interpentadal variability of temperature and salinity at intermediate depths of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited historical hydrographic observations for the world ocean for two pentads 1955 1959 and 1970 1974 and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the north atlantic ocean the subtropical gyre 500 1300 m of the north atlantic was colder and fresher during 1970 1974 compared to 1955 1959 the relatively large changes in temperature and salinity on constant depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces at intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970 1974 pentad in contrast the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970 1974 compared to 1955 1959 characteristic differences in temperature and salinity for both gyres were 0 5 c and 0 025 respectively analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26 5 surface suggesting that convection in this region may have been shallower in 1970 1974 compared to 1955 1959 below the 26 5 surface potential vorticity decreased through the 27 3 surface indicating that deeper than normal convection may have formed the water masses in these layers at 250 m depth relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the north atlantic ocean

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

effect of state of compensation on the relaxation of crustal plateaus on venus 1 building on nunes et al 2004 which examined the end member case of viscous relaxation of fully compensated crustal plateaus on venus we now investigate viscous relaxation for the other end member case of uncompensated topography the approach consists of modeling viscous flow in a crust mantle system by means of analytic and finite element methods differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle loss of topography occurs via fast subsidence due to isostatic adjustment and a slower lateral flow due to spreading of the thickened crust rims arise from the isostatic adjustment and rim dimensions depend on the thickness of the competent layer controlled by the thermal state of the system rim dimensions after 1 myr are 1 km high and 300 km wide for a surface temperature of 740 k and thermal gradient of 10 k km and 600 m high and 100 km wide for hotter crustal conditions although models predict the marginal circumferential hoop folding observed at crustal plateaus they do not account for the large radial grabens also observed adding a subcrustal zone of time dependent buoyancy causes a delay in the isostatic adjustment during this period of delay viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on venus consequently phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support lateral variation in heat flow and thickness of the competent layer may help address the latter and needs to be explored through both lithospheric and mantle convection models

delayed formation of the equatorial ridge on iapetus from a subsatellite created in a giant impact 1 the great equatorial ridge on saturn s moon iapetus is arguably the most perplexing landform in the solar system the ridge is a mountain range up to 20 km tall and sitting on the equator of iapetus and explaining its creation is an unresolved challenge models of its formation must satisfy three critical observations why the ridge 1 sits exactly on the equator 2 is found only on the equator and 3 is thus far found only on iapetus we argue that all previously proposed models fail to satisfy these observations and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around iapetus the orbit of this subsatellite would then decay once iapetus itself had despun due to tides raised by saturn until tidal forces from iapetus tore the subsatellite apart the resultant debris formed a transient ring around iapetus the material of which rained down on the surface to build the ridge by sequestering the material in a subsatellite with a tidally evolving orbit formation of the ridge is delayed which increases the likelihood of preservation against the high impact flux early in the solar system s history and allows the ridge to form on thick stiff lithosphere heat flow likely 1 mw m 2 required to support this massive load without apparent flexure this mechanism thus explains the three critical observations

crater relaxation as a probe of europa s interior viscous relaxation rates of craters are examined to discriminate among models of europa s interior it is shown that the presence of an insulating surface regolith is required for ice lithospheres that do not contain a liquid h2o layer without the presence of such an insulating regolith relaxation times for craters 100 km in diameter are excessively long 1 b y and thus inconsistent with the absence of such features in voyager images finite element calculations indicate that a thick insulating surface frost layer raising surface temperatures 40 k above the solar radiation ambient temperature of 92 k allows a 25 km thick surface viscous layer to relax 100 km diameter craters on the short timescales 100 m y required if the icy lithosphere is 100 km thick and without a liquid h2o layer but containing a subsolidus convecting region surface temperatures need to be raised 20 k by insulation to allow relaxation of craters in this size range to occur within the required time

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

thermally induced lineations on the asteroid eros evidence of orbit transfer 1 several systems of tectonic lineations have been observed on the surface of the asteroid 433 eros we suggest that many of these lineations may have resulted from thermal stresses associated with a secular change in surface temperature due to orbit migration we demonstrate how thermal diffusion can lead to tectonic features and how such features should be oriented with respect to eros complex geometry comparison to observed lineations suggests that some may be explained by thermal stresses such stresses represent a largely unexplored tectonic process on asteroids and could be used to gain rare insight into chaotic orbital dynamics if correct our results imply a gross orbital history over the last 100 myr involving slow several myr inward migration followed by more rapid outward migration to its present orbit

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

viscous flow rates of icy topography on the north polar layered deposits of mars we investigate the importance of viscous flow in shaping topography at the north polar layered deposits npld of mars by using finite element modeling to calculate the distribution of stresses and flow velocities present day impact craters on thenpldare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions this effect may be ignored when analyzing crater size frequency distributions scarps at the npldmargins where avalanches of dust and carbon dioxide frost occur are sufficiently steep high andwarm to experience significant viscous flow we find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm yr which are fast enough to significantly affect their slopeover kiloyear timescales alternatively the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow

impact cratering on titan ii global melt escaping ejecta and aqueous alteration of surface organics abstract new three dimensional hydrodynamic simulations of hypervelocity impacts into the crust of titan were undertaken to determine the fraction of liquid water generated on the surface of saturn s largest moon over its history and hence the potential for surface modification of hydrocarbons and nitriles by exposure to liquid water we model in detail an individual impact event in terms of ejecta produced and melt generated and use this to estimate melt production over titan s history taking into account the total flux of the impactors and its decay over time our estimates show that a global melt layer at any time after the very beginning of titan s history is improbable but transient melting local to newly formed craters has occurred over large parts of the surface local maxima of the melt are connected with the largest impact events we also calculate the amount of volatiles delivered at the impact with various impact velocities from 3 km s for possible hyperion fragments to 11 km s for jupiter family comets and their retention as a possible source of titan s atmosphere we find the probability of impact ejecta escaping titan with its modern dense and thick atmosphere is rather low and dispersal of titan organics throughout the rest of the solar system requires impactors tens of kilometers in diameter water ice melting and exposure of organics to liquid water has been widespread because of impacts but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation the largest impactors may breach an ammonia water mantle layer creating a circular albedo contrast rather than a crater

steric sea level variations during 1957 1994 importance of salinity 1 spatially averaged 50 s 65 n temperature and salinity changes in the 0 3000 m layer during the 1957 1994 period resulted in a sea level rise at a mean rate of about 0 55 mm per year about 10 of this rate is due to a decrease of the volume mean salinity the magnitude of total steric sea level tssl changes and the ratio of thermosteric and halosteric anomalies to tssl anomaly are nonuniform geographically salinity effects are critically important to the tssl changes in some regions of the ocean for example the thermosteric anomaly is nearly compensated by the halosteric anomaly in the subpolar north atlantic this fact will cause erroneous heat content estimates based on altimetric observations from space if a climatological salinity is assumed based on the present historical archive of salinity data a decrease in global mean salinity has occurred this increase of fresh water would cause sea level rise at a rate of 1 3 0 5 mm yr if the added water comes from sources other than floating sea ice

north atlantic sea level and circulation summary monthly sea levels from an extensive array of north atlantic tide gauges 26 50 n are examined the spatial scale of the sea level variations and the reasons for them are discussed one application of such a study is clearly in the design of a tide gauge network for monitoring eustatic changes of sea level the spatial scale of the sea level changes is large there is a coherent sealevel signal which can be traced along the eastern boundary of the north atlantic from newlyn 50 n to tenerife 28 n there are also two distinct groupings of tide gauges along the western boundary separated by cape hatteras the contribution of local air pressure and wind stress is quantified at each gauge through multiple regression techniques and the gains are then interpreted in terms of recent theoretical and numerical modelling studies for example the gains suggest that the wind forced boundary current along the nova scotian shelf is trapped to within about 16 km of the coast the influence of local meteorology cannot account for the large scale modes of variability the coherent signal along the eastern boundary is correlated with changes in the sverdrup transport of the north atlantic and hence the large scale wind field the two modes on the western boundary appear to be related to baroclinic boundary current variations the newlyn sea level record is finally corrected for some of the above effects to illustrate the utility of such a residual series in the identification of eustatic changes and vertical crustal movement

impact crater relaxation on dione and tethys and relation to past heat flow abstract relating relaxation of impact crater topography to past heat flow through the crusts of icy satellites is a technique that has been applied to satellites around jupiter and saturn we use global digital elevation models of the surfaces of dione and tethys generated from cassini data to obtain crater depth diameter d d data relaxation is found to affect craters down to smaller diameters on these satellites compared to rhea we perform relaxation simulations in order to assess the heat flow necessary to relax craters on dione and tethys to their present morphologies heat flows exceeding 60 mw m 2 are required to relax several craters on both satellites and relaxation appears to be subject to geographical controls on dione we define a relaxation dichotomy that separates the more relaxed craters in sparsely cratered plains from the less relaxed craters in heavily cratered terrain the configuration of this dichotomy resembles that of the structural geological dichotomy on enceladus implying that a similar resonance induced tidal heating mechanism concentrated in the southern hemisphere may have affected both satellites defining geographical distribution of relaxation on tethys is hindered by the presence of the young odysseus impact and its associated ejecta

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

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

time dependent flexure of the lithospheres on the icy satellites of jupiter and saturn abstract previous analyses into flexural deformation on the icy satellites of jupiter and saturn have assumed static elastic lithospheres viscous creep within the lithosphere however can cause evolution over time here we apply a finite element model that employs a time dependent elastic viscous plastic rheology in order to investigate flexure on icy satellites factors that affect this time dependent response are those that control creep rates surface temperature heat flow and grain size our results show that surface temperature is by far the dominant factor at higher surface temperatures 100 130 k the evolution of the deformation is such that the thickness of a modeled elastic lithosphere could vary by up to an order of magnitude depending on the time scale over which the deformation occurred because the flexure observed on icy satellites generally indicates transient high heat flow events our results indicate that the duration of the heat pulse is an important factor for the icy worlds of jupiter and saturn static models of lithospheric flexure should be used with caution

the status of ravine like incisions in the dry valleys of the pays de thelle paris basin france in the pays de thelle france ravines often cut the bottom of local chalk dry valleys morphometric and sedimentological analyses are used in an attempt to reconstruct the palaeoenvironments during the morphodynamic evolution of these small dry valleys as in other paris basin districts the dry valleys were formed during periglacial quaternary periods however some of the ravines look like fluvial incisions that have began since the subboreal and result more from recent uplift of the bray anticline than from human activities alone present agricultural practices which are not in accordance with respect for the environment contribute to the surface runoff that produces muddy floods along the dry valleys but they are not thought to be the cause of the ravines

finite element modeling of short wavelength folding on venus implications for the plume hypothesis for crustal plateau formation 1 the key geological observations and structural interpretations associated with the current plume hypothesis for venusian crustal plateau formation are that 1 short wavelength spatially periodic tectonic structures originated as mechanical instabilities in a regionally extensive surface brittle layer whose basal boundary was thermally generated and controlled 2 characteristic wavelengths of spatially periodic structures record the brittle layer thickness at the time of deformation and 3 structural wavelengths increased with time driven by brittle layer thickening in response to cooling the plume hypothesis accommodates these constraints by proposing that crustal plateaus formed above mantle plumes impinging on thin lithosphere it further suggests that surface temperatures temporarily elevated to 1000 k may have been required to maintain a sufficiently thin brittle layer for formation of the shortest wavelength structures we report here on finite element simulations designed to test the feasibility of the proposed thermal conditions specifically we model formation of short wavelength folds thought to have initiated as contractional layer instabilities early in the plateau formation process under high surface temperatures the finite element meshes develop semibrittle zones in which short wavelength folds can form but development of even modest structural relief requires unrealistically high total mesh shortening thus elevated surface temperatures inhibit development of short wavelength folds because the models effective integrated mechanical strength under such hot conditions is excessively low decreasing the surface temperature increases structural relief but produces tectonic wavelengths that are larger than those observed we conclude that a model with solely thermal control of mechanical properties cannot explain the observed structures

formation of the bulge of iapetus through long wavelength folding of the lithosphere abstract previous models that attempted to explain the formation of the pronounced oblate shape of iapetus suggested that it was a preserved rotational bulge these models found that heating was provided by short lived radioactive isotopes that decayed rapidly and allowed the excess flattening of the lithosphere to be locked in by a thickening lithosphere but placed severe timing constraints on the formation of iapetus and its bulge here we show that finite element simulations with an elastic viscous plastic rheology indicate it is possible to form the bulge through long wavelength folding of the lithosphere of iapetus during an epoch of contraction combined with a latitudinal surface temperature gradient in contrast to models of a frozen rotational bulge heat generated by long lived radioactive isotopes warms the interior which causes porosity loss and forces iapetus to compact by 10 our simulations are most successful when there is a 30 k temperature difference between the pole and the equator tectonic growth of the bulge is not sensitive to the time scale over which the moon contracts and lithospheric thickness primarily controls whether a fold can form not fold wavelength in addition long term simulations show that when no stress is applied the mechanical lithosphere is strong enough to support the bulge with negligible relaxation over billion year time scales

interpentadal variability of steric sea level and geopotential thickness of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited and objectively analyzed historical hydrographic observations for the north atlantic ocean for two periods 1955 1959 and 1970 1974 difference fields of steric sea level and geopotential thickness for the north atlantic ocean in the 0 to 1500 m depth range between the 1955 1959 and 1970 1974 pentads were computed we find that in the central portion of the subtropical gyre steric sea level decreased by up to 17 5 dyn cm whereas in the western subarctic gyre north of the gulf stream steric sea level increased by an amount up to 7 5 dyn cm from the earlier to the later pentad a decrease in steric sea level 5 dyn cm occurred along the eastern boundary of the north atlantic

crater relaxation on ganymede implications for ice rheology controversy has existed over whether or not viscous relaxation is an important process on the icy satellites previous models involved large extrapolations of newtonian flow laws for ice whereas ice is known to exhibit non newtonian behavior recently the flow law parameters for ice at the appropriate temperatures and stresses have been measured numerical modelling of the viscous relaxation of basins on ganymede using these parameters has given implausibly short relaxation times however this model treated ice as a purely viscous substance so that no elastic lithosphere could develop near the surface here we treat ice as a maxwell visco elastic material and numerically model the relaxation of basins on ganymede we find that realistic young s moduli lead to little relaxation occurring in basins even 4 0 ga after their formation further we demonstrate that within broad limits the near surface temperature gradient has little effect on this result finally through examination of the distribution of maxwell times in the vicinity of the crater we show that most viscous relaxation occurs early in the basin s history when stresses are high and thus maxwell time is short as stresses are relieved the maxwell time becomes long and relaxation essentially ceases

impact basin relaxation on rhea and iapetus and relation to past heat flow abstract evidence for relaxation of impact crater topography has been observed on many icy satellites including those of saturn and the magnitude of relaxation can be related to past heat flow e g moore j m schenk p m bruesch l s asphaug e mckinnon w b 2004 icarus 171 421 443 dombard a j mckinnon w b 2006 j geophys res 111 e01001 http dx doi org 10 1029 2005je002445 we use new global digital elevation models of the surfaces of rhea and iapetus generated from cassini data to obtain crater depth diameter data for both satellites and topographic profiles of large basins on each in addition to the factor of three lower amplitude of global topography on rhea compared to iapetus we show that basins on iapetus 100 km in diameter show little relaxation compared to similar sized basins on rhea because of the similar gravities of rhea and iapetus we show that iapetus basin morphologies can be used to represent the initial unrelaxed morphologies of the rhea basins and we use topographic profiles taken across selected basins to model heat flow on both satellites we find that iapetus has only experienced radiogenic heat flow since formation whereas rhea must have experienced heat flow reaching a few tens of mw m 2 although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes rhea experienced a different thermal history from iapetus which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system a recent model for the formation of saturn s mid sized icy satellites interior to and including rhea charnoz s et al 2011 icarus 216 535 550 describes how rhea s orbit would have expanded outwards after its accretion from a giant primordial ring which would have instigated early heating through rapid despinning and tidal interaction with saturn and other satellites rhea s basins would therefore be required to have formed within the first few tens of myr of rhea s formation in order to relax due of this heating and if so may provide an important anchor point for saturn system chronology none of these heating mechanisms are viable for iapetus in its isolated position far from saturn and as such it has remained dynamically inert since formation confirming conclusions based on thermal modeling of iapetus interior rapid and complete relaxation and subsequent erosion by bombardment of a first generation of large basins on rhea is regarded as an explanation for the lower counts of large basins on rhea relative to iapetus and the overall lower amplitude of topography on rhea compared to iapetus

testing the viability of topographic relaxation as a mechanism for the formation of lunar floor fractured craters historically two modes of origin for lunar floor fractured craters have been proposed subcrater laccolith emplacement and topographic relaxation to test the viability of the topographic relaxation model we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase an analog to lunar crustal materials trials were run under two situations 1 under extreme conditions e g high heat flow to determine the maximum degree of relaxation that the model can accommodate and 2 under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions we find that in the extreme case topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor fractured craters 100 km in diameter results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor fractured crater population these results indicate that topographic relaxation is not the primary mechanism for the formation of floor fractured craters because the topographic relaxation model does not apply for the vast majority of floor fractured craters i e those with diameters less than 60 km we conclude that the laccolith intrusion model is a more viable alternative

power law rheology of ice and the relaxation style and retention of craters on ganymede numerical models of viscous relaxation of craters in ice using rheological laws determined from laboratory data previously indicated excessively short relaxation times te the time required for crater depth to become 1 e of its initial value for craters on the icy satellites a typical value of te was 104 years for a 100 km diameter crater these time scales are totally incompatible with the large regions of heavily cratered terrain on the icy satellites we discuss the application of a numerical unite element model incorporating the most recent rheological data for ice at temperatures and pressures appropriate to the near surface regions of ganymede the model permits a full representation of a temperature and stress dependent power law viscosity for ice for temperature gradients in reasonable agreement with those obtained from thermal and structural models of ganymede we obtain relaxation times te 10 years for craters with diameters less than 100 km for all craters with diameters greater than 10 km the strong dependence of viscosity on stress was found to significantly shorten the relaxation time an attempt was also made to determine whether newtonian or non newtonian rheological laws dominated relaxation for craters of various sizes newtonian rheologies appear to dominate crater relaxation only for situations where the effective temperature is several tens of degrees warmer than the surface is observed to be perhaps because of the effect of an insulating regolith and where temperature gradients are small in this case viscous relaxation of craters with diameters smaller than 25 km is expected to be dominated by newtonian flow laws provided that the currently accepted parameters gross extrapolations from near melting temperature measurements are valid for the low temperatures of the icy satellites on the other hand larger diameter craters undergo at least initially viscous relaxation determined by a strongly non newtonian rheological law and have geologically short relaxation times te 107 years for a 300 km crater although these times may be extended by silicate suspensions and by dispersion hardening this presumably accounts for the absence of such craters in unmodified form on ganymede finally the large stress exponent n 4 7 of the current rheological data for ice indicates a strong role for non newtonian flow in determining the topography of viscously relaxing craterforms of this size in principle a threshold crater diameter and hence stress for the onset of non newtonian viscous behavior could be obtained from galileo images allowing estimates to be made of near surface temperatures and thermal gradients in ganymede s past in practice however any morphological variability is almost certainly too subtle to be detected from imagery the calculated relaxation times from this work are still much shorter than those indicated by the topographic relief of ganymede s surface this remains an unresolved problem possible ways in which the observations can be resolved with current laboratory data on ice deformation are discussed

the evolution of impact basins viscous relaxation of topographic relief we evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon we adopt a representative topographic profile of the orientale basin the youngest large impact basin on the moon as an estimate of the initial topography of older basins of similar horizontal dimensions and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography at wavelengths greater than the thickness of a high viscosity lithosphere both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins the effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths while the rate of relaxation decreases substantially for the fraction of long wavelength topography which is isostatically compensated as an initial condition these models are applied to two pre nectarian basins on the moon the tranquillitatis basin on the lunar nearside and the much larger and older south pole aitken basin on the farside the topographic profile of tranquillitatis after correcting for the effect of mare basalt fill is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit the large topographic relief of the larger farside basin on the contrary is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as tranquillitatis probably occurred a difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference such a nearside farside asymmetry in near surface temperature may date from the time of crust mantle differentiation and may have persisted because of the different histories of late stage impact basin formation and mare volcanism on the two hemispheres

relaxation of compensated topography and the evolution of crustal plateaus on venus 1 crustal plateaus dominant physiographic features on venus likely originate through dynamic mantle processes although a debate exists on whether they formed by mantle upwellings or downwellings regardless of the mode of formation several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high standing plateau to a low standing plateau with elevated margins we apply analytic and finite element models to test this hypothesis for isostatically compensated topography as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots geotherm values 5 k km 1 combined with a surface temperature of 740 k preclude relaxation within 109 years while geotherm values 20 k km 1 can yield relaxation times of 108 years or less hence significant relaxation requires hot conditions in order to occur within the appropriate 1 gyr timescale set by crustal plateau ages we also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance mantle temperatures 1400 k allow strong crust mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength thus preserving the original topographic shape during relaxation in either case relaxation of compensated plateaus does not yield elevated rims we propose that the state of compensation must be considered variable

comparison of central pit craters on mars mercury ganymede and the saturnian satellites we report on the first results of a large scale comparison study of central pit craters throughout the solar system focused on mars mercury ganymede rhea dione and tethys we have identified 10 more central pit craters on rhea dione and tethys than have previously been reported we see a general trend that the median ratio of the pit to crater diameter dp dc decreases with increasing gravity and decreasing volatile content of the crust floor pits are more common on volatile rich bodies while summit pits become more common as crustal volatile content decreases uplifted bedrock from below the crater floor occurs in the central peak upon which summit pits are found and in rims around floor pits which may or may not break the surface peaks on which summit pits are found on mars and mercury share similar characteristics to those of nonpitted central peaks indicating that some normal central peaks undergo an additional process to create summit pits martian floor pits do not appear to be the result of a central peak collapse as the median ratio of the peak to crater diameter dpk dc is about twice as high for central peaks summit pits than dp dc values for floor pits median dpk dc is twice as high for mars as for mercury reflecting differing crustal strength between the two bodies results indicate that a complicated interplay of crustal volatiles target strength surface gravity and impactor energy along with both uplift and collapse are involved in central pit formation multiple formation models may be required to explain the range of central pits seen throughout the solar system

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

extensional and compressional instabilities in icy satellite lithospheres a simple mechanical model of planetary lithospheres and asthenospheres is combined with the most recent experimental results concerning the brittle and ductile deformation of ice i h to determine the plausibility of forming the grooved terrains on ganymede enceladus and miranda by means of lithospheric instabilities the model assumes a lithosphere of uniform strength atop a substrate whose strength decreases exponentially with depth the thickness of the lithosphere is defined by the intersection of a near surface byerlee s law relationship for brittle failure with a standard exponential ductile flow law at depth for ganymede we find that the relatively high surface gravity warm temperatures render virtually unachievable an instability strong enough to account for the observed topographic relief however a sufficiently strong instability capable of producing features with the proper spacing could develop on a smaller colder body such as enceladus or miranda the requirements in this case would be a strain rate greater than about 10 14 sec 1 and a near surface thermal gradient of a few tens of degrees kelvin per kilometer the most reasonable way to accomplish these requirements is by the intrusive emplacement of a thick layer of warm ice beneath a preexisting lithosphere of about a kilometer or so in thickness accompanied almost contemporaneously by a strain event lasting on the order of 10 5 years for extension or 10 3 years for compression however observed properties of grooved terrains on enceladus and miranda such as nonuniform feature spacing and low crater counts force us to conclude that unstable deformation must be more complicated than this simple model would suggest if it is to explain these features

creep of water ices at planetary conditions a compilation many constitutive laws for the flow of ice have been published since the advent of the voyager explorations of the outer solar system conflicting data have occasionally come from different laboratories and refinement of experimental techniques has led to the publication of laws that supersede earlier ones in addition there are unpublished data from ongoing research that also amend the constitutive laws here we compile the most current laboratory derived flow laws for water ice phases i ii iii v and vi and ice i mixtures with hard particulates the rheology of interest is mainly that of steady state and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system advances in grain size dependent creep in ices i and ii as well as in phase transformations and metastability under differential stress are also included in this compilation at laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress temperature and pressure dependencies but with the exception of ice iii have fairly similar strengths hard particulates strengthen ice i significantly only at high particulate volume fractions ice iii has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability which may extend metastably well into what is nominally the ice ii field is located near likely geotherms of large icy moons

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

formation of mountains on io variable volcanism and thermal stresses abstract thermal stresses are potentially important drivers of io s tectonics and mountain building it has been hypothesized that sustained local or regional shut down of heat pipe volcanism on io could lead to deep crustal heating and large compressive stresses mckinnon w b schenk p m dombard a j 2001 geology 29 103 106 such large stresses would then be relieved by thrust faulting and uplifting of crustal blocks producing mountains like those observed on io here we analyze the tectonic consequences of the heat pipe model in detail considering both the initial thermal stress state of a basalt or peridotite crust created by heat pipe volcanism and relative roles of subsidence stresses due to burial of preexisting layers and thermal stresses arising from variable volcanism and changes in crustal lithosphere thickness we limit the magnitude of the potential subsidence stresses in our study because the magnitude of subsidence stresses can be quite large if not dominant results indicate that for a fixed crustal thickness the region of failure and faulting moves closer to the surface as eruption rate decreases and time increases when the crust melts at its base as volcanism decreases as might occur under steady state tidal heating resulting in crustal thinning the region of failure is brought even closer to the surface naturally when compressive subsidence stresses are included the vertical extent of crust in brittle failure thickens to include most of the lithosphere in contrast increases in eruption rate cause the extent of the region in compressional failure to decrease and be driven very deep in the crust in the absence of sufficient subsidence stress therefore regions of declining volcanism are more likely to produce mountains whereas regions of extensive or increasing volcanism are less likely to do so this is consistent with the observation of a global anticorrelation between mountains and volcanic centers on io finally we find that the choice of crustal composition rheology dry basalt vs dry peridotite has little effect on our results implying that basalt peridotite and komatiite are all similarly stiff in the io environment

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

hydrocode simulation of ganymede and europa cratering trends how thick is europa s crust abstract one of the continuing debates of outer solar system research centers on the thickness of europa s ice crust as it affects both the habitability and accessibility of its sub surface ocean here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to europan cratering trends and galileo derived topographic profiles to investigate the crustal thickness full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius craters produced in these thin shell simulations were consistently smaller than for larger ice thicknesses which will complicate inference of large impactor population sizes simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit pit morphology this work supports that summit pit craters noted on both rocky and icy bodies can be created by the presence of a weaker layer at depth we suggest that floor pits seen only on ice rich bodies require a different formation mechanism to summit pits pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow suggesting that the starting crater morphology for viscous relaxation isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow we find that the crater depth diameter trend of europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth with a corresponding geothermal gradient of 0 025 k m as this ice thickness estimate is below 10 km results from this work suggest that convective overturn of the surface ice may occur or have occurred on europa making the development of indigenous life a possibility

central pit craters on ganymede central pit craters are common on mars ganymede and callisto and thus are generally believed to require target volatiles in their formation the purpose of this study is to identify the environmental conditions under which central pit craters form on ganymede we have conducted a study of 471 central pit craters with diameters between 5 and 150 km on ganymede and compared the results to 1604 central pit craters on mars diameter range 5 160 km both floor and summit pits occur on mars whereas floor pits dominate on ganymede central peak craters are found in similar locations and diameter ranges as central pit craters on mars and overlap in location and at diameters 60 km on ganymede central pit craters show no regional variations on either ganymede or mars and are not concentrated on specific geologic units central pit craters show a range of preservation states indicating that conditions favoring central pit formation have existed since crater retaining surfaces have existed on ganymede and mars central pit craters on ganymede are generally about three times larger than those on mars probably due to gravity scaling although target characteristics and resolution also may play a role central pits tend to be larger relative to their parent crater on ganymede than on mars probably because of ganymede s purer ice crust a transition to different characteristics occurs in ganymede s icy crust at depths of 4 7 km based on the larger pit to crater diameter relationship for craters in the 70 130 km diameter range and lack of central peaks in craters larger than 60 km diameter we use our results to constrain the proposed formation models for central pits on these two bodies our results are most consistent with the melt drainage model for central pit formation

impact basin relaxation at iapetus abstract we investigate impact basin relaxation on iapetus by combining a 3d thermal evolution model robuchon g choblet g tobie g cadek o sotin c grasset o 2010 icarus 207 959 971 with a spherical axisymmetric viscoelastic relaxation code zhong s paulson a wahr j 2003 geophys j int 155 679 695 due to the progressive cooling of iapetus younger basins relax less than older basins for an ice reference viscosity of 10 14 pa s an 800 km diameter basin relaxes by 30 if it formed in the first 50 myr but by 10 if it formed at 1 2 gyr bigger basins relax more rapidly than smaller ones because the inferred thickness of the ice shell exceeds the diameter of all but the largest basins considered stereo topography shows that all basins 600 km in diameter or smaller are relaxed by 25 or less our model can match the relaxation of all the basins considered within error by assuming a single basin formation age 4 36 ga for our nominal viscosity this result is consistent with crater counts which show no detectable age variation between the basins examined

modeling the morphological diversity of impact craters on icy satellites abstract impact craters on icy satellites display a wide range of morphologies some of which have no counterpart on rocky bodies numerical simulation studies have struggled to reproduce the diversity of features such as central pits and transitions in crater depth with increasing diameter observed on the icy galilean satellites the transitions in crater depth at diameters of about 26 and 150 km on ganymede and callisto have been interpreted as reflecting subsurface structure using the cth shock physics code we model the formation of craters with diameters between 400 m and about 200 km on ganymede using different subsurface temperature profiles our calculations include recent improvements in the model equation of state for h 2 o and quasi static strength parameters for ice we find that the shock induced formation of dense high pressure polymorphs ices vi and vii creates a gap in the crater excavation flow which we call discontinuous excavation for craters larger than about 20 km discontinuous excavation concentrates a hot plug of material 270 k and mostly on the melting curve in the center of the crater floor the size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters and we propose that a genetic link exists between them we also derive depth versus diameter curves for different internal temperature profiles in a 120 k isothermal crust calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior hence the depth to diameter transition at about 26 km reflects thermal weakening of ice finally simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean

viscous relaxation of impact craters on icy planetary surfaces determination of viscosity variation with depth spacecraft images show that the icy galilean satellites have surfaces with very low topographic relief impact craters on ganymede and callisto are anomalously shallow and are characterized by sharp well defined rims and domed floors these morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth under these conditions large craters relax more rapidly than small craters therefore explaining a possible underabundance of large craters viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters the results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies

on the resurfacing of ganymede by liquid water volcanism abstract a long popular model for producing ganymede s bright terrain involves flooding of low lying graben with liquid water slush or warm soft ice the model suffers from major problems however including the absence of obvious near surface heat sources the negative buoyancy of liquid water and the lack of a mechanism for confining the flows to graben floors we present new models for cryovolcanic resurfacing to overcome these difficulties tidal heating within an ancient laplace like orbital resonance showman and malhotra 1997 icarus 127 93 showman et al 1997 icarus 129 367 provides a plausible heat source and could allow partial melting to occur as shallow as 5 10 km depth our favored mechanism for delivering this water to the surface invokes the fact that topography such as a global set of graben causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows graben despite the negative buoyancy of the liquid these eruptions can occur only within the topographic lows furthermore as the low areas become full the pressure gradients disappear and the resurfacing ceases this provides an explanation for the observed straight dark bright terrain boundaries water cannot overflow the graben so resurfacing rarely embays craters or other rough topography pure liquid water can be pumped to the surface from only 5 10 km depth but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush a challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows the resurfacing must occur before the graben topography disappears we also evaluate alternate resurfacing mechanisms such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate contaminated crust that is denser than liquid water and conclude that they are unlikely to explain ganymede s bright terrain

viscous relaxation of craters within the martian south polar layered deposits abstract the south polar layered deposits pld are of fundamental importance to martian climatology as they may comprise the largest reservoir of near surface water on mars the south pld exhibit relatively young crater retention surface ages which are widely attributed to recent resurfacing however we show that both constructional and destructional resurfacing mechanisms such as dust deposition and water ice sublimation respectively are inconsistent with the size depth and spatial distributions of south pld craters we demonstrate that another process viscous creep relaxation of dusty water ice is more compatible with the observed cratering of south pld surfaces the results of our finite element relaxation simulations suggest that despite their apparent youthfulness the pld have been stable for at least several hundred million and perhaps even over a billion years consequently our modeling implies that the time scales for the formation and preservation of the layers characteristic of the south and possibly north pld are much longer than generally assumed

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

relaxed impact craters on ganymede regional variation and high heat flows abstract viscously relaxed craters provide a window into the thermal history of ganymede a satellite with copious geologic signs of past high heat flows here we present measurements of relaxed craters in four regions for which suitable imaging exists near anshar sulcus tiamat sulcus northern marius regio and ganymede s south pole we describe a technique to measure apparent depth or depth of the crater with respect to the surrounding terrain elevation measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios see companion paper bland et al 2017 the presence of numerous substantially relaxed craters indicates high heat flows in excess of 30 40 mw m 2 over 2 gyr with many small

the inference of north atlantic circulation patterns from climatological hydrographic data we present a review of spiral techniques which have recently been developed for the determination of absolute velocity profiles from hydrographic observations a specific technique is then designed and applied to the north atlantic part of levitus 1982 climatological hydrographic atlas with the aim of estimating reference velocities and diffusivities for heat salt and potential vorticity these quantities are determined on a 1 grid from the local gradients of temperature and salinity under the constraints of the thermal wind relations and the conservation of the respective tracers including diapycnic and isopycnic mixing terms the estimation procedure includes the statistical framework of inverse modeling in the weighting of the constraints by the data noise variances and the determination of the covariances of the model parameters the resulting circulation pattern bears strong resemblance to the classical view of the north atlantic circulation as put foreward by wust 1935 and defant 1941 the upper layers are dominated by the gulf stream north atlantic current system with a broad subtropical gyre recirculation in the lower layer a western boundary current is fed from norwegian sea overflow penetrating the gibbs fracture zone and partly circulating around the labrador sea as a consequence of the climatological averaging the currents appear in broad shape with much reduced velocities in particular in the upper layer the vertical structure reveals an almost horizontal level of no motion pattern much along the concepts of defant 1941 diffusion coefficients were determined for an upper layer depth of mixed layer to 800 m depth and a lower layer 800 m to 2000 m the spatial pattern of these coefficients correlates with maps of eddy activity showing higher values in the strong current regimes and low values within the subtropical and subpolar gyre average values in the lower layer of the quiet regions are 10 5 m s and 10 m s for the diapycnal and isopycnal diffusivity respectively and 10 1 m s for the vertical diffusivity of vorticity which yields 10 m s for the lateral diffusivity of potential vorticity toward the regions of strong currents and in the upper layer these values roughly increase by an order of magnitude