constraints on the martian lithosphere from gravity and topography data localized spectral admittances of the large martian volcanoes are modeled by assuming that surface and subsurface loads are elastically supported by the lithosphere in order to model the case where the load density differs from that of the crust a new method for calculating gravity anomalies and lithospheric deflections is developed the modeled gravity anomalies depend upon the elastic thickness crustal thickness load density and crustal density and these parameters were exhaustively sampled in order to determine their effect on the misfit between the observed and modeled admittance function we find that the densities of the martian volcanoes are generally well constrained with values of 3200 100 kg m 3 which is considerably greater than those reported previously these higher densities are consistent with those of the martian basaltic meteorites which are believed to originate from the tharsis and elysium volcanic provinces the crustal density is constrained only beneath the elysium rise to be 3270 150 kg m 3 if this value is representative of the northern lowlands then pratt compensation is likely responsible for the approximately 6 km elevation difference between the northern and southern hemispheres the elastic thicknesses of the major martian volcanoes when subsurface loads are ignored are found to be the following elysium rise 56 20 km olympus mons 93 40 km alba patera 66 20 km and ascraeus mons 105 40 km we have also investigated the effects of subsurface loads allowing the bottom load to be located either in the crust as dense intrusive material or in the mantle as less dense material we found that all volcanoes except pavonis are better modeled with the presence of less dense material in the upper mantle which is indicative of either a mantle plume or a depleted mantle composition an active plume beneath the major volcanoes is consistent with recent analyses of cratering statistics on olympus mons and the elysium rise which indicate that some lava flows are as young as 10 30 myr as well as with the crystallization ages of the shergottites of which some are as young as 180 myr

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

impact features on europa results of the galileo europa mission gem during the galileo europa mission gem impact features on europa were observed with improved resolution and coverage was compared with voyager or the galileo nominal mission we surveyed all primary impact features 4 km in diameter seen on europa through orbit e19 the transition from simple to complex crater morphology occurs at a diameter of about 5 km we calculated the transient crater dimensions and excavation depths of all craters surveyed the largest impact feature tyre probably had a transient crater depth between 5 and 10 km and transported material to the surface from a depth of not greater than 4 km craters 30 km in diameter such as manannan and pwyll formed within targets whose immediate subcrater materials exhibited nonfluid behavior on time scales of the impact event and that are capable especially in the case of pwyll of supporting significant local topographic loads such as a central peak these craters are nevertheless quite shallow with very subdued floors and we suspect that manannan and pwyll s small depth to diameter ratios are due to the isostatic adjustment of large scale topography facilitated by warm plastically deformable ice at depth morphological similarities between callanish and tyre strongly imply that conclusions reached regarding callanish in j moore et al 1998 icarus135 127 145 also apply to tyre which was that callanish is the consequence of impact into target materials that are mechanically very weak at depth new evidence that callanish s circumferential rings formed before the proximal ejecta became immobile implies a low viscosity substrate at the time of impact we also report additional evidence that a component of the proximal ejecta of callanish was emplaced as a fluid our observations of pwyll secondaries support the conclusions stated in alpert and melosh 1999 that impacts on icy bodies eject smaller fragments and that fragment size decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities examination of pwyll s secondary craters reveals azimuthal variations with ejecta fragment sizes being larger near the center of a ray than off the ray our initial analysis of the characteristic size distribution of pwyll s secondary craters shows that they form a differential slope slightly shallower than 4 similar steep slopes for small craters on ganymede imply that small craters there are mostly formed by secondary impact and the jovian system may thus be deficient in small impacts relative to the environment of the terrestrial planets

simulated recovery of europa s global shape and tidal love numbers from altimetry and radio tracking during a dedicated flyby tour the fundamental scientific objectives for future spacecraft exploration of jupiter s moon europa include confirmation of the existence of subsurface ocean beneath the surface ice shell and constraints on the physical properties of the ocean here we conduct a comprehensive simulation of a multiple flyby mission we demonstrate that radio tracking data can provide an estimate of the gravitational tidal love number k2 with sufficient precision to confirm the presence of a liquid layer we further show that a capable long range laser altimeter can improve determination of the spacecraft position improve the k2 determination 1 error and enable the estimation of the planetary shape and love number h2 3 4 error which is directly related to the amplitude of the surface tidal deformation these measurements in addition to the global shape accurately constrained by the long altimetric profiles can yield further constraints on the interior structure of europa

the zonal harmonic model of polarity transitions a test using successive reversals a recently developed zonal model for the last geomagnetic field reversal which describes time and latitude dependent transitional behavior of intensity and inclination in terms of dominance of low order field harmonics was tested using a latitudinal and chronological succession of transition records the primary data were derived from a high resolution study of five matuyama to brunhes deep sea cores collected along a 40 meridional band in the north central pacific the transitions analyzed were the on sets and terminations of the olduvai and jaramillo events and the matuyama brunhes boundary supplementary data published by previous workers included a jaramillo onset record from the southern indian ocean and a transition recorded in the triassic chugwater formation of north america in a general sense the measured inclination and intensity records indicate the jaramillo transitions and the last reversal to be remarkably similar to each other and to differ from the olduvais crosscorrelation of the measured data sets with synthetic zonal harmonic records for the core latitudes indicates that the zonal harmonic model does predict the behavior of matuyama brunhes reversal in the northern hemisphere for the older reversals however the present model requires substantial modification this is particularly so in the case of the jaramillo onset although two inclination and intensity records of this transition from northern and southern hemisphere sediments strongly resemble each other the current model produces a synthetic record for one hemisphere that is incompatible with that hemisphere s measured data further to model zonally the olduvai reversals for which a wide latitudinal northern hemisphere sampling was available a substantially different ratio of low order multipole components from that of the standard model is required and the modeled solution was unsatisfactory at very low latitude a modified zonal harmonic approach which introduces a small sectoral component may offer a solution to the incompatibilities uncovered by the present study

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

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

saturn s icy satellites thermal and structural models abstract thermal histories of the small icy saturnian satellites mimas tethys dione rhea and iapetus are constructed by assuming that they formed as homogeneous ice silicate mixtures the models include effects of radiogenic and accretional heating conductive and subsolidus convective heat transfer and lithospheric growth accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating mimas is so small that its thermal evolution is essentially purely conductive at present it is a cold nearly isothermal body any subsolidus convection or thermal activity in mimas would have been confined to a brief period in its early history and would have been due to a warm formation the four largest satellites are big enough and contain sufficient heat producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions dione and rhea have convective interiors for most of their thermal histories while tethys and iapetus have mainly conductive thermal histories with early periods of convective 0activity the thermal histories of the five satellites for the last 4 by are independent of initial conditions at present they have cold conductive interiors the model thermal histories are qualitatively consistent with the appearances of these satellites mimas has an ancient heavily cratered surface tethys and probably iapetus have both heavily cratered and more lightly cratered areas and dione and rhea have extensively modified surfaces because of their similar sizes and densities mimas and enceladus are expected to have similar surfaces and thermal histories but instead enceladus has the most modified surface of all the small icy saturnian satellites our results suggest a heat source for enceladus in addition to radiogenic and accretional heating tidal dissipation is a possibility because the water ice in these bodies does not melt resurfacing must be accomplished by the melting of a low melting temperature minor component such as ammonia hydrate

shield volcano topography and the rheology of lava flows on io stereo and photoclinometry derived topography of shield like volcanoes on io indicate little relief 3 km and very low slopes 0 2 to 0 6 several shield volcanoes appear to be associated with broad rises of 1 to 3 km but only 5 shield volcanoes have been identified with steep flank slopes between 4 and 10 these steep slopes are restricted to within 20 30 km of the summit but where discernable most of the lava flows observed on these edifices occur on the outer flanks where slopes are less than a degree despite their abundance ionian shield volcanoes are among the flattest in the solar system the steepest volcanoes on io are most comparable to large venusian shield volcanoes using simplistic bingham rheologies we estimate the viscosity and yield strengths of ionian lavas yield strengths are estimated at 101 102 pa lower than most basaltic lavas viscosity estimates range from 103 to 105 pa s although these are probably upper limits actual values may have been as low as 100 pa s viscosity is sensitive to flow velocity which is poorly known on io the best constraint on flow velocity comes from observations of the 1997 pillan eruption which bracket the eruptive phase to 132 day maximum and more probably less than 50 days low slopes long run out distances and our estimated rheologic properties are consistent with but not proof of a low silica low viscosity high temperature composition for ionian lavas supporting arguments for low silica lava compositions such as basalt or komatiite we cannot eliminate sulfur on rheologic grounds however

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

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

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

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

structure of volcanic glasses from the nmr epr perspective a preliminary application to the neapolitan yellow tuff glass samples from alkali trachytic pumice of the lower member lm level lm1 and upper member um of the neapolitan yellow tuff at campi flegrei caldera were studied by nuclear magnetic resonance nmr and electron paramagnetic resonance epr spectroscopy the epr data show that the relative occupancy of the different iron sites varies in samples from the lower member lm1 and upper member of the eruptive sequence the lm1 glasses are characterized by a more homogeneous distribution of the cationic site population of iron compared with um and by a different fe3 al3 substitution in the c2v symmetrical sites of the silicate framework 29si mas nmr spectra on glasses indicate that lm1 glasses are more depolymerized than um glasses indicating the existence of structurally distinct magma batches 27al mas nmr spectra indicate the occurrence in um glasses of aluminum in both tetrahedral and octahedral coordination the observed alvi is believed to be a primary feature of the glass since 27al cp mas nmr experiments do not reveal detectable esa hydrated al the al in sixfold coordination could be ascribed either to the more abundant alkali oh terminations in um in which alkalies would be extracted from their charge balance role for al thus altering its fourfold structural position or to permanent compaction during fast cooling and decompression of high pressure melts 23na cp mas nmr experiments indicate the existence of na oh groups in both lm1 and um glasses relatively more abundant in the latter although preliminary the results indicate the possible application of atomic scale studies to variables that determine the regimes of explosive volcanism

gravity signals on europa from silicate shell density variations 1 future missions to jupiter s moon europa will attempt to measure the gravity field of this planetary body here we study the detectability of silicate shell density variations in the gravity field the first step in the gravity processing will be to remove the gravity signal of the ice shell the detection of the ice shell signal however can be technologically challenging depending on its thickness and compensation state since the predicted anomalies are only a few mgals or even smaller for long wavelength topography below degree 40 the signal of the silicate shell will likely dominate the gravity field assuming that the anomalies of the silicate shell are only caused by the ocean floor topography thus neglecting possible density anomalies in the mantle we should be able to detect the ocean floor signal even if its topographic variations are only a few hundred meters when studying the gravity signal of isolated midsize topographic features like volcanoes we find a good chance of detecting objects with a size of 75 200 km with measurement accuracy of 1 mgal owing to the large number of unknown parameters for the gravity inversion the reconstruction of a global ice water silicate interface shape is uncertain in particular as possible contributions to the gravity field from a low degree convecting mantle cannot be distinguished the comparison between the standard measurement technique of doppler tracking detecting the gravity anomalies and a microgradiometer measuring gravity gradients shows that the latter will not improve the detectability of the ocean floor structures

evolution of large shield volcanoes on venus 1 we studied the geologic history topographic expression and gravity signature of 29 large venusian shield volcanoes with similar morphologies in magellan synthetic aperture radar imagery while they appear similar in imagery 16 have a domical topographic expression and 13 have a central depression typical dimensions for the central depression are 150 km wide and 500 m deep the central depressions are probably not calderas resulting from collapse of a shallow magma chamber but instead are the result of a corona like sagging of a previously domical volcano the depressions all have some later volcanic filling all but one of the central depression volcanoes have been postdated by geologic features unrelated to the volcano while most of the domical volcanoes are at the top of the stratigraphic column analysis of the gravity signatures in the spatial and spectral domains shows a strong correlation between the absence of postdating features and the presence of dynamic support by an underlying plume we infer that the formation of the central depressions occurred as a result of cessation of dynamic support however there are some domical volcanoes whose geologic histories and gravity signatures also indicate that they are extinct so sagging of the central region apparently does not always occur when dynamic support is removed we suggest that the thickness of the elastic lithosphere may be a factor in determining whether a central depression forms when dynamic support is removed but the gravity data are of insufficient resolution to test this hypothesis with admittance methods

modeling the dynamic component of the geoid and topography of venus 1 we analyze the venusian geoid and topography to determine the relative importance of isostatic elastic and dynamic compensation mechanisms over different degree ranges the geoid power spectrum plotted on a log log scale shows a significant change in its slope at about degree 40 suggesting a transition from a predominantly dynamic compensation mechanism at lower degrees to an isostatic and or elastic mechanism at higher degrees we focus on the dynamic compensation in the lower degree interval we assume that 1 the flow is whole mantle in style 2 the long wavelength geoid and topography are of purely dynamic origin and 3 the density structure of venus mantle can be approximated by a model in which the mass anomaly distribution does not vary with depth solving the inverse problem for viscosity within the framework of internal loading theory we determine the families of viscosity models that are consistent with the observed geoid and topography between degrees 2 and 40 we find that a good fit to the data can be obtained not only for an isoviscous mantle without a pronounced lithosphere as suggested in some previous studies but also for models with a high viscosity lithosphere and a gradual increase in viscosity with depth in the mantle the overall viscosity increase across the mantle found for the latter group of models is only partially resolved but profiles with a 100 km thick lithosphere and a viscosity increasing with depth by a factor of 10 80 hence similar to viscosity profiles expected in the earth s mantle are among the best fitting models

the tidal response of europa the tidal response of europa to a time varying potential is determined by solving the quasi static equilibrium equations for a body composed of several uniform maxwell viscoelastic layers models in which europa s surface ice extends all the way to the silicate mantle have weak tidal responses 1 m vertical deflection while models in which a liquid ocean underlies the ice have about 30 m peak vertical deflection the phase lag of the tidal response is very small 2 in the case of an ocean but may be large if there is no ocean and the ice has a viscosity around 1013 pa s for models with an ocean the product of the thickness and strength of the ice determines the tidal response making it difficult to determine the thickness of the ice by observing the tidal deflection of europa

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

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

geophysical controls of chemical disequilibria in europa the ocean in jupiter s moon europa may have redox balance similar to earth s on earth low temperature hydration of crustal olivine produces substantial hydrogen comparable to any potential flux from volcanic activity here we compare hydrogen and oxygen production rates of the earth system with fluxes to europa s ocean even without volcanic hydrothermal activity water rock alteration in europa causes hydrogen fluxes 10 times smaller than earth s europa s ocean may have become reducing for a brief epoch for example after a thermal orbital resonance 2 gyr after accretion estimated oxidant flux to europa s ocean is comparable to estimated hydrogen fluxes europa s ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active as evidence of geologic activity and subduction implies

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

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

hydrothermal plume dynamics on europa implications for chaos formation 1 hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from europa s rocky interior through a liquid ocean to the base of its ice shell this process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer in contrast to earlier work we argue that europa s ocean should be treated as an unstratified fluid we have adapted and expanded upon existing work describing buoyant plumes in a rotating unstratified environment we discuss the scaling laws governing the flow and geometry of plumes on europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a europa like parameter regime we predict that hydrothermal plumes on europa are of a lateral scale at least 25 50 km comparable to large chaos regions they are too broad to be responsible for the formation of individual lenticulae plume heat fluxes 0 1 10 w m 2 are too weak to allow complete melt through of the ice layer current speeds in the plume 3 8 mm s are much slower than indicated by previous studies the observed movement of ice blocks in the conamara chaos region is unlikely to be driven by such weak flow index terms 6218 planetology solar system objects jovian satellites 5418 planetology solid surface planets heat flow 5430 planetology solid surface planets interiors 8147 4540 oceanography physical ice mechanics and air sea ice exchange processes 4568 oceanography physical turbulence diffusion and mixing processes keywords chaos europa hydrothermal plumes

the lunar crust global structure and signature of major basins new lunar gravity and topography data from the clementine mission provide a global bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar moho and therefore crustal thickness the large range of global crustal thickness 20 120 km is indicative of major spatial variations in melting of the lunar exterior and or significant impact related redistribution the 61 km average crustal thickness constrained by a depth to moho measured during the apollo 12 and 14 missions is preferentially distributed toward the farside accounting for much of the offset in center of figure from the center of mass while the average farside thickness is 12 km greater than the nearside the distribution is nonuniform with dramatic thinning beneath the farside south pole aitken basin with the global crustal thickness map as a constraint regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km in order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data the downward continuation of the bouguer gravity is stabilized by a three dimensional minimum slope and curvature algorithm both mare and non mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims the inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins for large 300 km diameter basins moho relief appears uncorrelated with diameter but is negatively correlated with basin age in several cases it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement suggesting that the lithosphere was strong enough to maintain their state of stress to the present

pits and uplifts on europa abstract a survey of depression and uplift features on europa based on galileo regional mapping images shows that these features come in a wide range of sizes with numbers increasing greatly with decreasing size down to the limits of resolution size distributions are similar in the northern leading and southern trailing hemispheres where they are distinctly different from the southern leading and northern trailing hemispheres suggesting an oblique antipodal symmetry pattern similar to that of chaotic and tectonic terrain this pattern is suggestive of polar wander uplifts are usually polygonal or irregular in shape and rarely are cracked patches of chaotic terrain which we had surveyed earlier are not included in the current study because their topography is generally unclear and because there is no a priori known genetic linkage with the pits and uplifts these results contradict generalizations based on the earlier pits spots and domes psd taxonomy most of the type examples for psds were simply patches of chaotic terrain selected from a limited portion of their full size range the use of the term lenticula to collectively describe psds is inconsistent with the iau definition of lenticula a small dark spot seen at low resolution pits and uplifts do not correlate with lenticulae although chaos often does properties of psds that have been widely cited as primary evidence for convective upwelling in thick ice e g that uplifts are generally dome shaped and often cracked that pits and domes are regularly spaced that there is a typical diameter of 10 km were premature and not supported by subsequent data most pits and uplifts are less than 10 km across so if they formed by diapirism or convective upwelling the sources must have been very shallow less than 5 km deep how they actually formed remains unknown

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

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

equatorial and mid latitude records of the last geomagnetic reversal from the atlantic ocean abstract three records of the matuyama brunhes reversal have been obtained from odp deep sea cores distributed along the west african margin in the atlantic ocean these studies in addition to the record from dsdp site 609b provide a latitudinal transect extending from the equator to 50 n simulations of natural smoothing by post depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records especially at the equator the results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition

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

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

effects of lithosphere on the long wavelength gravity anomalies and their implications for the formation of the tharsis rise on mars 1 the tharsis rise represents the most significant long wavelength gravity and topography anomalies on mars two competing models have been proposed to explain the origin of these anomalies in the first model the tharsis rise is attributed to loading of volcanic construction on the lithosphere while in the second model these anomalies are explained as dynamic effects of a one plume thermal structure below the tharsis rise in this study we seek to distinguish between these two models by formulating a generalized viscoelastic loading model that determines responses to loads at different depths with this new formulation we found that the instantaneous viscous flow loading formulation used to compute responses from a plume may significantly overestimate the geoid anomalies if the elastic thickness of the tharsis lithosphere is 150 km as inferred from gravity and topography data we suggest that the one plume structure if currently existing below the tharsis rise may be responsible for only a small fraction 10 of the tharsis geoid anomalies and that other processes including surface loading of volcanic construction may be the primary cause of the geoid anomalies the plume structure is inefficient in producing geoid anomalies because the long wavelength components of the plume structure are confined to a 300 km layer below the lithosphere in which the geoid response is minimal future studies on gravity and topography at different wavelengths may put constraints on possible contribution of a plume to the elevated tharsis topography

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

temperatures on europa from galileo photopolarimeter radiometer nighttime thermal anomalies galileo observations of europa s thermal emission show low latitude diurnal brightness temperatures in the range of 86 to 132 kelvin nighttime temperatures form an unexpected pattern with high temperatures on the bright ejecta blanket of the crater pwyll and an equatorial minimum in temperatures after sunset uncorrelated with surface albedo or geology the nighttime anomalies may be due to regional thermal inertia variations of an unknown origin which are equivalent to a two to threefold variation in thermal conductivity or to endogenic heat fluxes locally reaching 1 watt per square meter endogenic heat flow at this high level although consistent with some geological evidence is theoretically unlikely

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