Europa's Surface Research Articles

Bacterial spore morphology remains highly recognizable after exposure to simulated Enceladus and Europa surface conditions

The subsurface oceans of Enceladus and Europa are thought to be some of the best candidate environments for finding life beyond Earth. Realistically, the first missions aimed at searching for life on these worlds will likely be restricted to the shallow subsurface. Here, we investigated whether indicators of life, or biosignatures, deposited near the surface could persist long enough to be detected, given that the extremely harsh conditions there would tend to degrade them. We exposed Bacillus subtilis spores to Ocean World surface conditions and used electron microscopy combined with spectroscopic approaches to assess the preservation potential of structural and morphological biosignatures derived from spores. Our results show that spore structure is highly resilient in the face of extreme conditions long after they have been inactivated, suggesting that methods targeting cell morphology would be valuable components in a suite of life detection strategies used in future missions to Ocean Worlds.

Utility-Driven Approach to Onboard Scheduling and Execution for an Autonomous Europa Lander Mission

Jupiter’s icy moon Europa is among the most promising locations to explore for signs of extraterrestrial life in our solar system. However, searching for biosignatures on the surface of Europa presents an unprecedented set of challenges: immense uncertainty, limited energy, and few opportunities for operator feedback. Effectively carrying out a robotic science campaign under these conditions will require a system with a greater degree of autonomy than any planetary exploration mission to date. In particular, onboard scheduling and execution are required for robustness to uncertainty in surface conditions, variation in lander performance, and science discoveries to maximize the quantity and quality of science data returned to Earth during a fixed, limited lander lifetime. Here we represent a proposed Europa Lander surface mission as a utility-driven hierarchical task network and establish through analysis that onboard autonomy using automated mission planning with execution feedback and predictive task models results in mission execution that is more consistently productive compared to traditional static approaches. We design a simulated onboard autonomy framework built by integrating two software components—Multi-mission EXECutive and Europa Lander Autonomy Prototype—to properly simulate the Europa Lander domain and demonstrate empirically that the proposed planning and execution system is capable of commanding a set of realistic surface scenarios as part of a larger Europa Lander surface autonomy software prototype. We expect that an approach to scheduling and execution grounded in decision theory will be an enabling technology for future tightly constrained planetary surface missions.

Numerical Modelling of the Influence of Tidal Stresses on Fracture Patterns on the Surface of Europa

AbstractJupiter’s satellite Europa is covered by an ice shell exhibiting many surface features, including linear structures called lineae, which in this work are treated as fractures. A three-dimensional finite-element simulator is used to numerically model fracture nucleation, growth, and interaction, assuming the ice is an isotropic, linear elastic medium. Tidal stresses are exerted upon the ice through the Jovian orbital relationship. These stresses are calculated using a closed-form model derived from first principles. The fractures grow in response to stress concentration around their tips, and a damage criterion models the weakening of the ice matrix. Three-dimensional non-planar multiple fracture growth is modelled as a function of geometric multi-modal stress intensity factors computed at the fracture tips. Fracturing is evaluated over multi-scale periods, from days to millions of years, thus capturing multiple tidal effects. Fracture behaviour is modelled across the Europan surface in one domain. The patterns are dense clusters of lineae about the stress maxima with diffuse fracturing in outlying regions. Fractures are also modelled in the vicinity of subsurface meltwater lenses, where fractures form parallel to the surface in contrast to the usual perpendicular orientation. The resultant fracture patterns are qualitatively compared against images from NASA’s Galileo mission. This work contributes to the understanding of Europan lineae by illustrating how they behave in a fracture mechanics framework, and suggests interesting results regarding lineae interaction with meltwater lenses. This work is also a proof of concept for this modelling approach, and will serve as the framework for future work.

Sulfur Implantation into Water Ice with Propane: Implications for Organic Chemistry on the Surface of Europa

We performed experiments of implantation of energetic sulfur ions (105 keV) into 2:1 water:propane ices at 80 K and analyzed the resulting refractory organic matter with ultrahigh-resolution mass spectrometry. Our goal was to characterize the organic matter processed in the surface conditions of Europa, where it would receive a heavy flux of energetic particles, including sulfur ions, and determine whether organosulfurs could be formed in these conditions, using the simplest alkane that can exist in solid form on Europa’s surface. We find that the produced organic matter contains a large variety of both aliphatic and aromatic compounds (several thousand unique formulae), including polycyclic aromatic hydrocarbons (PAHs), with masses up to 900 amu. A large number of aromatic hydrocarbons is found along with oxygenated, mostly aliphatic, compounds. Organosulfurs are found in both CHS and CHOS form, demonstrating they can be formed from any organic compound through sulfur implantation. These organosulfurs’ properties (aromaticity, mass) appear similar to the rest of the organic matter, albeit their low quantity does not allow for a thorough comparison. Our results have implications for the type of refractory organic matter that could be observed by the JUICE and Europa Clipper space missions and how the surface of Europa could generate complex organics, including PAHs and organosulfurs, that could then enrich the subsurface ocean. In particular, they indicate that a large diversity of organic matter, including organosulfurs, can be formed from simple precursors in a geologically short time frame under the ion flux that reaches Europa.

Europa's Double Ridges Produced by Ice Wedging

AbstractDouble ridges are sprawling features observed globally across the icy surface of Europa. They consist of two topographic highs flanking a trough. The topographic relief of the ridges is approximately 100 m, and the ridges extend up to hundreds of kilometers in length. The interior structure and dynamics of Europa's ice shell are currently poorly constrained. Therefore, accurate models for the formation of these prominent surface features can be useful for determining how the ice shell operates. We hypothesize that double ridges form as a result of incremental ice wedging. We use both analytical and numerical finite element models to quantify the deformation that occurs as an ice wedge grows incrementally within the ice shell. We show that incremental growth of the ice wedge results in surface deformation that matches the size and shape of typical Europan double ridges, including their topographic relief and surrounding troughs. We find that as the depth of the ice wedge increases, double ridges become broader and shorter. We explore the possibility of local and non‐local sources for the liquid water that freezes to produce the wedge and ultimately argue in favor of local sources of liquid water within the ice shell.

Water dynamics in eutectic solutions of sodium chloride and magnesium sulfate: implications for life in Europa's subsurface ocean and ice shell.

Liquid water is essential for life as we know it and the coupling between water and biomolecular dynamics is crucial for life processes. Jupiter's moon Europa is a good candidate for searching for extraterrestrial life in our outer solar system, mainly because a liquid water salty ocean in contact with a rocky seafloor underlies its ice shell. Little, however, is known about the chemical composition of the subglacial ocean of Europa or the brine pockets within its ice shell and their impacts on water dynamics. Here, we employ 1H, 17O, 23Na and 35Cl NMR spectroscopy, especially NMR spin relaxation and diffusion methods, and investigate the mobility of water molecules and ions in eutectic solutions of magnesium sulfate and sodium chloride, two salts ubiquitously present on the surface of Europa, over a range of temperatures and pressures pertinent to Europa's subglacial ocean. The NMR data demonstrate the more pronounced effect of magnesium sulfate compared with sodium chloride on the mobility of water molecules. Even at its much lower eutectic temperature, the sodium chloride solution retains a relatively large level of water mobility. Our results highlight the higher potential of a sodium chloride-rich than magnesium sulfate-rich Europa's ocean to accommodate life and support life origination within the eutectic melts of Europa's ice shell.

A Complex Region of Europa's Surface With Hints of Recent Activity Revealed by Juno's Stellar Reference Unit

AbstractOn 29 September 2022 Juno's low‐light sensitive Stellar Reference Unit (SRU) captured a high‐resolution (256–340 m/pixel) broadband (450–1,100 nm) visible image of Europa's icy surface during the first close flyby of the Jovian moon since Galileo's last encounter in 2000. Collected at a sub‐spacecraft altitude of 412 km while the surface was illuminated only by Jupiter‐shine (incidence angle: 48–51°), the SRU image reveals a 3 × 104 km2 region between ∼0°–6°N and 43.5°–51°W in remarkable detail at the highest resolution to date. Prior coverage by Galileo under high‐sun conditions at 1 km resolution led to the characterization of the terrain as ridged plains with undifferentiated linea. The SRU image reveals a much richer and more complex picture. Intricate networks of cross‐cutting ridges and lineated bands surround an intriguing 37 km (east‐west) by 67 km (north‐south) chaos feature with a concentric fracture system, depressed matrix margins, and low‐albedo materials potentially associated with brine infiltration. The morphology and local relief of the chaos feature are consistent with formation in the collapse of ice overlying a salt‐rich lens of subsurface water. Low‐albedo deposits, similar to features previously associated with hypothesized cryovolcanic plume activity, flank nearby ridges. The SRU's high‐resolution view of many types of features in a single image allows us to explore their regional context and greatly improve the geologic mapping of this part of Europa's surface. The image reveals several relatively youthful features in a potentially dynamic region, providing baselines for candidate locations that future missions can investigate for present day surface activity.

Comparing the radiolytic oxidation of sulfur and chloride within ice on Europa and Mars

The surface of Europa is heavily irradiated by high-energy particles with the energy ranges of 1–100 MeV for primary electrons and ∼10 keV for secondary electrons. These irradiations may cause oxidation of surface materials, such as chlorides and sulfur-bearing species, through reactions with oxidative radicals formed by dissociation of H2O ice. Similar irradiation-induced oxidation could have occurred in near-surface ice on Mars. However, most of previous experiments simulated ∼10 keV electron irradiation onto ice materials on Europa and Mars, and few studies have investigated the oxidation of chlorides and sulfur by MeV electron irradiation. Here we performed laboratory experiments on MeV electron irradiation, as well as 10 keV electron and UV irradiations, onto mixtures of H2O ice and chlorides, or elemental sulfur, at low temperatures. Our experimental results indicate selective oxidation of sulfur against chlorides, with negligible formation of oxychlorides on irradiation of H2O ice and chloride mixtures, regardless of experimental conditions (temperature, electron energy, or presence of oxidants in ice). Sulfate is effectively formed by the electron irradiation. This selective oxidation may be caused by the different stabilities of atoms within irradiated H2O ice. If sulfate on Europa's surface is transported into the subsurface ocean, this could provide energy sources for possible chemoautotrophic life in the ocean. On Europa, sulfur would be one of key elements in redox chemistry on the surface and possibly in the subsurface ocean.

Potential signs of life on Europa

This paper discusses the possibility of life on Europa, the moon of Jupiter by analyzing the biological components, energy source, and ocean environment on Europa. The biological components, especially amino acids, have great possibilities of forming or have already existed on Europa since its compositive elements are present and the environment on Europa is very similar to the primitive Earth. Energy sources are expected to come from the hydrothermal vents at the ocean bottom, which are formed by tidal forces and tidal heating. The hydrothermal vents are rich in chemicals and elements and have the possibility of forming lives in the future. The ocean environment is thought to be rich in oxygen, which comes from the planet Jupiter, it is first reflected on the surface of Europa, but later on, transported to the ocean through the ice layer and a lake in between. Furthermore, the freezing point of the ocean might be affected by the fact that it contains salt, but not by the fact that it has very low air pressure. To conclude, there is a great possibility that lives exist on Europa, but it cannot be 100% positive since no specific experiments have been done throughout the research, and there are so many uncertainties in space that have not been discovered yet.

Energetic Electrons Near Europa From Juno JEDI Data

AbstractOptical remote sensing observations have suggested that the top layer of Europa's icy surface is heavily affected by external weathering agents. To model and understand these effects, it is necessary to characterize the environment as fully as possible. In this paper, we focus on one agent in the environment (energetic electrons). We show Juno electron data from its 2022 Europa flyby and other time periods. While the Juno sensor used here (Jupiter Energetic Particle Detector Instrument) was not designed to obtain high quality electron data in an intense radiation environment, it is possible to extract information such as how Europa blocks energetic particles from accessing some of the surrounding space. The decrease in charged particle flux in Europa's wake provides an upper limit on the precipitation fluxes of the same particles. We also report that electron pitch angle distributions near Europa for the single energy channel considered here are time variable and not isotropic.

Detectability of Local Water Reservoirs in Europa's Surface Layer Under Consideration of Coupled Induction

AbstractThe icy moon Europa is a primary target for the study of ocean worlds. Its subsurface ocean is expected to be subject to asymmetries on global scales (tidal deformation) and local scales (chaos regions, fractures). Here, we investigate the possibility to magnetic sound local asymmetries by calculating the induced magnetic fields generated by a radially symmetric ocean and a small, spherical water reservoir between the ocean and Europa's surface. The consideration of two conductive bodies introduces non‐linear magnetic field coupling between them. We construct an analytical model to describe the coupling between two conductive bodies and calculate the induced fields within the parameter space of possible conductivity values and icy crust thicknesses. Given the plasma magnetic field perturbations, we find that a reservoir cannot be detected during a flyby at 25 km altitude using electromagnetic induction. Potential detection of liquid water reservoirs can be achieved by deploying magnetometers on Europa's surface, where one magnetometer is placed directly on the target region of interest and a second one in the nearby vicinity as reference to distinguish from global asymmetries. With this method, the smallest reservoir that can be detected has a radius of 8 km and a conductivity of 30 S/m. Larger reservoirs are resolvable at lower conductivities, with a 20 km reservoir requiring a conductivity of approximately 5 S/m.

The distribution of CO2 on Europa indicates an internal source of carbon.

Jupiter's moon Europa has a subsurface ocean, the chemistry of which is largely unknown. Carbon dioxide (CO2) has previously been detected on the surface of Europa, but it was not possible to determine whether it originated from subsurface ocean chemistry, was delivered by impacts, or was produced on the surface by radiation processing of impact-delivered material. We mapped the distribution of CO2 on Europa using observations obtained with the James Webb Space Telescope (JWST). We found a concentration of CO2 within Tara Regio, a recently resurfaced terrain. This indicates that the CO2 is derived from an internal carbon source. We propose that the CO2 formed in the internal ocean, although we cannot rule out formation on the surface through radiolytic conversion of ocean-derived organics or carbonates.

Endogenous CO2 ice mixture on the surface of Europa and no detection of plume activity.

Jupiter's moon Europa has a subsurface ocean beneath an icy crust. Conditions within the ocean are unknown, and it is unclear whether it is connected to the surface. We observed Europa with the James Webb Space Telescope (JWST) to search for active release of material by probing its surface and atmosphere. A search for plumes yielded no detection of water, carbon monoxide, methanol, ethane, or methane fluorescence emissions. Four spectral features of carbon dioxide (CO2) ice were detected; their spectral shapes and distribution across Europa's surface indicate that the CO2 is mixed with other compounds and concentrated in Tara Regio. The 13CO2 absorption is consistent with an isotopic ratio of 12C/13C = 83 ± 19. We interpret these observations as indicating that carbon is sourced from within Europa.

Vitreous Magnesium Sulfate Hydrate as a Potential Mechanism for Preservation of Microbial Viability on Europa

Europa's subsurface ocean is postulated to contain appreciable amounts of Mg2+ and ions, among other species. Recent laboratory experiments have shown that when solutions containing these species freeze to Europa-relevant temperatures, they can form vitreous MgSO4 hydrate, which can remain stable at these temperatures. Since vitreous phases can protect cells from physical damage that can occur during crystallization, their presence on Europa could potentially preserve entrained microorganisms from the ocean below. However, to date, it remains unclear whether such materials actually impact microbial survival. In this work, experiments were performed in which the motile nonspore-forming Antarctic isolate Pseudoalteromonas haloplanktis in solutions of 0.1 M MgSO4 were frozen to Europa surface temperatures (100 K) under conditions that resulted in the formation of either vitreous or crystalline salt hydrates. We found that cells survived in both cases, exhibiting a 3-log reduction in viable cells in the crystalline salt hydrate case while only a 1.5-log reduction in the vitreous salt hydrate case. Scanning electron microscopy accordingly showed much higher degrees of membrane lysis and cellular damage in the crystalline salt hydrate than the vitreous case. Our results demonstrate the ability of a terrestrial oceanic microorganism to survive in MgSO4 solutions frozen to Europa surface temperatures, with enhanced viability in vitreous salt-hydrate-producing conditions versus crystalline. These findings suggest that future missions should target vitrified salt-rich environments for life detection due to this potential for preserving viable microorganisms that may be present and trapped in ocean world ices.

Tectonic analysis of a newly identified putative cryovolcanic field on Europa

More and more attention is devoted to the icy moons of the Solar System, including Europa, the second Galilean satellite of Jupiter, since the discovery of potential liquid water and the possibility of extra-terrestriallife harbored in its subsurface ocean below its icy crust. Along with the renaissanceof the study of icy satellites, the ongoing missions, such as Europa Clipper and JUICE (JUpiter ICy moons Explorer), are also part of such rejuvenation of icy satellite research.One of the leading research topics connected to Europa is understanding its surface renewal, including the interaction between the subsurface ocean and the icy crust. One of the longest-lasting and still unsettled debates related to Europa is about the nature of the potentially active cryovolcanism, which may play an essential role in the interaction between the surface of the Jovian moon and the underlying subsurface ocean. Our study focuses on the geological-geomorphological characterization of a newly identified putative cryovolcanic field found on the surface of Europa. Various volcanic structures, possibly in multiple stages of maturity, were identified. The executed geological analysis in the surroundings of the volcanoes suggests strong local influence during the formation of cryovolcanic cones working together with the overall global-scale stress fields appearing in the ice crust of Europa.

Surface‐Plasma Interactions at Europa in Draped Magnetospheric Fields: The Contribution of Energetic Electrons to Energy Deposition and Sputtering

AbstractWe calculate the time‐varying spatial distribution of energetic magnetospheric electron influx onto Europa's surface by combining a hybrid model of the moon's draped electromagnetic environment with a relativistic particle tracer. We generate maps of the energetic electron influx patterns at four distinct locations of Europa relative to the center of the Jovian magnetospheric current sheet. For a full synodic rotation of Jupiter, these results are applied to constrain the averaged number and energy influx patterns as well as the O2 sputtering rates associated with energetic electron precipitation. We also determine the relative contributions of magnetospheric ions and electrons to surface erosion and exospheric genesis at Europa. Our major results are: (a) Except for a small region near Europa's downstream apex, the moon's entire surface is exposed to heavy irradiation by magnetospheric electrons. (b) The spatial distribution of energetic electron influx onto Europa's surface is only slightly modified by field line draping and the induced magnetic field from the moon's subsurface ocean. (c) The contributions of magnetospheric electron and ion impacts to energy deposition onto Europa's surface are of the same order of magnitude. (d) Within uncertainties, impinging magnetospheric electrons and ions make similar contributions to O2 sputtering from Europa's surface. (e) The spatial distribution of electron energy influx and the observed concentrations of sulfuric acid (H2SO4) are only weakly correlated, suggesting that energy deposition by magnetospheric electron impacts is not a necessary agent for H2SO4 production within Europa's surface.

Ménec Fossae on Europa: A Strike‐Slip Tectonics Origin Above a Possible Shallow Water Reservoir

AbstractFaults and fractures may emplace fresh material onto Europa's surface, originating from shallow reservoirs within the ice shell or directly from the subsurface ocean. Ménec Fossae is a region of particular interest as it displays the interaction of several geological features, including bands, double ridges, chaotic terrains, and fossae, within a relatively small area. These features might affect the emplacement of buried material and subsequent exposure of fresh volatiles, prime targets for the upcoming JUICE and Europa Clipper missions in order to assess Europa's astrobiological potential. Previous studies have already revealed that a deep central trough is present at Ménec Fossae, flanked by several subparallel minor troughs and by a few asymmetrical scarps with lobate planforms. The presence of such features has motivated this study, given its potential to provide clear indications on the tectonic regime involved. Through detailed geomorphological‐structural mapping using Galileo Solid State Imager data and terrain analysis on Digital Terrain Models, we could develop a novel hypothesis on the formation mechanisms that might have been involved in the study area. We propose that Ménec Fossae has been shaped by transtensional (strike‐slip with an extensional component) tectonic activity, as indicated by the orientation and relationship of the tectonic features present. Likely, such transtensional tectonism occurred above or associated with shallow subsurface water, consistent with the overall morphology and topography of the study area and the presence of chaotic terrains and double ridges. These results strengthen the case for widely distributed shallow water reservoirs within Europa's ice shell.

On the identification of hyperhydrated sodium chloride hydrates, stable at icy moon conditions

Sodium chloride is expected to be found on many of the surfaces of icy moons like Europa and Ganymede. However, spectral identification remains elusive as the known NaCl-bearing phases cannot match current observations, which require higher number of water of hydration. Working at relevant conditions for icy worlds, we report the characterization of three "hyperhydrated" sodium chloride (SC) hydrates, and refined two crystal structures [2NaCl·17H2O (SC8.5); NaCl·13H2O (SC13)]. We found that the dissociation of Na+ and Cl- ions within these crystal lattices allows for the high incorporation of water molecules and thus explain their hyperhydration. This finding suggests that a great diversity of hyperhydrated crystalline phases of common salts might be found at similar conditions. Thermodynamic constraints indicate that SC8.5 is stable at room pressure below 235 K, and it could be the most abundant NaCl hydrate on icy moon surfaces like Europa, Titan, Ganymede, Callisto, Enceladus, or Ceres. The finding of these hyperhydrated structures represents a major update to the H2O-NaCl phase diagram. These hyperhydrated structures provide an explanation for the mismatch between the remote observations of the surface of Europa and Ganymede and previously available data on NaCl solids. It also underlines the urgent need for mineralogical exploration and spectral data on hyperhydrates at relevant conditions to help future icy world exploration by space missions.

Sulfur chemistry on the surface ice of Europa

Because Europa is embedded in Jupiter's magnetosphere, Europa's crust, which is mainly made of water ice, is continuously bombarded by energetic ions and electrons. The bombardment of the energetic particles not only sputters and dissociates water molecules on Europa's surface but also introduces impurities to its surface ice. Especially, sulfur ions deliver sulfur to the ice, which induces further chemical reactions in the ice and leads to the formation of sulfur-containing species. Observational and experimental studies have shown that sulfuric acid is formed from the sulfur ion implantation on ice. However, the sulfur chemistry on the surface ice of Europa is still poorly understood. In this study, we use a chemical-transport model to simulate chemical processes occurring in the surface ice of Europa during irradiation by ions and electrons. We show that sulfuric acid is the dominant species on the surface ice of Europa, whose mixing ratio may be as high as several percent in the first 100 μm of the ice. We also find that sulfur chemistry may play an important role in the formation of O2 on the surface ice of Europa. The results of our model can be used to estimate the transport rate of the key species (e.g., O2, H2O2, H2SO4) from Europa's surface ice to its subsurface ocean.

Crystallization Kinetics of Vitreous Magnesium Sulfate Hydrate and Implications for Europa’s Surface

Brines consistent with that expected for Europa’s global subsurface ocean have been shown to form vitreous salt hydrates when frozen. We report experiments examining the crystallization kinetics of vitreous MgSO4 hydrate in order to better understand the stability of such materials on the surface of Europa. Vitreous MgSO4 hydrates formed from a 2 M parent solution were found to crystallize into MgSO4·11H2O (meridianiite) upon annealing at 195–225 K. Raman spectroscopy was used to monitor the crystallization and reaction rates were determined from the growth of the crystalline fraction as a function of time. Arrhenius analysis yielded an activation energy of 60 ± 9 kJ mol−1 for the vitreous to crystalline transition, implying that such transformation does not occur spontaneously at Europa’s surface temperatures. If emplacement processes favor the formation of vitreous salt hydrates, they are likely to still be stable and could be an important non-ice component on Europa at present day.