Ocean tidal heating in icy satellites with solid shells

Introduction:Multiscale analysis is a powerful tool for investigating the surfaces and interiors of Solar System bodies. It is particularly valuable for studying the icy satellites of the outer planets, which are remote targets whose exploration relies heavily on remote sensing and therefore greatly benefit from complementary analog studies. These satellites are key objectives of current planetary missions: ESA’s JUICE and NASA’s Europa Clipper are both en route to the Jovian system, aiming to investigate their geological activity and potential habitability.Most icy satellites are thought to possess an outer ice shell overlying a subsurface liquid water ocean, sustained primarily by tidal heating, e.g. [1]. The ocean activity caused by tides drives deformation of the overlying brittle ice crust, leading to the formation of extensive networks of kilometer-scale fractures and strike-slip faults, particularly on Europa, Ganymede, and Enceladus [2][3]. Compressional structures are relatively rare [4]. These tectonic features are critically important, as they may serve as conduits between the surface and the internal ocean. Therefore, understanding both brittle and ductile deformation structures is essential for revealing the processes at work within the icy crust and the insights they offer.In this study, we explore how terrestrial glacial environments can serve as analogs for icy satellite tectonics, with a focus on strike-slip fault zones. We present structural data from the Priestley Glacier, Antarctica, where the eastern margin exhibits brittle structures caused by left-lateral strike-slip kinematics. [5] collected an ice core sample from this area, showing a well-developed Crystallographic Preferred Orientation (CPO) of the ice crystals throughout the 58-meter-deep core. This fabric correlates with larger-scale deformation patterns of the sinistral shear. These findings were used as a reference to interpret strike-slip zones on Europa that show macrostructural similarities to the Priestley margin.The aim of this study is to constrain the mechanical behavior and internal structure of strike-slip fault zones on icy satellites, particularly Europa, through multiscale structural analysis of terrestrial glacial analogs. This approach contributes to advancing our understanding of crustal deformation in icy satellites.Data and Methods:We conducted a multiscale analysis by mapping macro-scale features (e.g., fractures and faults) in satellite images of the Priestley Glacier, and micro-scale structures (e.g., subgrain boundaries and slip bands) in microscope images of a sample obtained from the Priestley Glacier ice core at a depth of ~52 m [5]. A strong correlation was found between surface structures and microstructural fabric, indicating the scalability of the deformation system. Both the microstructures and the CPO are consistent with synthetic strike-slip and compressional structures recognized at larger scale, suggesting that a transpressional component is active at the glacier margin depth, which is something not clearly observed at the macro scale.The strong CPO and the existence of subgrain boundaries suggest that deformation at approximately 50 m depth is governed by a ductile regime. These microstructures indicate that internal zones of weakness may exist even in the absence of brittle fracturing. Therefore, this may suggest that the oriented fabric and its associated defects could act as pathways for fluid migration within the ductile ice layer.Europa Application:We applied these insights to Europa, where long bands and ridges, interpreted as strike-slip zones, exhibit comparable structures and morphologies to those observed at Priestley Glacier. Four strike-slip zones were identified as analogs, where regional-scale structural mapping was performed. This comparison allowed us to estimate the depth of the brittle–ductile transition beneath these shear zones on Europa and to infer internal viscosity. Using available values for naturally deformed ice, we scaled viscosity estimates to Europa’s subsurface, providing new constraints on the mechanical behavior of its icy crust.Discussion and Conclusions:Our results help define the location of rheological variations within Europa’s icy crust, particularly across strike-slip zones. On the Priestley Glacier, the occurrence of an ice fabric consistent with transpression suggests that compressional deformation may primarily occur in the deeper, ductile layers rather than in the upper brittle portion. This leads us to make analog inferences for Europa's crust on shear zones. A three-dimensional tectonic model was developed, integrating both micro- and macro-scale data, to illustrate the structural and rheological architecture of Europa’s icy crust.This approach improves our understanding of icy satellite evolution, from surface features observed via remote sensing to deep crustal deformation. The findings support future scientific planning for targeted high-resolution imaging by the cameras of JUICE (JANUS [6]) and Europa Clipper ( EIS [7]) missions. Acknowledgements: This work is part of the DISCOVERIES project funded by INAF Mini Grants RSN3. The activity has been realized under the ASI-INAF contract 2023-6-HH.0.

Volcanic activity at Europa’s seafloor is one of the key questions regarding the habitability of its subsurface ocean. The suitable conditions for hydrothermalism on Europa’s seafloor are conditioned by the heat released from the underlying silicate mantle, supplied by both radiogenic and tidal heating. The orbital resonance between Io, Europa, and Ganymede forces their orbit and maintains non-zero eccentricities. Tidal heating due to eccentricity tides on Io is so extreme that it  produces intense volcanism.  Even though tidal heating  in Europa’s silicate mantle is expected to be much weaker than on Io due to its greater distance to Jupiter,   Běhounková et al. [1] showed that it may still be sufficient to maintain Europa’s mantle in a partially molten state  for several tens to hundred millions of years, particularly during periods of increased eccentricity. Due to inefficient melt transport through the thick lithosphere of Europa [2], melt produced during periods of enhanced eccentricity may accumulate and in turn affect the tidal heating, as it is the case for Io, implying a possible runaway melt process in the silicate interior of Europa. In this context, the goal of this study is to evaluate the effect of melt accumulation on the tidal heat production of Europa’s silicate mantle. For that purpose, we follow the approach developed to model the solid tides in Io’s partially molten interior [3], taking into account the effect of melt on the viscoelastic properties of the mantle. We adapt it to the context of Europa, corresponding to a deeper and thinner asthenosphere than on Io. We show that, whatever the partially molten layer thickness, melt accumulation increases tidal heat production and tidal dissipation even exceeds radiogenic heating. For equivalent volume of accumulated melt, the thinner the layer, the more pronounced this effect is. Our results show that the accumulation of melt, over timescales consistent with the 3D model prediction of Běhounková et al. [1], may significantly affect the tidal dissipation amplitude and its pattern. The potential presence of such melt accumulations may be tested by future measurements by Europa Clipper and JUICE from the combined analysis of gravimetric, altimetric and magnetic data, which might reveal long-wavelength anomalies which could be confronted to our model prediction. [1] Běhounková et al., GRL, 48(3), e2020GL090077 (2021),  [2] Bland and Elder, GRL, 49(5), e2021GL096939 (2022). [3] Kervazo et al., A&A, 650, A72 (2021) 

Convection driven by tidal and radiogenic heating in medium size icy satellites

Enceladus's crust as a non-uniform thin shell: II tidal dissipation

The goal of NASA’s Europa Clipper Mission is to investigate the habitability of the subsurface ocean within the Jovian moon Europa using a suite of ten investigations. The Europa Clipper Magnetometer (ECM) and Plasma Instrument for Magnetic Sounding (PIMS) investigations will be used in unison to characterize the thickness and electrical conductivity of Europa’s subsurface ocean and the thickness of the ice shell by sensing the induced magnetic field, driven by the strong time-varying magnetic field of the Jovian environment. However, these measurements will be obscured by the magnetic field originating from the Europa Clipper spacecraft. In this work, a magnetic field model of the Europa Clipper spacecraft is presented, characterized with over 260 individual magnetic sources comprising various ferromagnetic and soft-magnetic materials, compensation magnets, solenoids, and dynamic electrical currents flowing within the spacecraft. This model is used to evaluate the magnetic field at arbitrary points around the spacecraft, notably at the locations of the three fluxgate magnetometer sensors and four Faraday cups which make up ECM and PIMS, respectively. The model is also used to evaluate the magnetic field uncertainty at these locations via a Monte Carlo approach. Furthermore, both linear and non-linear gradiometry fitting methods are presented to demonstrate the ability to reliably disentangle the spacecraft field from the ambient using an array of three fluxgate magnetometer sensors mounted along an 8.5-meter (m) long boom. The method is also shown to be useful for optimizing the locations of the magnetometer sensors along the boom. Finally, we illustrate how the model can be used to visualize the magnetic field lines of the spacecraft, thus providing very insightful information for each investigation.

Tidal heating in icy satellite oceans

This article consists of three sections. The first discusses how we determine satellite internal structures and what we know about them. The primary probes of internal structure are measurements of magnetic induction, gravity, and topography, as well as rotation state and orientation. Enceladus, Europa, Ganymede, Callisto, Titan, and (perhaps) Pluto all have subsurface oceans; Callisto and Titan may be only incompletely differentiated. The second section describes dynamical processes that affect satellite interiors and surfaces: tidal and radioactive heating, flexure and relaxation, convection, cryovolcanism, true polar wander, non-synchronous rotation, orbital evolution, and impacts. The final section discusses how the satellites formed and evolved. Ancient tidal heating episodes and subsequent refreezing of a subsurface ocean are the likeliest explanation for the deformation observed at Ganymede, Tethys, Dione, Rhea, Miranda, Ariel, and Titania. The high heat output of Enceladus is a consequence of Saturn’s highly dissipative interior, but the dissipation rate is strongly frequency-dependent and does not necessarily imply that Saturn’s moons are young. Major remaining questions include the origins of Titan’s atmosphere and high eccentricity, the regular density progression in the Galilean satellites, and the orbital evolution of the Saturnian and Uranian moons.

The ESA Voyage 2050 Senior Committee in 2021 recommended a mission to the “Moons of the Giant Planets” as one of the upcoming ESA Large missions. The scientific goals for this mission include exploring the habitability of ocean worlds, searching for biosignatures, and studying the connection between moon interiors and near-surface environments, as well as their implications for the overall moon-planet system. This theme follows the breakthrough science from the NASA-ESA Cassini-Huygens mission and the expected scientific return from the ESA JUICE mission in Cosmic Vision. The mission to the “Moons of the Giant Planets” will be ESA’s fourth Large-class mission – L4. Following the recommendation by the Voyage 2050 Senior Committee, ESA convened an Expert Committee [1] to define the scientific scope and target for this mission, considering the characteristics of each moon and future planned missions to Jupiter and Saturn's ocean worlds. Scientists identified Saturn’s moon Enceladus as the most compelling target, followed by Saturn’s moon Titan and Jupiter’s moon Europa. Figure 1: Summary of the Scientific objectives versus their relevance and whether they will be addressed by planned missions (JUICE, Europa Clipper, and Dragonfly) for each target (Europa, Ganymede, Enceladus, and Titan). Dark blue marked areas indicate most relevant objectives that will not be addressed by other forthcoming missions and are thus of highest interest for the L4 mission (from [1]). The continued exploration of icy moons in the outer solar system suggests that subsurface oceans are common, although these oceans are hidden under thick ice shells. Their habitability can only be assessed indirectly. However, this changed with the Cassini mission in July 2005, which definitively detected water vapor plumes with jets of ice particles erupting from Enceladus during its third flyby. This was further supported by Magnetometer data returned at Cassini's first flyby in February 2005​[1], [2], [3], [4], [5]​. At its equator, which receives the most direct sunlight, surface temperatures on Enceladus average minus 193 degrees Celsius. The south pole, despite receiving less sunlight, was slightly warmer than the equator, at minus 188 degrees Celsius. The linear fractures, referred to as "tiger stripes," were as warm as minus 163 degrees Celsius in some areas, indicating active geological processes (​[2], [6], [7], [8], [9]​. These observations demonstrate that Enceladus is an active world. More importantly, it implies that Enceladus provides the opportunity to measure ocean water “in-situ” and directly search for biosignatures. Cassini demonstrated this capability by measuring the plume composition while passing through the plumes, despite the spacecraft and instruments not being designed for such measurements. It is now generally accepted that Enceladus has the three necessary conditions for a habitable environment capable of supporting life: the presence of liquid water, a source of energy, and a specific set of chemical elements ​[10], [11]​. The L4 mission will build on this legacy with a mission design and payload complement focused on exploring the habitability of Enceladus and searching for biosignatures. The mission will take a significant step forward by not only placing an orbiter around Enceladus but also deploying a lander on the moon's south pole area. No space agency has previously landed on Enceladus, offering substantial potential for new scientific discoveries, particularly concerning habitability. Landing on Enceladus presents unique challenges as its environment is both familiar and alien: The moon’s surface is coated with fine, icy particles originating from active cryovolcanic plumes—effectively “snow” precipitating from the subsurface ocean beneath the ice shell. These particles may continuously fall onto the landing site and the lander [12], [13], potentially delivering material rich in salts, organics, and even biosignatures directly into the scientific instruments Since March 2025, the ESA study team has been working with a newly selected Payload Working Group and the Expert Committee to refine the science requirements and identify key technologies to achieve the ambitious goals of ESA’s Mission to Enceladus [15]. This new mission will advance European expertise in several scientific and technological fields, including in-orbit assembly, operating in extreme environments, landing technologies, and novel scientific instrumentation. These revolutionary technologies will have wide-ranging applications beyond ESA’s space science programme.

Tidal dissipation is a major heat source for the icy satellites of the giant planets. Several icy satellites likely possess a subsurface ocean underneath an ice shell. Previous studies of tidal dissipation on icy satellites, however, have either assumed a static ocean or ignored the effect of the ice lid on subsurface ocean dynamics. In this study, we examine inertial effects on tidal deformation of satellites with a dynamic ocean overlain by an ice lid based on viscoelasto‐gravitational theory. Although ocean dynamics is treated in a simplified fashion, we find a resonant configuration when the phase velocity of ocean gravity waves is similar to that of the tidal bulge. This condition is achieved when a subsurface ocean is thin (<1 km). The enhanced deformation (increased h2 and k2 Love numbers) near the resonant configuration would lead to enhanced tidal heating in the solid lid. A static ocean formulation gives an accurate result only if the ocean thickness is much larger than the resonant thickness. The resonant configuration strongly depends on the properties of the shell, demonstrating the importance of the presence of a shell on tidal dissipation.

Basic shallowing‐upward cycles (shu‐cycles) and five‐bundled megacycles in the Latemar platform have been widely regarded as a model example for precessional and eccentricity forcing in the Mesozoic. Based on bio‐/chronostratigraphic data, recent studies questioned this particular type of Milankovitch forcing on the Latemar cyclic series. We present an integrated model that incorporates (i) new cyclostratigraphic data, (ii) new and existing bio‐/chronostratigraphic data and (iii) new spectral analyses. The basic shu‐cycles in the Latemar reflect sub‐Milankovitch control. Cycle bundles of 1 : 4–5 (megacycles : shu‐cycles) indicate precessional forcing. They do not reflect eccentricity superimposed on precessional forcing. Spectral analyses reveal highly significant ratios in the large‐scale cycle bundlings. Stacking patterns of 1 : 9.9 and 1 : 24.0 shu‐cycles represent obliquity and short eccentricity forcing. Both sub‐Milankovitch and Milankovitch forcing potentially controlled shallow subtidal carbonate deposition in Mesozoic greenhouse times. Cyclostratigraphic models require an integrated approach including bio‐ and chronostratigraphic data.

Does Titan’s long-wavelength topography contain information about subsurface ocean dynamics?

The icy satellites of the giant planets Jupiter and Saturn are among the most interesting celestial bodies in our Solar System. The interpretation of various remote sensing observations performed by the Voyager, Galileo and Cassini-Huygens missions strongly suggests that many icy satellites harbor a subsurface water ocean underneath the ice shell covering the satellites. Since the availability of liquid water is one of the prerequisites for the origin and evolution of life as we know it, the characterization of the physical properties of the putative subsurface oceans (and the overlying ice shells) has become a key research topic in planetary sciences. Due to the unavailability of direct means to explore these internal oceans, the relevant physical properties of the interior need to be derived from the combined interpretation of remote sensing measurements of observables such as the strength of the induced magnetic field, the amplitude of tidal deformations, the strength of gravity perturbations due to tides and the amplitude of forced longitudinal librations. This dissertation concentrates on the development of a unified and self-consistent physical model to determine the tidal and rotational response of a viscoelastic icy satellite with an internal water ocean. The developed tidal-rotational model is then applied to analyze the relation between the aforementioned observables (with the exception of the induced magnetic field) and the physical properties that characterize the internal structure of an icy satellite. The results in this thesis strongly suggest that the measurement of the libration amplitude of Europa’s shell with an accuracy of a few meters has the potential to provide a reasonable constraint on the rigidity of the ice-I shell in combination with measurements of the tidal response (Love numbers) at the surface. However, the modelling presented here is not extensive enough to support a strong conclusion regarding whether the ice shell thickness could be inferred from combined measurements of the libration amplitude and tidal Love numbers at the surface of Europa.

Large-scale dynamics and the fate of salts in Europa's icy shell

Tides on Europa: The membrane paradigm

Hydraulic head fluctuations in an intermediate depth coastal surface aquifer