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Multistage evolution and variable motion history of Agenor Linea, Europa
... Schmidt et al., 2022), as well as on ocean worlds such as Ganymede (e.g., Rossi et al., 2018), Enceladus (e.g., Rossi et al., 2020), and Titan (e.g., Burkhard et al., 2022;Matteoni et al., 2020). Europa, too, exhibits several examples of strike-slip structures, such as along Agenor Linea in the southern trailing hemisphere and Astypalaea Linea in the south polar region (Hoyer et al., 2014;Kattenhorn, 2004;Prockter et al., 2000;Tufts et al., 1999Tufts et al., , 2000. The identification of strike-slip faults on Europa has also revealed that certain fractures in the ice shell are not primarily caused by global tidal stresses but rather by lateral motions along strike-slip faults. ...
... Agenor Linea, a prominent bright band located in Europa's southern hemisphere (Figure 1), extends for approximately 1,500 km. This distinctive feature has experienced significant offset, with displacements reaching up to 20 km, and its current configuration may have been influenced by a complex history involving both strike-slip and dilational motion (Hoyer et al., 2014;Prockter et al., 2000). At its eastern tip, Agenor Linea displays a distinctive feature of at least six pronounced, curved trough-like tailcracks extending toward the southern direction, indicating right-lateral strike-slip motion (Kattenhorn, 2004;Kattenhorn & Marshall, 2006;Prockter et al., 2000). ...
... Therefore, bands are thought to represent a phenomenon analogous to Earth's mid-ocean ridge spreading centers (Prockter et al., 2002). Most models for bands' evolution include different stages of various tectonic regimes in a varying stress field (e.g., Hoyer et al., 2014), thus implying possible reactivation of structures (Kattenhorn & Hurford, 2009;Wesley Patterson & Head, 2010), that could also represent cases of episodic block plate tectonism . ...
Chaos terrains are geologically young and extensively disrupted surface features of Europa, thought to be an expression of the subsurface ocean interacting with the surface. The most prominent examples of this terrain on Europa are Conamara Chaos, and Thera and Thrace Maculae, all prime targets for the upcoming JUICE and Europa Clipper missions to assess the astrobiological potential of Europa. Of the three features, Thrace Macula is currently the least studied and understood. It intersects both Agenor Linea to the north and Libya Linea to the south, two important regional‐scale bands whose interaction with Thrace is yet to be fully unraveled, especially in terms of their relative ages of emplacement and activity. Using Galileo Solid State Imager data and Digital Terrain Models, we conducted detailed structural mapping and terrain analysis to develop a novel hypothesis on the mechanisms involved in the study area. We find that Thrace Macula is bordered along most sides by preexisting strike‐slip faults that have constrained its emplacement and areal distribution. We determine a sequence of events in the area involving the formation of Agenor Linea, followed by that of Libya Linea first and Thrace Macula later, and ultimately by strike‐slip tectonic activity likely driven by Libya Linea, that displaced a portion of Thrace Macula. Therefore, Thrace's subsurface material, uprising along faults postdating its formation, represents the freshest possible that could be sampled by future spacecraft in this region, a major consideration for the upcoming Europa Clipper mission.
... While global tidal stress models have conventionally been used to explain all fractures on Europa, not all tectonic feature types and orientations are consistent with these stress models, such as: (1) tensile fractures located in predicted compressive stress regimes (Figueredo and Greeley, 2000;Kattenhorn, 2002); (2) observed strike-slip sense counter to the predicted strike-slip sense (Hoppa et al., 1999a); (3) the orientations of geologically young features such as Agenor Linea (Prockter et al., 2000;Hoyer et al., 2014); and (4) the development of subsumption bands in Northern Falga Regio (Kattenhorn and Prockter, 2014). ...
... (Prockter et al., 2000). A tectonic reconstruction of Agenor Linea (Hoyer et al., 2014) identified four distinct deformation stages with progressively more clockwise successive extension directions. Agenor may have consecutively experienced periods of left-lateral oblique dilation, north-south oriented pure opening, right-lateral oblique dilation, and right-lateral strike-slip motions, with a cumulative dilation of ~30 kilometers. ...
... On the eastern tip of Agenor, tailcracks provide evidence of rightlateral strike-slip motion along Agenor (Prockter et al., 2000). Visible with higher resolution images (white rectangle, resolution is ~200 m/pix), different albedos represent different periods of dilation and right-lateral strike-slip motion (Hoyer et al., 2014). North of Agenor Linea, an apparent ~E-W linear boundary delineates regions of differing deformation style (white dashed line). ...
Europa, Jupiter's fourth-largest moon, has an anomalously young surface age (~40–90 million years old), and an extensively fractured surface. Conventional models for tectonic features on Europa have invoked global-scale tidal forcings (e.g., diurnal forcing, obliquity, nonsynchronous rotation, and true polar wander) as the mechanisms responsible for fracturing the icy shell. In an attempt to examine the complex history of deformation on Europa in the context of global tidal stress models, I examined a multitude of tectonic feature types, orientations, and ages across a broad region of Europa's anti-jovian hemisphere encompassing Argadnel Regio, a complex region of deformation consisting of an intertwining network of low albedo bands and ridges, and Agenor Linea, a ~1,500 km long band-like strike-slip fault. After mapping geologic feature types, orientations, and ages and comparing my observations with global tidal stress models, I found that Europa's oldest fractures (two sets of intersecting ridges oriented NE-SW and NW-SE) most closely align with the predicted stresses from two separate episodes of ~45° and ~15° of true polar wander. Dilational bands located to the north of Argadnel Regio that dilated pre-existing cycloids in a north-south extensional direction align more closely with a global stress field that would have been produced by a more recent stage of nonsynchronous rotation. While these models accounted for fracture sets in Europa's oldest terrain and younger dilated cycloids, many young tectonic features are not consistent with the predictions of true polar wander or nonsynchronous rotation stress fields, such as: 1) ~700–km-long, right-stepping én echelon bands with sigmoidal geometries within Argadnel Regio that are consistent with broadly-distributed, left-lateral shearing, 2) left-stepping én echelon bands younger than the ~700 km long sigmoidal bands that are consistent with right-lateral shearing, 3) clockwise rotations of circular rafts of material within Argadnel Regio also consistent with right-lateral shearing, and 4) bands (~5 km wide, ~10 km long) oriented ~045° and located ~100 km south of Agenor Linea that are consistent with left-lateral shearing of Agenor Linea and which pre-dates more recent right-lateral shearing of Agenor Linea. While these observations do not align with stresses from global tidal forcing, previous studies based on numerical and physical models have proposed plate tectonics as a potential mechanism responsible for fracturing and resurfacing Europa's icy crust. However, observational evidence (e.g., geologic evidence of translation or rotation across plate boundaries or broadly distributed lateral shearing within plates) is necessary to confirm the existence of plate tectonics on Europa. After extensively mapping bands, ridges, and other tectonic features across Argadnel Regio and Agenor Linea, I assert that young tectonic features align better with broadly distributed lateral shearing, a necessary component of plate tectonics, than with global tidal stress models. The misalignment of tectonic features with conventional global-tidal stress models, in addition to the presence of tectonic features resembling artifacts of broad-scale lateral shearing, suggests that deformation on Europa may need to be re-evaluated under a plate-tectonic paradigm in combination with global tidal stress models, occurring as contemporaneous processes. Plate tectonics would not only help explain complex deformation on Europa but would also provide a mechanism for recycling Europa's icy crust and maintaining the moon's young surface age.
... These features strongly resemble "tail cracks," which are found at the terminus of a strike-slip fault, where local extension caused by motion along the fault is accommodated by a set of fractures curving toward the side of the material that is under tension. Interpretation of the tail cracks at Agenor has led to the interpretation that the bright band slipped in a right-lateral direction (Prockter et al. 2000;Kattenhorn 2004;Hoyer et al. 2014). ...
On 2022 September 29 the Juno spacecraft passed Europa at 355 km, the first close pass since the Galileo flyby in 2000. Juno’s visible-light imager, JunoCam, collected four images, enabling cartographic, topographic, and surface geology analysis. The topography along the terminator is consistent with previously reported features that may indicate true polar wander. A bright band was discovered, and indicates global symmetry in the stress field that forms bright bands on Europa. The named feature Gwern is shown not to be an impact crater. Surface change detection shows no changes in 22 yr, although this is a difficult task considering differences between the JunoCam and Galileo imagers and very different viewing geometries. No active eruptions were detected.
... Schmidt et al., 2022) and other ocean worlds, Ganymede (e.g., Rossi et al., 2018), Enceladus (e.g., Rossi et al., 2020), and Titan (e.g., Burkhard et al., 2022;Matteoni et al., 2020). Several other examples exist on Europa as well, such as along Agenor Linea in the southern trailing hemisphere and Astypalaea Linea in the south polar region (Hoyer et al., 2014;Tufts et al., 1999Tufts et al., , 2000. In transtensional tectonic regimes, minor compressional features are common and expected (Fossen et al., 1994;Petit, 1987). ...
Faults 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.
... These intersecting bands appear to have formed from two separate episodes of diffuse, broad scale lateral shearing that initially formed a set of NW-SE oriented bands via right-lateral shearing, and later formed a set of sigmoidal bands oriented NE-SW via left-lateral shearing. Two episodes of opposite-sense shearing may also be observed in Agenor Linea (Hoyer et al., 2014), just to the north of the Libya study area. As the more recent episode of broad scale left-lateral shearing continued, the plates have rotated counter-clockwise, similar to plate motions within the southern portion of the Castalia Macula area and observed by Melton (2018) and Detelich and Kattenhorn (2022). ...
A nearly pole‐to‐pole survey near 140°E longitude on Europa revealed many areas that exhibit past lateral surface motions, and these areas were examined to determine whether the motions can be described by systems of rigid plates moving across Europa's surface. Three areas showing plate‐like behavior were examined in detail to determine the sequence of events that deformed the surface. All three areas were reconstructed to reveal the original pre‐plate motion surfaces by performing multi‐stage rotations of plates in spherical coordinates. Several motions observed along single plate boundaries were also noted in previous works, but this work links together isolated observations of lateral offsets into integrated systems of moving plates. Not all of the surveyed surface could be described by systems of rigid plates. There is evidence that the plate motions did not all happen at the same time, and that they are not happening today. We conclude that plate tectonic‐like behavior on Europa occurs episodically, in limited regions, with less than 100 km of lateral motion accommodated along any particular boundary before plate motions cease. Europa may represent a world perched on the theoretical boundary between stagnant and mobile lid convective behavior, or it may represent an additional example of the wide variations in possible planetary convective regimes. Differences in observed strike‐slip sense and plate rotation directions between the northern and southern hemispheres raise the question of whether tidal forces may influence plate motions.
... These intersecting bands appear to have formed from two separate episodes of diffuse, broad scale lateral shearing that initially formed a set of NW-SE oriented bands via right-lateral shearing, and later formed a set of sigmoidal bands oriented NE-SW via left-lateral shearing. Two episodes of opposite-sense shearing may also be observed in Agenor Linea (Hoyer et al., 2014), just to the north of the Libya study area. As the more recent episode of broad scale left-lateral shearing continued, the plates have rotated counter-clockwise, similar to plate motions within the southern portion of the Castalia Macula area and observed by Melton (2018) and Detelich and Kattenhorn (2022). ...
... Due to the difficulty of combining multiple DTMs in ASP, we only generated DTMs of Agenor Linea in SOCET SET ® , which precludes the types of comparisons described above; however, examination of the SOCET SET ® DTM can provide additional insight into how well we can know Europa's topography. The smallest features in the 50-m orthoimages that are clearly discernable in the DTM and hillshade are several 0.6-km wide troughs (referred to as "tension gashes" in Hoyer et al. [71]) within the eastern portion of Agenor that appear as one-post wide depressions. Several double ridges a few DTM posts across are also clearly visible outside the band on the eastern side of the DTM; however, these features actually appear narrower in the DTM than in the orthoimage (this may be real if the feature is highly sloped). ...
Jupiter’s moon Europa harbors one of the most likely environments for extant extraterrestrial life. Determining whether Europa is truly habitable requires understanding the structure and thickness of its ice shell, including the existence of perched water or brines. Stereo-derived topography from images acquired by NASA Galileo’s Solid State Imager (SSI) of Europa are often used as a constraint on ice shell structure and heat flow, but the uncertainty in such topography has, to date, not been rigorously assessed. To evaluate the current uncertainty in Europa’s topography we generated and compared digital terrain models (DTMs) of Europa from SSI images using both the open-source Ames Stereo Pipeline (ASP) software and the commercial SOCET SET® software. After first describing the criteria for assessing stereo quality in detail, we qualitatively and quantitatively describe both the horizontal resolution and vertical precision of the DTMs. We find that the horizontal resolution of the SOCET SET® DTMs is typically 8–11× the root mean square (RMS) pixel scale of the images, whereas the resolution of the ASP DTMs is 9–13× the maximum pixel scale of the images. We calculate the RMS difference between the ASP and SOCET SET® DTMs as a proxy for the expected vertical precision (EP), which is a function of the matching accuracy and stereo geometry. We consistently find that the matching accuracy is ~0.5 pixels, which is larger than well-established “rules of thumb” that state that the matching accuracy is 0.2–0.3 pixels. The true EP is therefore ~1.7× larger than might otherwise be assumed. In most cases, DTM errors are approximately normally distributed, and errors that are several times the derived EP occur as expected. However, in two DTMs, larger errors (differences) occur and correlate with real topography. These differences primarily result from manual editing of the SOCET SET® DTMs. The product of the DTM error and the resolution is typically 4–8 pixel2 if calculated using the RMS image scale for SOCET SET® DTMs and the maximum images scale for the ASP DTMs, which is consistent with recent work using martian data sets and suggests that the relationship applies more broadly. We evaluate how ASP parameters affect DTM quality and find that using a smaller subpixel refinement kernel results in DTMs with smaller (better) resolution but, in some cases, larger gaps, which are sometimes reduced by increasing the size of the correlation kernel. We conclude that users of ASP should always systematically evaluate the choice of parameters for a given dataset.
... While many of the ridges on Europa are straight, numerous examples of similar kinks exist (e.g., in double ridges or cycloids; Prockter and Patterson, 2009). In addition, transform boundaries (i.e., major strike-slip faults) are present (Hoyer et al., 2014); thus, it is possible that similar subduction geometries could have developed on Europa. ...
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.
... They also exhibit few, relatively younger, crosscutting features, implying geologic youthfulness. The Kattenhorn and Prockter (2014) study provided evidence for the third key process in the plate tectonics triad (i.e., subduction) which, combined with prior evidence of seafloor-spreading-like dilational band formation (Schenk and McKinnon, 1989;Sullivan et al., 1998;Tufts et al., 2000;Prockter et al., 2002) and transform-like strike-slip fault zones Sarid et al., 2002;Kattenhorn, 2004;Hoyer et al., 2014), suggest that the outer portion of Europa's ice shell comprises a regionally active mobile lid of distinct plates. This possibility would designate Europa as the only Solar System body other than Earth to have a system of plate tectonics that has modified its surface, and potentially accounts for its geologically young surface age. ...
A new modeling-based study by Johnson et al. (2017) lends support to the hypothesis that portions of Europa's surface may have been removed by the process of subduction, as suggested by Kattenhorn and Prockter (2014). Using a simple 1D model that tracks the thermal and density structure of a descending ice plate, Johnson et al. show that ice plates with 10% porosity and overall salt contents of ~5%, but which differ in salt content by ~2.5% from the surrounding reference ice shell, are non-buoyant and thus likely to sink through the underlying, convecting portion of the ice shell. The feasibility of subduction in an ice shell is critical to the existence of icy plate tectonics, which is hypothesized to exist at least locally on Europa, potentially making it the only other Solar System body other than Earth with a surface modified by plate tectonics.
... These intersecting bands appear to have formed from two separate episodes of diffuse, broad scale lateral shearing that initially formed a set of NW-SE oriented bands via right-lateral shearing, and later formed a set of sigmoidal bands oriented NE-SW via left-lateral shearing. Two episodes of opposite-sense shearing may also be observed in Agenor Linea (Hoyer et al., 2014), just to the north of the Libya study area. As the more recent episode of broad scale left-lateral shearing continued, the plates have rotated counter-clockwise, similar to plate motions within the southern portion of the Castalia Macula area and observed by Melton (2018) and Detelich and Kattenhorn (2022). ...
Observations of past plate tectonic - like motions in Europa’s icy lithosphere have been reported in previous studies. Quantifying the nature, age, and amount of plate motion is important for geophysical models of Europa’s ice shell and for astrobiology, since subsumed pates could drive the flow of nutrients into the subsurface ocean. We have used GPlates software (Williams et al., GSA Today 2012) and a mosaic of regional-resolution Galileo SSI data from orbits E11, E15, E17, and E19 to make interactive reconstructions of both the Northern Falga region (60N, 220W) and the Castalia Macula region (0N, 225W). The advantage of this method is that plate motions are calculated on a sphere, while still maintaining the original Galileo image pieces in their proper geographic locations.
Previous work on the Castalia Macula region (Patterson et al. J.Struct.Geol. 2006) and the adjacent Phaidra Linea region (Patterson and Ernst, LPSC 2011) found offsets along spreading boundaries, and then calculated the best fit finite rotations to close those offsets. Though this method is mathematically rigorous and gives a statistical goodness of fit, it is not easy to test multiple hypotheses for candidate piercing points or divisions of candidate plate boundaries. Through the interactive environment, we found that we could better account for observed offsets in this region by breaking it into 32 different plates. Patterson and Ernst broke the Phaidra region into 6 plates which exhibited nonrigid behavior, where our study breaks it into 16 rigid plates.
The Northern Falga Regio area is interesting due to the potential for large amounts of subsumption of Europa’s icy crust in this location. The previous reconstruction (Kattenhorn and Prockter, Nat.Geosci. 2014) was based on planar geometry, and we have replicated these results using a spherically-based reconstruction. We will present the plate maps and reconstructions for both of these regions, along with the best fit rotation poles.
... Dilational bands on Europa are thought to form from dilational opening and infill (Schenk and McKinnon, 1989;Pappalardo and Sullivan, 1996;Sullivan et al., 1998;Tufts et al., 2000;Culha et al., 2014) by fracturing of a brittle surface layer overlying a more ductile substrate (Schenk and McKinnon, 1989), but some also show evidence of strike-slip or oblique motion (Tufts et al., 1999(Tufts et al., , 2000Prockter et al., 2000Prockter et al., , 2002aGreenberg, 2004;Kattenhorn, 2004;Prockter and Patterson, 2009;Culha et al., 2014;Hoyer et al., 2014). The material external to the margins of dilational bands on Europa can be reconstructed (Schenk and McKinnon, 1989;Pappalardo and Sullivan, 1996;Sullivan et al., 1998), indicating that the band interiors consist of material from below that ascended to the surface (Prockter and Patterson, 2009). ...
... Our study is versatile enough to include most types of bands, but we do not include some such as gray bands [Kattenhorn and Hurford, 2009; Journal of Geophysical Research: Planets 10.1002/2013JE004526 Prockter and Patterson, 2009] and wide bands (e.g., Libya Linea and Thynia Linea). Different types of bands might form through different mechanisms, at different rates, or from different materials [Hoyer et al., 2013;Prockter and Pappalardo, 2000;Greeley et al., 1998;Marshall and Kattenhorn, 2005]. Nevertheless, if generalized, our results imply a mean strain of 7.6 ± 3.7%, Figueredo and Greeley [2004] conducted pole-to-pole mapping of all features on Europa between 80°W and 220°W longitude. ...
[1] Crosscutting relationships of tectonic lineaments on Europa record the history of surface deformation. We mapped the displacement and orientation of older features crosscut by two types of lineaments: bands and double ridges. These measurements allow us to determine both the strike-perpendicular and strikeparallel displacement along investigated features. Double ridges record both ridge-perpendicular contraction and expansion, with a mean of 0.16 ± 0.06 km of contraction based on the analysis of sixteen double ridges. Bands record expansion, with a mean of 3.33 ± 0.27 km for the six bands analyzed, but with perpendicular displacement less than their apparent morphologic widths of 324 km. The implied global surface strain for double ridges (including those that expand) and bands is 2.22 ± 0.76% contraction and 7.60 ± 3.7% expansion, respectively. Double ridges thus may accommodate part of the surface expansion recorded by bands. Most current models for double ridges do not predict contraction. The models that satisfy the observations for bands are “slow spreading” models, cryovolcanism, and folding.
Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.
Chaos terrains are geologically young and extensively disrupted surface features of Europa, thought to be an expression of the subsurface ocean interacting with the surface. The most prominent examples of this terrain on Europa are Conamara Chaos, and Thera and Thrace Maculae, all prime targets for the upcoming JUICE and Europa Clipper missions to assess the astrobiological potential of Europa. Of the three features, Thrace Macula is currently the least studied and understood. It intersects both Agenor Linea to the north and Libya Linea to the south, two important regional-scale bands whose interaction with Thrace is yet to be fully unraveled, especially in terms of their relative ages of emplacement and activity. Through detailed structural mapping using Galileo Solid State Imager data and terrain analysis on Digital Terrain Models, we here develop a novel hypothesis on the mechanisms that have been involved in the study area. We find that Thrace Macula is bordered along most sides by preexisting strike-slip faults that have constrained its emplacement and areal distribution. We determine a sequence of events in the area involving the formation of Agenor Linea, followed by that of Libya Linea first and Thrace Macula later, and ultimately by strike-slip tectonic activity driven by Libya Linea and displacing a portion of Thrace Macula. Therefore, Thrace’s subsurface material, uprising along faults postdating its formation, likely represents the freshest possible that could be sampled by future spacecraft in this region, a major consideration for the upcoming Europa Clipper mission.
The heavily fractured surface of Ganymede displays many morphologically distinctive regions of inferred distributed shear and strike-slip faulting that may be important to the structural development of its surface. To better understand the role of strike-slip tectonism in shaping this complex icy surface, we perform detailed mapping at nine sites using Galileo and Voyager imagery, noting key examples of strike-slip morphologies where present. These four morphological indicators are: en echelon structures, strike-slip duplexes, laterally offset pre-existing features, and possible strained craters. We map sites of both light, grooved terrain (Nun Sulci, Dardanus Sulcus, Tiamat Sulcus, Uruk Sulcus, and Arbela Sulcus), and terrains that are transitional from dark to light terrains (Nippur and Philus Sulci, Byblus Sulcus, Anshar Sulcus, and the Transitional Terrain of Northern Marius Regio). At least one, if not more, of the four strike-slip morphological indicators are observed at every site, suggesting strike-slip tectonism is indeed important to Ganymede's evolutionary history. Byblus Sulcus is the only mapped site where the presence of strike-slip indicators is limited to only a few en echelon structures; every other mapped site displays examples of at least two types, with Arbela Sulcus containing candidate examples of all four. In addition, quantification of morphological characteristics suggests related rotation between sites, as evidenced by the predominant NW/SE trend of mapped features within the light terrain present in five different sites (Nun, Tiamat, Uruk, Nippur/Philus, Byblus, and Anshar Sulcus). Moreover, incorporation of strike-slip tectonism with pre-existing observations of extensional behavior provides an improved, synoptic representation of Ganymede's tectonic history.
We present a theoretical model and discuss field-based observations from the Carboniferous Northumberland Basin (UK), describing the structures developed at stepovers associated with regionally oblique divergence (i.e., transtension zones). We show that these structures have significantly different geometries and evolution compared to those found in stepovers along strike-slip faults. The development of complex and heterogeneous patterns of structures, accommodating both brittle (fault/fracture mesh) and ductile deformation (folds), is observed at these sites. The dilational mesh structures in the stepover region experience a complex evolution due to finite strain-controlled switches from wrench- to extension-dominated transtension. This disrupts the development of a smoothly evolving structural fabric and may inhibit/perturb the development of a throughgoing fault linking adjacent fault segments. Markedly curvilinear and locally curviplanar folds, compartmentalized by strike-slip faults, are also developed. Significant amounts of hinge-parallel extension are accommodated by calcite-filled tensile veins and conjugate tension gash arrays.The fold-fracture associations described here contrast strongly with the more widely recognized patterns of strike-slip conjugate shear planes and extension fractures associated with folds developed in contractional and wrench tectonic settings. This represents a diagnostic feature that allows transtensional folds in both surface and subsurface environments to be distinguished from structures formed by later episodes of compressional or strike-slip inversion.
From 2007 the Russian Academy of Sciences and Roscosmos consider to develop a Europa surface element, in coordination with the Europa Jupiter System Mission (EJSM) international project planned to study the Jupiter system. The main scientific objectives of the Europa Lander are to search for the signatures of possible present and extinct life, in situ studies of the Europa internal structure, the surface and the environment. The mission includes the lander, and the relay orbiter, to be launched by Proton and carried to Jupiter with electric propulsion. The mass of scientific instruments on the lander is similar to 50 kg, and its planned lifetime is 60 days. A dedicated international Europa Lander Workshop (ELW) was held in Moscow in February 2009. Following the ELW materials and few recent developments, the paper describes the planned mission, including the science goals, technical design of the mission elements, the ballistic scheme, and the synergy between the Europa Lander and the EJSM. (C) 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.
Tidal heating models are linked to thermal convection models for ice having strongly temperature dependent viscosity. In the range of ice viscosity inferred from laboratory experiments, tidal forces will heat up rising diapirs on Europa. Partial melt produced in the rising diapirs is predicted to create disruption of near-surface materials and formation of lenticulae and chaos, even if the average ice layer thickness overlying an ocean is larger than 20 km.
Agenor Linea is a ~1500 km long, ~20-30 km wide geologically young zone
of deformation on Jupiter's icy moon, Europa. On the basis of recent
Galileo high-resolution images, we interpret Agenor Linea as a
strike-slip zone formed in three stages by a combination of lithospheric
separation, extension, and dextral horizontal shear. Agenor Linea
exhibits excellent examples of strike-slip duplexes in an icy
lithosphere, unobscured by vegetation and unaltered by erosion.
We utilize imaging data from the Galileo spacecraft to investigate band formation on one of Jupiter's moons, Europa. Bands are polygonal features first observed in Voyager data close to Europa's anti-Jovian point and represent areas where preexisting terrain has been pulled apart, allowing new material to move up into the gap. We examine the detailed morphology of several bands imaged at different resolutions and lighting geometries. We identify several distinct morphological characteristics, including central troughs, hummocky textures, and ridge and trough terrains, some of which are common among the bands studied. In many cases, bands have initiated along segments of one or more preexisting double ridges, ubiquitous within Europa's ridged plains. Distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice, rather than liquid water. Comparisons between Europan band morphologies and features found on terrestrial mid-ocean ridges reveal several similarities, including axial troughs, subcircular hummocks, normal faults, and indications of symmetrical spreading. We conclude that terrestrial mid-ocean ridge rifting is a good analogy for Europan band formation. If a terrestrial seafloor-spreading model is applicable to Europan bands, we speculate that band morphologies might be related to the relative rate of spreading of each band. Bands may have contributed significantly to the resurfacing of Europa. Europan bands we examine predate (but do not postdate) lenticulae and related features, implying that the style of resurfacing on Europa has changed over recent geological time in these regions.
It has been proposed that Jupiter's satellite Europa currently posesses a global subsurface ocean of liquid water. Galileo gravity data verify that the satellite is differentiated into an outer H2O layer about 100 km thick but cannot determine the current physical state of this layer (liquid or solid). Here we summarize the geological evidence regarding an extant subsurface ocean, concentrating on Galileo imaging data. We describe and assess nine pertinent lines of geological evidence: impact morphologies, lenticulae, cryovolcanic features, pull-apart bands, chaos, ridges, survace frosts, topography, and global tectonics. An internal ocean would be a simple and comprehensive explanation for a broad range of obserations; however, we cannot rule out the possibility that all of the surface morphologies could be due to prozesses in warm, soft ice with only localized or partial melting. Two different models of impact flux imly very different surface ages for Europa; the model favored here indicate an average age of ~ 50 Myr. Searches for evidence of current geological activity on Europa, such as plumes or surface changes, have yielded negative results to date. The current existence of global subsurface ocean, while attractive in explaining the observations, remains inconclusive. Future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean whithin Europa today.
Secondary cracks are commonly produced at stress concentration points at the tips of slipping interfaces such as faults. These so-called tailcracks form an antisymmetric pattern at opposite tips of the fault with a fracture geometry that is a mechanical indicator of the sense of slip, whether left-lateral or right-lateral. I present descriptions of tailcracks along numerous strike-slip faults on Europa. Two distinct styles of strike-slip faults are identified: ridge-like and band-like. The angles between faults and tailcracks are variable and are commonly less than the theoretical 70.5° angle that approximately characterizes many terrestrial examples involving fault surfaces that remain in contact during slip. Median tailcrack angles are lower for band-like faults (30°) than ridge-like faults (52°). In addition, the sense of curvature of band-like fault tailcracks is characteristically opposite to that of ridge-like faults. Analytical models of stress orientations around strike-slip faults on Europa indicate that these effects result from dilation during strike-slip motion. Band-like faults characteristically underwent concurrent dilation and shearing but this dynamic coupling is not characteristic of ridge-like faults. The implication is that strike-slip faulting was not a globally homogeneous process on Europa and as a result the morphologies of strike-slip faults are variable. Furthermore, band-like faults appear to corroborate the predictions of the tidal walking theory, exhibiting predominantly right-lateral motions in the southern hemisphere and left-lateral motions in the northern hemisphere. However, ridge-like faults do not obey this slip-sense convention, suggesting that the tidal walking theory may be appropriate for describing the evolution of dilational band-like faults but that ridge-like faults may have resulted from a different driving mechanism.
Europa is the primary target in NASA's future outer solar system exploration strategy. A tidally heated global ocean beneath its ice shell is important for astrobiological considerations; however, habitability requires a source of chemical nutrients. Europa's radiolytically processed surface is a potential source, but a means of delivery of compounds to the ocean is required. Past studies of Europa's surface have been unable to explain an abundance of extensional features (e.g., dilational bands) yet scant evidence of contraction. Moreover, the crater-based surface age (40-90 Myr) indicates one of the solar system's youngest surfaces, implying Europa's surface (3.09 x 107 km2) may have been recycled in this time frame (i.e., 0.3-0.8 km2 per year). We address this enigma by presenting evidence for subduction, and hence plate tectonics, on Europa. We reconstruct geologic features in a 106,000 km2 candidate region to show that the current surface configuration involved numerous translations and rotations of rigid plates. The reconstruction reveals ~100 km of missing surface that seemingly vanished along a 20-km-wide, band-like zone with unusual color characteristics. Mismatching geological features across this zone suggest an ~80-km-wide region may have subducted along a ≥300-km-long plate boundary. The subduction zone is arcuate, has no topographic expression at image resolutions, and is partially bounded by transform faults. The overriding plate has numerous strike-slip faults consistent with strain partitioning related to oblique convergence. The surface of the overriding plate is also pervasively dotted with isolated patches of disrupted terrain, which we interpret as erupted cryolava, implying a significant subsurface thermal perturbation related to the potential subduction. If a subduction model for Europa is accurate, buoyancy constraints and a lack of contractional topography imply that the subducting slab does not enter the ocean directly. We thus interpret a thin (~several km), brittle lid overlying a thicker, convecting ice layer, with plate motions and subduction restricted to the brittle lid. The subducting plate is presumably consumed at a rate conducive to complete subsumption into the convecting layer. On Earth, oceanic lithosphere recycling occurred along 55,000 km of subduction zones in <200 Myr at 20-80 mm/yr. On Europa, similar subduction rates (11-26 mm/yr), if valid, are possible for 30,000 km of subduction boundaries. Europa's surface area (~6% of Earth's) would accordingly recycle over a shorter time frame, consistent with the surface age. Our work potentially provides a new paradigm for interpreting Europa's surface features and age, and provides a mechanism to deliver nutrients from the surface to the ocean: crucial for astrobiology and habitability. If subduction exists, Europa would become the only other solar system body beyond Earth to exhibit plate tectonics, involving subduction (surface area removal), mid-ocean-ridge-like spreading (surface area creation at dilational bands), and transform motions. Such motions are presumably driven by convection in the deeper, warmer ice, evidenced by thermal upwellings at sites of chaos and lenticulae.
Geology: dark terrain, bright grooved terrain, polar caps, impact
structures; Surface composition: ice, dark material, other molecules;
Regolith properties: photometric properties, thermal and radar
characteristics; Interior evolution and global history.
Lineaments are thought to form as tensile cracks due to tidal stress,
which is driven mainly by Europa's eccentric orbit. However, this model
would not produce the wide range of lineament azimuths observed on
Europa unless the stress in a given region, or the conditions for fault
failure, change over time. In this work, we test the ability of two
mechanisms that would alter the stress field over time to account for
the observed lineament azimuths: non-synchronous rotation and spin pole
precession. First, we revisit previous analysis of lineaments and find
that an underlying assumption used to predict their azimuths was
inconsistently applied. After revising these predictions, we incorporate
the effects of a non-zero obliquity, which has been shown to influence
the formation of other tidal-tectonic features. We then expand our
analysis to include the effects of the time-variable phenomena, spin
pole precession and non-synchronous rotation of the ice shell. We also
consider additional failure assumptions to those used in previous work
on lineament azimuths. We test our models against the azimuths of
observed lineaments in the Bright Plains region of Europa. Without
obliquity, we find that non-synchronous rotation is insufficient to
explain the wide range of azimuths observed in this region. In the
presence of obliquity, we find that either spin pole precession or
non-synchronous rotation could produce wide variations in lineament
azimuths. However, neither model can independently account for the
observed distribution of azimuths in the Bright Plains region. In fact,
a model in which all of the lineaments are assumed to form at random
orientations outperforms the non-synchronous rotation model in our
statistical tests. The model with the highest likelihood of producing
the observations is one in which 45% of the lineaments formed as
predicted in the precession model, 55% formed at random orientations,
and older lineaments are less represented in the tectonic record. Given
the relative timescales expected for spin pole precession and
non-synchronous rotation, it makes sense that the signal of precession
is more apparent in the tectonic record. These results are in good
agreement with assessments of strike-slip faults; modeling of both types
of features indicates a large obliquity, the same spin pole direction
during the most recent epoch of tectonic activity, a similar percentage
of features that are not well-explained by the tidal model, and little
evidence of non-synchronous rotation.
The surface of Jupiter’s iconic icy satellite Europa is dominated by tectonic features that have formed in extension. In contrast, features that have formed in contraction appear to be rare. Thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain. In this paper we reexamine the amount of strain that can be accommodated during fold formation on Europa using a two-dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions. We find that, in contrast to previous assessments, relatively large strains can be accommodated by the development of relatively low-amplitude folds. Under our nominal conditions, creating peak-to-trough fold amplitudes of ∼200 m—consistent with Europa’s Astypalaea folds—requires ∼7% contraction of the lithosphere. Decreasing either the strain rate or the ice grain size allows the formation of larger-amplitude folds at smaller strains, but we find no cases where 200-m-amplitude folds form at less than ∼3% strain. Only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains ⩽3%. The requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation (generally 1–2% contractional strain) in which limited amplification occurs, and smaller amplitude growth rates overall. The requirement of several percent strain to produce even moderate-amplitude folds is consistent with previous numerical models of single-layer and lithospheric-scale viscoelastic folding under terrestrial conditions. Because the formation of large-amplitude folds requires relatively large strain, small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation. The accommodation of contractional strains via such pre-folding lithospheric thickening provides at least a partial solution to Europa’s apparent strain imbalance.
Agenor Linea is perhaps the youngest large linea on Europa's surface. Here we describe units based on geologic mapping of Agenor Linea using high-resolution Galileo SSI data.
We introduce a new model for producing strike-slip displacement on Europa, which we call shell tectonics. We invoke general principles of stress and failure along faults and include the influence of Europa's elastic shell when determining the response of faults to periodic tidal stress. We apply a Coulomb failure criterion to determine when and if failure will occur and adopt a linear elastic model for slip and stress release to determine the direction of net offsets along pre-existing faults. Our model reproduces the global-scale strike-slip fault pattern observed on Europa in which left-lateral faults dominate far north of the equator, right-lateral faults do so in the far south, and near-equatorial regions display a mixture of both types of faults. One of the most compelling attributes of the tidal walking model for strike-slip formation on Europa (Hoppa et al., 1999) is its ability to generate this global pattern. The shell tectonics model includes a more physical treatment of fault mechanics than tidal walking and makes a prediction of slip direction along faults by computing the net slip over several orbits. Also, several assumptions made in the tidal walking model are incorporated explicitly in the shell tectonics model. A strike-slip formation model with application to Enceladus has also been proposed (Smith-Konter & Pappalardo, 2008) that includes a mechanical treatment of faults but does not incorporate the effects of the elastic shell. Since this model should be equally applicable to Europa, we present predictions made using this plate-tectonics model along with our shell tectonics predictions. We find that a model neglecting the elastic shell effects does not agree as well with the observations. In addition to global predictions of slip direction, shell tectonics provides an estimate of the relative growth rates of faults and implications for seismicity and heating along faults.
The Walker Lane and northern part of the eastern California shear zone
form a boundary zone accommodating differential motion between the
Sierra Nevada and western Great Basin. Within the boundary zone, Global
Positioning System velocities show a westward increase from 2 3 mm/yr in
the central Great Basin to ˜14 mm/yr in the Sierra Nevada and a
clockwise rotation from west-northwest to northwest. In the same region,
incremental extensional strain axes recorded by earthquake focal
mechanisms and fault-slip inversion show an east to west
counterclockwise rotation of 50°, from parallel with the velocity
field in the central Great Basin to nearly orthogonal to the velocity
field along the eastern flank of the Sierra Nevada. Unlike plane-strain
deformation within the central Great Basin, the progressive deviation
between the orientation of extension axes and the velocity field in the
boundary zone is a product of nonplane strain. The boundary zone records
active constriction formed in conditions varying from wrench-dominated
to extension-dominated transtension.
We investigate arrays of echelon cracks in rock (veins, joints, and dikes) formed by dilation. Individual cracks are interpreted as cross-sections of blade-like cracks at the leading edges of a parent fracture. Two morphological end members are distinguished as having straight or highly curved propagation paths as seen in such cross-sections. Bridges of rock between overlapping straight crack paths bend to accommodate crack dilation. Sigmoidal curvature of the bridges is achieved in a phase of increased dilation as the aspect ratio of bridges increases and their resistance to bending drops. Large bending strains within bridges may lead to cross fractures that link adjacent cracks. Good estimates of the dilation and shear displacement over arrays may be obtained directly from measurements of bridges. In contrast, the sigmoidal form of cracks of the other end member is caused by their curving propagation paths. These cracks may link as one crack tip intersects the adjacent crack wall. Some arrays of sigmoidal echelon cracks are associated with shear zones, but those analyzed here are not.
Sedimentary basins developed under conditions of strike-slip or transtension are subject to significant rotational strains, yet faults developed in such regimes are commonly explained using simplified models that ignore rotation. The heaves of extensional faults developed provide a means of quantifying this rotation. For ideal strike-slip (simple shear), the apparent stretch due to fault heaves can be related simply to shear strain. At shear strains (γ) above 1.0, previously formed extensional faults begin to show inversion as reverse faults, becoming fully inverted at γ=2.0. In transtensional basins, the apparent stretch is related, in addition, to the initial orientation of the faults, which may itself be related to the incremental strain. In the Stellarton basin of Nova Scotia, Canada, fault heaves and orientations can be measured from subsurface mine plans. Measurements of these quantities indicate that strain was only mildly transtensional, with a small (<10°) angle of divergence (α). The measurement of fault heaves potentially provides detailed information on strain wherever strike-slip or transtensional basins have been explored in detail by seismic or other subsurface methods.
Geological mapping of regional and high-resolution Galileo images reveals a variety of units and structures within Ganymede's dark terrain. We have made a detailed study of areas within Galileo, Marius, and Nicholson Regiones in order to investigate the style of tectonic deformation experienced by dark terrain adjacent to swaths of grooved terrain. Dark terrain appears to become fractured as a precursor to grooved terrain formation; in places, dark fractured swaths are recognized which have similar characteristics to brighter grooved terrain swaths. Tectonic deformation may be, but is not always, focused through preexisting weaknesses caused by impact craters and furrows. A prominent groove lane, Anshar Sulcus, is inferred to have formed by the process of hanging wall rollover, accompanied by a small amount of right-lateral horizontal shear offset resulting from NE-SW extension.
We have used the open-source program SatStress to develop a graphic user interface (GUI) for calculating tidal stresses on the surface of a satellite with both elastic and viscoelastic rheology. SatStress GUI will eventually be open-source.
We performed 2D numerical simulations of oscillatory tidal flexing to study the interrelationship between tidal dissipation (calculated using the Maxwell model) and a heterogeneous temperature structure in Europa’s ice shell. Our 2D simulations show that, if the temperature is spatially uniform, the tidal dissipation rate peaks when the Maxwell time is close to the tidal period, consistent with previous studies. The tidal dissipation rate in a convective plume encased in a different background temperature depends on both the plume and background temperature. At a fixed background temperature, the dissipation increases strongly with plume temperature at low temperatures, peaks, and then decreases with temperature near the melting point when a melting-temperature viscosity of Pa s is used; however, the peak occurs at significantly higher temperature in this heterogeneous case than in a homogeneous medium for equivalent rheology. For constant plume temperature, the dissipation rate in a plume decreases as the surrounding temperature increases; plumes that are warmer than their surroundings can exhibit enhanced heating not only relative to their surroundings but relative to the Maxwell-model prediction for a homogeneous medium at the plume temperature. These results have important implications for thermal feedbacks in Europa’s ice shell.
This review of Europan tectonics previews a chapter of the forthcoming
text "Europa". After the Voyager flyby of the icy moon Europa in 1979,
models were developed that attributed pervasive surface fracturing to
the effects of tidal forcing due to the gravitational pull of Jupiter.
The late 1990s Galileo mission returned high resolution coverage of the
surface, allowing a diverse range of tectonic features to be identified.
Subsequent description, interpretation, and modeling of these features
has resulted in significant developments in five key themes: (1) What
drives the tectonics? (2) What are the formation mechanisms of the
various types of tectonic features? (3) What are the implications for a
subsurface ocean? (4) What is the nature and thickness of the ice shell?
(5) Is Europa currently tectonically active? We highlight key
developments pertaining to these fundamental issues, focusing on the
following elements: (1) Many fracture patterns can be correlated with
theoretical stress fields induced by diurnal tidal forcing and long-term
effects of nonsynchronous rotation of the ice shell; however, these
driving mechanisms alone cannot explain all fracturing. The tectonic
fabric has likely been affected by additional contributing effects:
tidal despinning, orbital evolution, interior differentiation, polar
wander, finite obliquity, stresses due to shell thickening, endogenic
forcing by convection and diapirism, and secondary effects driven by
strike-slip faulting and plate flexure. (2) Due to the prevalence of
global tension, a low lithostatic gradient, and the inherent weakness of
ice, tectonic features likely have predominantly extensional primary
formation mechanisms (e.g. surface fractures, ridges, and normal
faults). There has been no categorical documentation of fracture
development by compressive shearing. Even so, the constantly changing
nature of the tidal stress field results in shearing reactivation of
cracks being important for the morphologic and mechanical development of
tectonic features. Hence, strike-slip faults are relatively common.
Also, frictional shearing and heating has likely contributed to the
construction of edifices along crack margins (i.e., ridges). If Europa
has not recently expanded, crustal convergence (although elusive in
Galileo images) is required to balance out new surface material created
at spreading bands and may be accommodated locally along ridges or
convergence bands. (3) Chains of concatenated curved cracks called
cycloids provide convincing evidence of a subsurface ocean in that they
must be the result of diurnal forcing of sufficient tidal amplitude to
break the ice during a large portion of the Europan orbit, suggesting a
tidally responding ocean beneath the ice shell. (4) Fracture mechanics
reveals that the brittle portion of the ice shell is likely no more than
a few km thick, but convection driven diapirism and crater morphologies
necessitate a thicker shell overall (up to about 30 km). It is not
known if fractures are able to penetrate this entire shell thickness.
The brittle layer acts as a stagnant lid to plastic deformation in the
ductile portion of the ice shell, resulting in localized brittle
deformation. (5) Tectonic resurfacing has dominated the <70 my of
visible geologic history. No evidence exists that Europa is currently
tectonically active; however, this may be more a failing of the current
state of the science rather than a lack of probability. A tectonically
based answer to this question lies in a thorough analysis of
geologically young surface fractures but would benefit from far more
extensive coverage of the surface via a return mission to Europa.
We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa as secondary to the primary impacts of comets on Io, Europa, and Ganymede. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa's surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. The mass of basaltic impact ejecta from Io to reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa's ice crust. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady–Kipp fragments originally developed by Melosh. Our model successfully reproduces the observed size-number distributions of small craters on both Mars and Europa. However, the model predicts that a significant fraction of the 200–500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from Io that had gone into orbit about Jupiter. This prediction is not supported by observation, which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters. Iogenic basalts are also interesting because they provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa's surface.
The process of tide-driven walking, proposed as a major mechanism for strike–slip displacement on Europa, is modeled using a finite-element numerical simulation of the behavior of viscoelastic material. For material parameters that are plausible for the water ice composing Europa's crust, the simulation confirms earlier analytic results for strike–slip displacement along a crack that penetrates down to the liquid water substrate. The finite element code permits testing other cases as well. Of considerable interest is whether tidal walking can operate if a crack penetrates not to liquid but only as far as warm, relatively viscous ice. In such a case, significant displacement can be driven, but only if the threshold value of the compressive force needed to lock the fault is near the value of the overburden stress at the bottom of the crack. Such special conditions are not needed for displacement if the crack penetrates to the underlying ocean.
Secondary fractures at the tips of strike-slip faults are common in the ice shell of Europa. Large magnitude perturbed stress fields must therefore be considered to be a viable driving mechanism for the development of part of the fracture sequence. Fault motions produce extensional and compressional quadrants around the fault tips. Theoretically, these quadrants can be associated with tensile and compressive deformational features (i.e. cracks and anti-cracks), respectively. Accordingly, we describe examples of both types of deformation at fault tips on Europa in the form of extensional tailcracks and compressional anti-cracks. The characteristics of these features with respect to the plane of the fault create a fingerprint for the mechanics of fault slip accumulation when compared with linear elastic fracture mechanics (LEFM) models of perturbed stress fields around fault tips. Tailcrack kink angles and curving geometry can be used to determine whether opening accompanies sliding motion. Kink angles in the 50 70° range are common along strike-slip faults that resemble ridges, and indicate that little to no opening accompanied sliding. In contrast, tailcrack kink angles are closer to 30° for strike-slip faults that resemble bands, with tailcrack curvatures opposite to ridge-like fault examples, indicating that these faults undergo significant dilation and infill during fault slip episodes. Anti-cracks, which may result from compression and volume reduction of porous near-surface ice, have geometries that further constrain fault motion history, corroborating the results of tailcrack analysis. The angular separation between anti-cracks and tailcracks are similar to LEFM predictions, indicating the absence of cohesive end-zones near the tips of Europan faults, hence suggesting homogeneous frictional properties along the fault length. Tailcrack analysis can be applied to the interpretation of cycloidal ridges: chains of arcuate cracks on Europa that are separated by sharp kinks called cusps. Cusp angles are reminiscent of tailcrack kink angles along ridge-like strike-slip faults. Cycloid growth in a temporally variable tidal stress field ultimately resolves shear stresses onto the near-tip region of a growing cycloid segment. Thus, resultant slip and associated tailcrack development may be the driving force behind the initiation of the succeeding arcuate segment, hence facilitating the ongoing propagation of the cycloid chain.
Although a single model currently exists to explain the development of curved Europan cycloids, there have been no systematic studies of the range of morphologies and quantifiable geometric parameters of cycloidal features. We address variations in geometry along individual cycloid segments, characterizing differences in cusp styles and angles, and addressing the morphologic aspects of cycloid segments and cusps. In so doing, we illustrate how geometric and morphologic evidence imply a formation mechanism that differs from the existing model in several aspects. The current model states that cycloids are initiated as tensile fractures that grow in a curved path in response to rotating diurnal tidal stresses on Europa. However, the geometry of a cycloid cusp necessitates that shear stress was resolved onto the existing cycloid segment by the rotating diurnal stresses at the instant of cusp formation. Furthermore, we observe that cycloid cusps have a strikingly similar geometry to tailcracks that developed at the tips of many ridge-like strike-slip faults on Europa in response to shearing at the fault tip. We suggest that this similarity in geometries can be attributed to an identical formation mechanism whereby cycloid cusps form by a tailcracking process. We therefore present a revised, mechanically-based model for cycloid formation that retains the basic premise that crack growth is governed by diurnal stresses, but describes the development of cycloid cusps in response to resolved shear stresses at the tips of existing cycloid segments. The ratio of normal to shear stress at the time of tailcrack formation dictates the cusp angle and, over longer time periods, influences the morphologic evolution of the cycloid segment as it is repeatedly reworked by tidal stresses.
Estimates of the thickness of the ice shell of Europa range from 30 km. The higher values are generally assumed to be estimates of the entire ice shell thickness, which may include a lower ductile layer of ice, whereas many of the smaller thickness estimates are based on analyses that only consider that portion of the ice layer that behaves elastically at a particular strain rate. One example of the latter is flexure analysis, in which the elastic ice layer is modeled as a plate or sphere that is flexed under the weight of a surface load. We present calculations based on flexure analysis in which we model the elastic ice layer as flexing under a line-load caused by ridges. We use precisely located, parallel flanking cracks as indicators of the location of greatest tensile stress induced by flexure. Our elastic thickness results are spatially variable: ˜500 2200 m (two sites) and ˜200 1000 m (one site). Thorough analysis of Europan flexure studies performed by various researchers shows that the type of model selected causes the greatest variability in the thickness results, followed by the choice of Young's modulus, which is poorly constrained for the Europan ice shell. Comparing our results to those of previously published flexure analyses for Europa, we infer spatial variability in the elastic ice thickness (at the time of load emplacement), with smooth bands having the thinnest elastic ice thickness of all areas studied. Because analysis of flexure-induced fracturing can only reveal the elastic thickness at the time of load emplacement, calculated thickness variability between features having different ages may also reflect a temporal variability in the thickness of Europa's ice shell.
[1] We suggest that a crack that developed into a wedge-shaped band within the icy crust of Europa (at ∼28°S, 170°W [see Schenk and McKinnon, 1989, Figure 4; Prockter et al., 1999, Figure 1]) originated under an applied compressive stress through the operation of a frictional sliding mechanism. We analyze this suggestion using a scale-independent, sliding crack model and obtain an estimate of the maximum compressive stress to initiate the crack that compares reasonably well with earlier estimates [Helfenstein and Parmentier, 1985; McEwen, 1986; Greenberg et al., 1998] of Europan crustal stresses that are based upon elastic deformation of a shell. We then show through an application of both power law dislocation creep and diffusion creep that colder, near-surface Europan ice appears to be capable of supporting the deduced stresses with little relaxation over a period from 104 to 105 years.
The northern leading hemisphere of Europa was imaged at regional mapping resolution (~230 m/pixel) by the Galileo spacecraft SSI camera. We produced geologic maps from a regional-scale mosaic and a high resolution inset of this region. Twelve geologic units were sufficient to produce correlative geologic maps at both regional and local scales. Stratigraphic relationships indicate four major episodes in the geologic history of this area: The first episode includes background plains formation and modification, followed by a second period of extensive lineament formation. The third episode included extensive chaotic disruption of the surface at low and middle latitudes. The final episode includes the formation of new sets of ridges and bands, especially at high latitudes. The low crater density indicates that all these episodes in Europa's geologic history are geologically recent. The latitudinal distribution of chaos areas broadly matches that of areas of crustal thinning from models of tidal heating. We show that stress directions rotated clockwise with time, which is consistent with predictions from global stress models involving tidal deformation and nonsynchronous rotation of Europa's crust. On the basis of the change in lineament orientation with time, the reconstructed longitudinal positions of the studied area indicate that Europa's crust completed a full rotation relative to the tidally deformed interior.
The background lineated plains on Europa are locally highly modified and destroyed in regions known as chaos and lenticulae. Produced there are (1) isolated fragments and polygons of background material which rotate and translate, (2) matrix, which fills in the areas between the fragments and polygons, and (3) surface discolorations. Using observations and constraints from high-resolution Galileo images, we find that a model for the formation of these terrains which involves mobilization and migration of brines, and a possible percolation phase transition as the Europan lithosphere is warmed, can readily explain the vast majority of their characteristics. In addition, the presence of melt fractions of a few percent in the adjacent ice framework may enhance the creep rate and the accompanying deformation rates. The characteristics and distribution of lenticulae suggest that among the strong candidates for heat sources for brine migration and ice mobilization processes is diapirism linked to solid-state convection in a layer underlying a brittle lid and possibly overlying a liquid water layer.
We obtained a spectrum of Europa from 2224 to 3302 Å with high signal to noise and a resolution of 6.3 Å with the Hubble space telescope faint object spectrograph. We detect a broad absorption feature centered at 2800 Å, seen previously at lower signal to noise in International Ultraviolet Explorer spectra of Europa. No additional absorption features are detected. We compare the observed spectrum to laboratory spectra of sulfur ions in water ice and sulfur dioxide ice on water ice. The absorption in Europa is a closer match to sulfur dioxide ice. The poor fit of ice irradiated by sulfur ions may indicate that a direct source of SO2 is required to explain the observed absorption.
Two global issues regarding Europa are addressed by a survey of strike-slip faults. First, a common type of terrain that appears to represent convergent sites of surface removal, which may help compensate for substantial widespread dilation along tectonic bands elsewhere, thus helping resolve the problem of conserving global surface area, is identified. Second, evidence for polar wander may provide the first confirmation of that theoretically predicted phenomenon. These results, among others, come from an extensive survey of strike-slip faults over the portion of the surface where Galileo images at 200-m/pixel resolution were obtained for regional mapping purposes. The images cover two broad swaths that run from the far north to the far south, one in the leading hemisphere and the other in the trailing hemisphere. Among the faults that have been mapped are a fault 170 km long with a strike-slip offset of 83 km, the greatest yet identified on Europa, and a quasi-circular strike-slip fault that surrounds a 500-km-wide plate, which has undergone rotation as a rigid unit. Reconstruction of specific examples of strike slip reveals sites of lateral convergence. Because Europa is unique in many ways, these sites are not similar to compression features on other bodies, which may explain why they had previously been difficult to identify. The distribution of strike slip in both hemispheres, when compared with predictions of the theory of tidal walking, provides evidence for polar wander: The crust of Europa appears to have slid as a single unit relative to the spin axis, such that the site on the crust that was previously at the north rotational pole has wandered, probably during the last few million years, to a location currently in the leading hemisphere, about 30° away from the spin axis. Such polar wander probably also explains symmetry patterns in the distribution of chaotic terrain, pits, and uplift features.
Potential contractional folds on Jupiter's icy moon Europa have been identified. The best example is at the extensional band Astypalaea Linea, where a series of subtle topographic undulations, 25 km in wavelength, possess parasitic tectonic features that support a folding origin. A scenario has been qualitatively proposed, whereby folds form via unstable contraction of the icy lithosphere, compensate for extension elsewhere on Europa, and then subsequently relax. Here, we quantitatively address this scenario, applying a model for viscous-plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation. Our results suggest that the lithosphere of Europa could indeed be unstable, but the low required surface temperatures limit fold formation to higher latitudes, and the high required driving stresses (∼9–10 MPa) are difficult to achieve on the satellite. The depth to the brittle–ductile transition is well constrained, and high thermal gradients are indicated, implying heat flows near 100 mW m−2. In addition, topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of Europa's surface. Given the paucity of identified folds, we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on Europa.
The dilemma of the surface-area budget on Europa is resolved by identification of sites of crustal convergence, which have balanced the continual and common creation of new surface along dilational bands and pull-aparts. Convergence bands are characterized by a distinctive, albeit subdued, morphology. The prominent, unusual lineament Agenor is one of several examples. We also find diametrically opposite Agenor a similar bright linear feature surrounded by markings that allow reconstruction, which shows it to be a convergence feature. Until recently, identification of convergence sites was difficult because these features are subtle and do not exhibit structures (like the Himalayas or plate subduction) familiar from convergence of thick solid crusts on terrestrial planets.
We investigate the response of conductive and convective ice shells on Europa to variations of heat flux and interior tidal-heating rate. We present numerical simulations of convection in Europa's ice shell with Newtonian, temperature-dependent viscosity and tidal heating. Modest variations in the heat flux supplied to the base of a convective ice shell, F , can cause large variations of the ice-shell thickness δ. In contrast, for a conductive ice shell, large F involves relatively small δ. We demonstrate that, for a fluid with temperature-dependent viscosity, the heat flux undergoes a finite-amplitude jump at the critical Rayleigh number Ra cr . This jump implies that, for a range of heat fluxes relevant to Europa, two equilibrium states—corresponding to a thin, conductive shell and a thick, convective shell—exist for a given heat flux. We show that, as a result, modest variations in heat flux near the critical Rayleigh number can force the ice shell to switch between the thin, conductive and thick, convective configurations over a ∼10 7 -year interval, with thickness changes of up to ∼10–30 km. Depending on the orbital and thermal history, such switches might occur repeatedly. However, existing evolution models based on parameterized-convection schemes have to date not allowed these transitions to occur. Rapid thickening of the ice shell would cause radial expansion of Europa, which could produce extensional tectonic features such as fractures or bands. Furthermore, based on interpretations for how features such as chaos and ridges are formed, several authors have suggested that Europa's ice shell has recently undergone changes in thickness. Our model provides a mechanism for such changes to occur.
1] A cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects: thermal contraction of the ice due to cooling and the expansion of the shell due to the ice-water volume change. The former effect generates near-surface compression and deeper extension; the second effect generates extension only. In both cases, stresses are smaller at depth due to viscous creep. The resulting combined stresses are extensional except at shallow (<1 km) depths in thin ice shells. For ice shells thicker than 45 km, stresses are extensional throughout. The extensional stresses exceed 10 MPa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs. The dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on Europa and other large icy satellites. However, after 100 Myr of conductive cooling the maximum theoretical elastic strains for Europa are $0.35%, which are probably insufficient to explain the total amount of observed extension.
Cycloidal crack patterns on Europa are influenced by tides induced by orbital eccentricity, which in turn is driven by the Laplace orbital resonance. Their shapes potentially record the location of their formation (relative to the direction of Jupiter), as well as the parameters of crack formation. Hoppa et al.] modeled several cycloid chains using a fixed set of material parameters, but some details did not fit. We now allow material parameters to vary for each arc of an observed cycloid. In general, with minimal variation of model parameters between the arcs, fits are greatly improved. Furthermore, accounting for tidal stress accumulated during non-synchronous rotation, in addition to diurnal stress, allows even better fits. Even with the added freedom in the model our fits allow us to better constrain the location where each cycloid may have formed. Our results support Hoppa et al.'s finding that only a few cracks form ridges per cycle of non-synchronous rotation in the region examined, probably because cracking relieves built up stress until further substantial rotation occurs.
We use stereo-derived topography of extensional bands on Europa to show that these features can be elevated by 100–150 m with respect to the surroundings, and that the positive topography sometimes extends beyond the band margins. Lateral variations in shell thickness cannot maintain the observed topography for timescales greater than 0.1 Myr. Lateral density variations can maintain the observed topography indefinitely; mean density contrasts of 5 and 50 kg m-3 are required for shell thicknesses of 20 and 2 km, respectively. Density variations caused by temperature contrasts require either present-day heating or that bands are young features (<1 Myr old). Stratigraphic analyses suggest that these mechanisms are unlikely. The observation that bands form from ridges may be explained by an episode of shear-heating on ridges weakening the ridge area, and leading to strain localization during extension. Fracture porosity is likely to persist over Myr timescales in the top one-third to one-quarter of the conductive part of the ice shell. Lateral variations in this porosity (of order 20%) are the most likely mechanism for producing band topography if the ice shell is thin (approximate2 km); porosity variations of 2% or less are required if the shell is thicker (approximate20 km). If the ice shell is thick, lateral variations in salt content are a more likely mechanism. Warm ice will tend to lose dense, low-melting temperature phases and be buoyant relative to colder, salt-rich ice. Thus, lateral density variations will arise naturally if bands have been the sites of either localized heating or upwelling of warm ice during extension.
Cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites. Volcanic brine petrogenesis in ammonia-deficient satellites should mainly involve eutectic melting in relevant salt-water systems. Carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids. C1 chondrites contain about one-fifth by mass of salts in various states of hydration. Many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system H2OMgSO4Na2SO4. Minor components include sulfates of K, Ni, Mn, and Ca. Chondrites also contain abundant carbonates, but these are probably not very important in brine magmatism due to their low solubilities under expected conditions. Chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of Cl/S. Soluble salts may have profound effects on the geology and structure of icy satellites and asteroids. In some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate-rich mantle and crustal layers. In other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities. Europa probably has a layered crust composed of anhydrous MgNa sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated Mg and Na sulfates near the surface. Ganymede may have a crust about 300 km thick composed of a 10:1 ratio of ice: mirabilite, and a mantle 500 km thick composed of 50% ice phases plus 50% hydrated Mg and Na sulfates.
We produced geologic maps from two regional mosaics of Galileo images across the leading and trailing hemispheres of Europa in order to investigate the temporal distribution of units in the visible geologic record. Five principal terrain types were identified (plains, bands, ridges, chaos, and crater materials), which are interpreted to result from (1) tectonic fracturing and lineament building, (2) cryovolcanic reworking of surface units, with possible emplacement of sub-surface materials, and (3) impact cratering. The geologic histories of both mapped areas are essentially similar and reflect some common trends: Tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments, the opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges and ridge complexes along prominent fractures in the ice. It also appears that lineaments are narrower and more widely spaced with time. The lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation. In contrast, the degree of cryovolcanic resurfacing appears to increase with time, as chaos formation dominates the later parts of the geologic record. These trends, and the transition from tectonic- to cryovolcanic-dominated resurfacing could be attributed to the gradual thickening of Europa's cryosphere during the visible geologic history, that comprises the last 2% or 30–80 Myr of Europa's history: An originally thin, brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes; the degree of fracturing and plate displacements decreased with time in a thickening shell, and lineaments became narrower and more widely spaced; formation of chaos regions could have occurred where the thickness threshold for solid-state convection was exceeded, and can be aided by preferential tidal heating of more ductile ice. In a long-term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite, or cyclic or irregular episodes of tectonic and cryovolcanic activity.
Astypalaea Linea is an 810-km strike-slip fault, located near the south pole of Europa. In length, it rivals the San Andreas Fault in California, and it is the largest strike-slip fault yet known on Europa. The fault was discovered using Voyager 2 images, based upon the presence of familiar strike-slip features including linearity, pull-aparts, and possible braids, and upon the offset of multiple piercing points. Fault displacement is 42 km, right-lateral, in the southern and central parts and probably throughout. Pull-aparts present along the fault trace probably are gaps in the lithosphere bounded by vertical cracks, and which opened due to fault motion and filled with material from below. Crosscutting relationships suggest the fault to be of intermediate relative age.The fault may have initiated as a crack due to tension from combined diurnal tides and nonsynchronous rotation, according to the tectonic model of R. Greenberg et al. (1998a, Icarus135, 64–78). Under the influence of varying diurnal tides, strike-slip offset may have occurred through a process called “walking,” which depends upon an inelastic lithospheric response to displacement. Alternatively, fault displacement may have been driven by currents in the theorized Europan ocean, which may have created simple shear structures such as braids.The discovery of Astypalaea Linea extends the geographical range of lateral motion on Europa. Such motion requires the presence of a decoupling zone of ductile ice or liquid water, a sufficiently rigid lithosphere, and a mechanism to consume surface area.
Right-lateral structural offsets of ∼25 km have been identified on the icy Galilean satellite Europa. These occur along dark lineaments oriented orthogonally to wedge-shaped bands, which are also ∼25 km wide. Wedge-shaped bands are interpreted as dilated tension fractures, which formed as crustal blocks (or plates) 50–100 km across separated and slipped past each other along flanking strike-slip faults. This style of deformation does not appear to be characteristic of other lineament types, and with the exception of Earth appears to be unique to Europa. Together, the subparallel wedge-shaped b bands form a broad NW-SE trending belt, ∼1500 km long and less than 500 km across, near the anti-Jovian point. This belt is interpreted as a major crustal fracture (or rift) zone, with a pole of rotation (determined by the strike-slip faults) near 47°S, 144°W, and an approximate NE-SW direction of maximum tensile stress. Extension may have been areally compensated at Agenor Linea, a bright band of possible compressional origin. Global expansion, tidal distortion, and nonsynchronous rotation do not explain the inferred minimum principal (i.e., least compressive) stress directions. Alternatively, fracturing near the anti-Jovian point may be a result of (i) solid-state convection in the lower ice crust (possibly triggered by uneven heat flow from the silicate interior), (ii) rotation of the icy shell about the sub- and anti-Jovian points induced by latitudinal lithospheric thickness variations, or (iii) preferential strain accumulation from the rest of the icy shell. No significant distortion of the crustal blocks occurred during fracturing and rotation, indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing. Decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere, not a silicate one, and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior (which we show is likely) unless the ice shell rotates independently. Decoupling could have been due to either warm ice or liquid water near the base of the icy crust. Mechanical bounds on lithospheric thickness (a few to ∼10 km) lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water.
This study shows dependence of the surface diurnal tidal stress on the internal structure of Europa. Its purpose is to investigate possibility of cracking of the icy shell. The stress is evaluated under the plausible model of the internal structure constrained by the gravity field data. The possible effective stress at the sub-Jovian point decreases with thickening of the shell, while it does not depend on the core radius and the thickness of the H2O layer. The range of this value is from 0.095 to 0.161 MPa, which does not exceed the tensile strength of ice. The stress required for the surface cracking would be mainly due to longer period deformations, especially non-synchronous rotation. And/or the actual strength of the ice at the surface would be smaller because of the preexisting cracks than that at a laboratory of the same temperature.
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 , 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.