Europa's Ice Research Articles

Thermal and Chemical Evolution of Small, Shallow Water Bodies in Europa's Ice Shell

AbstractThe highly modified surface of Europa's ice shell hides a global, salty ocean that may hold conditions favorable for life, and cycles in the ice shell likely impact those conditions by allowing material transfer between the surface and subsurface ocean. The Galileo spacecraft observed a number of young geologic features that indicated Europa's ice shell was recently or presently active, including lenticulae. Lenticulae, a collection of quasi‐elliptically shaped surface disruptions, have been suggested to form above relatively small bodies of water (∼10 km diameter) emplaced in the ice shell as shallow as 1 km below the surface. Here, we use numerical models to quantify the longevity of small bodies of water in Europa's ice shell and explore their chemical evolution to understand how shallow water impacts the composition of the ice shell and regional surface geology over time. We find that lenticulae are more geologically transient than previously described, and that if the reservoirs contain salts, up to meters‐thick layers of salt may precipitate during the solidification process. Our results imply that, if depressions are the first signs of liquid water emplaced in the ice shell, lenticulae may be actively forming at present day, suggesting liquid water may still be present in Europa's shallow ice which could form water plumes and affect its habitability.

The Growth of Europa's Icy Shell: Convection and Crystallization

AbstractThe thickness of Europa's icy shell, which controls its thermal and rheological structure, cannot currently be directly measured. However, it can be estimated through indirect methods, including geodynamic modeling. We simulate the crystallization of the ice shell from a liquid water ocean and its subsequent thermal equilibration in the presence of tidal heat. We evaluate the effects of solid‐state stagnant lid convection on the shell's thermal equilibrium thickness and shell formation timing. The thermal equilibrium thickness of the ice shell ranges from 5 to 30 km, depending on the imposed tidal flexure strain rate. The ice lithospheric thickness remains less than 8 km, independently of imposed tidal flexure strain rate, overall shell thickness, and surface temperature, suggesting the Europan ice lithospheric strength is globally uniform. Thermal convection significantly reduces the thermal equilibration time scale, with the Europan ice shell able to fully crystallize out from a liquid water ocean in less than 2 Myr. Heterogeneous thermal equilibrium thickness values across the satellite suggest that widespread crystallization and melting may occur at the base of the shell. Rapid melting and crystallization timing suggest that the young surface age of the Europan ice shell may be the result of rapid catastrophic whole‐shell melting and recrystallization entirely recycling the surface.

Laboratory predictions for the night-side surface ice glow of Europa

Europa’s surface continuously experiences high fluxes of charged particles due to the presence of Jupiter’s strong magnetic field. These high-energy charged particles, including electrons, interact with the ice- and salt-rich surface, resulting in complex physical and chemical processes. Here, we report that Europa ice analogues emit characteristic spectral signatures in the visible region when exposed to high-energy electron radiation. The strongest emission (ice glow) we observed was centred at ~525 nm. We found that the presence of sodium chloride and carbonate strongly quenched, while epsomite enhanced, the radiation-induced ice glow. These emission characteristics could be used to determine the chemical composition of Europa’s surface during night-time low-altitude fly-bys of spacecraft such as the Europa Clipper. We estimate that the Europa Clipper Wide Angle Camera could record between 500 and 280,000 counts per second through different colour filters, depending on the chemical composition of Europa’s surface. Though we focus here on Europa, our study may be relevant to other bodies exposed to high doses of ionizing radiation, such as Io and Ganymede. With its extreme radiation environment, rich surface geology and compositional diversity, the radiation-induced ice glow on Europa could enable more precise surface characterization and provide unique night-time views. Based on laboratory experiments and predictions, the Europa Clipper mission is expected to detect the surface ices on the night side of Jupiter’s moon Europa glowing in the dark, with an intensity that can be used to determine their composition.

Brine Migration and Impact‐Induced Cryovolcanism on Europa

AbstractDespite evidence for plumes on Jupiter's moon Europa, no surface features have been definitively identified as the source of the plumes to date. Furthermore, it remains unknown whether the activity originates from near‐surface water reservoirs within the ice shell or if it is sourced from the underlying global ocean. Here we investigate brine pocket migration, studied previously in the context of sea ice on Earth, as a process for transporting brine along thermal gradients. We show that the fracture system located in the center of Europa's Manannán crater is consistent with the formation of a subsurface brine reservoir. After the initial impact, residual aqueous melt concentrated via brine pocket migration as the target material cooled. Freezing and overpressurization then resulted in a cryovolcanic eruption. The volume of the emptied reservoir and the critical composition at the end of migration provide further constraints on the average salinity of Europa's ice shell.

Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

AbstractCompositional heterogeneities within Europa's ice shell likely impact the dynamics and habitability of the ice and subsurface ocean, but the total inventory and distribution of impurities within the shell are unknown. In sea ice on Earth, the thermochemical environment at the ice‐ocean interface governs impurity entrainment into the ice. Here, we simulate Europa's ice‐ocean interface and bound the impurity load (1.053–14.72 g/kg [parts per thousand weight percent, or ppt] bulk ice shell salinity) and bulk salinity profile of the ice shell. We derive constitutive equations that predict ice composition as a function of the ice shell thermal gradient and ocean composition. We show that evolving solidification rates of the ocean and hydrologic features within the shell produce compositional variations (ice bulk salinities of 5–50% of the ocean salinity) that can affect the material properties of the ice. As the shell thickens, less salt is entrained at the ice‐ocean interface, which implies Europa's ice shell is compositionally homogeneous below ~1 km. Conversely, the solidification of water filled fractures or lenses introduces substantial compositional variations within the ice shell, creating gradients in mechanical and thermal properties within the ice shell that could help initiate and sustain geological activity. Our results suggest that ocean materials entrained within Europa's ice shell affect the formation of geologic terrain and that these structures could be confirmed by planned spacecraft observations.

A Very Young Age for True Polar Wander on Europa From Related Fracturing

AbstractEn echelon fissures 100–300 km long on Europa are found to be concentric and external to arcuate troughs previously attributed to true polar wander (TPW) of Europa's ice shell, strengthening the case for TPW. Fissures are composed of parallel faults distributed over 10‐to‐20‐km‐wide zones, with deformation focused in a main fissure 1–2 km wide and up to 200 m deep. Fissures crosscut all known terrains, including (apparently) ejecta of bright ray crater Manannan, establishing that fissures and by inference TPW are among the most recent geologic events on Europa. Very late ~70° of TPW shell rotation requires that most observed structures on Europa are not in their original configuration with respect to other stress regimes, requiring complete reanalysis of Europa's strain history. If reorientation happened recently, we predict that any crater distribution asymmetries and shell thickness variations measured by Europa Clipper will be offset from expected equilibrium patterns.

Tidal Walking on Europa's Strike‐Slip Faults—Insight From Numerical Modeling

AbstractTidal walking has been proposed as a mechanism inducing lateral offset on preexisting strike‐slip faults on Europa by tidal forcing. We test this hypothesis numerically by modeling a part of Europa's ice shell with an embedded strike‐slip fault. Our model involves two coupled processes: (i) slip at the fault and deformation of the ice shell on the tidal timescale and (ii) thermal evolution of the ice shell on the timescale of tens of thousands of years. The fault is characterized by the Mohr‐Coulomb criterion allowing to determine self‐consistently the activation depth of the fault. On the tidal timescale, the ice shell is described by the Maxwell viscoelasticity; on the convection timescale, the ice is treated as a non‐Newtonian viscous fluid. We show that tidal walking is capable of producing surface lateral offset of the order of kilometers over 100 thousand years provided that the active part of the fault penetrates the high‐viscosity part of the shell. Such conditions are likely not met for the current amplitude of the tidal forcing and for the estimated ice shell thickness. We show that either larger forcing amplitude (e.g., due to higher eccentricity of the moon) or partial flooding of the fault zone by water from the ocean is required to produce the observed offset. We demonstrate that thermo‐mechanical coupling can significantly enhance the efficiency of tidal walking and we investigate conditions for which the fault's activity can result in observable surface thermal signatures.

Estimating the Magnitude of Cyclic Slip on Strike‐Slip faults on Europa

AbstractStrike‐slip faults on Europa may slide back and forth in response to diurnal tidal stresses, which could generate significant frictional heating near the surface. Previous shear heating models assumed fault sliding rates a priori, without showing how the sliding rate is connected to the resolved stresses acting on the fault. Here, I calculate the cyclic displacement along tidally driven faults. I use a Mohr‐Coulomb failure criterion to determine the frictional failure depth, which varies throughout the tidal cycle. The displacement on the fault is calculated assuming an elastic broken plate model. The magnitude of cyclic displacements along a fault depends upon the coefficient of friction and the shear modulus of the ice shell. If Europa's ice shell is weak, diurnal tidal stress can cause faults on Europa to slide back and forth by ~0.1 to 2 m each cycle. Such large amounts of cyclic slip may be enough to frictionally heat the ice and potentially produce near‐surface melting. If Europa's ice shell has the strength of intact ice, faults become less responsive to cyclic tidal stresses and would only slide 0.01 to 0.2 m per cycle.

Ridged plains on Europa reveal a compressive past

Europa's young surface implies relatively recent resurfacing. Ridged plains, which make up >50% of Europa's surface, have not yet been fully analyzed for a potential formation mechanism. Because ridged plains dominate Europa's surface, this terrain is key to understanding how Europa has resurfaced and how the resurfacing mechanisms may have evolved through time. In this work, we create a new high-resolution topographic model and a two-layer physical analog model to investigate the formation of ridged plains. We find that the ridged plains most closely resemble the compressional physical analog experiments which generate folds. Specifically, the analog experiments with a brittle layer <1/16th the thickness of the underlying ductile-layer best explain the morphological observations of the ridged plains on Europa. Based on the scaling relationships, we find that at the time of the ridged plains formation, the brittle part of the ice shell of Europa's ice shell was ~200 m thick and the total ice-shell thickness, including both the ductile and brittle parts, was >~3000 m. Compared to the predicted current ice shell thickness, this would imply that Europa's ice-shell has thickened through the visible surface history.

The surface temperature of Europa

Previous estimates of the annual mean surface temperature of Jupiter's moon, Europa, neglected the effect of the eccentricity of Jupiter's orbit around the Sun, the effect of the emissivity and heat capacity of Europa's ice, the effect of the eclipse of Europa (i.e., the relative time that Europa is within the shadow of Jupiter), the effect of Jupiter's radiation, and the effect of Europa's internal heating. Other studies concentrated on the diurnal cycle but neglected some of the above factors. In addition, to our knowledge, the seasonal cycle of the surface temperature of Europa was not estimated. Here we systematically estimate the diurnal, seasonal and annual mean surface temperature of Europa, when Europa's obliquity, emissivity, heat capacity, and eclipse, as well as Jupiter's radiation, internal heating, and eccentricity, are all taken into account. For a typical internal heating rate of 0.05Wm−2, the equator, pole, and the global and mean annual mean surface temperatures are 96 K, 46 K, and 90 K, respectively. We found that the temperature at the high latitudes is significantly affected by the internal heating, especially during the winter solstice, suggesting that measurements of high latitude surface temperatures can be used to constrain the internal heating. We also estimate the incoming solar radiation to Enceladus, the moon of Saturn.

A magnetically driven equatorial jet in Europa’s ocean

During recent decades, data from space missions have provided strong evidence of deep liquid oceans underneath a thin outer icy crust on several moons of Jupiter, particularly Europa. But these observations have also raised many unanswered questions regarding the oceanic motions generated under the ice, or the mechanisms leading to the geological features observed on Europa. By means of direct numerical simulations of Europa's interior, we show here that Jupiter's magnetic field generates a retrograde oceanic jet at the equator, which may influence the global dynamics of Europa's ocean and contribute to the formation of some of its surface features by applying a unidirectional torque on Europa's ice shell.

Doppler-based discrimination of radar sounder target scattering properties: A case study of subsurface water geometry in Europa's ice shell

Radar sounding reflectometry is a powerful method of inferring subsurface geophysical properties of planetary bodies. The upcoming Europa Clipper Mission will employ a dual frequency radar sounder to image and characterize Europa's subglacial environment. In terrestrial subglacial radar sounding, characteristic Doppler signatures of radar returns have been used to discriminate between different subglacial features (Peters et al., 2007). We propose that analogous techniques can be used in conjunction with traditional reflectometry methods to discriminate between different proposed models of subglacial and englacial water distribution within Europa's ice shell. We introduce the synthetic aperture radar (SAR) processing considerations that are relevant for such a technique, propose a simple hypothesis test for Doppler-based subglacial target discrimination, and briefly discuss the implications and applicability of such a technique for planetary radar sounding.

Leeb hardness of salty Europa ice analogs exposed to high-energy electrons

Constraint of Europa's surface material properties is critical for future lander missions to Europa, particularly for drilling and sample retrieval. Surface ice on Europa, which is thought to contain water, hydrated acids and salts, and other trace materials at temperatures of ~80 K at the polar regions to ~120 K at the equator, is drenched in radiation from Jupiter's magnetosphere. Because Europa's ice composition and surface structure are not yet fully characterized, technological requirements for mobility, drilling, and caching mechanisms are also not well constrained. To begin to understand the physical properties of ice under these extreme environments, we have initiated a set of laboratory studies to examine the effect of temperature, composition, and radiation on ice hardness. By utilizing a commercially-available Leeb rebound hardness tester, we have determined that ice-containing salts such as NaCl and MgSO4 exhibit increased hardness relative to pure water ices at Europa-relevant temperatures. However, exposure of these ices to 10–25 MeV electron radiation (such as is received by the trailing hemisphere of Europa) led to measurable softening, which indicates that the ‘worst-case scenarios’ (upper limits) for hardness of ice on Europa may be somewhat tempered by surface radiation effects. We found that hydrated MgSO4 (close to 1:1 by mass with H2O) is the hardest of these materials at Europa-relevant temperatures and the Leeb Hardness (HL) ranges from 477 to 527.

Insights into Europa's ocean composition derived from its surface expression

The global ocean believed to exist beneath Europa's thick ice shell is often cited as one of the most likely places in the solar system to find evidence of extraterrestrial habitable environments or even extant life. However, given the technical challenges and the costs associated with outer solar system exploration, direct analysis of the ocean appears unlikely in the foreseeable future. Therefore, constraints on the chemical composition of the ocean will need to be inferred, for example, from analysis of the surface ice, either via remote sensing or landed surface missions. In this paper, we combine our recent body of work studying the chemistry of frozen putative Europa ocean brines with new experiments to develop predictions of the sequence by which the hydrated minerals form when a four-ionic component ocean (Na+, Cl−, Mg2+, and SO42−) freezes as a function of relative ionic concentrations and pH. This in turn provides a means to begin linking observed surface chemistry and the chemical environment of the subsurface ocean as well as insight into endogenic versus exogenic origin of detected species.

Europa's ice tectonics: New insights from physical wax experiments with implications for subduction initiation and global resurfacing processes

Jupiter's Moon Europa has one of the youngest geological surfaces in our solar system with an age of 40–90 Ma, implying an intense history of resurfacing. The surface of Europa indeed shows abundant evidence of tectonic deformation related to extension, strike-slip, and shortening. However, observed features related to shortening are scarce compared with pervasive extensive extensional features such as dilational bands, and do not suffice as the sole mechanism for recycling aging terranes. Recently, evidence for potential plate tectonics, associated with subduction zones, has been discovered on Europa; this could be responsible for recycling most of Europa's surface. However, basic physical parameters needed to initiate subduction on Europa, such as thickness of the brittle layer, deformation rates, and orientation of pre-existing zones of weakness at which subduction could start, are not well understood. Here, we aim to better understand the process and the conditions that could lead to initiation of subduction on Europa through physical experiments, using wax to simulate Europa's two-layered (i.e. convective) icy crust. By deforming the wax, strain on Europa's surface—possibly caused by diurnal tides or its nonsynchronous rotation—is simulated. Our results indicate that subduction could initiate over a broad range of surface thicknesses and deformation rates above a minimum conductive layer thickness, but is strongly dependent on the orientation of the pre-existing zones of weakness. Very thin conductive layer experiments, however, result in a previously undescribed process that we term ductile rolldown, which creates surface features similar to double ridges observed on Europa. Thus, subduction and ductile rolldown represent physically plausible mechanisms that could play a critical role in resurfacing Europa throughout its geologic history. These results could yield significant implications for Europa's thermal history and evolution, habitability, and future spacecraft missions.

Expected Seismicity and the Seismic Noise Environment of Europa

AbstractSeismic data will be a vital geophysical constraint on internal structure of Europa if we land instruments on the surface. Quantifying expected seismic activity on Europa both in terms of large, recognizable signals and ambient background noise is important for understanding dynamics of the moon, as well as interpretation of potential future data. Seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans. We define a range of models of seismic activity in Europa's ice shell by assuming each model follows a Gutenberg‐Richter relationship with varying parameters. A range of cumulative seismic moment release between 1016 and 1018 Nm/yr is defined by scaling tidal dissipation energy to tectonic events on the Earth's moon. Random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self‐consistent models of the interior structure of Europa. Spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records. While the range of seismicity models predicts noise levels that vary by 80 dB, we show that most noise estimates are below the self‐noise floor of high‐frequency geophones but may be recorded by more sensitive instruments. The largest expected signals exceed background noise by ∼50 dB. Noise records may allow for constraints on interior structure through autocorrelation. Models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise.

Porosity and Salt Content Determine if Subduction Can Occur in Europa's Ice Shell

AbstractMotivated by recent evidence for subduction in Europa's ice shell, we explore the geophysical feasibility of this process. Here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct. We also vary the initial salt content in Europa's ice shell and determine the buoyancy of our simulated subducting slab. We find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in Europa's ice shell is actually possible. Generally, we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction. If salt contents are laterally homogenous, and Europa has a reasonable surface porosity of ϕ0 = 0.1, the conductive portion of Europa's shell must have salt contents exceeding ~22% for subduction to occur. However, if salt contents are laterally heterogeneous, with salt contents varying by a few percent, subduction may occur for a surface porosity of ϕ0 = 0.1 and overall salt contents of ~5%. Thus, we argue that under plausible conditions, subduction in Europa's ice shell is possible. Moreover, assuming that subduction is actively occurring or has occurred in Europa's recent past provides important constraints on the structure and composition of the ice shell.

Impact Crater Morphology and the Structure of Europa's Ice Shell

AbstractWe performed numerical simulations of impact crater formation on Europa to infer the thickness and structure of its ice shell. The simulations were performed using iSALE to test both the conductive ice shell over ocean and the conductive lid over warm convective ice scenarios for a variety of conditions. The modeled crater depth‐diameter is strongly dependent on the thermal gradient and temperature of the warm convective ice. Our results indicate that both a fully conductive (thin) shell and a conductive‐convective (thick) shell can reproduce the observed crater depth‐diameter and morphologies. For the conductive ice shell over ocean, the best fit is an approximately 8 km thick conductive ice shell. Depending on the temperature (255–265 K) and therefore strength of warm convective ice, the thickness of the conductive ice lid is estimated at 5–7 km. If central features within the crater, such as pits and domes, form during crater collapse, our simulations are in better agreement with the fully conductive shell (thin shell). If central features form well after the impact, however, our simulations suggest that a conductive‐convective shell (thick shell) is more likely. Although our study does not provide a firm conclusion regarding the thickness of Europa's ice shell, our work indicates that Valhalla class multiring basins on Europa may provide robust constraints on the thickness of Europa's ice shell.

Assessing the potential for measuring Europa's tidal Love number h2 using radar sounder and topographic imager data

The tidal Love number h2 is a key geophysical measurement for the characterization of Europa's interior, especially of its outer ice shell if a subsurface ocean is present. We performed numerical simulations to assess the potential for estimating h2 using altimetric measurements with a combination of radar sounding and stereo imaging data. The measurement principle exploits both delay and Doppler information in the radar surface return in combination with topography from a digital terrain model (DTM). The resulting radar range measurements at cross-over locations can be used in combination with radio science Doppler data for an improved trajectory solution and for estimating the h2 Love number. Our simulation results suggest that the absolute accuracy of h2 from the joint analysis of REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) surface return and EIS (Europa Imaging System) DTM data will be in the range of 0.04–0.17 assuming full radio link coverage. The error is controlled by the SNR budget and DTM quality, both dependent on the surface properties of Europa. We estimate that this would unambiguously confirm (or reject) the global ocean hypothesis and, in combination with a nominal radio-science based measurement of the tidal Love number k2, constrain the thickness of Europa's outer ice shell to up to ±15 km.

Radar attenuation in Europa's ice shell: Obstacles and opportunities for constraining the shell thickness and its thermal structure

AbstractYoung surface and possible recent endogenic activity make Europa one of the most exciting solar system bodies and a primary target for spacecraft exploration. Future Europa missions are expected to carry ice‐penetrating radar instruments designed to investigate its subsurface thermophysical structure. Several authors have addressed the radar sounders' performance at icy moons, often ignoring the complex structure of a realistic ice shell. Here we explore the variation in two‐way radar attenuation for a variety of potential thermal structures of Europa's shell (determined by reference viscosity, activation energy, tidal heating, surface temperature, and shell thickness) as well as for low and high loss temperature‐dependent attenuation model. We found that (i) for all investigated ice shell thicknesses (5–30 km), the radar sounder will penetrate between 15% and 100% of the total thickness, (ii) the maximum penetration depth varies laterally, with deepest penetration possible through cold downwellings, (iii) direct ocean detection might be possible for shells of up to 15 km thick if the signal travels through cold downwelling ice or the shell is conductive, (iv) even if the ice/ocean interface is not directly detected, penetration through most of the shell could constrain the deep shell structure through returns from deep non‐ocean interfaces or the loss of signal itself, and (v) for all plausible ice shells, the two‐way attenuation to the eutectic point is dB which shows a robust potential for longitudinal investigation of the ice shell's shallow thermophysical structure.