does folding accommodate europa s contractional strain the effect of surface temperature on fold formation in ice lithospheres 1 on europa the mechanism by which contractional strains are accommodated remains a mystery subtle folds have been observed in the polar regions but if folding is a dominant accommodation mechanism ubiquitous large amplitude folds might be expected across europa s surface here we use finite element modeling to show that fold growth rates are a strong function of surface temperature warm temperatures result in lower rates whereas cold temperatures result in higher rates combining diurnally averaged europan surface temperatures derived from galileo photopolarimeter radiometer data and new numerical simulations we show that forming moderate amplitude folds requires roughly twice the contraction or shortening in europa s equatorial latitudes relative to polar latitudes relatively large contractional strains in europa s equatorial and midlatitudes can therefore be accommodated primarily through passive shortening without producing obvious folds lithospheric folding and or passive shortening therefore remain plausible mechanisms for strain accommodation on europa

rheological properties of paraffin as an analogue material for viscous crustal deformation the rheological stratification induced by temperature variations within the crust requires analogue materials with temperature sensitive viscosity to properly simulate the change of mechanical properties with depth rheological calibration experiments on commercial paraffin wax with melting point of 53 1 c show that this material is a nonlinear fluid 1 6 n 2 1 for homologous temperatures t tm 0 70 and an approximately newtonian fluid 1 0 n 1 3 for t tm 0 70 the decrease in the stress exponent n is probably associated with the phase transition of paraffin effective viscosity values are strongly temperature dependent varying by nine orders of magnitude from 1010 to 10 pa s in the temperature range between 30 and 52 c the rheological properties of paraffin make this material useful for modelling crustal rheology and tectonic processes when linear dimensions and time scale down by a factor of 106 and 1011 respectively

pressure solution in carbonate source rocks and its control on petroleum generation and migration abstract based on detailed geochemical and petrographic studies of conventional cores of a triassic age carbonate source rock formation from three well sites at increasing maturity stages it is shown that the effects of carbonate diagenesis especially pressure solution during relatively shallow burial stages exercise major controls on the processes of petroleum generation and migration during deeper burial pressure solution leads to an internal redistribution of organic matter within this source rock formation in the form of local concentrations as solution seams and stylolites they provide very favourable conditions for effective petroleum generation and migration due to their internal structure and composition three dimensional arrangement of kerogen surrounded by carbonate cemented rims as tight seals the geochemical and petrographic effects of petroleum generation and expulsion are documented e g the expulsion of petroleum occurred as a separate phase fluid at high efficiencies 60 80 for the normal and isoprenoid alkanes and was facilitated by fractures presumably resulting from pore fluid overpressuring a five stage conceptual model is proposed to explain how the effects of pressure solution achieved during relatively shallow burial stages exercised controls on the extent and efficiency of petroleum generation and expulsion during deeper burial stages

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

strike slip faults on europa global shear patterns driven by tidal stress abstract 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

forming europa s folds strain requirements for the production of large amplitude deformation abstract the surface of jupiter s iconic icy satellite europa is dominated by tectonic features that have formed in extension in contrast features that have formed in contraction appear to be rare thus there exists an apparent imbalance between the observed surface expression and relative magnitudes of extensional and contractional strain in this paper we reexamine the amount of strain that can be accommodated during fold formation on europa using a two dimensional finite element model that allows the examination of fold formation at large strains and under a broad range of initial conditions we find that in contrast to previous assessments relatively large strains can be accommodated by the development of relatively low amplitude folds under our nominal conditions creating peak to trough fold amplitudes of 200 m consistent with europa s astypalaea folds requires 7 contraction of the lithosphere decreasing either the strain rate or the ice grain size allows the formation of larger amplitude folds at smaller strains but we find no cases where 200 m amplitude folds form at less than 3 strain only if both of these conditions are met simultaneously do large amplitude folds form at contractional strains 3 the requirement of larger strains relative to previous analytical models results from the combined effects of an initial period of fold nucleation generally 1 2 contractional strain in which limited amplification occurs and smaller amplitude growth rates overall the requirement of several percent strain to produce even moderate amplitude folds is consistent with previous numerical models of single layer and lithospheric scale viscoelastic folding under terrestrial conditions because the formation of large amplitude folds requires relatively large strain small but significant amounts of contraction can be accommodated predominantly through uniform thickening of the lithosphere during the nucleation phase of fold growth without the formation of obvious surface deformation the accommodation of contractional strains via such pre folding lithospheric thickening provides at least a partial solution to europa s apparent strain imbalance

geological history of the tyre region of europa a regional perspective on europan surface features and ice thickness galileo images of the tyre macula region of europa at regional 170 m pixel and local 40 m pixel scales allow mapping and understanding of surface processes and landforms ridged plains doublet and complex ridges shallow pits domes chaos areas impact structures tilted blocks and massifs and young fracture systems indicate a complex history of surface deformation on europa regional and local morphologies of the tyre region of europa suggest that an impactor penetrated through several kilometers of water ice to a mobile layer below the surface morphology was initially dominated by formation of ridged plains followed by development of ridge bands and doublet ridges with chaos and fracture formation dominating the latter part of the geologic history of the tyre region two distinct types of chaos have been identified which along with upwarped dome materials appear to represent a continuum of features domes platy chaos knobby chaos resulting from increasing degrees of surface disruption associated with local lithospheric heating and thinning local and regional stratigraphic relationships block heights and the morphology of the tyre impact structure suggest the presence of low viscosity ice or liquid water beneath a thin several kilometers surface ice shell at the time of the impact the very low impact crater density on the surface of europa suggests that this thin shell has either formed or been thoroughly resurfaced in the very recent past

thermal equilibrium states of europa s ice shell implications for internal ocean thickness and surface heat flow abstract ice shell thickness and ocean depth are calculated for steady state models of tidal dissipation in europa s ice shell using the present day values of the orbital elements the tidal dissipation rate is obtained using a viscoelastic maxwell rheology for the ice the viscosity of which has been varied over a wide range and is found to strongly increase if an inviscid internal ocean is present to determine steady state values the tidal dissipation rate is equated to the heat transfer rate through the ice shell calculated from a parameterized model of convective heat transfer or from a thermal conduction model if the ice layer is found to be stable against convection although high dissipation rates and heat fluxes of up to 300 mwm 2 are in principle possible for europa these values are unrealistic because the states for which they are obtained are thermodynamically unstable equilibrium models have surface heat flows around 20 mwm 2 and ice layer thicknesses around 30 km which is significantly less than the total thickness of the h2o layer these results support models of europa with ice shells a few tens of kilometers thick and around 100 km thick subsurface oceans

thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes a thin outer ice shell on jupiter s large moon europa would imply easy exchange between the surface and any organic or biotic material in its putative subsurface ocean1 2 3 4 the thickness of the outer ice shell is poorly constrained however with model dependent estimates ranging from a few kilometres5 6 to ten or more kilometres7 here i present measurements of depths of impact craters on europa ganymede and callisto that reveal two anomalous transitions in crater shape with diameter the first transition is probably related to temperature dependent ductility of the crust at shallow depths 7 8 km on europa the second transition is attributed to the influence of subsurface oceans on all three satellites3 8 9 which constrains europa s icy shell to be at least 19 km thick the icy lithospheres of ganymede and callisto are equally ice rich but europa s icy shell has a thermal structure about 0 25 0 5 times the thicknesses of ganymede s or callisto s shells depending on epoch the appearances of the craters on europa are inconsistent with thin ice shell models1 and indicate that exchange of oceanic and surface material could be difficult

fault offsets and lateral crustal movement on europa evidence for a mobile ice shell abstract right lateral structural offsets of 25 km have been identified on the icy galilean satellite europa these occur along dark lineaments oriented orthogonally to wedge shaped bands which are also 25 km wide wedge shaped bands are interpreted as dilated tension fractures which formed as crustal blocks or plates 50 100 km across separated and slipped past each other along flanking strike slip faults this style of deformation does not appear to be characteristic of other lineament types and with the exception of earth appears to be unique to europa together the subparallel wedge shaped b bands form a broad nw se trending belt 1500 km long and less than 500 km across near the anti jovian point this belt is interpreted as a major crustal fracture or rift zone with a pole of rotation determined by the strike slip faults near 47 s 144 w and an approximate ne sw direction of maximum tensile stress extension may have been areally compensated at agenor linea a bright band of possible compressional origin global expansion tidal distortion and nonsynchronous rotation do not explain the inferred minimum principal i e least compressive stress directions alternatively fracturing near the anti jovian point may be a result of i solid state convection in the lower ice crust possibly triggered by uneven heat flow from the silicate interior ii rotation of the icy shell about the sub and anti jovian points induced by latitudinal lithospheric thickness variations or iii preferential strain accumulation from the rest of the icy shell no significant distortion of the crustal blocks occurred during fracturing and rotation indicating that the icy crust was probably mechanically decoupled from the silicate interior in this region over the time scale of fracturing decoupling on a global scale is also likely the simple geometry of lineaments argues for formation in an icy lithosphere not a silicate one and other evidence for fracturing caused by nonsynchronous rotation stress is not compatible with a tidally locked silicate interior which we show is likely unless the ice shell rotates independently decoupling could have been due to either warm ice or liquid water near the base of the icy crust mechanical bounds on lithospheric thickness a few to 10 km lead to heat flow estimates that admit both possibilities but favor decoupling by liquid water

polar wander and surface convergence of europa s ice shell evidence from a survey of strike slip displacement 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

flanking fractures and the formation of double ridges on europa abstract europa a satellite of jupiter is one of the most intriguing worlds in the solar system its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times double ridges are a common landform and appear to have formed over a significant fraction of the satellite s observed geologic history thus understanding their formation is critical to unraveling europa s history and many models have been proposed to explain their creation a clue to the formation of ridges may lie in evidence for flexure of the lithosphere in response to a load imposed by the ridge itself marginal troughs and subparallel flanking fractures when this flexure has been modeled a simple elastic lithosphere has typically been assumed however the generally thin lithospheres suggested by these models require very high heat flows that are inconsistent with europa s expected thermal budget of order 1 w m 2 vs of order 10 mw m 2 each of the proposed formational models however predicts a thermal anomaly that may facilitate the flexure of europa s lithosphere here we simulate this flexure in the presence of these anomalies as a means to evaluate the different models of ridge formation we find that nearly all models of double ridge formation are inconsistent with the observation of flexure specifically the flanking fractures except for a cryovolcanic model in which the growing ridge is underlain by a cryomagmatic sill that locally heats and thins the lithosphere

estimates of ice thickness in the conamara chaos region of europa 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

implications of shear heating and fracture zones for ridge formation on europa 1 we present 2d and 3d numerical simulations of convection to test the role of shear heating and fracture zones on europan ridge formation our simulations show that a pre existing fracture zone promotes upwelling and lithospheric thinning leading to topographic uplift of 50 m shear heating also promotes lithospheric thinning and buoyant ascent producing a ridge like feature with topography up to 120 m topography remains linear along strike even under the influence of heterogeneous 3d convection within the ice shell although the central trough is not reproduced in the simulations our results support the idea that shear heating can produce ridge like structures on europa

oceans in the icy galilean satellites of jupiter abstract equilibrium models of heat transfer by heat conduction and thermal convection show that the three satellites of jupiter europa ganymede and callisto may have internal oceans underneath ice shells tens of kilometers to more than a hundred kilometers thick a wide range of rheology and heat transfer parameter values and present day heat production rates have been considered the rheology was cast in terms of a reference viscosity 0 calculated at the melting temperature and the rate of change a of viscosity with inverse homologous temperature the temperature dependence of the thermal conductivity k of ice i has been taken into account by calculating the average conductivity along the temperature profile heating rates are based on a chondritic radiogenic heating rate of 4 5 pw kg 1 but have been varied around this value over a wide range the phase diagrams of h2o ice i and h2o 5 wt nh3 ice have been considered the ice i models are worst case scenarios for the existence of a subsurface liquid water ocean because ice i has the highest possible melting temperature and the highest thermal conductivity of candidate ices and the assumption of equilibrium ignores the contribution to ice shell heating from deep interior cooling in the context of ice i models we find that europa is the satellite most likely to have a subsurface liquid ocean even with radiogenic heating alone the ocean is tens of kilometers thick in the nominal model if tidal heating is invoked the ocean will be much thicker and the ice shell will be a few tens of kilometers thick ganymede and callisto have frozen their oceans in the nominal ice i models but since these models represent the worst case scenario it is conceivable that these satellites also have oceans at the present time the most important factor working against the existence of subsurface oceans is contamination of the outer ice shell by rock rock increases the density and the pressure gradient and shifts the triple point of ice i to shallower depths where the temperature is likely to be lower then the triple point temperature according to present knowledge of ice phase diagrams ammonia produces one of the largest reductions of the melting temperature if we assume a bulk concentration of 5 wt ammonia we find that all the satellites have substantial oceans for a model of europa heated only by radiogenic decay the ice shell will be a few tens of kilometers thinner than in the ice i case the underlying rock mantle will limit the depth of the ocean to 80 100 km for ganymede and callisto the ice i shell on top of the h2o nh3 ocean will be around 60 to 80 km thick and the oceans may be 200 to 350 km deep previous models have suggested that efficient convection in the ice will freeze any existing ocean the present conclusions are different mainly because they are based on a parameterization of convective heat transport in fluids with strongly temperature dependent viscosity rather than a parameterization derived from constant viscosity convection models the present parameterization introduces a conductive stagnant lid at the expense of the thickness of the convecting sublayer if the latter exists at all the stagnant lid causes the temperature in the sublayer to be warmer than in a comparable constant viscosity convecting layer we have further modified the parameterization to account for the strong increase in homologous temperature and therefore decrease in viscosity with depth along an adiabat this modification causes even thicker stagnant lids and further elevated temperatures in the well mixed sublayer it is the stagnant lid and the comparatively large temperature in the sublayer that frustrates ocean freezing

multistage evolution and variable motion history of agenor linea europa abstract europa the second galilean moon of jupiter is composed of a silicate mantle and an ice shell which overlies a supposed subsurface ocean the surface of europa is scarred with fractures varying in morphology dimensions and geometry we focus on agenor linea a 1500 km bright band that extends across europa s antijovian to trailing southern hemisphere agenor linea is morphologically a band like strike slip fault however it experienced at least three evolutionary growth phases marked by three zones of varying albedo each with a different geological history structures within the band material and kinematic indicators are consistent with formation through oblique left lateral dilation followed by normal dilation and right lateral strike slip movement or combinations thereof these motions produced a cumulative maximum right lateral offset of 29 5 km and a maximum band width of 34 km we interpret agenor linea to have formed primarily in response to the combined effects of nonsynchronous rotation stress and diurnal tidal flexing if so its orientation is optimal for right lateral oblique opening in the current global stress field consistent with its most recent kinematic behavior a small amount of offset of relatively young tension fractures that postdate cryospheric disruption by lenticulae suggest the possibility of ongoing activity along agenor in contrast to a previously published model in which strike slip duplexing was the primary process of band formation this study shows that initial dilation of three distinct morphological zones under disparate stress orientations preceded strike slip motion that resulted in both localized and distributed deformation features within the zones across the width of the band

fracture zone tectonics at zabargad island red sea egypt abstract zabargad island which lies along the western margin of the red sea rift is a remarkable place because it provides fresh exposures of undepleted mantle peridotite how this peridotite came to be exposed on zabargad remains unclear our field mapping indicates that most of the contacts between peridotite and the adjacent bodies of pan african gneiss and cretaceous zabargad formation on the island are now high angle brittle faults zabargad formation strata have been complexly folded partly in response to this faulting overall the array of high angle faults and associated folds on the island resembles those found in cross rift transfer zones we suggest therefore that the zabargad fracture zone a band of submarine escarpments on the floor of the red sea north of the island crosses zabargad island and has actively resolved differential movement between the central red sea rift and the northern red sea rift the final stage of uplift that brought the unusual peridotite to the earth s surface is related to shallow crustal transpression which may have inverted an earlier transtensional regime

numerical simulations of convection in europa s ice shell implications for surface features 1 europa s icy surface displays numerous small 5 to 30 km diameter pits spots and uplifts that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in europa s ice shell including temperature dependent viscosity and tidal heating ice shells 15 and 50 km thick are considered consistent with several estimates of the shell thickness on europa the convection produces deep pits consistent with some of the observed features when the lithospheric viscosity is 103 105 times greater than that of the underlying ice but greater viscosity contrasts lead to topography insufficient to explain the observed pits if ductile creep is the only deformation mechanism these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 1010 however the strength of europa s surface may be low enough for plastic deformation to play a role in the convection opening the possibility that convection could produce some of europa s pits for plausible viscosities 1013 pa s at the melting temperature the pits are 100 300 m deep and 10 20 km in diameter greater or lesser viscosities lead to wider or narrower pits respectively none of our simulations produced isolated uplifts of any diameter however so these probably formed by another mechanism the convection can induce surface stresses 1 bar which exceeds the inferred strength of europa s crust and indicates the likelihood of surface disruption the maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0 05 0 07 w m 2

empirical constraints on the salinity of the europan ocean and implications for a thin ice shell induced electrical currents within europa inferred from galileo spacecraft magnetometer instrument data have been interpreted as due to a salty europan ocean published compositional models for europa s ocean based on aqueous leaching of carbonaceous chondrites range over five orders of magnitude in predicted magnesium sulfate concentrations we combine the galileo spacecraft magnetometer derived oceanic conductivities and radio doppler data derived interior models with laboratory conductivity vs concentration data for both magnesium sulfate solutions and terrestrial seawater to determine empirically the range of salt concentrations permitted for europa s ocean solutions for both a three layer spherical model and a five layer half space model that satisfy current preferred best fits to magnetometer data imply high near saturation salt concentrations and require a europan ice shell of less than 15 km thick with a best fit at 4 km ice thickness adding a conductive core and mantle has a negligible effect on the amplitude when ocean conductivities are greater than a few siemens per meter similarly we find that including a realistic ionosphere has a negligible effect we examine the implications of these results for the subsurface habitability of europa

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

thickness of a europan ice shell from impact crater simulations several impact craters on jupiter s satellite europa exhibit central peaks on the terrestrial planets central peaks consist of fractured but competent rock uplifted during cratering therefore the observation of central peaks on europa indicates that an ice layer must be sufficiently thick that the impact events did not completely penetrate it we conducted numerical simulations of vapor and melt production during cratering of water ice layers overlying liquid water to estimate the thickness of europa s icy crust because impacts disrupt material well beyond the zone of partial melting our simulations put a lower limit on ice thickness at the locations and times of impact we conclude that the ice must be more than 3 to 4 kilometers thick

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

flexure of europa s lithosphere due to ridge loading abstract using photoclinometry topographic profiles across europan ridges have been produced these profiles allow the identification of bulges in the terrain adjacent to the ridges the bulges are assumed to have been produced by flexure of the elastic lithosphere due to the load of the ridges which lie along cracks in the crust the distance from the crack to these fore bulges depends on the thickness of the elastic plate being flexed based on a survey of ridges in galileo images with resolution

europa morphological characteristics of ridges and triple bands from galileo data e4 and e6 and assessment of a linear diapirism model 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

folds on europa implications for crustal cycling and accommodation of extension regional scale undulations with associated small scale secondary structures are inferred to be folds on jupiter s moon europa formation is consistent with stresses from tidal deformation potentially triggering compressional instability of a region of locally high thermal gradient folds may compensate for extension elsewhere on europa and then relax away over time

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

subsurface oceans on europa and callisto constraints from galileo magnetometer observations magnetic field perturbations measured during galileo flybys of europa and callisto are consistent with dipole fields induced by the temporal variations of the ambient jovian magnetospheric field these fields are close to those expected for perfectly conducting moons we investigate the implications of these observations for the electrical structure of the moon s interiors using a simple shell model it is found that europa and callisto must possess regions where the conductivity exceeds 0 06 and 0 02 s m at a depth of less than 200 and 300 km below the surface respectively this conductivity is unattainable in ice or silicates unless the ice layer is at least partially molten or very large temperature gradients can be maintained below the ice an ionosphere or a cloud of pick up ions are probably also insufficiently conductive global earth like oceans under the surface of both moons could account for the observations provided they are at least a few kilometers thick c 2000 academic press

double ridges on europa accommodate some of the missing surface contraction 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 strike parallel displacement along investigated features double ridges record both ridge perpendicular contraction and expansion with a mean of 0 16 0 06 km needs space of contraction based on the analysis of 16 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 3 24 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

hydrocode simulation of ganymede and europa cratering trends how thick is europa s crust abstract one of the continuing debates of outer solar system research centers on the thickness of europa s ice crust as it affects both the habitability and accessibility of its sub surface ocean here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to europan cratering trends and galileo derived topographic profiles to investigate the crustal thickness full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius craters produced in these thin shell simulations were consistently smaller than for larger ice thicknesses which will complicate inference of large impactor population sizes simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit pit morphology this work supports that summit pit craters noted on both rocky and icy bodies can be created by the presence of a weaker layer at depth we suggest that floor pits seen only on ice rich bodies require a different formation mechanism to summit pits pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow suggesting that the starting crater morphology for viscous relaxation isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow we find that the crater depth diameter trend of europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth with a corresponding geothermal gradient of 0 025 k m as this ice thickness estimate is below 10 km results from this work suggest that convective overturn of the surface ice may occur or have occurred on europa making the development of indigenous life a possibility

thermal orbital evolution of io and europa coupled thermal orbital evolution models of europa and io are presented it is assumed that io europa and ganymede evolve in the laplace resonance and that tidal dissipation of orbital energy is an internal heat source for both io and europa while dissipation in io occurs in the mantle as in the mantle dissipation model of segatz et al 1988 icarus 75 187 two models for europa are considered in the first model dissipation occurs in the silicate mantle while in the second model dissipation occurs in the ice shell in the latter model ice shell melting and variations of the shell thickness above an ocean are explicitly included the rheology of both the ice and the rock is cast in terms of a viscoelastic maxwell rheology with viscosity and shear modulus depending on the average temperature of the dissipating layer heat transfer by convection is calculated using a parameterization for strongly temperature dependent viscosity convection both models are consistent with the present orbital elements of io europa and ganymede it is shown that there may be phases of quasi steady evolution with large or small dissipation rates in comparison with radiogenic heating phases with runaway heating or cooling and oscillatory phases during which the eccentricity and the tidal heating rate will oscillate europa s ice thickness varies between roughly 3 and 70 km dissipation in the silicate layer or 10 and 60 km dissipation in the ice layer suggesting that europa s ocean existed for geological timescales the variation in ice thickness including both convective and purely conductive phases may be reflected in the formation of different geological surface features on europa both models suggest that at present europa s ice thickness is several tens of km thick and is increasing while the eccentricity decreases implying that the satellites evolve out of resonance including lithospheric growth in the models makes it impossible to match the high heat flux constraint for io other heat transfer processes than conduction through the lithosphere must be important for the present io

pits and uplifts on europa abstract a survey of depression and uplift features on europa based on galileo regional mapping images shows that these features come in a wide range of sizes with numbers increasing greatly with decreasing size down to the limits of resolution size distributions are similar in the northern leading and southern trailing hemispheres where they are distinctly different from the southern leading and northern trailing hemispheres suggesting an oblique antipodal symmetry pattern similar to that of chaotic and tectonic terrain this pattern is suggestive of polar wander uplifts are usually polygonal or irregular in shape and rarely are cracked patches of chaotic terrain which we had surveyed earlier are not included in the current study because their topography is generally unclear and because there is no a priori known genetic linkage with the pits and uplifts these results contradict generalizations based on the earlier pits spots and domes psd taxonomy most of the type examples for psds were simply patches of chaotic terrain selected from a limited portion of their full size range the use of the term lenticula to collectively describe psds is inconsistent with the iau definition of lenticula a small dark spot seen at low resolution pits and uplifts do not correlate with lenticulae although chaos often does properties of psds that have been widely cited as primary evidence for convective upwelling in thick ice e g that uplifts are generally dome shaped and often cracked that pits and domes are regularly spaced that there is a typical diameter of 10 km were premature and not supported by subsequent data most pits and uplifts are less than 10 km across so if they formed by diapirism or convective upwelling the sources must have been very shallow less than 5 km deep how they actually formed remains unknown

formation of ridges on europa above crystallizing water bodies inside the ice shell abstract jupiter s second galilean satellite europa is a moon sized body with an icy shell and global ocean approximately 100 km thick surrounding a rocky interior its surface displays extensive tectonic activity in a geologically recent past europa s most ubiquitous surface features double ridges have a central trough flanked by two raised edifices double ridges can extend hundreds of kilometers and appear genetically related to cracks formed in the europan ice shell the origin of the raised flanks has been the center of much debate and many models have been proposed there are also ridges without a central trough single ridges these ridges are far less common than their double ridge counterparts however there are locations where along strike changes in ridge type appear to occur we explore an elastic model in which the ridges form in response to crystallization of a liquid water intrusion in our model liquid water fills tension cracks that open in the europan crust in response to tidal stress or perhaps overpressure of a subsurface ocean the crack would be long and essentially continuous similar to dikes on earth explaining the remarkable continuity and lack of segmentation of europan ridges the freezing of the water would cause a volume expansion compressing and buckling the adjacent crust we find that the geometry of the intruding water body controls the shape of the resulting ridges with single ridges forming above sill like intrusions and double ridges above dike like intrusions in order to match the ridge heights observed for double ridges we would need approximately 1 5 km2 of water intruded at a shallow depth in the ice shell potentially over the course of multiple events deeper intrusions result in a broader lower amplitude ridge than shallow intrusions

impact breaching of europa s ice constraints from numerical modeling numerical simulations show that impactors can penetrate europa s ice creating conduits to the underlying ocean breaching becomes inevitable when transient cavity depth exceeds 90 of ice thickness results indicate that a 0 5 km comet would penetrate 5 km ice and a 5 km comet could breach 40 km ice thinner ice would be breached more frequently thicker ice less so but even the 40 km upper estimates for ice thickness would be penetrated by comets with recurrence intervals less than 250 ma if actual ice thickness is 8 13 km indicated by comparing europan and simulated crater geometries the ocean could be exposed by impactors in the range 0 7 1 5 km which have recurrence intervals 3 to 7 ma thus it seems that europa s ice has been penetrated often in the past and possibly in geologically recent time the largest known impact sites callanish and tyre probably represent transition from craters to penetrating impacts the geomorphic expression of full penetration must exist on the surface chaos terrain is a candidate astrobiological materials could be transported to the ocean via these impact created conduits

tidally heated convection constraints on europa s ice shell thickness 1 the thickness of europa s ice shell is constrained with numerical experiments of thermal convection including heterogeneous tidal heating thermal convection occurs in the stagnant lid regime with most of the tidal heating located in the bottom half of the layer the addition of tidal heating mainly results in the increase of the temperature of the well mixed interior and in the decrease of the heat flux at the base of the ice layer in many simulations the ice in hot plumes is heated up to its melting point this induces episodic upwellings 0 5 ma of partially molten ice up to the base of the conductive lid with possible implications for the formation of lenticulae and chaos regions the thickness of the convective ice shell in equilibrium with the heat flow from the silicate core is estimated to be about 20 25 km tidal dissipation and surface temperature variations create lateral variations of the ice shell thickness of about 5 km with maxima near the equator at the jovian and anti jovian points and minima at midlatitudes surface heat flux is about 35 40 mw m 2 it is almost constant all over europa s surface even though the tidal dissipation rate is four times larger at the poles than at the equator

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses 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 surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates 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 a 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 within europa today

galileo magnetometer measurements a stronger case for a subsurface ocean at europa on 3 january 2000 the galileo spacecraft passed close to europa when it was located far south of jupiter s magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward jupiter this pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of europa s internal field the galileo magnetometer measured changes in the magnetic field predicted if a current carrying outer shell such as a planet scale liquid ocean is present beneath the icy surface the evidence that europa s field varies temporally strengthens the argument that a liquid ocean exists beneath the present day surface

formation of cycloidal features on europa 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

europa s differentiated internal structure inferences from four galileo encounters radio doppler data from four encounters of the galileo spacecraft with the jovian moon europa have been used to refine models of europa s interior europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice liquid outer shell but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice liquid shell the size of a metallic core is uncertain because of its unknown composition but it could be as large as about 50 percent of europa s radius the thickness of europa s outer shell of water ice liquid must lie in the range of about 80 to 170 kilometers

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

considerations for effusive cryovolcanism on europa the post galileo perspective 1 cryovolcanic resurfacing is a popular mechanism to explain relatively young surface units on icy satellites of jupiter saturn uranus and neptune prior to the galileo data acquired between 1996 and 2001 europa was thought to have undergone significant cryovolcanic resurfacing facilitated by a global ocean beneath the icy surface however close examination of galileo data at resolutions much better than those of voyager images show that many of the features previously thought to be cryovolcanic are commonly best explained by other formative mechanisms including tectonism and diapirism in this study i present an examination of the characteristics of a variety of europan surface features for which effusive cryovolcanism is a possible origin including apparently lobate flows certain elliptical to circular lenticulae and low lying smooth low albedo surfaces a review of cryovolcanic eruption theory together with galileo data analysis of europan surface geology and composition indicates that cryovolcanism is a viable though not unequivocal explanation for some of these features some constraints on cryomagma properties and lithospheric structure are offered for these cases the presence of small volume low viscosity effusions is supported by observations and modeling some positive relief lenticulae could represent more viscous effusions although diapirism may be a preferable explanation however strong evidence is lacking for cryovolcanic resurfacing on a large scale on the basis of our experience with galileo images of europa and ganymede voyager era inferences for widespread cryovolcanism on icy satellites may be overstated and will need to be carefully reexamined in the light of new data from upcoming spacecraft missions

effects of plasticity on convection in an ice shell implications for europa abstract europa s surface exhibits numerous pits uplifts and disrupted chaos terrains that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in an ice shell including the effects of plasticity which provides a simple continuum representation for brittle or semibrittle deformation along discrete fractures plastic deformation occurs when stresses reach a specified yield stress at lower stresses the fluid flow follows a newtonian temperature dependent viscosity four distinct modes of behavior can occur for yield stresses exceeding 1 bar plastic effects are negligible and stagnant lid convection with no surface motion and minimal topography results at intermediate yield stresses a stagnant lid forms but deforms plastically leading to surface velocities up to several millimeters per year slightly smaller yield stresses allow episodic catastrophic overturns of the upper conductive lid with transient stagnant lids forming in between overturn events the smallest yield stresses allow continual recycling of the upper lid with simultaneous gradual ascent of warm ice to the surface and descent of cold near surface ice into the interior the exact yield stresses over which each regime occurs depend on the ice shell thickness melting temperature viscosity and activation energy for viscous creep to form hummocky matrix and translate chaos plates by several kilometers substantial surface strain must accompany chaos formation and the three plasticity dominated convection modes described here can provide such deformation our simulations suggest that if yield stresses of 0 2 1 bar are relevant to europa then convection in europa s ice shell can produce chaos like structures at the surface however our simulations have difficulty explaining europa s numerous pits and uplifts when plasticity forces the upper lid to participate in the convection dynamic topography of 50 100 m amplitude results but the topographic structures generally have diameters of 30 100 km an order of magnitude wider than typical pits and uplifts none of our simulations produced isolated pits or uplifts of any diameter

evidence for subduction in the ice shell of europa despite widespread evidence for extension there have been few signs of contraction on the icy surface of jupiter s europa evidence for a subduction like convergent boundary suggests that europa may have active plate tectonics