on the origin of a wedge crack within the icy crust of europa 1 we suggest that a crack that developed into a wedge shaped band within the icy crust of europa at 28 s 170 w see schenk and mckinnon 1989 figure 4 prockter et al 1999 figure 1 originated under an applied compressive stress through the operation of a frictional sliding mechanism we analyze this suggestion using a scale independent sliding crack model and obtain an estimate of the maximum compressive stress to initiate the crack that compares reasonably well with earlier estimates helfenstein and parmentier 1985 mcewen 1986 greenberg et al 1998 of europan crustal stresses that are based upon elastic deformation of a shell we then show through an application of both power law dislocation creep and diffusion creep that colder near surface europan ice appears to be capable of supporting the deduced stresses with little relaxation over a period from 104 to 105 years

the temperature of europa s subsurface water ocean a 100 km deep liquid water ocean probably underlies the icy exterior of jupiter s satellite europa the long term persistence of a liquid ocean beneat ha n ice shell presents a thermal conundrum is the temperature of the ocean equal to the freezing point of water at the bottom of the ice shell or is it equal t o the somewhat warmer temperature at which water attains its maximum density we argue that most of the ocean is at the temperature of maximum density and that the bulk of the vigorously convecting ocean is separated from the bottom of the ice shell by a thin stratosphere of stably stratified water whi ch is at the freezing point and therefore buoyant if europa s subsurface water ocean is warm it could explain the widespread geologic evidence for app arent melt through events observed on its surface and may constrain the overall age of its surface

numerical modeling of endogenic thermal anomalies on europa abstract a variety of recent resurfacing features have been observed on europa which may produce thermal anomalies detectable by a future mission however the likelihood of such a detection depends on their size and lifetimes the results of this numerical study suggest that the lifetime of a thermal anomaly associated with the emplacement of 100 m of water onto the surface of europa is several hundred years and 10 years for 10 m of water if warm ice is emplaced on the surface instead of liquid water these lifetimes decrease by up to a factor of two exploration of model parameters indicates that a thin insulating surface layer can double thermal anomaly lifetimes anomalies emplaced at a latitude of 80 can remain detectable nearly a factor of two longer than those at equatorial latitudes and anomalies on the night side can remain detectable for up to 20 longer than those on the day side high temperatures are very short lived as the surface ice cools very rapidly to below 200 k due to sublimation cooling assuming steady state resurfacing the number of detectable thermal anomalies associated with the emplacement of 100 m of water would be on the order of 10 if the typical resurfacing area is 15 km 2 if recent resurfacing is dominated by chaos regions with typical areas of 100 to 1000 km 2 and lifetimes of 1000 to 4000 years the number of detectable thermal anomalies would be on the order of 1 to 10

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

persistence of thin ice regions in europa s ice crust 1 extensive data from planetary spacecraft as well as celestial mechanics models support the existence of a subsurface ocean on europa 100 km thick maintained by a tidal heat flux models in which the overlying ice crust is less than 20 km thick permit breaches in the ice due to impact events or thermal plumes from the tidally heated core we apply a two dimensional thermal model to the analysis of the refreezing of a hole in the ice crust following a breach event our model incorporates heat produced by tidal heating of europa in two ways a basal heat flux from europa s silicate and iron core together with volumetric heating of the ice shell we compare our refreezing timescales to those obtained from a model where viscous flow in the base of the ice crust fills the hole we find that catastrophic breaches in europa s ice crust may produce regions of relatively thin ice persisting up to 1 my these breaches are closed by viscous flow when radii are small 10 50 km and by conductive refreezing for larger radii especially if europa s crust has a high basal heat flow due to a hot core detection of the ice ocean interface by orbital detection of the temperature anomalies or radar sounding would be most probable in the vicinity of these events

the role of episodic overturn in generating the surface geology and heat flow on enceladus saturn s satellite enceladus shows higher heat loss than expected and a wide range of surface ages numerical simulations indicate that occasional catastrophic overturn events could be responsible for both observations by recycling portions of the icy lid to the interior which would cause transiently enhanced heat loss

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

cycloid crack sequences on europa relationship to stress history and constraints on growth mechanics based on cusp angles 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

plate motion on europa and nonrigid behavior of the icy lithosphere the castalia macula region 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

radar interferometry for measuring tidal strains across cracks on europa 1 a major uncertainty in understanding the interaction between the surface of europa and its ocean below is the present day activity of fractures using well constrained models for tidal strain and a force balance in a cracked shell we estimate the shear and normal displacement of cracks that penetrate upward from the base of the shell if more than half of the plate is fractured then surface displacements having amplitudes of 3 to 30 cm will be localized in a band 1 to 100 km from the crack plate spreading will occur if more than 85 of the plate is fractured the pattern of deformation is sensitive to both the percentage of plate that is cracked and the total thickness of the shell repeat pass radar interferometry could easily detect and map the activity of the cracks during a short experiment from a variety of suitable orbits with repeating ground tracks

estimates of europa s ice shell thickness from elastically supported topography 1 the thickness of europa s solid ice shell is uncertain and has important implications for europa s habitability and thermal history and the design of future spacecraft missions here we obtain an estimate of the ice shell thickness from observations of a plateau sw of cilix impact crater stereo topographic profiles suggest that the plateau is flexurally supported with an effective elastic thickness te of 6 2 5 km for a conductive temperature profile this te value implies a solid ice shell thickness of 15 9 20 km if the shell is convecting this estimate is a lower bound combined with independent estimates we infer a probable shell thickness of 25 km the shell thickness is likely to be uniform over the entire satellite

evidence for shear failure in forming near equatorial lineae on europa 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

is there evidence for polar wander on europa abstract the orientations of the albedo lineaments bands and lineations on europa s surface have been compared in previous studies with the global stress fields set up by orbital eccentricity orbital recession and nonsynchronous rotation of these orbital and rotational effects nonsynchronous rotation combined with an offsetting of the tidal bulge comes closest to providing agreement between the stress field generated and the lineation orientations if the lineations trace tension or extension fractures mcewen 1986 nature321 49 51 however inferred minimum principal stress directions for a broad region of wedge shaped bands near the anti jove point cannot satisfactorily be accounted for by any of the stress fields above but are consistent with the stresses resulting from a rotation of europa s ice shell about an axis through the sub and anti jove points clockwise as seen from the anti jove hemisphere p m schenk and w b mckinnon 1989 icarus79 75 100 calculations by ojakangas and stevenson 1989 icarus81 220 241 of the thermal state of europa s ice shell indicate that spatial variations in the thickness of the shell may cause it to undergo such a reorientation we have investigated whether any reorientation of the shell about an axis through the sub and anti jove points produces a stress field consistent with the full global set of prominent lineations on europa s surface we find that no such reorientation provides a good fit between the lineations and plausible fracture orientations derived from the principal stress trajectories topographic ridges identified in a limited zone south of the anti jove point are roughly consistent with compression due to clockwise polar wander but the orientations of these ridges may be strongly biased by illumination direction within the limitations of the presently available imagery nonsynchronous rotation is still the most likely cause of the prominant fractures on europa s surface and the best specific albeit regionally limited tectonic evidence consistent with recent polar wander remains the wedge shaped bands

lineament azimuths on europa implications for obliquity and non synchronous rotation abstract 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 nonsynchronous rotation on the deformational history and ice shell properties in the south polar terrain of enceladus abstract 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

expected seismicity and the seismic noise environment of europa seismic data will be a vital geophysical constraint on internal structure of europa if we land instruments on the surface quantifying expected seismic activity on europa both in terms of large recognizable signals and ambient background noise is important for understanding dynamics of the moon as well as interpretation of potential future data seismic energy sources will likely include cracking in the ice shell and turbulent motion in the oceans we define a range of models of seismic activity in europa s ice shell by assuming each model follows a gutenberg richter relationship with varying parameters a range of cumulative seismic moment release between 10 16 and 10 18 nm yr is defined by scaling tidal dissipation energy to tectonic events on the earth s moon random catalogs are generated and used to create synthetic continuous noise records through numerical wave propagation in thermodynamically self consistent models of the interior structure of europa spectral characteristics of the noise are calculated by determining probabilistic power spectral densities of the synthetic records while the range of seismicity models predicts noise levels that vary by 80 db we show that most noise estimates are below the self noise floor of high frequency geophones but may be recorded by more sensitive instruments the largest expected signals exceed background noise by 50 db noise records may allow for constraints on interior structure through autocorrelation models of seismic noise generated by pressure variations at the base of the ice shell due to turbulent motions in the subsurface ocean may also generate observable seismic noise

the ridges of europa extensions of adjacent topography onto their flanks abstract the surface of europa displays numerous generations of intersecting arrays of linear ridges at some locations along these ridges 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 here we consider how the morphology of the overprinted topography may have been inherited by the more recent ridges an analogous process occurs along some sediment starved convergent plate boundaries 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 in addition 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 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 more recent ridges may provide a crucial diagnostic of dominant ridge building mechanisms when and if 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 constraints on ridge formation processes and their diversity

patterns of fracture and tidal stresses on europa abstract the hypothesis that lineaments on europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands triple bands and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity if short reticulate dark band nnear the anti jove point are tension cracks which formed in response to tidal distortion they could only have been produced by orbital eccentricity long arcuate dark band and triple bands peripheral to the anti jove point orientations which suggest that they are strike slip faults which formed in response to orbital recession if cuspate ridges are compressional features their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume stresses due to orbital eccentricit could have produced tension cracks near the anti jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed surface fracturing if a consequence of tidal deformation places important constraints on the orbital evolution of europa

dynamical investigation of a thickening ice shell implications for the icy moon europa abstract current astrobiological exploration of the outer solar system is largely driven by understanding planetary ice ocean systems such as those of europa and enceladus the ice shell thickness that affects the dynamics of the ice ocean system is determined by the balance between heat loss via convection and conduction and the heat production via tidal dissipation and radiogenic decay in this study we investigate how ice shells of such ocean moons grow and how convection inside them evolves as the system cools we perform geodynamical calculations to investigate the ice shell thickness and growth rate as the system cools from a warm initial condition we study both diffuse and localized end member modes of tidal dissipation within the ice shell we find that as the ice shell cools and thickens there is a reconfiguration of the convection patterns increasing the size of convection cells that leads to a relatively significant increase in the ice shell growth rate

heat flow lenticulae spacing and possibility of convection in the ice shell of europa abstract two opposing models to explain the geological features observed on europa s surface have been proposed the thin shell model states that the ice shell is only a few kilometers thick transfers heat by conduction only and can become locally thinner until it exposes an underlying ocean on the satellite s surface according to the thick shell model the ice shell may be several tens of kilometers thick and have a lower convective layer above which there is a cold stagnant lid that dissipates heat by conduction whichever the case from magnetic data there is strong support for the presence of a layer of salty liquid water under the ice the present study was performed to examine whether the possibility of convection is theoretically consistent with surface heat flows of 100 200 mw m 2 deduced from a thin brittle lithosphere and with the typical spacing of 15 23 km proposed for the features usually known as lenticulae it was obtained that under europa s ice shell conditions convection could occur and also account for high heat flows due to tidal heating of the convective nearly isothermal interior but only if the dominant water ice rheology is superplastic flow with activation energy of 49 kj mol 1 this is the rheology thought dominant in the warm interior of the ice shell in this case the ice shell would be 15 50 km thick furthermore in this scenario explaining the origin of the lenticulae related to convective processes requires ice grain size close to 1 mm and ice thickness around 15 20 km

constraints on europa s rotational dynamics from modeling of tidally driven fractures abstract 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

recent tectonic activity on pluto driven by phase changes in the ice shell the new horizons spacecraft has found evidence for geologic activity on the surface of pluto including extensional tectonic deformation of its water ice bedrock see moore et al 2016 one mechanism that could drive extensional tectonic activity is global surface expansion due to the partial freezing of an ocean we use updated physical properties for pluto and simulate its thermal evolution to understand the survival of a possible subsurface ocean for thermal conductivities of rock less than 3 w m 1 k 1 an ocean forms and at least partially freezes leading to recent extensional stresses in the ice shell in scenarios where the ocean freezes and the ice shell is thicker than 260 km ice ii forms and causes global volume contraction since there is no evidence for recent compressional tectonic features we argue that ice ii has not formed and that pluto s ocean has likely survived to present day

tidal reorientation and the fracturing of jupiter s moon europa the most striking characteristic of europa is the network of long linear albedo markings over the surface suggestive of global scale tectonic processes various explanations for the fractures have been proposed freezing and expansion of an early liquid water ocean1 planetary expansion due to dehydration of hydrated silicates2 localization by weak points in the crust generated by impacts3 and a combination of stresses due to planetary volume change and tidal distortions from orbital recession and orbital eccentricity4 5 calculations by yoder6 and greenberg and weidenschilling7 have shown that europa may rotate slightly more rapidly than the synchronous rate with a rotation period reorientation through 360 ranging from 20 to 103 yr if a liquid mantle is present or up to 1010 yr if the satellite is essentially solid7 helfen stein and parmentier8 modelled the stresses due to nonsynchronous rotation and concluded that this could explain the long fractures in part of the anti jovian hemisphere in this note i present a global map of lineaments with long arc lengths 20 or 550 km and compare the lineament orientations to the tensile stress trajectories due to tidal distortions changes in the lengths of three principal semiaxes and to nonsynchronous rotation longitudinal reorientation of two of the principal semiaxes an excellent orthogonal fit to the lineaments is achieved by the stresses due to nonsynchronous rotation with the axis radial to jupiter located 25 east of its present position this fit suggests that nonsynchronous rotation occurred at some time in europa s history

global scale surface spectral variations on titan seen from cassini vims we present global scale maps of titan from the visual and infrared mapping spectrometer vims instrument on cassini we map at 64 near infrared wavelengths simultaneously covering the atmospheric windows at 0 94 1 08 1 28 1 6 2 0 2 8 and 5 m with a typical resolution of 50 km pixel or a typical total integration time of 1 s our maps have five to ten times the resolution of ground based maps better spectral resolution across most windows coverage in multiple atmospheric windows and represent the first spatially resolved maps of titan at 5 m the vims maps provide context and surface spectral information in support of other cassini instruments we note a strong latitudinal dependence in the spectral character of titan s surface and partition the surface into 9 spectral units that we describe in terms of spectral and spatial characteristics

porosity and salt content determine if subduction can occur in europa s ice shell motivated by recent evidence for subduction in europa s ice shell we explore the geophysical feasibility of this process here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct we also vary the initial salt content in europa s ice shell and determine the buoyancy of our simulated subducting slab we find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in europa s ice shell is actually possible generally we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction if salt contents are laterally homogenous and europa has a reasonable surface porosity of 0 0 1 the conductive portion of europa s shell must have salt contents exceeding 22 for subduction to occur however if salt contents are laterally heterogeneous with salt contents varying by a few percent subduction may occur for a surface porosity of 0 0 1 and overall salt contents of 5 thus we argue that under plausible conditions subduction in europa s ice shell is possible moreover assuming that subduction is actively occurring or has occurred in europa s recent past provides important constraints on the structure and composition of the ice shell

nonsynchronous rotation evidence and fracture history in the bright plains region europa abstract 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

evidence for separation across a gray band on europa abstract thynia linea a gray band on europa is found to be a 25 km wide and 900 km long region of lithospheric separation which has been infilled by relatively dark material roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width displacement azimuths indicate a best fit pole of opening near 79 s 200 w however displacement magnitude decreases toward either end of the gray band indicating that it is more akin to a tear in a nonrigid europan lithosphere opening was in response to nw se directed tensile stress in accord with the stress predictions of nonsynchronous rotation observations of thynia linea are consistent with a laterally mobile brittle lithosphere decoupled from ductile or liquid material below as previously suggested to account for opening of wedge shaped bands in europa s antijovian region lithospheric separation and contemporaneous emplacement of new material offers a possible volcano tectonic scenario for resurfacing europa if this process is ongoing resurfacing can be accomplished on a time scale consistent with the satellite s surface age if one gray band or zone of wedge shaped bands forms every 10 3 10 4 yr and becomes unrecognizable with age this would imply that features on europa brighten with age as through continuous deposition of frost onto the surface

rapid changes in the east asian monsoon during the last interglacial in the bohai sea coastal zone china the bohai sea coastal zone of china faces the pacific ocean to the east and the eurasian continent to the west hence this region is influenced by both the ocean and continental landmasses the bohai sea coastal zone has significant monsoon climate characteristics and a strong sensitivity to climate change the miaodao stratigraphical section mds contains historical information about climate features in the region especially the high frequency variations during the last interglacial sea level changes and the evolution of the east asian monsoon by analyzing the ages of various sedimentary facies in combination with proxy paleoclimatic indices i e grain 63 mu m fraction average grain size mz clay silt sand content sc d magnetic susceptibility and the ratios na2o al2o3 and al2o3 tofe sio2 in the fifth segments of the mds from the last interglacial mds5 we conclude that subsections 5a 5c and 5e were controlled by summer monsoons whereas subsections 5b and 5d were formed when winter monsoons prevailed these results were similar to oxygen isotope analyses from previous studies including the spectral mapping project specmap and the north greenland ice core project northgrip five and a half comparable oscillations in proxy indices that were dated to ca 116 1 118 3 121 2 122 7 125 9 and 128 7 ka occurred within the mds 5e subsection when winter monsoon winds strengthened this millennial scale climate variability during the eemian period may have reached up to ten and a half oscillations with a quasi periodicity of approximately a 1 470 year cycle during the late glacial period this rapid period of climate change has been recorded in northern and central europe central asia and in east asia the climate influencing mechanism was probably initiated by changes in solar activity and driven by the east asian monsoon and sea level oscillations

evidence for a weakly stratified europan ocean sustained by seafloor heat flux we interpret chaos type features on the surface of europa as melt through structures formed by rotationally confined steady and or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor smaller lenticular features in the vicinity of chaos type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites the ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above seafloor heating maintained by tidal dissipation in the rocky interior generates an estimated global heat flux of 8 7 1012 w and limits the mean ice thickness to 2 5 km for seafloor heat sources with radii r that are less than the ocean s deformation radius rd nd n is the brunt vaisala frequency d is the water depth and is the coriolis parameter the diameters of chaos type regions are expected to diminish from o 100 km within equatorial regions to o 10 km at high latitudes assuming spatially uniform water depth and density structure where r rd the scale of the source region determines the scale of the melt through features provided there is sufficient time before refreezing ice rafts in large melt through regions are imbedded in episodes of preferentially anticyclonic circulation corresponding to clockwise counterclockwise motions in the northern southern hemisphere we calculate that 1021 j were required to melt the ice in the 100 km diameter conamara chaos region and that for a steady localized heat flux f 1011 w 1 of the global heat flux it took 1000 years for the initial melt through to occur assuming that ice raft displacements in conamara chaos occurred during a major melt through event maximum current speeds in the region were o 10 cm s 1 and refreezing occurred within 20 hours a lack of well defined ice drift in other major melt through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion

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

detectability of thermal signatures associated with active formation of chaos terrain on europa abstract a recent study by schmidt et al 2011 suggests that thera macula one of the chaos regions on europa may be actively forming over a large liquid water lens such a process could conceivably produce a thermal anomaly detectable by a future europa orbiter or flyby mission allowing for a direct verification of this finding here we present a set of models that quantitatively assess the surface and subsurface temperatures associated with an actively resurfacing chaos region using constraints from thera macula the results of this numerical study suggest that the surface temperature over an active chaos region can be as high as 200 k however low resolution galileo photo polarimeter radiometer ppr observations indicate temperatures below 120 k over thera macula this suggests that thera macula is not currently active unless an insulating layer of at least a few centimeters in thickness is present or activity is confined to small regions reducing the overall intensity of the thermal signature alternatively thera may have been cooling for at least 10 100 yr and still contain a subsurface lake which can take 300 000 yr to crystallize according to the present study a more sensitive instrument capable of detecting anomalies 5 k above ambient could detect activity at thera macula even if an insulating layer of 50 cm is present

europa s small impactor flux and seismic detection predictions abstract europa is an attractive target for future lander missions due to its dynamic surface and potentially habitable sub surface environment seismology has the potential to provide powerful new constraints on the internal structure using natural sources such as faults or meteorite impacts here we predict how many meteorite impacts are likely to be detected using a single seismic station on europa to inform future mission planning efforts to this end we derive 1 the current small impactor flux on europa from jupiter impact rate observations and models 2 a crater diameter versus impactor energy scaling relation for icy moons by merging previous experiments and simulations and 3 scaling relations for seismic signal amplitudes as a function of distance from the impact site for a given crater size based on analogue explosive data obtained on earth s ice sheets finally seismic amplitudes are compared to predicted noise levels and seismometer performance to determine detection rates we predict detection of 0 002 20 small local impacts per year based on p waves travelling directly through the ice crust larger regional and global scale impact events detected through mantle refracted waves are predicted to be extremely rare 10 8 1 detections per year so are unlikely to be detected by a short duration mission estimated ranges include uncertainties from internal seismic attenuation impactor flux and seismic amplitude scaling internal attenuation is the most significant unknown and produces extreme uncertainties in the mantle refracted p wave amplitudes our nominal best guess attenuation model predicts 0 002 5 local direct p detections and 6 10 6 0 2 mantle refracted detections per year given that a plausible europa landed mission will only last around 30 days we conclude that impacts should not be relied upon for a seismic exploration of europa for future seismic exploration faulting due to stresses in the rigid outer ice shell is likely to be a much more viable mechanism for probing europa s interior

fault induced perturbed stress fields and associated tensile and compressive deformation at fault tips in the ice shell of europa implications for fault mechanics 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 50e70 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 2005 elsevier ltd all rights reserved

formation of ganymede s grooved terrain by convection driven resurfacing the heat flux and strain rate inferred for grooved terrain formation on ganymede can be produced in a convecting ice shell 10 100 km thick with weak near surface ice smooth linear grooves may have formed by convection driven lithospheric spreading and long wavelength compressional folds may form atop convective downwellings and would possibly be detectable with mapping from esa s upcoming jupiter icy moon explorer mission

seismic detectability of a subsurface ocean on europa abstract a seismometer placed on jupiter s moon europa can use passive monitoring to determine the presence or absence of a possible liquid water ocean beneath europa s surface ice and estimates of the thickness of the ice shell itself using probable seismic velocities for europa s ice it is demonstrated that surface waves propagating in the frequency band of 0 1 to 0 5 hz wave periods of 2 to 10 s can discriminate ice shells varying from 5 to 20 km in thickness miniature light weight seismometers with appropriate sensitivities are available

planetary science europa awakening brines percolating in the icy crust of jupiter s moon europa may be responsible for the satellite s enigmatic chaotic terrains a new model predicts that one such terrain is currently forming over shallow subsurface water see letter p 502

eruptions arising from tidally controlled periodic openings of rifts on enceladus the cassini flyby of 14 july 2005 revealed plumes of water vapour and ice associated with the tiger stripe features on the surface of saturn s icy moon enceladus since then the challenge has been to explain the nature of the plumes and the forces driving them two papers this week offer an explanation that accounts for both the plume characteristics and the presence of hot spots without the need to assume the existence of near surface liquid water a requirement of some previous models nimmo et al identify tidally driven lateral fault motions near the tiger stripes as the most likely drivers of heat and vapour production and hurford et al show that as enceladus orbits saturn the parent planet s tides make the satellite s ice flex this may cause the tiger stripes to open and close periodically exposing volatile gases and allowing them to be released plumes of gas have been seen near the south polar region of enceladus a small icy satellite of saturn and observations revealed large rifts in the crust informally called tiger stripes hurford et al report that during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions plume activity is expected to vary periodically in 2005 plumes were detected near the south polar region of enceladus1 a small icy satellite of saturn observations of the south pole revealed large rifts in the crust informally called tiger stripes which exhibit higher temperatures than the surrounding terrain and are probably sources of the observed eruptions2 models of the ultimate interior source for the eruptions are under consideration1 3 4 5 other models of an expanding plume6 require eruptions from discrete sources as well as less voluminous eruptions from a more extended source to match the observations no physical mechanism that matches the observations has been identified to control these eruptions here we report a mechanism in which temporal variations in tidal stress open and close the tiger stripe rifts governing the timing of eruptions during each orbit every portion of each tiger stripe rift spends about half the time in tension which allows the rift to open exposing volatiles and allowing eruptions in a complementary process periodic shear stress along the rifts also generates heat along their lengths7 8 9 which has the capacity to enhance eruptions plume activity is expected to vary periodically affecting the injection of material into saturn s e ring10 and its formation evolution and structure moreover the stresses controlling eruptions imply that enceladus icy shell behaves as a thin elastic layer perhaps only a few tens of kilometres thick

strike slip fault evolution on europa evidence from tailcrack geometries secondary cracks are commonly produced at stress concentration points at the tips of slipping interfaces such as faults these so called tailcracks form an antisymmetric pattern at opposite tips of the fault with a fracture geometry that is a mechanical indicator of the sense of slip whether left lateral or right lateral i present descriptions of tailcracks along numerous strike slip faults on europa two distinct styles of strike slip faults are identified ridge like and band like the angles between faults and tailcracks are variable and are commonly less than the theoretical 70 5 angle that approximately characterizes many terrestrial examples involving fault surfaces that remain in contact during slip median tailcrack angles are lower for band like faults 30 than ridge like faults 52 in addition the sense of curvature of band like fault tailcracks is characteristically opposite to that of ridge like faults analytical models of stress orientations around strike slip faults on europa indicate that these effects result from dilation during strike slip motion band like faults characteristically underwent concurrent dilation and shearing but this dynamic coupling is not characteristic of ridge like faults the implication is that strike slip faulting was not a globally homogeneous process on europa and as a result the morphologies of strike slip faults are variable furthermore band like faults appear to corroborate the predictions of the tidal walking theory exhibiting predominantly right lateral motions in the southern hemisphere and left lateral motions in the northern hemisphere however ridge like faults do not obey this slip sense convention suggesting that the tidal walking theory may be appropriate for describing the evolution of dilational band like faults but that ridge like faults may have resulted from a different driving mechanism

europa 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

probing europa s interior with natural sound sources europa s interior structure may be determined by relatively simple and robust seismo acoustic echo sounding techniques the strategy is to use ice cracking events or impacts that are hypothesized to occur regularly on europa s surface as sources of opportunity a single passive geophone on europa s surface may then be used to estimate the thickness of its ice shell and the depth of its ocean by measuring the travel time of seismo acoustic reflections from the corresponding internal strata quantitative analysis is presented with full field seismo acoustic modeling of the europan environment this includes models for europan ambient noise and conditions on signal to noise ratio necessary for the proposed technique to be feasible the possibility of determining europa s ice layer thickness by surface wave and modal analysis with a single geophone is also investigated

on the origins of band topography europa we use stereo derived topography of extensional bands on europa to show that these features can be elevated by 100 150 m with respect to the surroundings and that the positive topography sometimes extends beyond the band margins lateral variations in shell thickness cannot maintain the observed topography for timescales greater than 0 1 myr lateral density variations can maintain the observed topography indefinitely mean density contrasts of 5 and 50 kg m 3 are required for shell thicknesses of 20 and 2 km respectively density variations caused by temperature contrasts require either present day heating or that bands are young features 1 myr old stratigraphic analyses suggest that these mechanisms are unlikely the observation that bands form from ridges may be explained by an episode of shear heating on ridges weakening the ridge area and leading to strain localization during extension fracture porosity is likely to persist over myr timescales in the top one third to one quarter of the conductive part of the ice shell lateral variations in this porosity of order 20 are the most likely mechanism for producing band topography if the ice shell is thin 2 km porosity variations of 2 or less are required if the shell is thicker 20 km if the ice shell is thick lateral variations in salt content are a more likely mechanism warm ice will tend to lose dense low melting temperature phases and be buoyant relative to colder salt rich ice thus lateral density variations will arise naturally if bands have been the sites of either localized heating or upwelling of warm ice during extension

impact penetration of europa s ice crust as a mechanism for formation of chaos terrain ice thickness estimates and impactor dynamics indicate that some impacts must breach europas ice crust and outcomes of impact experiments using ice over water targets range from simple craters to chaos like destroyed zones depending on impact energy and ice competence firstorder impacts into thick ice or at low impact energy produce craters second order impacts punch through the ice making holes that resemble raft free chaos areas third order impacts into thinnest ice or at highest energy produce large irregular raft filled zones similar to platy chaos other evidence for an impact origin for chaos areas comes from the size frequency distribution of chaos craters on europa which matches the impact production functions of ganymede and callisto and from small craters around the large chaos area thera macula which decrease in average size and density per unit area as a function of distance from theras center there are no tiny chaos areas and no craters 50 km diameter this suggests that small impactors never penetrate whereas large ones berpenetrators 2 5 km diameter at average impact velocity always do existence of both craters and chaos areas in the size range 2 40 km diameter points to spatial temporal variation in crust thickness but in this size range craters are progressively outnumbered by chaos areas at larger diameters suggesting that probability of penetration increases with increasing scale of impact if chaos areas do represent impact sites then europas surface is older than previously thought the recalculated resurfacing age is 480 302 960 ma greater than prior estimates but still very young by solar system standards

the search for current geologic activity on europa observational evidence and theoretical arguments suggest that jupiter s satellite europa could be geologically active and possess an ocean of liquid water beneath its surface at the present time we have searched for evidence of current geologic activity on europa in the form of active plumes venting material above the surface and by comparison of voyager and galileo images to look for any changes on the surface so far we have observed no plumes and have detected no definitive changes the lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km2 y 1 in the regions analyzed assuming alterations will cover contiguous areas of at least 4 km2 over a period of 20 years assuming this as a constant globally uniform resurfacing rate leads to a minimum average surface age of 30 million years we also suggest that the lack of obvious circular albedo patterns on the surface due to plumes coupled with the presence of bright rayed craters such as pwyll and the predicted sputtering erosion rate implies that no large scale plume activity has taken place over at least the last few thousand years we thus conclude that if europa s surface is currently active any changes must be relatively small in spatial scale or episodic in nature rather than continuous to detect potential small scale surface changes we need high resolution comparisons between the galileo data and future europa orbiter images

ocean worlds in the outer solar system many outer solar system bodies are thought to harbor liquid water oceans beneath their ice shells this article first reviews how such oceans are detected we then discuss how they are maintained when they formed and what the oceans likely characteristics are we focus in particular on europa ganymede callisto titan and enceladus bodies for which there is direct evidence of subsurface oceans we also consider candidate ocean worlds such as pluto and triton

tectonic processes on europa tidal stresses mechanical response and visible features abstract 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

polar wander of an ice shell on europa an ice shell on europa that is decoupled from the silicate core by a layer of liquid water has a thermal equilibrium thickness profile that varies with position over its surface because of spatial variations in the surface temperature and tidal dissipation within the ice g w ojakangas and d j stevenson 1989 icarus 81 220 241 the second spherical harmonic degree components of these thickness variations and of any fossil rotational and tidal bulges present on the shell contribute to the inertia tensor of the body the problem is that of a planetary elastic lithosphere that is topographically loaded from below following r j willemann and d l turcotte 1981 proc lunar planet sci b 12 837 851 we develop equations describing the variations in the inertia tensor of a body when second harmonic degree topography is added to the base of the crust for an ice shell on europa it is found that a state of thermal equilibrium may involve an unusual orientation of the principal axes of inertia when hydrostatic bulges are ignored in which the intermediate and maximum principal moments are interchanged to reach the preferred orientation for synchronous satellites a thermal equilibrium shell must execute a net reorientation of 90 about the satellite planet direction we present a simple model of rigid synchronously rotating satellite in a circular orbit for which the principal moment difference b c increases with time becoming positive for t 0 the model demonstrates that the expected reorientation is indeed dynamically favored we then consider a more realistic model in which europa s shell and ocean are assumed to reorient as a single entity independently of the core hindered only by viscous dissipation within the shell such coupling of the shell with the ocean and lack of coupling with the core is suggested by g w ojakangas 1988 coupled thermal and dynamical evolution of planetary bodies ph d thesis california institute of technology the model suggests that friction in the shell eliminates the possibility of polar wander unless a low conductivity regolith increases the near surface temperature by a few tens of degrees so the ice just below the regolith behaves viscously on the polar wander time scale however if the equator to pole near surface temperature difference is decreased by more than a critical model dependent amount the shell s inertia tensor no longer has an unstable form in thermal equilibrium alternatively polar wander may occur if preexisting surface fractures e g due to tidal stresses extend to a depth where the ice behaves viscously these fractures must be lubricated perhaps by liquid water from below if the temperature t f at the base of the regolith or the surface fractures is 125 k the model suggests that polar wander occurs on a time scale 2 x 10 6 years decreasing as t f increases after b c becomes postive in the absence of dissipation polar wander would occur in few x 10 3 years large scale polar wander may occur episodically separated by periods on the order of the thermal diffusion time for the shell 10 7 years although a state of slow continuous drifting of the pole is also possible the time scales of viscous flow of topography at the base of the ice is also near 10 7 years polar wander is a very effective means for fracturing the ice and may have contributed to the observed global fracture systems in europa s ice

a fracture history on enceladus provides evidence for a global ocean 1 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

mechanics of tidally driven fractures in europa s ice shell abstract a fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden it is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on europa such penetration is found to be very unlikely in the absence of an ocean a cycloidal crack would then form as a sequence of near instantaneous discrete failures each extending roughly the brittle layer thickness in range linked with a much lower apparent propagation speed set by the moving tidal stress field the implications of this porous ice fracture model for ice penetrating radar scattering loss and seismic activity are quantified

a revised model for cycloid growth mechanics on europa evidence from surface morphologies and geometries 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

petrology and fluid inclusions of a quartz kyanite segregation in the main thrust zone of the himalayas fluid inclusion data are compared to p t estimates from coexisting minerals in a quartz kyanite segregation and in the surrounding garnet bearing micaschist mineral assemblages notably plagioclase garnet kyanite quartz record a retromorphic evolution from 550 600 c 7 8 kb to 450 500 c 3 4 kb the density of the early fluid in the segregation h2o 9 to 15 mole co2 is compatible with this last set of values it suggests a continuity of the evolution in the metamorphic rock and in the segregation and an early separation of the during postmetamorphic uplift

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

gas driven water volcanism and the resurfacing of europa we have analyzed the problem of how pathways are created for the resurfacing by water or volatiles for a europa model in which there is an ocean underlying a thin ice shell 10 km thickness open cracks propagating downward cannot reach the ocean unless the relative tension in the ice is both very high 100 bar and applied very rapidly 10 4 10 5sec there is no known way to satisfy these criteria water filled cracks propagating upward can approach the surface for relative tensions of 5 10 bar comparable to the stresses induced by the eccentricity tide exsolution of plausible gas species co 2 co ch 4 so 2 can then occur at the crack tip the application of linear elastic fracture mechanics shows that the gas filled portion of the crack pinches off from the water filled portion and may rise rapidly to the surface entraining some water many of these cracks may end up as intrusions rather than extrusions because of partial freeze out of co 2 and water on the walls of the crack the resulting eruption is initially gas dominated but may include a subsequent less extended eruption of foam there may not be a direct relationship between the resurfacing process described here and the currently known geologic features observed on the surface the main problem with this or any model for tapping the ocean is the propagation of cracks through the soft ice immediately overlying the ocean

an upper bound on pluto s heat flux from a lack of flexural response of its normal faults abstract the topography of rifts on icy bodies can be used to probe their internal properties uplifted and curved rift flanks occur when the elastic thickness te of the crust is low indicative of high heat flow out of the body stereo topography of pluto shows no evidence of rift flank uplift associated with large extensional graben to the west of sputnik planitia modeling the amount of topographic deflection expected from varying elastic thicknesses yields a conservative lower bound on te of 8 km at the time of deformation the maximum implied heat flux since graben formation is 66 85 mwm 2 this upper bound is consistent with the predicted paleo heat fluxes from radioactive decay 6 mwm 2 but likely only marginally consistent with nearby apparently viscously relaxed impact craters additionally we also analyze the shear stresses on these faults and find that pluto s faults only need to support low stresses in the range of 100 300 kpa to explain their dimensions

geometry and spatial distribution of lenticulae on europa the surface of europa contains several types of roughly elliptical features collectively called lenticulae lenticulae may have positive relief domes or negative relief pits may disrupt the crust chaos or discolor the surface spots some lenticulae have attributes of both domes and chaos dome chaos we map the location dimensions and shapes of all lenticulae and their interactions with other lenticulae and lineaments we find that 1 pits and domes have similar sizes 2 chaos are larger than the other lenticulae 3 pits are clustered within the trailing antijovian quadrant and the leading subjovian quadrant whereas domes dome chaos and chaos terrains are more uniformly distributed 4 the areal density for all lenticulae is not uniform 5 lenticulae do not divert the path of younger lineaments such as ridges taken together these observations are consistent with conceptual models in which lenticulae are created by intrusion of liquid water bodies or convection within the ice shell additionally the observations are consistent with the notion that each type of lenticula is a surface expression of dynamics within the ice shell at a different stage of lenticulae evolution the similar size and shape of pits and domes suggests that one may evolve into the other because domes are more numerous and more uniformly distributed than pits they are more likely to represent the end stage of this evolution assuming the end stage leaves a longer lasting surface expression models also predict that larger features are more likely to disrupt the crust which is consistent with dome chaos and chaos being larger than pits and domes we find no examples of lineaments offsetting pits but lineaments do cross some chaos pits also have a preferred northwest southeast elongation whereas domes dome chaos and chaos do not have a preferred orientation if lenticulae orientation is influenced by crustal stress then pits may have formed during a shorter time interval than the other features as a result pits may sample a shorter more recent time period than domes dome chaos and chaos consistent with pits being the earliest stage in the evolution of lenticulae we find no strong evidence that lineaments are deflected by lenticulae implying either that the stresses created by lenticulae are too small to influence lineaments or that the complete evolution of lenticulae occurs on a time scale that is short compared to the time between the formation of lineaments at a given location

constraints on the subsurface structure of europa abstract the identification of anti jovian wedge shaped bands as tension cracks separating thin lithospheric blocks that have rotated apart is used with accepted failure criteria and ductile flow laws to quantitatively derive conductive thermal gradients in the brittle crust and minimum amounts of differentiation of water on europa the interpretation of wedge shaped bands as tension cracks that have allowed the rotation and translation of large blocks of lithosphere is based on offsets of preexisting lineations and the ability to fit blocks of europa s surface back together without gap or overlap the size of the blocks and their undeformed state suggest the lithosphere was greater than a few kilometers thick but less than a few tens of kilometers thick to have allowed the rotation to occur and that it was decoupled from the silicate interior at the time of rotation analysis of the griffith criterion for tensile and shear failure of intact materials and byerlee s law for the shear failure of prefractured materials shows that the slip on preexisting fractures with dips greater than 25 will occur at lower stress differences than those required to break intact ice or rock the formation of tension cracks on europa requires a intact lithosphere with no preexisting fractures or subsurface mechanical discontinuities confining pressure and the tensile strength of ice limit the depth to which tension cracks can form and thus the thickness of the brittle lithosphere under europan conditions to 6 km with a corresponding maximum stress difference of 10 mpa below the brittle lithosphere brittle ductile tr transition the ice deforms by ductile creep which is thermally activated the ductile yield stress curve must intersect the tensile failure curve no deeper than 6 km or tension cracks would not have formed shear failure occurs below this depth this tensile brittle ductile intersection depth constrains thermal gradients on europa to 3 7 km for tensile lithospheres of 6 3 km thick respectively for the assumed ductile flow laws and material constants the estimate of 3 km is a minimum because faster strain rates than assumed and strengthening of ice by inclusion of silicates each raises gradients by about a third and possible though unlikely fluid pressure can raise gradients by about a degree per kilometer for tensile lithospheres less than 6 km thick given a fluid pressure ratio of 0 5 our results limit the possible effects of surface warming processes such as insulating regoliths or a solid state greenhouse to less than 10 30 on europa at the time of fracturing a minimum of about one quarter of europa s water is required to have differentiated to have allowed complete decoupling of the ice lithosphere from the underlying silicate mantle

layering and double diffusion style convection in europa s ocean abstract we assess the effects of fluid composition and depth dependent hydrostatic pressure on dynamics in a europan ocean primarily composed of aqueous na 2 so 4 and mgso 4 for both a saturated and a dilute ocean we observe a salinity and pressure dependent check on buoyancy in putative upwellings which could act as a mechanism for storing heat in the ocean s base uptake of salt to a warmed parcel of water from the seafloor environment causes upwellings to lose buoyancy before reaching the base of the overlying ice implying a tendency toward ocean stratification we make an analogy to double diffusive convecting systems observed in the red sea and elsewhere in earth s waters using currently understood parameterizations for onset and stability we estimate a range of temperatures and salinities for which double diffusive convection may occur in europa s ocean in the red sea boundary layers separating convecting zones have been observed to move upward as the lower layer acquires heat and salt we examine the possibility of stratification and double diffusive convection as mechanisms for heat storage in europa s ocean and possible correlation with recently inferred changes in ice surface alteration style over the last 30 80 myr

geologic mapping of the northern leading hemisphere of europa from galileo solid state imaging data 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

tidal tectonic processes and their implications for the character of europa s icy crust 1 jupiter s satellite europa is covered by an icy crust likely overlying a deep global liquid water ocean the mutually dependent relationship between the orbit and tidal processes controls europa s rotation heating and stress observations of the surface and investigation of tidal tectonic processes indicate that the dominant types of surface terrain have been created by frequent repeated exposure of an underlying ocean to the surface surface lineaments including ubiquitous cycloidal chains of arcs features are correlated with tidal stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude stratigraphy of tectonic features combined with models of how and where they formed demonstrates a moderate rate of nonsynchronous rotation tidal driving of strike slip displacement in which surface areas shear passed one another by daily 85 hours on europa tidal stresses suggests that cracks penetrate to a liquid layer the characteristic ridge sets that cover tectonic terrain may be built by tidal pumping of fluid and slush to the surface on a daily basis widespread tectonic dilation creates new surface as material rises from below in addition to those tectonic terrains nearly half of the surface is chaotic terrain with morphology and other characteristics indicative of melt through from below consistent with the tectonic indications of thin ice and plausible tidal heating rates formation of both chaotic and tectonic terrains has continually resurfaced the satellite while connecting the ocean to the surface surface colorants correlate with locations where ocean water reached the surface such as along large scale ridge systems and around chaotic terrain as a result of tides liquid water may have bathed crustal cracks and surfaces with heat transporting and mixing substances vertically such daily transported materials would include substances from the oceanic reservoir oxidants and fuels created at the surface as well as any organisms and their chemical products thus a variety of habitable environments likely exist in the crust deactivation of individual cracks after thousands of years due to the tidally driven nonsynchronous rotation constrains processes like ridge building and strike slip displacement and would require any living organisms to adapt to such change

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

glimpsing eruptions on europa second closest to jupiter after io of the four large galilean satellites icy europa is one of the strangest objects in the solar system 1 on page 171 of this issue roth et al 2 present strong evidence for ongoing eruptions of plumes of water vapor from europa s surface this is a potentially major discovery making europa only the fourth object in the solar system known to exhibit ongoing internally powered geological activity after earth europa s volcanic neighbor moon io and saturn s icy moon enceladus

the orbital thermal evolution and global expansion of ganymede article history the tectonically and cryovolcanically resurfaced terrains of ganymede attest to the satellite s turbulent geologic history yet the ultimate cause of its geologic violence remains unknown one plausible scenario suggests that the galilean satellites passed through one or more laplace like resonances before evolving into the current laplace resonance passage through such a resonance can excite ganymede s eccentricity leading to tidal dissipation within the ice shell to evaluate the effects of resonance passage on ganymede s thermal history we model the coupled orbital thermal evolution of ganymede both with and without passage through a laplace like resonance in the absence of tidal dissipation radiogenic heating alone is capable of creating large internal oceans within ganymede if the ice grain size is 1 mm or greater for larger grain sizes oceans will exist into the present epoch the inclusion of tidal dissipation significantly alters ganymede s thermal history and for some parameters e g ice grain size tidal q of jupiter a thin ice shell 5 to 20 km can be maintained throughout the period of resonance passage the pulse of tidal heating that accompanies laplace like resonance capture can cause up to 2 5 volumetric expansion of the satellite and contemporaneous formation of near surface partial melt the presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in ganymede s ice shell larger stresses result if the ice shell rotates non synchronously the combined effects of satellite expansion its associated tensile stress rapid formation of near surface partial melt and tidal stress due to an eccentric orbit may be responsible for creating ganymede s unique surface features

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

crack azimuths on europa the g1 lineament sequence revisited abstract the tectonic sequence in the anti jovian area covered by regional mapping images from galileo s orbit e15 is determined from a study of cross cutting relationships among lineament features the sequence is used to test earlier results from orbit g1 based on lower resolution images which appeared to display a progressive change in azimuthal orientation over about 90 in a clockwise sense such a progression is consistent with expected stress variations that would accompany plausible non synchronous rotation the more recent data provide a more complete record than the g1 data did we find that to fit the sequence into a continual clockwise change of orientation would require at least 1000 5 cycles of azimuthal rotation if due to non synchronous rotation of europa this result implies that we are seeing back further into the tectonic record than the g1 results had suggested the three sets of orientations found by geissler et al now appear to have been spaced out over several cycles not during a fraction of one cycle while our more complete sequence of lineament formation is consistent with non synchronous rotation a statistical test shows that it cannot be construed as independent evidence other lines of evidence do support non synchronous rotation but azimuths of crack sequences do not show it probably because only a couple of cracks form in a given region in any given non synchronous rotation period

global resurfacing of uranus s moon miranda by convection miranda an icy moon of uranus is one of the most visually striking and enigmatic bodies in the solar system three polygonal shaped regions of intense deformation dubbed coronae dominate the surface of miranda here we use numerical methods to show that sluggish lid convection in miranda s ice shell powered by tidal heating can simultaneously match the global distribution of coronae the concentric deformation pattern and the estimated heat flow during formation the expected rheological conditions in miranda s ice shell lead to the development of low order convection that produces surface deformation patterns similar to those observed we find that satellite core size strongly controls convection geometry and that loworder convection patterns are much more stable for core radii less than half the satellite radius

dynamics of rifting and modes of extension on icy satellites 1 a simple numerical model of extension in icy satellite shells is developed thinning of the ice weakens the shell promoting further extension if lateral flow in the lower part of the shell is unimportant extension is opposed and wide rifts are generated if lateral flow is rapid localized extension is favored and narrow rifts are produced thick shells or high strain rates favor the development of narrow rifts low strain rates favor wide rifting it is proposed that bands extensional features on europa are narrow rifts while groove lanes on ganymede are wide rifts the existence of wide rifting on ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4 8 km and a strain rate of 10 15 s 1 strain rates at the time of rifting whether this shell thickness applies to present day europa depends on the age of band formation which is poorly known the difference between rifting behavior on ganymede and europa is due to either higher strain rates or higher shell thicknesses on europa during rifting the mean stresses required to cause the observed rifting are 0 2 mpa for ganymede and 0 3 mpa for europa these values are comparable to estimates derived from flexural features index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8109 tectonophysics continental tectonics extensional 0905 8160 tectonophysics rheology general keywords europa ganymede extension faulting ice

segmented lineaments on europa implications for the formation of ridge complexes and bright bands 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

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

tides on europa and the thickness of europa s icy shell 1 it has been shown previously that measurements of tides on jupiter s moon europa can be used to determine whether there is a liquid ocean beneath this moon s icy outer shell in this paper we examine the further possibility of constraining the thickness of the icy shell in the case where a liquid ocean exists by combining measurements of tidal gravity obtained from tracking an orbiting spacecraft with measurements of vertical tidal surface displacements obtained from a precise onboard altimeter by simulating a 1 month europa mapping mission we demonstrate that this combination of tidal measurements would provide a much better estimate of ice thickness than could be obtained using either tracking or altimeter measurements alone the thickness value inferred from the combined data would also require an estimate of the shear modulus of europa s icy shell this introduces an additional uncertainty in the thickness estimate that is approximately proportional to the uncertainty in the inverse of the shear modulus