymede’s interior.
Bibliography
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How to cite: Marzolini, A. and Rovira-Navarro, M.: Characterizing Ganymede’s Interior with Gravity, Tidal, Rotation and Magnetic Observations, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-804, https://doi.org/10.5194/epsc-dps2025-804, 2025.
Introduction:
Volatile material, released from planetary surfaces, provides fundamental information to understand the past and ongoing processes related to the formation and evolution of planetary bodies. The emission of volatiles opens a window on the composition of planetary interiors and allows the search for biologically relevant molecules.
An important role in the study of volatile release is played by the evidence of plume emissions from icy satellites, as those observed on Enceladus [1-9] and possibly Europa [10,11]. These plumes outgas from surface fractures [1,2] and are primarily composed of water vapor and icy grains, organics, salts and other gases [9]. The origin of such cryovolcanic activity is strongly thought to be the salty subsurface ocean beneath the icy crust of such moons [12].
In this work, we simulate the plume emission through a Smoothed Particle Hydrodynamics (SPH) numerical model considering the Enceladus case, a well-studied phenomenon thanks to models and the Cassini-Huygens mission measurements.
Methods:
We simulate a plume on Enceladus outgassing from a surface fracture through a SPH [13-15] model. It is a Lagrangian (mesh-free) method, where volume elements and a statistical approach are used to follow spatial and temporal variations of physical quantities. The model integrates the hydrodynamics equations for the evolution of density, velocity, and thermo-kinetic energy. We also model phase transitions from water vapor to single-sized icy grains and their possible sublimation, considering the dynamical viscous coupling between the two components. We include interactions with boundaries (fracture walls and Enceladus surface), the heating effect of the solar radiation on released particles, Enceladus’ gravity, and Saturn’s tidal field. The mentioned processes are important since they can alter the thermal and dynamical behavior of the volatile material [16, 17].
We start by considering an amount of saturated water vapor at the triple point that can outgas from a single rectangular fracture with a section of 200 m2 [3] and 5 km deep, located at Enceladus' South Pole. We explore the role of icy-grain size (1 μm and 100 μm) in shaping the properties of the plume.
Results:
Within the fracture, the initial reservoir of vapor can efficiently form icy grains that rapidly dominate the plume content. After emission, the solar radiation favors the sublimation of part of the icy grains to vapor that can rapidly expand in all directions, while the remaining icy grains are less widespread. Simulations assuming a large grain size (100 μm) show a more collimated ice component, due to the weaker viscous coupling with the vapor as compared to the small grain size (1 μm) case, where the coupling is more important. The obtained average velocity of the plume is of the same order of magnitude as the observed one, namely hundreds of m/s [5], with the vapor faster than icy grains. The mass loss from the surface fracture increases in a few seconds and reaches values compatible with the expected ones [8].
The assumptions of a fixed amount of material causes an overall decrease in the mass loss with time, and the density perturbations inside the fracture associated with the phase transition treatment produce a superimposed short-term temporal variability (Figure 1).
Figure 1: Mass loss from the surface fracture for the simulation with 1-micron-sized icy grains
The thermal conduction with the cold Enceladus’ surface is efficient and favors the deposition of surface ice, with a symmetrical distribution around the fracture. This allows the evaluation of the deposition rate as a function of the distance from the emitting region (Figure 2). The obtained values are consistent with those reported in the literature [4].
Figure 2: Ice deposition rate at different times as a function of the projected radial distance from the emitting region, for the simulation with 1-micron-sized icy grains.
Conclusions and perspectives:
Our simulations produce results consistent with literature knowledge regarding Enceladus plumes, validating our implementation of physical processes in the SPH method. This opens the possibility to explore the release of volatiles from different planetary objects through our approach. In such a view, we are further refining the model, mainly for phase transitions and boundary interactions for which we can take advantage of Eulerian models for surface and subsurface thermophysical characterization [18, 19]. Moreover, the presence of a dusty component with a size distribution is important for several targets. In particular, for application to cometary activity as well as to characterize the evolution of a small object post-impact plume over the Moon’s surface. Similarly, this model is applicable to the sublimation of surface ice and the release of gas-ice-dust mixtures due to drilling-induced activities, planned for missions to the Moon and Mars.
Finally, the presented model is suitable for Europa's case. It can provide a tool to interpret and plan the future possible plume observations by JUICE and Europa Clipper missions.
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[19] M. Formisano et al. 2024, PSS, 251, 105969.
Acknowledgments: This work was partially supported by ISSI within the project “Thermophysical Characterization of Ice-Rich Areas on the Surface of Specific Planetary Bodies: Conditions for the Formation of a Transient Exosphere”.
How to cite: Teodori, M., Maggioni, L., Magni, G., Formisano, M., De Sanctis, M. C., Altieri, F., D'Aversa, E., and Ciarniello, M.: Simulations of Enceladus’ plumes with a Smoothed Particle Hydrodynamics model, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-308, https://doi.org/10.5194/epsc-dps2025-308, 2025.
Introduction
Ganymede, the largest moon in the solar system, has an icy crust that covers a large ocean underneath. Its surface is subdivided into Dark Terrain (DT) and Light Terrain (LT). The low-albedo DT forms about one-third of Ganymede´s surface and is roughly 4 Gya old (e.g., Schenk et al. 2004; Baby et al. 2023). The younger LT, that covers two-thirds of the surface, fills the gaps between DT and provides evidence of extension of the crust (Steinbrügge et al. 2025). Both DT and LT are fragmented. Resurfacing of Ganymede has been taken place by tectonic processes and cryovolcanism. Here we present a statistical analysis of fragmentation of the DT and LT that may shed light on the rheological properties, fragmentation mode and crustal thickness of Ganymede´s icy crust.
Methods
We used Ganymede´s global remote sensing mosaic published by Kresten et al. (2022). This mosaic is composed of Voyager, Galileo, and Juno imageries with different resolutions, ranging from <1km/pxl to 5km/pxl (Kresten et al. 2021). Collins et al. (2013) mapped the blocks of DT and LT as polygones, available in a GIS format. We used this data set and analyzed the size-frequency distribution of these blocks, separated for LT and DT. There are many small polygones, and fewer large ones, and this relationship is visualized by plotting the number of blocks larger than a given size against the area itself on a log-log scale. Using cumulative fragment area-frequency diagrams we fitted power-laws with an exponent -b, if the distributions follow linear regressions in the double logarithmic space. A power law distribution indicates a scale-invariant behavior.
Fig. 1 Fragment size - frequency distribution of DT blocks
Results
Figure 1 shows the cumulative area-frequency distribution of DT blocks separated for the different resolutions. The negative power law slopes range between b = 0.58 and 0.75. The power law fits are robust over 2-3 orders of magnitude. To smaller block sizes a deviation from the power-law regression occurs. This cut-off is slightly affected by resolution and occurs at a threshold block size of around 3000-6000 square kilometers.
Fig. 2 Fragment size - frequency distribution of LT blocks.
The fragment size-frequency distribution of LT blocks is distinctly different from that of the DT (Fig. 2). Resolution only slightly affects the critical cut-off area, where a deviation from a power law exists. However, this cut-off is poorly defined as the slope changes gradationally and it starts at larger areas. The negative power law exponent b ranges between 1.6-1.85 for different resolutions. If all data are combined the area frequency distribution of LT blocks can be better described by an exponential function.
Discussion
The distinctly different fragment size distributions of DT and LT blocks may indicate different rheological properties of both terrain types. This means that the terrains are mechanically different as a whole and imply that the dark ice is not only a near-surface feature. The DT show a lower number of fragments in all size classes. Although these terrains are older than the LT, they are less fragmented. This suggests that they are mechanically stronger than the LT. Due to their higher strength, the deformation is localized in the area of the LT. But why should the DT have a higher strength than the LT? The concentration of dust and non-ice particles in the DT may inhibits grain growth of ice crystals, so that the DT may contain smaller grain sizes. Zhang et al. (2024) showed that the strength of ice increases with a decrease in grain size.
We also discuss whether the critical cut-off size of fragments from the power-law distribution may provide information on the thickness of the icy crust. The idea behind this is that the crustal thickness should not exceed the length of the short side of an ice fragment, otherwise there is a risk of toppling of the ice block in an extensional setting (if the block is not stabilized by adjacent blocks). Based on this simplified approach, crustal thicknesses should range between 50-130 km, which is in the range of published data (Schubert et al. 2004). Interestingly, this would imply that the thickness of the DT should be less than that of the LT.
We will compare the fragment size distributions of LT and DT with those of crushed shelf ice in Antarctica and Greenland, both showing scale invariance over many orders of magnitude (Weiss, 2001). Even the 52 lithospheric plates on Earth have a robust fractal structure and obey a power law distribution (Sornette & Pisarenko 2003).
References
Baby, N. R., et al. (2023). Stratigraphy, Crater Size–Frequency Distribution, and Chronology of Selected Areas of Ganymede’s Light and Dark Terrains. The Planetary Science Journal, 4(9), 162. https://doi.org/10.3847/PSJ/acebed
Collins, G. C., et al. (2013). Global Geologic Map of Ganymede, p. 3237 US Department of the Interior, US Geological Survey.
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Kersten, E. et al. (2021). Controlled Global Ganymede Mosaic from Voyager and Galileo Images. Planetary and Space Science 206: 105310.
Schenk, P. M., et al. (2004). Ages and interiors: the cratering record of the Galilean satellites. In: Bagenal, F., Dowling, T. E., McKinnon, W. B. (Eds.), Jupiter. The planet, satellites, and magnetosphere. Cambridge Univ. Press, Cambridge, pp. 427–456.
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Steinbrügge, G., et al. (2025) Tidal Deformation and tidal evolution of Ganymede. In Volwerk, M., et al. (eds.) Ganymede, 46-58, Cambridge University Press.
Weiss, J. (2001) Fracture and fragmentation of ice: a fractal analysis of scale invariance. Engineering Fracture Mechanics 68, 1975-2012.
Zhang, Y., et al. (2024). Effect of grain size on the uniaxial compressive strength of ice forming with different wind speeds in a cold laboratory. Water 16, 2049.https://doi.org/10.3390/w16142049.
How to cite: Kenkmann, T., Baby, N., Karagoz, O., Stephan, K., Hauber, E., and Wagner, R.: Statistical analysis of fragmentation of the Dark and Light Terrain on Ganymede and possible implications, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-954, https://doi.org/10.5194/epsc-dps2025-954, 2025.