Article

A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries

October 2005
Icarus 177(2):341-366

DOI:10.1016/j.icarus.2005.02.022

Authors:

University of Hawaiʻi at Hilo

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.

Tectonics of Europa

Chapter

Full-text available

Jan 2009

Europa has experienced significant tectonic disruption over its visible history. The descrip- tion, interpretation, and modeling of tectonic features imaged by the Voyager and Galileo mis- sions have resulted in significant developments in four key areas addressed in this chapter: (1) The characteristics and formation mechanisms of the various types of tectonic features; (2) the driving force behind the tectonics; (3) the geological evolution of its surface; and (4) the ques- tion of ongoing tectonics. We elaborate upon these themes, focusing on the following elements: (1) The prevalence of global tension, combined with the inherent weakness of ice, has resulted in a wealth of extensional tectonic features. Crustal convergence features are less obvious but are seemingly necessary for a balanced surface area budget in light of the large amount of ex- tension. Strike-slip faults are relatively common but may not imply primary compressive shear failure, as the constantly changing nature of the tidal stress field likely promotes shearing re- activation of preexisting cracks. Frictional shearing and heating thus contributed to the mor- phologic and mechanical evolution of tectonic features. (2) Many fracture patterns can be correlated with theoretical stress fields induced by diurnal tidal forcing and long-term effects of nonsynchronous rotation of the icy shell; however, these driving mechanisms alone prob- ably cannot explain all fracturing. Additional sources of stress may have been associated with orbital evolution, polar wander, finite obliquity, ice shell thickening, endogenic forcing by convection and diapirism, and secondary effects driven by strike-slip faulting and plate flex- ure. (3) Tectonic resurfacing has dominated the ~40–90 m.y. of visible geological history. A gradual decrease in tectonic activity through time coincided with an increase in cryomagmatism and thermal convection in the icy shell, implying shell thickening. Hence, tectonic resurfacing gave way to cryomagmatic resurfacing through the development of broad areas of crustal dis- ruption called chaos. (4) There is no definitive evidence for active tectonics; however, some tectonic features have been noted to postdate chaos. A thickening icy shell equates to a de- creased tidal response in the underlying ocean, but stresses associated with icy shell expansion may still sufficiently augment the contemporary tidal stress state to allow active tectonics.

Tectonics of Europa

Article

Nov 2007

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.

Constraints on Europa’s rotational dynamics from modeling of tidally-driven fractures

Article

Dec 2010

Cycloids, arcuate features observed on Europa’s surface, have been interpreted as tensile cracks that form in response to diurnal tidal stress caused by Europa’s orbital eccentricity. Stress from non-synchronous rotation may also contribute to tidal stress, and its influence on cycloid shapes has been investigated as well. Obliquity, fast precession, and physical libration would contribute to tidal stress but have often been neglected because they were expected to be negligibly small. However, more sophisticated analyses that include the influence of Jupiter’s other large satellites and the state of Europa’s interior indicate that perhaps these rotational parameters are large enough to alter the tidal stress field and the formation of tidally-driven fractures. We test tidal models that include obliquity, fast precession, stress due to non-synchronous rotation, and physical libration by comparing how well each model reproduces observed cycloids. To do this, we have designed and implemented an automated parameter-searching algorithm that relies on a quantitative measure of fit quality, which we use to identify the best fits to observed cycloids. We then apply statistical techniques to determine the tidal model best supported by the data. By incorporating obliquity, fits to observed southern hemisphere cycloids improve, and we can reproduce equatorial and equator-crossing cycloids. Furthermore, we find that obliquity plus physical libration is the tidal model best supported by the data. With this model, the obliquities range from 0.32° to 1.35°. The libration amplitudes are 0.72–2.44°, and the libration phases are −6.04° to 17.72° with one outlier at 84.5°. The variability in obliquity is expected if Europa’s ice shell is mechanically decoupled from the interior, and the libration amplitudes are plausible in the presence of a subsurface ocean. Indeed, the presence of a decoupling ocean may result in feedbacks that cause all of these rotational parameters to become time-variable.

Cycloid crack sequences on Europa: Relationship to stress history and constraints on growth mechanics based on cusp angles

Article

Jan 2008

The original model developed to explain cycloidal cracks on Europa interprets cycloids as tensile fractures that grow in a curved path in response to the constantly rotating diurnal tidal stress field. Cusps form when a new cycloid crack segment propagates at an angle to the first in response to a rotation of the principal tidal stress orientation during a period of no crack growth. A recent revised model states that a cycloid cusp forms through the creation of a secondary fracture called a tailcrack at the tip of an existing cycloid segment during shearing motion induced by the rotating tidal stress field. As the tailcrack propagates away from the cusp, it becomes the next cycloid segment in the chain. The qualitative tailcrack model uniquely accounts for the normal and shear stresses that mechanically must resolve onto the tip of an existing cycloid segment at the instant of cusp formation. In this work, we provide a quantitative framework and test of the hitherto purely conceptual tailcrack model. We first present a relative age sequence inferred from geologic mapping of multiply cross-cutting cycloids in Europa's trailing hemisphere and place this into the context of the global stress history. The age sequence requires a cumulative minimum of 630° of shell reorientation due to nonsynchronous rotation to account for the observed range of orientations of cycloids of different ages. We determined the back-rotated longitudes of formation of two cycloid chain examples and used mathematical modeling of europan tidal stresses to show that the tailcrack model for cusp formation is not only viable, but places constraints on the overall development of a cycloid chain by controlling the timing of cusp development within Europa's orbit. For all cusps analyzed, the exact ratio of resolved shear to normal stress required to form the cusp angles by a process of tailcracking, as governed by the principles of linear elastic fracture mechanics, is produced at the tip of a shearing cycloid segment during Europa's orbit. Cusp formation occurs after the point in the orbit at which the maximum tensile principal tidal stress occurs, implying that tensile tidal stresses are not directly responsible for cusp development. Instead, cusps develop when a tailcrack forms at the tip of a cycloid segment in response to the highly perturbed stress field induced during concomitant opening and shearing at the tip of the cycloid segment.

Regional Scale Tectonic Features at Argadnel Regio and Agenor Linea, Europa: Evidence in Support of a Plate Tectonic Paradigm.

Thesis

Full-text available

Sep 2021

Charlie Detelich

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.

Obliquity, Precession, and Fracture Mechanics: Implications of Europa's Global Cycloid Population

Article

Full-text available

Mar 2021

We catalog the global inventory of Europa's cycloids, arcuate fractures whose paths have been linked to diurnal tidal stress, and use the locations and orientations of their cusps to further test the formation mechanism of cycloids and constrain Europa's rotation state. We find that the global distribution of cycloids is better explained by a precessing spin pole than longitude translation due to nonsynchronous rotation, which is consistent with studies of Europa's strike‐slip faults and linear fractures. We also find that a small obliquity can reproduce the orientations of most cycloid cusps at their current locations, outperforming a model using stress from eccentricity alone. Matching cycloid locations and cusp orientations under these conditions implies a variable failure threshold and that most cusps form at stresses that are 50%–90% of the local peak tidal stress, suggesting that periodic deformation is causing failure through fatigue. The fact that cycloids are not observed in some regions, despite the low failure stresses implied by some cusp orientations, suggests an additional control on cusp formation that was not included in our simplified model. We hypothesize that the larger amplitude of tidal stress in some regions leads to fatigue‐induced failure after fewer tidal cycles, leading to a build‐up of cycloids in those regions. Why more linear fractures do not seem limited in location or density is still unknown. More physically informed constraints on how ice fails under low, periodic deformation and across generations of fractures would be helpful in interpreting Europa's cycloids.

Crevasse Propagation on Brittle Ice: Application to Cycloids on Europa

Article

Full-text available

Nov 2019
GEOPHYS RES LETT

Existing lineaments on the surface of the Jovian moon Europa are thought to be the result of ongoing brittle crack formation in the elastic regime. Arcuate features are called cycloids and can be modeled using linear elastic fracture mechanics. Here, we build on existing terrestrial models of rift propagation and extend them to cycloids on the moon. We propose that these cracks tend to grow as a series of nearly instantaneous events, spaced by periods of inactivity. The behavior is similar to what is observed on Antarctic ice shelves, where rifts can remain dormant for years. We argue that dormant periods between growth events could explain the presence of cycloids on Europa even without invoking secular motion of the crust. Furthermore, being able to model propagation events and their timing should help future missions exploring the moon.

Multistage evolution and variable motion history of Agenor Linea, Europa

Article

Apr 2014

Multi-Stage Dilational and Shearing History of Agenor Linea, Europa

Article

Full-text available

Mar 2013

Agenor Linea formed in at least three stages under different stress conditions. The first two stages were dilational; the third stage dextral transtension.

Science Potential from a Europa Lander

Article

Full-text available

Aug 2013
ASTROBIOLOGY

Abstract The prospect of a future soft landing on the surface of Europa is enticing, as it would create science opportunities that could not be achieved through flyby or orbital remote sensing, with direct relevance to Europa's potential habitability. Here, we summarize the science of a Europa lander concept, as developed by our NASA-commissioned Science Definition Team. The science concept concentrates on observations that can best be achieved by in situ examination of Europa from its surface. We discuss the suggested science objectives and investigations for a Europa lander mission, along with a model planning payload of instruments that could address these objectives. The highest priority is active sampling of Europa's non-ice material from at least two different depths (0.5-2 cm and 5-10 cm) to understand its detailed composition and chemistry and the specific nature of salts, any organic materials, and other contaminants. A secondary focus is geophysical prospecting of Europa, through seismology and magnetometry, to probe the satellite's ice shell and ocean. Finally, the surface geology can be characterized in situ at a human scale. A Europa lander could take advantage of the complex radiation environment of the satellite, landing where modeling suggests that radiation is about an order of magnitude less intense than in other regions. However, to choose a landing site that is safe and would yield the maximum science return, thorough reconnaissance of Europa would be required prior to selecting a scientifically optimized landing site. Key Words: Mission-Planetary science-Ice-Europa-Icy moon. Astrobiology 13, 740-773.

Interpretation and analysis of planetary structures

Article

Jun 2010
J STRUCT GEOL

Structural geology is an integral part of planetary science. Planetary structures provide the framework for determining the character and sequence of crustal deformation while simultaneously establishing the observational basis required to test geodynamic hypotheses for the deformation of planetary and satellite lithospheres. The availability of datasets that record spatial and topographic information with a resolution that matches or, in many cases, exceeds, what is available for Earth-based studies permits the deformation of several planets and satellites to be investigated down to the local or outcrop scales. The geometry and kinematics of common planetary structures such as joints, igneous dikes, deformation bands, faults, and folds can be determined with confidence from their distinctive morphologic and topographic signatures, enabling the structural histories and deformation magnitudes to be determined. Segmentation, displacement profiles, relay ramps, footwall anticlines, displacement-controlled depocenters, and other well-known characteristics of terrestrial normal fault and graben systems reveal the sequence and processes of fault growth in numerous planetary examples. Systems of thrust faults having both blind and surface-breaking components are important elements on several bodies including Mercury, the Moon, and Mars. Strike-slip faults have been identified on bodies including Mars and Europa with oblique extension found on Ganymede. Using field-based studies of Earth-based structures as a guide, planetary structures provide a means to explore and evaluate the causative stresses. Despite the wide range in structural styles across the solar system, plate tectonics is recognized only on the Earth, with the other planets and satellites deforming in the absence of large-scale horizontal motions and attendant plate recycling.

Searching for Jupiter and Saturn analog planets with potential icy exomoons -- a Panspermia perspective

Preprint

Full-text available

Jun 2024

Balazs Bradak

Considering the possibility of complex organic molecules and microbial life appearing under the ice shell of those satellites in the Solar system, this study investigates the possible analog sources (targeting the potential ice satellite hosting, Jupiter and Saturn-like planets in exoplanet databases) and the transport of such bioaerosols in an attempt to support or contradict Panspermia, a fringe theory about the fertilization of Earth. Along many general parameters of the candidate planets, the host star, and the star system, additional factors thought to be related to Panspermia were also considered (e.g., the evolution of icy satellites, the frequency of impact related ejection, the traveling time from a source, and so on), revealing the following results. Eleven exosystems, with candidate gas giants hosting icy satellites, were found in a database listing more than 5000 exoplanets. The exomoons of the oldest systems (c.a. > 8 Ga; HD 191939, HD 4203, and HD 34445) could have developed rapidly considering the short formation time (~100 Myrs), which may result in the overlap of the putative early biological evolution and asteroid bombardment phase, providing a higher chance to bioaerosol ejection to space due to frequent collisions. However, the direct transfer might have occurred too early, even before our Solar system was formed, which prevented the fertilization of the latter. A longer formation of the exomoons (~3 Gyrs) significantly reduces the chance of ejection into space by asteroid impacts, which become less frequent over time, but increased the chance of arrival in time to the Solar system. Younger systems, such as HD 217107, HD 219828, HD 140901, and HD 156279 (c.a. 4 to 6 Ga), are better candidates of putative microbe source if the short icy satellite formation, along the higher impact and ejection frequency are expected considering direct transport from those systems.

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

Article

Full-text available

Jun 2024
ROCK MECH ROCK ENG

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

Introduction to Dione’s Wispy Terrain as a Putative Model Region for “Micro” Wilson Cycles on Icy Satellites

Article

Full-text available

Oct 2023

The Wispy Terrain is the region of chasmata characterized by quasi-parallel fault systems, formed by extensional and shear stresses of the icy crust of Dione, a moon of Saturn. Besides the basic, satellite-scale geological mapping and very general definition of the phenomenon, only a few studies focus on the Wispy Terrain and its chasmata from the angle of detailed tectonic reconstruction, with others mainly targeting, e.g., the timing of its formation. This study provides a detailed geological and cryotectonic analysis in the surroundings of the Eurotas and Palatine Chasmata and proposes additional, until now, unidentified tectonic processes and a formation model. The relationship between fragmentary impact craters and tectonic features indicates other newly suspected tectonic movements, namely thrust, and splay and décollement fault systems. In contrast to the commonly expected and identified dilatational processes, such fault types show compression and are characteristic of subduction in a terrestrial environment. Theoretically, the appearance of such tectonic processes means that the already-known rift and the newly discovered subsumption (subduction-like) processes may appear together in the Wispy Terrain. The appearance of both features may suggest the presence of some of the components (phases) of a Wilson cycle analog cryotectonic cycle (or possibly cycles) in icy planetary bodies like Dione.

Tectonic analysis of a newly identified putative cryovolcanic field on Europa

Article

Aug 2023
ADV SPACE RES

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

LineaMapper: A deep learning-powered tool for mapping linear surface features on Europa

Article

Full-text available

Jul 2023
ICARUS

As discontinuities of the smooth icy surface, linear surface features might be directly or indirectly linked to Europa’s subsurface ocean. Mapping and categorising Europa’s lineaments is a means of retrieving information that could be linked to their formation history. As of today, planetary mapping is mainly conducted manually, which is tedious and subject to human bias once data sets become large. Mapping is further complicated by the heterogeneous quality and coverage of the available image data. Here, we train LineaMapper, a convolutional neural network (Mask R-CNN), to conduct instance segmentation of the four main units of linear surface features on Europa: bands, double ridges, ridge complexes and undifferentiated lineae. LineaMapper is trained on the basis of 15 mosaics from the Galileo solid-state imager data, yielding 930 training tiles. With LineaMapper, we provide a new method that facilitates detailed mapping of lineaments in Galileo images. LineaMapper could be applied to data to be returned by the Europa Imaging System (EIS) onboard the Europa Clipper mission. We validate LineaMapper v1.0 on an independent test set. On this test set, LineaMapper shows an overall higher precision than recall. In other words, there are more non-detections of actual lineaments than there are false detections of lineaments. The model shows the most correct predictions for double ridges (highest precision), while the most complete detections happen for ridge complexes (highest recall), compared with the ground truth. In some cases, LineaMapper preserves the cross-cutting relationships. The biggest strength of LineaMapper lies in its speed and tunable output. In the future, LineaMapper can be retrained, fine tuned and applied to similar looking features, for example wrinkle ridges on Venus, ridges on other planets and moons or even dust devil tracks on Mars

Deformation patterns of icy satellite crusts: Insights for tectonic balancing and fluid migration through structural analysis of terrestrial analogues

Article

Jun 2023
ICARUS

The formation of cone chains in the Chryse Planitia region on Mars and the thermodynamic aspects of this process

Article

Feb 2023
ICARUS

Separation of quasi-continuous and periodic components of lineament formation at the Belus – Phoenix - Rhadamanthys Linea “triangle” on Europa

Article

Feb 2023
ICARUS

The first images of Jupiter's moon Europa from the Voyager missions sparked the curiosity about the lineament system appearing at the surface. Curiosity quickly turned into profound interest, following the discovery of its subsurface ocean, which may harbour life below the thick ice crust. This study revisits Europa and reinvestigates its surface using high-resolution Galileo data and mapping one of its most characteristic surface patterns: the complex network of lineaments. The analysis of the morphological type of over two hundred lineaments and their crosscutting relationship based on relative age indicate the influence of cyclical tidal force, orbital forcing, and the nonsynchronous rotation triggered periodic stress in the formation of three characteristic lineaments-generations, along with additional and unaccounted forces that may contribute to surface renewal. Despite ongoing debate, which suggests that Earth-like tectonism (e.g., subduction) is less likely plausible on Europa, the result of this study calls for some process acting along with tidal forces, causing shortening, horizontal movement and increasing extension in the ice plate located in the region of a suspected subduction (or low-relief subsumption) zone and may be part of the “subducting” crust. This study raises new questions and encourages additional scientific discussions, which, along with the Europa Clipper and JUICE missions, will help to understand the nature of Europa's ocean and ice-tectonic processes.

Toward the identification of potential planetary-scale tidal-cryotectonic cycles in Europa

Poster

Jun 2022

More and more attention has been raising toward the icy moons of the Solar System since the discovery of their potential liquid water [1], and the astrobiology potential below their surface ice [2]. In addition, the ongoing preparation of JUICE (JUpiter ICy moons Explorer) mission is boosting the research as well. Before the hopefully successful JUICE mission, the re-evaluation of the existing data from new viewpoints may help to understand some segment of the cryotectonic processes and its relation to the subsurface ocean of Europa. One of the many research topics, connected to the study of Europa, the second Galilean satellites of Jupiter is the interpretation of the cryo-tectonic features on its surface and their relation to tidal processes triggered by Jupiter [3] and moon-moon interaction as well [4]. Following the detailed studies around the early 2000s, [5, 6] the goal of this research is to revisit Europa and re-investigate its surface by mapping one of its most characteristic surface patterns and test an approach toward an established “linea-stratigraphy” and the identification of planetary-scale tidal-cryotectonic cycles. The base map of the studied area is used trough the “Planetary WMS service hosted by Astrogeology, USGS of Jupiter's moon Europa” [7]. The focus of the research is the complex lineage system in various locations, including the Awnn and Balgatan Regio and the neighborhood of the Belus – Phoenix - Rhadamanthys (BPR) Linea triangle [8]. Based on the recent geological map of the moon [9], the regions are defined by “ridged plains” with “bands”, “high albedo bands”, “undifferentiated lineas” and “chaos” terrains. During the separation of the linea generations at the BPR triangle location (source image: PDS image atlas, Galileo mission; image 1865r) , parameters, such as the i) physical-morphological appearance; ii) “linear feature stratigraphy”, i.e., the appearance of overlapping and transform faults; iii) impact crater density; and iv) unique morphological indicators were considered. Under- (“stratigraphically older”) and overlying (“stratigraphically younger”) lines were counted in the case of the sampled linear features from Awnn and Balgatan Regio (only lower resolution image was avaiable) and the ratio of the two parameters were calculated. To avoid the possible bias due to the length of the lines, the length parameter was used on the calculated over/underlying lines ratio as a normalizer to decrease its possible effect on the results. Furthermore the distribution of the normalized ratio was analyzed by histograms (Fig. 1a and b). Theoretically, the distribution peaks on the histogram indicates characteristic groups of linea, which belongs to the same linea-generation, i.e., formed at the same time and are overlaying and underlaid by similar amount of other linea. Based on the distribution peaks in the histograms seven and nine possible ratio-categories were separated. These categories may represent stratigraphical units, which indicates various linea-generations (Fig. 1a and b). Various allocation and linea morphology were identified, such as the regional-scale “cycloidal ridges” [3] and smaller asymmetrical Y fault patterns, which may indicate quasi-circular shape dome forming-like processes [10]. The observed linea-generations at Awnn and Balgatan Regio may indicate regional(/palenetary)-size, repeating bulge of the ice-crust, possibly triggered by the allocation of water-mass in the subsurface hydrosphere below the ice shell of Europa, driven by “tidal-pumping” [3] in daily basis and, based our results, in long-term tidal periods as well.

The Geological History of Enceladus

Chapter

Jan 2018

Activation of preexisting transverse structures in an evolving magmatic rift in East Africa

Article

Apr 2018
J STRUCT GEOL

Inherited crustal weaknesses have long been recognized as important factors in strain localization and basin development in the East African Rift System (EARS). However, the timing and kinematics (e.g., sense of slip) of transverse (rift-oblique) faults that exploit these weaknesses are debated, and thus the roles of inherited weaknesses at different stages of rift basin evolution are often overlooked. The mechanics of transverse faulting were addressed through an analysis of the Kordjya fault of the Magadi basin (Kenya Rift). Fault kinematics were investigated from field and remote-sensing data collected on fault and joint systems. Our analysis indicates that the Kordjya fault consists of a complex system of predominantly NNE-striking, rift-parallel fault segments that collectively form a NNW-trending array of en echelon faults. The transverse Kordjya fault therefore reactivated existing rift-parallel faults in ∼1 Ma lavas as oblique-normal faults with a component of sinistral shear. In all, these fault motions accommodate dip-slip on an underlying transverse structure that exploits the Aswa basement shear zone. This study shows that transverse faults may be activated through a complex interplay among magma-assisted strain localization, preexisting structures, and local stress rotations. Rather than forming during rift initiation, transverse structures can develop after the establishment of pervasive rift-parallel fault systems, and may exhibit dip-slip kinematics when activated from local stress rotations. The Kordjya fault is shown here to form a kinematic linkage that transfers strain to a newly developing center of concentrated magmatism and normal faulting. It is concluded that recently activated transverse faults not only reveal the effects of inherited basement weaknesses on fault development, but also provide important clues regarding developing magmatic and tectonic systems as young continental rift basins evolve.

Double ridges on Europa accommodate some of the missing surface contraction

Article

Mar 2014

[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.

Identifying Contraction and Expansion Along Double Ridges and Bands on Europa with StrikeSlip Displacements

Conference Paper

Full-text available

Mar 2013

Introduction: Stresses from diurnal tides and possibly from non-synchronous rotation cause Europa’s surface to expand during high tide and contract during low tide. These stresses may be sufficient to create cracks [1] that ultimately lead to the development of the various lineaments preserved on the surface. A variety of models have been proposed to create lineaments, including diapirism [2], volumetric expansion to make bands [3], folding of ductile ice [4] shearing to make ridges [5], opening-mode fractures [6], and shearing of opening mode fractures [7]. Each of these predicts different lineament-normal displacements. Our objective is to quantify such displacements, and their variation along strike and with tectonic feature. Method: kinematics We exploit cross-cutting relationships to map both strike-slip and normal displacements. Faults were mapped using images collected by the Galileo SSI spacecraft. In particular, we measure the relationship between the main lineament and the older lineaments it crosses. Variation in the offset of crossing lineaments allows derivation of both the magnitude and direction of along-strike and normal displacement of the main lineament. On geometric grounds we expect !!! = !! + !cos (!!) (1.1) where !!! is the total (measured) offset, !! is the strike-slip offset, !! is the incidence angle of the crossing feature, and ! is the lineament-normal expansion or contraction component. The strike- slip segment is positive if the lineament is a right- lateral strike-slip fault and negative if it is left- lateral. The component of the normal displacement (!) is positive if the lineament has expanded and negative if it has contracted. Total amount of expansion for n features is given by: ! = !sin (!) (1.2) where sin (!) is the average of sin (!!) for all incidence angles. Positive ! is expansion and negative ! is contraction. Coulter performed a similar analysis to identify the slip and feature- normal displacement for ridges, though not bands [8]. Our analysis differs in the approach to fit models to the data and hence the displacements that are best resolved. Results: We have mapped 8 lineament systems (some are displayed in Figure 1) and have observed a correlation between lineament morphology and strike-slip/normal displacement. Bands typically record divergence while double ridges preserve convergence. There is no observed variation in strike-slip displacement with distance along the lineament, implying that cumulative strike-slip displacement is not a key variable. Double ridges. Double ridges show right or left lateral strike-slip with convergence.Lineament 1 is a right-lateral double ridge. It shows contraction, ! = −0.36 ± 0.11 km. Bands: Bands also express both right and left lateral motion and involve a component of expansion. Ridge Bands show expansion and strike-slip as well, suggesting a similar formation mechanism to normal bands. It is recognized that normal bands are formed by expansion and strike- slip faults [1][2][3][4][7]. Our results confirm this. If a similar mechanism formed both morphologic classes, one may represent an older or faster forming unit. As an example, lineament 2 is a cycloid that later experienced dilation. The center of the lineament appears as a double ridge, but it only intersects 2 lineaments, so it is still a good example of a band. There is evidence that the lineament is expanding, ! = 3.40 ± 0.62 km. The main difference between ridges and bands, in addition to showing convergence vs. divergence, is that the magnitude of normal displacement, ! , is much larger for bands than for ridges. Hoppa et al. (1999, 2000) noted that lineaments above 35°N are left lateral and those below are right lateral [9]. Our results are consistent with these findings. Discussion: Our measurements indicate that what ever process created double ridge lineaments led to net contraction. It is not clear that any of the proposed mechanisms for making ridges produces contraction. Mechanisms involving eruption [10], diapirism [2], folding [4], or tidal pumping of water to the surface [11] should also produce extension; the shear-heating model [5] does not predict lineament normal displacement. Coulter [8] did not find that double ridge features have the same kinematics, with some displaying extension, others contraction. Ridge bands visually appear as multiple double ridge lineament lined up parallel to each other, but the results show that the feature is expanding (whereas double ridge features appear to be contracting). Lineament 2, ridge bands, and other mapped bands have large strike-slip offsets, supporting the notion [12] that shearing plays a role in forming ridges within bands. Mapping multiples of each type of band will reveal which parameters form each type of band. Mapping additional lineament systems will help to elucidate additional correlations between morphology and geometry, leading to a detailed description of lineament formation mechanisms. References: [1] Hoppa et al. (1999) ICARUS, 141, 287-298. [2] Prokter, L. M. et al. (2002) Journal of Geophysical Research, 107, 26. [4] Manga, M. and Sinton, A. (2004) Journal of Geophysical Research, 109, 15. [5] Nimmo F. and Gaidos E. (2002) Journal of Geophysical Research, 107, 5021. [6]Coulter, C. E. (2009) Masters Thesis, Univ. of Idaho. [9] Hoppa et al. (2000) Journal of Geophysical Research, 105, 22617-22627. [10] Fagents, S. A. (2003) Journal of Geophysical Research, 108, 19. [10] Greenberg, R. (2002) Reviews of Geophysics, 40, 19. [12] Gaidos, E. J. and Nimmo, F. (2000) Nature, 405, 637.

Ridges on Ridges: Extensions of Adjacent Topography onto the Flanks of Europan Ridges

Article

Oct 2012
EARTH PLANET SC LETT

At some locations along ridges on Europa, older ridges on adjacent terrain appear to extend up the flank of a more recent ridge. It has thus been suggested that the ridges may have formed by upturning of that adjacent terrain. However, the newer ridges generally appear to be material deposited over the older terrain. How might material that was pushed over and buried the earlier surface have inherited the topography of the underlying material? At some sediment-starved subduction zones on Earth, where the poorly consolidated material of a frontal prism of an overriding plate is pushed over preexisting ridges and seamounts on the downgoing plate, the overriding plate inherits the morphology of the downgoing plate even though the actual extension of that topography has been underthrust and buried. A well-studied example lies offshore of Costa Rica where the Caribbean plate overrides the Cocos plate. Experiments show other mechanisms as well: Mass-wasting down a flank can result in extensions of adjacent ridges thanks to the geometry imposed by a constant angle of repose. More pronounced extensions of the older ridges result if the new ridge grows as it is bulldozed from behind (i.e. from the central groove of a double ridge on Europa). The shapes of the ridge extensions are distinctly different in these latter two cases. If tidal pumping extrudes material to the surface at the center of a double ridge, it might drive the latter mechanism. The ridge extensions observed on the flanks of newer ridges may be a definitive, and perhaps crucial, diagnostic of dominant ridge-building mechanisms when additional images are obtained at high resolution from future exploration. In additional to their morphology, the distribution of ridge extensions at only isolated locales may also provide important constraints on the diversity of ridge formation processes.

Geodynamics of Europa's Icy Shell

Article

Full-text available

Jan 2009

Processes that operate within Europa's floating icy shell and leave their signature on the surface are largely governed by the thermal and mechanical properties of the shell. We review how geodynamic models for icy shell dynamics can be integrated with observations to constrain the present and past properties of the icy shell. The near-surface exhibits brittle or elastic behavior, while deeper within the shell viscous processes, including tidal heating, lateral flow, and possibly convection, dominate. Given the large amplitude of topography on Europa, the icy shell is probably more than several kilometers thick at present. However, there are both theoretical models and observational evidence suggesting that the icy shell has been thickening over time, explaining the predominance of extensional features and the young (probably ~50 m.y.) surface age. Geophysical measurements on future missions will be able to determine the present thickness of the icy shell, and possibly its thermal structure.

A fracture history on Enceladus provides evidence for a global ocean

Article

Full-text available

Sep 2011

The region surrounding the south pole of Saturn's moon Enceladus shows a young, pervasively fractured surface that emanates enough heat to be detected by the Cassini spacecraft. To explain the elevated heat and eruptive icy plumes originating from large cracks (informally called ``tiger stripes'') in the surface, many models implicitly assume a global liquid ocean beneath the surface. Here we show that the fracture patterns in the south-polar terrain (SPT) of Enceladus are inconsistent with contemporary stress fields, but instead formed in a temporally varying global stress field related to nonsynchronous rotation of a floating ice shell above a global liquid ocean. This finding increase to at least three the number of outer planet satellites likely to possess a subsurface liquid water layer.

Fault-induced perturbed stress fields and associated tensile and compressive deformation at fault tips in the ice shell of Europa: implications for fault mechanics

Article

Dec 2006
J STRUCT GEOL

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.

Plate motion on Europa and nonrigid behavior of the Icy lithosphere: The Castalia Macula region

Article

Apr 2006
J STRUCT GEOL

We have developed a generalized quantitative technique for determining the finite pole of rotation between two rigid plates and use it to critically examine differing reconstructions of a region surrounding a prominent dark spot on Europa, Castalia Macula. This region is located near the equator of Europa's trailing hemisphere and has been suggested as a site where crustal convergence may have occurred. Previous reconstructions of the region have indicated that a ridge set and/or a band-like complex that define a collection of tectonic plates in the region accommodated surface contraction. However, a critical examination of the differences between these reconstructions has been complicated by the lack of a finite pole of rotation for the plates involved in either reconstruction. We have applied our modeling technique, coupled with a detailed examination of the morphology and cross-cutting relationships involving this ridge set and band-like complex, to determine if a unique reconstruction exists for several tectonic plates in this region. The cross-cutting relationships involving the ridge set also allow us to test the general assumption that plates behave rigidly on Europa. Assuming rigid behavior, our results suggest that a unique reconstruction does exist, indicating the ridge set accommodated surface contraction. However, analysis performed to test the assumption of plate rigidity indicates that one or more of the plates in the region did not behave rigidly. This does not rule out surface contraction along the ridge set but does indicate that a component of nonrigid behavior must be considered.

Cycloidal cracks on Europa: Improved modeling and non-synchronous rotation implications

Article

Jan 2007
ICARUS

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.

Segmented lineaments on Europa: Implications for the formation of ridge complexes and bright bands

Article

Feb 2010

We describe several segmented lineaments on Europa’s surface. These lineaments are extensive, stretching for 100s–1000s of km, and have ridge complex or bright band morphologies. The geometries of the segmented portions of these features are diagnostic of the remote normal and shear stress environment in which they formed and, therefore, constrain ridge complex and bright band formation mechanisms. Analysis of four ridge complexes indicates that they formed in a remote normal stress environment that was tensile and isotropic (or nearly so) and that these lineaments may have formed in a manner more analogous to bands on Europa than to ridges. The stress environment associated with these ridge complexes may also explain the anastomosing nature of their interior morphology. Analysis of two bright bands indicate that one formed in a remote normal stress environment that was tensile and the other was reactivated under a combination of remote tensile normal stress and remote sinistral shear stress. Aspects of the morphologies of these features also indicate that bright bands likely have complex deformation histories that can include multiple episodes of reactivation.

Evidence and concerns about a latent, embryonic phase tectonic evolution and the existence of the young subsurface ocean on Mimas

Article

Aug 2024
ICARUS

The icy moons of outer Solar system gas and ice giants have been in the spotlight of scientific and common interest because of the possibility of a subsurface ocean hidden under their ice shells, which may harbor extraterrestrial life. The patterns of various lineaments on the surface of icy satellites may indicate active tectonic processes and the interaction between the surface and the subsurface ocean, which allows material transport toward the habitat of putative alien life. In the case of Mimas, one of the moons of Saturn, new models challenge the long-standing conclusion about the satellite being an inactive snowball, suggesting the existence of a young ocean hiding under its ice shell. Unfortunately, no observable evidence has been found implying ongoing or probably already stopped tectonic activity and the theoretical subsurface ocean. Here, we present the first structural geological map of the icy satellite, with the signs of various tectonic features, along with a simple crosscutting chronology of lineaments formation. In accordance with the theoretical young age of the subsurface ocean, the observed phenomena are described as putative lineaments, ridges, and troughs. Such simple tectonic features are identified as young compared to complex structures, such as bands appearing on other satellites. The pattern of the linear features seems to overlap with the allocation of various modeled global nonlinear tidal dissipation patterns. In such a way, it may indicate the possible existence of the theoretical subsurface stealth ocean. With such an evolving young subsurface ocean and the barely recognizable pattern of simple lineaments, Mimas may represent a newly recognized group of icy satellites showing the early, latent, embryonic phase of tectonic activity. In contrast, the overlapping and crosscutting relations between craters and the observed features may raise some concerns about the “recent” formation of such linear features, indicating possibly long-time dormant or already stopped tectonic processes at the very early, embryonic phase of lineament formation billions of years ago. Despite the possible additional triggers of tectonic processes on icy satellites (e.g., surface crust mobility by plastic subsurface deformation), the presented geological investigation brought a new angle and additional evidence about a possible stealth ocean hiding under the crater-covered surface of Mimas, regardless of its geological age and recent state of evolution. Undoubtedly, the results and the raised concerns will trigger more intense investigations on Mimas and other, similar icy satellites in the Solar system.

The formation of Enceladus' Tiger Stripe Fractures from eccentricity tides

Article

Aug 2020
EARTH PLANET SC LETT

Enceladus has a young, tectonically active south polar region, which is erupting material from a prominent set of fractures called Tiger Stripes. No comparable activity is observed at the north pole, which is heavily cratered with limited tectonism. Given the many lines of evidence supporting a global ocean under Enceladus' icy shell, the reason for the dichotomy in geologic activity is unclear. We model the formation of the Tiger Stripes as tidally-driven fractures and examine the magnitudes of tidal stresses with different ice shell structures in order to explore whether and how tidal stress might explain Enceladus' distribution of tectonic activity. We find that eccentricity-driven tidal stresses would produce fractures of nearly identical orientations to the observed Tiger Stripe Fractures and that a 10-km difference in ice shell thickness between the north and south poles can result in substantially different tidal stress magnitudes, providing a natural explanation for the hemispheric dichotomy in tectonic activity on Enceladus. Finally, we synthesize these results with Enceladus' global geologic record to offer insight into the evolution of this enigmatic moon.

Implications of nonsynchronous rotation on the deformational history and ice shell properties in the south polar terrain of Enceladus

Article

Mar 2019

Numerous studies of the south polar region of Saturn's moon Enceladus have focused on the four main fractures (the tiger stripes), their associated water plume, and the effects of diurnal tidal stresses on these fractures. However, due to their small magnitude (10–100 kPa), diurnal stresses alone are unable to completely explain the initial formation of the tiger stripes and the many additional fractures in the region. We suggest nonsynchronous rotation (NSR) stresses, induced by a freely rotating ice shell over a global ocean, are large enough to overcome the tensile strength of the ice shell and initiate fracturing on Enceladus. Using the tidal stress calculation program SatStressGUI, we demonstrate the dependence of the magnitude of the NSR stress on the thickness of the ice shell, particularly the brittle outer layer of the ice shell. We suggest the brittle layer in the south polar region is 2–4 km thick whereas in the more northern regions, it is 6–8 km. The difference is most likely due to the elevated energy flux at the south pole, compared to the regions to the north, resulting in a warmer and thinner ice shell in the south polar region. The thinner ice around the south pole permits NSR tensile stresses of ∼5 MPa, enough to fracture the ice. However, the thicker ice to the north reduces the NSR stress to less than 2 MPa making fracturing less likely. The difference in ice shell thickness, and resultant NSR stress, can help explain the activity in the south, but relative quiescence in the north. Our modeled NSR stress magnitudes and orientations suggests a NSR period on the order of 1 Myr and advocate that the tiger stripes have rotated ∼45° clockwise about the south pole since their initial formation in response to NSR of the ice shell. If the ice shell is rotating at a constant rate, the tiger stripes would thus be up to ∼100,000 years old. An apparent decreasing angular separation between consecutive fracture sets that formed in the south polar region suggests an ice shell that has weakened in strength over the course of its recent geologic history; this could be a result of increased heating, thinning of the ice shell, or a combination of the two. As the ice shell thinned, the NSR stresses were enhanced, making fracturing more likely after a smaller amount of NSR compared to the previous fracturing episode.

Cycloid (Europa)

Chapter

Jan 2015

Alyssa Rhoden

The distribution and characterization of strike-slip faults on Enceladus

Article

Mar 2016

Emily S. Martin

Strike-slip faulting is typically characterized by lateral offsets on icy satellites of the outer solar system. However, strike-slip faults on Enceladus lack these typical lateral offsets and instead are marked by the presence of tailcracks or en echelon cracks. These features are used here to develop the first near-global distribution of strike-slip faults on Enceladus. Strike-slip faults on Enceladus fall into three broad categories: tectonic terrain boundaries, reactivated linear features, and primary strike-slip faults. All three types of strike-slip faults are found predominantly, or within close proximity to, the antipodal cratered terrains on the Saturnian and anti-Saturnian hemispheres. Stress modeling suggests that strike-slip faulting on Enceladus is not controlled by nonsynchronous rotation, as on Europa, suggesting a fundamentally different process driving Enceladus's strike-slip faulting. The motion along strike-slip faults at tectonic terrain boundaries suggests large-scale northward migration of the ice shell on the leading hemisphere of Enceladus, occurring perpendicular to the opening direction of the tiger stripes in the south polar terrain.

Cycloid (Europa)

Chapter

Sep 2014

Alyssa Rhoden

DefinitionChains of arcuate segments linked by cusps (Kattenhorn and Hurford 2009)Related TermsCycloidal ridge, Cycloidal crack, Cycloidal band, FlexusDescriptionA cycloid is a series of linked arcuate segments. Cycloids have been observed on Europa at both global and regional scales and with a variety of morphologies. Some appear to be hairline fractures, while others have well-developed flanking ridges. Several wide bands have been identified with the characteristic linked segment morphology of cycloids, and many of these also exhibit strike-slip offsets (Kattenhorn and Hurford 2009, and references therein).MorphometryWidths similar to other cracks, ridges, and bands. Ridges consisting of multiple segments up to >1,000 km long (Fig. 1).Fig. 1Three southern hemisphere cycloids that have been modeled using tidal stress (Hoppa et al. 1999; Hurford et al. 2007; Rhoden et al. 2010). These cycloids also exhibit well-developed flanking ridges, and their characteristic cusp morphology is hig ...

Fracture and Stress Evolution on Europa: New Insights Into Fracture Interpretation and Ice Thickness Estimates Using Fracture Mechanics Analyses. Principal Investigator

Article

Simon Kattenhorn

Preamble This report serves as the final technical report for the above mentioned project. The grant period began on January, 1,2002 and finished on June 3 0 ~ , 2004. The project personnel consisted of the PI, a co-investigator who served in an advisory capacity, and two full-time graduate students who completed MS thesis projects funded by the grant. The project ultimately led to the development of 6 full-length papers (1 published, 1 in press, 2 acceptedin revision, and 2 in preparation), 9 conference presentations with abstracts, and 1 invited seminar presentation at the Lunar and Planetary Institute in Houston, Texas. The success of the project, as outlined below, has enabled the strengthening of the planetary geoscience research program in the Department of Geological Sciences at the University of Idaho. I have successfully competed for two additional grant awards through the NASA-Idaho Space Grant Consortium and NASA-EPSCoR, and have developed national recognition through collaborations and interactions with numerous peer researchers. Furthermore, my planetary geoscience research program now attracts several graduate student applicants per year and has acquired two additional current graduate students working on research projects (one as an MS thesis and one as a PhD dissertation). A follow-up grant proposal is pending at NASA PG&G.

Lineament azimuths on Europa: Implications for obliquity and non-synchronous rotation

Article

Sep 2013

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.

Implications of Cycloid History in the Northern Trailing Hemisphere of Europa

Article

Jan 2006

A CW rotation of cycloids through time is compatible with 600 degrees of nonsynchronous rotation of the Europan ice shell. Up to eight cycloids formed in a single rotation cycle. Non-cycloidal cracks continued to form after cycloids first developed.

Searching for Evidence of Active Tectonics on Europa

Article

Mar 2009

Evidence of recent tectonic activity on Europa logically starts with the geologically young, ridgeless surface fractures. The temporal relationship between young fractures and their orientations could yield information about recent tectonic activity.

Europa's icy shell: Past and present state, and future exploration

Article

Oct 2005
ICARUS

After nearly a decade of investigation, it is becoming increasingly clear that the geologic history of Europa's surface and its floating ice shell are considerably more complex than originally thought when Galileo returned its first images in 1996. The shell is compositionally and perhaps mechanically heterogeneous, and has a thickness that has probably varied laterally and over time. The evolving shell thickness depends on the thermal and thus orbital evolution of the satellite, and exerts a major influence on the style of ice shell deformation. Thus, it may be ultimately possible to use geological observations to constrain the orbital history of Europa's good demonstration of the cross-disciplinary nature of planetary sciences. As the reports presented here make clear, the Galileo data have provided important clues to Europa's evolution, and will continue to do so for the foreseeable future. At the same time, many of the lessons learned from the Galileo mission will be directly applicable to the saturnian satellites, currently being investigated by the Cassini spacecraft. Nonetheless, it is clear that many questions of fundamental importance to Europa will remain unresolved with the available data. Mapping coverage of Europa's surface is uneven (in the extreme), ranging from 10 m in a few locations to ∼1-4 km over 75% of the surface. Topographic data is even more limited (and based solely on stereo and shape-from-shading). Infrared mapping spectroscopy is limited and resolved gravity data are non-existent. Chief among the unresolved questions are the lateral and temporal variability of the (floating) icy shell, and its current state (including thickness). The mechanical and thermal properties of ice are also relevant to understanding the observed geologic features but are relatively poorly understood and require dedicated investigation. Our understanding of these properties will also influence the ability of radar sounding to penetrate an ice shell of uncertain composition (e.g., Chyba et al., 1998). These questions, coupled with Europa's potential habitability, are likely to ensure that a return to Jupiter and Europa in particular remain at the top of NASA's exploration priorities.

Determination of ice crust thickness from flanking cracks along ridges on Europa

Conference Paper

Full-text available

Mar 2002

Introduction: The response of a planetary lithosphere to an applied load, such as a mountain or range of mountains, is regularly modeled as a plate deflecting under a mathematically defined load. The deflection caused by the load depends on the strength of the plate, which is defined by elastic parameters of the lithospheric material and by its thickness [1, 2]. A "forebulge", where the crust warps upward at the surface, is characteristic of the deflection profile. The forebulge location has been used to characterize the deflection profile and to calculate crustal thickness at the time of load emplacement for locations on Earth [3-5], Mars [6, 7], the Moon [6], and Europa [8, 9]. However, the exact location of a forebulge crest can be difficult to identify in planetary surface images. Stresses caused by the downward deflection of the lithosphere can produce more readily identifiable ex-tensional features such as cracks and graben between the forebulge and the load. The locations of maximum stresses can be correlated with the location of exten-sional features [10]. This correlation allows litho-spheric elastic thickness at the time of load emplacement to be calculated. On Europa, the exact thickness of the ice crust is a contentious issue. Various methods have been em-ployed to determine thickness, with results ranging from 0.2-30 km [11]. Lithospheric Flexure Due to Line Loading: Some of the many ridges of Europa are expected to load the ice crust enough to cause deflection [8, 12, 13]. Prior to the Galileo mission, Pappalardo and Coon [12] used a line load model developed by Turcotte and Schubert [2] to hypothesize that cracks might be seen flanking ridges caused by the load of the ridges, and that the flanking cracks would be located ~15 to 35 km from the ridge if the lithospheric thickness is ~2 to 6 km. More recent studies used the same method to determine the elastic thickness of Europa's ice crust at two locations in Conamara Chaos: (1) 100 m to 500 m at a mound load [9]; and (2) 123 m to 353 m at a ridge load ("Ridge R") [8]. In both cases, these estimates of ice crust thickness were determined using the distance from the load to the forebulge, which is difficult to locate precisely in Europa images. Deflection and Stress Due to Line Loading: For a line load on a broken plate (such as is produced by a ridge), the deflection profile is w=w0e-x/acos(x/a) [2], where w0 is the maximum deflection at the load, x is the distance from the ridge, and the flexural parameter a =[Eh3/(3rg(1-n2)]1/4. The flexural parameter comprises Young's modulus E, the thickness of the plate h, density of the plate r, the gravitational acceleration on the satellite g, and Poisson's ratio n. Maximizing the deflection equation gives the distance to the forebulge as xb =3πa/4. The stress profile is obtained from the deflection using the relationships: (1) strain e=-y(d2w/dx2), where y is the horizontal distance from the center of the plate (downward is positive); and (2) stress s=e E/(1-n). The stress profile can then be maximized to find the distance from the load to the maximum tensile stress. For a broken lithosphere, this maximum tensile stress occurs at the surface at xs=πa/4. Tensile features are most likely to form at xs and can be used to determine the plate thickness h by rearranging the above equations to obtain h=(3(4xs/p)4rg(1-n2)/E)1/3. Stress and Flexure on Europa: Deflection due to line loading and the induced stress at the surface are shown for three hypothetical crustal thicknesses in Figure 1. In these calculations, appropriate values have been used for Europa: E is 6x109 Pa, n is 0.3; g is 1.35 m/s and r is 1186 kg/m3 [9]. The stress profiles in Figure 1 (reddish colors) show that maximum tensile stress occurs closer to the load than does the forebulge (deflection profiles in blue). These regions of maximum stress are most likely to fail in tension, resulting in cracks. Regions of surface compression beyond the forebulge could manifest in folding or strike slip motion along preexisting cracks. Additionally, since the stresses at the bottom of the plate are of the same magnitude but of opposite sign, tensile stresses would occur at the bottom of the plate at this location. However, these tensile stresses would probably be too small to overcome lithostatic stresses and cause fracturing from below unless liquid water is present below the ice crust with pressures approaching lithostatic. Lithospheric Thickness Calculations: Androgeos Linea in the Bright Plains region (Figure 2), with clearly defined flanking cracks, has been identified as an example of a line load causing lithospheric flexure [8, 13, 14]. In this analysis, the distances from the center of the ridge to the flanking cracks were measured in five locations and used to calculate crustal elastic thickness at the time of crack formation. Using the potential ranges of E for the Europan ice crust of 6x107 Pa to 6x109Pa [9], calculated ice thickness ranges from 468 m to 2530 m. Tufts [8] identified two additional ridges with flanking cracks, which he called Ridge R and Ridge C2r. Ridge R is located at 8.4N, 271W and was imaged during the E6 orbit of the Galileo spacecraft. Using the forebulge distance, xb, Tufts determined the crustal elastic thickness at Ridge R to be in the range 123 m to 353 m. We have used the more precisely measurable distance to the flanking cracks, xs, at three locations along the ridge, and a broader range of crustal elastic moduli, as described above, to calculate the ice crust thickness to fall in the range 191 m to 1119 m. Ridge C2r, at 4.7N, 325.7W, was imaged during Galileo orbit E4. While Tufts noted the flanking cracks, no calculations of ice crust thickness were pre-sented. By using the flanking cracks as markers for the xs distance at four locations, we calculated an elastic thickness range of 421 m to 2633 m. Results Summary: A summary of the results from calculations at the three locations studied is given in the following Table: Location xs ave (m) h max (m) h min (m) Androgeos Lin., 14.7N, 273.4W 2926 2530 468 Ridge R, 8.4N, 271W 1134 1119 191 Ridge C2r, 4.7N, 325.7W 2687 2633 421 Discussion: A broken lithosphere was selected for these calculations based on the ridge formation model of Greenberg et al. [13]. If the ridges are formed by other processes and still create a load on the litho-sphere, a continuous plate, which can support more than twice the load of a broken plate, may provide a better model. In such a scenario, ice crust thicknesses are smaller, ranging from ~200 m to 1 km (Androgeos and Ridge C2r) and from ~85 m to 400 m (Ridge R). If the ridges are formed by other processes such as diapiric uplift [15, 16], then the loading on the plate would be imposed from below and flexure would fol-low a different pattern. The thickness of the ice crust as calculated here represents the thickness at the time of ridge formation and relates only to the thickness of the elastic portion of the crust. It does not address the thickness of a ductile layer below the crust, if one exists. References: [1] Urgural, A.C. (1999) Stresses in Plates and Shells, pp. 71–95. [2] Turcotte, D.L., and Schubert, G. (1982) Geodynamics, pp. 112–129. [3] Harris, R.N. and Chapman, D.S. (1994) JGR. 99, 9297–9317. [4] Lambeck, K., and Nakibouglu, S.M. (1980) JGR, 85, 6403–6418. [5] Caldwell, J.G., et al. (1976) Earth & Planet. Sci. Let. 31, pp. 239–246. [6] Shultz, R.A. and Zuber, M.T. (1994) JGR 99, 14,691–14,702. [7] Banerdt W.B., et al (1992) Mars, pp. 249-297. [8] Tufts, R.B. (1998) Lithospheric Displacement Features on Europa and their Interpretation. [9] Williams, K.K. and Greeley, R. (1998) Geophys. Res. Let. 25, 4273–4276. [10] Comer, R.P. et al. (1985) Rev. of Geophys. 23, 61–92. [11] Pappalardo., R.T., et al. (1999) JGR 104 24,015–24,056. [12] Pappalardo, R. and Coon, M.D. (1996) LPSC XXVII. [13] Greenberg, R., et al. (1998), Icarus 135, 64–78. [14] Kattenhorn, S.A. (2001) LPSC XXXII. [15] Head, J.W., et al. (1999) JGR 104, 24,223-24,236. [16] Pappalardo, R.T., and Head, J.W. (2001) LPSC XXXII, 1866. Acknowledgements: This work was supported by a fellowship from the NASA–Idaho Space Grant Consortium.

Secondary fracturing of Europa's crust in response to combined slip and dilation along strike-slip faults

Conference Paper

Full-text available

Mar 2003

Simon A. Kattenhorn

Introduction: A commonly observed feature in faulted terrestrial rocks is the occurrence of secondary fractures alongside faults. Depending on exact morphology, such fractures have been termed tail cracks, wing cracks, kinks, or horsetail fractures [1, 2], and typically form at the tip of a slipping fault or around small jogs or steps along a fault surface. The location and orientation of secondary fracturing with respect to the fault plane or the fault tip (Fig. 1) can be used to determine if fault motion is left-lateral or right-lateral [1, 3]. Figure 1. Tail cracks at the tips of a slipping portion of a pre-existing lineament. These secondary fractures, henceforth referred to as tail cracks, develop in response to concentrations of stress around the periphery of a slipping fault. The orientations of these perturbed stresses differ from the regional stress orientations responsible for fault slip. This causes secondary fracturing orientations to be rotated out of the plane of the primary fault. These orientations can be predicted analytically using principles of linear elastic fracture mechanics, assuming that the faulted material is behaving elastically. Ice has been documented to behave elastically under conditions of low temperatures, low confining pressures and high strain rates [4]. These conditions typify deformation of Europa's ice shell, which is subject to a constantly rotating diurnal stress field induced by the gravitational pull of Jupiter [5]. These tidal stresses on Europa are sufficient to drive strike-slip faulting [6]. Accordingly, strike-slip faults are well documented on Europa [6-10], with lengths of up to 810 km. My examination of many of these faults has revealed that tail cracks are also a common phenomenon along Europan strike-slip faults (Fig. 1) and reveal details about the nature of the stress field that existed at the time of fault motion. Figure 2. Tail cracks or horsetail fractures at the southern end of Agenor Linea [9]. The sense of tail cracking indicates right-lateral motion. Tail Crack Geometry: The geometry of the tail cracks in Fig. 1 is concave towards the region beyond the fault tips, as is commonly observed in terrestrial examples [1, 2]. However, the sense of curvature in the Europan example in Fig. 2 is opposite to this. Furthermore, the "take-off angle" (q) of the tail cracks away from the fault (i.e. relative orientations of fault and tail cracks) is relatively small in Fig. 2 compared to terrestrial tail cracks, which are commonly oriented at about 70° to the fault plane. Take-off angles have been calculated to vary theoretically [1, 3] based on the fracture mode, which describes the relative amounts of sliding and opening along a fracture or fault. I have extended such analyses to account for the sense of curvature of tail cracks, and thus describe the Europan crustal stress state at the time of fault motion, as well as the physical behavior of the fault during slip events. Theoretical Treatment: The stress tensor can be calculated at any arbitrary point in an elastic body containing a slipping discontinuity using the modified Westergaard stress functions [11]. The generalized form of the crack stress function for any mode of crack motion is given by fm(z)=Am[(z2 - a2)1/2 - z] + Bmz, where the crack length is 2a, m is the mode of failure (I, II, or III), and z is the complex variable z = x2 + ix1. The constants Am and Bm describe the nature of the loading of the crack (failure mode), and are given by: Am = [DsI, -iDsII, -iDsIII] = [(sr11 - sc11), -i(sr12 - sc12), -i(sr13 - sc13)] Bm = [(sr11 + sr22)/2, 0, sr23 - isr13)] where the r and c superscripts refer to the remote and crack components of the stress tensor, respectively. The complex number i is equal to (-1)1/2. For the two-dimensional case, the components of the stress tensor are directly related to the crack stress function through the relationships: s11 = Re[f'I] + x1Im[f''I + f''II] - C s12 = -Im[f'II] - x1Re[f''I + f''II] - D s22 = Re[f'I + 2f'II] - x1Im[f''I + f''II] + C where Re and Im are real and imaginary parts, and the constants C and D are given by C = (sr22 - sr11)/2 and D = -sr12. These stress tensor components can then be used to calculate the orientations of principal stresses at any coordinate point (x1, x2) around a slipping crack [11]. Figure 3. Analytical modeling results showing principal stress trajectories around right-lateral strike-slip faults (blue) in an elastic body for a range of boundary conditions. Locations and morphology of resultant tail cracks are shown in red. As the mode I/mode II ratio increases from left to right, the amount of fault dilation increases. For the pure mode II case on the left, there is no dilation of the fault. Tail crack morphology at the tip of Agenor Linea in Fig. 2 resembles the tail crack morphology for the mode I/mode II ratio = 2 modeling result. Long tic trajectories represent the maximum tensile stress direction. Effect of Fault Dilation on Tail Crack Geometry: Using the above equations, principal stress orientations were calculated in an elastic material around a slipping strike-slip fault, approximating fault slip in Europa's ice shell. Several loading conditions were considered, ranging from the pure shear case (pure mode II - no fault dilation), to cases where increasing fault dilation accompanied strike-slip motion (increasing mode I/mode II ratio). The orientations of principal stresses and resultant tail crack shapes are shown in Fig. 3. As the amount of fault dilation increases, the tail crack curvature changes. The pure mode II case resembles many terrestrial examples, as idealized in Fig. 1, with a take-off angle of 70.5° [1]. It implies frictional contact of the fault surfaces during slip. However, with increasing fault dilation accompanying slip, tail crack curvatures begin to resemble those at the tip of Agenor Linea (Fig. 2), showing decreasing take-off angles and changing tail crack curvatures. The Agenor example resembles the result for a mode I/mode II ratio of 2, which is predicted to have a take-off angle of ~5° [1]. This result implies that Agenor Linea was dilating at the time that tail crack growth was occurring at its tip during fault slip. Discussion: The current thinking on strike-slip fault kinematics on Europa is that fault slip is driven by a diurnal tidal process that induces tidal walking of the faults [6]. This process involves a repetitive cycle of fault motions that include dilation during slip, consistent with the model results presented here. This implies that a significant amount of fault-normal tension is needed during fault slip episodes. The mode I/mode II ratio of 2 suggested here for Agenor Linea is consistent with the ratio of normal to shear stresses suggested for the fault tidal walking model [6]. References: [1] Cruikshank, K.M. et al. (1991) J. Struct. Geol. 13, 865-886. [2] Willemse, E.J.M. & Pollard, D.D. (1998) JGR 103, 2427-2438. [3] Pollard, D.D. & Aydin, A. (1988) GSA Bull. 100, 1181-1204. [4] Rist, M.A. & Murrell, S.A.F. (1994) J. Glaciology 40, 305-318. [5] Greenberg, R. et al. (1998) Icarus, 135, 64-78. [6] Hoppa, G. et al. (1999) Icarus 141, 287-298. [7] Tufts, B.R. et al. (1999) Icarus 141, 53-64. [8] Hoppa, G. et al. (2000) JGR 105, 22,617-22,627. [9] Prockter, L.M. (2000) JGR 105, 9483-9488. [10] Sarid, A.R. et al. (2002) Icarus 158, 24-41. [11] Pollard, D.D. & Segall, P. (1987) Fracture Mechanics of Rock, 277-349. Acknowledgements: This work was supported by NASA grant number NAG5-11495.

What is (and isn't) wrong with both the tension and shear failure models for the formation of lineae on Europa

Conference Paper

Full-text available

Feb 2004

Simon A. Kattenhorn

Introduction: An unresolved problem in the interpretation of lineae on Europa is whether they formed as tension- or shear-fractures. Voyager image analyses led to hypotheses that Europan lineaments are tension cracks induced by tidal deformation of the ice crust [1]. This interpretation continued with Galileo image analyses, with lineae being classified as crust-penetrating tension cracks [2]. Tension fracturing has also been an implicit assumption of nonsynchronous rotation (NSR) studies [e.g. 1-4]. However, recent hypotheses invoke shear failure to explain lineae development [5-6]. If a shear failure mechanism is correct, it will be necessary to re-evaluate any models for the evolution of Europa's crust that are based on tensile failure models, such as NSR estimates. For this reason, it is imperative that the mechanism by which fractures are initiated on Europa be unambiguously unraveled. A logical starting point is an evaluation of the pros and cons of each failure model, highlighting the lines of evidence that are needed to fully justify either model. Types of Lineae: Numerous classification schemes have been developed to describe the range of lineae morphologies observed on Europa's surface [2-4, 7]. It is generally accepted that there is an evolutionary sequence of lineae development from fractures/isolated troughs, to proto-ridges/raised-flank troughs, to double ridges, and finally complex ridges. Strike-Slip Faults. Many lineae on Europa show lateral offsets of relatively older structures. These strike-slip faults vary considerably in both morphology and size. Many resemble double ridges [6, 8] having small offsets (~100s of m); others are hundreds of km long and occur as several km-wide, internally deformed, dilational bands with offsets of up to 10s of km [9-10]. These features attest to the fact that shear failure is a major deformation mechanism on Europa. Formation Mechanisms: A number of mechanisms have been proposed to explain the origin of lineae [11]. The two most prominent models are the tension and the shear failure models; however, neither model is undeniably more acceptable than the other. Tension Models. Tension fracturing is assumed to be the result of stretching of the ice crust in response to the combined effects of diurnal tides and NSR (Fig. 1). Tensile stresses of <1 MPa are predicted to occur [2]. Orientations of fractures are dictated by principal tidal stress orientations. For such a model, there is only one preferred orientation of fractures forming at any one point in time. Figure 1. Principal tidal stresses at 1/8 orbit after apo-jove with 1° of NSR [2]. Gray areas are equatorial compressive zones. Red lines show expected locations and orientation ranges of tension fractures. Mapped regions: orange: [3], red: [4], blue and green: [6]. Shear Models. Shear failure is hypothesized to occur in equatorial compressive zones (ECZs), where tidal stresses are predicted to be compressive (Fig. 1). Frictional heating during shearing causes melting and possibly extrusion that gradually builds up ridges [5]. The Coulomb criterion for shear failure allows for a conjugate set of shear fractures. In the shear failure model, there are thus two potential orientations of fractures forming at any one point in time. In Support of Tensile Failure Models: Other than in ECZs, tensile tidal stresses are common. Brittle materials are characteristically weaker in tension than in compression and ice on Europa is hypothesized to have a low tensile strength (<1 MPa) [2]. Orientations of major lineae agree remarkably well with orientations of tidal stresses (adjusting for NSR reorientation of the ice shell). Fracture mapping in both the leading and trailing hemispheres has shown a consistent rotation of fracture orientations through time [3-4], and no ambiguous cross-cutting relationships, agreeing with NSR model predictions for tensile fracturing. Problems with Tensile Failure Models: There are also many aspects of lineae that are not supported by tensile failure models. The most obvious problem arises with lineae having orientations that could only have occurred in the ECZs. In several mapped regions affected by ECZs (Fig. 1), lineae should have orientations within ~30° of E-W. However, such orientations are rare (Fig. 2), except for smooth bands, which are typically ~E-W oriented, extensional features [3-4]. Also unclear is how surface fractures evolve into ridges. Models for ridge development that invoke tapping into an underlying ocean have been criticized because they implicitly require a thin ice shell, which is inconsistent with mounting evidence for convection-driven diapirism in the crust, implying an ice thickness of at least 15km [12]. Figure 2. Expected range of orientations of tension cracks (light blue shaded areas) in the regions of the boxes in Fig. 1. Colored arrows show actual ranges of lineae in these regions (color scheme as in Fig. 1). In Support of Shear Failure Models: The obvious appeal of shear failure models is that they can account for lineae development in the absence of tensile stresses in the crust. Furthermore, lineae orientations (Fig. 2) seem to agree with predicted conjugate set orientations in the ECZs (NW-SE and NE-SW) rather than those predicted by tension models [6]. Shear activity is supported by the existence of major strike-slip faults. If slip events are rapid, frictional heating along the fault walls provide a source for ridge-building material without the need to tap into an underlying ocean. Problems with Shear Failure Models: At present, there is no convincing geological evidence that conjugate sets of similar-aged lineae exist in near-equatorial regions. This may be a reflection of a lack of explicit identification of such features rather than a lack of the features themselves. In a conjugate set, neither fault is more or less likely to form than the other; therefore, conjugate sets should show ambiguous cross-cutting relationships. Evidence of this has not been documented. Furthermore, inconsistent shear sense along features with identical orientations [6] is inconsistent with Coulomb failure predictions. The typical lack of offsets along most double ridges is also difficult to reconcile with lineae evolving as shear fractures. Finally, lineae at latitudes >±40°, where tensile stresses occur, are not morphologically different from ECZ lineae, raising the possibility that they have identical formation mechanisms (whether in tension or shear). Discussion: Detailed mapping from Galileo images must continue across all latitudes and longitudes to clarify fracture sequences, cross-cutting relationships, the mechanics of fracture propagation, and contrary fracture behaviors in different locations. For example, [6] suggest that there is no clear sequence of rotating fracture orientations through time in the E4 and E6 regions, but rather superposed conjugate sets. In contrast, I have found no ambiguous cross-cutting relationships or evidence of conjugate fracture sets in the Bright Plains (BP) region, very near the E6 region of [6]. The BP shows a clear time progression of resolved shear sense on fractures in different orientations, in response to the rotating NSR stress field. The angles between BP double ridges with stratigraphically similar ages are not constant, ranging from 19-86°. The angle, q, between conjugate faults is purely a function of the coefficient of sliding friction of Europan ice, m, such that q = tan-1 (1/m). For a reasonable range of m [13], q should be restricted to 60-90°, suggesting that BP double ridges are not conjugate sets (but not disproving that they may be shear fractures). Fracture orientations in the BP (red in Fig. 2) do not fall within a stress field that permits tension fracturing, which may either imply shear fracturing, or may entail the existence of a stress component (such as fluid pressure) that superimposes tidal stresses. Finally, assuming that all fractures in the BP are tension fractures, the amount of NSR is estimated to be as much as 900° [4]. But if BP double ridges are conjugate shear fractures, NSR estimates must be reduced by at least 180°. This discrepancy clearly indicates that our inferences about the rotational history of Europa are inherently flawed by our lack of certainty about the origin of lineae. References: [1] Helfenstein, P. and Parmentier, E.M. (1985) Icarus, 61, 175-184. [2] Greenberg, R. et al. (1998) Icarus, 135, 64-78. [3] Figueredo, P.H. and Greeley, R. (2000) JGR, 105, 22,629-22,646. [4] Kattenhorn, S.A. (2002) Icarus, 157, 490-506. [5] Nimmo, F. and Gaidos, E. (2002) JGR, 107, E4, 10.1029/ 2000JE001476. [6] Spaun, N.A. et al. (2003) JGR, 108, E6, 10.1029/2001JE001499, 1-21. [7] Head, J.W. et al. (1999) JGR, 104, E10, 24,223-24,236. [8] Hoppa, G. et al. (2000) JGR, 105, 22,617-22,627. [9] Schenk, P.M. and McKinnon, W.B. (1989) Icarus, 79, 75-100. [10] Sarid, A.R. et al. (2002) Icarus, 158, 24-41. [11] Pappalardo, R.T. et al. (1999) JGR, 104, 24,015-24,056. [12] Barr, A.C. and Pappalardo, R.T. (2002) GSA Abs. Progs. 34, (6), 36-37. [13] Rist, M.A. et al. (1994) Annal. Glaciol., 19, 131-137. Acknowledgements: Supported by NASA grant NAG5-11495 and NASA Idaho EPSCoR NCC5-577.

Strike-slip duplexing on Jupiter's icy moon Europa

Article

Full-text available

Apr 2000

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.

Geologic mapping of Europa

Article

Full-text available

Sep 2000

Galileo data enable the major geological units, structures, and surface features to be identified on Europa. These include five primary units (plains, chaos, band, ridge, and crater materials) and their subunits, along with various tectonic structures such as faults. Plains units are the most widespread. Ridged plains material spans a wide range of geological ages, including the oldest recognizable features on Europa, and appears to represent a style of tectonic resurfacing, rather than cryovolcanism. Smooth plains material typically embays other terrains and units, possibly as a type of fluid emplacement, and is among the youngest material units observed. At global scales, plains are typically mapped as undifferentiated plains material, although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size. Chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity; chaos is characterized by blocks of icy material set in a hummocky matrix. Band material is arrayed in linear, curvilinear, wedge-shaped, or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain. Bilateral symmetry observed in some bands and the relationships with the surrounding units suggest that band material forms by the lithosphere fracturing, spreading apart, and infilling with material derived from the subsurface. Ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales. Ridge material includes single ridges, doublet ridges, and ridge complexes. Ridge materials are considered to represent tectonic processes, possibly accompanied by the extrusion or intrusion of subsurface materials, such as diapirs. The tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales. Crater materials include various interior (smooth central, rough inner, and annular massif) and exterior (continuous ejecta) subunits. Structural features and landforms are shown with conventional symbols. Type localities for the units are identified, along with suggestions for portraying the features on geological maps, including colors and letter abbreviations for material units. Implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of Europa and contribute to an eventual global synthesis of its complex geology. On the basis of initial mapping results, a stratigraphic sequence is suggested in which ridged plains form the oldest unit on Europa, followed by development of band material and individual ridges. Band materials tend to be somewhat older than ridges, but in many areas the two units formed simultaneously. Similarly, the formation of most chaos follows the development of ridged plains; although chaos is among the youngest materials on Europa, some chaos units might have formed contemporaneously

Evidence for shear failure in forming near-equatorial lineae on Europa

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Jan 1029

1] Global stress models for Europa are unable to readily explain the orientations of lineae in the equatorial region of Europa's trailing hemisphere if lineae originate as tension cracks. Our analysis of two equatorial, trailing regions reveals that lineae are predominantly oriented NE and NW, and E-W lineae are rare, contrary to predictions of stress and formation models. The measured orientations are consistent with an origin by shear failure. The studied regions are located near the point of maximum differential stress and minimum surface tensile stresses, where shear faulting may surmount tension fracturing. Several types of lineae are recognized; their relative abundance is inferred to have changed with time, consistent with formation models suggesting lineae evolve from simple troughs to complex ridges. The opening of crevasse-like tensile fractures is not required for generation of all lineae; the observations are consistent with ridge formation models where troughs (formed in tension or shear) experience shear heating from tidal deformation, allowing warm ice to buoyantly uplift, creating ridges. The stratigraphic relationships indicate the following: ridged plains formed first, followed by continued formation of a wide range of lineae, and lastly emplacement of lenticulae and continued linea formation. This sequence is consistent with an early thin, brittle lithosphere that thickened with time and was subject to repetitive diurnal tides during $30°–90° of nonsynchronous rotation. Ultimately, the thickened shell underwent thermally induced solid-state convection, producing lenticulae. The young surface age of Europa implies that this entire stratigraphic sequence was emplaced in the geologically recent past.

Slightly Curved or Kinked Cracks

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Jan 1980

A solution is presented for the elastic stress intensity factors at the tip of a slightly curved or kinked two-dimensional crack. The solution is accurate to first order in the deviation of the crack surface from a straight line and is carried out by perturbation procedures analogous to those of Banichuk [1] and Goldstein and Salganik [2, 3]. Comparison with exact solutions for circular arc cracks and straight cracks with kinks indicates that the first order solution is numerically accurate for considerable deviations from straightness. The solution is applied to fromulate an equation for the path of crack growth, on the assumption that the path is characterized by pure Mode I conditions (i.e., K II=0) at the advancing tip. This method confirms the dependence of the stability, under Mode I loading, of a straight crack path on the sign of the non-singular stress term, representing tensile stress T acting parallel to the crack, in the Irwin-Williams expansion of the crack tip field. The straight path is shown to be stable under Mode I loading for TT>0.

Morphology of Europan bands at high resolution: A mid-ocean ridge-type rift mechanism

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May 2002

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.

Does Europa have a subsurface ocean? Evaluation of the geological evidence

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Oct 1999

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.

Strike-slip fault evolution on Europa: Evidence from tailcrack geometries

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Full-text available

Dec 2004

Simon A. Kattenhorn

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.

Evolution of Lineaments on Europa: Clues from Galileo Multispectral Imaging Observations

Article

Full-text available

Sep 1998

Four distinct classes of lineaments can be described on the basis of Galileo's improved spectral and spatial coverage of Europa: (1) incipient cracks are narrow (<1.6 km wide) and are generally colorless fractures which tend to crosscut other lineament types, (2) ridges have raised relief, tend to be wider (3–6 km) than the cracks, and are distinct in color from the icy plains they transect, (3) triple bands have dim medial stripes similar in color and width to the ridges and broad (8–12 km wide) diffuse low-albedo margins tainted by a red–brown nonice contaminant, and (4) ancient bands are generally similar in width to ridges and triple bands and have colors intermediate between those of triple bands and the undisturbed icy plains. They are only slightly darker than the plains at visible wavelengths—hence, they were largely undetected by Voyager—but are distinctly brighter than the plains in the infrared, precluding the possibility that they have simply faded from their formerly darker appearance.The morphologies, spectral properties, and orientations of the lineaments vary systematically with age, suggesting that these four classes represent different stages of development in an evolutionary sequence. Lineament formation appears to be the dominant resurfacing mechanism on Europa, and every landscape that has escaped erasure by heating from below is imprinted with generation after generation of intersecting ridges at various scales and orientations. Relatively recent fractures expose coarse-grained, clean ice in the shallow subsurface, possibly accounting for the distinctive color of the satellite in comparison to the other icy moons of Jupiter. The process of lineament formation may be continuing today; the bright band Agenor Linea is among the leading candidates for current activity.

Tectonic modeling of the formation of Europan ridges

Article

Jan 1998

Fracture localization along faults with spatially varying friction

Article

Jan 1997

Michele L Cooke

Opening-mode splay fractures have been observed within clusters near fault tips. The spatial distribution of splay fractures along faults influences fluid flow and lends insight into the mechanical processes of faulting. Slip gradients along faults produce stress concentrations which promote the development of opening-mode splay fractures along faults. Since the slip distribution depends on the distribution of frictional strength along faults, spatial variations in the frictional properties may influence fracture localization. Variations in friction coefficient along faults can reduce the stress singularities at fault tips and promote the development of multiple fractures inwards from the fault tips. The conditions that promote splay crack localization are examined using analytical and numerical fault models. Single splay fractures develop at locations of abrupt friction coefficient change and/or at fault tips when the frictign coefficient near the tips is less than a critical value. Fault models with linearly increasing friction coefficient toward the fault tip promote the development of multiple splay fractures within broad zones near the tip.

Compressive Anti-Cracks at the Tips of Strike-Slip Faults on Europa and Implications for Fault Mechanics

Article

Jan 2005

S. A. Kattenhorn

Compressive quadrants at the tips of strike-slip faults on Europa show evidence of convergent structures, or anti-cracks, that partly accommodate the effects of the creation of new surface area by plate-spreading mechanisms.

Classification of Europan Ridges and Troughs and a Possible Genetic Sequence

Article

Jan 1998

Theoretical displacements and stresses near fractures in rock: with applications to faults, joints, veins, dikes, and solution surfaces

Article

Dec 1987

Overview of Europa's Icy Shell: Questions of Thickness, Composition, Rheology, Tectonics, and Astrobiological Potential

Article

Feb 2004

William Mckinnon

Evidence for large-scale strike-slip faulting on Venus

Article

Jun 1998
GEOLOGY

Radar images obtained from the Magellan mapping mission reveal that much of the surface of Venus has undergone tectonic deformation. Though normal and thrust faults are common, there has been little evidence of discrete, large-offset strike-slip faults on Venus. We present a clear example of large-scale strike-slip faulting in Lavinia Planitia, Venus. Our observations of the spatial and temporal relationships between the Molpadia Linea deformation belt and an associated fracture system indicate that the fractures were formed as the result of strike-slip faulting localized along the belt. Global tectonism on Venus is dominated by vertical deformation; we attribute this example of horizontal deformation to a reorientation of the regional stress field associated with mantle downwelling.

Fundamental of Rock Mechanics

Article

Jan 1979

Trough-Bounding Ridge Pairs on Europa -- Considerations for an Endogenic Model of Formation

Article

Jan 1998

Fracture localization along faults with spatially varying friction

Article

Oct 1997

Michele L Cooke

Opening-mode splay fractures have been observed within clusters near fault tips. The spatial distribution of splay fractures along faults influences fluid flow and lends insight into the mechanical processes of faulting. Slip gradients along faults produce stress concentrations which promote the development of opening-mode splay fractures along faults. Since the slip distribution depends on the distribution of frictional strength along faults, spatial variations in the frictional properties may influence fracture localization. Variations in friction coefficient along faults can reduce the stress singularities at fault tips and promote the development of multiple fractures inwards from the fault tips. The conditions that promote splay crack localization are examined using analytical and numerical fault models. Single splay fractures develop at locations of abrupt friction coefficient change and/or at fault tips when the friction coefficient near the tips is less than a critical value. Fault models with linearly increasing friction coefficient toward the fault tip promote the development of multiple splay fractures within broad zones near the tip.

On the orientation and patterns of wing cracks and solution surfaces at the tips of a sliding flaw or fault

Article

Feb 1998

Sliding along a preexisting flaw can result in the formation of tensile cracks where stresses concentrate near the flaw tips. These tensile cracks are referred to as wing cracks and are generally oriented oblique to the preexisting flaw. Previous studies based on linear elastic fracture mechanics (LEFM) showed that the kink angle depends on the ratio of normal to shear loading on the flaw. We present analytical solutions for cohesive end zone (CEZ) flaw models and find that the relationship between kink angle and load differs significantly from that for LEFM flaws. Furthermore, the remote flaw-parallel normal stress may significantly reduce or increase the kink angle, especially for CEZ flaws with large end zones. These results suggest that multiple interpretations are possible for some measured kink angles. In some materials, solution surfaces may form at the tip of the sliding flaw. By considering the angle between wing cracks and solution surfaces it is possible to determine whether the LEFM or CEZ model is more appropriate and thus to provide a better constrained interpretation of the boundary conditions that accompanied sliding. For some CEZ flaws the stress state in the cohesive end zone is nearly homogeneous, possibly promoting formation of arrays of opening mode cracks and solution surfaces that together form a shear zone. The CEZ flaw model can explain some orientations and patterns of cracks and solution surfaces commonly observed along natural faults that cannot be accounted for with the LEFM model.

Numerical Modeling of Plate Motion: Unraveling Europa's Tectonic History

Article

Feb 2004

A San Andreas-sized Strike-slip Fault on Europa

Article

Aug 1996
ICARUS

Astypalaea Linea, a lineament in the extreme southern hemisphere of Europa, has been found to be a global-scale strike-slip fault, based on a palinspastic reconstruction of landscape on reprojected Voyager 2 images. The fault accommodates 35 km of right-lateral offset and extends at least 810 km - a length comparable to the San Andreas Fault in California. It exhibits familiar strike-slip features including braids and pull-aparts. Straight segments of the fault are concentric about an Euler pole provisionally located at (-48deg , 247.25deg W). Spanning over 29deg from (-60deg ,191deg W) to (-78.5deg , 268.5deg W) Astypalaea Linea is the longest strike-slip fault yet identified on Europa. The fault is consistent with differential stress magnitudes and stress directions predicted for high Europan latitudes due to possible non- synchronous rotation (tidal bulge in its present location) (Greenberg and Weidenschilling, 1984; Helfenstein and Parmentier, 1985). Extension on neighboring gray band Thynia Linea matches the same stress field (Pappalardo and Sullivan, 1996); thus, Astypalaea Linea and Thynia Linea may be part of a south polar deformation zone which acts as a "structural set" (Lucchitta and Soderblom, 1982). Analogous structures may exist at the Europan north pole, although factors such as a possible global structural dichotomy (Lucchitta and Soderblom, 1981) may affect their occurrence. Lateral crustal motion, as implied by the fault, is consistent with a subsurface viscous horizon structurally decoupling the outer layer of the icy Jovian satellite from its interior (e.g. Schenk and McKinnon, 1989).

The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges

Article

Oct 2005
ICARUS

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.

On the origin of a wedge crack within the icy crust of Europa

Article

Nov 2002

E. M. Schulson

[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.

Europa's Rate of Rotation Derived from the Tectonic Sequence in the Astypalaea Region

Article

Sep 2001
ICARUS

The most recent visible tectonic features in the Astypalaea region in the southern hemisphere of Europa are a set of cycloidal ridges, three of which have cross-cutting relationships that define a time sequence for their formation. The longitudes at which each of these features formed, which may be different from the current location due to rotation of Europa, are constrained by models of their formation. Reconciling the time sequence with the inferred longitudes of formation appears to require that

Geologic mapping of the northern leading hemisphere of Europa from Galileo solid-state imaging data

Article

Sep 2000

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.

Distribution of strike-slip faults on Europa

Article

Sep 2000

Study of four different regions on Europa imaged by the Galileo spacecraft during its first 15 orbits has revealed 117 strike-slip faults. Europa appears to form preferentially right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere. This observation is consistent with a model where diurnal tides due to orbital eccentricity drive strike-slip motion through a process of ``walking,'' in which faults open and close out of phase with alternating right-and left-lateral shear. Lineaments that record both left-and right-lateral motion (e.g., Agave Linea) may record the accommodation of compression in nearby chaotic zones. Nearly all identified strike-slip faults were associated with double ridges or bands, and few were detected along ridgeless cracks. Thus the depth of cracks without ridges does not appear to have penetrated to the low-viscosity decoupling layer, required for diurnal displacement, but cracks that have developed ridges do extend down to such a level. This result supports a model for ridge formation that requires cracks to penetrate to a decoupling layer, such as a liquid water ocean.

Europa: Morphological characteristics of ridges and triple bands from Galileo data (E4 and E6) and assessment of a linear diapirism model

Article

Oct 1999

Galileo solid-state imaging (SSI) images of Europa provide high-resolution data on the morphological characteristics of ridges and permit the development of nomenclatural schemes for their description and classification. Key observations are that ridges (1) are remarkably consistent in their along-strike linearity, width, and height, (2) form long linear features in which preexisting structures can sometimes be traced up the outer slopes of the ridges and in other cases appear to be buried, (3) contain narrow apical zones of small-scale, ridge-parallel faulting, (4) are sometimes flanked by narrow troughs and ridge-parallel fractures, and (5) often display associated color variations. On the basis of the characteristics and associated features of ridges, we find that a process in which initial fracturing (most plausibly related to tidal deformation) of a brittle layer overlying a buoyant ductile substrate leading to linear diapiric upwelling provides a consistent explanation for the observed features. In this process the upwelling linear diapir causes flexure (bending and faulting) of the region marginal to the fracture, the deformation and uplift of adjacent plains material and its preexisting structures to form the apical part of the ridge, the exposure of the inner walls of the crack, and the mass wasting of the inner and outer walls of the ridge to modify, but often not completely destroy, the preexisting structure of the adjacent plains. Specifically, this mechanism can account for many characteristics of the ridges, including their linearity, their consistent and regular morphology over their great lateral extent, their positive topography, the presence of preexisting structure on the outer ridges (caused by upbowing of background ridged plains), the formation of marginal troughs (as diapiric rim synclines), the detailed nature of their outer and inner slopes (caused largely by faulting and mass-wasting processes), and their sequential formation with multiple orientations (related to tidal deformation processes). Linear diapirism also provides a possible explanation for color and albedo characteristics, related to thermal effects of the upwelling warm ice (e.g., inducing volatile migration and grain-size variations). As the vast majority of deformation is vertical in this scenario, this mechanism minimizes the necessity for complementary compressional deformation required by some other models.

Estimates of ice thickness in the Conamara Chaos Region of Europa

Article

Dec 1998

Europa, the second Galilean moon outward from Jupiter, is similar in size to Earth's moon but has an outer water-ice layer on the order of 100 km thick. Images acquired by the Galileo spacecraft reveal domical features and iceberg-like blocks in some areas that suggest the ice was regionally thin at the time the features formed. This paper applies buoyancy and flexural models to some of the features to estimate ice thickness. Assuming that the ice blocks were floating in a liquid sublayer, the buoyancy model suggests an ice layer 0.2-3.0 km thick. Also, the flexural models suggest that only the upper 0.1-0.5 km of that ice layer responded elastically to stresses, acting as an elastic lithosphere.

Polar Wander and Surface Convergence of Europa's Ice Shell: Evidence from a Survey of Strike-Slip Displacement

Article

Jul 2002
ICARUS

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.

Strike-slip motion and double ridge formation on Europa

Article

Apr 2002

1] There is abundant observational evidence for strike-slip displacement on the surface of Europa. Strike-slip motion between crustal blocks produces shear heating and an increase in temperature. We model the shear heating within the ice crust using a two-dimensional, finite difference formulation, with a near-surface brittle layer of constant specified thickness and a Newtonian ductile layer beneath. We obtain a maximum temperature anomaly of 66 K for a brittle layer thickness of 2 km and shear velocity of 6 Â 10 À7 m s À1 . Such a velocity is appropriate for diurnal (85 hour) tidal motion. The local increase in temperature may cause

100 m uplift around the shear zone through thermal buoyancy. The stresses required to produce velocities of order 10 À7 m s À1 are similar to estimates for present-day tidal stresses on Europa (10 4 –10 5 Pa). Brittle layer thicknesses >2 km are unlikely to persist at active shear zones because of the effect of shear heating. Shear velocities greater than or equal to

10 À6 m s À1 will give rise to melting at shallow depths. The removal of material by downwards percolation of meltwater may cause surface collapse along the shear zone; inward motion, leading to compression, may also result. The combination of thermally or compression-induced uplift and melt-related collapse may be responsible for the pervasive double ridges seen on Europa's surface.

Resurfacing history of Europa from pole-to-pole geological mapping

Article

Feb 2004
ICARUS

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.

The geometry of echelon fractures in rock: Implications from laboratory and numerical experiments

Article

Mar 1993
J STRUCT GEOL

The traces of echelon joints, veins and dikes in rock range from curving to straight. Theoretical analyses using boundary element numerical methods have concluded that straight, open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress. We present a series of laboratory experiments which investigate the two-dimensional propagation paths of echelon fractures in PMMA plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array. The experimental results support the theoretical conclusions and verify the accuracy of the numerical method.

Astypalaea Linea: A Large-Scale Strike-Slip Fault on Europa

Article

Sep 1999

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.

Analysis of minor fractures associated with joints and faulted joints

Article

Dec 1991
J STRUCT GEOL

In this paper, we use fracture mechanics to interpret conditions responsible for secondary cracks that adorn joints and faulted joints in the Entrada Sandstone in Arches National Park, U.S.A. Because the joints in most places accommodated shearing offsets of a few mm to perhaps 1 dm, and thus became faulted joints, some of the minor cracks are due to faulting. However, in a few places where the shearing was zero, one can examine minor cracks due solely to interaction of joint segments at the time they formed.

Lithospheric Dilation on Europa

Article

Jul 2000

Lithospheric dilation on Europa has occurred at ridges, bands, and various hybrid lineaments on a global scale over a large part of the geological age of the surface. Dilational ridges (Class 2 in the R. Greenberg et al. (1998, Icarus135, 64–78) taxonomy) are elevated, are usually a few kilometers across, and may have a lineated or hummocky interior and a pronounced medial groove. Bands are lower and usually wider than Class 2 ridges, and may have a lineated interior with no prominent medial groove. Some lineaments have characteristics of both ridges and bands. The character of Class 2 ridges, bands, and hybrid forms suggests that they are dilational gaps in the lithosphere, filled from below, and that they constitute a morphological continuum with Class 2 ridges and bands as end-members. These relationships may be explained by a model in which external forcing superimposes a secular dilation on the tidal cycle that opens and closes cracks each Europan day, resulting in incomplete closure with accumulation and possible extrusion of new ice fill. Where the lineament ultimately falls on the morphological continuum—especially how much it is elevated above ambient terrain—depends upon the ratio of daily secular dilation to the amplitude of the cyclic tidal separation. We call this ratio the “dilation quotient”. Changes in the dilation quotient during the active life of the lineament will create variable lineament forms. One driver for dilation is tidal “walking” of strike-slip faults, which dilates linked nonparallel cracks. That process is prominent in the 800-km-long strike-slip fault Astypalaea Linea. A subsurface liquid water ocean allows the decoupling needed for horizontal displacements and is the source for the ice that fills the dilated lineaments.

Tectonic Processes on Europa: Tidal Stresses, Mechanical Response, and Visible Features

Article

Sep 1998

Europa's orbital eccentricity, driven by the resonance with Io and Ganymede, results in “diurnal” tides (3.5-day period) and possibly in nonsynchronous rotation. Both diurnal variation and nonsynchronous rotation can create significant stress fields on Europa's surface, and both effects may produce cracking. Patterns and time sequences of apparent tectonic features on Europa include lineaments that correlate with both sources of stress, if we take into account nonsynchronous rotation, after initial crack formation, by amounts ranging up to several tens of degrees. For example, the crosscutting time sequence of features in the Cadmus and Minos Linea region is consistent with a combined diurnal and nonsynchronous tensile-stress field, as it evolves during tens of degrees of nonsynchronous rotation. Constraints on the rotation rate from comparing Voyager and Galileo images show that significant rotation requires >104yr, but could be fast enough to have allowed significant rotation since the last global resurfacing, even if such resurfacing was as recent as a few million years ago. Once cracking is initiated, diurnal tides work cracks so that they open and close daily. Although the daily effect is small, over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures, according to a model in which underlying liquid water fills the open cracks, partially freezes, and is extruded during the daily closing of the cracks. Thus, several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer.

Strike–Slip Faults on Europa: Global Shear Patterns Driven by Tidal Stress

Article

Oct 1999
ICARUS

Diurnal tides due to orbital eccentricity may drive strike–slip motion on Europa through a process of “walking” in which faults open and close out of phase with alternate right- and left-lateral shear. Mapping of five different regions on Europa has revealed 121 strike–slip faults, including Astypalaea Linea, a 800-km-long fault with 42 km of right-lateral offset. At high southern latitudes near Astypalaea Linea all of the strike slip faults identified were right-lateral. Europa appears to preferentially form right-lateral faults in the southern hemisphere and left-lateral faults in the northern hemisphere, consistent with tidal walking. At the five locations, nonsynchronous rotation explains the azimuthal orientations and distribution of sense of shear, which fit formation ∼60° to 90° west of their current positions. Alternatively, stress due to differential rotation might also explain the observed shear patterns. Nearly all identified strike–slip faults were associated with double ridges or bands, but few were detected along ridgeless cracks (even older ones). Thus, cracks without ridges may not have penetrated to a decoupling layer, consistent with the models for ridge formation that require cracks to penetrate to a liquid water ocean.

Nonsynchronous Rotation Evidence and Fracture History in the Bright Plains Region, Europa

Article

Jun 2002

Simon A. Kattenhorn

A geologic map for the Bright Plains in the Conamara Chaos region of Europa is presented and is used to unravel a detailed fracture sequence using cross-cutting relationships and fracture mechanics principles. Fracture orientations in the Bright Plains region rotated with time, consistently in a clockwise sense. This conclusion agrees with the observations of other researchers' northern Europan hemisphere investigations and points strongly toward the fracture sequence being controlled by the effect of nonsynchronous rotation, whereby the outer ice crust of Europa rotates slightly faster than the satellite's interior. This is convincing evidence that Europa's crust has been decoupled from the interior, possibly due to the presence of a liquid ocean beneath the crust.Tidal stresses induced in the ice crust by the combined effects of nonsynchronous rotation and diurnal tidal flexing can be calculated using the assumption that the crust behaves elastically over relatively short time scales (i.e., no viscous relaxation of stresses). The fracture orientations in the Bright Plains area were compared to a global scale tidal stress field to determine the longitudes at which each fracture set developed. The fracture sequence points strongly to the Bright Plains region of the crust having rotated at least 720° (and perhaps up to 900°) with respect to the satellite's interior during the visible fracture history. This amount exceeds previously published estimates of nonsynchronous rotation. The orientations of the most recent surface fractures are incompatible with the current state of stress in the Bright Plains region, implying a period of a few thousand years since the most recent fracturing events based on existing nonsynchronous rotation rate estimates.

Cycloids and Wedges: Global Patterns from Tidal Stress on Europa

Conference Paper

Mar 2003

Ice triaxial deformation and fracture

Article

Jan 1994
J GLACIOL

An experimental investigation into the mechanical behaviour of polycrystalline ice in triaxial compression has been conducted using conditions generally favourable to brittle fracture and microcracking. Under triaxial stresses at high strain rate, ice failure occurs by abrupt shear fracturing, generally inclined at about 45° to the maximum principal stress. At -20°C, such failure is suppressed by the imposition of a small confining pressure, allowing a transition to ductile-type flow accompanied by distributed microcracking, but at -40°C shear fracture persists under confinement of up to at least 50 mPa. For low confining pressures (<10 MPa), brittle strength is strongly pressure-dependent; above this it is pressure-independent. Evidence is presented that suggests this may reflect a change from a fracture process influenced by friction to fracture initiated by localized yielding. y

Secondary Fracturing of Europa's Crust in Response to Combined Slip and Dilation Along Strike-Slip Faults

Article

Feb 2003

S. A. Kattenhorn

A commonly observed feature in faulted terrestrial rocks is the occurrence of secondary fractures alongside faults. Depending on exact morphology, such fractures have been termed tail cracks, wing cracks, kinks, or horsetail fractures, and typically form at the tip of a slipping fault or around small jogs or steps along a fault surface. The location and orientation of secondary fracturing with respect to the fault plane or the fault tip can be used to determine if fault motion is left-lateral or right-lateral.

What is (and Isn't) Wrong with Both the Tension and Shear Failure Models for the Formation of Lineae on Europa

Article

Feb 2004

S. A. Kattenhorn

An unresolved problem in the interpretation of lineae on Europa is whether they formed as tension- or shear-fractures. Voyager image analyses led to hypotheses that Europan lineaments are tension cracks induced by tidal deformation of the ice crust. This interpretation continued with Galileo image analyses, with lineae being classified as crust- penetrating tension cracks. Tension fracturing has also been an implicit assumption of nonsynchronous rotation (NSR) studies. However, recent hypotheses invoke shear failure to explain lineae development. If a shear failure mechanism is correct, it will be necessary to re-evaluate any models for the evolution of Europa's crust that are based on tensile failure models, such as NSR estimates. For this reason, it is imperative that the mechanism by which fractures are initiated on Europa be unambiguously unraveled. A logical starting point is an evaluation of the pros and cons of each failure model, highlighting the lines of evidence that are needed to fully justify either model.

The geology of Europa

Article

Feb 1982

The map units and lineations of Europa are detailed, and the geologic processes, and history, and thick and thin ice models of the satellite are discussed. It is concluded that Europa lacks evidence of a horizontally stratified crust; the geology appears characterized by disruption of the crust and intrusions into an icy shell. The surface consists of plains and mottled terrain, the former being older. Numerous straight and curved lineations, streaks, stripes, and bands cross EuropA's surface on a global and surface scale. Most lineations appear related to fractures in the crust. Five fresh craters in the 10 to 30 km diameter range are visible. The dark spots, stripes, and bands that appear to have replaced sections of the crust suggest that material was transported to the surface from the subjacent silicate lithosphere. The apparent low density of craters superposed on Europa's surface suggests that the surface is about 100 million years old.

Formation of Cycloidal Features on Europa

Article

Oct 1999

Cycloidal patterns are widely distributed on the surface of Jupiter's moon Europa. Tensile cracks may have developed such a pattern in response to diurnal variations in tidal stress in Europa's outer ice shell. When the tensile strength of the ice is reached, a crack may occur. Propagating cracks would move across an ever-changing stress field, following a curving path to a place and time where the tensile stress was insufficient to continue the propagation. A few hours later, when the stress at the end of the crack again exceeded the strength, propagation would continue in a new direction. Thus, one arcuate segment of the cycloidal chain would be produced during each day on Europa. For this model to work, the tensile strength of Europa's ice crust must be less than 40 kilopascals, and there must be a thick fluid layer below the ice to allow sufficient tidal amplitude.

Determination of Ice Crust Thickness from Flanking Cracks Along Ridges on Europa

Article

May 2002

We use equations describing the deflection of an elastic plate below a line load to estimate ice crust thickness below ridges on Europa. Using a range of elastic parameters, ice thickness is calculated to fall in the range 0.2 2.6 km. Additional information is contained in the original extended abstract.

Cycloids and wedges: Global patterns from tidal stress on Europa Determination of ice thickness from flanking cracks along ridges on Europa The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges

Jan 2002
397-412

G D Bart
R Greenberg
G V Hoppa
Rom ] Cd
S E Billings
S A Kattenhorn
Rom ] Cd
S E Billings
S A Kattenhorn

Bart, G.D., Greenberg, R., Hoppa, G.V., 2003. Cycloids and wedges: Global patterns from tidal stress on Europa. Proc. Lunar Sci. Conf. 34. Abstract #1396 [CD-ROM]. Billings, S.E., Kattenhorn, S.A., 2002. Determination of ice thickness from flanking cracks along ridges on Europa. Proc. Lunar Sci. Conf. 33. Ab-stract #1813 [CD-ROM]. Billings, S.E., Kattenhorn, S.A., 2005. The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges. Icarus 177, 397–412.

Jan 2000
75-97

B R Tufts
R Greenberg
G V Hoppa
P E Geissler

Tufts, B.R., Greenberg, R., Hoppa, G.V., Geissler, P.E., 2000. Lithospheric dilation on Europa. Icarus 146, 75–97.

A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries

Abstract

No full-text available

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