weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

dielectric properties of jovian satellite ice analogs for subsurface radar exploration a review the first european mission dedicated to the exploration of jupiter and its icy moons jupiter icy moons explorer juice will be launched in 2022 and will reach its final destination in 2030 the main goals of this mission are to understand the internal structure of the icy crusts of three galilean satellites europa ganymede and callisto and ultimately to detect europa s subsurface ocean which is believed to be the closest to the surface among those hypothesized to exist on these moons juice will be equipped with the 9 mhz subsurface penetrating radar rime radar for icy moon exploration which is designed to image the ice down to a depth of 9 km moreover a parallel mission to europa which will host onboard reason radar for europa assessment and sounding ocean to near surface equipped with 9mhz and 60mhz antennas has been recently approved by nasa the success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that in turn depend on the dielectric properties of the materials composing the icy satellite crusts in the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions first we discuss the experimental results on pure and doped water ice in the framework of the jaccard theory highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation we then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures carbon dioxide ice and ammonia ice building on this review we have selected the most suitable data to compute dielectric attenuation velocity vertical resolution and reflection coefficients for such icy moon environments with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders we present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of europa and discuss the physical and geological nature of various dielectric targets potentially detectable with rime finally we briefly highlight several unresolved issues that should be addressed in near future to improve our capability to produce realistic electromagnetic models of icy moon crusts the present review is of interest for the geophysical exploration of all solar system bodies including the earth where ice can be present at the surface or at relatively shallow depths

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

ice melting and downward transport of meltwater by two phase flow in europa s ice shell with its young surface very few impact craters and the abundance of tectonic and cryovolcanic features europa has likely been subjected to relatively recent endogenic activity morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface which may result from enhanced tidal heating a major issue concerns the thermal gravitational stability of these water reservoirs here we investigate the conditions under which water can be generated and transported through europa s ice shell we address particularly the downward two phase flow by solving the equations for a two phase mixture of water ice and liquid water in one dimensional geometry in the case of purely temperate ice we show that water is transported downward very efficiently in the form of successive porosity waves the time needed to transport the water from the subsurface region to the underlying ocean varies between 1 and 100 kyr depending mostly on the ice permeability we further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell within less than a few hundred kiloyears our calculations indicate that liquid water will be largely absent in the near subsurface with the possible exception of cold conductive regions subjected to strong tidal friction recently active double ridges subjected to large tidally driven strike slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on europa

predicting radar attenuation within the antarctic ice sheet abstract to better understand the ability of ice penetrating radar to diagnose the subglacial environment from bed returned power we model the englacial radar attenuation of antarctic ice first we use a one dimensional thermo mechanical model to evaluate the sensitivity of the depth averaged attenuation rates to ice temperature as a function of surface accumulation rate geothermal flux and ice thickness we find that attenuation is most sensitive to variations in geothermal flux and accumulation rate when the bed temperature is close to the pressure melting point but even if geothermal flux and accumulation rate remain fixed attenuation can easily vary with ice thickness such high sensitivities show that one should not assume a uniform attenuation rate in the radar data analysis then using ensembles of modeled ice temperatures with different boundary conditions we generate multiple attenuation predictions for the antarctic ice sheet and evaluate the resulting uncertainties the largest contributor to uncertainty in these predictions is the geothermal flux this uncertainty is localized within the deeper half of the ice sheet by combining these temperature ensembles with ice core chemistry data we show that the sea salt adds little to the attenuation but the contribution from acids accounts for 29 inland to 53 coast of the total attenuation we conclude that improving radar diagnosis of the subglacial environment using bed returned power requires both 1 better data interpretation algorithms that account for attenuation variations and 2 better constraints of geothermal flux and bulk chemistry

radar detectability of a subsurface ocean on europa abstract a spacecraft in orbit around jupiter s moon europa can use ice penetrating radar to probe for a possible liquid water ocean beneath europa s surface ice and to characterize other important subsurface structure consideration of available constraints on the properties of europa s ice possible subsurface temperature gradients and possible impurities in the ice places an upper limit of about 10 km on the depth to which an ocean might be detectable with an orbiting radar

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

mars north polar deposits stratigraphy age and geodynamical response the shallow radar sharad on the mars reconnaissance orbiter has imaged the internal stratigraphy of the north polar layered deposits of mars radar reflections within the deposits reveal a laterally continuous deposition of layers which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice the packet interpacket structure can be explained by approximately million year periodicities in mars obliquity or orbital eccentricity the observed 100 meter maximum deflection of the underlying substrate in response to the ice load implies that the present day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers alternatively the response to the load may be in a transient state controlled by mantle viscosity both scenarios probably require that mars has a subchondritic abundance of heat producing elements

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

active formation of chaos terrain over shallow subsurface water on europa the galileo spacecraft revealed a number of chaos regions on jupiter s moon europa where the surface terrain appears to have been disrupted from below in many places the surface contains sharp edged blocks or rafts of ice that have at some point been flipped or rotated some characteristics of these regions have been hard to explain such as the fact that the archetypal conamara chaos stands above its surroundings and contains matrix domes schmidt et al apply lessons learned from analogous processes within earth s subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water lenses that are perched only 3 kilometres deep within the ice shell the data suggest that ice water interactions and freeze out give rise to the varied morphology of chaos terrains implying that more water is involved than has been previously appreciated for instance the sunken topography of thera macula a large chaos area may indicate that europa is actively resurfacing over a lens comparable in volume to north america s great lakes europa the innermost icy satellite of jupiter has a tortured young surface1 2 3 4 and sustains a liquid water ocean1 2 3 4 5 6 below an ice shell of highly debated thickness1 2 3 4 5 7 8 9 10 quasi circular areas of ice disruption called chaos terrains are unique to europa and both their formation and the ice shell thickness depend on europa s thermal state1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 17 no model so far has been able to explain why features such as conamara chaos stand above surrounding terrain and contain matrix domes10 18 melt through of a thin few kilometre shell3 7 8 is thermodynamically improbable and cannot raise the ice10 18 the buoyancy of material rising as either plumes of warm pure ice called diapirs1 9 10 11 12 13 14 15 or convective cells16 17 in a thick 10 kilometres shell is insufficient to produce the observed chaos heights and no single plume can create matrix domes10 18 here we report an analysis of archival data from europa guided by processes observed within earth s subglacial volcanoes and ice shelves the data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres our results suggest that ice water interactions and freeze out give rise to the diverse morphologies and topography of chaos terrains the sunken topography of thera macula indicates that europa is actively resurfacing over a lens comparable in volume to the great lakes in north america

radar absorption due to impurities in antarctic ice measurements of radar echo strength have been carried out on ronne and george vi ice shelves antarctica the differences in ice column absorption between the two sites cannot be resolved by considering ice temperatures alone we present an absorption model that resolves the discrepancy by considering both the ice temperature and the ice impurity concentrations in antarctic coastal sites the concentrations of acid and sea salt impurities both of which affect absorption have a marked spatial variability and can dominate the absorption budget using the absorption model impurity levels in the ice column at the george vi site are deduced from the radar strength measurements and found to be typical of levels measured in antarctic ice at similar elevations and distances from the sea in particular the measurements suggest that the concentrations of biogenically derived acids are much lower than the unusually high levels found on the east coast of the antarctic peninsula

europa s crust and ocean origin composition and the prospects for life abstract we have considered a wide array of scenarios for europa s chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below we postulate that following formation of the jovian system the europan evolutionary sequence has as its major links a initial carbonaceous chondrite rock b global primordial aqueous differentiation and formation of an impure primordial hydrous crust c brine evolution and intracrustal differentiation d degassing of europa s mantle and gas venting e hydrothermal processes and f chemical surface alteration our models were developed in the context of constraints provided by galileo imaging near infrared reflectance spectroscopy and gravity and magnetometer data low temperature aqueous differentiation from a carbonaceous ci or cm chondrite precursor without further chemical processing would result in a crust ocean enriched in magnesium sulfate and sodium sulfate consistent with galileo spectroscopy within the bounds of this simple model a wide range of possible layered structures may result the final state depends on the details of intracrustal differentiation devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean crust compositions e g an ocean crust of sodium carbonate or sulfuric acid or a crust containing abundant clathrate hydrates realistic chemical physical evolution scenarios differ greatly in detailed predictions but they generally call for a highly impure and chemically layered crust some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and or cryovolcanism we describe some plausible geological consequences of the physical chemical structures predicted from these scenarios these predicted consequences and observed aspects of europa s geology may serve as a basis for further analys is and discrimination among several alternative scenarios most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms

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

sharad sounding radar on the mars reconnaissance orbiter 1 sharad shallow radar is a sounding radar provided by agenzia spaziale italiana asi as a facility instrument on the mars reconnaissance orbiter mission its 20 mhz center frequency and 10 mhz bandwidth complement the lower frequency relatively narrower bandwidth capability of the marsis sounding radar a joint italian u s team has guided the experiment development and is responsible for data analysis and interpretation the radar transmits signals at a 700 hz pulse repetition frequency prf and collects reflections from both the surface and near subsurface of mars vertical and horizontal resolutions are respectively 15 m free space and 3 6 km cross track by 0 3 1 km along track the scientific objective of sharad is to map in selected locales dielectric interfaces to at least several hundred meters depth in the martian subsurface and to interpret these results in terms of the occurrence and distribution of expected materials including competent rock soil water and ice a signal to noise ratio of 50 db for a specular surface return is achieved with 10 w of radiated power by using range and azimuth focusing in ground data processing preprocessed data as well as range and azimuth focused data will be formatted according to planetary data system pds standards and be made available from the asi science data center asdc and from the geosciences node of the planetary data system pds important targets for sharad include the polar layered deposits sedimentary stacks especially in terra meridiani buried channel systems buried impact craters volcanic complexes and shallow ice deposits in equilibrium with the atmosphere

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

radar attenuation in europa s ice shell obstacles and opportunities for constraining the shell thickness and its thermal structure young surface and possible recent endogenic activity make europa one of the most exciting solar system bodies and a primary target for spacecraft exploration future europa missions are expected to carry ice penetrating radar instruments designed to investigate its subsurface thermophysical structure several authors have addressed the radar sounders performance at icy moons often ignoring the complex structure of a realistic ice shell here we explore the variation in two way radar attenuation for a variety of potential thermal structures of europa s shell determined by reference viscosity activation energy tidal heating surface temperature and shell thickness as well as for low and high loss temperature dependent attenuation model we found that i for all investigated ice shell thicknesses 5 30 km the radar sounder will penetrate between 15 and 100 of the total thickness ii the maximum penetration depth varies laterally with deepest penetration possible through cold downwellings iii direct ocean detection might be possible for shells of up to 15 km thick if the signal travels through cold downwelling ice or the shell is conductive iv even if the ice ocean interface is not directly detected penetration through most of the shell could constrain the deep shell structure through returns from deep non ocean interfaces or the loss of signal itself and v for all plausible ice shells the two way attenuation to the eutectic point is 30 db which shows a robust potential for longitudinal investigation of the ice shell s shallow thermophysical structure

tidally induced melting events as the origin of south pole activity on enceladus abstract the intense activity at the south pole of enceladus hints at an internal water reservoir however there is no direct evidence of liquid water at present and its long term stability in the interior remains problematic by modeling heat production and transfer in the ice shell in a spherical geometry we show that tidal heating naturally leads to a concentration of convective hot upwellings in the south polar region favoring the preservation of liquid water at depth we show that large volumes of water are produced within the ice shell at the south pole during periods of elevated orbital eccentricity 3 5 times the present day value strong lateral variations in the melt production and crystallization rates result in stress concentration in the south polar region thus providing an explanation for the tectonic activity observed today we predict that an internal ocean may be sustained over the long term as the consequence of repeated periods with elevated orbital eccentricity leading to episodic melting and resurfacing events

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

illite smectite transformation in detrital glaucony during burial diagenesis of sandstone a study from siri canyon danish north sea detrital glaucony in the palaeogene glauconitic sandstones in siri canyon danish north sea has been analysed from 15 exploration wells by x ray diffraction electron microprobe and scanning electron microscopy these sandstones consist of mixed layer illite smectite and have a large variability in chemical composition and structure in the most shallow wells ca 1700 m the glaucony is rich in fe and consists of mixed layer illite smectite with random interstratification r 0 in the depth interval from 1700 to 2000 m the composition changes as si is incorporated the structure changes to ordered r 1 further increase in burial leads to the loss of fe ordered r 3 mixed layer illite smectite is recognized from burial depths of 2200 m the proportion of illite in illite smectite mixed layers increases only slightly with depth and temperature although the structural changes generally are associated with chemical changes they can also take place isochemically when the detrital glaucony is tightly embedded in earlier cement which prevented chemical exchange the glaucony transformation in the siri canyon sandstones partly reflects a supply of si and partly significant loss of fe thus the glaucony transformation relates to the general diagenesis of the host sandstone these sandstones are cemented by microquartz at an early stage followed by precipitation of fe rich grain coating berthierine or chlorite

onset of convection in the icy galilean satellites influence of rheology 1 ice i exhibits a complex rheology at temperature and pressure conditions appropriate for the interiors of the ice i shells of europa ganymede and callisto we use numerical methods and existing parameterizations of the critical rayleigh number to determine the conditions required to trigger convection in an ice i shell with each of the stress temperature and grain size dependent rheologies measured in laboratory experiments by goldsby and kohlstedt 2001 the critical rayleigh number depends on the ice grain size and the amplitude and wavelength of temperature perturbation issued to an initially conductive ice i shell if the shells have an assumed uniform grain size 0 4 mm deformation during initial plume growth is accommodated by newtonian volume diffusion if the ice grain size is between 0 4 mm and 3 cm deformation during plume growth is accommodated by weakly non newtonian grain boundary sliding where the critical ice shell thickness for convection depends on the amplitude of temperature perturbation to the 0 5 power if the ice grain size exceeds 2 cm convection can not occur in the ice i shells of the galilean satellites regardless of the amplitude or wavelength of temperature perturbation if the grain size in a convecting ice i shell evolves to effective values greater than 2 cm convection will cease if the ice shell has a grain size large enough to permit flow by dislocation creep the ice is too stiff to permit convection even in the thickest possible ice i shell consideration of the complex ice rheology implies that estimates of the grain size in the satellites and knowledge of their initial thermal states are required when judging the convective instability of their ice i shells

upper carboniferous and lower permian tectonostratigraphy on the southern margin of the central north sea hydrocarbon exploration in uk licence blocks 31 26 and 39 2 on the southern margin of the central north sea in 1996 and 1997 proved the presence of a palaeozoic sedimentary succession previously undescribed in the area biostratigraphic data from the sediments and radiometric dating of interbedded basaltic lavas indicate that the succession spans the upper carboniferous westphalian b to lower permian asselian interval this has been informally split into three sedimentary units the lower flora sandstone is of westphalian b c age this clastic interval is overlain by the first of two main lavas which is in turn overlain by the westphalian c d to stephanian upper flora sandstone the flora sandstones are interpreted to be lithostratigraphic equivalents to the schooner formation of the southern north sea the flora sandstones are separated from the grensen formation by an angular unconformity possibly related to variscan asturic inversion the grensen formation is in turn overlain by a second basaltic lava flow both basaltic units are assigned to the inge volcanics formation rotliegend group petrographic data indicate that the grensen formation also forms part of the rotliegend group a major extensional event of permian age post dates all of these strata extensional faults are planed by a regional unconformity saalian unconformity that is overstepped by permian to cretaceous strata

surface slope control on firn density at thwaites glacier west antarctica results from airborne radar sounding we derive the surface density variations over thwaites glacier from a statistical analysis of airborne high capability radar sounder surface returns acquired in the 2004 2005 summer we produce a 5 km gridded map with an estimated 12 5 kg m 3 accuracy the background pattern of densities decreases inland from 480 kg m 3 to 350 kg m 3 a remarkable 30 km wide vein shaped anomaly of up to 570 kg m 3 is located 100 km from the coastline density anomalies correspond with surface slope breaks but are not necessarily coincident with smaller slopes they could result from complex wind driven snow redistribution and or refreezing of former snowmelt this inversion technique can significantly improve surface mass balance calculations to understanding of glacier dynamics at regional scales and is valuable to verify and improve antarctic climate models it is also a promising approach for future surface analysis of icy moons by planetary radars

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

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

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

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

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

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

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

radar soundings of the subsurface of mars the martian subsurface has been probed to kilometer depths by the mars advanced radar for subsurface and ionospheric sounding instrument aboard the mars express orbiter signals penetrate the polar layered deposits probably imaging the base of the deposits data from the northern lowlands of chryse planitia have revealed a shallowly buried quasi circular structure about 250 kilometers in diameter that is interpreted to be an impact basin in addition a planar reflector associated with the basin structure may indicate the presence of a low loss deposit that is more than 1 kilometer thick