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Cassini observations imply a global ocean underneath Enceladus' ice shell, with hydrothermal seafloor activity. Previous numerical simulations showed that convection in Enceladus' unconsolidated core produces heterogeneous seafloor heat flux explaining the South Pole ice thinning and plume activity. While there is evidence for efficient hydrothermal transport from the seafloor to the plumes, the ocean dynamics is strongly debated. Here, we perform 3-D numerical simulations of the ocean with a very heterogeneous bottom boundary condition from 3-D hydrothermal core simulations. We show that a strong zonal flow inhibits low-latitudes heat transfer while it remains efficient in polar regions, which explains the ice shell variations derived from gravity and topography observations. Using passive tracers, we predict rising times of hours to weeks, compatible with previous predictions. Our simulations confirm that a strong heterogeneous seafloor heat flux concentrates upwellings at the South Pole, efficiently transporting potential bio-signatures from hydrothermal vents to erupting plumes.

International audience; Enceladus is the first planetary object for which direct sampling of a subsurface water reservoir, likely habitable, has been performed. Over a decade of flybys and seven flythroughs of its watery plume, the Cassini spacecraft determined that Enceladus possesses all the ingredients for life. The existence of active eruptions blasting fresh water into space, makes Enceladus the easiest target in the search for life elsewhere in the Solar System. Flying again through the plume with more advanced instruments, landing at the surface near active sources and collecting a sample for return to Earth are the natural next steps for assessing whether life emerges in this active world. Characterizing this habitable world also requires detailed mapping and monitoring of its tidally-induced activity, from the orbit as well as from the surface using complementary platforms. Such ambitious goals may be achieved in the future in the framework of ESA large or medium-class missions in partnership with other international agencies, in the same spirit of the successful Cassini-Huygens mission. For all these reasons, exploring habitable ocean worlds, with Enceladus as a primary target, should be a priority topic of the ESA Voyage 2050 programme.

We present a White Paper with a science theme concept of ocean world evolution and habitability proposed in response to ESA’s Voyage 2050 Call with a focus on Titan and Enceladus in the Saturn system. Ocean worlds in the outer Solar System that possess subsurface liquid water oceans are considered to be prime targets for extra-terrestrial life and offer windows into Solar System evolution and habitability. The Cassini-Huygens mission to the Saturn system (2004–2017) revealed Titan with its organic-rich evolving world with terrestrial features and Enceladus with its active aqueous environment to be ideal candidates to investigate ocean world evolution and habitability. Additionally, this White Paper presents a baseline for a multiple flyby mission with a focused payload as an example of how ocean world evolution and habitability in the Saturn system could be investigated building on the heritage of the Cassini-Huygens mission and complementing the recently selected NASA Dragonfly mission.

Of all the myriad environments in our Solar System, the least explored are the distant Ice Giants Uranus and Neptune, and their diverse satellite and ring systems. These ‘intermediate-sized’ worlds are the last remaining class of Solar System planet to be characterised by a dedicated robotic mission, and may shape the paradigm for the most common outcome of planetary formation throughout our galaxy. In response to the 2019 European Space Agency call for scientific themes in the 2030s and 2040s (known as Voyage 2050), we advocated that an international partnership mission to explore an Ice Giant should be a cornerstone of ESA’s science planning in the coming decade, targeting launch opportunities in the early 2030s. This article summarises the inter-disciplinary science opportunities presented in that White Paper [1], and briefly describes developments since 2019.

JUICE is the first large mission chosen in the framework of ESA’s Cosmic Vision 2015-2025 program. The focus of JUICE is to characterize the conditions that might have led to the emergence of habitable environments among the Jovian icy satellites, in particular Europa, Callisto and Ganymede. In addition, JUICE will also perform a multidisciplinary investigation of the Jupiter system as an archetype for gas giants. To address those key science objectives, the spacecraft payload consists of 10 state-of-the-art instruments (and one experiment that use the spacecraft telecommunication system with ground-based instruments) that will perform remote and in-situ measurements of Jupiter, its moon and their environment. From a trajectory’s point of view, the mission calls for a three-year orbital survey of the Jupiter System designed as a sequence of 67 orbits around the planet of different periods and inclinations, and includes 35 flybys of Europa (2), Ganymede (12) and Callisto (21). The second part of the mission will devote an additional 9 months in orbit around Ganymede for an in-depth characterization of Ganymede as a planetary object and possible habitat. By its touring nature and due to the diversity of science targets and different disciplines involved in the mission, the development of an observation plan will often face conflicting observations opportunities, also limited in resources as a consequence of the large helio- and geo-centric distances at play. The Science Operations Center, together with the Project Scientist and the Science Working Team, are in charge of setting up and developing a science planning strategy for the mission to ensure that the prime science objectives of the mission are addressed while remaining compatible with the mission constraints. The JUICE Science Planning Process for the Jupiter tour consists of two main pillars: Segmentation of the trajectory. Based on the Cassini heritage, this activity relies on the analysis of all science opportunities along the tour to subsequently support the decision on selecting a prime scientific objective that will get priority in terms of resources (data volume, power and pointing) for a specific time window (a.k.a segment). Taking into account known operational activities’ constraints (navigation, wheel off-loading etc) and high-level resources’ estimate, an analysis of the expected scientific coverage and resource status along the tour is performed, and several iterations between the different disciplines experts (through the science working groups) take place to converge towards a plan that should ensure that prime science objectives of the mission are fulfilled within the operational constraints of the mission. Detailed sizing case analysis for science operations: For specific operationally of scientifically challenging sections of the segmentation, in-depth analysis is needed in order to refine resources assumptions (attitude, power and data volume) made at segmentation level. Those detailed analysis, performed down to observation level, is done in collaboration with the instrument teams. The present work will describe the different steps and groups involved in the science planning, the current status of the segmentation, and the detailed analysis of the first JUICE Europa flybys will be presented.

The moon-coverage tool is a Python package currently used by ESA-SOC for the JUICE mission to plan and visualize Jupiter’s moons surface coverage. It supports instrument footprint projections and region of interest intersections. Introduction: One of the key elements to assess the quality of an observation plan is to visualize its spatial coverage at different scales (global, regional and local). All the planetary missions, past, current and future use JPL/NAIF SPICE kernels to describe the position of the spacecraft and its orientation in space [1]. These files are usually produced by space agencies and require advanced users to interpret them to know when and how a given surface feature is visible on a planetary body. A planning tool in Python: To simplify the identification of these opportunities, we developed the moon-coverage, a Python package built on top of spiceypy [2] that provides an object-oriented approach to perform spacecraft trajectory computations (Fig. 1), instrument field of view projections (Fig. 2) and region of interest intersections (Fig. 3). Originally developed for the ESA-JUICE mission, the tool can now handle any space mission (Europa-Clipper, BepiColombo, EnVision, Juno…). Figure 1: JUICE temporal sequence of Europa flyby 7E1 (July 2nd 2032 with crema 5.0b23.1). The top panel represents the altitude of the spacecraft and the bottom the local incident angle. The orange color corresponds to the segment when the local incidence is lower than 90° (day side). Figure 2: JUICE/MAJIS IR slit footprint during 7E1 flyby below 5,000 km. The trajectory is color-coded as a function of altitude and pixel scale. Figure 3: ESA/JUICE (red) and NASA/Europa Clipper (cyan) cumulative flybys over Callisto. The groundtrack below 750 km altitude are represented as solid lines. The large rectangles correspond to known regions of interest and are highlighted when the spacecraft groundtrack is intersecting them [3]. Resources: The source-code of the moon-coverage is publicly available on the JUICE Gitlab and distributed under open-source BSD license (https://moon-coverage.univ-nantes.fr). It is continuously tested on Python 3.8+ version and regularly deployed on PyPI. An extensive documentation is also available online with many examples than can be reproduced in Jupyter environments locally or ESA DataLabs. Future developments: Currently the spacecraft/instrument pointing is based on the default camera kernels (ck). In a future release, we will allow the user to provide a Planning Timeline Request file (PTR) to perform custom adjustment of the attitude. We will also support planetary projections to display the result in polar/orthographic/sinusoidal views. Acknowledgments: The moon-coverage is under active development at LPG (CNRS) and funded by ESA under 4000127262/19/ES/CM contract. References: [1] Acton (1996) PSS, [2] Annex et al. (2020) JOSS, [3] Stephan et al. (2021) PSS

The high number of discovered close-in planets motivates the improvement of tidal modeling.Among the five thousand exoplanets discovered up to now, half of them have an orbital period of less than 10 days and therefore experience some form of tidal evolution. Besides, the habitable zone of ultra-cool dwarfs is located close-in (e.g. TRAPPIST-1, Gillon et al. 2017), so planets in the habitable zone of these dwarfs are expected to undergo tidal evolution. By impacting the orbital and rotational dynamics of close-in planets, tides have an impact on their habitability. Indeed, tides drive the spin evolution of planets, influence their thermal state by internal friction (Henning & Hurford 2014, Bolmont et al. 2020), and drive their orbital evolution by exchange of angular momentum. This leads to the migration of the planets, their eccentricity and obliquity damping, and the precession of their orbits ( e.g Hut 1981, Bolmont et al. 2011, 2012, Ragozzine & Wolf 2009). Solid planetary tides also act to synchronize the spin of planets on circular orbits with their orbital motion. It is generally assumed that close-in planets (i.e. experiencing strong tidal interactions) have reached this state of synchronization, also called the 1:1 spin-orbit resonance (hereafter SOR). However, in the case of an eccentric or inclined orbit, tides can trap the spin in higher rotation rates, e.g. in the 3:2 or the 2:1 SOR. If a planet has an atmosphere, another tidal mechanism should be taken into account: the thermal tide, which is caused by the differential heating between day and night sides (Correia & Laskar 2001, Auclair-Desrotour 2017a). In the case of thick atmospheres such as that of Venus, the thermal tides acting on the atmosphere can be as strong as the gravitational tides acting on the solid core (Auclair-Desrotour et al. 2017b) and can both de-synchronize the planet and increase the spin inclination. The competition between the two tides, solid and thermal can be seen in figure [1]. Figure [1]: The two tidal contributions, gravitational and thermal, acting in opposition to each other. The solide tide drives the spin to synchronization, while the thermal tide drives it away from it. Tidal model To understand the evolution of exoplanets, we need a consistent tidal model which encapsulates the complex response of rocky planets to stress. Most models assume that planets are made of weakly viscous fluid (e.g. Hut 1981, Goldreich & Soter 1966) even for rocky planets. However, it has been shown that they do not reproduce the correct behavior for highly viscous solid bodies (Henning et al. 2009; Efroimsky & Makarov 2013). In contrast, we use here a model which accounts for more realistic anelastic behavior for the rocks (such as the Andrade rheology) for a Venus-like planet (Dumoulin et al. 2017). We choose here an approach developed by Kaula (1964), which consists in using a decomposition of the tidal potential into Fourier harmonic modes. For the thermal tide, we use the analytical formulation of Leconte et al (2015), which aims at reproducing Venus today. Our developments were added to the ESPEM code (Benbakoura et al. 2019) to compute the orbital and rotational evolution of a Venus-like planet around a Sun-like star. 2) A complex rotational evolution: Figure [2] shows the evolution of a Venus-like planet, with and without an atmosphere, orbiting a Sun-like planet with an initial obliquity of 50 degrees and a rotation of about 100 days and on a circular orbit. No atmosphere (red curve): When the planet does not have an atmosphere, the rotation spins down to synchronization in 100 million years. Interestingly and despite the absence of an eccentricity, Figure [2] shows that the obliquity of the planet allows the presence of the 2:0 spin-orbit resonance in which the planet spends a few 10 million years. The planet leaves this resonance when the obliquity becomes too low to maintain this configuration stable. With atmosphere (blue curve): The blue curve of Fig. [2] shows the effect of a strong CO2 atmosphere on the evolution of the spin and the obliquity. The evolution here is more complex. First, the combination of the two tides damps the spin toward the 2:1 SOR while increasing the obliquity. When the obliquity becomes high enough, the thermal torque begins to counter-balance the solid torque. The thermal tides continue to increase the obliquity and the spin of the planet until an equilibrium state between the solid and thermal tides is found. Depending on the atmospheric parameters, the evolutions we obtain are complex, with spin-orbit resonances, coming either from the eccentricity or from the obliquity of the planet. I will show the variety of different spin evolutions of a Venus-like planet, and how it depends on the presence of an eccentricity or obliquity and the presence or not of a thick atmosphere. Figure [2] Simulation of a Venus-like planet orbiting a Sun-like star, without an atmosphere (in red) and with an atmosphere (in blue). Top panel: evolution of the ratio of the planetary spin Ω and the mean motion n. Bottom panel: evolution of the obliquity of the planet (defined as the angle between the equatorial plane and the orbital plane).

Introduction The thermal tides are the main cause of the atmospheric pressure variations on Venus, and are expected to generate temporal variations of the low-degree coefficients of the gravity field of the planet (Bills et al., 2020). However, these gravity variations have not yet been detected since the current Venus gravity solution is not accurate enough (Konopliv et al., 1996). In this study, we model the gravity signature of the thermal tides due to surface pressure variations and analyze whether it can be retrieved with the future tracking data of the EnVision spacecraft. Modeling the thermal tides The thermal tides are computed from the output of the Venus Climate Database (Lebonnois et al., 2021). The atmospheric pressure variations are derived as variations around the mean value over the tidal period of 117 Earth days (Figure 1). The associated gravity potential is computed using the following relationships (McCarthy and Petit, 2004): where Clm(t)and Slm(t)are the time-varying part of the Stokes coefficients of the gravitational potential (land mbeing the degree and the order of the spherical harmonics expansion), R, Mand g, are the radius, the mass and the gravitational acceleration of the planet, respectively, and is the load Love number of degree l. The spherical harmonics expansion of the load variations are derived from the surface pressure variations provided by the VCD (Figure 1). This load Love number depends on the rheological properties of the internal layers of the planet (core, mantle, crust). These rheological properties mainly take into account the compressibility and the viscosity of these different layers (Dumoulin et al., 2017; Tobie et al., 2019). The EnVision gravity experiment. The EnVision spacecraft orbit is an elliptical orbit with an altitude range between 220 km and 515 km and an inclination of 88 degrees allowing for high-resolution mapping of the Venus gravity field (Rosenblatt et al., 2021). The EnVision gravity experiment relies on the two-way radio link established on daily passages of at least 3.5 hours long to guarantee the data download required by the EnVision payload. A very stable reference X-band frequency (at 7.1 GHz) is generated at the ground station and sent to the spacecraft, which then sends back to the station a coherent downlink frequency (X-band at 8.4 GHz) thanks to the radio-transponder of the spacecraft telecommunication system. An additional Ka-band downlink coherent frequency (32 GHz) is also sent back to Earth to support the telemetry volume requirements. This two-way X/X-Ka radio link provides a precise Doppler tracking of the EnVision spacecraft over the six Venusian days of the mission science phase. Figure 1: Simulation of the time-varying part of the gravity field due to the surface pressure variations. Left: Map of the pressure variations extracted from the Venus Climate Database – VCD (Lebonnois et al., 2021). It is centered on longitude 180° while the sub-solar point at longitude zero degree. The main signal is dominated by the second-degree harmonics (Bills et al., 2020). Right:time-variations of the second-degree harmonics of the gravitational potential due to the pressure variations over one tidal cycle (117 Earth-days). Simulations of the retrieval of time-varying gravitational potential from the future EnVision tracking data On the basis of the modeling of the thermal tides, we perform simulations of the EnVision gravity experiment following the procedure described in details in Rosenblatt et al. (2021). We thus use the GINS (Géodésie par Intégrations Numériques Simultanées) software developed by CNES (Marty et al., 2009). We simulate the EnVision tracking data with and without the contribution of the time-varying gravitational potential generated by the thermal tides. Then, we perform least squares fit of the difference between both simulated tracking dataset in order to assess the capacity to retrieve the temporal variations of the gravity field due to the thermal tides. The Doppler noise budget and other source of errors in the Precise Orbit Determination process are also taken into account as in Rosenblatt et al. (2021). The goal of this study is in particular to assess our ability to correct the effect of the thermal tides on the estimation of the k2 tidal potential Love number (real and imaginary part). References: Bills B.G. et al. (2020), Icarus, 340, article id. 113568 ; Dumoulin C., et al. (2017), Planets, 122 (6), 1338-1352; Konopliv A.S. & Sjogren W.L. (1996). JPL Publication 96-2 1996; Jet Propulsion Laboratory: Pasadena, CA, USA ; Lebonnois S. et al. (2021). The Venus Climate Database. EPSC2021-234, Europlanet Science Congress 2021, virtual meeting, 13-24 September, 2021; Marty J.C. et al. (2009), Planet. Space Sci. 2009,57, 350–363; McCarthy D.D. & Petit G. (Eds.) IERS Conventions (2003); IERS Technical Note 32; BKG: Frankfurt/Main, Germany, 2004; Rosenblatt P. et al. (2021),Remote Sensing, vol. 13, 1624; Tobie G. et al. (2019), Astronomy & Astrophysics, 630, id.A70, 11 pp.

The water-rich layer of icy moons of the outer solar system contains a variety of organic (e.g. CH4, NH3, CO2) and inorganic (e.g. NaCl, MgSO4, Na2SO4, MgCl2) solutes detected by robotic mission like NASA Cassini and Galileo probes. Many of the geological processes on these moons, like cryovolcanisms, tectonics, oceanic evolution, involves processes of partial melting and fractional crystallization that are controlled by the stability of solid phases (ices and hydrates) and their eutectic. The ices and pure water thermodynamic are now accurately constrained at the high pressure and low temperature found inside these moons by the SeaFreeze numerical representation. However, the thermodynamics and equilibria in salty aqueous systems relevant for icy moons remains underexplored. SeaFreeze is used broadly in the planetary science community and beyond, the next logical step is the representation of the effect of salts (e.g. anti-freeze effects, eutectics, density, formation of hydrates, water activity), that requires important experimental effort. We will discuss new results from sound speed measurements, isochoric freezing and in-situ single crystal X-Ray diffraction experiments at high pressure and low temperatures that allowed us to constrain the equilibria (eutectics and phases stability) as well as solution thermodynamics in the H2O-(Na-,Cl+,Mg2+,SO42-) systems. We will present how these new data are implemented into SeaFreeze to allow for a comprehensive representation of water-salts systems from the cryogenic conditions at the surface to the incredible pressures found in icy worlds. We will discuss the implication for chemical physics/mineralogy as well as for the upcoming exploration of icy worlds of our solar system by NASA and ESA robotic space missions.

Volcanic activity at Europa’s seafloor is one of the key question 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, either supplied by both radiogenic and tidal heating. The orbital resonance between Io, Europa, and Ganymede forces their orbit and maintain non-zero eccentricities. As a consequence, Io, and to a lesser extent, Europa, undergo large tidal forcing during their journey around Jupiter. Because of its greater distance from Jupiter, a smaller mantle size and the decoupling effect due to the presence of the subsurface water ocean [1], mechanical energy produced by tides in Europa’s mantle is much less than that produced within Io [1, 2]. However, as demonstrated by the 3D approach of Běhounková et al. [3], the combination of tidal and radiogenic heating could maintain a partially molten state within Europa’s mantle for several tens to hundred millions of years, particularly during periods of increased eccentricity [4]. Due to inefficient melt transport through the thick lithosphere of Europa [5], melt produced during periods of enhanced eccentricity may accumulate and in turn affect the tidal heating, as it is the case for Io [e.g. 6], 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 Europa’s mantle tidal heat production, based on the predictions from the 3D simulations of Běhounková et al. [3]. For that purpose, we follow the approach developed to model the solid tides in Io’s partially molten interior [6], 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 asthenosphere than on Io. We use the 3D model of Běhounková et al. [3] to assess in a manner consistent with mantle dynamics the depths where melting should occur as well as the production rates, which should be seen here as minimum values, and estimate the effect of melt accumulation in Europa’s mantle on local dissipation rate and re-assess the consequences in terms of global melt production rate. Based on the production rates predicted by Běhounková et al. [3], a 100 km thick layer with a melt fraction of 20% would form in 140 million years. In order to evaluate the impact of such an accumulation zone of melt, we considered different thicknesses of partially molten layers (25, 50, 100, 200 km), plated at the base of the lithosphere, and investigate melt fraction from 0 to 30 %, corresponding to different degrees of accumulation of melt products. We show that, whatever the partially molten layer thickness, melt accumulation increase tidal heat production and even exceed radiogenic heating. For equivalent volume of accumulated melt, the thinner the layer, the more pronounced this effect is (Figure 1). Our results show that the accumulation of melt, over timescales consistent with the 3D model prediction of Běhounková et al. [3], may significantly affect the tidal dissipation amplitude and its pattern. The potential presence fo 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. Future work is, however, needed to better understand the link between melt generation and extraction in the context of Europa's mantle. Figure 1 : Tidal power produced in Europa’s mantle with a partially molten layer of 25 (blue), 50 (cyan), 100 (green) and 200 (orange) km thickness. The expected range for radiogenic heating is represented for comparison by the red dashed lines and the black dashed line corresponds to the tidal power for no melt. The melt fraction value required in order to exceed the radiogenic power (black line) for each partially molten layer thickness is indicated, with the corresponding pattern of tidal heat flux integrated over the mantle. [1] Tobie, G. et al., Icarus (2005).[2] Sotin, C. et al. Europa (2009).[3] Běhounková et al., GRL (2021).[4] Hussmann and Spohn , Icarus (2004).[5] Bland and Elder, GRL (2022).[6] Kervazo et al., A&A (2021). Acknowledgements: This research received funding from the French “Agence Nationale de Recherche” A.N.R. (OASIS project, ANR-16-CE31-0023-01,( G.T., G.C., M.K., C.D.), from CNES (JUICE and Europa Clipper missions, G.T., G.C., M.K., C.D.) and from Czech Science Foundation through project No. 22-20388S (M.B.).

Observations from the Cassini spacecraft imply the presence of a global salty ocean underneath Enceladus’ ice shell, with ongoing hydrothermal activity at its seafloor. Previous numerical simulations showed that convection in Enceladus’ unconsolidated core generates enough dissipation to sustain the ocean and produces a strongly heterogeneous heat flow pattern at the seafloor that looks promising for explaining the surface ice topography. Yet, how the ocean in-between connects the deeper core to the surface ice shell is unknown. The dynamics of the ocean is thus the missing piece of the Enceladus puzzle towards a fully self-consistent model of the heat transfer in this moon. We fill this gap by performing 3D numerical simulations of the buried ocean of Enceladus. We show that, for the strongly heterogeneous heat flow pattern expected at the seafloor from the unconsolidated core model, azimuthal deflection of strongly localised plumes by the Coriolis force inhibits the heat transfer at low latitudes. The resulting heat flow pattern at the ocean/ice interface is qualitatively consistent with the surface ice topography and in decent quantitative agreement with an ice shell model derived from observations. Using passively advected tracers particles, we predict rising times for hydrothermal products of the order of 1-2 months, in line with previous observations-based predictions. We are thus now able to draw a complete self-consistent model of the heat transfer in Enceladus, explaining the global characteristics of this moon: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity.

The main objective of this research project is to develop super-resolution mapping methods to increase the spatial resolution of surface maps of icy moons of the Solar System by combining all the overlapping observations acquired during a given planetary mission. Cassini-Huygens was the first dedicated mission to Saturn, where it spent 13 years studying the gas giant and its system of moons and rings creating a treasure-trove of data. The focus of this study is to create global surface cartographic products in the infrared by evaluating the Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) hyperspectral dataset in its entirety. We will first focus our efforts on developing, testing and validating the methods on Enceladus, a prime astrobiological candidate, which, in contrast to Titan, does not require any atmospheric corrections. We offer a new module in Python for creating the navigation cubes necessary in merging individual cubes into global mosaics, increasing the accuracy of the covered area and maximising the number of pixels used (e.g. limb-crossing pixels). Moreover, this new module, allows us to explore the Enceladus VIMS dataset in its entirety. We then determine a surface photometric function which mitigates variations in the brightness due to the changing geometric conditions of the flybys. With a precise and comprehensive navigation and an accurate surface photometric correction, we can apply the super-resolution method and present some preliminary maps with enhanced spatial resolution and signal-to-noise ratio.

Models for the internal structure of the icy satellites Ganymede and Titan, as derived from the data of the Galileo and Cassini-Huygens space missions, suggest that both moons are differentiated with a hydrosphere of ices and liquid water overlaying an inner rocky core. The presence of significant amounts of Insoluble Organic Matter (IOM) in this silicate layer (in quantities consistent with those found in chondrites) has recently been advanced to properly explain the density and moment of inertia of these moons [1]. Interestingly, laboratory experiments at room pressure have shown that the pyrolysis of IOM (starting from temperatures as low as 500 K) gradually releases volatiles such as H2O, CO/CO2, CH4, H2S, and SO2, with possible N-bearing compounds such as N2, NOx and NH3 [2, 3, 4]. This evolution of the IOM could have a defining impact on the habitability and chemical evolution of icy worlds, including the formation of an atmosphere. However, the effect on these thermal reactions of the high pressures found inside large icy worlds remain largely unknown. The purpose of this study is to analyze the chemical and physical evolution of the IOM under the combined pressure and temperature conditions expected inside Titan and Ganymede (pressures from 0.5 to 7 GPa and temperatures up to 1200 K). Figure 1: Species produced by IOM dissociation at high pressure and high temperature (blue) compared to those produced by pyrolysis at ambient pressure in Kuga et al. (2014) (green) and Okumura and Mimura (2011) (red). We conducted anvil cell experiments on mixtures of IOM with water at temperatures up to 773 K and pressure up to 8 GPa. The IOM, with a composition of C100H93N65O61, was synthetized at the Nebulotron (CRPG, France [3]), an ultra-high vacuum chamber using a radiofrequency plasma to ionize a N2-CO gas mixture. Systematic pressure and temperature monitoring, and in situ Raman spectroscopy analyses, were conducted during the experiments to characterize the evolution of the samples. Additional infrared analyses were conducted to compare the initial organic matter (as loaded in the anvil cell) with the residual IOM collected at the end of some of the experiments. During our high-pressure experiments, elevated temperatures led to the production of C- and N-bearing species, as was reported by others during the pyrolysis of dry IOM at room pressure. Our IOM-water mixtures, however, yielded NH3 (rather than N2) as the main N-bearing molecule. Furthermore, CO2 was never observed in our samples; instead, CO3 (as carbonic acid and/or carbonate ions) was identified as the main C-bearing species alongside CH4 (Figure 1). Overall, the degradation of the IOM at high pressure appears to start at slightly higher temperature, although additional experiments are needed to confirm this result (in particular for the formation of CO3 species). Evidence of the restructuration of the IOM appeared in both Raman and infrared spectroscopy. Our results support that the thermal dissociation of the IOM inside Titan may have contributed to the formation of its atmosphere [5,6]. These results will also prove useful in assessing the chemical evolution of the hydrosphere of icy worlds, notably regarding the formation of gas hydrates inside their high-pressure ice layers. Acknowledgements: This research is founded by CNRS 80 PRIME program. This work also acknowledges the financial support from CNES (Centre National d’Etudes Spatiales, France) in preparation of the ESA JUICE mission. References: [1] Néri et al. (2020) A carbonaceous chondrite and cometary origin for icy moons of Jupiter and Saturn. Earth and Planetary Science Letters, 530 :115920. [2] Okumura and Mimura (2011) Gradual and stepwise pyrolysis of insoluble organic matter from the Murchison meteorite revealing chemical structure and isotopic distribution. Geochimica et Cosmochimica Acta, 75(22) :7063–7080. [3] Kuga et al. (2014) Nitrogen isotopic fractionation during abiotic synthesis of organic solid particles. Earth and Planetary Science Letters, 393:2–13. [4] Franklin (1949) A study of the fine structure of carbonaceous solids by measurements of true and apparent densities. Part I. Coals. Transactions of the Faraday society, 45:274–286. [5] Tobie et al. (2012) Titan’s bulk composition constrained by Cassini-Huygens: implication for internal outgassing. The Astrophysical Journal, 752(2):125. [6] Miller et al. (2019) Contributions from accreted organics to Titan’s atmosphere: new insights from cometary and chondritic data. The Astrophysical Journal, 871(1):59.

Comet nuclei, when stored in the transneptunian region, are subject to heating at temperatures from 30 to 50 K over the age of the solar system. The timescale for sublimated volatiles to escape the objects at these temperatures is long though [1], so that a gas phase remains in contact to an icy matrix on such long timescales. Once these nuclei enter the inner solar system and become active, subsurface sublimation puts once again a gas phase in contact of the porous and tortuous ice structure of cometary material. In this context, the formation of clathrate hydrates may be considered as a plausible trapping mechanism of these gases, occurring in subsurface layers, and allowing some of the most volatile species to subsequently survive in cometary material at temperatures higher than the sublimation temperature of the corresponding pure solid [2]. Gas hydrates only form and remain stable in specific temperature and pressure regimes that depend on the nature of the guest molecules [3]. Theoretical phase diagram of clathrate hydrates show that it would be possible to form clathrates at very low pressure (10-10 bar) and temperature (< 80 K), but there is a critical lack of experimental data using these preparation methods [4]. Could clathrate hydrates be formed under conditions relevant to the interior of comet nuclei? The formation and characterisation of these ice-like structures under such conditions could provide valuable experimental evidence for understanding the preservation of some volatile species during the thermally-induced evolution of comets. In an effort to assess whether hydrates may play a role in maintaining volatile species in cometary material, FTIR spectroscopic identification of several species have been performed. We present results related to carbon dioxide and methane hydrates, in conditions relevant to cometary nuclei, i.e. at low temperature (10 K) and pressure (base pressure 10-7 mbar) regimes. To understand the nature of the gas hydrates formed under these conditions, vibrational spectra of distinct gas/ice interactions (clathrate hydrate, gas in/on water ice) were compared. The behaviour of the water crystalline skeleton interactions with the trapped molecules at different temperatures, as well as the influence of the gas mixture and the deposition method, will be presented. [1] Prialnik et al. (2004) in Comets II, Festou, Keller and Weaver (Eds.), 359-387 [2] Mandt et al. (2017) in Comets as Tracers of Solar System Formation and Evolution, Mandt, Mousis, Bockelee-Morvan and Russel (Eds.) [3] Sloan E., Nature 426, 353-359 (2003) [4] Choukroun et al. (2003) in The Science of Solar System Ices, Gudipati and Castillo- Roguez (Eds.), 409-454 This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No 802699)

The Cassini-Huygens mission that explored the Saturn system during the period 2004–2017 revolutionized our understanding of Titan, the only known moon with a dense atmosphere and the only body, besides Earth, with stable surface liquids. Its predominantly nitrogen atmosphere also contains a few percent of methane that is photolyzed on short geological timescales to form ethane and more complex organic molecules. The presence of a significant amount of methane and40Ar, the decay product of40K, argues for exchange processes from the interior to the surface. Here we review the information that constrains Titan's interior structure. Gravity and orbital data suggest that Titan is an ocean world, which implies differentiation into a hydrosphere and a rocky core. The mass and gravity data complemented by equations of state constrain the ocean density and composition as well as the hydrosphere thickness. We present end-member models, review the dynamics of each layer, and discuss the global evolution consistent with the Cassini-Huygens data. ▪ Titan is the only moon with a dense atmosphere where organic molecules are synthesized and have sedimented at the surface. ▪ The Cassini-Huygens mission demonstrated that Titan is an ocean world with an internal water shell and liquid hydrocarbon seas at the poles. ▪ Interactions between water, rock, and organics may have occurred during most of Titan's evolution, which has strong astrobiological implications. ▪ Data collected by the Dragonfly mission and comparison with the JUpiter ICy moons Explorer (JUICE) data for Ganymede will further reveal Titan's astrobiology potential.

The most active icy worlds such as Europa or Enceladus are predicted to host extensive aqueous alteration driven by water–rock interactions at elevated temperatures1,2,3. On the other hand, it is assumed that such alteration is kinetically inhibited at the subzero temperatures of other icy worlds, such as the mid-sized moons of Saturn and Uranus or trans-Neptunian objects1,4. Here we perform aqueous alteration experiments on a chondrite-analogue material (olivine) and find that chemical alteration processes are still efficient at temperatures as low as −20 °C, as the presence of an unfrozen water film still allows olivine to dissolve in partially frozen alkaline solutions. We infer that aqueous alteration may be enhanced by salts and ammonia present in icy worlds, and therefore remains a geologically rapid process even at subzero temperatures. Our results imply that the primary chondritic minerals in most icy bodies exceeding 400–500 km in diameter will be completely altered to hydrous secondary minerals early in their evolutionary histories.

Context: Saturn's massive gravity is expected to causes a tide in Titan's atmosphere, producing a surface pressure variation through the orbit of Titan and tidal winds in the troposphere. The future Dragonfly mission could analyse this exotic meteorological phenomenon. Aims: We analyse the effect of Saturn's tides on Titan's atmosphere and interior to determine how pressure measurements by Dragonfly could constrain Titan's interior. Methods: We model atmospheric tides with analytical calculations and with a 3D Global Climate Model (the IPSL-Titan GCM), including the tidal response of the interior. Results: We predict that the Love numbers of Titan's interior should verify 1 + Re(k2 - h2) ~ 0.02-0.1 and Im(k2 - h2) < 0.04. The deformation of Titan's interior should therefore strongly weaken gravitational atmospheric tides, yielding a residual surface pressure amplitude of only ~ 5 Pa, with a phase shift of 5-20 hours. Tidal winds are very weak, of the order of 3*10^-4 m/s in the lower troposphere. Finally, constraints from Dragonfly data may permit the real and the imaginary parts of k2 - h2 to be estimated with a precision of ~0.01-0.03. Conclusions: Measurements of pressure variations by Dragonfly over the whole mission could give valuable constraints on the thickness of Titan's ice shell, and via geophysical models, its heat flux and the density of Titan's internal ocean., Comment: Accepted for publication in A&A. 10 pages, 8 figures, 1 table

Orbital geophysical investigations of Enceladus are critical to understanding its energy balance. We identified key science questions for the geophysical exploration of Enceladus, answering which would support future assessment of Enceladus' astrobiological potential. Using a Bayesian framework, we explored how science requirements map to measurement requirements. We performed mission simulations to study the sensitivity of a single spacecraft and dual spacecraft configurations to static gravity and tidal Love numbers of Enceladus. We find that mapping Enceladus' gravity field, improving the accuracy of the physical libration amplitude, and measuring Enceladus' tidal response would provide critical constraints on the internal structure, and establish a framework for assessing Enceladus' long-term habitability. This kind of investigation could be carried out as part of a life search mission at little additional resource requirements., 21 pages, 6 figures. A paper submitted to PSJ, which is an extension of the white paper previously submitted to the decadal survey

International audience; The discovery of a liquid ocean on Saturn's small moon Enceladus and evidence of modern hydrothermal activity provide an unexpected new environment in which to expand the search for life. However, as with the age of the moons themselves, the age of the liquid ocean and any hydrothermal activity therein remains an area of debate. Based on physical and chemical observations from the Cassini mission we can apply known mineral dissolution rates, estimated water-rock ratios from Enceladus' observed density, and variable water flow rates within the rocky core to constrain duration of active serpentinization, and therefore, the maximum age of the liquid water circulation in the core. On this basis we developed a 1-D reactive transport model to compare the effect of initial olivine percentage, grain size, temperature, and flow rate on timespans of primary olivine alteration in a rocky core the size and density of Enceladus'. In most cases, olivine alteration and precipitation of hydrous secondary minerals results in a water-limited alteration regime. An alteration front that propagates in the direction of water flow then controls the overall rate of olivine alteration. Of the parameters explored, high initial olivine percentages and slow fluid flow rates were the strongest predictors of long serpentinization times, while temperature and grain size had a smaller effect. The annual global H 2 production rate in all model cases (> 1 ×10 12 moles yr-1) is several orders of magnitude greater than the minimum H 2 release rate calculated from the observed H 2 in Enceladus' plume (1 ×10 9 moles yr-1), suggesting that any ongoing active serpentinization processes in the core are likely nearing completion. The longest timescales indicate the potential for olivine alteration and H 2 production for up to ~75 Myr, consistent with weathering rates of terrestrial peridotite massifs. If the H 2 produced from Enceladus is sourced from primary mineral alteration, these results suggest that hydrothermal activity in the core of Enceladus developed only very recently-even as recent as within the past 100 Myr.

INTRODUCTION The thermo-mechanical evolution of Titan's ice shell is primarily controlled by the mode of the heat transfer in the ice shell and the amount of heat coming from the ocean. Numerical models of thermal convection in Titan's ocean have suggested that the heat flux from the core can be strongly affected by the Coriolis effect, leading to a latitudinal gradient in the heat flux at the base of the ice shell [1,2,3]. These heat flux variations are of the order of mW/m2 and may induce phase changes at the ice/ocean interface, affecting the thickness of the ice shell. Until now, the effect of lateral variations in the heat flux has been studied under the assumption that the ice shell is highly viscous and the heat transfer in the ice occurs by conduction. In such a case, the heat flux from the ocean is negatively correlated with the ice shell thickness [3]. A higher-than-average heat flux in polar regions then causes melting of ice and thinning of the ice shell, possibly explaining the fact that the poles on Titan are about 300 m lower than the equator [4]. The issue of whether Titan's ice shell is convecting or not has not yet been resolved. The mode of heat transfer depends on the ice shell thickness and the viscosity of ice, which in turn depends on the grain size and the temperature of the ocean. Since these parameters are not accurately known, the convection model cannot be excluded from analysis. The response of the convecting ice shell to heat flux variations from the ocean is likely to be different from that described for the conduction model [5]. When heat is transferred by convection, the ice shell is on average warmer and less viscous than in the conductive case. The loss/gain of ice mass at the ice/ocean interface caused by melting/freezing is compensated by flow of low-viscosity ice and, therefore, the shape of the ice/ocean interface remains unchanged. The shape of the surface topography is difficult to assess a priori because it may depend on the vigor of convection. METHOD In this study, we investigate the effect of spatial variations in the heat flux from the ocean on the behavior of a convecting ice layer. The models discussed below have been obtained by simultaneously solving the momentum equation, the transport equation for temperature and the mass conservation equation for an incompressible fluid. The temperature is fixed to 90 K on the upper boundary and to 265 K on the bottom (ice/ocean) boundary. The computational domain is 100 km thick and has an aspect ratio of 16:1. We assume that the deformation of ice is controlled by diffusion creep and the viscosity of ice only depends on temperature and grain size. The surface is treated as a material boundary while the ice/ocean interface is open and its shape can change due to phase changes. The heat flux variations from the ocean enter the problem through the energy conservation law which relates the jump in the heat flux across the boundary with the relative motion of the ice/ocean interface [3,6,7]. Note that the heat flux across the phase boundary is generally not continuous (as assumed, for example, by [8]), but depends on the velocity of ice flow, which cannot be neglected if the viscosity of ice is low (< 1016 Pa.s). The lateral variation of the heat flux imposed at the bottom boundary of the domain is scaled to correspond to 50 % of the average heat flux (Fig. 1a). RESULTS In Figs. 1 and 2, we compare the results obtained for a grain size of 2 mm and 3 mm. Although these values are close to each other, the models represent two end-member cases discussed above. While in the former case (2 mm), the heat transfer is controlled by convection, in the latter case (3 mm), convection does not develop and the heat is transferred by conduction. Inspection of Figs. 1 and 2 shows that the convection model gives the opposite sign of the surface topography than the conduction model (Fig. 1b) and predicts a significant (8 K) increase in ice temperature above the positive heat flux anomaly (Fig. 2a). The conduction and convection models also give different orientations of tectonic stresses - extension in the case of convection and compression in the case of conduction (Fig. 2b). Both models predict only small amplitudes of the geoid (Fig. 2c), suggesting that the equilibrium state can be approximated by either Airy or Pratt isostatic model, depending on the mode of heat transfer. CONCLUSIONS Variations in the heat flux from the ocean can significantly influence Titan’s topography and the distribution of stress near the surface. The topography is negatively correlated with the heat flux when the heat in the ice shell is transferred by conduction, whereas a positive correlation and more complex topography response is found for the convective heat transfer. In contrast to the topography, the heat flux variations have only a small effect on the geoid. A careful analysis of the geoid and topography data collected by Cassini could thus provides an insight into the processes occurring in Titan's interior and helps answer the question of which heat transfer mode dominates in Titan's ice shell. ACKNOWLEDGEMENTS This research was supported by the Czech Science Foundation through project No. 19-10809S. M.K. acknowledges the support from the Charles University project SVV-2020-260581. K.K. was supported by the Charles University Research program No. UNCE/SCI/023. G.C. and G.T. acknowledge the support from the ANR COLOSSe project. REFERENCES [1] Soderlund, 2019, Geophys. Res. Lett. 46, 8700–8710. [2] Amit et al., 2020, Icarus 338, 113509. [3] Kvorka et al., 2018, Icarus 310, 149–164. [4] Corlies et al., 2017, Geophys. Res. Lett. 44, 11754–11761. [5] Nimmo and Bills, 2010, Icarus 201, 896–904. [6] Čadek et al., 2019a, Icarus 319, 476–484. [7] Čadek et al., 2019b, Geophys. Res. Lett. 46, 14299–14306. [8] Ojakangas and Stevenson, 1989, Icarus 81, 242–270.

Analysis of Enceladus’ gravity, topography and libration data suggests that the thickness of the ice crust significantly varies, which may have important consequences for the heat transfer across and the tidal dissipation within the ice shell. Understanding these processes is a critical prerequisite for analyzing Enceladus’ thermal evolution and assessing the long-term stability of its subsurface ocean. Here, we investigate the impact of ice shell thickness variations on the tidal deformation of the moon and the associated heat production using a finite-element model that includes faults (“tiger stripes”) in the south polar region (SPR). Since the tidal deformation and the thermal structure of the ice shell are coupled through temperature and deviatoric stress, we simultaneously solve the equations governing the anelastic deformation of ice and also equations which model conductive heat transfer in the ice shell. We find that tidal heating is concentrated in a narrow low viscosity zone near the base of the ice shell and along faults. Outside the SPR, the thickness of this zone is about 1/10 of the local ice thickness and the associated volumetric heating is ≲ 1 0 − 6 W/m 3 , corresponding to less than 1 GW of dissipated power. In the SPR, the tidal effects are enhanced by the combined action of faults and ice shell thinning. Although the volumetric heating in this relatively small region may be larger than 10 −4 W/m 3 , the total heat production in this region does not exceed 1.1 GW. Our computations show that tidal heating in the ice shell can explain only a small fraction of Enceladus’ heat production derived from astrometric observations, implying that Enceladus’ heat engine is powered by dissipation in the core or in the ocean.

We present a new model of Enceladus’ internal structure based on the recent shape model by Tajeddine et al. (2017) and the gravity model by Iess et al. (2014). Assuming that the rocky core is homogeneous and in hydrostatic equilibrium, we derive a set of structural models that accurately reproduce the main characteristics of Enceladus’ gravity field. In order to restrict the range of acceptable models we analyze the degree of compensation (ratio of the bottom to the surface load) at spherical harmonic degrees well constrained by the gravity data. In agreement with previous studies, we find that Enceladus’ ice shell is close to equilibrium, with the degree of compensation approaching one for models with a hydrostatic core having a radius between 190 km and 195 km. By computing the flow of ice driven by variations in hydrostatic pressure on the ice water/interface, we demonstrate that the ice shell is in steady state, as suggested by the gravity and shape data, only if the viscosity of ice at the melting temperature is equal to or higher than 3 × 1014 Pa s, corresponding to diffusion creep with a grain size of 1 mm or larger. The topographic anomalies are maintained by phase changes at the ice/water interface with a melting/freezing rate of a few mm/yr or smaller. This process is controlled by the heat flux at the top of the ocean, characterized by a strong degree-2 zonal component with amplitudes exceeding 60 mW/m2 in both south and north polar regions.

13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology; International audience; We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration.

International audience; Jupiter's icy moon Europa harbors underneath tectonically modified ice shell (Figueredo & Greeley, 2004; Kattenhorn & Prockter, 2014) a salty ocean (Kivelson et al., 2000) in direct contact with a rocky interior that may still be active (Moore & Hussmann, 2009). Such an oceanic environment makes Europa a primary target in the search for a habitable world beyond Earth (Hand et al., 2009). The chemical evolution of Europa's ocean and its habitability is conditioned by the interaction with the rocky seafloor (Vance et al., 2016). It depends on the heat released from the deep interior to the seafloor (Altair et al., 2018), and hence by the intensity of magmatic activity. BĚHOUNKOVÁ ET AL.

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s., Comment: arXiv admin note: substantial text overlap with arXiv:1908.01374

Context. Io’s spectacular activity is driven by tidal dissipation within its interior, which may undergo a large amount of melting. While tidal dissipation models of planetary interiors classically assume that anelastic dissipation is associated only with shear deformation, seismological observation of the Earth has revealed that bulk dissipation might be important in the case of partial melting. Aims. Although tidal dissipation in a partially molten layer within Io’s mantle has been widely studied in order to explain its abnormally high heat flux, bulk dissipation has never been included. The aim of this study is to investigate the role of melt presence on both shear and bulk dissipation, and the consequences for the heat budget and spatial pattern of Io’s tidal heating. Methods. The solid tides of Io are computed using a viscoelastic compressible framework. The constitutive equation including bulk dissipation is derived and a synthetic rheological law for the dependence of the viscous and elastic parameters on the melt fraction is used to account for the softening of a partially molten silicate layer. Results. Bulk dissipation is found to be negligible for melt fraction below a critical value called rheological critical melt fraction. This corresponds to a sharp transition from the solid behavior to the liquid behavior, which typically occurs for melt fractions ranging between 25 and 40%. Above rheological critical melt fraction, bulk dissipation is found to enhance tidal heating up to a factor of ten. The thinner the partially molten layer, the greater the effect. The addition of bulk dissipation also drastically modifies the spatial pattern of tidal dissipation for partially molten layers. Conclusions. Bulk dissipation can significantly affect the heat budget of Io, possibly contributing from 50 to 90% of the global tidal heat power. More generally, bulk dissipation may play a key role in the tidally induced activity of extrasolar lava worlds.

The South Polar Terrain (SPT) of Enceladus is a site with eruptions of gas and water ice particle plumes, which indicate internal geodynamic activity. These eruptions are located along a series of tectonic structures, that is, the Tiger Stripe Fractures (TSF), which are composed of regularly spaced, linear depressions. The SPT is surrounded by sinuous chains of ridges and troughs (the Marginal Zone). To unravel the tectonics that affect the region and its evolution, we performed specific structural mapping and quantitative analyses of brittle features from remotely sensed images. The results are consistent with a block rotation model, in which several tectonic regimes coexist. The TSF are left-lateral strike-slip faults that bound rigid elongated blocks. The blocks rotate clockwise and are enclosed in a regional scale right-lateral kinematic framework expressed in the Marginal Zone. These two opposite and complementary kinematic regimes induce transtensional and transpressional regimes within the SPT. An evolutionary tectonic model is proposed for the past and future evolution of the SPT. This model confirms the role of tectonic-related kinematics in icy satellites and contributes to preparations for future missions.

Ganymede is the largest icy moon in the solar system. The Galileo spacecraft discovered the presence of a magnetic field that is generated in an iron-rich core (Kivelson et al., 1996). Gravity data from the Galileo mission suggest a low value of the reduced Moment of Inertia (MoI) of 0.3115 (Anderson et al., 1996), which indicates a high degree of differentiation. Schubert et al. (1996) proposed a three-layer structure: an iron-rich liquid core, coherent with magnetic data, a silicate mantle, and a hydrosphere. The present work investigates the thermal evolution of Ganymede’s rocky core to assess the conditions under which the rocky interior can differentiate, leading to the formation of an iron core. The first step of the study addresses the possible end-members for the rocky composition. Two chondritic compositions have been proposed for Ganymede’s rocky interior: LL chondrite (Kronrod, Kuskov, 2006) that can explain the low Fe/Si ratio inferred from the density derived from mass and moment of inertia (MoI) or a carbonaceous chondrite (CI type) which is more probable during the accretion beyond the snow line (Néri et al., 2020). Using the bulk elementary composition and the abundances of the different phases, the elementary composition of each phase is calculated. The silicate fractions have a very similar Mg# and the main difference is the much larger fraction of iron-rich phases in the CI chondrite. Perple_X,a thermodynamic calculation tool that determines the phases from elementary composition,was used to study the mineralogical evolution of the silicate phase in the (P,T) domain relevant to Ganymede. In the case of the LL composition, we consider the hydration of the rock during the accretion and differ- entiation between the rocky core and the hydrosphere. The second step simulates the thermal evolution of the rocky core starting after the differentiation of the hydrosphere (primitive core). The decay of long-lived radioactive elements (K, U, Th) provides thermal energy that heats up the interior. Thermal energy can be transferred by either conduction or convection. The onset of convection in a solid material depends on its viscosity (ratio of stress to strain rate) that depends on parameters such as pressure, temperature, grain size, and stress. Hydrated silicates have a viscosity that strongly depends on stress whereas dry silicates have a viscosity that depends mainly on temperature. More than 30 numerical simulations were performed to investigate the effect of parameters including the time of formation of the primitive core (or time of differentiation of the hydrosphere), the type of chondrite origin (LL chondrites have a larger amount of radioactive elements), the initial temperature profile, and the sensitivity to the rheology parameters. The effect of numerical parameters such as initial perturbations in temperature and strain rate and maximum viscosity contrasts were also investigated. Simulations performed in 3D spherical geometry show that, in the domain of investigated parameters, convection does not happen in the hydrated silicates before dehydration. Dehydration occurs in the center leading to a structure in two layers: an upper layer, about 150 km thick, of hydrated silicates sitting on top of a dry silicates core. As temperature increases in the core, the eutectic temperature of the Fe-FeS sys- tem (Buono, Walker, 2011) is reached before convection in the dry silicates starts. Such an event happens between 1.5 and 2.5 Gyr, leading to a potential formation of the iron-rich liquid core. The percolation of the iron-rich liquid phase would have significant effect on the core dynamics as the deep silicates would become less dense than the upper layer of hydrated silicates. Even without this effect, we observe an onset of convection when the temperature in the core reaches 1600 K when dry silicates have a viscosity low enough for convection to start. We note that convection does not start in all models: when the differentiation of the hydrosphere is late the rocky interior remains in a con- ductive state until present, the rocky interior remains in a conductive state until present. The differentiation of the iron core and its effect on the interior dynamics is not yet implemented in the model. It would lead to an earlier onset of the convection. The convection step is short on geological time steps (a few 10s to 100s of Myr). It has two consequences. First, it dehydrates the upper layer and eventually allows for silicate melt- ing. Second, it cools down the interior very efficiently, reducing the temperature and stopping the convection process. By bringing the newly dehydrated iron-rich silicates in the center, it may lead to a second step in the formation of the iron core. It also creates a pulse in heat flux that may have a major consequence on Ganymede’s global interior dynamics. The upcoming ESA mission JUICE that will orbit Ganymede will provide additional information on the interior structure of Ganymede that will help understand the evolution of its core. Such models can be extended to the evolution of the silicate cores of other icy moons such as Titan and Europa that will be visited by the Dragonfly mission and the Europa Clipper mission, respectively. Anderson JD, Lau EL, Sjogren WL, Schubert G, Moore WB., 1996. Nature. 384, 6609. 541– 543. Buono AS, Walker D, 2011.Geochimica et Cosmochimica Acta. 75, 8. 2072–2087. Kivelson MG, Khurana KK, Russell CT, Walker RJ, Warnecke J, Coroniti FV, Polanskey C, Southwood DJ, Schubert G. 1996. Nature. 384, 6609. 537–541. Kronrod VA, Kuskov OL., 2006. Geochemistry International. 44, 6. 529–546. Néri A, Guyot F, Reynard B, Sotin C, 2020. Earth and Planetary Science Letters. 530. 115920. Schubert G, Zhang K, Kivelson MG, Anderson JD, 1996. Nature. 384, 6609. 544–545.

IntroductionThe presence of an internal ocean in direct contact with silicates and possible seafloor magmatic activity drew attention to the potential habitability of Jupiter's moon, Europa. Here, we address the conditions for melt production in its silicate mantle until the present day, an essential ingredient for assessing the habitability potential of Europa.MethodTo reach our goal we employ Oedipus/Antigone (Choblet et al., 2007; Běhounková et al., 2010) – a numerical tool solving consistently thermal convection and tidal dissipation in 3d planetary shells. For the thermal convection part, the mass, momentum and energy conservation equations for viscous material are solved in the extended Boussinesq approximation. The solved equations are supplemented by an instantaneous melt extraction model once the solidus temperature is reached. The efficiency of heat transfer is mainly determined by viscosity, characterized by viscosity at the melting point 𝜂melt and strong temperature dependency. We consider both radiogenic and tidal heating as energy sources. The former is computed from long-lived radioisotope abundances assuming concentrations based on LL (upper limit) and CM (lower limit) chondrites. Two families of models are tested: homogeneous distribution and a model with a simple 50:50 partitioning of radioactive elements between the crust and the mantle. The time-evolution follows the decay law and is progressively diminishing with time. The latter is controlled by internal structure and Europa’s orbital eccentricity. For the tidal dissipation part, the mass and momentum conservation of tidal deformations are solved over the tidal time scale assuming a Maxwell viscoelastic rheology with a temperature-dependent effective viscosity capturing an Andrade-like behaviour. The orbital eccentricity and the eccentricity evolution are parameterized. Two families of eccentricity models are taken into account assuming either constant or periodically varying eccentricity (cf. Hussmann and Spohn, 2004).ResultsOur simulations show that the evolution of Europa's silicate mantle and associated melt production are controlled by three primary parameters: i) the viscosity at the melting temperature 𝜂melt, ii) the amount of radiogenic heating and its partitioning, and iii) Europa’s orbital eccentricity. For viscosity equal or larger than 1020Pa.s, no onset of convection is observed, the melt production is concentrated into the layer above the core-mantle boundary, and melt production can be sustained until the present day. For lower viscosity (𝜂melt=1018 or 1019 Pa.s), more effective heat transfer is obtained. All the performed simulations follow a similar pattern. The onset of convection is observed within 1 Gyr and 100 Myr for 1019 Pa.s and 1018 Pa.s, respectively. Due to the large volumetric heating, the mantle temperature is close to the dry silicate solidus except in the cold lid and the temperature differences are relatively small below the lid. The onset of convection can, therefore, occur after a relatively long time, especially for 𝜂melt=1019 Pa.s, and the exact timing is sensitive to the radiogenic heating model. The onset of convection is characterized by abrupt changes in melt production and in temperature, and by increased flux from the core. The temperature during the onset is represented by long-wavelength features following the pattern of heterogeneous tidal dissipation. After the onset of convection, the melting production concentrates into the layer just under the stagnant lid, the thickness of this layer depends on the viscosity.For models with constant eccentricity equal to the present-day value, no melt production is expected at present (except for non-convective models with high reference viscosity). Only simulations with enhanced eccentricity can lead to melt production during the last billion years. Increased tidal dissipation during periods of enhanced eccentricity leads to prolonged melt production at high latitudes, even at present.We predict that melt can be currently produced in the polar regions if Europa experienced a recent period of enhanced eccentricity. This would also imply the eccentricity is presently decreasing. The volume of melt that is produced during such a magmatic pulse and the associated volatile release depends on the assumed initial rock composition, which determines the abundance in radioactive elements and volatile species. ConclusionsWe have investigated the thermal evolution of Europa's mantle and conditions for sustaining melt production until the present. Present-day seafloor magmatism might be possible if Europa experienced a period of increased eccentricity in the recent past. Such recent magmatism should influence the topography of the seafloor and composition of the ocean (Zolotov and Kargel, 2009; Dombard and Sessa 2019). Characterization of the ocean composition, and possible detection of gravity anomalies, hydrothermally derived volatile species at high latitude, and changes in Europa’s orbital motion by Europa Clipper and JUICE may confirm possible ongoing large-scale seafloor activity. AcknowledgementsThe research leading to these results received funding from the Czech Science Foundation through project No. 19-10809S, from the French ”Agence Nationale de Recherche” ANR (OASIS project, ANR-16-CE31-0023-01), from CNES (JUICE and Europa Clipper missions), from the Région Pays de la Loire - GeoPlanet consortium project (No. 2016-10982). The computations were carried out in IT4Innovations National Supercomputing Center (project no. LM2015070).ReferencesBehounkova, M., Tobie, G., Choblet, G., and Cadek, O.: Coupling mantle convection and tidal dissipation: Applications to Enceladus and Earth-like planets. Journal of Geophysical Research Planets, 115, E09011, (2010).Choblet, G., Cadek, O., Couturier, F. and Dumoulin, C. OEDIPUS: A new tool to study the dynamics of planetary interiors. Geophysical Journal International 170, 9–30 (2007). Dombard, A.J., and Sessa, A.M. Gravity measurements are key in addressing the habitability of a subsurface ocean in Jupiter's Moon Europa, Icarus 325, 31–38 (2019).Hussmann, H. and Spohn, T.: Thermal-orbital evolution of Io and Europa. Icarus, 171, 391–410, (2004).Zolotov, M. Y. and Kargel, J.: On the chemical composition of Europa’s icy shell, ocean, and underlying rocks. Europa, edited by R.T. Pappalardo, W.B. McKinnon, and K. Khurana, University of Arizona Press, Tucson, AZ, pp. 431–458 (2009).

Uranus and Neptune, and their diverse satellite and ring systems, represent the least explored environments of our Solar System, and yet may provide the archetype for the most common outcome of planetary formation throughout our galaxy. Ice Giants will be the last remaining class of Solar System planet to have a dedicated orbital explorer, and international efforts are under way to realise such an ambitious mission in the coming decades. In 2019, the European Space Agency released a call for scientific themes for its strategic science planning process for the 2030s and 2040s, known as Voyage 2050. We used this opportunity to review our present-day knowledge of the Uranus and Neptune systems, producing a revised and updated set of scientific questions and motivations for their exploration. This review article describes how such a mission could explore their origins, ice-rich interiors, dynamic atmospheres, unique magnetospheres, and myriad icy satellites, to address questions at the heart of modern planetary science. These two worlds are superb examples of how planets with shared origins can exhibit remarkably different evolutionary paths: Neptune as the archetype for Ice Giants, whereas Uranus may be atypical. Exploring Uranus' natural satellites and Neptune's captured moon Triton could reveal how Ocean Worlds form and remain active, redefining the extent of the habitable zone in our Solar System. For these reasons and more, we advocate that an Ice Giant System explorer should become a strategic cornerstone mission within ESA's Voyage 2050 programme, in partnership with international collaborators, and targeting launch opportunities in the early 2030s., Comment: 34 pages, 9 figures, accepted to Planetary and Space Science

We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration., 13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology

We use rotating convection simulations in a thin spherical shell to study fluid dynamics in subsurface oceans of icy moons. We find two types of persistent results, characterized by larger outer boundary heat flux either at polar regions or at the equatorial region. Simulations corresponding to larger Rossby numbers result in polar cooling with moderate lateral heterogeneity in heat flux, whereas lower Rossby numbers give equatorial cooling with more pronounced heat flux heterogeneity. The polar cooling scenario is in agreement with inferences for the heat flux at the top of Titan's ocean, which may provide a dynamical constraint for the vigor of convection in this layer. Our results may help unraveling the internal dynamics and the interactions among the different layers within the hydrosphere of Titan. Possible implications for the deep interiors of other icy moons are envisaged.

Pressures in the hydrospheres of large ocean worlds extend to ranges exceeding those in Earth deepest oceans. In this regime, dense water ices and other high-pressure phases become thermodynamically stable and can influence planetary processes at a global scale. The presence of high-pressure ices sets large icy worlds apart from other smaller water-rich worlds and complicates their study. Here we provide an overview of the unique physical states, thermodynamics, dynamic regimes, and evolution scenarios specific to large ocean worlds where high-pressure ice polymorphs form. We start by (i) describing the current state of knowledge for the interior states of large icy worlds in our solar system (i.e. Ganymede, Titan and Callisto). Then we (ii) discuss the thermodynamic and physical specifics of the relevant high–pressure materials, including ices, aqueous fluids and hydrates. While doing this we (iii) describe the current state of the art in modeling and understanding the dynamic regimes of high-pressure ice mantles. Based on these considerations we (iv) explore the different evolution scenarios for large icy worlds in our solar system. We (v) conclude by discussing the implications of what we know on chemical transport from the silicate core, extrapolation to exoplanetary candidate ocean worlds, limitations to habitability, differentiation diversity, and perspectives for future space exploration missions and experimental measurements.

Saturn’s moon Enceladus harbours a global water ocean¹, which lies under an ice crust and above a rocky core². Through warm cracks in the crust³ a cryo-volcanic plume ejects ice grains and vapour into space⁴–⁷ that contain materials originating from the ocean⁸,⁹. Hydrothermal activity is suspected to occur deep inside the porous core¹⁰–¹², powered by tidal dissipation¹³. So far, only simple organic compounds with molecular masses mostly below 50 atomic mass units have been observed in plume material⁶,¹⁴,¹⁵. Here we report observations of emitted ice grains containing concentrated and complex macromolecular organic material with molecular masses above 200 atomic mass units. The data constrain the macromolecular structure of organics detected in the ice grains and suggest the presence of a thin organic-rich film on top of the oceanic water table, where organic nucleation cores generated by the bursting of bubbles allow the probing of Enceladus’ organic inventory in enhanced concentrations.

Ganymede, the largest moon in the solar system, has a fully differentiated interior with a layer of high-pressure (HP) ice between its deep ocean and silicate mantle. In this paper, we study the dynamics of this layer using a numerical model of two-phase ice–water mixture in two-dimensional Cartesian geometry. While focusing on the generation of water at the silicate/HP ice interface and its upward migration towards the ocean, we investigate the effect of bottom heat flux, the layer thickness, and the HP ice viscosity and permeability. Our results suggest that melt can be generated at the silicate/HP ice interface for small layer thickness ( ≲ 200 km) and high values of heat flux ( ≳ 20 mW m − 2 ) and viscosity ( ≳ 1015 Pa s). Once generated, the water is transported through the layer by the upwelling plumes. Depending on the vigor of convection, it stays liquid or it may freeze before melting again as the plume reaches the temperate (partially molten) layer at the boundary with the ocean. The thickness of this layer as well as the amount of melt that is extracted from it is controlled by the permeability of the HP ice. This process constitutes a means of transporting volatiles and salts that might have dissolved into the melt present at the silicate/HP ice interface. As the moon cools down, the HP ice layer becomes less permeable because the heat flux from the silicates decreases and the HP ice layer thickens.

Geophysical data from the Cassini spacecraft imply the presence of a global ocean underneath the ice shell of Enceladus 1 , only a few kilometres below the surface in the South Polar Terrain 2–4 . Chemical analyses indicate that the ocean is salty 5 and is fed by ongoing hydrothermal activity 6–8 . In order to explain these observations, an abnormally high heat power (>20 billion watts) is required, as well as a mechanism to focus endogenic activity at the south pole 9,10 . Here, we show that more than 10 GW of heat can be generated by tidal friction inside the unconsolidated rocky core. Water transport in the tidally heated permeable core results in hot narrow upwellings with temperatures exceeding 363 K, characterized by powerful (1–5 GW) hotspots at the seafloor, particularly at the south pole. The release of heat in narrow regions favours intense interaction between water and rock, and the transport of hydrothermal products from the core to the plume sources. We are thus able to explain the main global characteristics of Enceladus: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity. We predict that this endogenic activity can be sustained for tens of millions to billions of years. Tidal forcing within a very porous (unconsolidated) core can generate enough energy to drive all the observed global features of Enceladus. This activity can be sustained up to several billion years.

We propose a dynamic model relating the non-hydrostatic long-wavelength topography on Enceladus to loading at the base of the ice shell. In dynamic equilibrium, the surface topography induced by a bottom load is very close to that inferred for Airy isostasy. However, due to the small size of the moon the relaxation to equilibrium is significantly longer than the maximum Maxwell relaxation time. During the relaxation, the upper load/bottom load ratio can be either smaller than 1 or larger than 1, depending on the initial state and the loading history.

As a prospective study for a future exploration of Venus, we compute the tidal response of Venus' interior assuming various mantle compositions and temperature profiles representative of different scenarios of Venus' formation and evolution. The mantle density and seismic velocities are modeled from thermodynamical equilibria of mantle minerals and used to predict the moment of inertia, Love numbers and tide-induced phase lag characterizing the signature of the internal structure in the gravity field. The viscoelasticity of the mantle is parameterized using an Andrade rheology. From the models considered here, the moment of inertia lies in the range of 0.327 to 0.342, corresponding to a core radius of 2900 to 3450 km. Viscoelasticity of the mantle strongly increases the potential Love number relative to previously published elastic models. Due to the anelasticity effects, we show that the possibility of a completely solid metal core inside Venus cannot be ruled out based on the available estimate of k2 from the Magellan mission [Konopliv and Yoder, 1996]. A Love number k2 lower than 0.27 would indicate the presence of a fully solid iron core while, for larger values, solutions with an entirely or partially liquid core are possible. Precise determination of the Love numbers, k2 and h2, together with an estimate of the tidal phase lag, are required to determine the state and size of the core, as well as the composition and viscosity of the mantle.