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Exospheres, the tenuous gas environments surrounding planets, planetary satellites, and cometary comae, play a significant role in mediating the interactions of these astronomical bodies with their surrounding space environments. This paper presents a comprehensive review of both analytical and numerical methods employed in modeling exospheres. The paper explores analytical models, including the Chamberlain and Haser models, which have significantly contributed to our understanding of exospheres of planets, planetary satellites, and cometary comae. Despite their simplicity, these models provide baselines for more complex simulations. Numerical methods, particularly the Direct Simulation Monte Carlo (DSMC) method, have proven to be highly effective in capturing the detailed dynamics of exospheres under non-equilibrium conditions. The DSMC method's capacity to incorporate a wide range of physical processes, such as particle collisions, chemical reactions, and surface interactions, makes it an indispensable tool in planetary science. The Adaptive Mesh Particle Simulator (AMPS), which employs the DSMC method, has demonstrated its versatility and effectiveness in simulating gases in planetary and satellite exospheres and dusty gas cometary comae. It provides a detailed characterization of the physical processes that govern these environments. Additionally, the multi-fluid model BATSRUS has been effective in modeling neutral gases in cometary comae, as discussed in the paper. The paper presents methodologies of exosphere modeling and illustrates them with specific examples, including the modeling of the Enceladus plume, the sodium exosphere of the Moon, the coma of comet 67P/Churyumov-Gerasimenko, and the hot oxygen corona of Mars and Venus. [ABSTRACT FROM AUTHOR]

Icy moons of our Solar system are currently one of the focuses of the leading space agencies in the search for extraterrestrial life. Especially, Enceladus is a prime target because of the presence of geyser-like jets of water emanating from its south pole. Future missions to Enceladus would benefit from inter-moon transfers of the Saturnian system to reach Enceladus. However, such trajectories are challenging to design, due to the highly nonlinear, often chaotic, multi-body dynamics with close encounters of the planetary moons. The trajectory design is even more complex when low-thrust propulsion is used due to the long-duration, multi-revolution nature of the solutions. The approach outlined in this paper confronts this shortcoming by exploiting multi-body dynamics to construct a 'resonant hopping' trajectory, which is then optimized using indirect methods. Unstable resonant orbits are pre-computed and provide a starting point to a multiple indirect shooting method in a forward–backward fashion. The insights from multi-body dynamics steer the algorithm to a near-ballistic solution. The use of indirect optimization allows the design of long-duration, multi-revolution low-thrust trajectory, with a limited number of optimization variables. Finally, forward–backward shooting technique reduces the sensitivities to constraints satisfaction. This strategy is employed to design a low-thrust inter-moon transfer between a close resonant orbit of Tethys and a near rectilinear halo orbit around Enceladus. [ABSTRACT FROM AUTHOR]

A mechanism of formation of plasma-dust system above the surface of Enceladus, the Saturn moon, illuminated by the solar radiation is proposed. It is demonstrated that the photoelectric effect caused by the sunlight and the influence of the solar-wind plasma create conditions for lifting of dust particles above the surface of the moon as a result of electrostatic repulsion. Based on a self-consistent model describing the electrostatic field and plasma components, including photoelectrons emitted from the Enceladus surface and those emitted from the surface of the dust particles, distribution functions of photoelectrons, dust particles, and their electrostatic charges are determined as functions of altitude and the angle between the local normal and the direction to the Sun. Also determined are the altitude profiles of the electrostatic fields for the corresponding angles between the local normal and the direction of the solar radiation. It is demonstrated that the photoelectric effect plays an important role in formation of dusty plasma near the Enceladus surface despite considerable distance from the Sun. It is established that concentration of photoelectrons above the Enceladus surface can exceed concentration of electrons and ions of the solar wind by an order of magnitude, and the size of the levitating particles is larger than the characteristic size of dust particles lifted above the surface of the Moon due to the fact that Enceladus is much smaller than the Moon. On the contrary, the size of particles levitating above Enceladus is much smaller than the size of particles levitating above the surface of celestial objects smaller than the Enceladus, e.g., the Martian satellites Phobos and Deimos. [ABSTRACT FROM AUTHOR]

Spacecraft trajectories near the south pole of Enceladus violate the Brillouin sphere associated with the convergence radius of spherical harmonics models. In this study, a shifted coordinate frame is demonstrated to ensure a convergent model is available in regions of operational interest. Hypothetical experiments are performed around a simulated celestial body where the truth exterior gravity fields are known exactly. The divergence of the harmonics below the Brillouin sphere of the unshifted models is confirmed, while the shifted harmonics model converges. The method is next applied to the Cassini-derived gravity field for Enceladus, including uncertainties. Using these low-degree and low-order reference models, expected for use in an operational setting, the results show that the shifted and body-centered harmonics models agree to near machine precision for all evaluations at or above the surface, and no divergence is noticed. The results imply that mission designers and navigation engineers can safely use a traditional spherical harmonics field for Enceladus, even in regions that dip below the Brillouin sphere. For low-flying missions to celestial bodies besides Enceladus, the experiments conducted in this study can be repeated. The need for an alternative to the traditional spherical harmonics, such as the presented shifted model, increases for bodies that are increasingly non-spherical and orbits that are deeper inside the Brillouin sphere. [ABSTRACT FROM AUTHOR]

Saturn's mid-sized icy moons have complex relationships with Saturn's interior, the rings, and with each other, which can be expressed in their shapes, interiors, and geology. Observations of their physical states can, thus, provide important constraints on the ages and formation mechanism(s) of the moons, which in turn informs our understanding of the formation and evolution of Saturn and its rings. Here, we describe the cratering records of the mid-sized moons and the value and limitations of their use for constraining the histories of the moons. We also discuss observational constraints on the interior structures of the moons and geologically-derived inferences on their thermal budgets through time. Overall, the geologic records of the moons (with the exception of Mimas) include evidence of epochs of high heat flows, short- and long-lived subsurface oceans, extensional tectonics, and considerable cratering. Curiously, Mimas presents no clear evidence of an ocean within its surface geology, but its rotation and orbit indicate a present-day ocean. While the moons need not be primordial to produce the observed levels of interior evolution and geologic activity, there is likely a minimum age associated with their development that has yet to be determined. Uncertainties in the populations impacting the moons makes it challenging to further constrain their formation timeframes using craters, whereas the characteristics of their cores and other geologic inferences of their thermal evolutions may help narrow down their potential histories. Disruptive collisions may have also played an important role in the formation and evolution of Saturn's mid-sized moons, and even the rings of Saturn, although more sophisticated modeling is needed to determine the collision conditions that produce rings and moons that fit the observational constraints. Overall, the existence and physical characteristics of Saturn's mid-sized moons provide critical benchmarks for the development of formation theories. [ABSTRACT FROM AUTHOR]

Organic and inorganic cyanides are widely distributed in nature, yet not much is known about the ability of microorganisms to use these compounds as a source of nitrogen and/or carbon at high temperatures (>80 °C). Here we studied the capacity of organic and inorganic cyanides to support growth of an hyperthermophilic Pyrococcus strain isolated from Deception Island, Antarctica. This microorganism was capable of growing with aromatic nitriles, aliphatic nitriles, heterocyclic nitriles, amino aromatic nitriles and inorganic cyanides as nitrogen and/or carbon source. This is the first report of an hyperthermophilic microorganism able to incorporate these compounds in its nitrogen and carbon metabolism. Based on enzymatic activity and genomic information, it is possibly that cells of this Pyrococcus strain growing with nitriles or cyanide, might use the carboxylic acid and/or the ammonia generated through the nitrilase enzymatic activity, as a carbon and/or nitrogen source respectively. This work expands the temperature range at which microorganisms can use organic and inorganic cyanides to growth, having important implications to understand microbial metabolisms that can support life on Earth and the possibility to support life elsewhere. [ABSTRACT FROM AUTHOR]

• We study the chemistry of the core and ocean of Enceladus based on different core compositions. • We study the hydrothermal products from the ocean-core interaction in the context of the chemistry of the plume of Enceladus. • We evaluate the chemistry of the ocean based on its possible pH values and the saturation indices of mineral products. • We find that the actual composition of the core poses a somewhat wide range of possible compositional scenarios. Measurements made by the Cassini spacecraft instruments were able to reveal the composition of the geysers of the Saturn moon, Enceladus, among which salts (sodium chloride, sodium bicarbonate and/ or sodium carbonate) and traces of silica could be the result of hydrothermal processes from the interaction of an inner liquid ocean with the core of Enceladus. The chemistry of the ocean should be closely connected to the chemistry of the core. Even though. the actual composition of the core is unknown, Enceladus has been characterised as a moon with a silicate core and different authors have estimated the properties of the ocean. A core with a silicate composition alone, does not necessarily justify most of the observed species of the plume. Given the many uncertainties, in the current work, we study the possible chemistry of the core and the ocean in the context of a four-layered Enceladus (dry core, hydrated core, ocean and crust) with three different compositional scenarios for the core: primordial (represented by an ordinary chondrite), composite (represented by a carbonaceous chondrite with an igneous inclusion) and non primordial (epresented by a given peridotite). The scenarios comprise a set of minerals that interact with a primordial ocean (either pure or enriched water) at a given temperature and pressure, producing a series of secondary minerals and compounds, some of them, coincident with the chemical species observed in the geysers. Specifically, we make a chemical speciation for each proposed compositional scenario with the software PhreeqC, however our analysis is limited to the study of interactions that reach a given equilibrium. In particular, output values like the potential of hydrogen of the ocean or the saturation indices of mineral products may give us hints about the chemistry of the ocean and the core. We find that, based on the species observed in the plume, the actual composition of the core (and the ocean as well) poses a somewhat wide range of possible compositional scenarios and each of our proposed scenarios and their products justify, to some extent, the observations of the plume. [ABSTRACT FROM AUTHOR]

There is considerable interest in sending a mission to Enceladus to sample its erupting materials, which are sourced from its ocean, a proposed habitable environment. However, we lack resolution between competing ascent and eruption models, which offer differing consequences and challenges for mission sampling and access strategies. We report a new Enceladus ascent and eruption model, "Cryo‐Erupt," where ascent from ocean to jet is driven by the exsolution and expansion of dissolved gases from ascending water within conduits. This mechanism shares many similarities with some forms of terrestrial activity, including explosive silicate volcanism, cold‐water geysers and "limnic" eruptions. This preliminary study suggests that this mode of ascent and eruption is viable and broadly consistent with a range of observations including the apparent co‐existence of point‐ (jet) and fissure‐ (curtain) sourced activity as well as strong contrasts in velocity and ice‐to‐vapor ratio between the plume and the jets feeding it. However, it requires the co‐existence of a sublimation plume as an additive component to the broader plume. The outcomes of the Cryo‐Erupt model differ in terms of conduit physical and chemical processes from previously proposed boiling interface eruption models, for example, predicting larger dynamic pressures and narrower conduits, which could present challenges for direct robotic access. Due to the lack of a static boiling interface or wall condensation, bulk composition is unlikely to change appreciably during ascent from the ocean‐conduit interface to the jet, potentially simplifying the interpretation of samples collected in space or on Enceladus' surface. Plain Language Summary: High‐speed jets from giant ice fissures on Saturn's moon Enceladus feed a large plume, which is of interest to scientists because it contains salts and organic compounds, which are evidence of a subsurface ocean that may possibly host life. However, it is unclear how the chemistry of the plume material (gas and grains) might be altered as this material moves from the ocean into space, and to what extent samples from the plume are representative of ocean composition. Previous models to predict this behavior mostly relied on boiling of water as the primary way that gas and droplets are ejected, but these models do not take into account all of the physics involved, and do not fully reproduce what Cassini observed at Enceladus. We propose a new model that instead invokes dissolved gas molecules expanding, similar to explosive volcanoes on Earth and essentially the same mechanism that causes cans of soda to explode upon opening if shaken. We predict that the erupting jets would largely preserve bulk ocean content and thus would be the best place to study ocean content, in contrast with the broader plume, which would have more water that has sublimated from the surface. Key Points: Recent studies on the ascent and eruption of Enceladus' plume have neglected the role of exsolution and expansion of dissolved volatilesVolatile‐driven direct ocean‐to‐jet liquid water ascent is generally consistent with observations if combined with a sublimation co‐plumeThis mode of ascent could preserve ocean bulk content in jets, leading to jet sampling strategies being preferred for future missions [ABSTRACT FROM AUTHOR]

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]

The Cassini mission provided evidence for a global subsurface ocean and ongoing hydrothermal activity on Enceladus, based on results from Cassini's mass spectrometers. Laboratory simulations of hydrothermal conditions on icy moons are needed to further constrain the composition of ejected ice grains containing hydrothermally altered organic material. Here, we present results from our newly established facility to simulate the processing of ocean material within the temperature range 80–150°C and the pressure range 80–130 bar, representing conditions suggested for the water–rock interface on Enceladus. With this new facility, we investigate the hydrothermal processing of triglycine (GGG) peptide and, for the first time, analyse the extracted samples using laser-induced liquid beam ion desorption (LILBID) mass spectrometry, a laboratory analogue for impact ionization mass spectrometry of ice grains in space. We outline an approach to elucidate hydrothermally processed GGG in ice grains ejected from icy moons based on characteristic differences between GGG anion and cation mass spectra. These differences are linked to hydrothermal processing and thus provide a fingerprint of hydrothermal activity on extraterrestrial bodies. These results will serve as important guidelines for biosignatures potentially obtained by a future Enceladus mission and the SUrface Dust Analyzer (SUDA) instrument onboard Europa Clipper. This article is part of the theme issue 'Dust in the Solar System and beyond'. [ABSTRACT FROM AUTHOR]

Icy moons with subsurface oceans of liquid water rank among the most promising astrobiological targets in our Solar System. In this work, we assess the feasibility of deploying laser sail technology in precursor life-detection missions. We investigate such laser sail missions to Enceladus and Europa, as these two moons emit plumes that seem accessible to in situ sampling. Our study suggests that GigaWatt laser technology could accelerate a 100 kg probe to a speed of ∼ 30 km s − 1 , thereupon reaching Europa on timescales of 1–4 years and Enceladus with flight times of 3–6 years. Although the ideal latitudes for the laser array vary, placing the requisite infrastructure close to either the Antarctic or Arctic Circles might represent technically viable options for an Enceladus mission. Crucially, we determine that the minimum encounter velocities with these moons (about 6 km s − 1 ) may be near-optimal for detecting biomolecular building blocks (e.g., amino acids) in the plumes by means of a mass spectrometer akin to the Surface Dust Analyzer onboard the Europa Clipper mission. In summary, icy moons in the Solar System are potentially well-suited for exploration via the laser sail architecture approach, especially where low encounter speeds and/or multiple missions are desirable. • Light sail astrobiology mission architecture to analyze Enceladus and Europa formulated. • GigaWatt laser technology capable of achieving flight times of ≤ 6 years. • Encounter velocities with plumes of icy moons compatible with optimal detection of biomolecules. • Potential of light sail flotillas for Solar System exploration discussed. [ABSTRACT FROM AUTHOR]

Astrobiology research seeks to understand how life begins and evolves, and to determine whether life exist elsewhere in the universe. The discovery of diverse ocean worlds has significantly expanded the number of planetary bodies in the Solar System that could potentially contain life. Of the recognized ocean worlds, Saturn's moon Enceladus stands out because it appears to meet all requirements to sustain life. For that reason, robotic mission concepts are being developed to determine whether Enceladus' ocean is inhabited. The theory of organic chemical evolution (OCE) represents an ideal framework to guide this exploration strategy, articulating investigations and associated measurements of organic matter in the subsurface ocean. Within this reference frame, the immediate priority with the lowest science risk would be to understand molecular and structural properties of bulk organic matter in the ocean, and search for metabolic precursors and biochemical building blocks, both free and bound. This could be supplemented with "high‐risk, high‐reward" searches for functional polymers, catalytic activity, and cell‐like objects with traits indicative of evolutionary adaptations. The theory of OCE provides a robust scientific foundation for the astrobiological exploration of ocean worlds, fostering a productive path to discovery with lower mission risk that could be implemented with existing technology. Strong synergies between astrobiology and Earth‐bound research could ensue from this exploration strategy particularly in the context of terrestrial analog studies and laboratory simulations. Plain Language Summary: There is a diversity of "ocean worlds" in our Solar System, which are of great scientific interest. Enceladus, a small moon of Saturn, has a global subsurface ocean that could sustain life and contains complex organic matter. To further understand the biological potential of Enceladus, and other ocean worlds, we need to consider how abiotic and prebiotic chemistry in Enceladus's ocean might play a role in the origin of life. The theory of organic chemical evolution provides the ideal framework to address this question. The top priority would be to study the organic inventory in the ocean, and to search for the basic building blocks of life, as well as simple compounds involved in metabolic processes. Next, we should search for complex polymers and cell‐like structures with traits suggesting Darwinian evolution. This exploration strategy is a solid foundation for discovery and can be done with current technology, which lowers the risk and complexity of spaceflight missions. Key Points: Enceladus is one of the most compelling destinations in the solar system for exobiology explorationThe theory of organic chemical evolution provides a framework for the continued and systematic exploration of Enceladus and other ocean worldsWith this exploration framework biotic, abiotic, and prebiotic scenarios are all possible outcomes with profound implications [ABSTRACT FROM AUTHOR]

All known life on Earth inhabits environments that maintain conditions between certain extremes of temperature, chemical composition, energy availability, and so on (Chapter 6). Life may have emerged in similar environments elsewhere in the Solar System and beyond. The ongoing search for life elsewhere mainly focuses on those environments most likely to support life, now or in the past—that is, potentially habitable environments. Discussion of habitability is necessarily based on what we know about life on Earth, as it is our only example. This chapter gives an overview of the known and presumed requirements for life on Earth and discusses how these requirements can be used to assess the potential habitability of planetary bodies across the Solar System and beyond. We first consider the chemical requirements of life and potential feedback effects that the presence of life can have on habitable conditions, and then the planetary, stellar, and temporal requirements for habitability. We then review the state of knowledge on the potential habitability of bodies across the Solar System and exoplanets, with a particular focus on Mars, Venus, Europa, and Enceladus. While reviewing the case for the potential habitability of each body, we summarize the most prominent and impactful studies that have informed the perspective on where habitable environments are likely to be found. [ABSTRACT FROM AUTHOR]

Enceladus is of interest to astrobiology and the search for life since it is thought to host active hydrothermal activity and habitable conditions. It is also possible that the organics detected on Enceladus may indicate an active prebiotic or biotic system; in particular, the conditions on Enceladus may favor mineral-driven protometabolic reactions. When including metabolism-related biosignatures in Enceladus mission concepts, it is necessary to base these in a clearer understanding of how these signatures could also be produced prebiotically. In addition, postulating which biological metabolisms to look for on Enceladus requires a non-Earth-centric approach since the details of biological metabolic pathways are heavily shaped by adaptation to geochemical conditions over the planet's history. Creating metabolism-related organic detection objectives for Enceladus missions, therefore, requires consideration of how metabolic systems may operate differently on another world, while basing these speculations on observed Earth-specific microbial processes. In addition, advances in origin-of-life research can play a critical role in distinguishing between interpretations of any future organic detections on Enceladus, and the discovery of an extant prebiotic system would be a transformative astrobiological event in its own right. [ABSTRACT FROM AUTHOR]

Tidal interactions between moons and planets can have major effects on the orbits, spins, and thermal evolution of the moons. In the Saturn system, tidal dissipation in the planet transfers angular momentum from Saturn to the moons, causing them to migrate outwards. The rate of migration is determined by the mechanism of dissipation within the planet, which is closely tied to the planet's uncertain structure. We review current knowledge of giant planet internal structure and evolution, which has improved thanks to data from the Juno and Cassini missions. We discuss general principles of tidal dissipation, describing both equilibrium and dynamical tides, and how dissipation can occur in a solid core or a fluid envelope. Finally, we discuss the possibility of resonance locking, whereby a moon can lock into resonance with a planetary oscillation mode, producing enhanced tidal migration relative to classical theories, and possibly explaining recent measurements of moon migration rates. [ABSTRACT FROM AUTHOR]

Beneath Enceladus' ice crust lies an ocean which might host habitable conditions. Here, the scale and productivity of a notional Enceladean methanogenic biosphere are computed as a function of the core‐to‐ocean flux of hydrogen and the ratio between abiotic and biotic methane in Enceladus' space plume. Habitats with an ocean‐top pH range of 8–9 have up to 40%–60% probability of being energy‐limited. Those at pH > 9 are increasingly uninhabitable, and those <8.5 are increasingly likely to host exponential growth, possibly leading to compositional inconsistencies between the ocean and Cassini gas observations. In those cases, energy‐based habitability models cannot infer an inhabited Enceladus consistent with both Earth life and Cassini measurements without including additional microbial growth limiters such as nutrient limitation, toxicity, or spatial constraints. If methanogens are isolated to a 350 K seafloor habitat and consume 10 mol s−1 of H2, the most probable biomass is 103, 103.7 kg C with ocean‐top pH 8,9, respectively. Biomass production consistent with space plume fluxes is 104–106 kgC yr−1—milligrams of cellular carbon per kilogram of H2O ejected—but requires that >50% of the space plume methane is biotic. Alternative scenarios are presented, and biomass is generally lower when habitat temperature is higher. Ocean biomass density cannot yet be reliably estimated owing to uncertainties in the scale and physicochemical properties of Enceladus' putative habitats. Evaluating abiotic to biotic ratios in plume methane and organic material could help identify false negative results from life detection missions and constrain the scale of an underlying biosphere. Plain Language Summary: The Cassini spacecraft observed energy and nutrient sources for life erupting from a water ocean below the icy exterior of Saturn's moon Enceladus. Future spacecraft planned for Enceladus, Europa and Titan will obtain similar measurements in greater detail. In this work, we calculate the probability that different suites of environments inside Enceladus' ocean are habitable for methanogens, an ancient type of organism which may be able to survive there. Three scenarios are possible: uninhabitable, habitable, and energy‐saturated. Next, we extend this calculation to show that the possible biosphere is small but productive. Microbial waste, generated at the seafloor, could be ejected from the ocean and consequently snow down onto Enceladus' surface. Mass spectrometers on future spacecraft could constrain these possibilities further and may be critical to inferring biosignatures when cell densities are too low to detect. Key Points: Spatial habitability on Enceladus is not guaranteed, and additional modeling parameters are needed to evaluate possible vacant habitatsCell turnover is important for life detection missions and can be constrained more tightly than total biomassAbiotic to biotic ratios of space plume carbon are critical as biosignatures and for assessing the scale of Enceladus' notional biosphere [ABSTRACT FROM AUTHOR]

Future missions to Enceladus would benefit from multi-moon tours that leverage V ∞ on resonant orbits to progressively transfer between moons. Such resonance hopping trajectories present a vast search space for global optimization due to the different combinations of available resonances and flyby velocities. The proposed multi-objective tour design algorithm optimizes entire moon tours from Titan to Enceladus via grid-based dynamic programming, in which the computation time is significantly reduced by discretization of the design variables and pre-computation of a database of V ∞ -leveraging transfers. The result unveils a complete trade space of the moon tour design to Enceladus, and the obtained solution is validated in a full-ephemeris model. [ABSTRACT FROM AUTHOR]

The freezing point of water is negatively dependent on pressure; therefore in any ocean without external forcing it is warmest at the surface and grows colder with depth. Below floating ice on Earth (e.g., ice shelves or sea ice), this pressure dependence combines with gradients in the ice draft to drive an ice redistribution process termed the "ice pump": submerged ice melts, upwells, and then refreezes at shallower depths. Ice pumping is an exchange process between the ocean and overhead ice that results in unique ice compositions and textures and influences the distribution of sub‐ice habitats on Earth. Here, we scale recent observations from Earth's ice shelves to planetary conditions and find that ice pumping is expected for a wide range of possible sub‐ice shell pressures and salinity at other ocean worlds such as Europa and Enceladus. We show how ice pumping would affect hypothetical basal ice shell topography and ice thickness under varying ocean conditions and demonstrate how remote sensing of the ice shell draft can be used to estimate temperature gradients in the upper ocean ahead of in situ exploration. For example, the approximately 22 km gradient observed in Enceladus' ice shell draft between the south pole and the equator suggests a temperature differential of 0.18 K at the base of the ice shell. These concepts can extend the interpretation of observations from upcoming ocean world missions, and link ice shell topography to ice‐ocean material exchange processes that may prove important to overall ocean world habitability. Plain Language Summary: The freezing point of water depends on pressure. As pressure increases, the freezing point decreases, which can influence the melting or freezing of ice in an ocean. A helpful way to conceptualize this dependency is to recall that water expands as it freezes. As pressure increases, this expansion requires more work to displace the higher pressure surroundings, so the water must be even colder to freeze—a decrease in the freezing point. If ice is submerged, the deeper ice where the freezing point is colder can melt faster. This forms freshened meltwater that may rise to shallower depths where it is now colder than the shallower, lower pressure freezing point and can refreeze underwater. This process is referred to as an "ice pump", because it acts to equilibrate topography in submerged ice. In ice‐covered oceans on Earth, the ice pump is an important process that influences the composition and texture of the ice, and therefore the sub‐ice ecosystems. Here, we find that ice pumping is also likely at other ocean worlds in our solar system where it may similarly influence potential sub‐ice ecosystems and show how observations of planetary ice shell thicknesses can be used to bound ocean conditions. Key Points: When ice is submerged, a melting and freezing exchange process termed the "ice pump" can affect ice composition, texture, and thicknessWe find that ice pumping is likely beneath the ice shells of several ocean worlds in our solar systemThe ice pump concept enables inversion between ocean world ice shell thickness and ice‐ocean interface temperature ranges [ABSTRACT FROM AUTHOR]

The inferred density of Enceladus' core, together with evidence of hydrothermal activity within the moon, suggests that the core is porous. Tidal dissipation in an unconsolidated core has been proposed as the main source of Enceladus' geological activity. However, the tidal response of its core has generally been modeled assuming it behaves viscoelastically rather than poroviscoelastically. In this work, we analyze the poroviscoelastic response to better constrain the distribution of tidal dissipation within Enceladus. A poroviscoelastic body has a different tidal response than a viscoelastic one; pressure within the pores alters the stress field and induces a Darcian porous flow. This flow represents an additional pathway for energy dissipation. Using Biot's theory of poroviscoelasticity, we develop a new framework to obtain the tidal response of a spherically symmetric, self-gravitating moon with porous layers and apply it to Enceladus. We show that the boundary conditions at the interface of the core and overlying ocean play a key role in the tidal response. The ocean hinders the development of a large-amplitude Darcian flow, making negligible the Darcian contribution to the dissipation budget. We therefore infer that Enceladus' core can be the source of its geological activity only if it has a low rigidity and a very low viscosity. A future mission to Enceladus could test this hypothesis by measuring the phase lags of tidally induced changes of gravitational potential and surface displacements.

Planetary exploration mission concepts that include flying directly through material for collection and/or analysis are becoming increasingly common, with Enceladus water ice particles offering a particularly high‐value target. Despite this interest, understanding and predicting what happens to ice samples upon impacting a surface at the high‐ and hyper‐velocities expected for these missions remains a critical knowledge gap. We describe a set of custom simulations using the Hot Optimal Transportation Meshfree method that was implemented to better understand ice impacts. We then compare the simulations with relevant experimental results from the Aerosol Impact Spectrometer. These simulations and experiments illustrate the complex relationship between different energy dissipation mechanisms and how they affect the fate of the particle. These results highlight the importance of understanding the implications of this complex physics on successful sample collection and transfer in order to achieve the scientific goals of the mission. Plain Language Summary: Many mission concepts exploring planets, moons and other bodies include flying directly through material such as gas, ice particles, or dust grains for collection and/or analysis. In particular, Saturn's moon Enceladus has a plume of water ice particles that appear to be sourced from its subsurface ocean, making them valuable to study for understanding the ocean composition, as well as its potential to host life. Currently, we have limited insights into what will happen to ice particles that impact a spacecraft moving through a plume. The outcome of these impacts is critical to achieving the scientific goals proposed for these missions. We describe a set of simulations complemented by experiments to elucidate the fate of these impacted particles. Simulation results highlight the complex interplay between the different energy dissipation mechanisms, and the high sensitivity to particle size, speed, incidence angle, and temperature. Experiments are consistent with the simulations, showing the similar trends for rebounding, sticking, particle fracturing, and phase change. Missions targeting ice particle plume flythrough architectures must consider the implications of the impacts along with their uncertainties in order to select credible methods for meeting the scientific goals of any mission. Key Points: Impacts of ice particles on spacecraft result in complex energy dissipation mechanisms that affect the fate of the particleMissions targeting ice particle plume fly‐through architectures must consider the implications of the impacts to select credible methodsCustom simulations and complementary impact experiments were used to gain insights into the outcome of high‐velocity ice particle impacts [ABSTRACT FROM AUTHOR]

Constraining the spatial variability of the thickness of the ice shell of Enceladus (i.e., the crust) is central to our understanding of the internal dynamics and evolution of this small Saturnian moon. In this study, we develop a new methodology to infer regional variations in crustal thickness using measurements of tidally‐driven elastic strain that could be made in the future. As proof of concept, we recover thickness variations from synthetic finite‐element crustal models subjected to diurnal eccentricity tides. We demonstrate recovery of crustal thickness to within ∼2 km of true values across the crust with ∼10% error in derived spherical harmonic coefficients at degrees l ≤ 12. Our computed uncertainty is significantly smaller than the inherent ∼10 km ambiguity associated with crustal thickness derived solely from gravity and topography measurements. Therefore, future measurements of elastic strain can provide a robust approach to probe crustal structure at Enceladus. Plain Language Summary: Inferences of the thickness of Enceladus's ice shell—or crust—can provide valuable insights for understanding the processes which control the long‐term evolution and heating of this moon of Saturn. We develop a new method to infer regional variations in crustal thickness at Enceladus using proposed measurements of deformation caused by tidal interactions with Saturn. Using models of Enceladus's ice shell, we demonstrate recovery of crustal thickness with a deviation of ∼2 km (i.e., ∼10%) relative to input values. Our approach to inferring crustal thickness complements traditional methods that rely solely on analyzing gravity and surface topography to constrain the crustal structure of the satellite. Key Points: Determinations of variations in ice shell (i.e., crustal) thickness are crucial for understanding the dynamics and evolution of EnceladusWe develop a new method to infer spatially‐varying crustal thickness using proposed measurements of tidally‐driven elastic strainUsing our method, we demonstrate recovery of crustal thickness to within ∼2 km of true values with ∼10% accuracy over length scales >60 km [ABSTRACT FROM AUTHOR]

Water parcels close to their freezing point contract and become heavy on warming if they are sufficiently fresh (salinity less than 22g kg−1 for earth's ocean), but expand and become buoyant when salty (salinity greater than 22g kg−1). We explore the resulting divergent behavior of hydrothermal plumes in fresh versus salty icy ocean worlds, with particular emphasis on Europa and Enceladus. Large, salty, putative Europa‐like oceans, develop buoyant plumes which rise upwards in the water column when energized by localized hydrothermal vents. Instead, small, fresher, putative Enceladus‐like oceans, can develop bottom‐hugging gravity currents when heated near the freezing point, due to the anomalous contraction of fluid parcels on warming. Such a bottom‐filling regime would most likely be a transient stage in the evolution of an icy moon over geological time. The contrasting dynamics are highlighted and rationalized in terms of key non‐dimensional numbers with a focus on the ability of ocean to carry bio‐markers from the hydrothermal activity at the bottom to the ice shell at the top. Finally, the implications of our study for prioritizing future missions to icy moons are discussed. An advantage of a mission to a large icy moon (e.g., Europa), rather than a smaller target (e.g., Enceladus), is that a larger moon's ocean would likely support buoyant convection, which could bring signatures of seafloor venting to the outer ice‐shell regardless of that ocean's salinity. For smaller icy moons, the nature of convection would hinge on its assumed salinity. Plain Language Summary: Oceans on icy moons such as Enceladus and Europa may potentially have many of the conditions required for life. The possible existence of hydrothermal vents on the ocean floors of these moons are prime candidates as sources of biological activity. Here we explore the conditions in which heating at bottom vents might lead to convection that could carry biomarkers from the bottom of the ocean up to the ice. If the water is salty (perhaps Europa), upward‐reaching plumy convection results which, if buoyancy fluxes are sufficiently large, can reach right up to the icy shell covering the ocean, allowing us, perhaps, to draw inferences about the interior ocean by observing the ice shell. If the background salinity is low, however, (perhaps Enceladus) heating close to the freezing point of water leads to dense, bottom‐hugging density currents or very weak plumes. This will likely be a transient state in the evolution of the ocean of such a moon in which the bottom layer slowly warms up, but with much diminished communication between the bottom and the top. An advantage for any future mission to a large ocean world, is that it would likely host buoyant convection regardless of the ocean's salinity. Key Points: Salty oceans near the freezing point develop buoyant plumes which rise in the water column when energized by localized hydrothermal ventsBuoyant plumes become diluted by turbulence and baroclinic instability as they rise upwardsIn a transient phase, fresh oceans near the freezing point can develop bottom‐hugging currents as warm fluid parcels contract on heating [ABSTRACT FROM AUTHOR]

We present a novel approach to modeling the tidal response of icy moons with subsurface oceans. The problem is solved in the time domain and the flow in the ocean is calculated simultaneously with the deformation of the core and the ice shell. To simplify the calculations, we assume that the internal density interfaces are spherical and the effective viscosity of water is equal to or greater than 100 Pa s. The method is used to study the effect of an unconsolidated core on tidal dissipation in Enceladus' ocean. We show that the partitioning of tidal heating between the core and the ocean strongly depends on the thickness of the ocean layer. If the ocean thickness is significantly greater than 1 km, heat production is dominated by tidal dissipation in the core and the amount of heat produced in the ocean is negligible. In contrast, when the ocean thickness is less than about 1 km, tidal heating in the core diminishes and dissipation in the ocean increases, leaving the total heat production unchanged. Extrapolation of our results to realistic conditions indicates that tidal flow is turbulent which suggests that the linearized Navier‐Stokes equation may not be appropriate for modeling the tidal response of icy moons. Finally, we compare our results with those obtained by solving the Laplace tidal equations and discuss the limitations of the two‐dimensional models of ocean circulation. Plain Language Summary: The origin of the heat powering Enceladus' geological activity and preventing its ocean from freezing has been debated since the discovery of a plume of icy particles above Enceladus' south pole in 2005. Here, we evaluate the heat generated by tides in Enceladus' ocean assuming that the internal density interfaces are spherical and the flow in the ocean is primarily driven by the deformation of Enceladus' unconsolidated core. We find that the heat production in the ocean can explain only a small fraction of Enceladus' heat budget under the present day conditions (i.e., for an ocean thickness of about 40 km) but can be as high as 25 GW if the thickness of the ocean layer is less than about 1 km. Analysis of the flow field suggests that the simplifying assumptions often used in previous studies may not be appropriate. In particular, we show that, regardless of ocean thickness, the dissipation rate obtained by solving the shallow water equations corrected for the dampening effect of the ice shell can be significantly different from that obtained by solving the three‐dimensional Navier‐Stokes equations. Key Points: The heat production in the ocean depends on the ocean thickness and the material properties of the coreDissipation is likely to be negligible for ocean thicknesses >1 km but can exceed 20 GW if the ocean is thin and the core is easy to deformThe shallow water approach, widely used in previous studies, can lead to incorrect results regardless of ocean thickness [ABSTRACT FROM AUTHOR]

An ideal life detection instrument would have high sensitivity but be insensitive to abiotic processes and would be capable of detecting life with alternate molecular structures. In this study, we propose that catalytic activity can be the basis of a nearly ideal life detection instrument. There are several advantages to catalysis as an agnostic life detection method. Demonstrating catalysis does not necessarily require culturing/growing the alien life and in fact may persist even in dead biomass for some time, and the amplification by catalysis is large even by minute amounts of catalysts and, hence, can be readily detected against abiotic background rates. In specific, we propose a hydrolytic catalysis detection instrument that could detect activity in samples of extraterrestrial organic material from unknown life. The instrument uses chromogenic assay-based detection of various hydrolytic catalytic activities, which are matched to corresponding artificial substrates having the same, chromogenic (preferably fluorescent) upon release, group; D- and L-enantiomers of these substrates can be used to also answer the question whether unknown life is chiral. Since catalysis is a time-proportional product-concentration amplification process, hydrolytic catalytic activity can be measured on a sample of even a minute size, and with instruments based on, for example, optofluidic chip technology. [ABSTRACT FROM AUTHOR]

Growing evidence of the potential habitability of Ocean Worlds across our solar system is motivating the advancement of technologies capable of detecting life as we know it—sharing a common ancestry or physicochemical origin with life on Earth—or don't know it, representing a distinct emergence of life different than our one known example. Here, we propose the Electronic Life-detection Instrument for Enceladus/Europa (ELIE), a solid-state single-molecule instrument payload that aims to search for life based on the detection of amino acids and informational polymers (IPs) at the parts per billion to trillion level. As a first proof-of-principle in a laboratory environment, we demonstrate the single-molecule detection of the amino acid L-proline at a 10 μM concentration in a compact system. Based on ELIE's solid-state quantum electronic tunneling sensing mechanism, we further propose the quantum property of the HOMO–LUMO gap (energy difference between a molecule's highest energy-occupied molecular orbital and lowest energy-unoccupied molecular orbital) as a novel metric to assess amino acid complexity. Finally, we assess the potential of ELIE to discriminate between abiotically and biotically derived α-amino acid abundance distributions to reduce the false positive risk for life detection. Nanogap technology can also be applied to the detection of nucleobases and short sequences of IPs such as, but not limited to, RNA and DNA. Future missions may utilize ELIE to target preserved biosignatures on the surface of Mars, extant life in its deep subsurface, or life or its biosignatures in a plume, surface, or subsurface of ice moons such as Enceladus or Europa. One-Sentence Summary: A solid-state nanogap can determine the abundance distribution of amino acids, detect nucleic acids, and shows potential for detecting life as we know it and life as we don't know it. [ABSTRACT FROM AUTHOR]

Enceladus and Europa, icy moons of Saturn and Jupiter, respectively, are believed to be habitable with liquid water oceans and therefore are of interest for future life detection missions and mission concepts. With the limited data from missions to these moons, many studies have sought to better constrain these conditions. With these constraints, researchers have, based on modeling and experimental studies, hypothesized a number of possible metabolisms that could exist on Europa and Enceladus if these worlds host life. The most often hypothesized metabolisms are methanogenesis for Enceladus and methane oxidation/sulfate reduction on Europa. Here, we outline, review, and compare the best estimated conditions of each moon's ocean. We then discuss the hypothetical metabolisms that have been suggested to be present on these moons, based on laboratory studies and Earth analogs. We also detail different detection methods that could be used to detect these hypothetical metabolic reactions and make recommendations for future research and considerations for future missions. [ABSTRACT FROM AUTHOR]

The thickness of the outer ice shell plays an important role in several geodynamical processes at ocean worlds. Here, we show that observations of tidally driven diurnal surface displacements can constrain the mean ice shell thickness, d∼ice ${\tilde{d}}_{\mathit{ice}}$. Such estimates are sensitive to any significant structural features that break spherical symmetry such as faults and lateral variation in ice shell thickness and structure. We develop a finite‐element model of Enceladus to calculate diurnal tidal displacements for a range of d∼ice ${\tilde{d}}_{\mathit{ice}}$ values in the presence of such structural heterogeneities. Consistent with results from prior studies, we find that the presence of variations in ice shell thickness can significantly amplify deformation in thinned regions. If major faults are also activated by tidal forcing—such as Tiger Stripes on Enceladus—their characteristic surface displacement patterns could easily be measured using modern geodetic methods. Within the family of Enceladus models explored, estimates of d∼ice ${\tilde{d}}_{\mathit{ice}}$ that assume spherical symmetry a priori can deviate from the true value by as much as ∼41% when structural heterogeneities are present. Additionally, we show that crustal heterogeneities near the South Pole produce differences of up to 35% between Love numbers evaluated at different spherical harmonic orders. A ∼41% range in estimates of d∼ice ${\tilde{d}}_{\mathit{ice}}$ from Love numbers is smaller than that found with approaches relying on static gravity and topography (∼250%) or analyzing diurnal libration amplitudes (∼85%) to infer d∼ice ${\tilde{d}}_{\mathit{ice}}$ at Enceladus. As such, we find that analysis of diurnal tidal deformation is a relatively robust approach to inferring mean crustal thickness. Plain Language Summary: For ocean worlds such as Enceladus, it is useful to determine the thicknesses of the outer ice crust—as it provides information about the depth of the ocean, the thermal evolution of the body, and the rate at which material at the surface can be recycled to the ocean by burial processes. By measuring the deformation of the surface in response to tidal forces, we can infer the mean ice shell thickness at Enceladus. Here, we show that the presence of large fault systems (such as the Tiger Stripes) or variations in the thickness of the ice shell (i.e., structural heterogeneities) affects Enceladus's response to tides. We find that estimates of ice shell thickness that ignore the potential impact of structural heterogeneities can deviate from true thickness values by up to 41%. This deviation is smaller than that found with other approaches that rely on analyzing gravity and topography (∼250%) or the periodic rigid rotation of the ice shell (∼85%) to infer Enceladus's mean ice shell thickness. As such, despite the presence of heterogeneities, measurements of tidal deformation at Enceladus would be a powerful probe of subsurface structure. Key Points: We show faults, crustal weak zones, and thickness variations in the ice shell measurably impact tidal deformation at EnceladusWe find that structural heterogeneities could bias inferences of ice shell thickness using diurnal Love numbers by up to 41%Heterogeneities near the South Pole could drive differences of up to 35% between Love number values at different spherical harmonic orders [ABSTRACT FROM AUTHOR]

Settlement of the Outer Planets of the Solar System will take humans far away from the Sun, its warmth, and the light on which humans depend. Most settlements will be small mining enclaves or research stations until well into the future. Lengthy travel times will satisfy the human needs for hope, patience, acceptance, self-study, and contemplation of God's role in the universe. The authors explore the potential roles of theology and religion in Deep Space, the Asteroids, and the icy moons of Jupiter and Saturn where life might be found in the liquid oceans under ice that is kilometers deep. [ABSTRACT FROM AUTHOR]

The surface morphology of airless, ice‐covered moons of the outer solar system, such as Europa, Enceladus, and Callisto, is not well known at centimeter‐ to meter‐scales. Ice and snow erode differently on such worlds in part because sublimation is the dominant process. On Earth, ice penitentes have been observed in sublimation‐driven environments, and may provide a guide for similar formations on ice‐covered worlds. Penitentes are blade‐like snow features observed on Earth in high‐altitude, low‐latitude snowfields. Models of penitente formation on Earth break down within the free‐molecular regime of airless bodies, leaving a major gap in understanding whether such morphologies can form on their surfaces. To investigate the morphologic evolution of icy bodies, we developed a Sublimation Monte Carlo (SMC) model that enables a numerical approach to modeling exosphere‐surface interactions at free‐molecular conditions. The SMC model uses Monte Carlo tracking of molecules emitted from the surface to determine the net molecular interchange that drives surface changes. We validated results against experiments, matching the evolution of pre‐formed penitentes as they receded in height and became less pronounced. Our results reveal the importance of molecular redeposition on topology, indicating that the stable morphology of isothermal topographies is a planar morphology on regions of net sublimation, regardless of initial surface shape. A study of parametrically varying temperature profiles for sinusoidal penitentes resulted in the following requirement for penitente growth: the trough temperature must exceed the peak temperature by a threshold value, which notably depends on the surface aspect ratio and peak temperature. Plain Language Summary: The surfaces of airless, ice‐covered moons of the outer solar system are not well known at the centimeter‐ to meter‐scale because no spacecraft has yet imaged these worlds at that scale. Ice and snow evolve and erode differently on such worlds in large part because sublimation is the dominant process. On Earth, ice penitentes, which are blade‐like structures, have been observed in sublimation‐driven environments, and as such may provide a guide for similar formations on ice‐covered worlds. Our modeling results, however, show that icy surfaces on worlds like Jupiter's moons Europa and Callisto, and Saturn's moon Enceladus, do not easily form structures analogous to penitentes. Rather, the sublimation of water molecules off a rough isothermal surface leads to a flat, icy surface on regions of net sublimation. Under the conditions we studied, in order for penitentes or related blade‐like structures to form, we find that the temperature of the trough regions in the ice must be a few degrees Celsius higher than the temperature of the blade tips. This condition is hard to generate naturally on pure ice patches of icy bodies. Consequently, we predict that penitentes will not be found in pure ice regions of these distant worlds. Key Points: We present a sublimation model capable of simulating surface morphology changes over geologic time scales on airless worldsFor regions of net sublimation, the stable morphology of isothermal topographies is a flat surface regardless of initial geometryOur model shows good agreement with a cryogenic vacuum chamber experiment, showing that pre‐formed snow ridges decay with time [ABSTRACT FROM AUTHOR]

The plume of Enceladus is thought to originate from the dispersion of a liquid source beneath the icy crust. Cryovolcanic activity on Enceladus may present a direct way of accessing material originating from the potentially habitable subsurface ocean. One way to test the hypothesis of whether life is present within the ocean of Enceladus would be to investigate the plume material for the presence of microbial life. In this study, we investigated the entrainment of Bacillus subtilis within Enceladus-like fluids under boiling conditions caused by exposure of the fluids to low pressure. We show that boiling, associated with exposure of a fluid to low pressure, works as a mechanism for dispersing bacteria in Enceladus plume-like environments. Exposure of Enceladus-type fluids (0.01–0.1 molal Na2CO3 and 0.05–0.2 molal NaCl) to low pressure (5 mbar) results in the dispersion of bacteria in droplets that evaporate to produce particles of salt. We find that, for particles with radius (r) ≤ 10 μm, the number of dispersed particles containing cells was between 7.7% and 10.9%. However, for larger particles 10 < r ≤ 50 μm, 64.4% and 56.4% contained cells for lower and upper end-member solutions, respectively. Our results suggest that the gravity-induced size sorting of plume particles will result in plume deposits closer to the vent source containing a larger volume of biological material than within the plume. If life is present in the ocean of Enceladus, we would expect that it would be effectively entrained and deposited on the surface; therefore, it would be accessible with a surface-lander-based instrument. [ABSTRACT FROM AUTHOR]

Some of the icy moons of the solar system with a subsurface ocean, such as Europa and Enceladus, are the targets of future space missions that search for potential extraterrestrial life forms. While the ice shells that envelop these moons have been studied by several spacecrafts, the oceans beneath them remain unreachable. To better constrain the habitability conditions of these moons, we must understand the interactions between their frozen crusts, liquid layers, and silicate mantles. To that end, astrobiologists rely on planetary field analogues, for which the polar regions of Earth have proven to be great candidates. This review shows how spectroscopy is a powerful tool in space missions to detect potential biosignatures, in particular on the aforementioned moons, and how the polar regions of the Earth are being used as planetary field analogues for these extra-terrestrial environments. [ABSTRACT FROM AUTHOR]

Enceladus is a dynamic icy moon of Saturn and a leading target for future planetary missions focused on the search for life beyond Earth. For such missions, instruments that can provide geophysical and geochemical context for ice shell and ocean processes are critical to evaluate whether conditions are suitable for life and biosignature detection. Radar sounding is a powerful geophysical technique to probe the thermophysical and chemical properties of icy moons, like Enceladus, and to investigate the subsurface context for the exchange of material and energy between their subsurface oceans, ice shells, and plumes. To inform the scientific potential and instrument performance demands of such a radar‐sounding investigation of Enceladus' ice shell, we adapt and extend previous radar attenuation analysis done for Europa to the configuration and conditions of Enceladus. We also discuss how attenuation (both as an obstacle for the detection of ice shell reflectors and as a signal itself) can help constrain the thermal, physical, and chemical configuration of Enceladus' ice shell and reveal the processes governing the moon's ocean/shell/plume system. Plain Language Summary: Enceladus, a tiny icy moon of Saturn, has been attracting attention since the discovery of its spectacular water jet activity. The jets, sampling the moon's hidden deep ocean, have revealed conditions favorable for the existence of life below the cold outer ice shell. Understanding the long‐term persistence of the ocean, the jets, and their connection to the ice shell and ocean requires reliable knowledge of the shell's physical, thermal, and chemical properties. Ice penetrating radar is a powerful tool for studying ice shells in the planned missions to Jupiter's icy moons. In this paper, we expand and adapt the radar‐based approach to Enceladus in light of the established constraints on the shell's structure and chemistry. We find that although the shell's chemical composition makes direct radar detection of the ice‐ocean interface challenging, the shell's thermal structure enables constraining the thickness—and potentially chemistry—by future missions using echoes from water within the shell. Key Points: We explore radar attenuation within Enceladus' ice shell as a function of its chemistry using a 3D model of its shape and thermal structureDirect radar detection of the ice‐ocean interface is improbable for chloride‐rich shells except for the thin south‐polar regionThe NH3 eutectic isotherm is a comparatively easy‐to‐detect interface, with the potential to constrain the shell thickness and composition [ABSTRACT FROM AUTHOR]

Observational data suggest that the ice shell on Enceladus is thicker at the equator than at the pole, indicating an equator-to-pole ice flow. If the ice shell is in an equilibrium state, the mass transport of the ice flow must be balanced by the freezing and melting of the ice shell, which in turn is modulated by the ocean heat transport. Here we use a numerical ocean model to study the ice–ocean interaction and ocean circulation on Enceladus with different salinities. We find that salinity fundamentally determines the ocean stratification. A stratified layer forms in the low-salinity ocean, affecting the ocean circulation and heat transport. However, in the absence of tidal heating in the ice shell, the ocean heat transport is found to always be toward lower latitudes, resulting in freezing at the poles, which cannot maintain the ice shell geometry against the equator-to-pole ice flow. The simulation results suggest that either the ice shell on Enceladus is not in an equilibrium state or tidal dissipation in the ice shell is important in maintaining the ice shell geometry. The simulations also suggest that a positive feedback between cross-equatorial ocean heat transport and ice melting results in spontaneous symmetry breaking between the two hemispheres. This feedback may play a role in the observed interhemispheric asymmetry in the ice shell.

A key objective for the future exploration of the icy moon Enceladus is the characterization of the habitable conditions in its internal ocean. Radio science instrumentation on board a spacecraft in a low-altitude orbit about Enceladus would enable gravity measurements that are fundamental to providing constraints on its internal structure. We present here the concept of operations and expected results of the gravity investigation for a New Frontiers–class mission. Numerical simulations are carried out to determine the gravity field in spherical harmonics to degree and order 30 and the Love number k _2 and its phase. By combining Enceladus’s shape measured by Cassini and the geophysical constraints obtained through the processing of the simulated radio science data, a Bayesian inference network is used for the interior model inversion. Our results indicate that the gravity investigation would enable tight constraints on core radius and density, ocean depth and density, and ice shell rigidity. By assuming a high core rigidity and a preliminary modeling of dissipation in the ice shell, our interior model inversion also yields information on the ice shell viscosity. Further data on the hydrosphere properties might be gathered through optical navigation data by accurately measuring Enceladus’s orientation model.

Characterizing the geochemistry of Europa and Enceladus is a key step for astrobiology investigations looking for evidence of life in their subsurface oceans. Transition metals with several oxidation states, such as iron, may be tracers of the oxidation state of icy ocean moon interiors. Their detection, as well as the characterization of their oxidation states, on the moons’ (plume) ice grains would bring valuable new information about the geochemistry of both the subsurface oceans and surface processes. Impact ionization mass spectrometers such as the SUDA instrument on board Europa Clipper can analyze ice grains ejected from icy moons’ surfaces and detect ocean-derived salts therein. Here we record mass spectra analogs for SUDA using the Laser Induced Liquid Beam Ion Desorption technique for Fe ^2+ and Fe ^3+ salts (both sulfates and chlorides). We show that impact ionization mass spectrometers have the capability to detect and differentiate ferrous (Fe ^2+ ) from ferric (Fe ^3+ ) ions in both cation and anion modes owing to their tendency to form distinct ionic complexes with characteristic spectral features. Peaks bearing Fe ^3+ , such as [Fe ^3+ (OH) _2 ] ^+ and [Fe ^3+ (OH) _a Cl _b ] ^− , are particularly important to discriminate between the two oxidation states of iron in the sample. The recorded analog spectra may allow the characterization of the oxidation state of the oceans of Europa and Enceladus with implications for hydrothermal processes and potential metabolic pathways for life forms in their subsurface oceans.

Water vapor produces a series of diagnostic emission lines in the near-infrared between 2.60 and 2.75 μ m. The Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft detected this emission signal from Enceladus’s plume, and so VIMS observations provide information about the variability of the plume’s water-vapor content. Using a data set of 249 spectral cubes with relatively high signal-to-noise ratios, we confirmed the strength of this water-vapor emission feature corresponds to a line-of-sight column density of order 10 ^20 molecules m ^−2 , which is consistent with previous measurements from Cassini’s Ultraviolet Imaging Spectrograph. Comparing observations made at different times indicates that the water-vapor flux is unlikely to vary systematically with Enceladus’s orbital phase, unlike the particle flux, which does vary with orbital phase. However, variations in the column density on longer and shorter timescales cannot be ruled out and merit further investigation.

The Cassini spacecraft detected a soup of organics in the plume of Saturn’s moon Enceladus. Those compounds could provide building blocks for the potential emergence or sustenance of microbial life in Enceladus’ subsurface ocean. However, the sources and stabilities of organics in Enceladus’ ocean are still poorly understood. Here, we perform nonequilibrium thermodynamic calculations to assess the energetics of abiotic synthesis for a broad spectrum of small organic molecules under both cold oceanic and hydrothermal conditions on Enceladus. Most of the organics that we studied are thermodynamically favorable to synthesize at micromolal dissolved concentrations over wide ranges of pH (8.5–11) and redox conditions. This suggests that many of the organic compounds detected by Cassini may be derived from reactions of CO _2 and H _2 . However, some widely assumed precursors of biomolecules, such as formaldehyde, hydrogen cyanide, and acetylene, are found to be unstable and therefore unfavorable to be synthesized. We found that higher temperatures also seem to favor the synthesis of organic species under Enceladus hydrothermal conditions. Detection of thermodynamically unstable species in the plume might reflect artifacts introduced during high-speed spacecraft flybys and/or active synthesis via degradation of primordial chondritic organics, or biological activities.

Evidence for the beneficial role of impacts in the creation of urable or habitable environments on Earth prompts the question of whether meteorite impacts could play a similar role at other potentially urable/habitable worlds like Enceladus, Europa, and Titan. In this work, we demonstrate that to first order, impact conditions on these worlds are likely to have been consistent with the survival of organic compounds and/or sufficient for promoting synthesis in impact melt. We also calculate melt production and freezing times for crater sizes found at Enceladus, Europa, and Titan and find that even the smallest craters at these worlds offer the potential to study the evolution of chemical pathways within impact melt. These first-order calculations point to a critical need to investigate these processes at higher fidelity with lab experiments, sophisticated thermodynamic and chemical modeling, and, eventually, in situ investigations by missions.

We provide a comprehensive study of Enceladus’s time-variable magnetic field environment and identify in measurements of the Cassini spacecraft signatures that appear to be consistent with induced fields from the moon’s interior. Therefore, we first analyze the background field Enceladus is exposed to within 21 flybys and 50 crossings of the moon’s orbit by the Cassini spacecraft. Considering magnetic field variability due to Enceladus’s eccentric orbit, Saturn’s planetary period oscillations, and local time effects within the magnetospheric current sheet, we construct predictive, time-variable background fields near Enceladus with a correlation coefficient of 0.75 and larger compared to the measured background fields. Subsequently, we build a geophysically based electrical conductivity model of Enceladus’s ocean from the equation of state for saline water and mixing laws for a porous core permeated by water. Using this conductivity model and the derived time-variable fields, we calculate expected induced fields. For close flybys, we identify within mostly plume-dominated magnetic field perturbations of 10–30 nT much smaller perturbations of 1–3 nT, which could be consistent with induction. The flybys over Enceladus’s north pole are best suited for induction studies, and the associated measurements suggest that a conductivity of the ocean with 1–3 S m ^–1 is not sufficient to produce an adequate induction response, but they support a highly conductive, porous core of 20–30 S m ^–1 and/or a more conductive ocean. Our study also provides strategies for future magnetic sounding of Enceladus.

In this article, we summarize the work of the NASA Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group. The aim of this group is to assemble the scientific framework that will guide the exploration of ocean worlds, and to identify and prioritize science objectives for ocean worlds over the next several decades. The overarching goal of an Ocean Worlds exploration program as defined by ROW is to "identify ocean worlds, characterize their oceans, evaluate their habitability, search for life, and ultimately understand any life we find." The ROW team supports the creation of an exploration program that studies the full spectrum of ocean worlds, that is, not just the exploration of known ocean worlds such as Europa but candidate ocean worlds such as Triton as well. The ROW team finds that the confirmed ocean worlds Enceladus, Titan, and Europa are the highest priority bodies to target in the near term to address ROW goals. Triton is the highest priority candidate ocean world to target in the near term. A major finding of this study is that, to map out a coherent Ocean Worlds Program, significant input is required from studies here on Earth; rigorous Research and Analysis studies are called for to enable some future ocean worlds missions to be thoughtfully planned and undertaken. A second finding is that progress needs to be made in the area of collaborations between Earth ocean scientists and extraterrestrial ocean scientists.

Some of the major discoveries of the recent Cassini-Huygens mission have put Titan and Enceladus firmly on the Solar System map. The mission has revolutionised our view of Solar System satellites, arguably matching their scientific importance with that of their host planet. While Cassini-Huygens has made big surprises in revealing Titan's organically rich environment and Enceladus' cryovolcanism, the mission's success naturally leads us to further probe these findings. We advocate the acknowledgement of Titan and Enceladus science as highly relevant to ESA's long-term roadmap, as logical follow-on to Cassini-Huygens. In this White Paper, we will outline important science questions regarding these satellites and identify the science themes we recommend ESA cover during the Voyage 2050 planning cycle. Addressing these science themes would make major advancements to the present knowledge we have about the Solar System, its formation, evolution, and likelihood that other habitable environments exist outside the Earth's biosphere. [ABSTRACT FROM AUTHOR]