crack azimuths on europa time sequence in the southern leading face the formation sequence of prominent ridges and other tectonic lineaments on the southern portion of the leading hemisphere of europa is determined from cross cutting relationships these selected features formed fairly recently relative to most of the surface older lineaments no longer retain clear evidence of cross cutting if we assume that this sequence represents the order of formation of cracks that underlie each lineament and that the orientation of each crack was determined by tidal stress whose azimuth varies monotonically counter clockwise with time the azimuth must have rotated more than 740 which would correspond to the change in tidal stress over two periods of nonsynchronous rotation relative to the direction of jupiter however that interpretation is not necessarily compelling because the observed orientations of cross cutting lineaments are not densely spaced over these cycles in fact the sequence would fit nearly as well into an arbitrary model with rotation in the opposite sense from that predicted by theory this tectonic record may have formed over many more rotational cycles such that typically only a few cracks form per cycle which would be consistent with evidence from considerations of cycloidal crack patterns sets of cracks that cluster near certain azimuth orientations appear to be parts of globe encircling lineament systems and may result from other effects perhaps polar wander that occurred rapidly relative to nonsynchronous rotation

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

can earth like plate tectonics occur in ocean world ice shells abstract the outer h2o ice shell of europa a galilean satellite of jupiter thought to exhibit a global interior water ocean shows evidence for plate like surface motions along tectonic boundaries in this study we investigate whether forces similar to those that drive plate tectonics on earth may drive plate motions on icy satellites we focus on whether the forces of slab pull and ridge push driven by thermal and compositional buoyancy can overcome the mechanical forces resisting plate motions to sustain interaction between surface ice and the interior ocean we find that earth like plate tectonics is implausible within the outer ice shell of icy satellites because the forces resisting self sustaining plate cycling are of significantly greater order than the driving forces in the case of ganymede and triton if it has a thick shell plate tectonics would require warm low viscosity slabs to remain intact for timescales that are orders of magnitude longer than those of interior convection

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

the implications of tides on the mimas ocean hypothesis we investigate whether a present day global ocean within mimas is compatible with the lack of tectonic activity on its surface by computing tidal stresses for ocean bearing interior structure models derived from observed librations we find that for the suite of compatible rheological models peak surface tidal stresses caused by mimas high eccentricity would range from a factor of two smaller to an order of magnitude larger than those on tidally active europa thermal stresses from a freezing ocean or a past higher eccentricity would enhance present day tidal stresses exceeding the magnitudes associated with europa s ubiquitous tidally driven fractures and in some cases the failure strength of ice in laboratory studies therefore in order for mimas to have an ocean its ice shell cannot fail at the stress values implied for europa furthermore if mimas ocean is freezing out the ice shell must also be able to withstand thermal stresses that could be an order of magnitude higher than the failure strength of laboratory ice samples in light of these challenges we consider an ocean free mimas to be the most straightforward model best supported by our tidal stress analysis

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

fracture penetration in planetary ice shells abstract the proposed past eruption of liquid water on europa and ongoing eruption of water vapor and ice on enceladus have led to discussion about the feasibility of cracking a planetary ice shell we use a boundary element method to model crack penetration in an ice shell subjected to tension and hydrostatic compression we consider the presence of a region at the base of the ice shell in which the far field extensional stresses vanish due to viscoelastic relaxation impeding the penetration of fractures towards a subsurface ocean the maximum extent of fracture penetration can be limited by hydrostatic pressure or by the presence of the unstressed basal layer depending on its thickness our results indicate that europa s ice shell is likely to be cracked under 1 3 mpa tension only if it is 2 5 km thick enceladus ice shell may be completely cracked if it is capable of supporting 1 3 mpa tension and is less than 25 km thick

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

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

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

three layered models of ganymede and callisto compositions structures and aspects of evolution abstract three layered structural models are determined for ganymede and callisto each consists of a rock core a mixed ice rock lower mantle and a pure ice upper mantle this structure results from differentiation subsequent to accretional melting attention is given to evaluating various candidates for the rock component and three alternatives representing various degrees of silicate hydration and oxidation are modeled and incorporated structures are calculated on the basis of a 250 k isotherm which is a reasonablev approximation to the gentle adiabats expected to occur in icy satellites differentiation of an ice rock satellite generally involves an increase in radius and the three layered approach allows this process to be examined in some detail it is determined that satellite expansion is most significant early in the process and much less so as differentiation proceeds to completion if tectonics are due to global expansion distinguishing on this basis between a completely differentiated satellite and one that is only partially differentiated is difficult the postaccretional global expansion of ganymede which may have left a tectonic record was probably limited to 1 in radius in agreement with observed limits useful quantities such as silicate mass and volume fraction uncompressed density j2 c22 binding energy and surface heat flow are also determined nonhydrostatic contributions to j2 and c22 are estimated and shown to be nonnegligible encounters with jupiter orbiting spacecraft are unlikely to determine callisto s degree of central condensation we conclude by calculating the relative likelihood of postaccretional melting caused by radiogenic heating three layered satellites have generally hotter interiors because additional thermal boundary layers divide the separately convecting upper and lower mantles inhibiting heat transport ganymedes and callistos that are less than about 1 3 differentiated by mass should experience a second episode of melting as these boundary layers are either above the level of the water ice minimum melting temperature or intersect the melting curve at deeper levels runaway differentiation to at least a depth corresponding to a pressure in the ice v stability field is likely the main point here is that if satellite tectonics are tied to differentiation by melting or its aftermath as in the instability following ocean closure of kirk and stevenson moderate or small amounts of accretional differentiation are unlikely to explain an absence of tectonics as in callisto because extensive differentiation ultimately occurs

an experimental investigation of divergent bow waves simulated by a two dimensional plus temporal wave marker technique divergent ship bow waves were simulated experimentally with a two dimensional wavemaker that employs a flexible wave board the wavemaker was programmed so that the wave board created a time sequence of shapes that simulated the line of intersection between one side of the hull of a slender ship model moving at constant speed and an imaginary vertical plane oriented normal to the ship model track the time history of the water surface shape was measured with a cinematic laser induced fluorescence technique for eight froude numbers f d u where u is the forward speed of the equivalent three dimensional ship model g the acceleration of gravity and d the ship model draft the waves produced ranged from small amplitude non breaking waves at the lowest froude numbers to plunging breakers at the highest froude numbers these waves are strongly forced and at the higher froude numbers begin breaking before leaving the wave board the time histories of various geometric characteristics of the water surface shape including the hull contact line the wave crest the plunging jet and the splash zone which is here defined as both the turbulent zone on the front face of the wave in the spilling breakers and the turbulent zone generated ahead of the jet impact point in the plunging breakers were measured the phase speed of the primary wave generated during each run ranged from 2 56 u wl where u wl is the maximum speed of the wave board at the undisturbed water level in the tank at the lowest froude number to about 1 7 u wl at the three highest froude numbers the maximum heights of the primary wave the contact point on the wavemaker and the splash zone increased in a nearly linear fashion with increasing f d in the cases with plunging jets the jet tip trajectory was parabolic with a vertical acceleration ranging from 0 6 g at f d 1 467 to 0 8 g at f d 1 817 the highest froude number

variability of rhodes and ierapetra gyres during levantine intermediate water experiment observations and model results 1 this work is aimed at studying the variability of rhodes and ierapetra gyres during the levantine intermediate water experiment sea surface temperature maps derived from global area coverage advanced very high resolution radiometer avhrr data in the framework of the pathfinder project and sea level anomalies derived from topex poseidon altimeter were used to track the evolution both in terms of dimension and position of the cyclonic rhode gyre and ierapetra anticyclone the analysis was performed for the period from october 1994 to the end of april 1995 i e from the preconditioning to the spreading phase of levantine intermediate water the description of the surface fields was complemented by a temporal evolution study of the rhodes and ierapetra gyres hydrological vertical structure european centre for medium range weather forecasts avhrr derived fluxes have been used as forcing for a numerical model to study the variability of the mixed layer in both gyres the simulated upper layer dynamics remarkably matched remotely sensed and in situ observations this work shows that using a combined analysis of observational data and model simulations we were able to propose a dynamical scenario for the winter of 1995 where the levantine basin was characterized by short intense heat loss events occurring during at the beginning of december the intense air sea interaction contributed to deep instead of intermediate mixing as confirmed by remote sensing in situ data and model experiments a hypothesis on the role of the lateral contribution on the water column buoyancy content in the rhodes gyre was formulated

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

ridged plains on europa reveal a compressive past abstract europa s young surface implies relatively recent resurfacing ridged plains which make up 50 of europa s surface have not yet been fully analyzed for a potential formation mechanism because ridged plains dominate europa s surface this terrain is key to understanding how europa has resurfaced and how the resurfacing mechanisms may have evolved through time in this work we create a new high resolution topographic model and a two layer physical analog model to investigate the formation of ridged plains we find that the ridged plains most closely resemble the compressional physical analog experiments which generate folds specifically the analog experiments with a brittle layer 3000 m compared to the predicted current ice shell thickness this would imply that europa s ice shell has thickened through the visible surface history

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

mountains on titan modeling and observations 1 we have developed a thermal model of titan s interior to study changes in volume during partial freezing or melting of a subsurface ocean due to heat flux variations from the interior we find that the long term cooling of titan can cause global volume contraction v v 0 01 we then simulate two dimensional contractional deformation of titan s icy lithosphere finding that contractional deformation can produce tectonic activity and fold formation folds could potentially achieve a topographic height of several kilometers for high local strain 0 16 and for high temperature gradients in the ice i shell order of 10 k km 1 corresponding to an ancient high heat flux from the interior order of 0 02 0 06 w m 2 examination of synthetic aperture radar sar imagery obtained by cassini radar shows possible evidence of contractional tectonism in the equatorial regions of titan although the moderate resolution of the cassini sar imagery does not permit an unambiguous geological interpretation

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

the interior and orbital evolution of charon as preserved in its geologic record abstract pluto and its largest satellite charon currently orbit in a mutually synchronous state both bodies continuously show the same face to one another this orbital configuration is a natural end state for bodies that have undergone tidal dissipation in order to achieve this state both bodies would have experienced tidal heating and stress with the extent of tidal activity controlled by the orbital evolution of pluto and charon and by the interior structure and rheology of each body as the secondary charon would have experienced a larger tidal response than pluto which may have manifested as observable tectonism unfortunately there are few constraints on the interiors of pluto and charon in addition the pathway by which charon came to occupy its present orbital state is uncertain if charon s orbit experienced a high eccentricity phase as suggested by some orbital evolution models tidal effects would have likely been more significant therefore we determine the conditions under which charon could have experienced tidally driven geologic activity and the extent to which upcoming new horizons spacecraft observations could be used to constrain charon s internal structure and orbital evolution using plausible interior structure models that include an ocean layer we find that tidally driven tensile fractures would likely have formed on charon if its eccentricity were on the order of 0 01 especially if charon were orbiting closer to pluto than at present such fractures could display a variety of azimuths near the equator and near the poles with the range of azimuths in a given region dependent on longitude east west trending fractures should dominate at mid latitudes the fracture patterns we predict indicate that charon s surface geology could provide constraints on the thickness and viscosity of charon s ice shell at the time of fracture formation

causes characteristics and consequences of convective diapirism on europa 1 we investigate the hypothesis that the 10 km diameter dome shaped features seen on europa s surface are caused by strongly temperature dependent convection in which upwellings form isolated diapirs or thermals we use the observed lower limit on dome diameter of 4 km to deduce that the conductive stagnant lid thickness must be 5 km such a lid thickness implies a minimum surface heat flux of 90 mwm 2 compatible with recent estimates of tidal heating we also use the mean observed dome diameter to infer a lower thermal boundary layer thickness of 1 km we find that the ice is probably deforming in the diffusion creep regime with a grain size in the range 0 02 0 06 mm the fraction of internal heating is 0 5 the ice viscosity 10 12 10 13 pa s and the crustal solidification rate 5km ma index terms 6218 planetology solar system objects jovian satellites 5430 planetology solid surface planets interiors 8147 5418 planetology solid surface planets heat flow 5455 planetology solid surface planets origin and evolution citation nimmo f and m manga causes characteristics and consequences of convective diapirism on europa geophys res lett 29 23 2109 doi 10 1029 2002gl015754 2002

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

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

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

stresses generated in cooling viscoelastic ice shells application to europa 1 a cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects thermal contraction of the ice due to cooling and the expansion of the shell due to the ice water volume change the former effect generates near surface compression and deeper extension the second effect generates extension only in both cases stresses are smaller at depth due to viscous creep the resulting combined stresses are extensional except at shallow 1 km depths in thin ice shells for ice shells thicker than 45 km stresses are extensional throughout the extensional stresses exceed 10 mpa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs the dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on europa and other large icy satellites however after 100 myr of conductive cooling the maximum theoretical elastic strains for europa are 0 35 which are probably insufficient to explain the total amount of observed extension

thermal evolution of pluto and implications for surface tectonics and a subsurface ocean abstract determining whether or not pluto possesses or once possessed a subsurface ocean is crucial to understanding its astrobiological potential in this study we use a 3d convection model to investigate pluto s thermal and spin evolution and the present day observational consequences of different evolutionary pathways we test the sensitivity of our model results to different initial temperature profiles initial spin periods silicate potassium concentrations and ice reference viscosities the ice reference viscosity is the primary factor controlling whether or not an ocean develops and whether that ocean survives to the present day in most of our models present day pluto consists of a convective ice shell without an ocean however if the reference viscosity is higher than 5 10 15 pa s the shell will be conductive and an ocean should be present for the nominal potassium concentration the present day ocean and conductive shell thickness are both about 165 km in conductive cases an ocean will be present unless the potassium content of the silicate mantle is less than 10 of its nominal value if pluto never developed an ocean predominantly extensional surface tectonics should result and a fossil rotational bulge will be present for the cases which possess or once possessed an ocean no fossil bulge should exist a present day ocean implies that compressional surface stresses should dominate perhaps with minor recent extension an ocean that formed and then re froze should result in a roughly equal balance between older compressional and younger extensional features these predictions may be tested by the new horizons mission

thermal stresses in planetary elastic lithospheres the role of thermal stresses in the tectonics of the moon is reexamined in this paper a model is used that considers a spherical elastic shell overlying a fluid core it is shown that the thermal stresses generated by temperature changes within the shell usually dominate over the thermal contraction or expansion of the core during the entire evolution of the moon the cooling of the lithosphere is likely to have dominated the cooling of the interior and the result would be tensional lithospheric thermal stresses however during the recent evolution of the moon the change in the near surface thermal stresses is compressional it is argued that the surface compressional features on mercury are not due to the thermal contraction of the interior

computations of overturning waves the numerical method of longuet higgins cokelet 1976 for waves on deep water is extended to account for a horizontal bottom contour and used to investigate breaking waves in water of finite depth it is demonstrated that a variety of overturning motions may be generated ranging from the projection of a small scale jet at the wave crest of the type that might initiate a spilling breaker to large scale plunging breakers involving a significant portion of the wave although there seems to be a continuous transition between these wave types a remarkable similarity is noticed in the overturning regions of many of the waves three high resolution computations are also discussed the results are presented in the form of interrelated space velocity and acceleration plane plots which enable the time evolution of individual fluid particles to be followed these computations should be found useful for the testing of analytical theories and may also be applied for example to studies of slamming forces on shipping and coastal structures

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

the origin of domes on europa the role of thermally induced compositional diapirism 1 the surface of jupiter s moon europa is peppered by topographic domes interpreted as sites of intrusion and extrusion diapirism is consistent with dome morphology but thermal buoyancy alone cannot produce sufficient driving pressures to create the observed dome elevations instead diapirs may initiate by thermal convection that induces compositional segregation exclusion of impurities from warm upwellings allows sufficient buoyancy for icy plumes to create the observed surface topography provided the ice shell has a small effective elastic thickness 0 2 to 0 5 km and contains low eutectic point impurities at the few percent level this model suggests that the ice shell may be depleted in impurities over time

implications from ithaca chasma for the thermal and orbital history of tethys 1 flexural modeling of ithaca chasma on tethys suggests that the elastic thickness and the surface heat flux were 5 7 km and 18 30 mw m2 respectively when the feature formed 4 gyr b p tidal heating is a plausible heat source suggesting that tethys eccentricity was higher in the past depending on tethys internal structure eccentricities in the range 0 001 to 0 02 are sufficient to have generated the inferred heat flux because the eccentricity damping timescale is short 20 myr it is unlikely that this eccentricity was primordial more likely tethys eccentricity increased during passage through a 3 2 resonance with dione in equilibrium this resonance produces 2 5 mw m2 suggesting that non equilibrium or periodic heating was important similar to the case for enceladus at present

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

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

folding of europa s icy lithosphere an analysis of viscous plastic buckling and subsequent topographic relaxation abstract potential contractional folds on jupiter s icy moon europa have been identified the best example is at the extensional band astypalaea linea where a series of subtle topographic undulations 25 km in wavelength possess parasitic tectonic features that support a folding origin a scenario has been qualitatively proposed whereby folds form via unstable contraction of the icy lithosphere compensate for extension elsewhere on europa and then subsequently relax here we quantitatively address this scenario applying a model for viscous plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation our results suggest that the lithosphere of europa could indeed be unstable but the low required surface temperatures limit fold formation to higher latitudes and the high required driving stresses 9 10 mpa are difficult to achieve on the satellite the depth to the brittle ductile transition is well constrained and high thermal gradients are indicated implying heat flows near 100 mw m 2 in addition topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of europa s surface given the paucity of identified folds we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on europa

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

shell thickness variations and the long wavelength topography of titan the long wavelength topography of titan has an amplitude larger than that expected from tidal and rotational distortions at its current distance from saturn this topography is associated with small gravity anomalies indicating a high degree of compensation both observations can be explained if titan has a floating isostatically compensated ice shell with a spatially varying thickness the spatial variations arise because of laterally variable tidal heating within the ice shell models incorporating shell thickness variations result in an improved fit to the observations and a degree two tidal love number h2t consistent with expectations without requiring titan to have moved away from saturn our preferred models have a mean shell thickness of 100 km in agreement with the observed gravity anomalies and a heat flux appropriate to a chondritic titan shell thickness variations are eliminated by convection we therefore conclude that titan s ice shell is not convecting at the present day

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

lithospheric stresses due to radiogenic heating of an ice silicate planetary body implications for ganymede s tectonic evolution thermal evolution models of differentiated and undifferentiated ice silicate bodies containing longlived radiogenic heat sources are examined lithospheric stresses arise due to volume change of the interior and temperature change in the lithosphere for an undifferentiated body the surface stress peaks early in the evolution while in the differentiated case stresses peak later and continue to accumulate for longer periods of time the variation of near surface stress with depth shows that stresses for the undifferentiated body initially penetrate to great depths but rapidly concentrate within a few kilometers of the surface for the differentiated body elastic stresses never accumulate at a depth greater than a few kilometers these models are applied to consider long term radioactive heating as a possible mechanism of tectonic activity and bright terrain formation on ganymede

double ridges on europa accommodate some of the missing surface contraction crosscutting relationships of tectonic lineaments on europa record the history of surface deformation we mapped the displacement and orientation of older features crosscut by two types of lineaments bands and double ridges these measurements allow us to determine both the strike perpendicular and strike parallel displacement along investigated features double ridges record both ridge perpendicular contraction and expansion with a mean of 0 16 0 06 km needs space of contraction based on the analysis of 16 double ridges bands record expansion with a mean of 3 33 0 27 km for the six bands analyzed but with perpendicular displacement less than their apparent morphologic widths of 3 24 km the implied global surface strain for double ridges including those that expand and bands is 2 22 0 76 contraction and 7 60 3 7 expansion respectively double ridges thus may accommodate part of the surface expansion recorded by bands most current models for double ridges do not predict contraction the models that satisfy the observations for bands are slow spreading models cryovolcanism and folding

morphology of europan bands at high resolution a mid ocean ridge type rift mechanism 1 we utilize imaging data from the galileo spacecraft to investigate band formation on one of jupiter s moons europa bands are polygonal features first observed in voyager data close to europa s anti jovian point and represent areas where preexisting terrain has been pulled apart allowing new material to move up into the gap we examine the detailed morphology of several bands imaged at different resolutions and lighting geometries we identify several distinct morphological characteristics including central troughs hummocky textures and ridge and trough terrains some of which are common among the bands studied in many cases bands have initiated along segments of one or more preexisting double ridges ubiquitous within europa s ridged plains distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice rather than liquid water comparisons between europan band morphologies and features found on terrestrial mid ocean ridges reveal several similarities including axial troughs subcircular hummocks normal faults and indications of symmetrical spreading we conclude that terrestrial mid ocean ridge rifting is a good analogy for europan band formation if a terrestrial seafloor spreading model is applicable to europan bands we speculate that band morphologies might be related to the relative rate of spreading of each band bands may have contributed significantly to the resurfacing of europa europan bands we examine predate but do not postdate lenticulae and related features implying that the style of resurfacing on europa has changed over recent geological time in these regions

pressurized oceans and the eruption of liquid water on europa and enceladus 1 it is difficult for liquid water to erupt onto the surface of icy satellites such as europa and enceladus because liquid water is more dense than ice if the ice shell thickens the volume expansion of ice upon freezing increases pressure in the subsurface ocean the excess pressure is determined by a balance between compression of ocean water and elastic expansion of the ice shell we show that on europa the freezing of 1 10 km of ice generates tangential stresses that exceed the tensile strength of ice excess pressure however is insufficient for liquid water to erupt to the surface within smaller icy satellites such as enceladus ocean pressure can become large enough to cause an eruption of large amounts of liquid water

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

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

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 tectonics of titan global structural mapping from cassini radar abstract the cassini radar mapper has imaged elevated mountain ridge belts on titan with a linear to arcuate morphology indicative of a tectonic origin systematic geomorphologic mapping of the ridges in synthetic aperture radar sar images reveals that the orientation of ridges is globally e w and the ridges are more common near the equator than the poles comparison with a global topographic map reveals the equatorial ridges are found to lie preferentially at higher than average elevations we conclude the most reasonable formation scenario for titan s ridges is that contractional tectonism built the ridges and thickened the icy lithosphere near the equator causing regional uplift the combination of global and regional tectonic events likely contractional in nature followed by erosion aeolian activity and enhanced sedimentation at mid to high latitudes would have led to regional infilling and perhaps covering of some mountain features thus shaping titan s tectonic landforms and surface morphology into what we see today

formation of corner waves in the wake of a partially submerged bluff body we study theoretically and numerically the downstream flow near the corner of a bluff body partially submerged at a deadrise depth rm delta h into a uniform stream of velocity u in the presence of gravity g when the froude number mathit fr u sqrt g rm delta h is large a three dimensional steady plunging wave which is referred to as a corner wave forms near the corner developing downstream in a similar way to a two dimensional plunging wave evolving in time we have performed an asymptotic analysis of the flow near this corner to describe the wave s initial evolution and to clarify the physical mechanism that leads to its formation using the two dimensions plus time approximation the problem reduces to one similar to dam break flow with a wet bed in front of the dam the analysis shows that at leading order the problem admits a self similar formulation when the size of the wave is small compared with the height difference rm delta h the essential feature of the self similar solution is the formation of a mushroom shaped jet from which two smaller lateral jets stem however numerical simulations show that this self similar solution is questionable from the physical point of view as the two lateral jets plunge onto the free surface leading to a self intersecting flow the physical mechanism leading to the formation of the mushroom shaped structure is discussed

eastern mediterranean transient and reversal of the ionian sea circulation 1 maps of the remotely sensed absolute dynamic topography adt and sea surface temperature sst over the eastern mediterranean in the period june 1993 to june 2001 show a dramatic and sudden change in the sea surface structure this change which is presumably associated with the relaxation of the eastern mediterranean transient emt had taken place by the middle of 1997 and consisted of a complete reversal of the ionian upper layer circulation from anticyclonic to cyclonic in the emt phase sst and adt maps suggest enhanced communication between the ionian and the aegean basins as well as aegean waters spreading northward along the eastern ionian flank negative wind vorticity in the ionian suggests that the switch of the upper layer circulation from anticyclonic to cyclonic cannot be explained in terms of the wind forcing therefore we suggest the predominance of the baroclinic vorticity production term in determining the upper layer circulation inversion

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

thermal stress tectonics on the satellites of saturn and uranus most of the major saturnian and uranian satellites have undergone some form of extensional tectonism and or resurfacing early in their histories we examine the stresses created by the early thermal evolution of these satellites and consider the geologic implications of these stresses starting from an initial accretional temperature profile we calculate the satellites temperature profiles as they evolve by conduction solid state convection when it occurs is handled using a simple parameterized convection scheme the effects of radiogenic heating and release of heat of phase change are included the thermal stresses built up in the satellites are then calculated assuming the satellites to have a maxwell viscoelastic rheology the stress calculation takes into account both the effects of thermal stresses and of shifts in the position of the ice i ice ii phase boundary in some satellites if accretion takes place in a cold gas free environment the stresses that build up in the smaller satellites mimas miranda and enceladus are minor in the larger satellites however stresses are large enough to cause fracturing and extensional tectonism under these circumstances the stresses result largely from rapid cooling contraction of a thin outer shell the ice ii ice i phase transition also contributes significantly for the largest satellites if accretion occurs in a warm gaseous nebula that dissipates rapidly stresses large enough to cause failure are attained in all of the satellites in all cases the material immediately underlying the zone of failure is the warmest in the satellite and hence may be buoyant and mobile enough to be extruded to the surface through fractures created by the extension we conclude that thermal stresses following accretion led to a substantial early episode of extensional tectonism and perhaps also associated resurfacing on many of the saturnian and uranian satellites if evidence of this early episode of extensional tectonism has survived subsequent impacts then the process may have been responsible for much of the early tectonism and resurfacing observed on these satellites if not then it may only be responsible for the very early global resurfacing that some of the satellites are inferred to have undergone