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

resurfacing history of europa from pole to pole geological mapping abstract we produced geologic maps from two regional mosaics of galileo images across the leading and trailing hemispheres of europa in order to investigate the temporal distribution of units in the visible geologic record five principal terrain types were identified plains bands ridges chaos and crater materials which are interpreted to result from 1 tectonic fracturing and lineament building 2 cryovolcanic reworking of surface units with possible emplacement of sub surface materials and 3 impact cratering the geologic histories of both mapped areas are essentially similar and reflect some common trends tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments the opening of wide bands with infilling of inter plate gaps and the buildup of ridges and ridge complexes along prominent fractures in the ice it also appears that lineaments are narrower and more widely spaced with time the lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation in contrast the degree of cryovolcanic resurfacing appears to increase with time as chaos formation dominates the later parts of the geologic record these trends and the transition from tectonic to cryovolcanic dominated resurfacing could be attributed to the gradual thickening of europa s cryosphere during the visible geologic history that comprises the last 2 or 30 80 myr of europa s history an originally thin brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes the degree of fracturing and plate displacements decreased with time in a thickening shell and lineaments became narrower and more widely spaced formation of chaos regions could have occurred where the thickness threshold for solid state convection was exceeded and can be aided by preferential tidal heating of more ductile ice in a long term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite or cyclic or irregular episodes of tectonic and cryovolcanic activity

the dynamics of extensional sedimentary basins constraints from subsidence inversion on the continents sedimentary basins are generally formed by small amounts of lithospheric extension extension might cease because the lithospheric mantle has cooled and strengthened or because the driving force is removed in the former case a relationship should exist between the maximum strain rate during extension and the final stretching factor attained we explore how this relationship depends on the rheology of the lithospheric mantle and of the crust as well as on sedimentation and prior rifting events we also present strain rate information obtained by inverting subsidence data from a variety of sedimentary basins there is evidence of a systematic relationship between strain rate and stretching factor in each basin implying that lithospheric rheology is consistent with the rheology of olivine measured in laboratory experiments our data are difficult to explain if removal of the driving force governs the magnitude of stretching factors

tidal heating in an internal ocean model of europa considerable evidence suggests that europe s internal structure might consist of a liquid water layer that decouples a thin 30 km overlying ice lithosphere from an underlying silicate core1 2 a lack of impact features extremely subdued topography and positive spectroscopic identification of h2o all imply recent resurfacing by water in addition curvilinear features resembling cracks are ubiquitous over the surface their orientations are broadly consistent with tidally controlled tectonic activity3 compositional models of europea indicate that the h2o layer could be over 100 km thick1 cassen et al 4 first showed how tidal heating if sufficiently intense might stabilize a thin 30 km ice lithosphere over the internal ocean and prevent the water from freezing other models of europa do not include an internal ocean and instead have an ice layer perhaps as thick as 100 km or as thin as a few kilometres resting directly on the silicate interior5 6 however based on studies of crater relaxation thomas and schubert7 have shown the version of this model with a thin ice layer to be unlikely the plausibility of the internal ocean model vis a vis the thick ice model depends critically on the level of tidal heating in europa here we present new calculations of tidal heating based on a more realistic three layer model of europa the tidal distortion of a decoupled ice lithosphere is only half that previously thought at the current value of orbital eccentricity tidal heating is only marginally able to prevent the internal ocean from freezing

polar wander and surface convergence of europa s ice shell evidence from a survey of strike slip displacement two global issues regarding europa are addressed by a survey of strike slip faults first a common type of terrain that appears to represent convergent sites of surface removal which may help compensate for substantial widespread dilation along tectonic bands elsewhere thus helping resolve the problem of conserving global surface area is identified second evidence for polar wander may provide the first confirmation of that theoretically predicted phenomenon these results among others come from an extensive survey of strike slip faults over the portion of the surface where galileo images at 200 m pixel resolution were obtained for regional mapping purposes the images cover two broad swaths that run from the far north to the far south one in the leading hemisphere and the other in the trailing hemisphere among the faults that have been mapped are a fault 170 km long with a strike slip offset of 83 km the greatest yet identified on europa and a quasi circular strike slip fault that surrounds a 500 km wide plate which has undergone rotation as a rigid unit reconstruction of specific examples of strike slip reveals sites of lateral convergence because europa is unique in many ways these sites are not similar to compression features on other bodies which may explain why they had previously been difficult to identify the distribution of strike slip in both hemispheres when compared with predictions of the theory of tidal walking provides evidence for polar wander the crust of europa appears to have slid as a single unit relative to the spin axis such that the site on the crust that was previously at the north rotational pole has wandered probably during the last few million years to a location currently in the leading hemisphere about 30 away from the spin axis such polar wander probably also explains symmetry patterns in the distribution of chaotic terrain pits and uplift features

stability and abundance of the trisulfur radical ion s 3 in hydrothermal fluids the interpretation of sulfur behavior in geological fluids and melts is based on a long standing paradigm that sulfate sulfide and sulfur dioxide are the major sulfur compounds this paradigm was recently challenged by the discovery of the trisulfur ion s 3 in aqueous s bearing fluids from laboratory experiments at elevated temperatures however the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions here we used in situ raman spectroscopy on model thiosulfate sulfide and sulfate aqueous solutions across a wide range of sulfur concentration 0 5 10 0 wt acidity ph 3 8 temperature 200 500 degrees c and pressure 15 1500 bar to identify the different sulfur species and determine their concentrations results show that in the low density 0 2 g cm 3 vapor phase h2s is the only detectable sulfur form by contrast in the denser liquid and supercritical fluid phase together with sulfide and sulfate the trisulfur radical ion s 3 is a ubiquitous and thermodynamically stable species from 200 degrees c to at least 500 degrees c in addition the disulfur radical ion s is detected at 450 500 degrees c in most solutions and polymeric molecular sulfur with a maximum abundance around 300 degrees c in s rich solutions these results combined with revised literature data allow the thermodynamic properties of s 3 to be constrained enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids these predictions suggest that s 3 may account for up to 10 of total dissolved sulfur s tot at 300 500 degrees c in fluids from arc related magmatic hydrothermal systems and more than 50 s tot at 600 700 degrees c in s rich fluids produced via prograde metamorphism of pyrite bearing rocks the trisulfur ion may favor the mobility of sulfur itself and associated metals au cu pt mo in geological fluids over a large range of depth and provide the source of these elements for orogenic au and porphyry epithermal cu au mo deposits furthermore the ubiquity of s 3 in aqueous sulfate sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass dependent and mass independent fractionation of sulfur isotopes

an experimental estimate of europa s ocean composition independent of galileo orbital remote sensing we have conducted a series of experiments designed to simulate in the laboratory the development of any subsurface aqueous phase on europa in our theoretical experimental approach we select a single natural sample a cm meteorite that based on cosmochemical considerations we consider to approximately represent the bulk material that accreted to form europa we then subject the sample to a hot water leaching procedure designed to simulate low to moderate temperature aqueous alteration the resulting leach solution was then subjected to a series of sequential fractional crystallization steps producing a series of ices and residual brines then all this brines and ices are multiply analyzed for na ca mg sr ba fe mn k cl and so4 results were found to be remarkably consistent between brines and ices in the same stages of crystallization and also between stages we found that any putative aqueous phase below europa s ice crust is probably a brine with cations na mg ca k fe and anions so4 cl our results are in harmony with inferences drawn from one of the two main current interpretations of the orbital spectral data but cannot definitively rule out inferences drawn from the alternative interpretation this is so because the mineralogy of the top 200 m may not reflect the chemical composition of bodies of brine below the solid surface owing to extensive alteration caused by magnetospheric bombardment

a two dimensional inverse model for extensional sedimentary basins 2 application 1 we apply the two dimensional strain rate inversion algorithm described in the companion paper to five sedimentary basins located world wide this general algorithm makes no assumptions about the number duration or intensity of rifting episodes within a basin instead the distribution of strain rate is allowed to vary smoothly through space and time until the misfit between observed and predicted stratigraphy is minimized potentially important two dimensional effects such as lateral heat flow and elastic thickness have been incorporated in each case observed subsidence profiles can be automatically and accurately fitted the calculated distributions of strain rate are corroborated by independent information about the number and duration of rifting episodes peak strain rate estimates are comparable to present day measurements in actively extending basins such as the aegean sea and the basin and range province temporal and spatial strain rate patterns from a large number of sedimentary basins in diverse settings will help to constrain the dynamical evolution of thinning continental lithosphere in all five sedimentary basins the smallest acceptable misfit is achieved when the elastic thickness is less than 2 4 km this surprising result indicates that the lithosphere beneath sedimentary basins remain weak during and after rifting it also underscores the importance of inverting data sets so that the existence uniqueness and resolution of solutions can be analyzed our two dimensional approach is one small step toward a generalized three dimensional inverse model of sedimentary basins

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

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

habitability of europa s crust the role of tidal tectonic processes jupiter s satellite europa has been identified as one of the most likely sites for life in the solar system the tidal tectonic processes that appear to have governed europa s geology seem to require interaction with an ocean under only a very thin crust providing a variety of evolving environmental niches the mutually dependent relationship between orbital evolution and tidal processes in turn controls europa s rotation heating and stress surface lineaments are correlated with global stress patterns demonstrating that they form by crustal cracking but only if a substantial ocean is present to give adequate tidal amplitude tidal driving of strike slip faulting indicates that cracks penetrate to a fluid layer which is possible only with a very thin ice crust the characteristic ridge sets that cover tectonic terrain are likely 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 chaotic terrain has morphology and other characteristics indicative of melt through from below surface colorants correlate with locations such as along large scale ridge systems and around chaotic terrain where ocean water reached the surface this model implies that as a result of tides liquid water regularly bathed crustal cracks and surfaces with heat and whatever nutrients are included in the oceanic chemistry creating a variety of habitable environments the processes were recent and thus most likely continue today longer term evolution of environmental conditions provided the need for adaptation and opportunity for evolution

europa stratigraphy and geological history of the anti jovian region from galileo e14 solid state imaging data a tidal bulge and associated deformation are predicted to occur in europa s anti jovian region voyager data showed polygonal low albedo bands in this region it was demonstrated that when the low albedo material comprising these bands was removed the margins could be reconstructed suggesting that fracturing and lateral separation has occurred higher resolution galileo images of a 300 000 km2 portion of the anti jovian region permit recognition and mapping of a variety of features and documentation of the geological history we identify background ridged plains several distinctive types of bands and ridges lenticulae and regions of chaotic terrain these images are at relatively low incidence angles thus albedo variations predominate over topography this presents a challenge to morphologically based mapping but does not preclude development of a regional stratigraphy stratigraphic relationships show that dark bands crosscut the bright ridged plains and are themselves disrupted by lenticulae and chaos suggesting a geological history beginning with background ridged plains followed by a period dominated by band formation with the most recent history predominantly characterized by formation of lenticulae and chaos the paucity of impact craters in this region indicates that the entire sequence of units is geologically recent

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

a two dimensional inverse model for extensional sedimentary basins 1 theory 1 a simple two dimensional inverse model for extracting the spatial and temporal variation of strain rate from extensional sedimentary basins is described our starting point is a forward model which allows basin stratigraphy to be calculated from any given strain rate distribution this forward model includes potentially important effects such as flexural rigidity and the two dimensional conduction advection of heat conversely inverse modeling determines strain rate variation by minimizing the misfit between predicted and observed patterns of basin subsidence no a priori assumptions about the number duration or intensity of rifting episodes are necessary instead strain rate is allowed to vary smoothly throughout time and space an important restriction is that vertical lines remain vertical but this restriction can be relaxed if deemed necessary we have successfully inverted different synthetic sedimentary basin geometries which were generated by forward modeling sensitivity analysis and noise tests demonstrate that strain rate patterns can be recovered with confidence the relationship between stratigraphic misfit and elastic thickness shows that usually an upper limit but not a lower limit for elastic thickness can be retrieved our general approach is applicable to any extensional sedimentary basin including highly extended passive margins and it should be regarded as the stepping stone to more sophisticated two dimensional and three dimensional inverse modeling in the companion paper we apply our algorithm to a set of phanerozoic extensional sedimentary basins located worldwide

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

tectonics and water on europa the tectonics of europa one of jupiter s moons are complex this satellite probably hosts a subsurface water ocean but the thickness of the outer ice crust is poorly constrained and the episodic presence of liquid water at the surface is debated we argue that some surface features of europa are formed by soft ice that is heated by viscous dissipation of tidal motion along faults and do not depend on a shallow ocean our model suggests that transient pockets of liquid water or brine could form at shallow depths in the crust

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses a global subsurface ocean of liquid water galileo gravity data verify that the satellite is differentiated into an outer h2o layer about 100 km thick but cannot determine the current physical state of this layer liquid or solid here we summarize the geological evidence regarding an extant subsurface ocean concentrating on galileo imaging data we describe and assess nine pertinent lines of geological evidence impact morphologies lenticulae cryovolcanic features pull apart bands chaos ridges surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates an average age of 50 myr searches for evidence of current geological activity on europa such as plumes or surface changes have yielded negative results to date the current existence of a global subsurface ocean while attractive in explaining the observations remains inconclusive future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within europa today

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

the influence of distributed sources and upwelling on the baroclinic structure of the abyssal circulation abstract the study of a simple continuously stratified model of the abyssal ocean driven by upwelling out of the abyss into the main thermocline and by source fluid entering the basin through northern western and southern bound aries is reported our approach divides the deep mean into an inviscid interior and frictional boundary layer regions the interior circulation is fully determined by the horizontal distribution of upwelling and by the net vertical distribution of sources entering the basin forcing by the local upwelling drives a barotropic velocity field and remote forcing by both the upwelling and sources generates an underlying baroclinic flow which can be considerably stronger and of opposite sign at some depths the boundary current functions to redistribute around the perimeter fluid entering the boundary regions either through the basin walls or from the interior in contrast to the interior flow it depends also on the geographical location of sources the boundary current is divided

estimates of ice thickness in the conamara chaos region of europa europa the second galilean moon outward from jupiter is similar in size to earth s moon but has an outer water ice layer on the order of 100 km thick images acquired by the galileo spacecraft reveal domical features and iceberg like blocks in some areas that suggest the ice was regionally thin at the time the features formed this paper applies buoyancy and flexural models to some of the features to estimate ice thickness assuming that the ice blocks were floating in a liquid sublayer the buoyancy model suggests an ice layer 0 2 3 0 km thick also the flexural models suggest that only the upper 0 1 0 5 km of that ice layer responded elastically to stresses acting as an elastic lithosphere