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This study presents a comprehensive assessment of the geomorphology, crater distributions, and tectonic structures within Enceladus' cratered terrains. We analyzed the distributions of impact craters and tectonic structures in seven regions of interest to inform an interpretation of the geological history of this terrain in the context of Enceladus' global evolution. We found that the tectonic structures, including both ancient, subdued troughs and young, narrow fractures, point to a cratered terrain that not only experienced early tectonic modification but also shows evidence of recent geological activity. Ancient troughs present in the equatorial cratered terrains are similar in scale and orientation to troughs present in the Leading and Trailing Hemisphere Terrains, an observation that supports possible non‐synchronous rotation of the ice shell. A dearth of impact craters in the equatorial regions as identified previously does not hold for craters <3 km in diameter in the anti‐Saturnian hemisphere. The anomalous presence of excess small craters in this region could be due to secondary or sesquinary impacts from a catastrophic event occurring at Enceladus or a neighboring moon. Finally, narrow fractures are pervasive across the cratered terrains and are most commonly oriented parallel or sub‐parallel to the most proximal cratered terrain boundary. This directionality of pervasive recent fracturing could be related to the vertical movement of an isostatically uncompensated ice shell. Enceladus' cratered terrains provide insight into the long‐term evolution of the satellite, an important component to assessing its role in Solar System evolution and its potential for habitability. Plain Language Summary: We present a comprehensive characterization of the oldest terrain on Enceladus, the cratered terrains. We evaluated structural features and impact craters on the surface, including their morphology and spatial distributions. This analysis informs how we interpret the formation and change of these surface features in the context of how Enceladus as a whole has changed over time. We found that there are two main types of tectonic structures in the cratered terrains, including one set of relatively old troughs and at least one set of narrow fractures that is very young. This finding indicates that not only have the cratered terrains experienced early tectonic activity, but were also active recently. The ancient set of troughs are similar to those present in younger terrains on Enceladus, suggesting that the ice shell has rotated relative to its tidally locked position about Saturn. The comparatively young, narrow fractures in the cratered terrains are widespread and could have formed due to a vertically shifting ice shell. Enceladus' cratered terrains provide insight into what long‐term changes have taken place on the surface, an important component to understanding how Enceladus fits into Solar System formation and its past or present potential to host life. Key Points: Systematic review of Enceladus' cratered terrains results in a stratigraphic framework for geologic mapping and age determinationTroughs in the equatorial cratered terrains are similar to tectonized equatorial regions, suggesting non‐synchronous ice shell rotationAncient, subdued troughs and young, narrow fractures point to a cratered terrain that experienced both early and recent tectonic activity [ABSTRACT FROM AUTHOR]

We determined the frictional strength and stability of polycrystalline ice and ice‐ammonia to constrain fault behavior on icy satellites such as Enceladus and Europa. Friction experiments including velocity steps and slide‐hold‐slide tests were conducted to measure the steady‐state coefficient of friction, velocity dependence and healing between temperatures of 98 and 248 K at a normal stress of 100 kPa. Rate‐state friction parameters determined from velocity steps provide stability values. The friction results are used to infer fault strength and frictional heating of an icy crust with depth for both a pure ice crust and one containing ammonia. We find a reduced coefficient of friction for an ammonia‐bearing crust and stronger velocity dependence in the presence of partial melt. The temperature dependence of fault stability maps a seismogenic zone with depth analogous to the synoptic model for terrestrial fault stability, where we find instability between 0.7 and 3.9 km with a return to stability from 4.6 km if we assume a 6 km ice shell. We consider the role of sliding velocity and fault thickness on localized frictional heating in both systems and estimate the depth of melt generation in an ammonia‐bearing crust. Our results imply that faults at conditions similar to icy satellites can be seismogenic. Plain Language Summary: We performed laboratory friction experiments with ice at conditions (e.g., temperature, composition, stresses) analogous to tidally driven faults on icy satellites in the outer solar system. We observed a temperature dependence on the coefficient of friction of ice under these conditions, which may help improve shear heating models on icy faults. We use the velocity dependence of friction observed in the experiments to constrain the depths (based on temperature) in a 6 km ice shell where seismic activity may occur. We find instability between 0.7 and 3.9 km with a return to stability from 4.6 km. We explored the effect of partial melt on ice friction and fault stability using ice‐ammonia mixtures and found that the presence of ammonia decreases the coefficient of friction while the presence of partial melt increases the velocity dependence. Key Points: Velocity weakening friction behavior is observed below 220 K, indicating a potential seismogenic zone with depth on icy moonsWe establish temperature dependence of the coefficient of friction for water ice that can be used to improve shear heating modelsThe effect of solid‐state impurities on friction is constrained using homologous temperature and melt fraction with ice + ammonia mixtures [ABSTRACT FROM AUTHOR]

Global‐scale periodic deformation has been studied using the (visco)elastic gravitational theory, which assumes a planetary body consists of solid or liquid layers. Recent planetary exploration missions, however, suggest that a global layer of a mixture of solid and liquid exists in several planetary bodies. This study provides a theory of periodic deformation of such a layer unifying the viscoelastic gravitational theory with the theory of poroelasticity without introducing additional constraints. The governing equation system and a formulation suitable for numerical calculation are given. Equations used to calculate the energy dissipation rate are also given. The analytical solutions for a homogeneous sphere are obtained using an eigenvalue approach. Simple numerical calculations assuming a homogeneous sphere reveal that a numerical instability occurs if a thick porous layer, a low permeability, or a high frequency is assumed. This instability can be avoided by choosing an appropriate interior structure model that is numerically equivalent. Different simple numerical calculations adopting a multilayered, radially varying interior profile reveal that the radial profile of the tidal heating rate for a fluid‐saturated porous layer and that for a low‐viscosity solid layer are completely different. In addition, the radial variation in porosity can lead to a factor of ∼100 increase in the local heating rate. These results indicate that future studies should consider a wider variety of detailed interior structure models. Plain Language Summary: Global periodic deformations, such as tidal and seismic motions, are fundamental phenomena that have been studied for a long time. While planetary exploration missions suggest the presence of solid‐liquid mixed layers in solar system bodies, a comprehensive theory describing deformation of such a layer has not been established. In this study, a classic geophysical approach and a classic engineering approach are unified to investigate the dynamics of such a layer. A full set of equations as well as analytical solutions are derived. A numerical issue that is likely to be encountered under typical calculation conditions is discussed. A simple calculation assuming tides on the Saturnian satellite Enceladus indicates that the radial variation in porosity can lead to a significant increase in the tidal heating rate in a shallow part of the water‐saturated rocky core. The theory established in this study will be valuable for future detailed studies. Key Points: The theory for periodic deformation of a planetary body with a global, fluid‐saturated porous layer is establishedThe procedure to make numerical calculations accurate and stable is givenThe importance of the direct calculation of the fluid flow and that of the radial porosity profile are demonstrated [ABSTRACT FROM AUTHOR]

The Cassini spacecraft demonstrated that Saturn's small moon Enceladus may harbor hydrothermal activity. In particular, molecular hydrogen production could result from water‐rock interactions in a tidally heated, water‐filled porous rocky core. The lifetime of such reactions is key to assess the habitability potential of Enceladus and to constrain plausible durations of the active stage in a context where the evolution of the moon is debated. Although it has recently been suggested that the serpentinization timescale does not exceed a few hundred million years, this estimation was based on assumptions regarding silicate dissolution kinetics that are prone to overestimate the actual reactivity of primary silicates. Here, we investigated plausible rate‐limiting mechanisms governing fluid‐rock interactions that could delay the completion of Enceladus' core serpentinization. In particular, we considered the impact of (a) various secondary mineral assemblages on the Gibbs free energy of Fe‐bearing silicate dissolution and associated dissolution rates; (b) rate‐laws alternative to the transition state theory; (c) diffusion in nanoporous secondary assemblages; (d) slow water supply. Overall, our results confirm that serpentinization timescales never exceed 500 Myr, and indicate that fluid flow ultimately sets the tempo for serpentinization. Only unreasonable grain sizes in Enceladus' core (>1 m) or unexpectedly low diffusivity of secondary coatings covering primary silicates would be consistent with serpentinization durations of several billion years. We thus suggest that either the hydrothermal activity has developed recently on Enceladus, or alternative processes (pyrolysis of insoluble organic matter, microbial activity) must be tested to explain the observed H2 flux in Enceladus' plume. Plain Language Summary: Enceladus is a small icy moon orbiting Saturn that presents a striking activity with jets emanating from parallel fractures in the tectonized south polar terrain. The amplitude of the moon's rotation indicates the presence of a global internal ocean, and chemical analyses by the Cassini spacecraft of the materials ejected from the interior suggest the ongoing occurrence of high‐temperature water‐rock interactions, possibly in the porous rocky core. In particular, this conjecture stems from the detection of molecular hydrogen (H2), a well‐known by‐product of the aqueous alteration of iron‐bearing silicates. The lifetime of such reactions is key to assess both the habitability potential of Enceladus and to constrain plausible durations of the present active stage in a context where the evolution of the moon is debated. Here, we modeled such lifetimes, exploring various scenarios for the rate‐limiting mechanisms of the process. Overall, we found that the timescales of hydrogen production never exceed 500 Myr, and are ultimately controlled by the water supply in the core. We thus suggest that either the hydrothermal activity has developed recently on Enceladus, or that alternative processes including microbial activity must be tested to explain the observed H2 flux in Enceladus' plume. Key Points: The dissolution of Fe‐silicates in the Enceladus' core takes place at far‐from‐equilibrium conditions, resulting in elevated reaction ratesThe duration of H2 generation resulting from serpentinization of Enceladus' core does not exceed 500 MyrAlternative processes must be explored to explain the observed rate of H2 emission in Enceladus' plume [ABSTRACT FROM AUTHOR]

In recent years, Enceladus' subsurface ocean has become a tantalizing case study for potentially habitable conditions in an extraterrestrial ocean world. However, we still know very little about its subsurface conditions. Its oceanic composition is difficult to characterize with current data and estimates are highly dependent on model‐based interpretations which are also not yet tightly constrained. In light of these uncertainties, we consider a wide selection of the inferred parameter spaces to quantify the energy available to putative hydrogenotrophic methanogens on Enceladus in the bulk ocean at cool and elevated temperatures. We estimate the instantaneous power supply their metabolism could provide in these conditions and compare it to the expected power demands of life on Earth. To be habitable for methanogens a 273 K ocean with relatively high salt content must have pH < 10, and a relatively low salt ocean must have pH < 8 at 273 K, or pH < 9 when heated to >360 K. Some combinations meet the power demands of exponential growth, but large swathes of the parameter space appear energetically uninhabitable. The habitability of the Enceladus ocean for methanogens appears to be a delicate balance between its temperature, pH, salinity, and concentrations of carbonates, nutrients, and dissolved gases (particularly H2). Many of these parameters are co‐dependent; variation in any one of them could tip the balance into uninhabitable conditions. Further constraining these variables should be a priority for future missions to ocean worlds to enable definitive assessments of their habitability. Plain Language Summary: Observations of Enceladus in recent years have revealed tantalizing details of its potentially habitable subsurface ocean, allowing its conditions to be resolved in unprecedented detail compared to other icy moons. Still, the variation in possible parameters below its icy shell is huge, ranging from the cold bulk ocean with a pH similar to seawater on Earth to potentially scalding alkaline fluids at its depths. The ocean contains the ingredients of an ancient metabolism that is used by life on Earth by organisms known as methanogens. In this study, we explore the instantaneous habitability of Enceladus' subsurface ocean to methanogens using a range of environmental parameters informed by data from the Cassini mission and modeling. In other words, we ask: if Cassini's observations offered a "snapshot" view of the ocean, does a habitable window exist within the uncertainty of the data, without considering as‐yet unconstrained (but still important) variables such as a supply of nutrients? Some parameter combinations appear habitable, but not all the combinations are suitable for methanogens as we know them on Earth. We identify the most important drivers of habitability in Enceladus' ocean and explain how they can be better constrained by future research or space missions. Key Points: •Key known drivers of the Enceladus ocean's habitability with respect to methanogens are identified and their uncertainties accounted for•There is energy available for methanogens in most cases if the pH of the bulk ocean is less than 10•Instantaneous microbial power supplies imply both habitable and uninhabitable conditions are possible [ABSTRACT FROM AUTHOR]

The submeter‐scale surface morphology of ice‐covered worlds is poorly constrained by observations, but is important for understanding their physics and chemistry. In this experiment, we exposed preformed snow structures to temperature, pressure, and solar irradiance conditions similar to those on Europa and Enceladus. The sample consisted of 2‐D sinusoidal penitente‐like ridges, 2.5 cm tall with 1.25 cm spacing. Over 1 month at 165–175 K, and 10−4 Pa, the ridges decreased in height by 20% and became more triangular and less sinusoidal, evolving toward a more flat surface. This despite the fact that the troughs appear brighter than the peaks, indicating focusing of light into the troughs. This suggests that penitentes are unlikely to be stable on airless worlds with free‐molecular flow conditions, which is significant for understanding the spectral and thermal properties of these worlds, and for understanding risks for future landed spacecraft. Plain Language Summary: Ice penitentes are blade‐like ice or snow structures formed by sublimation and heating from the Sun, often seen at high altitudes on Earth's subtropical mountains, particularly the Andes. A key question is whether they can arise in vacuum, on worlds without an atmosphere such as Europa and Enceladus. This is important for understanding the physics of these worlds, as well as for understanding the terrain for future landed spacecraft. We have no high‐resolution images of these worlds to tell if such structures exist, so we use laboratory experiments to simulate these icy surfaces. In our experiment, we start with a preformed set of 2.5 cm tall penitente‐like ridges, formed using a plastic mold filled with snow. Inside a vacuum chamber, a solar‐like light source illuminated the penitentes for 60 days. During the first 2 weeks of the experiment, we kept ice temperatures between −138°C and −113°C, and we saw no changes. During the second half of the test, we raised temperatures to −98°C to −108°C. The penitentes shrank in height by 20%, and become more triangular in shape. This is an indication, consistent with model predictions, that penitentes need an atmosphere to grow, and we may not expect them on Europa and Enceladus. Key Points: This is the first ice penitente experiment under hard vacuum (free‐molecular) conditionsPreformed penitentes undergo sublimation erosion, evolving toward a flat morphology, during simulated solar irradiancePeaks are warmer than troughs, consistent with diminishing peak heights, despite visibly brighter troughs indicating focusing of light [ABSTRACT FROM AUTHOR]

The plumes naturally erupting from the icy satellite Enceladus were sampled by the Cassini spacecraft in high‐speed fly‐bys, which gave evidence of salt. This raises the question of how salt behaves under high‐speed impact, and how it can best be sampled in future missions to such plumes. We present the results of 35 impacts onto aluminum targets by a variety of salts (NaCl, NaHCO3, MgSO4, and MgSO4·7H2O) at speeds from 0.26 to 7.3 km s−1. Using SEM‐EDX, identifiable projectile residue was found in craters at all speeds. It was possible to distinguish NaCl and NaHCO3 from each other, and from the magnesium sulfates, but not to separate the hydrous from anhydrous magnesium sulfates. Raman spectroscopy on the magnesium sulfates and NaHCO3 residues failed to find a signal at low impact speeds (<0.5 km s−1) where there was insufficient projectile material deposited at the impact sites. At intermediate speeds (0.5 to 2–3 km s−1), identifiable Raman spectra were found in the impact craters, but not at higher impact speeds, indicating a loss of structure during the high speed impacts. Thus, intact capture of identifiable salt residues on solid metal surfaces requires impact speeds between 0.75 and 2 km s−1. [ABSTRACT FROM AUTHOR]

The detection of a water-rich plume erupting from 'tiger stripes' near the Enceladus south polar region is an important discovery of geologically active satellites within the Solar system. In this work, we apply an analytical approach to model the plume structure, using as a diagnostic the CO2 distribution extracted from the Cassini Ion and Neutral Mass Spectrometer measurements made during the E14, E17, and E18 flybys. Special focus is placed on the modelling of the spike-like structures by including only sources with substantial contributions to the plume densities. Such a procedure reduces greatly the model complexity and helps to better constrain the source parameters. Both the model source rate and Mach number are found to vary considerably among different sources during the same flyby and also among different flybys for the same source, revealing a complicated spatio-temporal variability in the plume structure. Our analysis suggests a total escape rate of (1.0–7.1) × 1026 s−1 for CO2 and (2.4–6.6) × 1028 s−1 for H2O, in broad agreement with previous estimates. Of particular interest is our identification of a tentative correlation between the Mach number and thermal brightness, of which the latter is a tracer of the geological activity of the emission source. Such a relation should be able to provide more insights into the nature of geyser emission along the 'tiger stripes'. [ABSTRACT FROM AUTHOR]

Dissipation of tidal energy is expected to generate seismicity on icy‐ocean worlds; however, the distribution and timing of this seismic activity throughout an orbital cycle is not known. We used new observations from an icy‐ocean‐world analog environment on Earth to examine the relationship between tidally driven tensile stress and seismic activity within an ice shell. We investigated a pair of rifts within Antarctica's Ross Ice Shelf which are tidally stressed in a manner analogous to the orbital cycle of tidal stress experienced by Enceladus' Tiger Stripe Fractures. We found that seismic activity at the Antarctic rifts is sensitive to both the amplitude and the rate of tensile stress across the rifts. We combined these findings with calculated stress values along Enceladus' Tiger Stripe Fractures to predict seismic‐activity levels expected along the ice‐shell fractures. We predict a peak in seismicity along the four Tiger Stripe Fractures when Enceladus is 90°–120° past pericenter in its orbit around Saturn, at which point tensile stresses would reach ∼2/3 of their maximum value. We also used the magnitude distribution of icequakes along Antarctic rifts to investigate implications for the likely size of stick‐slip rupture patches along icy faults on Enceladus. Our findings predict that the Tiger Stripe Fractures should produce sustained, low‐magnitude seismic events that involve rupture along discrete portions of each fracture's total length. We predict that seismicity would fall to 50% of peak levels when stresses across the Tiger Stripe Fractures are dominantly compressional. Plain Language Summary: Saturn's moon Enceladus is covered in ice which contains many long fractures or rifts, including four fractures called the Tiger Stripes. As Enceladus orbits Saturn, tidal forces cause the Tiger Stripe Fractures to erupt tall geysers of water vapor. This suggests that there is liquid water somewhere beneath Enceladus' icy surface and that there must be motion along the fractures. We expect that this motion should generate icequakes, similar to earthquakes on Earth. In this study, we built a model to predict where and when icequakes occur along the Tiger Stripe Fractures, using what we know about ice in Antarctica. We use a large floating expanse of solid ice, the Ross Ice Shelf, which contains fractures that are similar to the Tiger Stripe Fractures. We investigate Antarctic icequakes and find that the majority of icequakes occur during falling tide, when the rifts are being stretched open. We predict that the majority of icequakes along the Tiger Stripe Fractures occur around 100° into Enceladus' orbit and that these icequakes are very small, but numerous. Our results are valuable to future landed missions to Enceladus or another icy‐ocean world, where icequakes will help us understand what lies beneath the ice. Key Points: We predict significant seismicity along all four Tiger Stripe Fractures on Enceladus, with peak seismic activity ∼100° past pericenterWe find that rift seismicity is correlated with both applied tensile stress and tensile‐stress (stretching) rateGutenberg–Richter b‐value estimates for icequakes at studied rifts are ≥1.2, suggesting that b‐values >1 are appropriate for icy shells [ABSTRACT FROM AUTHOR]

This research provides updated and validated spatial distributions of Enceladus-generated H2O, OH, and O neutral tori. These tori are produced by the primary neutral source, Enceladus, and consist of populations of coorbiting neutral particles that provide the bulk of heavy molecules to Saturn's (neutral particle dominant) magnetosphere. We conducted self-consistent neutral particle and plasma modeling that is constrained and validated by all relevant Cassini and Hubble Space Telescope observations. Our results include a three dimensional (3D) spatial distribution of all three neutral tori. Significant findings include (1) short-term plume variability combines to produce an average H2O source rate of ∼280 kg/s; (2) Enceladus plume variability does not impact the neutral tori distribution because it occurs on time scales less than the orbital periodicity (<17 h); (3) H2O source torus particles quickly dissociate in <65 days while only ∼22% are directly ionized; (4) the resulting OH torus dissociates in <40 days and only ∼11% are directly ionized; (5) ∼59% of the significantly dispersed O torus is directly ionized; and (6) the near plume environment is very important and relatively small changes (<0.5 eV) in the core electron population within the plume region dramatically change the H3O+ abundance. [ABSTRACT FROM AUTHOR]

Context. Carbonic acid (H2CO3) is a weak acid relevant to astrobiology which, to date, remains undetected in space. Experimental work has shown that the β-polymorph of H2CO3 forms under space relevant conditions through energetic (UV photon, electron, and cosmic ray) processing of CO2- and H2O-rich ices. Although its α-polymorph ice has been recently reassigned to the monomethyl ester of carbonic acid, a different form of H2CO3 ice may exist and is synthesized without irradiation through surface reactions involving CO molecules and OH radicals, that is to say γ-H2CO3. Aims. We aim to provide a systematic set of vacuum ultraviolet (VUV) photoabsorption spectroscopic data of pure carbonic acid that formed and was destroyed under conditions relevant to space in support of its future identification on the surface of icy objects in the Solar System by the upcoming Jupiter ICy moons Explorer mission and on interstellar dust by the James Webb Space Telescope spacecraft. Methods. We present VUV photoabsorption spectra of pure and mixed CO2 and H2O ices exposed to 1 keV electrons at 20 and 80 K to simulate different interstellar and Solar System environments. Ices were then annealed to obtain a layer of pure H2CO3 which was further exposed to 1 keV electrons at 20 and 80 K to monitor its destruction pathway. Fourier-transform infrared (FT-IR) spectroscopy was used as a secondary probe providing complementary information on the physicochemical changes within an ice. Results. Our laboratory work shows that the formation of solid H2CO3, CO, and O3 upon the energetic processing of CO2:H2O ice mixtures is temperature-dependent in the range between 20 and 80 K. The amorphous to crystalline phase transition of H2CO3 ice is investigated for the first time in the VUV spectral range by annealing the ice at 200 and 225 K. We have detected two photoabsorption bands at 139 and 200 nm, and we assigned them to β-H2CO3 and γ-H2CO3, respectively. We present VUV spectra of the electron irradiation of annealed H2CO3 ice at different temperatures leading to its decomposition into CO2, H2O, and CO ice. Laboratory results are compared to Cassini UltraViolet Imaging Spectrograph observations of the 70−90 K ice surface of Saturn's satellites Enceladus, Dione, and Rhea. [ABSTRACT FROM AUTHOR]

Plain Language Summary: The southern leading hemisphere of Enceladus contains a unique terrain characterized by blocks of older, cratered material surrounded by areas of younger ridged terrain. In order to, investigate how this terrain formed and its implications for the surface history of Enceladus, we create a structural map of the region, infer a formation mechanism, and apply a simplified analytical model. From the structural map, we draw a comparison to shear zones on Earth where coherent blocks are rotated while the material around them is deformed. This comparison allows us to calculate the heat flux required in the region at the time of formation. We find that the heat flux in the region of the cratered blocks may have been high when the terrain formed, similar to that in the South Polar Terrain of Enceladus today, or involved high stresses. The Leading Hemisphere Terrain (LHT) of Enceladus has experienced distributed deformation expressed by the pervasive development of >100 km wide and >100s km long ductile strike‐slip shear zones and sparsely distributed craters. Within the zones are locally preserved little‐deformed semirigid blocks up to 10–20 km in size with surfaces dominated by impact craters. The edges of these relic blocks display ductile drag folds and smeared tails of rigid fragments, which are indicative of vertical‐axis rotation associated with ductile strike‐slip shear. Kinematic analysis requires the development of the LHT to have been driven mainly by toroidal flow of the ice‐shell material. Additionally, the structural mapping quantifies the shear strain magnitude (2.8) and minimum strain rate (10−17 s−1). If the observed deformation induced by tidal stress, the estimated ice‐shell deformation requires a high heat flux (∼0.13 W/m2) and a thin brittle layer (∼4 km) during the development of the LHT. However, the observed deformation may have formed under high stresses, exceeding 1 MPa and a heat flux similar to the current background observed today. Key Points: Regional left lateral shear could be responsible for forming the extensively deformed southern Leading Hemisphere TerrainCratered blocks ("crater islands") on Enceladus could have formed through a process analogous to rotated clasts in shear zones on EarthThe resurfacing event that formed the Leading Hemisphere Terrain (LHT) likely resulted from an elevated, South Polar Terrain (SPT)‐like surface heat flux or high stresses (>1 MPa) [ABSTRACT FROM AUTHOR]

Saturn's satellite Enceladus is proposed to have a soda‐type subsurface ocean with temperature able to support life and an iron ore‐based core. Here, it was demonstrated that ocean chemistry related to Enceladus can support the development of Fe‐based hydrothermal vents, one of the places suggested to be the cradle of life. The Fe‐based chemical gardens were characterized with Fourier‐transform (FT)IR spectroscopy and XRD. The developed chemobrionic structures catalyzed the condensation polymerization of simple organic prebiotic molecules to kerogens. Further, they could passively catalyze the condensation of the prebiotic molecule formamide to larger polymers, suggesting that elementary biochemical precursors could have emerged in Enceladus. [ABSTRACT FROM AUTHOR]

Saturn's ice-covered moon Enceladus may contain the requisite conditions for life. Its potentially habitable subsurface ocean is vented into space as large cryovolcanic plumes that can be sampled by spacecraft, acting as a window to the ocean below. However, little is known about how Enceladus' ocean fluids evolve as they freeze. Using cryo-imaging techniques, we investigated solid phases produced by freezing simulated Enceladean ocean fluids at endmember cooling rates. Our results show that under flash-freezing conditions (>10 K s-1), Enceladus-relevant fluids undergo segregation, whereby the precipitation of ice templates the formation of brine vein networks. The high solute concentrations and confined nature of these brine veins means that salt crystallization is kinetically inhibited and glass formation (vitrification) can occur at lower cooling rates than typically required for vitrification of a bulk solution. Crystalline salts also form if flash-frozen fluids are re-warmed. The 10 Îm-scale distribution of salt phases produced by this mechanism differs markedly from that of gradually cooled (~1 K min-1) fluids, showing that they inherit a textural signature of their formation conditions. The mineralogy of cryogenic carbonates can be used as a probe for cooling rate and parent fluid pH. Our findings reveal possible endmember routes for solid phase production from Enceladus' ocean fluids and mechanisms for generating compositional heterogeneity within ice particles on a sub-10 Îm scale. This has implications for understanding how Enceladus' ocean constituents are incorporated into icy particles and delivered to space. [ABSTRACT FROM AUTHOR]

Saturn's moon Enceladus has a global subsurface ocean and a porous rocky core in which water‐rock reactions likely occur; it is thus regarded as a potentially habitable environment. For icy moons like Enceladus, tidal heating is considered to be the main heating mechanism, which has generally been modeled using viscoelastic solid rheologies in existing studies. Here we provide a new framework for calculating tidal heating based on a poroviscoelastic model in which the porous solid and interstitial fluid deformation are coupled. We show that the total heating rate predicted for a poroviscoelastic core is significantly larger than that predicted using a classical viscoelastic model for intermediate to large (>1014 Pa·s) rock viscosities. The periodic deformation of the porous rock matrix is accompanied by interstitial pore fluid flow, and the combined effects through viscous dissipation result in high heat fluxes particularly at the poles. The heat generated in the rock matrix is also enhanced due to the high compressibility of the porous matrix structure. For a sufficiently compressible core and high permeability, the total heat production can exceed 10 GW—a large fraction of the moon's total heat budget—without requiring unrealistically low solid viscosities. The partitioning of heating between rock and fluid constituents depends most sensitively on the viscosity of the rock matrix. As the core of Enceladus warms and weakens over time, pore fluid motion likely shifts from pressure‐driven local oscillations to buoyancy‐driven global hydrothermal convection, and the core transitions from fluid‐dominated to rock‐dominated heating. Plain Language Summary: Discoveries made by the Cassini spacecraft revealed that we may find life even in our own cosmic neighborhood in the ocean of Saturn's moon Enceladus. Recent studies infer that the fluid‐saturated rocky core of Enceladus harbors some of the processes that likely gave rise to life on Earth. Here we develop a physics‐based model to understand the heating in the water‐saturated porous core of Enceladus during periodic deformation under tidal forces. We find that the periodically loaded moon distributes the heat generated during its deformation in both its rocky and fluid part. When the core is strong and rigid (likely in the early history), heating in the fluid could be hundreds or thousands of times larger than that in the solid rock; when the core is severely weakened (such as in its later history), more heat is distributed in the solid rock than in the fluid. We also find that the coupling between pore fluid and solid rock may enhance the heating of the core for a larger range of material properties, in contrast with earlier studies. The enhanced heating could provide explanation for the high heat flux measured for Enceladus, and a potential hot period in its early history. Key Points: A poroviscoelastic model is developed to explore the interaction between permeable flow and rock deformation during tidal deformationThe poroviscoelastic core generates more heat in both the fluid and solid component, compared to a solid viscoelastic coreFor certain permeabilities and viscosities, the total heating rate reaches 25–40 GW, similar to the observed value [ABSTRACT FROM AUTHOR]

A growing number of satellites in the outer solar system likely have global oceans beneath their outer icy shells. While the presence of liquid water makes these ocean worlds compelling astrobiological targets, the exchange of heat and materials between the deep interior and the surface also plays a critical role in promoting habitable environments. In this article, we combine geophysical, geochemical, and geological observations of the Jovian satellites Europa, Ganymede, and Callisto as well as the Saturnian satellites Enceladus and Titan to summarize our current state of understanding of their interiors and surface exchange processes. Potential mechanisms for driving exchange processes upward from the ocean floor and downward from the satellite surface are then reviewed, which are primarily based on numerical models of ice shell and ocean dynamics and complemented by terrestrial analog studies. Future missions to explore these exo-oceans will further revolutionize our understanding of ice-ocean exchange processes and their implications for the habitability of these worlds. [ABSTRACT FROM AUTHOR]

Enceladus produces plumes of water vapor and ice particles whose gaseous H2O molecules dissociate into OH and O. The gas ejecta form a large toroidal planetary cloud colocated with and partially sourced by a similarly large torus of ice grain ejecta. Gas torus neutrals are ionized by charge exchange, solar ultraviolet, and electron impacts, producing the thermal water group ions, W+ (O+, OH+, H2O+, and H3O+), which become energized in Saturn's magnetosphere. We study the components of energetic (~96 keV) W+ using Cassini Charge‐Energy‐Mass Spectrometer (CHEMS) data from 78 near‐equatorial main ring current passes (dipole L = 7–16, ±10° in latitude) in 2004–2010. W+ fractional abundances are ~53% O+, ~22% OH+, ~22% H2O+, and ~3% H3O+, when averaged over L = 7–16, resulting in a mean water group mass of 16.7 amu. This large energetic O+ fractional abundance, over twice that observed for thermal O+ at L < 10, qualitatively agrees with the broad atomic O cloud observed by Cassini and predicted by some models. Fractional abundance ratios for O+/W+, OH+/W+, and H2O+/W+ vary little over L = 7–21. Our observation of energetic H2O+/W+ ≈ 22% out to L ~ 21 suggests that neutral H2O spreads throughout the magnetosphere rather than being confined to a narrow torus centered on Enceladus' orbit. Although nearly constant at <2% for L > 13, H3O+/W+ tends to increase significantly to ~4–5% at L ~ 7–8. Key Points: Average ~96 keV energetic ion fractional abundances at L = 7–16 are O+/W+ ~ 0.53, OH+/W+ ~ 0.22, H2O+/W+ ~ 0.22, and H3O+/W+ ~ 0.03Partial number densities of energetic O+, OH+, H2O+, and H3O+ peak at L ~ 9.5, and the ratios O+/W+, OH+/W+, and H2O+/W+ vary little with LAt L < 11, H3O+/W+ increases steadily with decreasing L, whereas, at L > 13, H3O+/W+ varies little with L [ABSTRACT FROM AUTHOR]

Recent planetary data and geophysical modeling suggest that hydrothermal activity is ongoing under the ice crust of Enceladus, one of Saturn's moons. According to these models, hydrothermal flow in the porous, rocky core of the satellite is driven by tidal deformation that induces dissipation and volumetric internal heating. Despite the effort in the modeling of Enceladus' interior, systematic understanding—and even basic scaling laws—of internally heated porous convection and hydrothermal activity are still lacking. In this article, using an idealized model of an internally heated porous medium, we explore numerically and theoretically the flows that develop close to and far from the onset of convection. In particular, we quantify heat‐transport efficiency by convective flows as well as the typical extent and intensity of heat flux anomalies created at the top of the porous layer. With our idealized model, we derive simple and general laws governing the temperature and hydrothermal velocity that can be driven in the oceans of icy moons. In the future, these laws could help better constraining models of the interior of Enceladus and other icy satellites. Plain Language Summary: Enceladus, one of Saturn's icy moons, is known from planetary data to be the site of ongoing hydrothermal activity. According to recent modeling, this activity is driven by tidal distortion throughout its porous rocky core, which causes friction and induces volumetric heating. As subsurface liquid water penetrates through the core, it warms, rises, and returns into the ocean through localized hot spots. We introduce an idealized model of this hydrothermal circulation in order to understand the formation of hot spots, their typical size, and their activity. We find that the hydrothermal flow in the porous core of Enceladus is about a few centimeters per year and is thus much slower than circulations in the Earth's ocean crust. As a result, the timescale for hot‐spot activity‐variations is as long as a few million years. Despite the slowness of the circulation, we predict that it drives oceanic plumes with velocity of the order of 1 cm·s−1. Key Points: We carry out numerical and theoretical analysis of an idealized model of tidallydriven hydrothermalactivity inside EnceladusWith numerical and theoretical analysis, we explore the flow that develops in a porous core with volumetric heatingOur model allows us to predict typical temperature, velocity, and heat‐flux anomalies at the bottom of Enceladus' subsurface ocean [ABSTRACT FROM AUTHOR]

In 2015, the New Horizons spacecraft flew past Pluto and its moon Charon, providing the first clear look at Charon's surface. New Horizons images revealed an ancient surface, a large, intricate canyon system, and many fractures, among other geologic features. Here, we assess whether tidal stresses played a significant role in the formation of Charon's tensile fractures. Although presently in a circular orbit, most scenarios for Charon's orbital evolution include an eccentric orbit for some period of time and possibly an internal ocean. Past work has shown that these conditions could have generated stresses comparable in magnitude to other tidally fractured moons, such as Europa and Enceladus. However, we find no correlation between observed fracture orientations and those predicted to form due to eccentricity driven tidal stress. It, thus, seems more likely that Charon's orbit circularized before its ocean froze and that either tidal stresses alone were insufficient to fracture the surface or subsequent resurfacing removed these ancient fractures. Plain language summary: Charon's surface displays many fractures that appear to have formed when the surface was pulled apart. The source of stress that created these fractures is currently debated. We test the hypothesis that, in the past, Charon had a slightly elliptical orbit, which would have stressed the surface, potentially creating the observed fractures. We find that this model provides a poor match to the observed fractures over a wide range of parameter values. These findings suggest that Charon's orbit was circular for most of its history and that its ocean froze out after the orbit became circular. The observed fractures were likely formed as part of the freezing process, which has been suggested for other icy moons with canyons and fracture systems similar to those seen on Charon, although the details of this process are not well understood. Key Points: Charon's tectonic features are inconsistent with eccentricity tidal stressesIf Charon had an internal ocean, it froze out after orbit circularizationCharon's fractures lack the equatorial symmetry of most tidal stress patterns [ABSTRACT FROM AUTHOR]

Enceladus' plume is the dominant source of neutrals and plasma in Saturn's magnetosphere. The plasma results from the ionization of icy particles and water vapor, which are vented into Saturn's inner magnetosphere through fissures in Enceladus' southern polar region. These fissures are subjected to tidal stresses that can vary as Enceladus moves in a slightly eccentric orbit around Saturn. Plume activity and brightness have also been shown to vary with the moon's orbital position, reaching a maximum when Enceladus is farthest away from Saturn in its orbit (the Enceladus orbital apoapsis). In this paper we will show that temporal variations in the thermal electron density distribution correlate with the position of Enceladus in its orbit around Saturn, with the strongest density enhancements in the vicinity of Enceladus when the moon is in the post‐apoapsis sector of its orbit. Key Point: There is evidence of enhanced electron densities in the near‐moon environment when Enceladus is in the post‐apoapsis sector of its orbit [ABSTRACT FROM AUTHOR]

Ion velocities have been measured during the Enceladus E3 and E5 flybys using the Cassini Plasma Spectrometer (CAPS) instrument on the Cassini spacecraft. Data from three sensors in the CAPS instrument have been examined from two flybys that occurred during 2008. Positive ion measurements from the CAPS Ion Beam Spectrometer and Ion Mass Spectrometer have been used to measure positive ion velocities. The CAPS Electron Spectrometer has been used to complement the positive ion findings with measurements of negative ion velocities. Two velocities for the positive ions are found, with the fast ions (2.3–5.8 km/s) originating from the high‐speed neutral gas emission and slow ions (0.2–2.2 km/s) associated with the low‐speed thermal gas emission from Enceladus. Negative ions were found to be near stationary or northerly traveling, implying a deceleration mechanism within the plume. A tentative detection of fast negative ions was also recorded for one of the flybys. These findings will aid in future modeling of plume dynamics. Key Points: Velocities of positive and negative ions have been measured in the Enceladus plume using CAPS sensorsTwo distinct ion velocities are found, in agreement with fast gas and slow thermal emissions of neutralsThermal negative ions are near stationary relative to Enceladus, implying a deceleration mechanism such as an ambipolar electric field [ABSTRACT FROM AUTHOR]

The icy shells of Enceladus and Europa consist from top down of cold (~100 K) ice at low (<0.1 MPa) pressure, cold ice at high pressure up to ~10 MPa, and warm ice near 273 K the base. The pressure ~10 MPa and temperature near 273 K of basal ice within Enceladus and Europa are similar to that within terrestrial glaciers that are known to have seismicity (icequakes). Warm ice easily melts during sliding so its icequakes are qualitatively explained and expected on these satellites if the macroscopic strain rates are comparable to those within terrestrial glaciers subjected to oceanic tides. However, cold ice at high pressures does not readily macroscopically melt during sliding. A dynamic weakening mechanism for crustal faults in rock may be applicable to Enceladus and Europa. Micron‐scale real contacts support ~0.4‐GPa shear tractions and normal tractions on rapidly sliding ice faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak in ice. The macroscopic friction depends on the average strength of the asperity tips during the lifetimes of contact. The strength of the asperity tips self‐organizes so that frictional heating balances the heat lost from the asperity tip by conduction. The macroscopic coefficient of friction at coseismic sliding velocities decreases to a modest fraction of the low‐velocity coefficient of friction, but does not approach zero. This velocity‐weakening mechanism likely allows major icequakes within the cold interiors of Europa and Enceladus. Key Points: Thermal weakening at asperity tips occurs within cold ice at sliding velocities above 0.1 m/sCoefficient of friction of thermally weakened asperities in cold ice is similar to that of crustal rockThermal weakening of faults with Europa and Enceladus likely allows large icequakes [ABSTRACT FROM AUTHOR]

Enceladus is characterized by a south polar hot spot associated with a large outflow of heat, the source of which remains unclear.We compute the heat generated via viscous dissipation resulting from tidal and (longitudinal) libration forcing in the moon's subsurface ocean using the linearized Navier-Stokes equation in a three-dimensional spherical model.We conclude that libration is the dominant cause of dissipation at the linear order, providing up to ~0.001 GWof heat to the ocean, which remains insufficient to explain the ~10 GWobserved by Cassini.We also illustrate how resonances with inertial modes can significantly augment the dissipation. Our work is an extension to Rovira-Navarro et al. (2019, https://doi. org/10.1016/j.icarus.2018.11.010) to include the effects of libration and the presence of the icy crust. The model developed here is readily applicable to the study of other moons with a subsurface ocean and planets with a liquid core. [ABSTRACT FROM AUTHOR]

The Cassini-Huygens mission to Saturn provided a close-up study of the gas giant planet, as well as its rings, moons, and magnetosphere. The Cassini spacecraft arrived at Saturn in 2004, dropped the Huygens probe to study the atmosphere and surface of Saturn’s planet-sized moon Titan, and orbited Saturn for the next 13 years. In 2017, when it was running low on fuel, Cassini was intentionally vaporized in Saturn’s atmosphere to protect the ocean moons, Enceladus and Titan, where it had discovered habitats potentially suitable for life. Mission findings include Enceladus’ south polar geysers, the source of Saturn’s E ring; Titan’s methane cycle, including rain that creates hydrocarbon lakes; dynamic rings containing ice, silicates, and organics; and Saturn’s differential rotation. This Review discusses highlights of Cassini’s investigations, including the mission’s final year. [ABSTRACT FROM AUTHOR]

Highlights • Tidal forces induce internal inertial waves in Europa's and Enceladus' oceans. • Internal waves focus energy along internal shear layers that propagate in the ocean. • Inertial waves can focus tidal dissipation at the poles. • Inertial waves can result in ocean currents of significant amplitude (a few cm/s). • For an ocean of constant thickness, inertial waves cannot prevent freezing. Abstract Some of the moons of the outer solar system harbour subsurface liquid oceans. Tidal dissipation plays an important role in preventing these oceans from freezing. In the past, most studies considered only tidal dissipation in the solid layers of these bodies (rock and ice). Recently, new studies considering tidal dissipation in the oceans of these moons have appeared. All of them make use of the shallow water approximation. However, the use of this approximation might not be adequate. Here we consider the linear non-hydrostatic three dimensional response of these oceans to tidal forcing with the full Coriolis force. To do so we consider an ocean of homogeneous density contained within a perfectly spherical shell and neglect the effect of the ice shell. We force the ocean with a time changing tidal potential and observe patterns of periodic inertial waves that take energy from the global tidal forcing and focus it along thin shear layers that propagate in the fluid. We focus on Europa and Enceladus, showing that inertial waves result in fluid flows of significant amplitude (a few cm/s). Nevertheless, we find that under the previously mentioned assumptions tidal dissipation due to inertial waves is several orders of magnitude smaller than Europa's radiogenic heating and Enceladus' observed heat flux. Finally, we propose additional dissipation mechanisms that might play a relevant role in Europa and Enceladus and could be further investigated. [ABSTRACT FROM AUTHOR]

Highlights • Formation of Saturn's rings and Mimas from the collision of a comet with an icy moon. • Saturn ring formation mechanism produces nearly pure ice rings recently. • Ring parent moon initially trapped in mean motion resonance heats Enceladus. Abstract The disruption of an icy moon in a collision with an interloping comet a few hundred million years ago is a simple way to create Saturn's rings. A ring parent moon with a mass comparable to Mimas could be trapped in mean motion resonance with Enceladus and Dione in an orbit near the current outer edge of the rings just beyond the Roche zone. I present collisional N-body simulations of cometary impacts that lead to the partial disruption of a differentiated moon with a rocky core and icy mantle. The core can survive largely intact while the debris from the mantle settles into a ring of predominantly ice particles straddling the orbital radius of the parent moon. The nascent ring spreads radially due to collisional viscosity while mass re-accretes onto the remnant rocky core to form a new moon that can be identified as Mimas. The icy debris that migrates into the Roche zone evolves into Saturn's ring system. Torques from tidal interaction with Saturn and resonant interactions with the rings push the recently formed Mimas outward to its current position on the same timescale of a few hundred million years. This scenario accounts for the high ice fraction observed in Saturn's rings and explains why the ring mass is comparable to the mass of Mimas. The prior existence of a ring parent moon in mean motion resonance results in a tidal heating rate for Enceladus in the recent past that is significantly larger than the current rate. [ABSTRACT FROM AUTHOR]

Highlights • Our Modeling of diurnal tidal stresses and nonsynchronous rotation stresses on Enceladus suggest the cold, brittle ice layer on South Polar terrain on Enceladus is 2–6 km thinner than the north polar region. • Nonsynchronous rotation stresses are large enough to initiate fracturing near Enceladus's south pole. • Enceladus's tiger stripes are on the order of 100,000 years old. Abstract Numerous studies of the south polar region of Saturn's moon Enceladus have focused on the four main fractures (the tiger stripes), their associated water plume, and the effects of diurnal tidal stresses on these fractures. However, due to their small magnitude (10–100 kPa), diurnal stresses alone are unable to completely explain the initial formation of the tiger stripes and the many additional fractures in the region. We suggest nonsynchronous rotation (NSR) stresses, induced by a freely rotating ice shell over a global ocean, are large enough to overcome the tensile strength of the ice shell and initiate fracturing on Enceladus. Using the tidal stress calculation program SatStressGUI, we demonstrate the dependence of the magnitude of the NSR stress on the thickness of the ice shell, particularly the brittle outer layer of the ice shell. We suggest the brittle layer in the south polar region is 2–4 km thick whereas in the more northern regions, it is 6–8 km. The difference is most likely due to the elevated energy flux at the south pole, compared to the regions to the north, resulting in a warmer and thinner ice shell in the south polar region. The thinner ice around the south pole permits NSR tensile stresses of ∼5 MPa, enough to fracture the ice. However, the thicker ice to the north reduces the NSR stress to less than 2 MPa making fracturing less likely. The difference in ice shell thickness, and resultant NSR stress, can help explain the activity in the south, but relative quiescence in the north. Our modeled NSR stress magnitudes and orientations suggests a NSR period on the order of 1 Myr and advocate that the tiger stripes have rotated ∼45° clockwise about the south pole since their initial formation in response to NSR of the ice shell. If the ice shell is rotating at a constant rate, the tiger stripes would thus be up to ∼100,000 years old. An apparent decreasing angular separation between consecutive fracture sets that formed in the south polar region suggests an ice shell that has weakened in strength over the course of its recent geologic history; this could be a result of increased heating, thinning of the ice shell, or a combination of the two. As the ice shell thinned, the NSR stresses were enhanced, making fracturing more likely after a smaller amount of NSR compared to the previous fracturing episode. [ABSTRACT FROM AUTHOR]

Highlights • Oceanic tidal heating from eccentricity forcing is strongly suppressed on Enceladus and other small satellites by the ice shell's mechanical forcing. • For large satellites, oceanic tidal heating from eccentricity forcing is enhanced due to the ice shell's self-gravity. • An ice shell increases oceanic obliquity tide heating on most satellites, except Triton, due to self-gravity and the flow properties of Rossby-Haurwitz waves. • Non-linear tidal heating in barotropic ocean tides is still found to be small compared to radiogenic decay, except for Triton. • Dynamic tidal forcing drives a time-varying pressure excess at the ocean surface due to the restrictive nature of the ice shell. Abstract Subsurface ocean tides act as a mechanism to dissipate tidal energy in icy satellite interiors. We numerically model the effect of an ice shell on ocean tides using non-linear bottom drag for the first time. We demonstrate that subsurface oceans experience tidal pressurization due to the confining nature of the ice shell, and find that Enceladus' eccentricity forcing can generate up to 2.2 kPa of pressure excess at the ocean surface. Existing free-surface oceanic energy dissipation scaling laws are extended to subsurface oceans, and are benchmarked against our numerical results to within 10 %. We show that for the large bodies Ganymede, Europa and Titan, an ice shell increases eccentricity tidal heating due to self-gravity, whereas the shell's suppressive mechanical forcing reduces eccentricity tide dissipation on Enceladus and Dione by several orders of magnitude due to their high effective rigidities. In contrast, the ice shell enhances obliquity-forced dissipation in all satellites investigated, except Triton, because the largely divergence-free ocean response is unaffected by the shell's rigidity but is still enhanced by self-gravity. We conclude that the fundamental difference in ocean response to obliquity and eccentricity forcing, combined with self-gravity, results in increased obliquity heating and suppressed eccentricity heating in small satellites. For large satellites with low effective rigidities, the type of ocean response is less important because the shell's mechanical forcing has little impact on the flow, whereas self-gravity will enhance the response, and consequently dissipation, regardless of the forcing. Overall, obliquity tides are likely to dominate the tidal heating budget of icy satellite oceans, remaining the most prominent source of fluid dissipation in subsurface barotropic ocean tides. [ABSTRACT FROM AUTHOR]

Highlights • Ice shell thickness on Enceladus ranges from 5 km to more than 30 km. • Ice thickness variations are maintained by phase changes at ice/water interface. • Gravity and shape data suggest that the shell is close to dynamic equilibrium. • Steady state is obtained only if basal viscosity is larger than 3.1014Pa s. • Amplitudes of ocean heat flux exceed 60 mW/m2 in polar regions. Abstract 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. [ABSTRACT FROM AUTHOR]

Highlights • We present the results of laboratory experiments with carbonate-hydroxide chemical gardens designed to reproduce the possible conditions of mineral precipitation at a hydrothermal vent in the alkaline ocean of Saturn's moon Enceladus. We describe the membranaceous and tubular structures that form in this self-organized system. • We analyse the resulting material morphology, composition and microstructure using scanning electron microscopy (SEM), micro-Raman spectroscopy, X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX). We find that, as well as more common minerals such as calcite and vaterite, a significant amount of gaylussite may form under conditions present in Enceladus' soda ocean. • We discuss how gravity differences may affect the growth of such chemical-garden structures on Enceladus compared to on Earth. Abstract We present the results of laboratory experiments with carbonate-hydroxide chemical gardens designed to reproduce the possible conditions of mineral precipitation at a hydrothermal vent in the alkaline ocean of Saturn's moon Enceladus. We describe the membranaceous and tubular structures that form in this self-organized system. We analyse the resulting material morphology, composition and microstructure using scanning electron microscopy (SEM), micro-Raman spectroscopy, X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDX). We find that, as well as more common minerals such as calcite and vaterite, a significant amount of gaylussite may form under conditions present in Enceladus' soda ocean. We discuss how gravity differences may affect the growth of such chemical-garden structures on Enceladus compared to on Earth. [ABSTRACT FROM AUTHOR]

Highlights • DSMC simulations of the Enceladus South polar plumes. • Parametrization of the outgassing flow parameters (number density, speed components, kinetic and rotational temperature) at 10 km above ground, where the flow becomes collisionless. • Parametric study of the outgassing flow parameters variations as a function of the vent exit conditions: vent radius, water mixture mass flow, water vapor/water ice mass ratio, water ice grain radius, water vapor and water ice exit speed, vent exit angle and flow temperature. • The DSMC results that have been run for this paper are available at an online repository. Abstract The field parameters – number density, velocity components and temperature – for the Enceladus geysers, and possibly similar jets from other bodies, such as Europa, Ceres or comets, are expensive to obtain using physically correct simulation methods, such as the Direct Simulation Monte Carlo (DSMC) approach. It would be very useful to be able to correctly reproduce all the different states that the flow is undergoing while expanding into vacuum, from a high density and collisional state near the surface to free-molecular and collisionless at high altitude without resorting to expensive DSMC simulations in every case. In this work, we consider a two-phase water vapor/grain mixture exiting a circular vent, assuming a uniform radius of the water ice grains, and study how the field parameters can be fitted at an altitude of 10 km, where the flow is collisionless. To do so, we define simple functional forms for each of these fitted parameters, and we study how their coefficients vary as a function of the vent exit parameters, i.e. the vent radius, the water mixture mass flow, the water vapor/water ice mass ratio, the water ice grain radius, the water vapor and water ice exit speed, the vent exit angle and flow temperature. We define polynomial approximations to model these variations. We show that all the vent parameters have nearly-independent influences on the radial profiles at 10 km, except for the water vapor and water ice exit speed, for which we considered cross-correlations. We finally show that the geyser field parameters can be reconstructed using our parametric study for variations of the vent parameters within the range considered here, and in a time frame of a few milliseconds. The results of the parametrizations presented in this study can now be used to propagate the geyser field parameters using computationally inexpensive free molecular/ballistic codes up to higher altitudes. The DSMC results that have been run for this paper are available at an online repository. [ABSTRACT FROM AUTHOR]

Highlights • Simulated Enceladus plume and developed surface-deposition profiles. • Deposition profile of highly tilted jets largely inconsistent with surface images. • Jets with a large zenith angle are unlikely to be active for longer than one year. Abstract Since the discovery of an ice particle plume erupting from the south polar terrain on Saturn's moon Enceladus, the geophysical mechanisms driving its activity have been the focus of substantial scientific research. The pattern and deposition rate of plume material on Enceladus' surface is of interest because it provides valuable information about the dynamics of the ice particle ejection as well as the surface erosion. Surface deposition maps derived from numerical plume simulations by Kempf et al. (2010) have been used by various researchers to interpret data obtained by various Cassini instruments. Here, an updated and detailed set of deposition maps is provided based on a deep-source plume model (Schmidt et al., 2008), for the eight ice-particle jets identified in Spitale and Porco (2007), the updated set of jets proposed in Porco et al. (2014), and a contrasting curtain-style plume proposed in Spitale et al. (2015). Methods for computing the surface deposition are detailed, and the structure of surface deposition patterns is shown to be consistent across changes in the production rate and size distribution of the plume. Maps are also provided of the surface deposition structure originating in each of the four Tiger Stripes. Finally, the differing approaches used in Porco et al. (2014) and Spitale et al. (2015) have given rise to a jets vs. curtains controversy regarding the emission structure of the Enceladus plume. Here we simulate each, leading to new insight that, over time, most emissions must be directed relatively orthogonal to the surface because jets "tilted" significantly away from orthogonal lead to surface deposition patterns inconsistent with surface images. Data for maps are available in HDF5 format for a variety of particle sizes at http://impact.colorado.edu/southworth_data. [ABSTRACT FROM AUTHOR]

Highlights • How Saturnian mid-sized moons avoid resonant capture during orbital evolution? • The tidal orbital evolution of these moons was investigated by N-body simulations. • The ring torque is a key to reproduce the current orbits of the mid-sized moons. • The stored heat in Enceladus during the evolution may explain the current heat flow. Abstract The formation and orbital evolution of Saturn's inner mid-sized moons – Rhea, Dione, Tethys, Enceladus, and Mimas – are still debated. The most puzzling aspects are 1) how the Tethys–Dione pair and the Mimas–Enceladus pair passed through their strong 3:2 mean-motion resonances during the tidal orbital evolution, and 2) the current strong heat flow from Enceladus, which is a few orders of magnitude higher than the tidal energy dissipation caused by the present orbital eccentricity of Enceladus. Here we perform N-body simulations of the moons' orbital evolution from various initial conditions – assuming that the moons were formed from Saturn's hypothetical massive ring – and investigate possible paths to solve the above difficulties. If the moons remain on nearly circular orbits and the influence of the rings is neglected, we find that the Tethys–Dione pair cannot avoid becoming trapped in the 2:1 and 3:2 mean-motion resonances as they recede from Saturn, and that the Tethys–Enceladus pair cannot avoid collisions after the resonance trapping, in case Saturn's quality factor is smaller than 15,000. These findings are inconsistent with the current orbital configuration. However, taking into account both the eccentricity excitation and the orbital expansion caused by the ring torque, we find that these resonance captures are avoided. With the relatively high eccentricity pumped up by the torque, Enceladus passes through all the mean-motion resonances with Tethys, and the Dione–Tethys pair passes through their 2:1 resonance and possibly the 3:2 resonance as well. After Enceladus resides beyond the 2:1 resonance with the outer ring edge, the eccentricity can be tidally damped. While this is a promising path of evolution, in most runs, Enceladus collides with Tethys by the excited eccentricity. There is a hint that a ring mass decrease (possibly due to Mimas formation) could avoid the collision between Enceladus and Tethys. The parameter survey taking into account detailed ring evolution and Mimas is left for future study. The heat that was tidally dissipated due to the eccentricity excitation by the ring torque in the past is stored in the moons and slowly radiated away through conductive transfer. The stored heat in Enceladus may account for the current anomalously high heat flow. [ABSTRACT FROM AUTHOR]

Highlights • We present the first map of CO 2 at the surface of Enceladus by Cassini VIMS. • The distribution of CO 2 in the Southern Polar Region suggests an endogenic origin and control by tectonic activity. • High surface temperature likely explains why the highest concentrations of CO 2 are measured between the Tiger Stripes and not directly on them. • Pure surface CO 2 could be vented at low-velocity (seeping) as a cold gas (∼70 to ∼119 K) from gas pockets that form between the internal ocean and the ice lid along the main active faults. • Complexed CO 2 is detected. • CO 2 clathrate hydrates, which form crystal structures, may originate in the internal ocean and be eventually released to the surface by gas pocket eruptions. Abstract We present the first map of CO 2 at the surface of Enceladus using data obtained by the Cassini Visible-Infrared Mapping Spectrometer (VIMS). In order to measure the weak and narrow CO 2 absorption band depths around 4.26 µm, we improved: (1) the calibration of VIMS spectra; (2) the calculation of geographic coordinates; and (3) the projection techniques. We averaged multiple observations of a given area to obtain a signal to noise ratio high enough to map the CO 2 abundances. CO 2 is reliably detected mostly in the South Polar Region. This region includes active faults (Tiger Stripes), the highest observed surface temperatures, and locations of active plume eruptions. The occurrence here of CO 2 is consistent with an endogenic origin controlled by tectonics. Both pure CO 2 ice and complexed CO 2 are detected from the positions of absorption bands between 4.27 and 4.24 µm. The highest concentrations of CO 2 are between the main active faults of the South Polar Region, where the surface temperature is low. Pure CO 2 ice deposits at the surface of Enceladus are best modeled by the formation of gas pockets below the icy crust and above the surface of the internal ocean. These pockets eventually release cold CO 2 gas (∼70 to ∼119 K) at low-velocity (seeping) between the Tiger Stripes [Matson et al., 2018, Icarus, 302, 18–26]. CO 2 clathrate hydrates may form in the ocean and may be subsequently released when a CO 2 gas pocket blows out and erupts. Other mechanisms may contribute to reinforcing the anti-correlation of the CO 2 distribution (of any type) with respect to the location of the Tiger Stripes, such as successive sublimation of CO 2 and condensation on colder areas, and partial frost cover by H 2 O releases from plume eruptions. [ABSTRACT FROM AUTHOR]

Abstract The Imaging Science Subsystem (ISS) on-board Cassini took a few high-resolution images of the icy Saturnian satellites Mimas and Enceladus over the last seven years of the Cassini mission during non-targeted flybys. We used the new Mimas images to improve the existing semi-controlled mosaic of Mimas. A new controlled Enceladus mosaic was published recently (Bland et al., 2015; Bland et al. in prep.). Both new mosaics are the baseline for improved atlases of Mimas in three tiles with a map scale of 1:1,000,000 and Enceladus in 15 tiles with a map scale of 1:400,000. The nomenclature for both satellites was proposed by the Cassini-ISS team and approved by the IAU. Highlights • The mosaics of Mimas and Enceladus were created using camera data from the recent Cassini orbits. • The final atlas of Mimas consisting of four tiles was created. • The final controlled atlas of Enceladus consisting of 15 tiles was created. [ABSTRACT FROM AUTHOR]

We provide new astrometric observations of Enceladus from images taken using the Cassini Imaging Science Subsystem (ISS) Narrow-Angle Camera. Astrometric reduction was performed using a new fully automatic camera repointing technique built within the Caviar software package. The new automatic procedure is based on the k-d tree technique and has been developed as an alternative to the manual technique currently available in Caviar. We describe the new procedure and demonstrate its effectiveness compared with manual processing. Repointing corrections were computed using reference stars from the UCAC4 and Gaia DR1 star catalogues. We compare the observed-minus-computed (O−C) residuals of Enceladus based on UCAC4 with those based on Gaia DR1, relative to the JPL SAT375 ephemeris and the IMCCE NOE ephemeris, respectively. We reduced all available Cassini ISS images of Enceladus during the period 2013–2017, producing a total of 1109 Cassini -centred astrometric observations. These are provided in right ascension (α) and declination (δ) in the International Celestial Reference Frame (ICRF). Estimated mean O−C residuals relative to JPL's SAT375 ephemeris were 0.7 km in α  × cos(δ) and 1.8 km in δ, with standard deviations of 2.4 and 3.2 km, respectively. No improvement in the precision of the observations was obtained when using the GAIA DR1 star catalogue compared with UCAC4. [ABSTRACT FROM AUTHOR]

Abstract The process by which the subsurface ocean on Enceladus is heated remains a puzzle. Tidal interaction with Saturn and Dione is the leading candidate but whether the dominant heating occurs in the solid core, ice crust or in the ocean itself is an outstanding question. Here we consider the driving effect of the longitudinal libration of the ice crust on the subsurface ocean and argue that the flow response should be turbulent even in the most benign situation of a smooth spherical ice shell when the motion of the boundary is only transmitted viscously. A rigorous upper bound on the turbulent viscous dissipation rate is then derived and used to argue that libration should be potent enough to explain the observed heat flux emanating out of Enceladus when the effects of tidal distortion and roughness of the ice crust are included. Highlights • Tides can directly drive fluid motions in bodies of rotating fluids. • The response in Enceladus's subsurface ocean should be turbulent. • An estimate of the turbulent dissipation rate indicates it should be sufficient to stop the subsurface ocean from freezing. [ABSTRACT FROM AUTHOR]

As a long-term energy source, tidal heating in subsurface oceans of icy satellites can influence their thermal, rotational, and orbital evolution, and the sustainability of oceans. We present a new theoretical treatment for tidal heating in thin subsurface oceans with overlying incompressible elastic shells of arbitrary thickness. The stabilizing effect of an overlying shell damps ocean tides, reducing tidal heating. This effect is more pronounced on Enceladus than on Europa because the effective rigidity on a small body like Enceladus is larger. For the range of likely shell and ocean thicknesses of Enceladus and Europa, the thin shell approximation of Beuthe (2016) is generally accurate to less than about 4%. Explaining Enceladus’ endogenic power radiated from the south polar terrain by ocean tidal heating requires ocean and shell thicknesses that are significantly smaller than the values inferred from gravity and topography constraints. The time-averaged surface distribution of ocean tidal heating is distinct from that due to dissipation in the solid shell, with higher dissipation near the equator and poles for eccentricity and obliquity forcing, respectively. This can lead to unique horizontal shell thickness variations if the shell is conductive. The surface displacement driven by eccentricity and obliquity forcing can have a phase lag relative to the forcing tidal potential due to the delayed ocean response. For Europa and Enceladus, eccentricity forcing generally produces greater tidal amplitudes due to the large eccentricity values relative to the obliquity values. Despite the small obliquity values, obliquity forcing generally produces larger phase lags due to the generation of Rossby–Haurwitz waves. If Europa’s shell and ocean are, respectively, 10 and 100 km thick, the tide amplitude and phase lag are 26.5 m and  <1° for eccentricity forcing, and <2.5 m and  <18° for obliquity forcing. Measurement of the obliquity phase lag (e.g. by Europa Clipper) would provide a probe of ocean thickness [ABSTRACT FROM AUTHOR]