The ESA L4 mission to Enceladus is a flagship astrobiology project under the Voyage 2050 “Moons of the Giant Planets” science theme, aiming to assess habitability and search for biosignatures on one of the solar system’s most promising ocean worlds. Building on the Cassini–Huygens legacy and recent JWST findings, L4 will place Enceladus within the broader context of the Saturn system through a comprehensive tour, including multiple flybys of Enceladus’ plumes, dedicated orbital observations, and a landing on the moon’s south polar region.

ESA’s L4 study of the Enceladus mission adopts a science-first mission design methodology in which a Science Traceability Matrix (STM) is used as the central tool to connect top-level science themes to science questions, measurable objectives, mission needs, and candidate payload elements. This approach is motivated by the exceptional astrobiological potential of Enceladus and by the need to evaluate mission architectures, payload options and ultimately cost against a transparent and complete set of science requirements.

The current STM (https://www.cosmos.esa.int/web/l4/science-traceability-matrix) is structured around three overarching science themes:

(A) habitability of Enceladus and surface–interior interaction,

(B) Enceladus’ interaction with the external environment and the Saturnian system

(C) prebiotic chemistry and biosignatures.

These themes are detailed into science questions and a detailed set of objectives each that include constraining interior structure, ice shell dynamics, aqueous reservoirs, hydrothermal exchange, heat budget, plume activity, surface composition, plume–environment interactions, organic inventory, and potential biosignatures. By mapping these objectives systematically, the STM provides the framework from which mission measurement needs and potential instrument capabilities can be derived.

The study therefore proceeds from science to design rather than from a preselected payload to available science return. Current work assesses a broad range of strawman payload complements and evaluates how well each complement addresses the STM, whether any science objectives remain uncovered, what system-level impacts follow from different payload choices, and where technology developments may be required. The payload has not yet been selected, and the present activity is explicitly focused on identifying boundary cases and robust candidate payload combinations ahead of subsequent industrial studies and a later payload call.

This STM-driven approach also informs mission design choices beyond the instrument suite itself. For example, reconnaissance and landing-site selection are linked to the need for safe and scientifically valuable surface access, while contamination-sensitive science related to organics and biosignatures affects landing and sampling strategies. Surface and sampling considerations are especially important because Enceladus presents unique environmental conditions, including plume-derived snowfall, cryogenic temperatures, vacuum, and potentially variable mechanical surface properties.

The presentation describes the current status of the ESA L4 Enceladus study, with emphasis on how the STM is being used to derive mission needs and evaluate potential payload options in a consistent and traceable way. This framework is intended to ensure that future mission and payload decisions remain tightly anchored to the scientific objectives that motivate an Enceladus flagship-class investigation.

How to cite: Helbert, J., Haag, M., Bründl, T.-M., Ordoubadian, B., Linder, M., and Wittig, S. and the L4 Expert committee and L4 Payload Working Group:: The ESA L4 Mission to Enceladus: Using a Science Traceability Matrix to Derive Mission Needs and Potential Payloads, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-264, https://doi.org/10.5194/epsc2026-264, 2026.

EPSC2026-264

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Previous flybys by NASA missions, namely Galileo and Juno, revealed that Ganymede, the largest moon of the Solar System, hosts a rather complex, dense ionosphere, more diverse than previously thought. Recent modelling work [1] has highlighted that ion-neutral chemistry (e.g. H₂⁺+H₂ ->H₃⁺+H) occurs and is effective at producing new ion species such as H₃O⁺ and H₃⁺. The latter was detected during the Juno flyby [2].

During its revolution around Jupiter, two main parameters are affecting Ganymede’s ionosphere: Ganymede’s local time and latitude in the Jupiter’s dipole frame (magnetic latitude). On one hand, as Ganymede is tidally locked, the subsolar point, where water is sublimated and plasma is produced in large quantities, drifts westwards in Ganymede's fixed frame when Ganymede revolves around Jupiter. On the other hand, depending on Ganymede’s location in the dipole field from Jupiter, whether Ganymede is above, within or below the plasma sheet, the plasma dynamics within Ganymede’s magnetosphere varies.

In support of the Juice mission, we propose to explore the response of Ganymede’s ionosphere to both independent parameters. Our model will be driven by different exospheric configurations – corresponding to different local times [3] – and by different electromagnetic configurations – corresponding to different locations within Jupiter’s dipole [4]. From this set of simulations, we will evaluate how the ionosphere's structure varies. In particular, we will assess the evolution of the ionospheric electrical conductances and the sputtering rates on the surface by ionospheric ions with these two parameters.

Both quantities are essential to quantify and constrain. The ionosphere (through its conductance) might contribute to the induced magnetic field signal and interfere with that of the subsurface ocean and its characterisation. Ion sputtering might be a significant source of the neutral exosphere [5].

[1] Beth et al., Ion-neutral chemistry at icy moons: the case of Ganymede, MNRAS, 2025

[2] Valek et al., In Situ Ion Composition Observations of Ganymede's Outflowing Ionosphere, GRL, 2022

[3] Leblanc et al., Ganymede's atmosphere as constrained by HST/STIS observations, Icarus, 2023

[4] Jia et al., Properties of Ganymede's magnetosphere inferred from improved three-dimensional MHD simulations, JGR, 2009

[5] Carnielli et al., Simulations of ion sputtering at Ganymede, Icarus, 2020

How to cite: Beth, A., Galand, M., Jia, X., Leblanc, F., and Modolo, R.: Variability of Ganymede’s ionosphere during its revolution around Jupiter: implications on ionospheric conductances and surface sputtering, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-262, https://doi.org/10.5194/epsc2026-262, 2026.

EPSC2026-262

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An image of Lyman-α (Lyα, 1216 Å) emission from Europa obtained with the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST/STIS) provided the first evidence for localized water vapor (H2O) aurora, potentially originating from plume outgassing. Subsequent HST/STIS observations revealed the presence of a global atomic hydrogen (H) exosphere at Europa. We present a comprehensive analysis of STIS Lyα observations of Europa acquired in 1999 and between 2012 and 2020 to search for localized auroral emissions and to constrain the properties of Europa’s H exosphere. We construct a model that accounts for all known sources of Lyα emission, including resonantly scattered sunlight from Europa’s H exosphere. To identify localized anomalies such from an outgassing plume, we subtract the modeled Lyα emission and analyze the residuals. 

We find evidence to support a persistent hydrogen exosphere at Europa, but no evidence of localized water vapor.

How to cite: Roth, L., Retherford, K., Saur, J., Strobel, D., Becker, T., Bergman, S., Blöcker, A., Carberry Mogan, S., Grava, C., Ivchenko, M., Joshi, S., McGrath, M., Nimmo, F., Paganini, L., Pryor, W., and Spencer, J.: Europa’s Lyman-α emissions: Insights into the Hydrogen Exosphere but No Sign of a Plume, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-780, https://doi.org/10.5194/epsc2026-780, 2026.

EPSC2026-780

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Introduction: Europa’s ice shell controls heat loss and surface–ocean chemical exchange, including the delivery of oxidants needed to sustain habitability-relevant redox gradients [1,2]. Whether this shell is mainly conductive or undergoes solid-state convection remains uncertain, but its dynamical state determines how efficiently material can be transported between the surface and the subsurface ocean [2]. Impacts can perturb this system by depositing heat in the shallow subsurface, locally reducing viscosity and  modifying the convection pattern. In some cases, they may even promote surface-to-ocean exchange through melt migration or breaching the ice shell [3,4]. 

Here, we quantify the long-term thermo-chemical response of Europa’s ice shell to impact-generated anomalies. Using impact-scaling laws [5], we test how post-impact convection and material transport depend on the ice grain size, impactor radius, and impactor salinity. 

Methods: We perform thermo-chemical convection simulations with the geodynamical code GAIA in a 2D cylindrical geometry [6]. The models solve the conservation of mass, momentum, energy, and composition for a 30 km ice shell thickness, with fixed temperatures of 110 K at the surface and 268 K at the ice–ocean interface. Each model is first evolved without impacts to define the background thermal state. The setup includes a composite ice rheology [7], pressure- and temperature-dependent thermal expansivity and conductivity [8,9], viscosity-dependent tidal heating [10], and grain sizes of 1 cm, 1 mm, and 0.1 mm. 

Impacts are imposed at t=25 kyr as localized, lens-shaped thermal anomalies superposed on the background temperature field (Fig. 1). Rather than resolving shock propagation, excavation, and melt dynamics, we track the long-term evolution impact-induced temperature anomaly estimated from scaling laws [5,11]. We assume a vertical incidence angle, an impact velocity of 15 km/s, and impactor radii of 0.5, 1.0, and 1.8 km, consistent with studies of impact-breaching events and multiring-basin formation on Europa [4,12].

Compositional effects are modeled by advecting impactor material with a particle-in-cell method [13]. We compare passive icy impactors with ice–NaCl impactors whose density depends on salinity, set to 1, 2, and 4 times the concentration of the  Earth ocean (Fig. 2). Surface mobilization is modeled through pseudo-plastic yielding: when convective stresses exceed a prescribed yield stress, the near-surface viscosity is reduced, mimicking stagnant-lid weakening or failure (Fig. 3).

Figure 1: Grain-size control on post-impact thermal evolution for a 1.8 km icy impactor. Columns show grain sizes of 1 cm, 1 mm, and 0.1 mm; rows show the pre-impact state, the anomaly at t=25 kyr, and the final state at t=30 Myr. 

Figure 2: Grain-size and salinity effects on impactor-material redistribution. Final-time snapshots show ice–NaCl impactors for grain sizes of 1 cm, 1 mm, and 0.1 mm, and salinities of 1, 2, and 4 times the concentration of the Earth ocean.

Figure 3: Surface mobilization as a function of grain size and yield stress for a 1.8 km impactor. Models are classified as mobilized when at least 5% of initially near-surface impactor material leaves its original surface region.

Results and discussion:  The purely thermal models show that impact-triggered convection is most effective when the shell is close to the convective threshold. For a grain size of 1 cm, the ice shell remains mostly conductive and the anomaly dissipates without initiating convection. For a grain size of 1 mm, a 1.8 km impactor generates a long-lived plume and raises the shell velocity toward ~3 cm/yr. For an already convective ice shell (grain size of 0.1 mm), the impact signal is absorbed into the background flow (Fig. 1). 

The thermo-chemical cases show that salinity changes both the amount and the style of material redistribution. For the intermediate grain sizes of 1 mm, material mixed below the initial anomaly depth increases from <1% of the shell volume at 1× Earth-Ocean (EO) salinity, to ~2–4.5% at 2×EO salinity, and ~5–7% at 4×EO salinity. However, the dynamical response also changes: at 1×EO salinity, sinking impactor material displaces the impact-triggered plume, whereas at 2 and 4×EO salinity the denser anomaly suppresses plume development. In the highly convective cases (grain size of 0.1 mm), the volume of mixed material reaches ~10–13% of the total ice shell volume, with little dependence on salinity; higher salinity mainly delays homogenization within the convective layer rather than increasing the final mixed volume (Fig. 2). 

Surface mobilization tests use yield stresses of 15–100 kPa plus a no-yield endmember, bracketing weak effective lithospheric strengths inferred from Europa cycloids and pseudo-plastic/fatigue arguments [14–16]. Thermal-only impacts mobilize the surface only for small yield stress values, whereas thermo-chemical impacts promote mobilization at higher yield stresses, suggesting that dense salt-bearing material can enhance stagnant-lid weakening and vertical exchange (Fig. 3).

Outlook: Future work will first use the modeled temperature and salinity variations to estimate their gravity signatures and assess whether impact-related structures could be relevant to Europa Clipper and JUICE observations [17–19]. We will then extend the models from single impacts to impact sequences based on published Europa impact-flux estimates, testing how repeated events affect heating, salt transport, and surface weakening. Finally, the geodynamic models will be combined with shock-physics impact models to better link imposed thermal anomalies to the impact process itself.

References: [1] Hand et al. (2007). [2] Vance et al. (2018). [3] Carnahan et al. (2022). [4] Cox and Bauer (2015). [5] Melosh (1989). [6] Hüttig et al. (2013). [7] Goldsby and Kohlstedt (2001). [8] Feistel and Wagner (2006). [9] Wolfenbarger et al. (2021). [10] Tobie et al. (2003). [11] Reese et al. (2002). [12] Wakita et al. (2024). [13] Plesa et al. (2015). [14] Hoppa et al. (1999). [15] Showman and Han (2005). [16] Hammond et al. (2018). [17] Pappalardo et al. (2024). [18] Roberts et al. (2023). [19] Van Hoolst et al. (2024).

How to cite: Izzo, D., Plesa, A.-C., Dai, K., Santangelo, S., Wünnemann, K., and Hussmann, H.: The Role of Impacts for Europa's Ice Shell Dynamics and Surface-to-Ocean Material Exchange, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1051, https://doi.org/10.5194/epsc2026-1051, 2026.

EPSC2026-1051

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Enceladus with its abundance of water, life building blocks, and organic compounds is among the solar system’s most exciting candidates for discovering extraterrestrial life. The Cosmic Dust Analyzer (CDA) aboard Cassini already measured impact rates and mass spectra during its Enceladus plume flythroughs. A more detailed study on habitability and biomarkers of the Enceladus icy surface and subsurface oceans could provide unprecedented insights on the origin of life in the solar system.

To deepen this understanding, the ESA L4 mission aims to investigate habitability and biomarkers with more detail. This is achieved by analyzing the chemical composition of the plumes ejected from the sub-surface ocean with an orbiter and by deploying a lander to the icy surface. The plume ejecta consist mostly of salt and ice grains, whose composition can be determined by mass spectroscopy.

The measurement of the plume’s pH value, availability of important building blocks of life (CHNOPS – carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur) in the plume grains, and isotope ratios can indicate the habitability of the sub-surface ocean. Meanwhile, the occurrence and distribution of certain amino acids, aromatic hydrocarbons and deviations of species concentrations from the chemical equilibrium in non-biologic systems can be understood as biomarkers for the presence of life.

The scientific objectives of the mission yield the requirements for the proposed instrument: the required mass range (1…400 amu) results from the measurement of single-atom life building blocks (1 amu of a Hydrogen) on the one hand and aromatic hydrocarbons (< 400 amu) on the other hand.  To distinguish the expected species from their isobaric interferants, a high resolution of m/dm>1000 is required. Furthermore, the polyvinylidene fluoride (PVDF) impact detector of CDA indicated a peak impact rate of ~3000/cm²/s during a flythrough of a plume. Therefore, the dedicated plume analyzer must consider target size and processing speed to enable unambiguous analysis of individual impacts.

As CDA was designed for a more versatile purpose, it did not meet the demands for an in-depth Enceladus plume investigation. The instrument’s resolution and mass range were not sufficient to unambiguously identify the species named above. The impact rate led to superposition and the oversight of several spectra. Nevertheless, the data from CDA and the lessons learned from the instrument provide a profound knowledge base for the design of an Enceladus Dust Analyzer, specialized for the application in Enceladus plumes, which is currently developed at the University of Stuttgart.

This Enceladus Dust Analyzer is a time-of-flight dust mass spectrometer, featuring a smaller sensitive area and higher mass resolution. The instrument is able to measure ice particle compositions under extreme conditions within the Enceladus plume. It measures cation or anion mass spectra of particles with sizes between a few tens of a nanometer up to many micrometers. It will be optimized to measure at moderate encounter speeds.

This talk aims to describe the architecture of the instrument, its current development status as well as the lessons learned from preceding dust analyzers. What are the scientific and instrument requirements? What are the challenges of the environment to perform reliable measurements in the vicinity of Enceladus?

How to cite: Komposch, M., Srama, R., and Simolka, J. and the Destiny+ Dust Analyzer Development Team: Time-of-Flight Dust Mass Spectrometry for Enceladus Plume Analysis: A Concept Study for the ESA L4 Mission, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1128, https://doi.org/10.5194/epsc2026-1128, 2026.

EPSC2026-1128

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Saturn's E ring harbours faint, quasi-periodic, inclined brightness structures called luminous bands, first characterised from Cassini VIMS and ISS observations during Enceladus flybys. We present the first automated, catalogue-scale survey of luminous bands, drawing on 23 Cassini flyby sequences spanning 2006–2012. Detection exploits the chromatic character of the bands by subtracting a broadband CLEAR-filter image from its science-filter counterpart to isolate wavelength-dependent brightness variations. Their quasi-periodic, inclined nature motivates a frequency-domain search using a zero-padded two-dimensional Fourier angular spectrum, with candidates assessed against a phase-randomised null ensemble. Of 573 processed images, 62 yield positive detections across eight encounter sequences. Six carry no prior published detection, including the first ISS detection for E13, and five are non-targeted flybys, demonstrating bands are recoverable from archival data. The observed band directions are compared against a diffraction model: 39 of the 62 detections agree to within 2 degrees, confirming the grating interpretation. Band contrasts decrease with Cassini–Enceladus distance, suggesting the strongest structure near the plume source. Notably, no detections occur in the morning ansa despite comparable coverage. A complementary VIMS spectral analysis reveals selective suppression of sub-micron grains on the morning-side ring, consistent with size-selective electromagnetic clearing by Lorentz forces, explaining the asymmetric detection rate.

How to cite: Cazaux, S., Matthieu, A., and Loicq, J.: Uncovering Peculiar Rainbows: A Catalogue-Scale Survey of Luminous Bands in Saturn's E Ring, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1192, https://doi.org/10.5194/epsc2026-1192, 2026.

EPSC2026-1192

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Cassini’s observations of Enceladus revealed active water-vapour and ice-grain plumes emerging from the Tiger Stripes, but the physical link between plume observables and the subsurface fracture system remains uncertain. Here, we present a new way to approach this problem derived from our laboratory experiments of the South Polar Terrain, which is seen to behave as a giant single-stream heat exchanger.

We performed a new experimental campaign using the CLAM setup, in which water vapour is driven through cooled 3D-printed crevasse analogues under low-pressure conditions. The experiments show a strong coupling between wall temperature, channel diameter, phase change, and plume evolution. We find that lower wall temperature increases the velocity of the vapour flow, while reducing our crevasse analogue diameter lowers its temperature.

We interpret these results with a "heat exchanger" model that follows the energy balance of vapour rising through icy crevasses, including convective cooling against cold walls, kinetic cooling from flow acceleration, and latent heat release from condensation and ice accretion. Using dimensionless heat-transfer parameters, we connect measurable plume properties such as mass-flow rate, vent temperature, velocity, and solid fraction to possible crevasse geometries and ice-shell thicknesses.

When applied to Enceladus, we show that our model can be used as a tool to identify which plume-origin scenarios are physically plausible. They suggest that the velocity and temperature of the jets can be directly linked to the crevasse depth and cross-section area. Our approach can therefore provide a direct link between plume's characteristics and crevasses morphology and can be expanded toward more realistic fracture geometries, evolving ice accretion, briny sources, and both jet-like and curtain-like eruptions, which will be instrumental to prepare plume models for future ocean-world missions.

How to cite: Bourgeois, Y. and Cazaux, S.: Enceladian Song of Ice and Vapour: Experimentally Modelling the South Polar Terrain as a Giant Heat Exchanger, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-405, https://doi.org/10.5194/epsc2026-405, 2026.

EPSC2026-405

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Vast subsurface oceans are thought to exist beneath the ice shells of several icy worlds in the Solar System.¹ A new era of detailed exploration of these environments is rapidly approaching, with NASA’s Europa Clipper and ESA’s JUICE (JUpiter ICy moons Explorer) missions both expected to arrive in the Jovian system in the early 2030s. Interpreting the data returned by these missions requires a detailed understanding of the geochemistry and physical behaviour of icy multicomponent systems under planetary interior conditions. While the properties of pure ice have been studied extensively, our understanding of salt-containing aqueous systems including those at elevated pressures remains comparatively limited.²

Among the salts of interest, magnesium chloride (MgCl₂) is particularly important, as it is the second most abundant salt in Earth’s oceans and is also considered highly relevant to the chemistry of icy moons.³ Despite this significance, only limited work has been carried out on the high-pressure polymorphism of MgCl₂ hydrates, especially within the pressure regime most relevant to icy moon interiors.^{4, 5}

Fig. 1 Phase diagram of H₂O-MgCl₂ at ambient pressure. Red line represents 34.6 w% solution equivalent to MgCl₂·10H₂O. Blue dashed line represents the experimental concentration of 34 w% equivalent to MgCl₂·10.3H₂O (adapted from 6)

Magnesium chloride forms a wide range of hydrate phases with the general formula MgCl₂·nH₂O. At ambient pressure, hydrate phases with n = 1, 2, 4, 6, 8, and 12 have been reported, demonstrating the considerable structural complexity of this system (see Figure 1). The first magnesium chloride hydrate structure to be determined was that of bischofite, MgCl₂·6H₂O, in 1934. High-pressure investigations have since revealed additional polymorphs, including MgCl₂·10H₂O. Most recently, a second decahydrate polymorph, MgCl₂·10H₂O-II, was identified under pressure, further highlighting the complex high-pressure behaviour of magnesium chloride hydrates.⁷

To gain further insights into the freezing behaviour of MgCl₂ solutions, neutron diffraction experiments were carried out at the ISIS Neutron and Muon Source using TiZr gas-pressure cells with argon employed as the pressure-transmitting medium.⁸ Pressures of up to 0.4 GPa were explored, covering the intermediate pressure regime, particularly relevant to icy moon interiors. The use of gas-pressure technology offers several important advantages compared to diamond-anvil approaches. Since the pressure is controlled directly through an external compressor, there is no need for an internal pressure marker. This enables the pressure to be determined and maintained with high accuracy throughout the experiment, including during heating and cooling cycles. Furthermore, the use of argon as the pressure medium ensures highly hydrostatic conditions and minimises non-hydrostatic stress effects that can broaden diffraction peaks or influence phase behaviour.

The gas-cell approach also allows the entire sample volume to be frozen. This is particularly important for binary systems such as MgCl₂–H₂O, where multiple crystalline phases can coexist and spatial inhomogeneities may otherwise complicate the interpretation of diffraction data. An additional advantage is that the full crystallisation sequence occurring over a broad temperature range can be monitored directly. Although the relatively long acquisition times for neutron diffraction patterns limited the temperature resolution to approximately 10°C, the measurements nevertheless enabled the progressive formation of the different hydrate and ice phases to be followed in situ.

Fig. 2 Neutron diffraction data of 31.66 w% MgCl₂ in D₂O collected at ambient pressure upon (a) cooling from 290 K to 200 K and (b) heating from 200 K to 290 K. Le Bail fits (red) of the experimental neutron diffraction data (grey) are shown at (c) 200 K and (d) 260 K. Rietveld simulations are shown below in black. The tick marks in (c) are for MgCl₂·12H₂O and (d) MgCl₂·12H₂O with MgCl₂·8H₂O on top.

The experiments were conducted using a MgCl₂ concentration close to the ambient-pressure saturation limit, corresponding to a MgCl₂ to D₂O ratio of 1:10.3. Upon cooling at ambient pressure, the expected MgCl₂·12D₂O phase crystallised following supercooling (Figure 2). In contrast, the anticipated octahydrate phase proved difficult to obtain and was observed only transiently upon subsequent heating and immediately prior to melting. This behaviour highlights the kinetic complexity of hydrate formation in the MgCl₂–H₂O system and demonstrates that the crystallisation of individual hydrates can be highly sensitive to temperature pathways.

Fig. 3 Neutron diffraction data of 31.66 w% MgCl₂ in D₂O upon cooling from 290 K to 200 K at pressures of (a) 0.1 GPa (b) 0.2 GPa (c) 0.3 GPa and (d) 0.4 GPa.

Remarkably, increasing the pressure to only 0.1 GPa fundamentally altered the crystallisation behaviour. Under these conditions, the newly discovered high-pressure decahydrate polymorph MgCl₂·10H₂O-II was obtained (Figure 3). The observation that such a comparatively small increase in pressure changes the preferred hydrate phase underlines the delicate balance of stability relationships in the MgCl₂–H₂O system. MgCl₂·10H₂O-II was subsequently also observed upon freezing the solution at 0.2, 0.3 and 0.4 GPa, indicating that this phase is the stable one over the investigated pressure range and at our chosen concentration.

With increasing pressure, the crystallisation of ice II was found to occur at lower temperatures relative to the MgCl₂ hydrate. This behaviour can be rationalised by the slight excess of water present in the investigated solution compositions. In addition, increasing pressure resulted in an increase of the eutectic temperature of the system. The accurate control of both pressure and temperature throughout the experiments further enabled the determination of reliable equations of state of MgCl₂·10H₂O-II, representing another important advantage of the gas-pressure neutron diffraction approach. These results provide essential constraints on the phase behaviour of MgCl₂ solutions under icy moon interior conditions, with direct implications for interpreting geophysical data from the JUICE and Europa Clipper missions.

(1)        Space Sci. Rev., 2020 216 7.
(2)        J. Chem. Phys., 2019 150. 
(3)        Astrophys. J., 2013 145 110.
(4)        Struct. Chem., 2015 71 74.
(5)        Cryst. Struct. Commun., 2019 75 1605.
(6)        Can. Mineral., 2009 47 457.
(7)        ACS Earth Space Chem., 2026 10 434.
(8)        High Press. Res., 2016 36 493.

How to cite: Beaumont, M., Li, J., Novoveska, M., Collings, I., Bull, C., and Salzmann, C.: Crystallization of Magnesium Chloride Solutions at Icy Moon Conditions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-510, https://doi.org/10.5194/epsc2026-510, 2026.

EPSC2026-510

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Icy worlds hosting subsurface oceans, such as Europa and Enceladus, are leading candidates in the search for habitable environments beyond Earth. Identifying cryovolcanic regions on the surface of icy worlds, where ocean-derived fluids are delivered to the surface, is critical for probing ocean chemistry and therefore assessing their potential habitability. Characterising these surface ocean deposits is a key objective of both European Space Agency’s JUICE and NASA’s Europa Clipper missions [1]. However, the mechanisms that transport ocean material to the surface, and the extent to which emplacement processes modify composition and structure of ocean material are poorly constrained.

NaCl is a major component that has been identified in the cryovolcanic plumes at Saturn’s moon Enceladus [2] and on the surface of Jupiter’s moon Europa [3]. For more than 150 years, the NaCl-H₂O phase diagram has only comprised of two known crystalline salt phases: NaCl (halite) and NaCl·2H₂O (hydrohalite). Our recent work has discovered a novel metastable NaCl dihydrate formed through rapid freezing at rates comparable to cryovolcanic emplacement [4]. This metastable hydrate is stable below ~190 K, indicating that it can form and persist on the surfaces of icy worlds. This discovery provides the first evidence that ocean derived material on icy worlds may form previously unrecognised mineral phases under cryovolcanic conditions.

In this study, we assessed the compositional and structural signatures produced by variations in cooling rate and brine concentration. Raman spectroscopy, along with X-ray and neutron diffraction, was used to determine the mineralogical composition of NaCl-H₂O ices formed over a range of cooling rates and concentrations. To investigate the effect of cooling rate on structure, we employed cryoSEM analysis. The aim of this study was to determine whether unique compositional and/or structural signature exist within NaCl-H₂O assemblage as a function of cooling rate. Detection of these signatures would therefore provide insight into the thermal history experiences by the material. 

Here, we present four distinct NaCl-H₂O assemblages, comprising of hydrohalite, metastable hydrate and amorphous phases, that form along unique thermal pathways controlled by the initial brine concentration and cooling rate. Our study also shows that these NaCl-H₂O assemblages produce distinct near-infrared spectral signatures detectable by upcoming orbital missions, highlighting the potential for this thermal proxy to be tested. CryoSEM analysis reveals that cooling rate also strongly influenced the structural morphology. Slower cooling rates produced thicker salt-bearing channels containing ordered microscale structure, whereas faster cooling rates produced thinner salt-bearing channels lacking microscale structure (Figure 1).  

Figure 1: CryoSEM images of a 2 mol kg^-1 NaCl-H₂O solution frozen at different cooling rates. The slow frozen image (left) shows a large salt thick salt channel with ordered microscale texture. In comparison, the flash frozen image (middle) at the same scale shows much thinner and widely distributed salt channels. The right-hand image shows no order microscale texture in the flash frozen sample.

Cryo-imaging also revealed that there are no observable changes to sample structural morphology during heating, even when heated to beyond the known metastable-stable phase transition at ~190 K. This demonstrates that the structure formed during initial cooling is preserved during the transition to the thermodynamically stable phase. Consequently, material on the surface of icy worlds that initially contained metastable phases that have since decomposed to the thermodynamically stable forms could still be identified on the basis of their structural characteristics. These results demonstrate that both phase assemblages and morphologies can record brine cooling rates on icy worlds, providing a means to reconstruct geological history of ocean-derived surface material. The identification of metastable phases on the surface of icy worlds, which form at the fastest cooling rates, would provide evidence for rapidly frozen ocean deposits and could help identify high priority targets for analysis for upcoming missions.

References: [1] Hendrix, A. R. et al. Astrobiology 19, 1–27 (2019) [2] Postberg et al., Nature (2009), 459, 1098–1101 [3] Trumbo et al., Science Advances, 5 (2019) [4] Hamp et al., J. Phys. Chem. Lett, 15, 50, 12301-12308 (2024)

How to cite: Hamp, R., Fox-Powell, M., Fawdon, P., Salzmann, C., Hogan, J., Beaumont, M., Perera, L., and Thompson, S.: Cooling history recorded in the structure and mineralogical composition of surface NaCl-H2O on icy worlds, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-826, https://doi.org/10.5194/epsc2026-826, 2026.

EPSC2026-826

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Icy moons exhibiting cryovolcanism provide rare opportunities to probe material originating from their interiors and assess the potential for habitable environments. Observations of the water vapour and ice plume at Enceladus’ south pole by the Cassini spacecraft revealed a salty subsurface ocean with a basic pH and traces of organic compounds. The leading plume formation mechanism involves the vaporisation of liquid water beneath the ice shell, followed by rapid ascent through near-vertical crevasses. However, significant uncertainties remain in linking plume composition to that of the subsurface ocean, and in identifying the processes that control plume formation and influence material transport. We conducted laboratory experiments designed to study Enceladus-like plumes and assess how the ocean composition and ice-shell conditions affect eruption dynamics. Our setup consists of a water reservoir and a narrow channel representing the subsurface ocean and ice fractures, placed inside a vacuum chamber to simulate the low-pressure environment. Our results show that lower crevasse temperatures lead to higher venting speeds and ice-to-vapour fractions. Moreover, gas exsolution from the liquid phase can accelerate the flow and loft droplets that act as nuclei for ice grain formation. These findings indicate that the velocity of a single eruption and its ice grain abundance are strongly controlled by thermal conditions and dissolved gas content, rather than solely by crevasse geometry.

How to cite: Giordano, F., Bourgeois, Y. R. A., Cazaux, S. M., Schrijer, F. F. J., Ligterink, N. F. W., McClure, M. K., and Mason, P. R. D.: Plume experiments to constrain Enceladus ocean composition and subsurface conditions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1089, https://doi.org/10.5194/epsc2026-1089, 2026.

EPSC2026-1089

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Ocean worlds, including Jupiter’s moon Europa, are key targets in the search for extraterrestrial life. One powerful approach involves searching for biochemical signatures at the molecular level - both specific biosignature molecules and their distribution patterns [1]. These include amino acids and fatty acids, the latter being considered as universal biomarkers of extraterrestrial life [2]. 

NASA’s upcoming Europa Clipper mission [3] will evaluate Europa’s habitability. The SUrface Dust Analyzer (SUDA [4]) is the onboard impact ionization mass spectrometer, which will provide compositional analyses of water ice grains, potentially rich in organic material derived from the moon’s subsurface ocean. SUDA has the capability to detect and distinguish between abiotic organic molecules [5,6] and (microbial) biosignatures [7,8,9] in Europa’s ice grains. However, if such signatures are indeed present in Europa’s surface ice, they are exposed to Jupiter’s harsh magnetospheric radiation and may thus be destroyed and/or modified by high-energy particles, mainly electrons [10]. It is therefore crucial to evaluate the impact of Europa’s radiation environment on the preservation of biosignatures in its surface ice and on their detectability with spaceborne instruments. 

Here, we assess the effects of electron irradiation on the mass spectral signatures of molecular biosignatures embedded in ice, as detectable by SUDA-type mass spectrometers. We irradiated amino acids and fatty acids in water ice with high-energy (10 MeV) electrons and recorded analogue mass spectra using Laser Induced Liquid Beam Ion Desorption (LILBID), a technique that accurately simulates SUDA-type mass spectra [11]. Samples were irradiated with doses from about 300 Gy to 3.5 MGy, i.e., exposure on the timescale of minutes up to a year on Europa’s surface at 0.1mm depth [12]. The irradiation experiments were conducted at conditions similar to those experienced by Europa’s surface ice (i.e., high vacuum [pressure of 10^-5-10^-6 mbar] and surface temperatures between 80 and 130 K). Samples consisted of (i) 4 amino acids (alanine, aspartic acid, glycine, lysine) at a concentration of 0.01 wt% each, (ii) fatty acids at simulated abiotic concentrations (all fatty acids at the same concentration) and (iii) fatty acids at simulated biotic concentrations (increased abundances for even carbon number fatty acids, especially C16 and C18, as compared to odd carbon number fatty acids [13]). 

Results show characteristic signals can be identified from both amino acids and fatty acids even at the highest tested dose, indicating that these biosignatures can persist under Europan surface conditions over long periods of time and still be detectable by SUDA. The intensities of peaks corresponding to high molecular mass amino acids was found to decrease with increasing radiation dose, indicating that these compounds undergo destruction due to the radiation, but they decreased with a lower degree than expected. Results will be further discussed in the context of surface location and depth on Europa, drawing implications for the detection of organic biosignatures with space missions such as Europa Clipper and other future icy moon missions [14]. 

[1] Yoffe, G. et al. (2026) Nat Astron https://doi.org/10.1038/s41550-026-02864-z

[2] Georgiou, C. D., & Deamer, D. W. (2014). Astrobiology, 14(6), 541-549

[3] Howell, S. M., & Pappalardo, R. T. (2020). Nature Communications, 11(1), 1311

[4] Kempf S., et al. (2025) Space Sci Rev 221, 10

[5] Napoleoni, M., et al. (2023a) ACS Earth and Space  Chemistry, 7(4), 735-752

[6] Napoleoni, M., et al. (2023b) ACS Earth and Space Chemistry, 7(9), 1675-1693

[7] Klenner, F., et al. (2020a) Astrobiology, 20(2), 179-189

[8] Klenner, F., et al. (2020b) Astrobiology, 20(10), 1168-1184

[9] Dannenmann, M., et al. (2023) Astrobiology, 23(1), 60-75

[10] Cooper, J. F., et al. (2001) Icarus, 149(1), 133-159 

[11] Klenner, F., et al. (2019) Rapid Communications in Mass Spectrometry, 33(22), 1751-1760

[12] Nordheim, T.A., Hand, K.P. & Paranicas, C. (2018) Nat Astron 2, 673–679 

[13] Dorn, E. D., Nealson, K. H., & Adami, C. (2011) Journal of Molecular Evolution, 72(3), 283-295

[14] Mousis, O., et al. (2022) The planetary science journal, 3(12), 268

How to cite: Napoleoni, M., Knolle, W., Lugovoy, E., Klenner, F., Abel, B., and Postberg, F.: Experiments for the Detection of Molecular Biosignatures in Irradiated Ice Grains with Europa Clipper’s SUDA Mass Spectrometer, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-805, https://doi.org/10.5194/epsc2026-805, 2026.

EPSC2026-805

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The Kronian moon Enceladus harbours a liquid ocean beneath its icy crust, making it of particular scientific interest. Data collected by the Cassini orbiter suggest the presence of the chemicals necessary to develop life as we know it, making it a key driver behind ESA’s Voyage 2050 programme. Libration-enhanced tidal heating keeps the moon’s interior sufficiently warm to sustain its liquid ocean (Hemingway et al., 2018). The icy crust thins at its southern pole, where fissures, dubbed the ‘Tiger Stripes’, jet water from the liquid ocean to the planet's surface, akin to geysers (C. C. Porco et al., 2006; Hemingway and Mittal, 2017). The jetted liquid water expands into droplets and freezes as it experiences the vacuum of space. The water-ice grains are distributed; the heavier grains fall back to the moon’s surface, while the lighter ones escape Enceladus and feed Saturn’s E-ring (Postberg et al., 2018). Hence, Enceladus’ surface undergoes a continuous renewal from water-ice grain depositions either from direct plume deposition or E-ring bombardment (Southworth et al., 2018). Near the plume’s source, the deposition could amount to 1mm/year with particle sizes ranging from 0.6 to 15 micrometre (Spitale et al., 2015).

These water-ice grains are unconsolidated and undergo a consolidation process over long periods of time (Blackford, 2007; Molaro et al., 2019), called sintering, which largely follows the processes described by Swinkels and Ashby (1981). Initially, two grains are brought into contact, followed by a neck-growth stage driven by multiple mechanisms until the grains become difficult to distinguish over time. The pores become isolated and closed off as they shrink over a longer period, resulting in the densification of the bulk, see figure 1.

Figure 1: The blue circles represent water-ice grains sintering together over time. The initial stage highlights an initially small neck formation, which grows in stage 1, through two highlighted transport processes until the pores coalesce in the final stage.

The grain size and local temperature significantly affect the sintering time. The average surface temperature of 72K prevents significant sintering over the geological timescale of the moon (Molaro et al., 2019). However, the elevated temperatures near the plumes would allow micron-sized crystalline water-ice grains to sinter over relatively short geological timescales (~15 years) and withstand pressures on the order of ten megapascals, indicating that Enceladus’ surface may exhibit locally different mechanical properties (Southworth et al., 2015; Choukroun et al., 2020). Furthermore, the porosity of the bulk material affects the bulk thermal properties through conduction and radiation, affecting the sintering rates between the grains (Ferrari and Lucas, 2016).

To understand the sintering processes and determine the mechanical properties of Enceladus’ surface, laboratory experiments have been used to study water-ice produced in low-temperature environments. Results indicate that icy granular surfaces are mechanically weaker at low temperature than at warmer temperatures (Gundlach et al., 2018; Molaro et al., 2019; Choukroun et al., 2020; van Veen, 2025; Fabbretti, 2026). Vapour transport is the dominant consolidation mechanism in early sintering stages, while surface diffusion may become dominant at much lower temperatures. In vacuum conditions, Fabbretti (2026) found an increased sintering rate between water-ice grains compared to the atmospheric findings of van Veen (2025). This indicates that the sintering behaviour of water-ice grains operates differently in the absence of an atmosphere.

This study aims to understand the mechanical properties of Enceladus’ surface analogues as well as low-temperature and low-pressure sintering of water-ice grains. We performed experiments in our PISCES (Plumes and Ices Simulation Chamber for Enceladus and other moonS) to investigate the sintering behaviour of water-ice grains under vacuum conditions. This novel experimental chamber is used to reach sub-mbar pressures and low temperatures (Bourgeois and Cazaux, 2025). Fine grains are produced through direct spray into the chamber or outside using liquid nitrogen, and are exposed to a near vacuum where they are mechanically evaluated. A penetrometer setup is used to measure the cone penetration resistance of the bulk material in a vacuum with respect to the grain size, porosity and sintering time. The L4 mission is in the works with respect to ESA’s Voyage 2050 programme. It is set to explore the Kronian moons with an orbiter and a lander (Helbert et al., 2025). Understanding the surface’s mechanical properties will help find an adequate landing site for the mission.

How to cite: Larguet, M., Cazaux, S., van den Heuvel, N., and Giordano, F.: Mechanical properties of Enceladus’ surface analogues, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-993, https://doi.org/10.5194/epsc2026-993, 2026.

EPSC2026-993

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Negative ions are emerging as key players in the chemistry of icy moon environments [1], yet their formation mechanisms remain difficult to fully constrain from space data alone. Here we present a dual approach of combining in-situ observations from the Cassini mission CAPS-ELS instrument with dedicated laboratory simulations to better understand negative ion behaviour under Enceladus-like conditions.

From the planetary perspective, we report new findings from the analysis of Cassini's Enceladus flyby E3 to build on former reports [2], focusing on the composition and chemical complexity of the plume environment. These results extend our understanding of the moon's subsurface ocean and reinforce the potential role of negative ions in forming complex organic molecules along a prebiotic chemistry pathway. We also introduce initial concepts for a next-generation instrument designed to detect and characterise negative ions in future L4 mission.

From the laboratory perspective, we present simulations performed at LATMOS designed using PAMPRE experiment [3] to reproduce Enceladus-like conditions and study negative ion formation mechanisms directly. Preliminary results demonstrate how gas composition influences ion formation in icy moon environments.

These complementary approaches highlight the power of combining planetary datasets with Earth-based laboratory experiments to unravel the chemistry of ocean worlds. We discuss how these approaches complement each other, where open questions remain and what a next generation mission would need to resolve them.

[1] Schaible, M. J. et al., A. (2024). Chapter 1: The Astrobiology Primer 3.0. Astrobiology, 24(S1):S–4–S–39.

[2] Coates, A. J. et al. (2010) ‘Negative ions in the Enceladus plume’, Icarus, 206(2), pp. 618–622. Available at: https://doi.org/10.1016/j.icarus.2009.07.013.

[3] Dubois, D. et al. (2020) ‘Positive ion chemistry in an N2-CH4 plasma discharge: Key precursors to the growth of Titan tholins’, Icarus, 338. Available at: https://doi.org/10.1016/j.icarus.2019.113437.

How to cite: Parsec-Wallis, A., Coates, A., Preston, L., Chatain, A., Nicolaou, G., Chatterjee, A., and Vettier, L.: Bridging Cassini observations and laboratory experiments to further analyse the plumes of Enceladus, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1009, https://doi.org/10.5194/epsc2026-1009, 2026.

EPSC2026-1009

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Ice dust emitted from Enceladus has been shown to be extremely informative source regarding the chemical composition of the subsurface ocean and its geochemical processes. When sampled by an impact ionization mass spectrometer during a flyby - without the need for a surface landing - these grains provide a means for particle-by-particle characterization. This approach avoids bulk sample dilution, allowing even rare molecules concentrated within a single particle to be detected, as exemplified by Cassini’s Cosmic Dust Analyzer (CDA). In preparation for ESA’s L4 mission, next-generation impact mass spectrometers are being considered to explore ocean chemistry, habitability, and astrobiology. The development and validation of these instruments require laboratory accelerated ice particles with archetypical biomolecules.

In this study, the Selected Ice Nanoparticle Accelerator (SELINA), coupled with a commercial Time-of-Flight (ToF) mass spectrometer, was used to accelerate 100–150 nm ice particles to terminal velocities of up to 3.6 km/s. Ice grains with embedded phospholipid membrane produced distinct single-impact mass spectra in negative mode. Mass assignment and mass-to-charge (m/z) calibration were supported by high resolution Orbitrap collision-induced dissociation (CID) studies of bare phospholipid and fatty acid ions.

At studied velocities the most intense peak corresponded to the PO₃^- ion with other prominent peaks included H₂PO₄^-, various phosphate fragments, fatty acids related ions, and phospholipid adduct. Additionally, impact ionization spectra of particles containing solvated fatty acids, phosphates, and acetates were recorded under similar conditions to better understand impact dynamics, specifically the notably low abundance of water clusters observed in negative mode.

The obtained results illustrate the imperative demand for high-resolution (Orbitrap-class)  impact ionization mass spectrometry probes, supported by extensive analogue databases, if complex biomolecules are to be successfully discovered during  ESA’s L4 mission.

How to cite: Spesyvyi, A., Wang, C., Barragán-Mayet, H., Charvat, A., Cebecauer, M., Olżyńska, A., Polášek, M., Žabka, J., and Abel, B.: SELINA Laboratory Impact Mass Spectrometry for Identification of Phospholipids and Other Organics in Dust from Enceladus, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-467, https://doi.org/10.5194/epsc2026-467, 2026.

EPSC2026-467

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Saturn’s geologically-active, icy moon Enceladus possesses a subsurface liquid water ocean beneath its icy crust, which is in contact with an unconsolidated rocky core. The ocean water percolates through the core where hydrothermal reactions potentially produce a multitude of organic compounds. Originating deep within the core, chemical species may rise from the ocean floor to the surface and are ejected into space in a plume of gas and ice grains through vents in the icy crust. Thus, markers of chemical or even biological processes occurring in the ocean or at larger depth could be mirrored in the plume composition and detected via spacecraft performing flybys through the plume. In this way, the Cosmic Dust Analyser (CDA; [1]) and the Ion and Neutral Mass Spectrometer (INMS, [2]) onboard the Cassini spacecraft sampled Enceladean material. A variety of organic and inorganic compounds, including salts and silica, and most of the  bioessential CHNOP(S) elements have been found in the erupted plume material [3,4,5,6,7,8,9]. The organic fraction of the compounds detected by CDA ranges from simple to complex, and aromatic rings represent a significant component of their molecular structures.

The analysis of ice grains from Enceladus [3,4], and returned samples from the carbonaceous asteroids Bennu and Ryugu [10,11,12], have revealed the presence of diverse organic compounds, including aromatic species, amines, and heterocyclic molecules. Aromatic compounds are amongst the most abundant and stable organic compounds in the universe and are thought to be involved in subsurface reaction chemistry on Enceladus [3]. Additionally, the polycyclic aromatic hydrocarbon (PAH) world hypothesis, aromatic groups may have acted as stabilising compounds within early pre-cellular container elements before the evolution of modern lipid membranes [13].  The identification of nitrogen-bearing heterocyclic compounds (i.e. nucleobases) in chondritic material from Bennu and Ryugu [10,11,12], with a more tentative detection of N-heterocycles in Enceladus plume ice grains [3], further highlights the importance of investigating the origin and chemical evolution of aromatic and heterocyclic compounds in extraterrestrial aqueous environments.

As part of the ERC-CoG Analogue Icy Moon Simulations (AIMS) project, we simulate high-temperature and -pressure processes under conditions though to be found at Enceladus’ hydrothermal seafloor, and investigate the alteration of organic compounds in such conditions [14]. This project will advance our understanding of the synthesis, degradation, alteration, and evolution of organic, inorganic and biogenic material in the ocean before ejection into the plume, which is vital for missions aiming to detect biosignatures. In this work, we present the latest updates on the organic composition of Enceladean ice grains detected by Cassini [3], as well as results from our new hydrothermal simulations using aromatic and heterocyclic compounds. This allows not only further constraint of Cassini data interpretation, but will also provide feedback for future missions visiting Enceladus, e.g. the future ESA’s L4 mission alongside the ongoing  ESA’s JUICE and NASA’s Europa-Clipper missions to Europa.

[1]Srama et al. (2004), Space Sci Rev 114, 465-518.

[2]Waite et al. (2004), Space Sci Rev (2004) 114, 113-231.

[3]Khawaja et al. (2025), Nature Astron. 9, 1662–1671

[4]Khawaja et al. (2019), MNRAS 489, 5231–5243

[5]Postberg & Khawaja et al. (2018), Nature 558, 564 – 567

[6]Postberg et al.(2023), Nature 618, 489–493

[7]Postberg et al. (2009), Nature 459, 1098–1101

[8]Hsu et al. (2015), Nature 519, 207-210

[9]Waite et al. (2009), Nature 460, 487-490

[10]Mojarro et al., (2025), PNAS, 122, 49

[11]Koga et al. (2026), Nature Astron. 10, 1038

[12]Glavin et al. (2025), Nature Astron. 9, 199–210

[13]Groen et al. (2012), Orig Life Evol Biosph 42, 295–306

[14]Khawaja, Hortal- S.nchez & O'Sullivan et al. (2024), RSTA 382, 2273.

How to cite: Khawaja, N., O'Sullivan, T., Hortal-Sanchez, L., Betton, Q., Ladd, J., Dannenmann, M., Napoleoni, M., Hillier, J., Beinlich, A., Postberg, F., and John, T.: Tracing Hydrothermal Alteration and Chemical Evolution of Aromatic and Heterocyclic Compounds in Enceladus Ice Grains, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1158, https://doi.org/10.5194/epsc2026-1158, 2026.

EPSC2026-1158

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Icy worlds are ubiquitous in the solar system. Among them, the Jovian and Saturnian moons Europa and Enceladus are considered the most habitable places as they host subsurface liquid oceans in direct contact with a rocky core below their icy shells. Moreover, Enceladus possesses valuable chemical ingredients, including at least five of the six basic elements for life - CHNOPS - and its porous rocky core hosts hydrothermal systems which, as sources of chemical disequilibria, are deemed necessary for the emergence of life. High-mass and low-mass organics have been detected in Enceladus icy grains with the Cosmic Dust Analyzer (CDA), a spaceborne impact mass-spectrometer. The structure of these organics comprises a variety of functional groups and moieties [1,2,3,4].

Spaceborne instruments such as CDA and future ones such as the SUrface Dust Analyzer (SUDA) onboard Europa Clipper will strongly benefit from data obtained in laboratory experiments, both in terms of data interpretation and performance.

In this context, our current understanding of the solar system strongly benefits from comparative planetology. Terrestrial analogue sites of icy ocean moons are specific locations on Earth that are similar in some important aspects (e.g., geological or environmental characteristics) to those on icy ocean moons, and can thus serve as natural laboratories. Their study brings substantial contribution to the scientific knowledge of ocean worlds, and can support space missions by e.g., informing on the habitability of extreme environments, the preservation of biosignatures in these environments and the relevance of specific analytical methods to detect them, as well as testing of instrumentation. The detailed analysis of terrestrial analogue samples with techniques relevant for space missions is thus a key milestone in the preparation of missions investigating habitability and searching for signs of life on icy ocean moons.

Here we present the first ever analysis of natural ice analogues with Laser Induced Liquid Beam Ion Desorption (LILBID) - a well-established method allowing the simulation of ice grains’ impact ionization mass spectra. Many LILBID spectra of synthetic samples have already been recorded to complement an expanding reference database [5] for e.g., Europa Clipper. Gas Chromatography linked to Mass Spectrometry (GC-MS), Raman and IR spectroscopy measurements were also carried out, in an effort to carry out an orthogonal investigation of the samples. Ice samples were collected from key locations in the Collins (a.k.a. Bellingshausen) glacier on King George Island, Antarctica, with support from the Instituto Antártico Uruguayo. Here we will discuss results from icy samples containing the microalgae Sanguina nivalis, a species highly adapted to the low temperatures of the glacier, limited nutrient availability and intense sunlight. Various low and high-mass molecular biosignatures are detected despite the complexity of the samples’ matrix, including a range of amino acids, fatty acids and larger lipids. Moreover, environmental adaptations to intense UV radiation and low temperature relevant to icy ocean worlds are reflected in the molecular biosignatures, with the respective detections of pigments and a high degree of unsaturation in the lipid profile. This marks the first ever detection of biosignatures in natural samples with LILBID. Our results highlight the detection capability of SUDA-type instruments in regards to biosignatures specifically reflecting icy moon conditions, despite the high salt levels of the ice matrix. This work and other measurements carried out at the Analogue Icy Moon Simulations (AIMS) laboratory will support preparation for ESA’s future L4 mission to Enceladus.

[1] F. Postberg et al. Nature 558, 564–568 (2018)

[2] F. Postberg et al. Nature 618, 489–493 (2023)

[3] N. Khawaja et al. Mon. Not. R. Astron. Soc 489, 4, 5231–5243 (2019)

[4] N. Khawaja et al. Nat Astron 9, 1662–1671 (2025)

[5] F. Klenner et al., Earth Space Sci., 9, e2022EA002313 (2022)

How to cite: Hortal Sánchez, L., Napoleoni, M., L. Finkel, P., Carrizo, D., Sánchez-García, L., Burr, D., Hofmann, F., Moreno Paz, M., Khawaja, N., Parro, V., and Postberg, F.: Detection of High-Mass Molecular Biosignatures in Antarctic Ices: Relevance for Icy Moons Exploration , Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-895, https://doi.org/10.5194/epsc2026-895, 2026.

EPSC2026-895

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Active plume emissions from icy moons provide direct access to subsurface ocean material (Porco et al., 2006), enabling compositional characterisation of potentially habitable environments. This work develops a multi-instrument framework for organic compound detection in Enceladus' plumes by integrating complementary Cassini VIMS, INMS, and CDA datasets across multiple flyby encounters.

VIMS provided spatially resolved infrared spectra of plume particles and surface deposits, characterising water ice properties, grain sizes, and spectral signatures of volatiles including CO₂ and organic compounds (Brown et al., 2006; Dhingra et al., 2017). INMS measured neutral gas composition through mass spectrometry, identifying species including ammonia, molecular hydrogen from serpentinisation, and radiogenic argon indicating water-rock interactions (Waite et al., 2009). CDA characterised solid ice grain composition through impact ionisation mass spectrometry, detecting high-molecular-weight organic compounds exceeding 200 Da beyond INMS instrumental limits (Postberg et al., 2009, 2018). Together, these instruments span complementary molecular weight regimes and physical phases.

Our framework processes VIMS cubes using background subtraction for high phase angle observations to isolate plume signals, whilst water ice crystallinity analysis constrains particle formation temperatures. INMS processing integrates mass channel data accounting for fragmentation patterns to reconstruct parent molecule abundances. Multi-flyby comparison across diverse encounter geometries reveals water dominance with high molecular diversity throughout the accessible mass range.

The combined detection of liquid water, diverse organic chemistry including complex macromolecular compounds, and chemical energy sources establishes Enceladus as a compelling astrobiology target potentially analogous to Earth's hydrothermal vent ecosystems. This framework enables direct transferability to Europa Clipper's SUDA instrument and future missions, providing a methodology for comprehensive ocean world characterisation.

Acknowledgments

This work was financially supported by LA/P/0056/2020 (IMS DOI https://doi.org/10.54499/LA/P/0056/2020) and CQE UID/00100/2025 (https://doi.org/10.54499/UID/00100/2025), UID/PRR/100/2025 (https://doi.org/10.54499/UID/PRR/00100/2025) and UID/PRR2/00100/2025 (https://doi.org/10.54499/UID/PRR2/00100/2025) funded by national funds through FCT/MECI (PIDDAC). The authors also acknowledge funding by the Portuguese Foundation for Science and Technology (FCT) through project UID/04434/2025, and project ORIGINS (2022.05284.PTDC).

References

• Brown, R. H., et al. (2006). Composition and physical properties of Enceladus' surface. Science, 311(5766), 1425-1428.

• Dhingra, D., et al. (2017). Spatially resolved near infrared observations of Enceladus' tiger stripe eruptions from Cassini VIMS. Icarus, 292, 1-12.

• Porco, C. C., et al. (2006). Cassini observes the active south pole of Enceladus. Science, 311(5766), 1393-1401.

• Waite, J. H., et al. (2009). Liquid water on Enceladus from observations of ammonia and ⁴⁰Ar in the plume. Nature, 460(7254), 487-490.

• Postberg, F., et al. (2009). Sodium salts in E-ring ice grains from an ocean below the surface of Enceladus. Nature, 459(7250), 1098-1101.

• Postberg, F., et al. (2018). Macromolecular organic compounds from the depths of Enceladus. Nature, 558(7711), 564-568.Multi-Instrument Spectral Framework for Biosignature Detection in Enceladus' Plumes

How to cite: Nunes, M., Machado, P., and Martins, Z.: Multi-Instrument Spectral Framework for Biosignature Detection in Enceladus' Plumes, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-289, https://doi.org/10.5194/epsc2026-289, 2026.

EPSC2026-289

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With the Europa Clipper and JUICE missions in the Jovian system, and Dragonfly mission planned for launch in 2028, large icy moons and their subglacial oceans draw a renewed attention. Dwarf planets, currently scrutinized with the JWST, also constitute icy worlds with potential oceans at various stages of their evolution. Icy moons and dwarf planets provide a "water-world" model for studying potential prebiotic chemistry or extraterrestrial habitats in their subglacial oceans, and understanding the formation and properties of postulated ocean-exoplanets. Icy moons and dwarf planets are formed in the outer solar system in very different physico-chemical conditions than the terrestrial planets in the inner solar system. In particular, carbon, the fourth most abundant element in the Solar photosphere, was detected in abundance in comets (45 wt% of the refractory dust; Bardyn et al. (2017), in plumes emanating from Enceladus (Postberg et al., 2018), and on Ceres (Marchi et al., 2019).

Using equation of states for ices, silicates, and organics, the value of the moment of inertia (MoI) of icy moons (Titan, Enceladus, Dione, Europa, Ganymede) and dwarf planets (Ceres, Haumea) suggest that organic molecules are a major compound of their refractory core (Reynard and Sotin, 2023). During the thermal evolution of the refractory core, the composition evolves and the density of the remaining carbonaceous compound increases as H and heteroatoms are released, eventually reaching the density of graphite (Fig. 1; Delarue et al. (2025).

Figure 1. Kinetic evolution of COM density. The curves represent the time evolution, from 100 Myr to present time, of the COM along with temperature profiles expected at such time (see text for details). The COM densities evolve quickly for the first hundred million years, and almost all the material is transformed within Titan’s core after 1 Gyr. The values are larger than those of Reynard & Sotin (2023) (dashed red lines) which were based on laboratory measurements without taking into account the kinetics. The denser the COM, the higher its concentration needed to match the moment of inertia factor.

Gravitational data inform on the present state of the refractory core, suggesting that the organic fraction could be a larger fraction of the initial body. In an effort to retrieve the initial fractions of ice, silicates, and organics, two models were developed. First a kinetic model (KINCAM-E) was developed to describe the transformation of the carbonaceous compound with time and temperature. This model is based on experimental data with long duration pyrolysis. Second, a thermo-chemical model was developed to describe the feedback of the carbonaceous compound evolution on the thermal evolution. This model includes the migration of volatiles produced by the degradation of the organic compound to the hydrosphere. It also includes the dehydration of the hydrated silicates as the temperature increases. In these models, the mass of the body is fixed and partitioned between silicates, ice, and organics. The evolution of the radius and other parameters is followed. Only models consistent with the present value of the radius and the MoI are retained. First applied to Titan, this thermo-chemical evolution model shows that the initial fraction of organics composing Titan is similar to cometary amount, for accepted values of a leaching parameter L₀ describing radioactive elements dissolution in the hydrosphere and differentiation delay (Fig.2). The model is applied to the icy Galilean satellites, and implications for future observations by JUICE and Europa Clipper will be discussed.

Figure 2. Composition of Titan and potential precursors (Sun, comets, IDP and UCAMM, CI and CV chondrites) in a ternary diagram (SSC: dry silicate-sulfide, IC: hydrosphere and water in silicates; C: pure carbon). Carbonaceous matter composition (COM) evolves from IOM-like to pure carbon during heating. Three orange ellipses represent model results for Titan assuming different values of L₀ and compatible with MoI. The center values of the ellipses correspond to the initial MoI estimate of 0.3414 (Iess et al., 2012). The blue area indicates the loosely defined compositional range of comets. For the Sun, the composition from Lodders (2021) is indicated by the largest symbol, and a recent carbon-rich estimate of composition (Truong et al., 2024) is shown as a dimmer symbol.

References

Bardyn, A., Baklouti, D., Cottin, H., Fray, N., Briois, C., Paquette, J., Stenzel, O., Engrand, C., Fischer, H., Hornung, K., Isnard, R., Langevin, Y., Lehto, H., Le Roy, L., Ligier, N., Merouane, S., Modica, P., Orthous-Daunay, F.-R., Rynö, J., Schulz, R., Silén, J., Thirkell, L., Varmuza, K., Zaprudin, B., Kissel, J., Hilchenbach, M., 2017. Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta. Monthly Notices of the Royal Astronomical Society 469, S712-S722.

Delarue, C., Reynard, B., Sotin, C., Fellah, C., Cardon, H., Montagnac, G., Confortini, G., Ferreiro Mählmann, R., 2025. Carbon-rich interiors of Ganymede and Titan: application of a kinetic model of carbonaceous organic matter transformation, EPSC, Helsinki (Finland), Finland.

Iess, L., Jacobson, R.A., Ducci, M., Stevenson, D.J., Lunine, J.I., Armstrong, J.W., Asmar, S.W., Racioppa, P., Rappaport, N.J., Tortora, P., 2012. The tides of Titan. Science 337, 457-459.

Marchi, S., Raponi, A., Prettyman, T.H., De Sanctis, M.C., Castillo-Rogez, J., Raymond, C.A., Ammannito, E., Bowling, T., Ciarniello, M., Kaplan, H., Palomba, E., Russell, C.T., Vinogradoff, V., Yamashita, N., 2019. An aqueously altered carbon-rich Ceres. Nature Astronomy 3, 140-145.

Postberg, F., Khawaja, N., Abel, B., Choblet, G., Glein, C.R., Gudipati, M.S., Henderson, B.L., Hsu, H.-W., Kempf, S., Klenner, F., Moragas-Klostermeyer, G., Magee, B., Nölle, L., Perry, M., Reviol, R., Schmidt, J., Srama, R., Stolz, F., Tobie, G., Trieloff, M., Waite, J.H., 2018. Macromolecular organic compounds from the depths of Enceladus. Nature 558, 564-568.

Reynard, B., Sotin, C., 2023. Carbon-rich icy moons and dwarf planets. Earth and Planetary Science Letters 612, 118172.

How to cite: Reynard, B., Confortini, G., Delarue, C., Kervaso, M., Pinceloup, M., and Sotin, C.: Thermo-chemical modelling of organic-rich icy worlds of the outer solar system, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-418, https://doi.org/10.5194/epsc2026-418, 2026.

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The Galilean moon Europa will be intensively explored in a few years by two upcoming space missions: JUICE and Europa Clipper. This icy moon, whose surface is covered by a thick ice shell, has attracted considerable attention because of its potential habitability and the likely presence of a subsurface liquid-water ocean. Despite the detection of potentially endogenic compounds on its surface such as salts or trapped volatiles (e.g. McCord et al. 2002, Fisher et al. 2015, Villanueva et al. 2023, Trumbo and Brown 2023…) the evolution and the current composition of such a subsurface ocean remain poorly constrained.

The evolution of Europa’s subsurface ocean was likely shaped by a combination of physical and chemical processes. In particular, interactions between the rocky mantle and the overlying water ocean may have strongly influenced the volatile inventory of the hydrosphere, as well as its pH and salinity. In addition, if Europa’s surface temperature remained sufficiently high shortly after accretion, equilibrium with a primordial atmosphere could also have modified the volatile distribution within the early ocean.

In the context of upcoming missions exploring the Galilean system, this study aims to establish links between future observations potentially probing Europa’s ocean composition and the moon’s formation and evolutionary history. Specifically, assuming Europa’s hydrosphere formed from the delivery of ice-rich planetesimals and solids, we investigate how the primordial volatile inventory evolved during the early stages of accretion by modeling the chemical evolution of its hydrosphere.

As Europa accumulates mass through accretion of surrounding material and bombardment by impactors, the surface temperature is calculated from a combination of impact heating and thermal input from the circumjovian disk (Bennacer et al. 2025). At each time step, we compute the composition of the primordial atmosphere and ocean, considering:

• The liquid-vapor equilibrium at the atmosphere-ocean interface
• The water-rock interactions occurring at the ocean-rocky mantle interface
• The influence of the rocks remaining in suspension in the ocean as Europa accretes material

Chemical equilibria associated with water-rock interactions are calculated using PHREEQ-C (Parkhurst and Appelo, 2013), while atmosphere-ocean exchange is modelled following the framework of Amsler Moulanier et al. (2025). The transport of species throughout the water column is modelled as well, using the diffusion transport scheme available in PHREEQ-C.

Our results provide an overview of how Europa’s accreted volatile inventory in the hydrosphere was influenced by the processes occurring during its early evolution. In particular, we highlight the key role of water-rock interactions in controlling the composition of the primordial atmosphere, the chemical evolution of the ocean, as well as its pH and salinity. In the context of the upcoming JUICE and Europa Clipper missions, this work provides various evolutionary pathways for Europa’s ocean composition, depending on the conditions of accretion, the initial volatile inventory and the composition of the rocky mantle. Comparing these results to future measurements potentially indicative of today’s ocean composition, we may better constrain and understand Europa’s evolution and formation conditions.

Figure 1 : Evolution of pH (left panel), species abundances (middle panel) and precipitated minerals (right panel) in one of the ocean’s layer during Europa’s accretion process (assuming a slow accretion in a warm CPD). During the modelling process, the ocean is divided in n layers, with here n=10. The results shown in this plot are for the modelled ocean layer highlighted in red on the left-hand side of the figure.

Acknowledgement:

The authors acknowledge the financial support of the SNSF under SNSF starting grant 218336.

References: 

- McCord, B. T. et al. , J. Geophys. Res., 107(E1), doi:10.1029/2000JE001453, 2002.

- P. D. Fischer et al 2015 AJ 150 164

- G. L. Villanueva et al., Endogenous CO₂ ice mixture on the surface of Europa and no detection of plume activity.Science381,1305-1308(2023).DOI:10.1126/science.adg4270

- Samantha K. Trumbo, Michael E. Brown, The distribution of CO₂ on Europa indicates an internal source of carbon.Science381,1308-1311(2023).DOI:10.1126/science.adg4155

- Alizée Amsler Moulanier et al 2025 Planet. Sci. J. 6 1

- Yannis Bennacer et al 2025 Planet. Sci. J. 6 138

- Parkhurst, D.L., and Appelo, C., 2013, Description of input and examples for PHREEQC version 3: A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods 6-A43, xx, 497 p., https://doi.org/10.3133/tm6A43.

How to cite: Amsler Moulanier, A., Sekine, Y., Bennacer, Y., Kadoya, S., Mousis, O., and Vorburger, A.: On the composition of Europa’s early hydrosphere during accretion., Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-950, https://doi.org/10.5194/epsc2026-950, 2026.

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Aromatic organic compounds have been detected in ice grains ejected in the plume of Enceladus [1-3], demonstrating that they are derived from the subsurface and could be expected in other ocean-bearing icy moons such as Europa. At least two populations of aromatic structures, fragments of high-mass macromolecular and isolated low-mass species, were observed by Cassini’s Cosmic Dust Analyser (CDA [4]) during hypervelocity impacts of ice grains. Interpretations of CDA mass spectra have relied on laboratory analogue experiments [5], but theoretical insights into molecular impact fragmentation through quantum chemistry would add an additional technique to the toolbox of habitability-investigating missions at icy ocean worlds, such as Europa Clipper and ESA’s L4 mission to Enceladus [6].

A common challenge faced in computational chemistry investigations is the laborious construction of the potential energy surface (PES) when exploring the dissociation of a molecule under space conditions. Many molecular fragmentation pathways can be accessed in energetic processes, e.g. radiolytic chemistry and impact ionisation of ice grains onto spaceborne mass spectrometers at hypervelocity. Previous theoretical work into understanding impact ionisation mass spectra has been conducted by tedious exploration of the full fragmentation space [7]. To the best of our knowledge, there is no general-purpose method currently available for systematically surveying the full fragmentation space of a given organic molecule with quantum chemistry.

Here, we introduce the Chemical Rupture Understanding through Molecular Bonding and Liberation Energetics (CRUMBLE) package, in which we compute the dissociation energies using a graph theoretical approach for enumerating all binary fragmentation channels of a given molecule. The fragment channels are ranked by the number of bonds broken or formed, or the thermodynamic dissociation energy. Fragment structures calculated at a given quantum chemical method are stored in a library that is queried to reduce the computational expense associated with future runs. With representative examples, we demonstrate that CRUMBLE enables researchers to quickly locate minima in the fragmentation space of molecules and generate valuable quantum chemical data for each fragment and its isomers to aid in mass spectrometric investigation. This package can be applied to many environments in space: Cassini, Europa Clipper, and possibly ESA’s L4 mission have sampled or will sample organic-bearing ice grains at icy ocean moons, and JAXA’s DESTINY+ mission will sample dust emitted by the near-Earth asteroids Phaethon and Apophis at hypervelocities with the DESTINY+ Dust Analyser (DDA [8]). Beyond spaceborne mass spectrometry, CRUMBLE is broadly applicable to any context in which the fragmentation space of a molecule is explored, including the dissociation of molecules in the interstellar medium and protoplanetary disks, and the top-down processing of large molecules into smaller fragments in energetic astrophysical environments.   

References:

[1] Postberg, F. et al. Nature, 2018. 558(7711): p. 564-568.

[2] Khawaja, N. et al., Monthly Notices of the Royal Astronomical Society, 2019. 489(4): p. 5231-5243.

[3] Khawaja, N. et al., Nature Astronomy, 2025. 9: p. 1662-1671

[4] Srama, R. et al., Space Science Reviews, 2004. 114(1-4): p. 465-518.

[5] Klenner, F. et al. Rapid Communications in Mass Spectrometry, 2019. 33(22): p. 1751-1760.

[6] Helbert, J. et al. The Mission to Enceladus – The ESA L4 mission. EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1307.

[7] O’Sullivan, T.R. et al., ACS Earth and Space Chemistry, 2026. 10(4), p. 1033-1046.

[8] Simolka, J. et al. Philosophical Transactions of the Royal Society A, 2024. 382(2273): 20230199.

How to cite: O'Sullivan, T. R., Bera, P. P., Khawaja, N., and Postberg, F.: CRUMBLE: An automated computational chemistry package for molecular fragmentation with applications to spaceborne mass spectrometry at icy moons, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-286, https://doi.org/10.5194/epsc2026-286, 2026.

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The Cosmic Dust Analyzer (CDA) on the Cassini spacecraft has convincingly demonstrated the scientific value of mass spectra of ice particles ejected by the plume on Saturn's ice moon Enceladus. Trace amounts of organic and inorganic molecules embedded in ice particles revealed invaluable insight into the chemical composition of the ocean beneath the moon's icy crust. However, it became quickly obvious that to address open questions about the astrobiological nature of the ocean requires impact ionisation mass spectrometers with a considerably higher mass resolution than that of the CDA instrument of m/Δm ~ 50. Other CDA shortcomings include target cleanliness issues and the low detection cadence of 1 impact per second.

The High Ice Flux Instrument (HIFI) is a reflectron-type impact mass spectrometer specifically designed for such applications. It has a mass resolution of 1000 to 2000 and has been optimized for using the electronics of the Surface Dust Analyzer instrument on Europa Clipper for recording the spectra. To ensure a high mass resolution HIFI has a long drift region and uses a set of electrostatic Einzel lenses to prevent the ion beam from diverging before entering the single stage reflectron region. The reflectron optics is composed of 23 precision machined electrostatic electrodes to guarantee a smooth reflecting field. In contrast to previous reflectron impact mass spectrometers such as CIDA enter the impacting particles the spectrometer through the reflectron to strike the target at a right angle. The target itself is a highly polished Titanium carrier coated with 250 nm of high purity Iridium (5 nm surface roughness). The high atomic mass of Iridium ensures that no target lines as well as target cluster lines appear in the mass range ≥ 200 u relevant for the compositional analysis of mineral and ice particles. 

Impact ionization mass spectrometers characterize the composition of microscopic particles hitting the instrument's metal target by recording Time-Of-Flight (TOF) mass spectra of the plasma created upon impact. The resulting TOF spectra show a strong dependence on the impact velocity. At very low speeds around 1 km/s, the spectra are dominated by lines of elements with very low ionization energy, mainly sodium and potassium. At velocities of a few km/s or more, the spectra are dominated by the mass lines associated with the impactor material and the target material.

Because dust impacts in interplanetary space are rare events, previous impact ionization mass spectrometers, such as the Surface Dust Analyzer (SUDA) on the payload of NASA's next flagship mission to the Jovian moon Europa, the Interstellar Dust Experiment (IDEX) on the payload of NASA's IMAP mission, or the Dust Detection System (DDS) on the payload of JAXA's Destiny+ mission to the asteroid Phaethon, have been optimized for a large sensitive area. However, there are dust environments where the dust number density is so high that the TOF mass spectra of successive impacts recorded by large area dust detectors begin to overlap. The Cosmic Dust Detector (CDA) onboard the Cassini spacecraft  recorded impact rates exceeding 1000 s^-1 in the ice particle plume of the Saturnian moon Enceladus (sensitive area ~36 cm²). Therefore, large area dust instruments are not capable of recording high quality mass spectra in dust-rich environments, such as cryovolcanoes on the Saturnian moon Enceladus  or comet trails. To prevent mass spectra from overlapping, instruments optimized for high impact rates must have a very small sensitive area.

The ideal ion optics for such an instrument is a classical reflectron optimized for impact plasma analysis. Each TOF spectrometer consists of an accelerator region, which provides all ions with the energy U_acc, and an ion detector at a distance L from the accelerator region - the drift length - to measure the flight times of the ions t = t₀ + a m^1/2, where t₀ is the ion launch time and a is the stretch parameter of the instrument. The initial thermal energy of an ion species of mass m leads to a spread Δt in their flight times, which decreases the resulting mass resolution m/Δm = t/2Δt of the instrument. A reflectron uses a repulsive electric field to reduce Δt. Ions launched with excess energy ΔU > 0 penetrate deeper into the repulsive field region and thus have a longer flight path than ions launched with zero initial energy, which partially compensates for the nonzero excess energy.

Cosmic dust particle impacts are a stochastic phenomenon, which means that there is no prior knowledge of either the impact or the ion launch time t₀. Therefore, techniques such as the extraction pulse method  for reducing the initial energy spread cannot be used in impact ionization mass spectroscopy.

Current state-of-the-art impact mass spectrometers have a mass resolution of 100 to 300, which is not sufficient to identify astrobiologically relevant materials such as amino acids embedded in ice grains. For example, in an impact mass spectrum of a shock-frozen ice grain from a 0.1 M NaCl solution containing 100 ppm argenine, which is a realistic assumption for the composition of the Enceladus ocean in the presence of life, argenine appears as the sodiated ion [Arg_Na]⁺ (m ~ 219.08 u), which is close to the salt cluster line [(NaCl)₂(NaOH)₂Na]⁺ (m ~ 218.89 u). The separation of the two lines requires a mass resolution of m/Δm \sim 219 u / 0.19 u = 1,153.

Here we report on the High Ice Flux Instrument (HIFI), which combines a mass resolution sufficient for the unambiguous identification of astrobiologically relevant traces of organic matter in ice particles with a small sensitive area. The instrument is a Time-Of-Flight (TOF) impact mass spectrometer with a single-stage reflectron ion optics, optimized for the acquisition of high-resolution mass spectra of ice particles during close spacecraft flybys through dust-rich environments.

How to cite: Kempf, S., Micheal, C., John, F., Scott, T., Zoltan, S., Hsian-Wen, H., Marshall, S., Morgan L., C., Libor, N., Bernd, A., Frank, P., and Jürgen, S.: Sniffing the Enceladus Plume: The High Ice Flux Instrument (HIFI) Compositional Analyzer., Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-779, https://doi.org/10.5194/epsc2026-779, 2026.

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Introduction: Icy worlds such as Europa, Enceladus and Ceres show evidence for subsurface liquids reaching the surface, either as plumes or effusive flows. Regions where liquids have been emplaced serve as potential archives of subsurface chemistry and habitability, making them prime targets for future missions. Fluids of a wide range of salt concentrations are expected at icy worlds, reflecting different potential liquid reservoirs and crysopheric processes. Despite their importance, little is understood about how saline fluids evolve if exposed to surface conditions. Previous experimental work has been limited to small scales, while models consider only equilibrium states or neglect phase changes. New large-scale experiments are therefore needed to better predict the behaviour of brine extrusions on icy worlds. We exposed large quantities (~50 kg) of NaCl and MgSO₄ brines at concentrations near their eutectics to pressures below their triple points and observed their physical and thermal evolution.

Results and Discussion: Both brines initially boiled during depressurization, which drove rapid cooling due to the loss of latent heat. However, vigorous boiling was soon interrupted by the formation of a thin ‘raft’ of salt crystals at the liquid-vapour interface. We found that the presence of the salt raft strongly limited evaporation, effectively halting further evaporative cooling of the brines.

Instead of gradually approaching the eutectic point, and hence complete solidification, the temperatures and bulk salinities of both experiments asymptotically approached the liquidus of their respective hydrates (hydrohalite, NaCl.2H₂O, in the NaCl case; meridianiite, MgSO₄.11H2O, in the MgSO₄ case) at concentrations approximately 3% higher than the eutectic. Ice was never observed to form in the NaCl experiment, and the temperature never decreased below 264 K. Instead, the system approached a steady state whereby hydrohalite formed at the surface, and then gradually sank, to be replaced by more surficial hydrohalite. In the MgSO₄ experiment, the eutectic point was initially reached, triggering massive ice and hydrate precipitation. However, the system quickly rebounded to a temperature above the eutectic, where a similar steady-state scenario was achieved; the experiment gradually approached a temperature of ~271 K, similarly buffered by the continual precipitation and sinking of hydrates.

Our results show that eutectic brines, if emplaced into low-pressure environments, resist evaporatively- driven freezing. Due to the formation of a salt raft which slows evaporation, eutectic liquids could be relatively long-lived in low-pressure environments, providing opportunity for them to pond and infiltrate pre-existing topography. Furthermore, although emplaced brines at icy worlds may freeze conductively from below, ice formation should not be expected in the upper 10s of cm simulated by these experiments. Instead, we predict the systems should continue to evaporate and precipitate hydrates until dryness, meaning that regions where high salinity brines have been emplaced could be indicated by salt evaporites rather than salt-bearing ices. For example, evaporation of high salinity brines offers a potential route to form the observed ice-free salt deposits in the Ceres faculae.

Our findings provide a new perspective on the possible longevity of liquid water under non-equilibrium scenarios on planetary surfaces, and indicate that quasi-stable transient liquids can play an important role in surface processes on icy worlds.  

References: [1] Quick et al. (2017) Icarus 284; [2] Postberg et al. (2009) Nature 459; [3] De Sanctis et al. (2019) Icarus 320; [4] Chivers et al. (2023) Planet. Sci. J. 4; [5] Steinbrügge et al. (2020) GRL 47

How to cite: Fox-Powell, M., Brož, P., Patočka, V., Sindhu, P., Hamp, R., Hogan, J., Sylvest, M., Emerland, Z., and Patel, M.: Large volumes of eutectic brines quasi-stable under near-vacuum conditions: implications for transient liquids on icy world surfaces, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1320, https://doi.org/10.5194/epsc2026-1320, 2026.

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Introduction

The presence of endogenic hotspots provides a measure of the level of geologic activity of icy moons, since they are indicative of ongoing resurfacing processes. However, to avoid misinterpreting thermal abnormalities, it is first necessary to understand passive thermal emission that is governed by the physical structure of materials. Two important thermophysical properties in such analysis are bolometric Bond albedo and thermal inertia: if diurnal temperature variations can be accurately modeled by adjusting these two parameters, a passive rather than endogenic origin is possible. In this work, we aim to constrain the thermal inertia and Bond albedo across the surfaces of Europa, Ganymede, and Callisto using brightness temperature observations recorded by the Galileo Photopolarimeter-Radiometer (PPR) instrument [1]. By presenting estimated values and their uncertainties for these thermophysical properties, we prepare for future thermal measurements carried out by both Europa Clipper and JUICE.

Methods

We first take average diurnal temperatures of the surfaces of Europa, Ganymede, and Callisto using brightness temperatures recorded by PPR. In this analysis, we use 29 datasets for Europa, notably increased beyond previous efforts [2], comparable to [3].  With less observations available, only 7 datasets each were used for both Ganymede and Callisto.

 By translating the observed radiance into brightness temperatures, the variation in temperature with local time is determined for latitude and longitude bins across each of the three moons. These diurnal curves are compared to those predicted by a 1-D thermal model [4] to determine what thermal inertias and Bond albedos can fit the data within a reduced chi-squared cut-off of χ²_red ≤1.0. This analysis is used to extensively quantify the uncertainty of the two thermophysical parameters derived from PPR data.

Results

Europa: Ensuring closed upper and lower limits within our χ²_red cut-off, we map albedo and thermal inertia for 33% and 24% of Europa's surface area into 6°x6° latitude/longitude bins (Fig. 1, left). We find a range of 0.375-0.75 for albedo and 20-110 J m^-2 K^-1 s^-1/2 (MKS) for thermal inertia, agreeing with [2] and [3]. Our uncertainty analysis indicates well-constrained estimates for albedo, with the average higher and lower ranges overlapping within their uncertainties: A_high = 0.11 ± 0.06 and A_low = 0.18 ± 0.12. Uncertainties for thermal inertia remain poorly constrained, with average higher and lower ranges being ­Γ_high = 103(+153/-103) MKS and Γ­_low = 17 ± 9 MKS.

Ganymede: Due to less available surface coverage, Ganymede’s surface is divided into 18°x6° longitude/latitude bins in order to meet the diurnal fitting routine requirements (Fig. 1, right). Preliminary results indicate the Bond albedo remains nearly uniform, with an average of 0.42 ± 0.07 across the surface and agreeing well with previous work [6]. Specifically, our χ²_red-deduced higher and lower uncertainty ranges of albedo are: A_high = 0.10 ± 0.06 and A_low = 0.14 ± 0.08. No apparent distinct surface variations in thermal inertia are as of yet discerned, with global values distributed across a range of 20-70 MKS. Upper limit thermal inertia estimates align with [6] within χ²_red-deduced uncertainty ranges of ­Γ_high = 57 ± 35 MKS and Γ­_low = 25 ± 11 MKS.

Callisto: With less PPR coverage available, Callisto fits are performed across grouped hemispheric regions, where the Jovian and anti-Jovian hemispheres are analyzed separately. Each hemisphere is divided into 10° latitude strips. Preliminary results for thermal inertia and Bond albedo indicate an overall agreement with lower-bound estimations from previous literature [5,6]. Further constraints are expected to be obtained at the time of the conference.

Conclusion

These results give an indication of the albedo and thermal inertia variation across Europa, Ganymede and Callisto. They aid in preparing for the arrival of Europa Clipper and JUICE to the Jupiter system by improving estimates for passive surface thermal properties and providing uncertainties of their values. This will enable future work to help discern what temperatures may lie above those expected from passive emission alone, providing a critical first step in the search for endogenic heating anomalies. Future work aims to refine the Ganymede and Callisto calculations using a two-component (ice and non-ice) analysis, and characterize the causes behind thermal inertia variations by modeling microphysical ice states for the three Galilean moons.

Fig. 1: Thermal Inertia (top) and Bond albedo (bottom) map of Europa (left) and preliminary results for Ganymede (right).

[1] Russell, E.E. et al., 1992. Space Sci Rev 60, 531-563.; [2] Rathbun, J.A. et al., 2010. Icarus 210, 763-769.; [3] Lange, L. et al., 2026. arXiv:2604.14374; [4] Lyster, D. et al., 2025. pp. EPSC-DPS2025-1479; [5] Meyer, C. et al., 2026. Planet. Sci. J. 7, 10; [6] Spencer, J.R. et al., 1989. Icarus 78(2), 337-354.

How to cite: Howes, S., Howett, C., and Lyster, D.: Galileo PPR Thermal Inertia & Albedo Measurements of Europa, Ganymede and Callisto, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1104, https://doi.org/10.5194/epsc2026-1104, 2026.

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The observation of active jets of ice grains and vapour emanating from four warm faults (called Tiger Stripes) at Enceladus’s South Pole was one of the major discoveries of the Cassini-Huygens mission (Porco et al. 2006, Spencer et al. 2006, Waite et al. 2006, Spahn et al. 2006).  Infrared mapping carried out by the Visual and Infrared Imaging Spectrometer (VIMS) on board the Cassini spacecraft provided information on surface composition, but also on the physical state (grain size and degree of crystallinity) near active faults (Brown et al. 2006, Jaumann et al., 2008; Taffin et al., 2012; Filacchione et al., 2016, Combes et al. 2018, Robidel et al. 2020). However, many spectral characteristics were not fully exploited by previous studies. Here, by acquiring laboratory infrared spectra of ice powder analogues, we identified several salt compounds, and potentially CO2 clathrate, at the surface, and with a higher concentration along active faults. Our analysis shows that the spectral signatures in the inter-stripe regions are consistent with fresh, cold, fine-grained ice deposits, while ice near the tiger stripes has been thermally processed. The higher concentration of salts observed along the tiger stripes, as well as the main spectral features of water ice, imply significant sublimation and sintering processes in the vicinity of active jet sources. These new results provide essential constraints for identifying the best landing site for a future mission to Enceladus and for anticipating the mechanical properties of the icy regolith.

How to cite: Tobie, G., Scordia, L., Munoz-Iglesias, V., Seignovert, B., Le Menn, E., Mode, N., Munoz-Granados, D., Closson, S., Herny, C., Artoni, R., Bollengier, O., Choblet, G., Le Mouélic, S., and Sotin, C.: New constraints on the composition and physical properties of the icy surface on Enceladus’ South Polar Terrain, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-878, https://doi.org/10.5194/epsc2026-878, 2026.

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Accurately characterizing the endogenic thermal signatures of icy moons requires a detailed understanding of surface heating driven by insolation, surface properties and local terrain. Due to the lack of high-resolution thermal observations, Europa’s surface temperatures have historically been sufficiently modelled using smooth surface or one-dimensional approximations [1]. However, Europa’s complex terrains alter local illumination conditions and surface temperatures. Here, we use TEMPEST, an open-source, python based thermophysical model [2, 3] to investigate the significance of radiative self-heating and light scattering on interpretation of Europa’s thermal IR emission, with a particular focus on detectability of near-surface liquid-water reservoirs by high spatial resolution (≤100 m/pixel) observations soon to be taken by Europa Clipper’s E-THEMIS instrument [4].

Passive heating by solar illumination was modelled under Europa-like conditions using TEMPEST, which solves a surface energy balance that includes solar flux, thermal emission, vertical heat conduction, and (optionally) scattering, radiative self-heating and macroscopic surface roughness. The 1D periodic conduction solver at its core is based on thermprojrs [5]. Comparing a smooth sphere (fig. 1a, c) to the topographic digital elevation model (DEM) (fig. 1b, d) reveals that while mean temperatures remain comparable, topography introduces substantial heterogeneity. For example, the DEM has a wider distribution of surface temperatures due to local slope and shadowing effects. Thermal radiance scales with Temperature4 (Stefan-Boltzmann law), so the small minority of hotter regions can dominate the thermal IR signal (fig. 2). We show that rough terrain emits significantly higher total radiance than a smooth sphere despite the similar mean kinetic temperature.

Simulated topographic temperature maps show that light scattering and radiative exchange with local terrain cause increased surface temperature heterogeneity, with local temperature increases in deep fractures as high as 35 K (fig. 3). Temperature heterogeneity can systematically bias mean temperature measurements, and localised mutual radiative heating can lead to apparently anomalous warm regions, potentially mimicking endogenic heating, or obscuring indications of conductive heating from near-surface water reservoirs. To place these effects in the context of E-THEMIS detectability, we will use first-order conductive heat-flux calculations to estimate the surface expressions of idealised subsurface liquid-water reservoirs at different depths and spatial scales. These calculations will provide test cases for distinguishing plausible endogenic thermal anomalies from topographically induced radiance variations. This work shows that if topography is ignored, the excess radiance caused by it could be misinterpreted as a region of lower thermal inertia (in daytime), or even an endogenic heat source (hotspot). Accurately modelling terrain reduces the risk of false positives when searching for plume sources or active regions.

Figure 1: Surface temperature maps for a section of an icy moon modelled as a smooth sphere (a) and using a digital elevation model (DEM) of Enceladus (b) using Europa thermal parameters. Histograms (c) and (d) show the distribution of surface temperatures within the above terrain samples.   

Figure 2: The integrated surface radiance from each terrain sample. The T4 dependence of radiance leads to significantly higher emission from the DEM terrain.

Figure 3: Simulated Europa surface temperatures without (a) and with (b) multiple scattering and radiative self-heating. Panel (c) shows the resulting temperature difference, with local increases up to 35 K in shadowed terrain. Enceladus DEM [6] provides an icy-terrain analogue for topographic heating effects.  Future work will repeat this analysis using Europa-specific topography where available.

References:

[1] Rathbun, J. A., Rodriguez, N. J., & Spencer, J. R. (2010). Galileo PPR observations of Europa: Hotspot detection limits and surface thermal properties. Icarus, 210(2), 763-769.

[2] Lyster, D., Howett, C., & Penn, J. (2025). TEMPEST: A Modular Thermophysical Model for Airless Bodies with Support for Surface Roughness and Non-Periodic Heating. EPSC-DPS 2025, 1479.

[3] Chivers, C.J., Hayne, P.O. and Schmidt, B.E. (2025). Prospects For Detecting Shallow Liquid Water Bodies At Europa Using E-Themis. LPSC 2025, 1226.

[4] Christensen, P.R., Spencer, J.R., Mehall, G.L., Patel, M., Anwar, S., Brick, M., Bowles, H., Farkas, Z., Fisher, T., Gjellum, D. and Holmes, A. (2024). The Europa thermal emission imaging system (E-THEMIS) investigation for the Europa clipper mission. Space Science Reviews, 220(4), 38.

[5] Spencer, J.R., Lebofsky, L.A., and Sykes, M.V. (1989). Systematic biases in radiometric diameter determinations. Icarus, 78(2), 337-354. 

[6] Park, R.S., Mastrodemos, N., Jacobson, R.A., Berne, A., Vaughan, A.T., Hemingway, D.J., Leonard, E.J., Castillo-Rogez, J.C., Cockell, C.S., Keane, J.T. and Konopliv, A.S. (2024). The global shape, gravity field, and libration of Enceladus. Journal of Geophysical Research: Planets, 129(1), e2023JE008054.

How to cite: Lyster, D. and Howett, C.: Thermophysical Modelling of Europa’s Surface: Influence of Topography on E-THEMIS Sensitivity to Localised Endogenic Heating, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-341, https://doi.org/10.5194/epsc2026-341, 2026.

EPSC2026-341

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Introduction. The icy satellites of Jupiter preserve a record of the physical and chemical interactions occurring between internal geologic activity, surface evolution, and the intense radiative environment imposed by Jupiter’s magnetosphere (e.g., [1, 2]). Understanding the composition and physical state of their surfaces is therefore essential not only to reconstruct their geological history, but also to investigate the coupling processes linking surface, subsurface, exosphere and the Jovian magnetosphere. In preparation for the arrival of the JUICE and Europa Clipper missions in the Jovian system, we present a spectroscopic analysis of Ganymede’s surface based on the merging of multiple visible-to-infrared hyperspectral datasets acquired from both ground-based observatories (VLT/MUSE, SPHERE, SINFONI) and space instrumentation (JWST/NIRSpec). By combining these datasets, we produce reflectance spectra spanning 0.5–5.3 µm over representative regions of Ganymede’s surface, enabling the simultaneous investigation of visible spectral slopes, water-ice absorption bands, Fresnel peaks, and infrared signatures of non-icy compounds.

Data and Methods. The observations were projected and photometrically corrected using an Oren–Nayar rough Lambertian model [3] before cross-calibration and spectral merging. The resulting dataset covers approximately 13% of Ganymede’s surface (both leading and trailing) at spatial resolutions ranging from 25 to 300 km/px. Spectral cubes are finally combined to the lowest resolution equivalent pixel to maintain consistency across the entire spectrum.

Spectral features. Spectral parameter maps reveal strong correlations between absorption features distributed across different wavelength ranges, demonstrating the scientific value of broadband spectral analyses. In particular, the combined analysis of the 1.03, 1.3, 1.5, 2, and 3 µm H₂O bands, together with the 1.65 µm crystalline-ice feature and the Fresnel peaks near 3.1 µm, reveals major spatial variations in ice grain size and crystallinity across the surface.

Our results indicate that coarse-grained crystalline ice dominates the ancient grooved and dark terrains, whereas fine-grained ice is preferentially associated with recent crater ejecta and polar regions. These trends likely reflect the competition between thermal recrystallization and irradiation-driven amorphization caused by charged particles trapped within Jupiter’s magnetosphere. The weakening or disappearance of the 1.65 µm band and Fresnel peaks in dark trailing-hemisphere terrains further suggests enhanced radiation damage and accumulation of amorphous ice in these regions. Additional spectral features associated with radiolytic species such as H₂O₂ and O₂ also display strong spatial correlations with the geometry of Ganymede’s magnetic field and plasma bombardment patterns. Those results are in line with previous studies on individual datasets (e.g., [4-7]).

Spectral modeling. To further constrain surface composition, we performed spectral modeling using three complementary approaches: areal (linear), intimate (nonlinear Hapke-based), and multiscale mixtures (an original and hybrid approach between areal and intimate mixtures). The combined spectral range allows simultaneous modeling of grain-size effects, continuum variations, water-ice crystallinity, and non-icy contaminants (Fig. 1). All models require mixtures dominated by water ice associated with sulfur-bearing phases, including sulfuric acid hydrates and polyhydrated magnesium sulfates. Older terrains appear enriched in Mg-sulfates, possibly linked to endogenous material, whereas younger and more irradiated terrains are preferentially enriched in sulfuric acid hydrates produced by radiolytic processing. Chloride salts remain poorly constrained in all models and are detected only at low abundance.

Lessons Learned. One of the most important outcomes of this study concerns the darkening agent responsible for the low albedo of Ganymede’s ancient terrains. Previous studies relying on linear mixing models suggested that dark material could represent a major fraction of the surface [6]. Our results show instead that intimate and multiscale mixtures reproduce the observations with much lower abundances of dark material (typically 1–10%), demonstrating that the inferred abundance strongly depends on the adopted radiative transfer formalism and grain-scale physics. This result has major implications for the interpretation of spectral observations of icy worlds, as it highlights the degeneracy between composition, grain size, and mixing geometry.

Beyond Ganymede itself, this work demonstrates the scientific potential of broadband visible-to-infrared spectral fusion for the exploration of icy satellites throughout the Jovian system to better constrain the physical processes that are expected to occur on the icy Moons under different irradiation and geological conditions. In particular, the ability to combine spectral diagnostics across the 0.5–5.5 µm range will be essential to disentangle the effects of composition and texture, identify minor compounds, and characterize the coupling between surface composition, geological activity, and exogenic processing on the Galilean moons. The methodology developed in this study also provides a direct framework for the future interpretation of MAJIS/JUICE and MISE/Europa Clipper observations [8,9].

Finally, our study highlights a critical limitation for future spectroscopic investigations: the lack of optical constants measured over extended spectral ranges and under cryogenic conditions representative of icy moon surfaces. The interpretation of forthcoming orbital datasets will therefore require a coordinated effort in laboratory spectroscopy to expand spectral libraries for hydrated salts, sulfur-bearing species, irradiated compounds, amorphous water ice, silicates, and candidate darkening agents. Such efforts will be essential to fully exploit the unprecedented spectral and spatial capabilities of JUICE and Europa Clipper and to achieve a comparative understanding of the icy worlds of the Jovian system.

References: [1] Pappalardo et al., 1998. Nature; [2] Schenk et al., 2001, Nature; [3] Oren & Nayar, 1995, Int J Comput Vis; [4] King et al., 2025, JGR: Planets; [5] King & Fletcher., 2022, JGR: Planets; [6] Ligier et al., 2019, Icarus; [7] Bockelée-Morvan et al., 2024, A&A; [8] Poulet et al., 2024, SSR; [9] Blainey et al., 2025, SSR

How to cite: Royer, C., Poulet, F., King, O., Bockelée-Morvan, D., and Fletcher, L.: Ganymede’s Surface Composition from 0.5 to 5.3 µm using combined VLT and JWSTdatasets and the resulting lessons learned, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-768, https://doi.org/10.5194/epsc2026-768, 2026.

EPSC2026-768

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Observations with Juno’s Microwave Radiometer (MWR) provided new constraints on the depth and subsurface structure of Europa’s ice shell.  The observed temperature gradient constrains the thickness of the thermally conductive part of the ice shell, and pores or cracks beneath the surface scatter microwaves, enabling characterization of the size and distribution of the scatterers.  Results assuming an ice shell of pure water ice, are consistent with a conductive ice shell thickness of 29±10 km, with negligible surface reflectivity, volume fraction of scatterers 0.045, scale height of scatterers 219 m, and scatterer size distribution power law index -3.96.  The size, depth, and volume fraction of the scatterers suggest they are not a likely source of carrying nutrients between the ocean and the surface.  Ice salinity of 15 mg/kg would reduce the estimate of the thickness by about 5 km.  A thermally convective layer would increase the total ice shell thickness but only slightly decrease the estimate of the conductive layer.  Recent analysis related to the variation of ice shell thickness and subsurface characteristics will be reported along with the more general results described above. 

How to cite: Bolton, S., Levin, S., Zhang, Z., Ermakov, A., Wolfenbarger, N., Akiba, R., Misra, S., McKinnon, B., and Stevenson, D.:  Constraints on the Thickness and Subsurface Characteristics of Europa’s Ice Shell by the Juno Microwave Radiometer, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-619, https://doi.org/10.5194/epsc2026-619, 2026.

EPSC2026-619

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Saturn’s ocean moon Enceladus is among the most compelling targets for future astrobiology-focused missions. Material erupted from the moon’s south polar “Tiger Stripe” fractures is thought to originate from a global subsurface ocean, providing a natural pathway for ocean-derived compounds to be sampled without penetrating the ice shell. Cassini observations demonstrated that Enceladus’ plume contains water vapor, ice grains, salts, organics, phosphates, and other compounds of astrobiological interest. Consequently, future landed or plume-sampling missions to Enceladus will seek to determine whether ocean-sourced organic molecules, and potentially biosignatures, can be preserved and detected in plume material.

However, once plume grains are erupted and deposited onto the surface, they are exposed to multiple space weathering agents. Solar ultraviolet radiation, Saturnian magnetospheric charged particles, and galactic cosmic rays can alter or destroy organic molecules contained within surface material. The degree of processing depends not only on radiation environment and molecular susceptibility, but also on the rate at which fresh plume fallout buries previously deposited material. Identifying locations and depths where organics may remain minimally processed is therefore key for mission planning and interpretation of future measurements.

Here we model the degradation and preservation of plume-deposited organics on Enceladus, using the amino acids glycine and phenylalanine as representative organic molecules and potential biosignature proxies. We combine solar UV photodestruction calculations, Geant4-based charged-particle irradiation simulations, and estimates of local plume deposition rates to produce a coupled model of irradiation and burial across the surface. Solar UV photolysis rates are calculated using experimental rate coefficients adjusted for seasonal variation in photon flux at selected locations on Enceladus, including equatorial and southern-hemisphere sites at leading and trailing longitudes. Because even small abundances of non-water-ice contaminants can strongly influence UV attenuation, we also examine the effect of trace tholin-like material mixed into the ice. Charged-particle processing is estimated using Cassini-derived magnetospheric electron spectra and a numerical transport model for the galactic cosmic ray flux at Saturn.

Our results show that preservation of plume-deposited organics is highly sensitive to burial rate and season. At low to moderate surface deposition rates, less than approximately 0.002 mm yr⁻¹, newly deposited surface material is strongly processed by solar UV radiation before burial can shield it from further exposure. In such regions, there may be no practical “safe sampling depth” at which pristine or minimally processed organics can be accessed, because material has already been altered before reaching depth. By contrast, in regions receiving the highest predicted plume deposition rates, especially during local winter when solar UV exposure is reduced, burial can occur rapidly enough to protect a significant fraction of the organic inventory. Under these conditions, the near-surface stratigraphy may consist of millimeter-scale layers that alternate between relatively well-preserved and heavily photoprocessed material, reflecting seasonal variations in irradiation during emplacement.

Our findings have implications for the mission design and sampling strategy of future Enceladus missions. A landed mission seeking minimally processed ocean-derived organics should prioritize regions of high plume fallout, where burial is most effective, and should consider the seasonal context of deposition and sampling. The most favorable strategy may be to collect plume grains directly before surface deposition, e.g., from a funnel on the surface or plume fly-through, thereby avoiding prolonged radiolytic and photolytic alteration. If surface sampling is required, targeting the uppermost approximately 0.1 mm of material in high-deposition regions during local winter may maximize the probability of detecting less-processed organics. More broadly, our results demonstrate that preservation potential at Enceladus is governed by the competition between surface irradiation and plume-driven burial, and that this balance should be incorporated into future landing site selection, sampling-depth requirements, and biosignature-detection strategies.

How to cite: Nordheim, T., Grayson, R., Regoli, L., Fayolle, E., Cable, M., MacKenzie, S., Hand, K., Wiemann, J., Howett, C., Choukroun, M., and Paranicas, C.: Degradation of plume-deposited organics at Enceladus and implications for future surface missions, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-741, https://doi.org/10.5194/epsc2026-741, 2026.

EPSC2026-741

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