Europa's Ice Shell Research Articles

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.

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.

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.

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.

Formation of lenticulae on Europa by saucer-shaped sills

Europa's surface contains numerous quasi-elliptical features called pits, domes, spots and small chaos. We propose that these features, collectively referred to as lenticulae, are the surface expression of saucer-shaped sills of liquid water in Europa's ice shell. In particular, the inclined sheets of water that surround a horizontal inner sill limit the lateral extent of intrusion, setting the lateral dimension of lenticulae. Furthermore, the inclined sheets disrupt the ice above the intrusion allowing the inner sill to thicken to produce the observed relief of lenticulae and to fracture the crust to form small chaos. Scaling relationships between sill depth and lateral extent imply that the hypothesized intrusions are, or were, 1–5 km below the surface. Liquid water is predicted to exist presently under pits and for a finite time under chaos and domes.

Material transport across Europa's ice shell

AbstractJupiter's moon Europa exhibits a deformed icy surface with salt deposits concentrated along the varied geological features. The topographic alignment of salt deposits has been speculated to indicate an endogenic sourcing of the material. Two‐way transport of salts from a liquid‐water ocean beneath the ice shell to the surface, and vice versa, has been speculated. We present dynamical models that demonstrate the incorporation of newly frozen ice into convective plumes within the ice shell, caused by convection within the ice shell that drives dynamic topography along the ice‐ocean boundary. The new ice that forms at the freezing front can be transported by the rising ice plumes toward the surface until it is blocked by a high‐viscosity lid at the surface. Weakening of the lid by tidal or tectonic forces could then lead to the surface detection of ocean trace chemistry captured in the newly formed ice.

Evidence for subduction in the ice shell of Europa

Despite widespread evidence for extension, there have been few signs of contraction on the icy surface of Jupiter’s Europa. Evidence for a subduction-like convergent boundary suggests that Europa may have active plate tectonics.

Ice melting and downward transport of meltwater by two‐phase flow in Europa's ice shell

With its young surface, very few impact craters, and the abundance of tectonic and cryovolcanic features, Europa has likely been subjected to relatively recent endogenic activity. Morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface, which may result from enhanced tidal heating. A major issue concerns the thermal/gravitational stability of these water reservoirs. Here we investigate the conditions under which water can be generated and transported through Europa's ice shell. We address particularly the downward two‐phase flow by solving the equations for a two‐phase mixture of water ice and liquid water in one‐dimensional geometry. In the case of purely temperate ice, we show that water is transported downward very efficiently in the form of successive porosity waves. The time needed to transport the water from the subsurface region to the underlying ocean varies between ∼1 and 100 kyr, depending mostly on the ice permeability. We further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell (within less than a few hundred kiloyears). Our calculations indicate that liquid water will be largely absent in the near subsurface, with the possible exception of cold conductive regions subjected to strong tidal friction. Recently active double ridges subjected to large tidally driven strike‐slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on Europa.

Domes, pits, and small chaos on Europa produced by water sills

AbstractPits, domes, and small chaos on Europa's surface are quasi‐circular features a few to a few tens of kilometers in diameter. We examine if injection of water sills into Europa's ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features. We study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill. At shallow depths, water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill. In contrast, a deep sill bends the underlying elastic layer and its weight does not affect its spreading. In that case, the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially. After emplacement, cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer. As a result, preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill (small chaos). Disruption of the surface also allows for partial isostatic compensation of water weight, leading to a topographic depression at the surface (pit), of the order of ~102 m. Complete water solidification finally causes expansion of the initial sill volume and results in an uplifted topography (dome) of ~102m.

Constraints on Europa's surface properties from primary and secondary crater morphology

We investigate the near‐surface properties of Europa's ice shell by examining small crater morphology; we use both primary and secondary craters for our analysis. For primary craters, the simple‐to‐complex transition provides an estimate of 0.19–0.36 MPa for the cratering effective strength. Through the aggregate statistics of over 100 profiles, we find that secondary craters on Europa tend toward smaller depth‐to‐diameter (d/D) ratios than primary craters, consistent with observations on other cratered surfaces (the Moon and Mars). In addition, we find that adjacent secondaries tend to be more shallow than distant secondaries, also consistent with trends seen on the Moon and Mars. The presence of numerous and far‐flung secondaries requires a solid, competent surface for spallation that is inconsistent with weak ice. Although the effective strength evaluated by crater scaling laws does not have a direct quantitative relationship to lab‐based measurements of shear or tensile strength, comparison to other observationally derived effective strengths and geologic materials suggests that the mechanical properties of Europa's surface ice should be consistent with terrestrial measurements of cold ice.

Modeling stresses on satellites due to nonsynchronous rotation and orbital eccentricity using gravitational potential theory

The tidal stress at the surface of a satellite is derived from the gravitational potential of the satellite's parent planet, assuming that the satellite is fully differentiated into a silicate core, a global subsurface ocean, and a decoupled, viscoelastic lithospheric shell. We consider two types of time variability for the tidal force acting on the shell: one caused by the satellite's eccentric orbit within the planet's gravitational field (diurnal tides), and one due to nonsynchronous rotation (NSR) of the shell relative to the satellite's core, which is presumed to be tidally locked. In calculating surface stresses, this method allows the Love numbers h and ℓ, describing the satellite's tidal response, to be specified independently; it allows the use of frequency-dependent viscoelastic rheologies (e.g. a Maxwell solid); and its mathematical form is amenable to the inclusion of stresses due to individual tides. The lithosphere can respond to NSR forcing either viscously or elastically depending on the value of the parameter Δ ≡ μ η ω , where μ and η are the shear modulus and viscosity of the shell respectively, and ω is the NSR forcing frequency. Δ is proportional to the ratio of the forcing period to the viscous relaxation time. When Δ ≫ 1 the response is nearly fluid; when Δ ≪ 1 it is nearly elastic. In the elastic case, tensile stresses due to NSR on Europa can be as large as ∼ 3.3 MPa , which dominate the ∼ 50 kPa stresses predicted to result from Europa's diurnal tides. The faster the viscous relaxation the smaller the NSR stresses, such that diurnal stresses dominate when Δ ≳ 100 . Given the uncertainty in current estimates of the NSR period and of the viscosity of Europa's ice shell, it is unclear which tide should be dominant. For Europa, tidal stresses are relatively insensitive both to the rheological structure beneath the ice layer and to the thickness of the icy shell. The phase shift between the tidal potential and the resulting stresses increases with Δ. This shift can displace the NSR stresses longitudinally by as much as 45° in the direction opposite of the satellite's rotation.

Fracture penetration in planetary ice shells

The proposed past eruption of liquid water on Europa and ongoing eruption of water vapor and ice on Enceladus have led to discussion about the feasibility of cracking a planetary ice shell. We use a boundary element method to model crack penetration in an ice shell subjected to tension and hydrostatic compression. We consider the presence of a region at the base of the ice shell in which the far-field extensional stresses vanish due to viscoelastic relaxation, impeding the penetration of fractures towards a subsurface ocean. The maximum extent of fracture penetration can be limited by hydrostatic pressure or by the presence of the unstressed basal layer, depending on its thickness. Our results indicate that Europa's ice shell is likely to be cracked under 1–3 MPa tension only if it is ⩽2.5 km thick. Enceladus' ice shell may be completely cracked if it is capable of supporting ∼1–3 MPa tension and is less than 25 km thick.

Reply to “Comment on ‘Mechanics of tidally driven fractures in Europa's ice shell’ ”

Both Lee et al. and Qin et al. consider propagation of a surface initiated tensile crack oriented vertically in an ice sheet of finite thickness with gravitational overburden. Lee et al. assume the crack walls are always in contact and bear normal stress from overburden. In this closed crack scenario, overburden stress increases linearly with depth just as in an ice half-space. Crack walls cannot sustain tension, so the effect of far field tension is concentrated in the material below the crack walls. This leads to the deep crack penetrations of Lee et al. Qin et al., however, assume an open crack scenario. They inappropriately apply normal stress to open crack walls which are exposed to vacuum and so physically cannot sustain a normal stress [Timoshenko, S.P., Goodier, J.N., 1970. Theory of Elasticity, third ed. McGraw–Hill, New York, p. 191]. Since this inappropriate normal stress is horizontally oriented it has the effect of artificially concentrating compressive stress in the material below the open crack. The severely limited crack propagation depths of Qin et al. result from this inappropriate boundary condition on an open crack wall.