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Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates.

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.

Introduction Titan is the only moon in the solar system with a thick atmosphere, dominated by nitrogen and organic compounds and methane- and ethane-based climatic cycles similar to the hydrological cycle on Earth. Hence, Titan is a prime target for planetary and astrobiological researches. Heaviest organic materials resulting from atmospheric chemistry (including high atomic number aerosols) precipitate onto the surface and are subject to geological processes (e.g., eolian and fluvial erosion) that lead to the formation of a variety of landscapes, including dune fields, river networks, mountains, labyrinth terrains, canyons, lakes and seas analogous to their terrestrial counterparts but in an exotic context. Its optically thick atmosphere, however, prevents the surface from being probed in the entirety of the near-infrared (NIR) range, and its composition is still largely unknown, or largely debated at the least, preventing to fully understand and quantify the geological processes at play. Incident and reflected solar radiations are indeed strongly affected by gaseous absorption and aerosol scattering in the NIR. Only where the methane absorption is the weakest, a few transmission windows allow the detection of radiation coming from the low atmosphere and the surface, making possible to retrieve the surface albedo. In the 0.88-5.11 μm range (VIMS-IR channel), the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board the Cassini spacecraft has shown that the surface can be observed in eight narrow transmission windows centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.69, and 2.78 μm, and in the 5.0-5.11 μm interval. Even in these transmission windows, residual gaseous absorption and increasing scattering from aerosols with decreasing wavelength make the analysis of the surface signal and the retrieving of surface albedo complex and delicate. In order to retrieve the surface albedo in the atmospheric windows in the most possible rigorous way, we have developed a radiative transfer (RT) model with up-to-date gaseous abundances profiles and absorption coefficients and improved photochemical aerosol optical properties. We validated our model using in situ observations of Huygens-DISR (Descent Imager / Spectral Radiometer) acquired during descent and once landed. We then applied our RT model to the Selk crater area (the Dragonfly mission landing area) in order to map the surface albedo and discuss the surface properties of the different geomorphological units of the region. Radiative transfer Our RT model is based on the SHDOM solver to solve the RT equations using the plan-parallel approximation. Vertical abundance profiles and absorption lines of CH4 and isotopes, CO, C2H2 and HCN are implemented using the most recent studies. Correlated-k coefficients are used to calculate gases absorption coefficients at VIMS-IR spectral sampling and resolution. Aerosols extinction profile and single scattering albedo are described using a fractal code developed by [1], allowing the aerosol fractal dimension to be varied. Aerosols phase function is modified using a multi-angular VIMS sequence (Sébastien Rodriguez, personal communication). Our model is validated using the in situ observations of Huygens-DISR acquired during the complete descent sequence and once landed. Application We applied our RT model to the Selk crater region by inverting aerosol opacity and surface albedo over 4 VIMS cubes (1578266417_1, 1575509158_1, 1578263500_1, 1578263152_1) acquired over the area. We built local maps of aerosol opacities and surface albedos of the Selk region by combining the 4 VIMS cubes on a geographically projected mosaic (see the mosaic of the 4 raw VIMS observations in Fig. 1). A few longitudinal profiles of the retrieved atmospheric properties are shown in Fig. 2. Slopes and seams between cubes of the aerosol opacities, originally due to varying observation geometries between flybys, have been entirely corrected, confirming the robustness of our RT model and making the retrieved surface albedo more reliable. Retrieved surface albedo have been then corrected for the photometry using in-situ observations ([3]). The resulting albedo maps of the regions are highly contrasted and homogeneous, most of the seams between cubes (due to residual surface photometry) being corrected (Fig. 3). Conclusion We developed and validated a new RT model for Cassini-VIMS observations of Titan with up-to-date atmospheric optical description. Coupled with an efficient inversion scheme, our model can be apply to the complete VIMS dataset for the retrieval of Titan’s atmospheric opacities and surface albedos at regional and global scales. References [1] Rannou, P., McKay, C., & Lorenz, R. 2003, Planetary and Space Science, 51, 963 [2] Karkoschka, E., Schröder, S. E., Tomasko, M. G., & Keller, H. U. 2012, Planetary and Space Science, 60, 342

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.

Internal structure models of dwarf planets and giant planets’ moons previously assumed essentially Earth-like silicate-metal cores surrounded by ice. Inner density models of the rocky cores of differentiated Ganymede and Titan, the largest icy moons in the solar system indicate the presence of a low-density component in addition to silicates and metal sulfide. Carbonaceous matter akin to coal formed from abundant organic matter in the outer solar system is a likely low-density component. Progressive gas release from coal may sustain up to present-day the replenishment of ice-oceanic layers in organics and volatiles. This accounts for widespread observation of nitrogen as well as light hydrocarbons to complex organic molecules at the surface, in the atmospheres, or in plumes emanating from moons and dwarf planets. Analysis of available density of rocky cores of other icy moons and dwarf planets also suggests the presence of a low-density carbonaceous component. We tested this hypothesis and found that rocky core densities in dwarf planets and icy moons are consistent with a mixture of chondritic silicate-sulfide rocks and a rock-free precursor composed of ices and carbonaceous matter in near-solar proportions. Thermal models taking into account the presence of carbonaceous matter is performed to evaluate its effects on the present-day structure of icy moons and dwarf planets.

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.

Titan, Saturn’s largest satellite, is the only icy moon with a dense atmosphere. This atmosphere is composed mainly of N2. Methane, the second most abundant constituent, would be depleted in only 3...

International audience; The dissolution of Fe-silicates in the Enceladus' core takes place at far-from-equilibrium conditions, resulting in elevated reaction rates.-The duration of H 2 generation resulting from serpentinization of Enceladus' core does not exceed 500 Myr.

Titan is unique in the solar system: it is an ocean world, an icy world, an organic world, and has a dense atmosphere. It is a geologically active world as well, with ongoing exogenic processes, such as rainfall, sediment transportation and deposition, erosion, and possible endogenic processes, such as tectonism and cryovolcanism. This combination of an organic and an ocean world makes Titan a prime target for astrobiological research, as biosignatures may be present in its surface, in impact melt deposits and in cryovolcanic flows, as well as in deep ice and water ocean underneath the outer ice shell. Impact craters are important sites in this context, as they may have allowed an exchange of materials between Titan's layers, in particular between the surface, composed of organic sediments over icy bedrock, and the subsurface ocean. It is also possible that impacts may have favored the advance of prebiotic chemical reactions themselves, by providing thermal energy that would allow these reactions to proceed. To investigate possible exchange pathways between the subsurface water ocean and the organic-rich surface, we modeled the formation of the largest crater on Titan, Menrva, with a diameter of ca. 425 km. The premise is that, given a large enough impact event, the resulting crater could breach into Titan's ice shell and reach the subsurface ocean, creating pathways connecting the surface and the ocean. Materials from the deep subsurface ocean, including salts and potential biosignatures of putative subsurface biota, could be transported to the surface. Likewise, atmospherically derived organic surface materials could be directly inserted into the ocean, where they could undergo aqueous hydrolysis to form potential astrobiological building blocks, such as amino acids. To study the formation of a Menrva-like impact crater, we staged numerical simulations using the iSALE-2D shock physics code. We varied assumed ice shell thickness from 50 to 125 km and assumed thermal structure over a range consistent with geophysical data. We analyze the implications and potential contributions of impact cratering as a process that can facilitate the exchange of surface organics with liquid water. Our findings indicate that melt and partial melt of ice took place in the central zone, reaching ca. 65 km depth and ca. 60 km away from the center of the structure. Furthermore, a volume of ca. 102 km3 of ocean water could be traced to depths as shallow as 10 km. These results highlight the potential for a significant exchange of materials from the surface (organics and ice) and the subsurface (water ocean), particularly in the crater's central area. Our studies suggest that large hypervelocity impacts are a viable and likely key mechanism to create pathways between the underground water ocean and Titan's organic-rich surface layer and atmosphere.

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

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories. References: [1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764. Acknowledgements: This work was conducted at the California Institute of Technology (Caltech) under contract with NASA. Y.M. and A.S. (partly) was supported by the Czech Science Foundation (grant no. 20-27624Y). ©2021 California Institute of Technology. Government sponsorship acknowledged.

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.[1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764.

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system., Submitted to the PSJ Focus Issue on Ocean World Exploration

Venus Corona and Tessera Explorer (VeCaTEx) would use an aerobot to descend repeatedly beneath the dense clouds for imaging targeted area of the surface in the near infrared to address six of the prime investigations prioritized by VEXAG. The technologies needed could be matured during the next decade.

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

In this chapter, we review the contribution of space missions to the determination of the elemental and isotopic composition of Earth, Moon and the terrestrial planets, with special emphasis on currently planned and future missions. We show how these missions are going to significantly contribute to, or sometimes revolutionise, our understanding of planetary evolution, from formation to the possible emergence of life. We start with the Earth, which is a unique habitable body with actual life, and that is strongly related to its atmosphere. The new wave of missions to the Moon is then reviewed, which are going to study its formation history, the structure and dynamics of its tenuous exosphere and the interaction of the Moon’s surface and exosphere with the different sources of plasma and radiation of its environment, including the solar wind and the escaping Earth’s upper atmosphere. Missions to study the noble gas atmospheres of the terrestrial planets, Venus and Mars, are then examined. These missions are expected to trace the evolutionary paths of these two noble gas atmospheres, with a special emphasis on understanding the effect of atmospheric escape on the fate of water. Future missions to these planets will be key to help us establishing a comparative view of the evolution of climates and habitability at Earth, Venus and Mars, one of the most important and challenging open questions of planetary science. Finally, as the detection and characterisation of exoplanets is currently revolutionising the scope of planetary science, we review the missions aiming to characterise the internal structure and the atmospheres of these exoplanets.

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.

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure.

International audience; Liquid water oceans are now predicted to exist beneath the icy shells of numerous worlds in the outer solar system. These ocean worlds are prime targets in our search for evidence of life beyond Earth, and specifically extant life. Here we review the conditions that may lead to several of these worlds being habitable, and provide a framework for the future exploration of these astrobiologically compelling targets.

We describe the prediction, design, execution and calibration of stellar and solar occultation observations of Saturn's rings by the Visual and Infrared Mapping Spectrometer (VIMS) instrument on the Cassini spacecraft. Particular attention is paid to the technique developed for onboard acquisition of the stellar target and to the geometric and photometric calibration of the data. Examples of both stellar and solar occultation data are presented, highlighting several aspects of the data as well as the different occultation geometries encountered during Cassini's 13 year orbital tour. Complete catalogs of ring stellar and solar occultations observed by Cassini-VIMS are presented, as a guide to the standard data sets which have been delivered to the Planetary Data System's Ring Moon Systems Node., 90 Pages, 22 Figures, Accepted for Publication in Icarus

The inner structure of icy moons comprises ices, liquid water, a silicate rocky core and sometimes an inner metallic core depending on thermal evolution and differentiation. Mineralogy and density models for the silicate part of the icy satellites cores were assessed assuming a carbonaceous chondritic (CI) bulk composition and using a free-energy minimization code and experiments [1]. Densities of other components, solid and liquid sulfides, carbonaceous matter, were evaluated from available equations of state. Model densities for silicates are larger than assessed from magnesian terrestrial minerals, by 200 to 600 kg/m3 for the hydrated silicates, and 300 to 500 kg/m3 for the dry silicates, due to the lower iron bulk concentration in terrestrial silicates as a lot of iron is segregated in the core.We find that CI density models of icy satellite cores taking into account only the silicate and metal/sulfide fraction cannot account for the observed densities and reduced moment of inertia of Titan and Ganymede without adding a lower density component. We propose that this low-density component is carbonaceous matter derived from insoluble organic matter, in proportion of ~30-40% in volume and 15-20% in mass. This proportion is compatible with contributions from CI and comets, making these primitive bodies including their carbonaceous matter component likely precursors of icy moons, and potentially of most of the objects formed behind the snow line of the solar system. Similar conclusions are reached for 1-Ceres when applying this compositional model, with even higher carbon content of the order of 25±5wt% in line with independent estimates [2]. It suggests that the building materials are similar for asteroid 1-Ceres and the icy moons of giant planets. [1]Neri et al., Earth Planet Sci Letters, 530 (2020) 115920[2]Zolotov, Icarus, 335 (2020) 113404

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.

We explore the possibility that Callisto’s ocean sits beneath its high-pressure ice, rather than above it. Oceans perched between ice phases are considered to be stable configurations for Ganymede,...