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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]

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]

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]

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]

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]

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]

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]

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]

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 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]

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]

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

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

Ocean worlds such as Europa and Enceladus are high priority targets in the search for past or extant life beyond Earth. Evidence of life may be preserved in samples of surface ice by processes such as deposition from active plumes, hydrofracturing, or thermal convection. Terrestrial life produces unique distributions of organic molecules that translate into recognizable biosignatures. Identification and quantification of these organic compounds can be achieved by separation science such as capillary electrophoresis coupled to mass spectrometry (CE‐MS). However, the data generated by such an instrument can be multiple orders of magnitude larger than what can be transmitted back to Earth during an ocean world's mission. This requires onboard science data analysis capabilities that summarize and prioritize CE‐MS observations with limited computational resources. In response, the autonomous capillary electrophoresis mass‐spectra examination (ACME) onboard science autonomy system was created for application to the ocean world's life surveyor (OWLS) instrument suite. ACME is able to compress raw mass spectra by two to three orders of magnitude while preserving most of its scientifically relevant information content. This summarization is achieved by the extraction of raw data surrounding autonomously identified ion peaks and the detection and parameterization of unique background regions. Prioritization of the summarized observations is then enabled by providing estimates of scientific utility, including presence of key target compounds, and the uniqueness of an observation relative to previous observations. Key Points: Onboard science autonomy makes high data rate instruments viable for interplanetary missions with limited bandwidthDeveloped onboard summarization and prioritization of data from CE‐MS raw files to compress raw mass spectra by orders of magnitudePrioritization of samples by science utility estimate and diversity allows to maximize science yield [ABSTRACT FROM AUTHOR]

Landed missions to icy worlds with a subsurface liquid water ocean must meet planetary protection requirements and ensure a sufficiently small likelihood of any microorganism-bearing part of the landed element reaching the ocean. A higher bound on this likelihood is set by the potential for radioisotope thermoelectric generator (RTG) power sources, the hottest possible landed element, to melt through the ice shell and reach the ocean. In this study, we quantify this potential as a function of three key parameters: surface temperature, ice shell thickness (i.e., heat flux through the shell), and thickness of a porous (insulating) snow or regolith cover. Although the model we describe can be applied to any ocean world, we present results in the context of a landed mission concept to the south polar terrain of Saturn's moon Enceladus. In this particular context, we discuss planetary protection considerations for landing site selection. The likelihood of forward microbial contamination of Enceladus' ocean by an RTG-powered landed mission can be made sufficiently low to not undermine compliance with the planetary protection policy. [ABSTRACT FROM AUTHOR]

• Enceladus' disk-averaged degree of linear polarization (DoLP) is calculated from Cassini data. • Enceladus' surface polarization gives insight into plume particle morphology. • A multiple-scattering simulation model rules out spherical plume particles. • Plume particles on the surface may be present in narrow size distributions. • Future work includes expanding the model to other geometries and possibilities. Enceladus, the sixth largest moon of Saturn, is known to be a geologically active icy body. Observations by NASA's Cassini spacecraft show that Enceladus has cryovolcanoes on its south pole as well as a global subsurface ocean hidden beneath its frozen crust. Photographs from NASA's Cassini Spacecraft during flybys of Enceladus document Enceladus's surface and the cryovolcano plumes. It is known that larger particles ejected from the cryovolcanoes deposit on the surface, while smaller particles escape into Saturn's E-ring. Cassini observations of the sunlight scattered by the plume particles on the surface may provide information about the plume composition and potentially the dynamics of the ocean below. This work presents Enceladus' disk-averaged degree of linear polarization (DoLP), which has yet to be analyzed, and compares it to spherical, spheroidal, cylindrical, and hexagonal ice particles simulated by light scattering codes ADDA and T-Matrix using a multiple-scattering approximation. The Pearson correlation coefficient is used to quantify the fit of each morphology to the surface polarization data in the GRN (569 nm) and MT2 (727 nm) wavelength filters. The polarization of Enceladus' surface implies the particles are likely non-spherical, and possibly occur in a narrow size distribution near a mean of two microns in radius. [ABSTRACT FROM AUTHOR]

The Ames Vertical Gun Range (AVGR) is a hypervelocity impact facility hosting a variable angle, two-stage, light-gas gun that can accelerate projectiles up to 7 km s−1. We have used the AVGR to produce impacts into supercooled (73–89 K) synthetic seawater ice targets inside a large impact chamber, whose pressure is 0.3 to 0.7 Torr. We report on the effective novel use of stainless steel meshes (49 μm, 100 μm, 149 μm, and 500 μm) to filter the ejecta plume for the ice grains of a desired size and demonstrate that reproducible temperature, speed, and particle size distribution can be achieved. The resulting ejecta plume of micrometer-sized ice particles (micro grains) from these impacts provides a method to simulate spacecraft encounters with ice particles found in the plumes of the outer Solar System, especially Enceladus. Image analysis of ejecta plume impacts onto aluminum (Al) foil enabled us to determine the ice particle diameter range. We find that the particles in the ejecta plume have speeds from 0.2 to 2 km s−1 and temperatures from 102 to 113 K (−171 to −160 °C). Furthermore, 61–97% by numberof the ice grains have equivalent diameter between 2.5 and 20 μm, that is, in the range 1 μm ≤ radii ≤10 μm, the upper size constrained by Cassini, i.e., ∼2 μm to 50 μm. Overall, the AVGR provides a high-fidelity simulation of ice micro grains impacting spacecraft surfaces and sample-collecting systems. This may be useful in the design, testing, and data analysis of future missions to Enceladus, Europa, and other icy moons. • The Ames Vertical Gun Range is used to create high speed ice particle plumes. • We have calibrated the size and speed of these ice plumes. • A way to test instruments for ice particle collection at Enceladus and Europa. [ABSTRACT FROM AUTHOR]

Enceladus, an icy moon of Saturn, consists of an ice shell, global subsurface ocean and a silicate interior. By sampling plume material, the Cassini spacecraft found evidence of ongoing water-rock reactions between the silicate interior and the subsurface ocean. These data showed that these reactions provide a source of bioessential elements to the ocean, making Enceladus one of the leading astrobiological targets in our Solar System. Understanding these water-rock reactions is critical in understanding the potential habitability of Enceladus. To study these reactions experimentally, a chemical simulant to represent the contemporary silicate interior of Enceladus has been designed. Based on the available interpretations of Cassini data about the density, chemical composition, and aqueous alteration of the interior, the chosen starting point for the simulant is a CI chondrite. However, Enceladus is still undergoing active aqueous alteration, thus its silicate mineral assemblage cannot have reached the fully altered assemblage seen in a CI chondrite. To account for this, adaptations have been made to a CI chondrite mineral assemblage, extrapolating back to an assemblage of less aqueously altered minerals whilst maintaining the same chemical composition in terms of major oxide phases. Thus, the chemical and mineralogical composition of this simulant represents a best estimate of the silicate components in the ongoing water rock interactions on Enceladus today. • Rocky interior of Enceladus may have formed from CI chondritic material. • Different periods of Enceladus' geological history can be investigated. • Experimental water-rock reactions will help constrain geochemical cycling. [ABSTRACT FROM AUTHOR]

Spacecraft flybys provide access to the chemical composition of the gaseous envelope of the planetary object. Typical relative encounter velocities range from km/s to tens of km/s in flybys. For speeds exceeding about 5 km/s, modern mass spectrometers analyzing the rapidly encountering gas suffer from intrinsic hypervelocity impact-induced fragmentation processes causing ambiguous results when analyzing complex molecules. In this case, instruments use an antechamber, inside which the incoming species collide many times with the chamber wall. These collisions cause the desired deceleration and thermalization of the gas molecules. However, these collisions also dissociate molecular bonds, thus fragmenting the molecules, and possibly forming new ones precluding scientists from inferring the actual chemical composition of the sampled gas. We developed a novel time-of-flight mass spectrometer that handles relative encounter velocities of up to 20 km/s omitting an antechamber and its related fragmentation. It analyzes the complete mass range of m/z 1 to 1000 at an instance. This innovation leads to unambiguous analysis of complex (organic) molecules. Applied to Enceladus, Europa or Io, it will provide reliable chemical composition datasets for exploration of the Solar System to determine its status, origin and evolution. [ABSTRACT FROM AUTHOR]

Metal complexes of porphyrins and porphyrin-type compounds are ubiquitous in all three domains of life, with hemes and chlorophylls being the best-known examples. Their diagenetic transformation products are found as geoporphyrins, in which the characteristic porphyrin core structure is retained and which can be up to 1.1 billion years old. Because of this, and their relative ease of detection, metalloporphyrins appear attractive as chemical biosignatures in the search for extraterrestrial life. In this study, we investigated the stability of solid chlorido(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III) [FeCl(oep)], which served as a model for heme-like molecules and iron geoporphyrins. [FeCl(oep)] was exposed to a variety of astrobiologically relevant extreme conditions, namely: aqueous acids and bases, oxidants, heat, and radiation. Key results are: (1) the [Fe(oep)]+ core is stable over the pH range 0.0–13.5 even at 80°C; (2) the oxidizing power follows the order ClO− > H2O2 > ClO3− > HNO3 > ClO4−; (3) in an inert atmosphere, the iron porphyrin is thermally stable to near 250°C; (4) at high temperatures, carbon dioxide gas is not inert but acts as an oxidant, forming carbon monoxide; (5) a decomposition layer is formed on ultraviolet irradiation and protects the [FeCl(oep)] underneath; (6) an NaCl/NaHCO3 salt mixture has a protective effect against X-rays; and (7) no such effect is observed when [FeCl(oep)] is exposed to iron ion particle radiation. The relevance to potential iron porphyrin biosignatures on Mars, Europa, and Enceladus is discussed. [ABSTRACT FROM AUTHOR]

Cassini revealed that Saturn's Moon Enceladus hosts a subsurface ocean that meets the accepted criteria for habitability with bio-essential elements and compounds, liquid water, and energy sources available in the environment. Whether these conditions are sufficiently abundant and collocated to support life remains unknown and cannot be determined from Cassini data. However, thanks to the plume of oceanic material emanating from Enceladus' south pole, a new mission to Enceladus could search for evidence of life without having to descend through kilometers of ice. In this article, we outline the science motivations for such a successor to Cassini, choosing the primary science goal to be determining whether Enceladus is inhabited and assuming a resource level equivalent to NASA's Flagship-class missions. We selected a set of potential biosignature measurements that are complementary and orthogonal to build a robust case for any life detection result. This result would be further informed by quantifications of the habitability of the environment through geochemical and geophysical investigations into the ocean and ice shell crust. This study demonstrates that Enceladus' plume offers an unparalleled opportunity for in situ exploration of an Ocean World and that the planetary science and astrobiology community is well equipped to take full advantage of it in the coming decades. [ABSTRACT FROM AUTHOR]

In this paper, we characterise the adsorption of ethylene, propanol and hexanal molecules on crystalline ice by grand canonical Monte Carlo simulations performed at 236 K, a temperature which is typical of some Enceladus' environments. We show that at low coverage of the ice surface, the adsorption of propanol and hexanal is driven by the interaction of these molecules with the ice phase and, as a consequence, the adsorbed molecules lie more or less parallel to the ice surface. On the other hand, upon saturation, the adsorbate–adsorbate interactions become more and more important and the molecules tend to become tilted with respect to the surface, the aliphatic chain pointing towards the gas phase, while the polar head of propanol and hexanal molecules always stays attached to the ice surface, irrespective of the coverage. By contrast, the ethylene molecules do not show any strong affinity for the ice surface because of the corresponding weak binding energy. These results are in good agreement with the major features provided by the available experiments on similar systems. [ABSTRACT FROM AUTHOR]

Water is a key ingredient to the potential for life, and several icy moons, such as Europa and Enceladus, are thought to harbor a dynamic subsurface ocean of liquid water. An ice probe or "cryobot" is a proposed technology to penetrate the surrounding ice shell and deliver a life-detection payload to these oceans. As the ice probe descends, it is surrounded by a thin melt film of liquid water. This paper focuses on the fluidics of the melt film for the case of a general ice probe shape, leading to a new approximate equation for the force balance constraints (validated to < 0. 4 % error by numerical model). In addition to the introduction of water by melting, the analysis allows for consideration of additional volumetric flow via a pumped jet of recirculated water. The general force balance equation is explored in more detail for the example of a cylindrical probe geometry with flat circular end caps. The thickness of the melt film below and to the side of the cylindrical probe is constrained by the force balance such that the probe typically operates in one of two asymptotic regimes, moving or stalled. [Display omitted] • Responds to community interest in accessing the oceans of Europa and Enceladus. • Advances models of mechanical and thermal behavior of a descending ice probe. • Generalizes previous work to an arbitrary probe shape. • Identifies two asymptotic regimes of probe behavior, moving or stalled. • Quantifies previously unmodeled inefficiency due to advective heat loss. [ABSTRACT FROM AUTHOR]

We propose a new model of Enceladus' tiger stripes in which the ice shell is modeled as an elastic system with Coulomb‐type frictional interfaces subjected to periodic tidal loading. We find that the diurnal tides produce a complex pattern of stress anomalies, characterized by a length scale of tens of km and the peak values exceeding 100 kPa. Friction delays the response of the system to tidal loading and leads to an asymmetry between the compression and extension phases. This asymmetry results in additional stress, constant in time and comparable in magnitude to the cyclic stress. This static stress field is characterized by compression in the direction perpendicular to the faults and may influence the evolution of the south polar region on geological time scales. The total heat flow generated by friction is 0.1–1 GW, accounting for only a small fraction of the heat power emitted from the tiger stripes. Plain Language Summary: The tiger stripes are four sub‐parallel, linear depressions in the south polar region (SPR) of Saturn's moon Enceladus, which are known for their vapor plumes containing organic molecules. The nature of the tiger stripes is not fully understood and remains a subject of intense debate. Here, we propose a new model of the tiger stripes in which Enceladus' ice shell is modeled as an elastic system with frictional interfaces subjected to periodic tidal loading. We find that the diurnal tides produce a complex pattern of stress anomalies, characterized by a length scale of tens of km and the peak values exceeding 100 kPa. Friction delays the system's response to tidal loading and leads to an asymmetry between the compression and extension phases. This asymmetry results in an additional stress, which is constant in time and comparable in magnitude to the cyclic stress. The static stress field is characterized by compression in the direction perpendicular to the faults, and its magnitude is large enough to influence the evolution of the SPR on geological time scales. The total heat flow generated by friction is 0.1–1 GW, accounting for only a small fraction of the heat power emitted from the tiger stripes. Key Points: Friction on faults delays the response of the system to tidal loading and leads to an asymmetry between the compression and extension phaseThe asymmetry results in additional stress, which is constant in time and comparable in magnitude to the cyclic stress (10–100 kPa)The frictional heating does not exceed 1 GW in total and is mainly concentrated in the central parts of the tiger stripes [ABSTRACT FROM AUTHOR]

We propose a new model of Enceladus' tiger stripes in which the ice shell is modeled as an elastic system with Coulomb‐type frictional interfaces subjected to periodic tidal loading. We find that the diurnal tides produce a complex pattern of stress anomalies, characterized by a length scale of tens of km and the peak values exceeding 100 kPa. Friction delays the response of the system to tidal loading and leads to an asymmetry between the compression and extension phases. This asymmetry results in additional stress, constant in time and comparable in magnitude to the cyclic stress. This static stress field is characterized by compression in the direction perpendicular to the faults and may influence the evolution of the south polar region on geological time scales. The total heat flow generated by friction is 0.1–1 GW, accounting for only a small fraction of the heat power emitted from the tiger stripes. Plain Language Summary: The tiger stripes are four sub‐parallel, linear depressions in the south polar region (SPR) of Saturn's moon Enceladus, which are known for their vapor plumes containing organic molecules. The nature of the tiger stripes is not fully understood and remains a subject of intense debate. Here, we propose a new model of the tiger stripes in which Enceladus' ice shell is modeled as an elastic system with frictional interfaces subjected to periodic tidal loading. We find that the diurnal tides produce a complex pattern of stress anomalies, characterized by a length scale of tens of km and the peak values exceeding 100 kPa. Friction delays the system's response to tidal loading and leads to an asymmetry between the compression and extension phases. This asymmetry results in an additional stress, which is constant in time and comparable in magnitude to the cyclic stress. The static stress field is characterized by compression in the direction perpendicular to the faults, and its magnitude is large enough to influence the evolution of the SPR on geological time scales. The total heat flow generated by friction is 0.1–1 GW, accounting for only a small fraction of the heat power emitted from the tiger stripes. Key Points: Friction on faults delays the response of the system to tidal loading and leads to an asymmetry between the compression and extension phaseThe asymmetry results in additional stress, which is constant in time and comparable in magnitude to the cyclic stress (10–100 kPa)The frictional heating does not exceed 1 GW in total and is mainly concentrated in the central parts of the tiger stripes [ABSTRACT FROM AUTHOR]

• Understanding Enceladus' plumes gives information about the possibility of life. • Previous studies have no consensus on size/shape of the plume particles. • Cryovolcano particles are likely larger than 2 µm on average. • Q-space analysis gives evidence that the particles are not spherical. • Future work includes comparisons of light scattering simulations to satellite data. Observations by NASA's Cassini spacecraft show that Enceladus, the sixth largest moon of Saturn, has cryovolcanoes on its south pole as well as a global subsurface ocean beneath Enceladus's icy crust. Photographs from the Imaging Science Subsystem (ISS) cameras aboard Cassini during flybys of Enceladus show cryovolcano plumes. The images are taken in several filters and with linear polarizers at various angles and orbital positions. Light scattered by the plume particles and observed by Cassini may provide information about the plume composition and potentially the dynamics of the ocean below. Here, we remove the coupling between orbital position and scattered plume intensity as well as analyze ISS data using light scattering models and the Q-space method. We suggest that the plume particles are generally larger than two microns in radius and nonspherical in shape. [ABSTRACT FROM AUTHOR]

Supersonic plumes of water vapour and icy particles have been observed by the Cassini spacecraft during several flybys over Enceladus. These plumes originate from the Tiger Stripes located in the South Polar Terrain (SPT), and indicate the presence of a subsurface ocean under the icy crust which is salty and contains complex organic molecules. Other characteristics of the plumes, such as the vent temperature, mass flow rate, velocity and mass fraction of icy particles can be used to determine the conditions in the channel, linking the subsurface ocean to the icy surface. In this paper, we developed a fluid dynamics model that accounts for nucleation, particle growth, wall accretion and sublimation. The channel behaves similarly to a converging–diverging nozzle, which forms supersonic plumes due to a pressure difference between the reservoir where the subsurface ocean is located and the exosphere. The geometry of the channel and its evolution with accretion of gas and sublimation of ice are studied to reproduce the characteristics of the plumes observed by Cassini. We first performed a parameter study on the channel geometry to determine how it influences the plumes' velocity, solid fraction and exit temperature. Our results show that the size of the icy particles is primarily dependent on the length of the channel, indicating that large particles (∼ 75 μ m) must originate from within a kilometer below the surface, while smaller particles (∼ 3 μ m) can originate from only hundreds of meters below the surface. We further show that the velocity of the flow, exit temperature and nucleation depend directly on the exit-to-throat size ratio. We find that the channel geometry evolves within a few tens of hours until an equilibrium is reached, when considering the accretion of gas to the walls, or sublimation of ice from the walls. As the channel closes due to accretion, the flow becomes thinner, which in turn reduces accretion. After around 70 h, the accretion is sufficiently slowed such that the geometry does not evolve anymore. This equilibrium geometry produces higher Mach numbers and a larger particle size and solid fraction compared to the initial geometry. • On Enceladus, a Saturnian moon, supersonic plumes of water ice and vapour have been observed. • These plumes indicate the presence of a liquid subsurface ocean hidden under kilometers of ice. • We studied numerically the plumes as they develop from the ocean to the surface through channels. • We found that the size of the icy grains observed in the plumes can constrain the length of the channels. • The plumes' velocity and temperature constrain the expansion (throat to exit) ratio of the channel. • Water accretion changes the channel's geometry within hours and increases the expansion ratio. [ABSTRACT FROM AUTHOR]

In this study we investigate the relative importance of endogenic vs. exogenic stresses in controlling the location and timing of active ice-shell deformation on Enceladus, which is expressed by cyclic plume eruptive flux along active fault zones (i.e., the tiger stripes). Although the variation of the eruption flux on Enceladus follows the periodicity of the diurnal tide, it remains unclear why there is a consistent phase delay of the observed peak eruption when compared to the predicted peak tidal stress. Here, we explore whether endogenic stresses in the ice shell are capable of explaining this observed phase delay. To achieve this goal, we performed geologic mapping along the tiger-stripe faults that host the erupting plumes. Using the fault kinematics established from our mapping, we determine the general stress state (i.e., the principal-stress directions) along the tiger-stripe faults. This knowledge in turn forms the basis for inferring the most likely plume-eruption mechanism. Our mapping shows that the tiger-stripe fractures are not tensile cracks but are instead left-slip fault zones locally displaying extensional fissures. This insight leads to a hypothesis that strike-slip faults and their local tensile cracks experience simultaneous shear and tensile failure, and that the tensional opening reaches maximum at the time of the peak plume flux. We quantified this hypothesis using a stress decomposition model that assesses (1) the relative importance in magnitude between the tectonic stress and tidal stress exerted on the tiger-stripe faults and (2) the role of ice shell properties such as the shear strengths, tensile strengths, and ice shell thickness in controlling the eruption phase delay. Using laboratory-determined ice strengths and the best estimate of the ice shell thickness at the South Polar Terrain of Enceladus, which hosts the tiger-stripe faults, our model results indicate that the endogenic tectonic stress is comparable in magnitude to the tidal stress. Although we cannot rule out warm-ice convection, true polar wander, and non-synchronous rotation as causes of endogenic stresses, the large variation in ice shell thickness makes the lateral gravitational-potential gradient the most plausible source of the endogenic stress required by our model results. • Geologic mapping of the tiger stripes suggest they are left-slip fault zones. • The tiger stripes experience simultaneous shear and tensile failure. • Local extensional structures host plume eruption centers, i.e., pull-apart basins. • Tectonic stress is comparable in magnitude to the tidal stress at the SPT. • Gravitational-potential gradient most plausible source of the endogenic stress. [ABSTRACT FROM AUTHOR]

The ability of tardigrades to survive impact shocks in the kilometer per second and gigapascal range was investigated. When rocks impact planetary surfaces, the impact speeds and shock pressures are in the kilometer per second and gigapascal range. This investigation tested whether tardigrades can survive in impacts typical of those that occur naturally in the Solar System. We found that they can survive impacts up to 0.9 km s−1, which is equivalent to 1.14 GPa shock pressure, but cannot survive impacts above this. This is significantly less than the static pressure limit and has implications for tardigrade survival in panspermia models. The potential survival of tardigrades in impacts of terrestrial impact ejecta on the Moon is shown to be impossible for the average lunar impact speed of such ejecta. However, a notable fraction (around 40%) of such ejecta impact at vertical speeds low enough to permit survival. Similarly, martian impact ejecta striking Phobos, for example, at a typical impact speed will not permit viable transfer of tardigrade-like organisms, but if a fraction of such material had a lower impact speed, survival may be possible. We also consider the implications of this for the collection of viable samples by spacecraft transiting the plumes of icy water worlds such as Europa and Enceladus. We have found the limit on survival of shocks to be around 1 GPa, which is instrumental in determining appropriate mission scenarios and collection methods for the acquisition of viable materials. [ABSTRACT FROM AUTHOR]

Amide bonds are fundamental products in biochemistry, forming peptides critical to protein formation, but amide bonds are also detected in sterile environments and abiotic synthesis experiments. The abiotic formation of amide bonds may represent a prerequisite to the origin of life. Here we report thermodynamic models that predict optimal conditions for amide bond synthesis across geologically relevant ranges of temperature, pressure, and pH. We modeled acetamide formation from acetic acid and ammonia as a simple analog to peptide bond formation, and tested this model with hydrothermal experiments examining analogous reactions of amides including benzanilide and related structures. We also expanded predictions for optimizing diglycine formation, revealing that in addition to synthesis becoming more favorable at near-ambient pressures (P sat) with increasing temperatures, the strongest thermodynamic drive exists at extremely high pressures (>15,000 bar) and decreasing temperatures. Beyond implications for life's origins, the reactants and products involved in simple amide formation reactions can potentially be used as geochemical tracers for planetary exploration of environments that may be habitable. [ABSTRACT FROM AUTHOR]

The atomic-scale fragmentation processes involved in molecules undergoing hypervelocity impacts (HVIs; defined as >3 km/s) are challenging to investigate via experiments and still not well understood. This is particularly relevant for the consistency of biosignals from small-molecular-weight neutral organic molecules obtained during solar system robotic missions sampling atmospheres and plumes at hypervelocities. Experimental measurements to replicate HVI effects on neutral molecules are challenging, both in terms of accelerating uncharged species and isolating the multiple transition states over very rapid timescales (<1 ps). Nonequilibrium first-principles-based simulations extend the range of what is possible with experiments. We report on high-fidelity simulations of the fragmentation of small organic biosignature molecules over the range v = 1−12 km/s, and demonstrate that the fragmentation fraction is a sensitive function of velocity, impact angle, molecular structure, impact surface material, and the presence of surrounding ice shells. Furthermore, we generate interpretable fragmentation pathways and spectra for velocity values above the fragmentation thresholds and reveal how organic molecules encased in ice grains, as would likely be the case for those in "ocean worlds," are preserved at even higher velocities than bare molecules. Our results place ideal spacecraft encounter velocities between 3 and 5 km/s for bare amino and fatty acids and within 4–6 km/s for the same species encased in ice grains and predict the onset of organic fragmentation in ice grains at >5 km/s, both consistent with recent experiments exploring HVI effects using impact-induced ionization and analysis via mass spectrometry and from the analysis of Enceladus organics in Cassini Data. From nanometer-sized ice Ih clusters, we establish that HVI energy is dissipated by ice casings through thermal resistance to the impact shock wave and that an upper fragmentation velocity limit exists at which ultimately any organic contents will be cleaved by the surrounding ice—this provides a fundamental path to characterize micrometer-sized ice grains. Altogether, these results provide quantifiable insights to bracket future instrument design and mission parameters. [ABSTRACT FROM AUTHOR]

This contribution investigates the properties of a category of orbits around Enceladus. The motivation is the interest in the in situ exploration of this moon following Cassini's detection of plumes of water and organic compounds close to its south pole. In a previous investigation, a set of heteroclinic connections were designed between halo orbits around the equilibrium points L 1 and L 2 of the circular restricted three-body problem with Saturn and Enceladus as primaries. The kinematical and geometrical characteristics of those trajectories makes them good candidates as science orbits for the extended observation of the surface of Enceladus: they are highly inclined, they approach the moon and they are maneuver-free. However, the low heights above the surface and the strong perturbing effect of Saturn impose a more careful look at their dynamics, in particular regarding the influence of the polar flattening of the primaries. Therefore, those solutions are here reconsidered by employing a dynamical model that includes the effect of the oblateness of Saturn and Enceladus, individually and in combination. The dynamical equivalents of the halo orbits around the equilibrium points L 1 and L 2 and their stable and unstable hyperbolic invariant manifolds are obtained in the perturbed models, and maneuver-free heteroclinic connections are identified in the new framework. A systematic comparison with the corresponding solutions of the unperturbed problem shows that qualitative and quantitative features are not significantly altered when the oblateness of the primaries is taken into account. Furthermore, it is found that the J 2 coefficient of Saturn plays a larger role than that of Enceladus. From a mission perspective, the results confirm the scientific value of the solutions obtained in the classical circular restricted three-body problem and suggests that this simpler model can be used in a preliminary feasibility analysis. • Orbits in the Saturn–Enceladus three-body problem with oblate primaries are computed. • Heteroclinic connections between libration point orbits around Enceladus are designed. • Performance of heteroclinics as science orbits is assessed. • Solutions offer long, uninterrupted views of lunar surface at low speeds. • Solutions confirm previous results in classical circular restricted three-body problem. [ABSTRACT FROM AUTHOR]

The Solar System is becoming increasingly accessible to exploration by robotic missions to search for life. However, astrobiologists currently lack well-defined frameworks to quantitatively assess the chemical space accessible to life in these alien environments. Such frameworks will be critical for developing concrete predictions needed for future mission planning, both to determine the potential viability of life on other worlds and to anticipate the molecular biosignatures that life could produce. Here, we describe how uniting existing methods provides a framework to study the accessibility of biochemical space across diverse planetary environments. Our approach combines observational data from planetary missions with genomic data catalogued from across Earth and analyzed using computational methods from network theory. To demonstrate this, we use 307 biochemical networks generated from genomic data collected across Earth and "seed" these networks with molecules confirmed to be present on Saturn's moon Enceladus. By expanding through known biochemical reaction space starting from these seed compounds, we are able to determine which products of Earth's biochemistry are, in principle, reachable from compounds available in the environment on Enceladus, and how this varies across different examples of life from Earth (organisms, ecosystems, planetary-scale biochemistry). While we find that none of the 307 prokaryotes analyzed meet the threshold for viability, the reaction space covered by this process can provide a map of possible targets for detection of Earth-like life on Enceladus, as well as targets for synthetic biology approaches to seed life on Enceladus. In cases where biochemistry is not viable because key compounds are missing, we identify the environmental precursors required to make it viable, thus providing a set of compounds to prioritize for detection in future planetary exploration missions aimed at assessing the ability of Enceladus to sustain Earth-like life or directed panspermia. [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]

In research, sometimes sheer happenstance and serendipity make for an unexpected discovery. Once revealed and if interesting enough, such a finding and its follow-up investigations can lead to advances by others that leave its originators 'scooped' and mulling about what next to do with their unpublished data, specifically what journals could it still be published in and be perceived as original. This is what occurred with us nearly 40 years ago with regard to our follow-up observations of acetylene fermentation and led us to concoct a 'cock-and-bull' story. We hypothesized about a plausible role for acetylene metabolism in the primordial biogeochemistry of Earth and the possibility of acetylene serving as a key life-sustaining substrate for alien microbes dwelling in the orbs of the outer solar system. With the passage of time, advances were made in whole-genome sequencing coupled with major in silico progress in bioinformatics. In parallel came the results of explorations of the outer solar system (i.e. the Cassini mission to Saturn and its moons). It now appears that these somewhat harebrained ideas of ours, arisen at first out of a sense of desperation, actually ring true in fact, and particularly well in song: 'Tell a tale of cock and bull, Of convincing detail full Tale tremendous, Heav'n defend us! What a tale of cock and bull!' From 'The Yeoman of the Guard' by Gilbert & Sullivan. [ABSTRACT FROM AUTHOR]

Mission analysis using aerogravity-assist (AGA) maneuver at Titan is presented in this paper. Considering Saturn arrival, Titan encounter, and different post-AGA orbits, the analysis uses vector diagrams to illustrate the relations among all conditions as well as design constraints. The analysis focuses on Enceladus mission concept and also present a summary result for Cassini-type Saturn system missions. Turn angles using AGA maneuver at Titan are quantified via numerical simulation, showing that 25-deg AGA turn angles are achievable with a Titan inbound excess velocity (V ∞ , IN ) of 10–20 km/s. Results show that for an Enceladus mission, V ∞ , IN of 10 km/s is the minimum velocity required for AGA maneuver to achieve an outbound transfer orbit directly to Enceladus; the corresponding peak g-load is less than 6 g, peak heat rate less than 200 W/cm2, and total heat load of around 25 kJ/cm2. For a Cassini-type Saturn mission, a minimum V ∞ , IN of 5 km/s can be used for Titan AGA. The results showed are for V ∞ , IN of 6 km/s; and the corresponding peak g-load, peak heat rate, and total heat load are 1.5 g, 20 W/cm2, and 5 kJ/cm2 respectively. • Design space for Saturn system missions using Titan aerogravity-assist is fully explored. • A novel method combining graphic approach using vector diagrams and numerical simulation is presented. • Readiness of entry vehicles and TPS technologies for using Titan aerogravity-assist maneuver is evaluated. [ABSTRACT FROM AUTHOR]

We used a deep-ultraviolet fluorescence mapping spectrometer, coupled to a drill system, to scan from the surface to 105 m depth into the Greenland ice sheet. The scan included firn and glacial ice and demonstrated that the instrument is able to determine small (mm) and large (cm) scale regions of organic matter concentration and discriminate spectral types of organic matter at high resolution. Both a linear point cloud scanning mode and a raster mapping mode were used to detect and localize microbial and organic matter "hotspots" embedded in the ice. Our instrument revealed diverse spectral signatures. Most hotspots were <20 mm in diameter, clearly isolated from other hotspots, and distributed stochastically; there was no evidence of layering in the ice at the fine scales examined (100 μm per pixel). The spectral signatures were consistent with organic matter fluorescence from microbes, lignins, fused-ring aromatic molecules, including polycyclic aromatic hydrocarbons, and biologically derived materials such as fulvic acids. In situ detection of organic matter hotspots in ice prevents loss of spatial information and signal dilution when compared with traditional bulk analysis of ice core meltwaters. Our methodology could be useful for detecting microbial and organic hotspots in terrestrial icy environments and on future missions to the Ocean Worlds of our Solar System. [ABSTRACT FROM AUTHOR]

Identifying and distinguishing between abiotic and biotic signatures of organic molecules such as amino acids and fatty acids is key to the search for life on extraterrestrial ocean worlds. Impact ionization mass spectrometers can potentially achieve this by sampling water ice grains formed from ocean water and ejected by moons such as Enceladus and Europa, thereby exploring the habitability of their subsurface oceans in spacecraft flybys. Here, we extend previous high-sensitivity laser-based analog experiments of biomolecules in pure water to investigate the mass spectra of amino acids and fatty acids at simulated abiotic and biotic relative abundances. To account for the complex background matrix expected to emerge from a salty Enceladean ocean that has been in extensive chemical exchange with a carbonaceous rocky core, other organic and inorganic constituents are added to the biosignature mixtures. We find that both amino acids and fatty acids produce sodiated molecular peaks in salty solutions. Under the soft ionization conditions expected for low-velocity (2–6 km/s) encounters of an orbiting spacecraft with ice grains, the unfragmented molecular spectral signatures of amino acids and fatty acids accurately reflect the original relative abundances of the parent molecules within the source solution, enabling characteristic abiotic and biotic relative abundance patterns to be identified. No critical interferences with other abiotic organic compounds were observed. Detection limits of the investigated biosignatures under Enceladus-like conditions are salinity dependent (decreasing sensitivity with increasing salinity), at the μM or nM level. The survivability and ionization efficiency of large organic molecules during impact ionization appear to be significantly improved when they are protected by a frozen water matrix. We infer from our experimental results that encounter velocities of 4–6 km/s are most appropriate for impact ionization mass spectrometers to detect and discriminate between abiotic and biotic signatures. [ABSTRACT FROM AUTHOR]

Enceladus and possibly Europa spew materials from their internal ocean into their exosphere, some of which are deposited back onto the surface of those Ocean Worlds. This setting provides a unique opportunity to seek traces of past or extant life in ice plume deposits on their surfaces. However, the design of lander missions and surface sampling techniques and the choice of sampling locations rely heavily on strength expectations. Here we present an experimental investigation of the evolution in strength of ice plume deposit analogs at several temperatures, as well as a model that predicts first‐order estimates of the strength of evolved ice plume deposits under geologic timescales relevant to Enceladus and Europa. These results suggest that plume deposits remain weak and poorly consolidated on Enceladus, while they may develop substantial strength (comparable to solid ice) within <100 Myr on Europa. Plain Language Summary: Enceladus and Europa are Ocean Worlds; they harbor an internal ocean beneath their ice shells. There is proof that a plume emits ocean materials out of Enceladus, similar to geysers on Earth, and some evidence for similar activity on Europa. Based on the composition of the plumes and the surface, both Enceladus and Europa are the leading outer solar system candidates for possibly harboring life. Areas where fresh plume materials are deposited would be the best location to search for traces of life on the surface. A major challenge in preparing mission concepts to explore these locations arises from the need to collect samples of the surface ice, while little is known at present about the mechanical properties of the surface. In this study, we prepared icy plume deposit analogs and let them evolve in the laboratory over extended periods of time to investigate the evolution of their strength. We find that plume deposits are likely to remain loose and exhibit a low strength over geologic timescales under Enceladus's conditions, suggesting that they would be relatively easy to sample. Conversely, under Europa's surface conditions, such plume deposits appear likely to develop a substantial strength. Key Points: The cone penetration resistance of fine‐grained porous ice held under isothermal conditions increases linearly over timeThe temperature dependence of the strengthening rate yields an activation energy similar to self‐diffusion at the surface of ice grainsPlume deposits would remain weak on Enceladus, while they may develop substantial strength within a few million years on Europa [ABSTRACT FROM AUTHOR]

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