Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa's water vapor aurora

Characterization of the Surfaces and Near-Surface Atmospheres of Ganymede, Europa and Callisto by JUICE

We present the state of the art on the study of surfaces and tenuous atmospheres of the icy Galilean satellites Ganymede, Europa and Callisto, from past and ongoing space exploration conducted with several spacecraft to recent telescopic observations, and we show how the ESA JUICE mission plans to explore these surfaces and atmospheres in detail with its scientific payload. The surface geology of the moons is the main evidence of their evolution and reflects the internal heating provided by tidal interactions. Surface composition is the result of endogenous and exogenous processes, with the former providing valuable information about the potential composition of shallow subsurface liquid pockets, possibly connected to deeper oceans. Finally, the icy Galilean moons have tenuous atmospheres that arise from charged particle sputtering affecting their surfaces. In the case of Europa, plumes of water vapour have also been reported, whose phenomenology at present is poorly understood and requires future close exploration. In the three main sections of the article, we discuss these topics, highlighting the key scientific objectives and investigations to be achieved by JUICE. Based on a recent predicted trajectory, we also show potential coverage maps and other examples of reference measurements. The scientific discussion and observation planning presented here are the outcome of the JUICE Working Group 2 (WG2): "Surfaces and Near-surface Exospheres of the Satellites, dust and rings".

Chapter 7: Assessing Habitability Beyond Earth

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.

Multi-Fluid MHD Simulations of Europa's Plasma Interaction: Effects of Variation in Europa's Atmosphere

Europa's plasma interaction is inextricably coupled to its O2 atmosphere by the chemical processes that generate plasma from the atmosphere and the sputtering of magnetospheric plasma against Europa's ice to generate O2. Observations of Europa's atmosphere admit a range of possible densities and spatial distributions (Hall et al., 1998, https://doi.org/10.1086/305604). To better understand this system, we must characterize how different possible configurations of the atmosphere affect the 3D magnetic fields and bulk plasma properties near Europa. To accomplish this, we conducted a parameter study using a multi-fluid magnetohydrodynamic model for Europa's plasma interaction (Harris et al., 2021, https://doi.org/10.1029/2020ja028888). We varied parameters of Europa's atmosphere, as well as the conditions of Jupiter's magnetosphere, over 18 simulations. As the scale height and density of Europa's atmosphere increase, the extent and density of the ionosphere increase as well, generating strong magnetic fields that shield Europa's surface from impinging plasma on the trailing hemisphere. We also calculate the precipitation rate of magnetospheric plasma onto Europa's surface. As the O2 column density increased from (1-2.5) × 1014 cm-2, the precipitation rate decreased sharply then leveled off at 2 × 1024 ions/s for simulations with low magnetospheric plasma density and 6.4 × 1024 ions/s for simulations with high magnetospheric plasma density. These results indicate that the coupling between Europa's plasma populations and its atmosphere leads to feedback that limits increases in the ionosphere density.

The 3D Direct Simulation Monte Carlo Study of Europa’s Gas Plume

Europa has been spotted as having water outgassing activities by space- and ground-based telescopes as well as reanalysis of the Galileo data. We adopt a 3D Direct Simulation Monte Carlo (DSMC) model to investigate the observed plume characteristics of Europa assuming that supersonic expansion originated from the subsurface vent. With a parametric study of the total gas production rate and initial gas bulk velocity, the gas number density, temperature and velocity information of the outgassing plumes from various case studies were derived. Our results show that the plume gases experience acceleration through mutual collisions and adiabatic cooling when exiting from the surface. The central part of the plume with relatively large gas production rates (1029 and 1030 H2O s−1) was found to sustain thermal equilibrium and near continuum condition. Column density maps integrated along two different viewing angles are presented to demonstrate the importance of the projection effect on remote sensing diagnostics. Finally, the density profiles at different altitudes are provided to prepare for observations of Europa’s plumes including upcoming spacecraft missions such as JUICE and Europa Clipper.

Laboratory exploration of mineral precipitates from Europa's subsurface ocean

The precipitation of hydrated phases from a chondrite-like Na-Mg-Ca-SO4-Cl solution is studied using in situ synchrotron X-ray powder diffraction, under rapid- (360 K h-1, T = 250-80 K, t = 3 h) and ultra-slow-freezing (0.3 K day-1, T = 273-245 K, t = 242 days) conditions. The precipitation sequence under slow cooling initially follows the predictions of equilibrium thermodynamics models. However, after ∼50 days at 245 K, the formation of the highly hydrated sulfate phase Na2Mg(SO4)2·16H2O, a relatively recent discovery in the Na2Mg(SO4)2-H2O system, was observed. Rapid freezing, on the other hand, produced an assemblage of multiple phases which formed within a very short timescale (≤4 min, ΔT = 2 K) and, although remaining present throughout, varied in their relative proportions with decreasing temperature. Mirabilite and meridianiite were the major phases, with pentahydrite, epsomite, hydrohalite, gypsum, blödite, konyaite and loweite also observed. Na2Mg(SO4)2·16H2O was again found to be present and increased in proportion relative to other phases as the temperature decreased. The results are discussed in relation to possible implications for life on Europa and application to other icy ocean worlds.

Strategies for Detecting Biological Molecules on Titan

Saturn's moon Titan has all the ingredients needed to produce "life as we know it." When exposed to liquid water, organic molecules analogous to those found on Titan produce a range of biomolecules such as amino acids. Titan thus provides a natural laboratory for studying the products of prebiotic chemistry. In this work, we examine the ideal locales to search for evidence of, or progression toward, life on Titan. We determine that the best sites to identify biological molecules are deposits of impact melt on the floors of large, fresh impact craters, specifically Sinlap, Selk, and Menrva craters. We find that it is not possible to identify biomolecules on Titan through remote sensing, but rather through in situ measurements capable of identifying a wide range of biological molecules. Given the nonuniformity of impact melt exposures on the floor of a weathered impact crater, the ideal lander would be capable of precision targeting. This would allow it to identify the locations of fresh impact melt deposits, and/or sites where the melt deposits have been exposed through erosion or mass wasting. Determining the extent of prebiotic chemistry within these melt deposits would help us to understand how life could originate on a world very different from Earth. Key Words: Titan-Prebiotic chemistry-Solar system exploration-Impact processes-Volcanism. Astrobiology 18, 571-585.

In Situ Field Sequencing and Life Detection in Remote (79°26'N) Canadian High Arctic Permafrost Ice Wedge Microbial Communities

Significant progress is being made in the development of the next generation of low cost life detection instrumentation with much smaller size, mass and energy requirements. Here, we describe in situ life detection and sequencing in the field in soils over laying ice wedges in polygonal permafrost terrain on Axel Heiberg Island, located in the Canadian high Arctic (79°26'N), an analog to the polygonal permafrost terrain observed on Mars. The life detection methods used here include (1) the cryo-iPlate for culturing microorganisms using diffusion of in situ nutrients into semi-solid media (2) a Microbial Activity Microassay (MAM) plate (BIOLOG Ecoplate) for detecting viable extant microorganisms through a colourimetric assay, and (3) the Oxford Nanopore MinION for nucleic acid detection and sequencing of environmental samples and the products of MAM plate and cryo-iPlate. We obtained 39 microbial isolates using the cryo-iPlate, which included several putatively novel strains based on the 16S rRNA gene, including a Pedobacter sp. (96% closest similarity in GenBank) which we partially genome sequenced using the MinION. The MAM plate successfully identified an active community capable of L-serine metabolism, which was used for metagenomic sequencing with the MinION to identify the active and enriched community. A metagenome on environmental ice wedge soil samples was completed, with base calling and uplink/downlink carried out via satellite internet. Validation of MinION sequencing using the Illumina MiSeq platform was consistent with the results obtained with the MinION. The instrumentation and technology utilized here is pre-existing, low cost, low mass, low volume, and offers the prospect of equipping micro-rovers and micro-penetrators with aggressive astrobiological capabilities. Since potentially habitable astrobiology targets have been identified (RSLs on Mars, near subsurface water ice on Mars, the plumes and oceans of Europa and Enceladus), future astrobiology missions will certainly target these areas and there is a need for direct life detection instrumentation.

Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes

Spacecraft observations suggest that the plumes of Saturn's moon Enceladus draw water from a subsurface ocean, but the sustainability of conduits linking ocean and surface is not understood. Observations show eruptions from "tiger stripe" fissures that are sustained (although tidally modulated) throughout each orbit, and since the 2005 discovery of the plumes. Peak plume flux lags peak tidal extension by ∼1 rad, suggestive of resonance. Here, we show that a model of the tiger stripes as tidally flexed slots that puncture the ice shell can simultaneously explain the persistence of the eruptions through the tidal cycle, the phase lag, and the total power output of the tiger stripe terrain, while suggesting that eruptions are maintained over geological timescales. The delay associated with flushing and refilling of O(1)-m-wide slots with ocean water causes erupted flux to lag tidal forcing and helps to buttress slots against closure, while tidally pumped in-slot flow leads to heating and mechanical disruption that staves off slot freezeout. Much narrower and much wider slots cannot be sustained. In the presence of long-lived slots, the 10(6)-y average power output of the tiger stripes is buffered by a feedback between ice melt-back and subsidence to O(10(10)) W, which is similar to observed power output, suggesting long-term stability. Turbulent dissipation makes testable predictions for the final flybys of Enceladus by Cassini Our model shows how open connections to an ocean can be reconciled with, and sustain, long-lived eruptions. Turbulent dissipation in long-lived slots helps maintain the ocean against freezing, maintains access by future Enceladus missions to ocean materials, and is plausibly the major energy source for tiger stripe activity.