salt inclusions in polar ice core location and chemical form of water soluble impurities abstract ice cores retrieved from ice sheets provide the most detailed records of climate over the past several hundred thousand years for example water soluble impurities have been used as proxies for atmospheric chemistry however numerical models warned that the distributions of the trace constituents are significantly altered by rapid diffusion through liquid veins along triple junctions during ice sheet flow although these calculations assumed that the trace constituents are preferentially confined to grain boundaries and form a vein network this assumption has not been confirmed experimentally using micro raman spectroscopy here we show that most of the soluble impurities are confined as salt inclusions within grains these inclusions are mainly composed of the sulfate salts na 2 so 4 10h 2 o mgso 4 12h 2 o and caso 4 2h 2 o we also found that methane sulfonate salts and other complex salts exist only in the glacial period ice our findings imply that these salts form within 185 m of the ice surface and then preserve their composition as the ice is pushed to greater depths this means that depth profiles of the trace constituents should be preserved in deep ice cores as reliable proxies for past climate

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

coupling of thermal evolution and despinning of early iapetus abstract the cassini mission revealed two spectacular characteristics of iapetus 1 a geologically old and high equatorial ridge which is unique in the solar system and 2 a large flattening of 35 km consistent with the equilibrium figure for a hydrostatic body rotating with a period of 16 h whereas the current spin period is 79 33 days this study describes three dimensional simulations of solid state convection within an undifferentiated iapetus it investigates the implications for the evolution of the interior thermal structure and its spin rate and global shape using radially layered viscoelastic models the role of the concentration in the short lived radiogenic element 26 al just after accretion is completed is specifically addressed the first result is to show that whatever the 26 al value convection occurs as suggested by castillo rogez et al castillo rogez j matson d sotin c johnson t lunine j thomas p 2007 icarus 190 179 202 convection reduces the warming of the interior compared to the conductive evolution and therefore limits the conditions for despinning in our calculations two conceptual linear viscoelastic models are used when considering a maxwell rheology the interior temperature viscosity never reaches a value high low enough to induce despinning in order to promote dissipation at low temperature a burgers rheology which includes an additional dissipation peak is introduced for favorable parameter values this latter rheology leads to despinning however only models associated with large amounts of short lived radiogenic elements 26 al 25 ppb lead to the observed flattening this suggests that the accretion process needs to be completed shortly after the formation of cais calcium aluminum rich inclusions 4 myr for 26 al varying between 72 and 46 ppb the observed flattening is obtained only for a limited range of initial spin period between 9 5 and 10 2 h for 26 al ranging between 30 and 15 ppb initial spin rates smaller than 8 5 h are required for smaller values of 26 al the body is too cold and viscous to acquire a significant flattening even if a rotation period close to the body disruption limit is considered even with a thin lithosphere during the early stage our simulations show that iapetus never reaches the equilibrium figure for a hydrostatic body due to the non zero rigidity of the lithosphere the 35 km value of the flattening is the result of the partial relaxation of an ancient larger flattening ranging between 45 and 80 km depending on the evolution of the lithosphere thickness mainly controlled by the radiogenic content a thin lithosphere is consistent with an early building of the equatorial ridge the lithosphere thickening due to interior cooling can explain the preservation of the ridge throughout the remaining evolution of iapetus

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

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

convection in a volatile nitrogen ice rich layer drives pluto s geological vigour the volatile ice filled basin informally named sputnik planum is central to pluto s geological activity this ice layer is organized into cells or polygons and it is now shown that convective overturn in a several kilometre thick layer of solid nitrogen can explain both the presence of the cells and their great width nasa s new horizons spacecraft has revealed fascinating details of the surface of pluto including a vast ice filled basin known as sputnik planum which is central to pluto s geological activity much of the surface of sputnik planum consisting mostly of nitrogen ice is divided into irregular polygons that are tens of kilometres in diameter and whose centres rise tens of metres above their sides two papers in this issue of nature analyse new horizons images of this polygonal terrain both conclude that it is continually being resurfaced by convection but arrive at contrasting models for the process alexander trowbridge et al report a parameterized convection model in which the nitrogen ice is vigorously convecting ten or more kilometres thick and about a million years old william mckinnon et al from the new horizons team show that sluggish lid convective overturn in a several kilometre thick layer of solid nitrogen can explain both the presence of the cells and their great width the vast deep volatile ice filled basin informally named sputnik planum is central to pluto s vigorous geological activity1 2 composed of molecular nitrogen methane and carbon monoxide ices3 but dominated by nitrogen ice this layer is organized into cells or polygons typically about 10 to 40 kilometres across that resemble the surface manifestation of solid state convection1 2 here we report on the basis of available rheological measurements4 that solid layers of nitrogen ice with a thickness in excess of about one kilometre should undergo convection for estimated present day heat flow conditions on pluto more importantly we show numerically that convective overturn in a several kilometre thick layer of solid nitrogen can explain the great lateral width of the cells the temperature dependence of nitrogen ice viscosity implies that the ice layer convects in the so called sluggish lid regime5 a unique convective mode not previously definitively observed in the solar system average surface horizontal velocities of a few centimetres a year imply surface transport or renewal times of about 500 000 years well under the ten million year upper limit crater retention age for sputnik planum2 similar convective surface renewal may also occur on other dwarf planets in the kuiper belt which may help to explain the high albedos shown by some of these bodies

meteoric smoke fallout over the holocene epoch revealed by iridium and platinum in greenland ice an iridium anomaly at the cretaceous tertiary boundary layer has been attributed to an extraterrestrial body that struck the earth some 65 million years ago1 it has been suggested that during this event the carrier of iridium was probably a micrometre sized silicate enclosed aggregate2 or the nanophase material of the vaporized impactor3 but the fate of platinum group elements such as iridium that regularly enter the atmosphere via ablating meteoroids remains largely unknown here we report a record of iridium and platinum fluxes on a climatic cycle timescale back to 128 000 years ago from a greenland ice core4 we find that unexpectedly constant fallout of extraterrestrial matter to greenland occurred during the holocene whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust laden atmosphere of the last glacial age we suggest that nanometre sized meteoric smoke particles5 6 formed from the recondensation of ablated meteoroids in the atmosphere at altitudes 70 kilometres are transported into the winter polar vortices by the mesospheric meridional circulation7 and are preferentially deposited in the polar ice caps this implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the holocene

heat flow lenticulae spacing and possibility of convection in the ice shell of europa abstract two opposing models to explain the geological features observed on europa s surface have been proposed the thin shell model states that the ice shell is only a few kilometers thick transfers heat by conduction only and can become locally thinner until it exposes an underlying ocean on the satellite s surface according to the thick shell model the ice shell may be several tens of kilometers thick and have a lower convective layer above which there is a cold stagnant lid that dissipates heat by conduction whichever the case from magnetic data there is strong support for the presence of a layer of salty liquid water under the ice the present study was performed to examine whether the possibility of convection is theoretically consistent with surface heat flows of 100 200 mw m 2 deduced from a thin brittle lithosphere and with the typical spacing of 15 23 km proposed for the features usually known as lenticulae it was obtained that under europa s ice shell conditions convection could occur and also account for high heat flows due to tidal heating of the convective nearly isothermal interior but only if the dominant water ice rheology is superplastic flow with activation energy of 49 kj mol 1 this is the rheology thought dominant in the warm interior of the ice shell in this case the ice shell would be 15 50 km thick furthermore in this scenario explaining the origin of the lenticulae related to convective processes requires ice grain size close to 1 mm and ice thickness around 15 20 km

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

convection in ice i shells and mantles with self consistent grain size 1 the viscosity of ice i is grain size dependent for temperature and stress conditions appropriate for ice i shells and mantles of large and midsized icy satellites satellite thermal evolution heat flux critical shell thickness for convection brittle ductile transition temperature and potential for surface deformation are therefore grain size dependent using measured grain sizes from terrestrial ice sheets experiencing temperature and strain rate conditions similar to convecting ice shells in icy satellites we develop two end member models of grain size and its evolution in the absence of non water ice impurities grain size in a convecting ice shell of a large icy satellite will evolve over time to an equilibrium value which we determine to be between 30 and 80 mm because of dynamic recrystallization such large grains imply viscous ice so convection in an ice shell thinner than 35 km in a large icy satellite such as ganymede and callisto would be sluggish at best and may cease after a few convective overturns if it starts soluble ions and silicate microparticles may keep grains small however of order 1 5 mm and permit convection in shells as thin as 30 km in large icy satellites the dynamic recrystallization model provides a plausible upper limit on grain size in a geodynamically evolved ice shell whereas grain sizes from dust contaminated terrestrial ice cores provide a plausible lower limit on grain size in an icy satellite on the basis of these constraints grain sizes in evolved ice shells of large water ice satellites in the absence of tidal forcing should be of order 1 mm to 10 cm

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

reply to comment by p duval and m montagnat on superplastic deformation of ice experimental observations 1 in a recent paper we reported the results of a detailed experimental investigation of the rheological properties of ice goldsby and kohlstedt 2001 one of the innovative aspects of our investigation was the use of fine grained samples that enabled us to explore deformation not only by grain size insensitive mechanisms involving grain matrix dislocation processes but also by grain size sensitive flow mechanisms involving grain boundary sliding gbs an exciting discovery in our research was an extensive creep regime in which gbs contributes substantially to the flow of ice most importantly this deformation regime in which basal slip is accommodated by gbs referred to in our paper as the superplastic flow regime governs the flow of ice over a wide range of temperature grain size and stress conditions found in glaciers ice sheets and icy planetary interiors 2 in their comment on our paper duval and montagnat 2002 repeat the objection that they had raised previously montagnat and duval 2000 concerning our conclusion that superplastic deformation controls the flow of glaciers and ice sheets they assert that gbs as a significant creep mechanism is not compatible with observations on the development of fabrics and microstructures in ice sheets special emphasis is placed instead on the role of grain boundary migration as a recovery process in glacier ice they then continue by arguing that basal slip accommodated by grain boundary migration gbm rather than by gbs is the dominant deformation mechanism in ice 3 in this reply we extend the results presented in detail by goldsby and kohlstedt 1997 and goldsby and kohlstedt 2001 to further demonstrate that gbs is a very important deformation process in natural ice bodies our reasoning is based on three points 1 the microstructures reported for samples from large natural ice bodies are remarkably similar to those observed in samples deformed in the laboratory in the superplastic flow regime 2 our constitutive equation for ice is in excellent agreement with the flow behavior determined from field measurements on glaciers and ice sheets 3 contrary to the assertion of montagnat and duval 2000 and duval and montagnat 2001 fabric development is fully compatible with deformation in the superplastic regime described in our paper in addition we note that while gbm associated with dynamic recrystallization is an important recovery mechanism in ice it is not a deformation mechanism and hence cannot accommodate basal slip

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

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

origin and development of textures and fabrics in basal ice at summit central greenland abstract detailed textural and crystallographic characteristics of the bottom 6 m of the grip core summit central greenland are presented the results are discussed in the light of present day fabric analogues ice fabrics are shown to retain the mark from the entrainment of locally formed ice by the growing ice sheet this mark is only partly overprinted by the present day stress configuration at the ice divide these results corroborate the model put forward by souchez et al 1 on stable isotopes and gas composition grounds in which the basal silty ice appears as a remnant of a growing stage of the ice sheet during this stage which probably represents the original build up ice of local origin formed at the ground surface was overridden by the main ice sheet and incorporated into its base preservation of remnants of ancient fabric patterns and occurrence of annealing recrystallization fabrics just above the first layer of silty ice are used amongst other arguments to demonstrate that this basal ice is not presently subject to large cumulative strains the implications for flow and dating models are discussed

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

on convection in ice i shells of outer solar system bodies with detailed application to callisto it has been argued that the dominant non newtonian creep mechanisms of water ice make the ice shell above callisto s ocean and by inference all radiogenically heated ice i shells in the outer solar system stable against solid state convective overturn conductive heat transport and internal melting oceans are therefore predicted to be or have been widespread among midsize and larger icy satellites and kuiper belt objects alternatively at low stresses where non newtonian viscosities can be arbitrarily large convective instabilities may arise in the diffusional creep regime for arbitrarily small temperature perturbations for callisto ice viscosities are low enough that convection is expected over most of geologic time above the internal liquid layer for plausible ice grain sizes a few mm the alternative for early callisto a conducting shell over a very deep ocean 450 km is not compatible with callisto s present partially differentiated state moreover if convection is occurring today the stagnant lid would be quite thick 100 km and compatible with the lack of active geology nevertheless callisto s steady state heat flow may have fallen below the convective minimum for its ice i shell late in solar system history in this case convection ends the ice shell melts back at its base and the internal ocean widens considerably the presence of such an ocean of order 200 km thick is compatible with callisto s moment of inertia but its formation would have caused an 0 25 radial expansion the tectonic effects of such a late slow expansion are not observed so convection likely persists in callisto possibly subcritically ganymede due to its greater size rock fraction and full differentiation has a substantially higher heat flow than callisto and has not reached this tectonic end state titan if differentiated and triton should be more similar to ganymede in this regard pluto like callisto may be near the tipping point for convective shutdown but uncertainties in its size and rock fraction prevent a more definitive assessment

downslope sediment transport by boiling liquid water under mars like conditions experiments and potential implications for martian gullies gullies are widespread morphological features on mars for which current changes have been observed liquid water has been one of the potential mechanisms to explain their formation and activity however under present day martian conditions liquid water is unstable and should only be transiently present in small amounts at the surface yet little attention has been paid to the mechanisms by which unstable water transports sediment under low atmospheric pressure here we present the results of laboratory experiments studying the interaction between liquid water owing over a sand bed under mars like atmospheric pressure c 9 mbar the experiments were performed in a mars simulation chamber at the open university uk in which we placed a test bed of ne sand at a 25 slope we chose to investigate the in uence of two parameters the temperature of the water and the temperature of the sand we performed 27 experiments with nine different combinations of water and sand temperatures ranging from 278 to 297 k under all experimental conditions the water was boiling we investigated and compared the types and timing of sediment transport events and the shapes characteristics and volumes of the resulting morphologies in agreement with previous laboratory studies we found that more intense boiling increased the volume of sediment transported for a given volume of water we found four main types of sediment transport entrainment by overland ow grain ejection grain avalanches and levitation of saturated sand pellets our results showed that increasing sand temperature was the main driving parameter in increasing the sand transport and in modifying the dominant sediment transport mechanism the temperature of the water played a negligible or minor role apart from the duration of sand ejection and avalanches which lasted longer at low water temperature at low sand temperature the majority of the sand was transported by overland ow of the liquid water at higher sand temperatures the transport was dominated by processes triggered by the boiling behaviour of the water at the highest temperatures sediment transport was dominated by the formation of levitating pellets dry avalanches and ejection of the sand grains this resulted in a transport volume about nine times greater at a sand temperature of 297 k compared with 278 k our heat transfer scaling shows that the boiling behaviour will be enhanced under martian low gravity resulting in more ef cient transport of sediment by levitating sand pellets even at temperatures close to the triple point our results showed that the boiling intensity played an important role in the physics of sediment transport by liquid water this implied that the amount of water required to produce morphological changes at the surface of mars could be lower than previously estimated by assuming stable liquid water boiling is a critical process to be considered when assessing gully formation and modi cation mechanisms mobilized by liquid water our work could have similar implications for any water formed landform on mars which could include recurring slope lineae dark dune ows and slope streaks

formation of ganymede s grooved terrain by convection driven resurfacing the heat flux and strain rate inferred for grooved terrain formation on ganymede can be produced in a convecting ice shell 10 100 km thick with weak near surface ice smooth linear grooves may have formed by convection driven lithospheric spreading and long wavelength compressional folds may form atop convective downwellings and would possibly be detectable with mapping from esa s upcoming jupiter icy moon explorer mission

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

observations of polar ice from the holocene and the glacial period using the scanning electron microscope samples taken from the dome c ice core antarctica and the grip ice core greenland are examined using the scanning electron microscope to determine their microstructure in both cores samples are taken from two differing climatic periods the holocene and the last glacial period many of the usual features observed in similar samples under the light microscope are observed including bubbles grain boundaries and clathrate hydrates features not resolvable using the light microscope are also found dust particles are found in situ eighty five per cent of those observed contained silicon which was generally associated with aluminium and magnesium an estimation is made of the relative proportions of dust particles located at grain boundaries and in the bulk of the ice grain at dome c a higher proportion than expected from a random distribution of particles was found located at grain boundaries although in greenland this was not found to be the case for most samples direct evidence is also presented indicating the role of dust particles and microscopical inclusions in impeding or pinning grain boundary migration soluble impurities are also detected at some triple junctions and grain boundaries

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

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

grain growth in polar ice ii application grain growth observed in polar ice that is not deforming rapidly can be accounted for if concentrations and distributions of extrinsic materials microparticles bubbles and dissolved impurities are characterized fully dissolved impurities segregate to grain boundaries and slow grain growth in all cold glacial ice the high concentration of soluble impurities in wisconsinan ice from the dome c antarctica ice core and perhaps other ice cores probably causes the small grain sizes observed in that ice microparticles have little effect on grain growth in ordinary ice in ice layers that appear dirty owing to concentrations of volcanic tephra such as in the byrd station antarctica ice core or of morainal material micro particles reduce grain growth rates significantly the relatively high vapor pressure of ice allows rapid growth and high mobility of intergranular necks so grain growth in firn is limited by boundary migration rather than by neck growth bubbles formed by pore close off at the firn ice transition are less mobile than grain boundaries causing bubble boundary separation whenever geometric constraints are satisfied however such separation reduces grain growth rates by only about 10 the observed linear increase of grain area with time is thus predicted by theory but the growth rate depends on soluble impurity concentrations as well as on temperature

geological mapping of sputnik planitia on pluto abstract the geology and stratigraphy of the feature on pluto informally named sputnik planitia is documented through geologic mapping at 1 2 000 000 scale all units that have been mapped are presently being affected to some degree by the action of flowing n2 ice the n2 ice plains of sputnik planitia display no impact craters and are undergoing constant resurfacing via convection glacial flow and sublimation condensation of atmospheric n2 onto the surface to form a bright mantle has occurred across broad swathes of sputnik planitia and appears to be partly controlled by pluto s obliquity cycles the action of n2 ice has been instrumental in affecting uplands terrain surrounding sputnik planitia and has played a key role in the disruption of sputnik planitia s western margin to form chains of blocky mountain ranges as well in the extensive erosion by glacial flow of the uplands to the east of sputnik planitia

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

the orbital thermal evolution and global expansion of ganymede article history the tectonically and cryovolcanically resurfaced terrains of ganymede attest to the satellite s turbulent geologic history yet the ultimate cause of its geologic violence remains unknown one plausible scenario suggests that the galilean satellites passed through one or more laplace like resonances before evolving into the current laplace resonance passage through such a resonance can excite ganymede s eccentricity leading to tidal dissipation within the ice shell to evaluate the effects of resonance passage on ganymede s thermal history we model the coupled orbital thermal evolution of ganymede both with and without passage through a laplace like resonance in the absence of tidal dissipation radiogenic heating alone is capable of creating large internal oceans within ganymede if the ice grain size is 1 mm or greater for larger grain sizes oceans will exist into the present epoch the inclusion of tidal dissipation significantly alters ganymede s thermal history and for some parameters e g ice grain size tidal q of jupiter a thin ice shell 5 to 20 km can be maintained throughout the period of resonance passage the pulse of tidal heating that accompanies laplace like resonance capture can cause up to 2 5 volumetric expansion of the satellite and contemporaneous formation of near surface partial melt the presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in ganymede s ice shell larger stresses result if the ice shell rotates non synchronously the combined effects of satellite expansion its associated tensile stress rapid formation of near surface partial melt and tidal stress due to an eccentric orbit may be responsible for creating ganymede s unique surface features

effects of anisotropic viscosity and texture development on convection in ice mantles 1 convection may occur in the ice shells of satellites in the outer solar system the style of convection rate of heat transport and resulting surface features depend on the rheology of ice which in turn depends on temperature grain size stress and crystallographic preferred orientation cpo here we study the effect of cpo development and anisotropic viscosity on convection by coupling a model of polycrystalline ice deformation with a macroscopic flow model despite having a first order effect on velocities and heat transport in a convecting ice shell fabric development is unlikely to be observable either directly by spacecraft based radar or indirectly based on changes in the wavelength or amplitude of dynamic topography

does titan s long wavelength topography contain information about subsurface ocean dynamics abstract the long wavelength topography of titan is characterized by relatively small amplitudes about 1 km peak to peak an anomalous equatorial bulge the poles are about 300 m lower than the equator and small gravity anomalies indicating a high degree of compensation in the past years the nature of titan s non hydrostatic topography has been addressed in several studies the topography has been interpreted in terms of isostatic or viscous models and discussed in connection with tidal heating in the ice shell and surface erosion here we present a model of the shape evolution of titan s ice shell driven by tidal heating in the shell and spatial variations of the heat flux from a subsurface ocean the model is obtained by solving a general set of equations coupling the viscoelastic flow of ice with the thermal evolution of the ice shell and phase transitions at the ice water interface the equations are solved in a domain with radially varying material properties and moving boundaries the motion of the boundaries is a consequence of ice flow within the shell melting and crystallization at the bottom boundary and erosion and deposition at the surface our model suggests that titan s anomalous topographic bulge can be explained by lateral variations of ocean heat flux of the order of 0 1 1 mw m 2 provided that the heat flux is stable over a period of at least 10 myr and the ice shell has a sufficiently high viscosity exceeding 1016 pa s at the base of the shell such a high value of viscosity implies that either the ice grains are coarse 10 mm or the temperature of the ocean is significantly by more than 40 k lower than the melting temperature of pure water ice the heat flux pattern predicted on top of the ocean is consistent with a flow characterized by upwelling of warm water in polar regions and downwelling of cold water at low latitudes the negative correlation between the topography and geoid at degree 3 reported in a previous study but not confirmed at higher degrees yet is shown to be compatible with erosion and deposition occurring at a rate of 0 01 0 1 mm yr 1 our results underline the importance of gravity and topography measurements for understanding titan s surface and deep interior processes

material transport across europa s ice shell jupiter s moon europa exhibits a deformed icy surface with salt deposits concentrated along the varied geological features the topographic alignment of salt deposits has been speculated to indicate an endogenic sourcing of the material two way transport of salts from a liquid water ocean beneath the ice shell to the surface and vice versa has been speculated we present dynamical models that demonstrate the incorporation of newly frozen ice into convective plumes within the ice shell caused by convection within the ice shell that drives dynamic topography along the ice ocean boundary the new ice that forms at the freezing front can be transported by the rising ice plumes toward the surface until it is blocked by a high viscosity lid at the surface weakening of the lid by tidal or tectonic forces could then lead to the surface detection of ocean trace chemistry captured in the newly formed ice