europa s south pole region a sequential reconstruction of surface modification processes abstract high resolution images of a region near the south pole of europa allow reconstruction of a series of events tectonic and thermal that displaced and modified the surface this approach is complementary to regional geological mapping which is based on less detailed images but covers a broader sample of the surface the events reconstructed in this locale resurfaced about half the area or more so they probably ranged over a substantial fraction of the age of europa s surface which in itself is quite young

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

hydrothermal systems on europa 1 we estimate the global hydrothermal heat transport on europa by considering the range of heat generation resulting from tidal stresses and radiogenic heat production within europa s rocky sub ocean lithosphere we show that if factors controlling tidally generated heat on europa are similar to those on io global hydrothermal heat output and its high temperature component may exceed that on earth on the individual system scale however the low acceleration of gravity on europa indicates that individual hydrothermal plumes are relatively weak such weak hydrothermal plumes are not likely to result in melt through events which have been hypothesized to form surface features such as conamara chaos studies of concentrated hydrothermal activity and the resulting interacting plumes are needed to address the possible impact of hydrothermal activity on europa s ice sheet

heat transfer of ascending cryomagma on europa abstract jupiter s moon europa has a relatively young surface 60 90 myr on average which may be due in part to cryovolcanic processes current models for both effusive and explosive cryovolcanism on europa may be expanded and enhanced by linking the potential for cryovolcanism at the surface to subsurface cryomagmatism the success of cryomagma transport through europa s crust depends critically on the rate of ascent relative to the rate of solidification the final transport distance of cryomagma is thus governed by initial melt volume ascent rate overall ascent distance transport mechanism i e diapirism diking or ascent in cylindrical conduits and melt temperature and composition the last two factors are especially critical in determining the budget of expendable energy before complete solidification here we use these factors as constraints to explore conditions under which cryomagma may arrive at europa s surface to facilitate cryovolcanism we find that 1 5 km radius warm ice diapirs ascending from the base of a 10 km thick stagnant lid can reach the shallow subsurface in a partially molten state cryomagma transport may be further facilitated if diapirs travel along pre heated ascent paths under certain conditions cryolava transported from 10 km depths in tabular dikes or pipe like conduits may reach the surface at temperatures exceeding 250 k ascent rates for these geometries may be high enough that isothermal transport is approached cryomagmas containing significant amounts of low eutectic impurities can also be delivered to europa s surface by propagating dikes or pipe like conduits

chaos on europa the characteristics of chaos regions on europa suggest they may be sites of melt through from below they are wide ranging in size location and age the largest are hundreds of kilometers across most are similar to conamara with a matrix reminiscent of frozen slush and often rafts of preexisting crust edges are of two types ramps perhaps the tapering of crustal thickness to zero or cliffs where rafts appear to have broken clear from the shore the small features called lenticulae generally appear to be small chaoses with textured matrix and occasional rafts and many domes may be small chaoses raised by isostatic compensation following refreezing of the crust the extent of chaoses often appears to be limited by ridge systems with the coastline parallel and set back by a distance comparable to the width of the ridge system preexisting ridges often survive as causeways or chains of rafts boundaries of chaoses are apparently not controlled by preexisting cracks consistent with formation by a thermal rather than mechanical process ridges may thicken the crust such that melt through is more likely but not always between ridge systems subsequent cracks and ridges form across preexisting chaoses ranging from fresh cases with few cracks or ridges across them with paths somewhat jagged as they meander among rafts to heavily dissected examples isolated tilted raft like blocks surrounded by densely ridged terrain may be relics of former chaotic terrain thus two fundamental resurfacing processes have alternated over europa s geological history melt through at various places and times forming chaos terrain and tectonic cracking and dilation building densely ridged and banded terrain mapping of chaos features based on morphology at 200 m shows that they correlate albeit imperfectly with dark regions in global 2 km resolution mosaics except dark regions due to ridge margins or craters extrapolating from our mapping of the 5 of europa covered by appropriate images at least 18 of the surface of europa is fresh appearing chaos an additional 4 is slightly modified chaos and much more older chaotic terrain has been overprinted by tectonic structures considerable area has been available globally to accommodate the expansion of crust that occurs along extensional ridges and bands chaos ubiquity suggests that europan geology has been dominated by the effects of having liquid water under a very thin ice shell with chaos regions being widespread indicators of occasional zero shell thickness

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

the stability against freezing of an internal liquid water ocean in callisto the discovery of the induced magnetic field of callisto one of jupiter s moons has been interpreted1 2 as evidence for a subsurface ocean even though the presence of such an ocean is difficult to understand in the context of existing theoretical models3 4 5 tidal heating should not be significant for callisto and in the absence of such heating it is difficult to see how this internal ocean could have survived until today without freezing1 6 previous work3 4 indicated that an outer ice layer on the ocean would be unstable against solid state convection which once begun would lead to total freezing of liquid water in about 108 years here i show that when a methodology7 for more physically reasonable water ice viscosities that is stress dependent non newtonian viscosities rather than the stress independent newtonian viscosities considered previously is adopted the outer ice shell becomes stable against convection this implies that a subsurface ocean could have survived up to the present without the need for invoking antifreeze substances or other special conditions

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

constraining the thickness of europa s water ice shell insights from tidal dissipation and conductive cooling abstract the time of crystallization of a 100 km thick ocean on europa is estimated using a stefan style solidification solution this solution is then extended to estimate the present thickness of the ice shell it is assumed that the shell is initially in a steady state conductive regime and the ocean is taken to be an infinite liquid half space cooling from above we find that in the absence of tidal heating and without the presence of low eutectic impurities to serve as anti freezes a 100 km thick ocean solidifies in about 64 myr conversely when considering the present thickness of europa s ice shell if tidal heating is included at a global dissipation rate of 1 tw the shell is found to be on average approximately 28 km thick however if this dissipative heating is solely restricted to the shell the local rate of heating may vary significantly due to crustal compositional heterogeneities and it is shown that this process may in turn produce thermal maxima in the crust which could lead to local melting and structural instabilities perhaps associated with the formation of chaos regions our approach is also extended to ganymede and callisto in order to estimate the time of solidification of their putative subsurface oceans and the current thicknesses of their ice i shells

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

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

tidal heating in icy satellite oceans abstract tidal heating plays a significant role in the evolution of many satellites in the outer solar system however it is unclear whether tidal dissipation in a global liquid ocean can represent a significant additional heat source tyler tyler r h 2008 nature 456 770 772 tyler r h 2009 geophys res lett 36 doi 10 1029 2009gl038300 suggested that obliquity tides could drive large scale flow in the oceans of europa and enceladus leading to significant heating a critical unknown in this previous work is what the tidal quality factor q of such an ocean should be the corresponding tidal dissipation spans orders of magnitude depending on the value of q assumed to address this issue we adopt an approach employed in terrestrial ocean modeling where a significant portion of tidal dissipation arises due to bottom drag with the drag coefficient o 0 001 being relatively well established from numerical solutions to the shallow water equations including nonlinear bottom drag we obtain scalings for the equivalent value of q as a function of this drag coefficient in addition we provide new scaling relations appropriate for the inclusion of ocean tidal heating in thermal orbital evolution models our approach is appropriate for situations in which the ocean bottom topography is much smaller than the ocean thickness using these novel scalings we calculate the ocean contribution to the overall thermal energy budgets for many of the outer solar system satellites although uncertainties such as ocean thickness and satellite obliquity remain we find that for most satellites it is unlikely that ocean tidal dissipation is important when compared to either radiogenic or solid body tidal heating of known satellites triton is the most likely icy satellite to have ocean tidal heating play a role in its present day thermal budget and long term thermal evolution

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

dielectric properties of granodiorite partially saturated with water and its correlation to the detection of seismic electric signals abstract transient electric signals emitted prior to earthquake occurrence are recorded at certain sites in the earth s crust termed sensitive these field observations enforce the laboratory investigation of the dielectric response of rocks forming these localities the dielectric relaxation of granodiorite rock coming from such a sensitive locality keratea greece reveals through complex impedance spectroscopy that the activation volume for relaxation of this rock is negative that so far has been reported only rarely this result however supports a theoretical model on the pre seismic electric signals and is likely to be correlated with the sensitivity of the site and hence with the selectivity

evaluation of models for the formation of chaotic terrain on europa chaotic terrain on europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate two end member models have been proposed for the mobile substrate a liquid water ocean or solid warm ductile ice direct contact between the brittle surface ice and an underlying ocean might be accomplished by melting through europa s ice shell but plausible sources of thermal energy are insufficient to locally melt through the ice shell we calculate that a liquid or partially liquid substrate is probably necessary for tilted blocks to be observed challenging a solid state model we propose that a model in which chaos areas on europa formed over bodies of melt or partial melt within the ice shell may be the best match to observational and thermal constraints such bodies of melt within the ice shell may be triggered by warm diapirs of relatively clean ice that partially melted an overlying layer of lower melting temperature such as ice contaminated with salts

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

a possible brine reservoir beneath occator crater thermal and compositional evolution and formation of the cerealia dome and vinalia faculae abstract the dawn spacecraft has imaged several putative cryovolcanic features on ceres and several lines of evidence point to past cryovolcanic activity at occator crater it is therefore possible that cryovolcanism played a key role in delivering sodium carbonate and chloride enriched brines to ceres surface in recent geological times the detection of a 200 km 200 km negative bouguer anomaly beneath occator suggests the presence of a low density region beneath the crater if this region is a residual partially crystallized cryomagma chamber excess pressures caused by its gradual freezing or stresses produced by the occator forming impact could have facilitated the delivery of cryolavas to the surface in the geologically recent past here the progressive solidification of a cryomagma chamber beneath occator and implications for the delivery of cryolavas to the surface has been explored models for the behavior of cryolavas at ceres surface and for the formation of the cerealia dome and vinalia faculae are also presented minimal crystallization of a subsurface fluid reservoir located at the crust mantle boundary could have driven cryolavas enriched in chloride salts and nh3 to ceres surface however cryolavas enriched in sodium carbonates would have had to have existed at shallower levels in the crust in order to be delivered to the surface via pressure driven ascent depending on the size of the reservoir cryolavas could have been driven to ceres surface for tens to hundreds of millions of years after cooling of the cryomagma chamber commenced the mineralogy at occator is suggestive of subsurface cryomagma reservoirs enriched in sodium carbonate and chloride salts aqueous solutions enriched in chloride salts and or ammonia would have arrived at the surface at warm enough temperatures to erupt if transported in propagating fractures that traveled at least 10 5 m s additionally if the cerealia dome was formed from viscous cryolava extrusions bulk kinematic lava viscosities may have been between 106 108 m2 s at the onset of relaxation plausible relaxation times to form the dome which are linked to bulk cryolava rheology are found to have ranged from 2 5 to 273 days moreover the low volatile content necessary to drive explosive eruptions on ceres supports the possibility that cerealia and vinalia faculae were emplaced as a consequence of ballistic eruptions the enigmatic geology of occator crater is consistent with a diversity of exchange processes operating on ceres in the geologically recent past further the processes that have occurred at occator could shed light on the changing geology associated with a relic ocean world future studies of occator and ceres should be undertaken with these results in mind

geologic mapping of europa galileo data enable the major geological units structures and surface features to be identified on europa these include five primary units plains chaos band ridge and crater materials and their subunits along with various tectonic structures such as faults plains units are the most widespread ridged plains material spans a wide range of geological ages including the oldest recognizable features on europa and appears to represent a style of tectonic resurfacing rather than cryovolcanism smooth plains material typically embays other terrains and units possibly as a type of fluid emplacement and is among the youngest material units observed at global scales plains are typically mapped as undifferentiated plains material although in some areas differences can be discerned in the near infrared which might be related to differences in ice grain size chaos material is composed of plains and other preexisting materials that have been severely disrupted by inferred internal activity chaos is characterized by blocks of icy material set in a hummocky matrix band material is arrayed in linear curvilinear wedge shaped or cuspate zones with contrasting albedo and surface textures with respect to the surrounding terrain bilateral symmetry observed in some bands and the relationships with the surrounding units suggest that band material forms by the lithosphere fracturing spreading apart and infilling with material derived from the subsurface ridge material is mapped as a unit on local and some regional maps but shown with symbols at global scales ridge material includes single ridges doublet ridges and ridge complexes ridge materials are considered to represent tectonic processes possibly accompanied by the extrusion or intrusion of subsurface materials such as diapirs the tectonic processes might be related to tidal flexing of the icy lithosphere on diurnal or longer timescales crater materials include various interior smooth central rough inner and annular massif and exterior continuous ejecta subunits structural features and landforms are shown with conventional symbols type localities for the units are identified along with suggestions for portraying the features on geological maps including colors and letter abbreviations for material units implementing these suggestions by the planetary mapping community would facilitate comparisons of maps for different parts of europa and contribute to an eventual global synthesis of its complex geology on the basis of initial mapping results a stratigraphic sequence is suggested in which ridged plains form the oldest unit on europa followed by development of band material and individual ridges band materials tend to be somewhat older than ridges but in many areas the two units formed simultaneously similarly the formation of most chaos follows the development of ridged plains although chaos is among the youngest materials on europa some chaos units might have formed contemporaneously with ridged plains smooth plains generally embay all other units and are late stage in the evolution of the surface c 1 craters are superposed on ridged plains but are crosscut by other materials including bands and ridges most c2 craters postdate all other units but a few c2 craters are cut by ridge material c3 craters constitute the youngest recognizable material on europa

europa initial galileo geological observations abstract images of europa from the galileo spacecraft show a surface with a complex history involving tectonic deformation impact cratering and possible emplacement of ice rich materials and perhaps liquids on the surface differences in impact crater distributions suggest that some areas have been resurfaced more recently than others europa could experience current cryovolcanic and tectonic activity global scale patterns of tectonic features suggest deformation resulting from non synchronous rotation of europa around jupiter some regions of the lithosphere have been fractured with icy plates separated and rotated into new positions the dimensions of these plates suggest that the depth to liquid or mobile ice was only a few kilometers at the time of disruption some surfaces have also been upwarped possibly by diapirs cryomagmatic intrusions or convective upwelling in some places this deformation has led to the development of chaotic terrain in which surface material has collapsed and or been eroded

distribution of chaotic terrain on europa the locations of chaotic terrain are mapped over all regions of europa for which we have adequate resolution 200 m or better and appropriate lighting geometry sun angle 35 from the local horizontal comprising 9 of the total surface of the satellite nearly 30 of the mapped area is occupied by chaotic terrain recognizable at 200 m resolution and sampling at higher resolution suggests that at least 10 more may be covered by small chaos features the largest contiguous area of chaotic terrain is 1300 km across chaotic terrain displays variations in freshness of appearance probably because of aging by fine scale tectonics resurfacing of previous chaotic terrain by larger scale tectonics or disruption by newer chaos are common chaos formation is not necessarily recent relative to tectonics both types of process appear to have gone on diachronously the size distribution shows no dominant or characteristic size and appears to have been controlled by competition from tectonics which has created terrain that occupies more than half of the surface of europa

europa tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting europa we show that tidal energy can be preferentially focused in rising plumes for viscosities in agreement with laboratory experiments when the plume cores reach the base of the outer cold brittle layer they spread laterally causing shallow melting disruption and formation of terrain similar to lenticulae and chaos we show that this mechanism can readily explain the major characteristics of chaos and lenticulae even if the average outer solid ice layer thickness is larger than 20 km

ice diapirs on europa implications for liquid water early examinations of galileo images of europa revealed features that look like low topographic domes these small 5 10 km in radius domes have been interpreted as surface manifestations of diapirs as a way to probe the subsurface structure of europa we investigate the possibility that thermally driven ice diapirism created these surface features we use a previously developed analytic model for rising diapirs to estimate the initial radii of the diapirs their initial depth the temperature of the medium through which they rise and their formation timescales we assume that the diapirs originate at the boundary layer between solid ice and whatever underlies it whether it be liquid water or solid rock our results show that the diapirs originate at a depth of no more than a few tens of kilometers since the h2o layer on europa is substantially thicker than this our results support the view that liquid water recently existed beneath the ice on europa at this location at a depth of no more than a few tens of kilometers further the ice must have been warm 250 270 k and was therefore likely to have been convecting

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

geology and origin of europa s mitten feature murias chaos 1 the mitten provisionally named murias chaos by the international astronomical union is a region of elevated chaos like terrain in the leading hemisphere of europa its origin had been explained under the currently debated theories of melting through a thin lithosphere or convection within a thick one galileo observations reveal several characteristics that suggest that the mitten is distinct from typical chaos terrain and point to a different formational process photoclinometric elevation estimates suggest that the mitten is slightly elevated with respect to the surrounding terrain geologic relations indicate that it must have raised significantly from the plains in its past resembling disrupted domes on europa s trailing hemisphere moreover the mitten material appears to have extruded onto the plains and flowed for tens of kilometers the area subsequently subsided as a result of isostatic adjustment viscous relaxation and or plains loading using plate flexure models we estimated the elastic lithosphere in the area to be several kilometers thick we propose that the mitten originated by the ascent and extrusion of a large thermal diapir thermal mechanical modeling shows that a mitten sized plume would remain sufficiently warm and buoyant to pierce through the crust and flow unconfined on the surface such a diapir probably had an initial radius between 5 and 8 km and an initial depth of 20 40 km consistent with a thick lithosphere model in this scenario the mitten appears to represent the surface expression of the rare ascent of a large diapir in contrast to lenticulae and chaos terrain which may form by isolated and clustered small diapirs respectively

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

the tidal response of ganymede and callisto with and without liquid water oceans calculations of the tidal responses of ganymede and callisto reveal that tidal amplitudes on these bodies may be as large as a few meters if a liquid ocean exists to decouple the surface ice from the interior tides on ganymede s surface can exceed 7 m peak to peak variation while on callisto the tidal amplitude can exceed 5 m in the presence of a liquid ocean without an ocean tidal amplitudes are less than 0 5 m on ganymede and less than 0 3 m on callisto an orbiting spacecraft using an altimeter for crossover analysis and doppler tracking from earth should be able to achieve sufficient accuracy to identify the tidal amplitude to within about a meter over the course of a few months observing tens of tidal cycles

tidal heating and the long term stability of a subsurface ocean on enceladus abstract tidal dissipation has been suggested as the heat source for the south polar thermal anomaly on enceladus we find that under present day conditions and assuming maxwellian behavior tidal dissipation is negligible in the silicate core dissipation may be significant in the ice shell if the shell is decoupled from the silicate core by a subsurface ocean we have run a series of self consistent convection and conduction models in 2d axisymmetric and 3d spherical geometry in which we include the spatially variable tidal heat production we find that in all cases the shell removes more heat from the interior than can be produced in the core by radioactive decay resulting in cooling of the interior and the freezing of any ocean under likely conditions a 40 km thick ocean made of pure water would freeze solid on a 30 ma timescale an ocean containing other chemical components will have a lower freezing point but even a water ammonia eutectic composition will only prolong the freezing not prevent it if the eccentricity of enceladus were higher e 0 015 in the past the increased dissipation in the ice shell may have been sufficient to maintain a liquid layer we cannot therefore rule out the presence of a transient ocean as a relic of an earlier era of greater heating if the eccentricity is periodically pumped up the ocean may have thickened and thinned on a similar timescale as the orbital evolution provided the ocean never froze completely we conclude that the current heat flux of enceladus and any possible subsurface ocean is not in steady state and is the remnant of an epoch of higher eccentricity and tidal dissipation

thermal state and complex geology of a heterogeneous salty crust of jupiter s satellite europa abstract the complex geology of europa is evidenced by many tectonic and cryomagmatic resurfacing structures some of which are painted into a more visible expression by exogenic alteration processes acting on the principal endogenic cryopetrology the surface materials emplaced and affected by this activity are mainly composed of water ice in some areas but in other places there are other minerals involved non ice minerals are visually recognized by their low albedo and reddish color either when first emplaced or more likely after alteration by europan weathering processes especially sublimation and alteration by ionizing radiation while red chromophoric material could be due to endogenic production of solid sulfur allotropes or other compounds most likely the red substance is an impurity produced by radiation alteration of hydrated sulfate salts or sulphuric acid of mainly internal origin if the non ice red materials or their precursors have a source in the satellite interior and if they are not merely trace contaminants then they can play an important role in the evolution of the icy crust including structural differentiation and the internal dynamics here we assume that these substances are major components of europa s cryo hydrosphere as some models have predicted they should be if this is an accurate assumption then these substances should not be neglected in physical chemical and biological models of europa even if major uncertainties remain as to the exact identity abundance and distribution of the non ice materials the physical chemical properties of the ice associated materials will contribute to the physical state of the crust today and in the geological past in order to model the influence of them on the thermal state and the geology we have determined the thermal properties of the hydrated salts our new lab data reveal very low thermal conductivities for hydrated salts compared to water ice lower conductivities of salty ice would produce steeper thermal gradients than in pure ice if there are salt rich layers inside the crust forming salt beds over the seafloor or a briny eutectic crust for instance the high thermal gradients may promote endogenic geological activity on the seafloor bedded salt accumulations may exhibit high thermochemical gradients metamorphic and magmatic processes and possible niches for thermophilic life at shallow suboceanic depths result from the calculated thermal profiles even if the ocean is very cold

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

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

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

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

radar dark impact crater related parabolas on venus characterization of deposits with magellan emissivity data 1 we have analyzed the radar dark parabola deposits associated with impact craters on the surface of venus using the results of the magellan radio experiments in both passive and active modes the extent of mantle deposits is found to be much wider than the dark features observed in synthetic aperture radar sar images using the emissivity data we have classified features coinciding with the radar dark parabolas into four types related to both differences in ejecta deposition styles for craters of different diameters and stages in the degradation of crater related deposits a model for the microwave properties of the mantled surfaces is proposed comparison between the distribution of mantle thickness predicted by the model and the previously published independent results based on modeling of the cratering process show good agreement when the loss tangent of the mantle material is assumed to be within the 0 001 0 005 range this value is similar to that of dry powders of terrestrial and lunar rocks on the basis of this work we conclude that radar dark diffuse features due to these mantles can be up to several meters thick if the loss tangent is assumed to be 0 003 thin deposits as little as 7 cm thick are found to be responsible for the deposits seen in the emissivity parabolas but not observed in the sar images these results provide a basis for the interpretation of the character and thickness of mantle deposits their states of degradation processes associated with their formation and degradation and guides to their use in establishing stratigraphic relationships and ages of underlying geologic units on venus the results will also be useful in interpreting the nature of surface deposits at previous landing sites and predicting the nature of deposits likely to be encountered at future landing sites

brine volcanism and the interior structures of asteroids and icy satellites abstract cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites volcanic brine petrogenesis in ammonia deficient satellites should mainly involve eutectic melting in relevant salt water systems carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids c1 chondrites contain about one fifth by mass of salts in various states of hydration many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system h2o mgso4 na2so4 minor components include sulfates of k ni mn and ca chondrites also contain abundant carbonates but these are probably not very important in brine magmatism due to their low solubilities under expected conditions chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of cl s soluble salts may have profound effects on the geology and structure of icy satellites and asteroids in some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate rich mantle and crustal layers in other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities europa probably has a layered crust composed of anhydrous mg na sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated mg and na sulfates near the surface ganymede may have a crust about 300 km thick composed of a 10 1 ratio of ice mirabilite and a mantle 500 km thick composed of 50 ice phases plus 50 hydrated mg and na sulfates

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

thermo compositional convection in europa s icy shell with salinity 1 we present two dimensional numerical simulations of thermo compositional convection to test the hypothesis that the combined buoyancy from both thermal and salinity contrasts in europa s ice shell can produce the numerous uplifts and pits on europa s surface our simulations show that uplifts and pits with amplitude of 100 500 m and diameters of 10 30 km similar to some of the observed features can be produced in a 10 30 km thick ice shell with 2 10 compositional density variations if the viscosity contrast due to temperature variation does not exceed 106 the pit and uplift formation time and lifetime are approximately proportional to the surface viscosity ranging from 104 years to 107 years for viscosity contrasts of 104 106 convection cannot produce substantial surface topography if the viscosity contrast exceeds 107 108 these results imply that thermo compositional convection can only produce europa s pits and uplifts if europa s surface is weak

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

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

a melt through model for chaos formation on europa the character of chaotic terrain on europa is consistent with its formation by the melting of a thin conducting ice shell from below tidal dissipation can provide adequate energy for such a process for example only a few percent of europa s predicted tidal heat spread over a region 200 km in diameter can lead to large melt regions within a few tens of thousands of years stronger more localized concentrations result in melt through in significantly shorter times i e a few hundred years the time scale for melt through is shorter than the time scale for the solid state viscous inflow of ice by several orders of magnitude in general modest concentrations of tidal heat can melt ice away faster than viscous inflow leading to melt through a mechanism to transmit these heat concentrations through the ocean is required for this model such heat transport could be the result of convective plumes in the ocean driven by seafloor volcanism or by the destabilization of a stratified ocean

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