a new look at the saturn system the voyager 2 images voyager 2 photography has complemented that of voyager i in revealing many additional characteristics of saturn and its satellites and rings saturn s atmosphere contains persistent oval cloud features reminiscent of features on jupiter smaller irregular features track out a pattern of zonal winds that is symmetric about saturn s equator and appears to extend to great depth winds are predominantly eastward and reach 500 meters per second at the equator titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths several satellites have been photographed at substantially improved resolution enceladus surface ranges from old densely cratered terrain to relatively young uncratered plains crossed by grooves and faults tethys has a crater 400 kilometers in diameter whose floor has domed to match tethys surface curvature and a deep trench that extends at least 270 around tethys circumference hyperion is cratered and irregular in shape iapetus bright trailing hemisphere includes several dark floored craters and phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours within saturn s rings the birth of a spoke has been observed and surprising azimuthal and time variability is found in the ringlet structure of the outer b ring these observations lead to speculations about saturn s internal structure and about the collisional and thermal history of the rings and satellites

rapid conversion of an oceanic spreading center to a subduction zone inferred from high precision geochronology where and how subduction zones initiate is a fundamental tectonic problem yet there are few well constrained geologic tests that address the tectonic settings and dynamics of the process numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system gurnis m hall c lavier l 2004 geochem geophys geosys 5 q07001 but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere instead other weak zones such as fracture zones have been invoked because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself in the western philippines we find that oceanic crust was less than 1 my old when it was underthrust and metamorphosed at the onset of subduction in palawan philippines implying forced subduction initiation at a spreading center this result shows that young and positively buoyant but weak lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older denser lithosphere and that plate motion rapidly changed from divergence to convergence

ground based observations of saturn s north polar spot and hexagon ground based observations of two conspicuous features near the north pole of saturn the polar vortex and the hexagonal wave structure were made from july 1990 to october 1991 10 years after their discovery during this period the polar spot drifted in longitude relative to system iii by 0 0353 per day on average superimposed on this mean motion the spot also underwent short term rapid excursions in longitude of up to 14 at rates of up to 1 per day the spot also exhibited irregular variations in its latitude location a combination of these data together with those obtained by voyager 1 and 2 in 1980 and 1981 shows that the spot drifted 0 0577 per day for the 11 year interval from 1980 to 1991 the large lifetime of both features indicates that they are insensitive to the strong variations in the seasonal heating of the cloud layers in the upper polar atmosphere

saturn s atmosphere at 1 10 kilometer resolution we present images of saturn from the final phases of the cassini mission including images with 0 5 km per pixel resolution as high as any saturn images ever taken notable features are puffy clouds resembling terrestrial cumulus shadows indicating cloud height dome and bowl shaped cloud structures indicating upwelling and downwelling in anticyclones and cyclones respectively and filaments which are thread like clouds that remain coherent over distances of 20 000 km from the coherence of the filaments we give upper bounds on the diffusivity and kinetic energy dissipation a radiative transfer analysis by sanz requena et al 2018 indicates that methane band imagery is most useful in determining cloud and haze properties in the 60 250 mbar pressure range our methane band imagery finds haze in this pressure range covering 64 74 planetocentric latitude filaments lie within the haze and cumulus clouds lie below it but pressure levels are uncertain below the 250 mbar level

saturn s zonal winds at cloud level we present an analysis of new cloud motion measurements in saturn s atmosphere from voyager 1 and 2 images and define a nominal zonal wind velocity vs latitude profile for the cloud top level that can be used as reference for the upcoming cassini mission improvements over previous studies include i twice the number of wind vectors used ii independent measurements of cloud tracers iii extended latitudinal coverage iv strict error control and evaluation and v a new zonal averaging methodology mean zonal wind velocities are given for the latitude range 81 to 71 planetographic in steps of 0 5 with a gap between 0 and 10 due to ring shadowing this research also includes the study of the morphology and motions of cloud systems in the equator and in the southern hemisphere which have not been described previously we give an ephemeris motion equation for several long lived features that could be observable if they survive until cassini arrives on saturn the long term behavior of the zonal wind profile is studied by comparing it with a complete set of wind velocities compiled from historical and modern ground based and hst observations significant changes are noted in the equatorial jet between this set and the nominal voyager profile we discuss in detail the possible origin for such differences

viscoelastic versus viscous deformation and the advection of pre stress summary the heaviside response of an incompressible viscous half space is compared to that of a viscoelastic half space that may or may not be compressible elastically it will be shown that the difference between viscous and viscoelastic response is detectable from uplift data provided that the age of the data goes back far enough it is also possible to resolve using the post glacial rebound data whether the viscous model or the viscoelastic model is preferred provided that the ice model is better constrained moreover the rate of solid fluid transition is wavenumber k dependent and the transition is not as rapid nor as small as was previously predicted finally the advection of pre stress is shown to be required in viscoelastic deformation problems of the earth and that it is responsible for viscoelastic gravitational relaxation

effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced surface motion on a spherical self gravitating maxwell earth abstract the effects of lateral variations in lithospheric thickness and mantle viscosity on surface motions on a spherical self gravitating incompressible viscoelastic earth are investigated using the coupled laplace finite element method the ice 4g deglaciation model is used together with gravitationally self consistent sea levels in realistic oceans to describe the load lateral variations in mantle viscosities and lithospheric thickness are inferred from seismic tomography model s20a using well known scaling relationships the laterally homogeneous background viscosity has viscosity increases across 670 km and 1330 km depths and is a simplification of peltier s vm2 model the effect of interaction between ice complexes in north america greenland eurasia and antarctica are investigated and found to be significant for tangential motion the predicted motion in laurentide and antarctica are also compared with results obtained with flat earth models and found that there is general agreement between the two types of modeling although spherical earth models are more realistic because they take into account the interaction between ice complexes and there is no problem with edge effects the effects of lateral heterogeneity on uplift rate are found to be large enough to be resolved by current gps technology it confirms that the effect of the reverse viscosity contrast in the transition zone under the laurentian and fennoscandian craton may mask the effect of lateral heterogeneity in the neighboring layers on uplift rate for tangential motion lateral heterogeneity is found to be able to overprint the divergent motion from the centers of rebound predicted in laterally homogeneous models this finding is new for laurentide and is due to the contribution from the deep part of the lower mantle although the contribution from the upper mantle is also important on tangential motion its effects are compromised by the effects from the other layers

grace gravity data constrain ancient ice geometries and continental dynamics over laurentia the free air gravity trend over canada derived from the gravity recovery and climate experiment grace satellite mission robustly isolates the gravity signal associated with glacial isostatic adjustment gia from the longer time scale mantle convection process this trend proves that the ancient laurentian ice complex was composed of two large domes to the west and east of hudson bay in accord with one of two classes of earlier reconstructions moreover gia models that reconcile the peak rates contribute 25 to 45 to the observed static gravity field which represents an important boundary condition on the buoyancy of the continental tectosphere

regional perturbations in a global background model of glacial isostasy abstract in schotman and vermeersen 2005 earth planet sci lett 236 the effect of crustal and asthenospheric low viscosity zones lvzs on geoid heights was shown as predicted by models of glacial isostatic adjustment gia the governing equations were solved analytically in the spectral domain which makes the method used accurate and fast however it does not allow for large lateral variations in earth stratification as the properties of shallow lvzs can be expected to vary laterally we have developed a finite element model based on abaqus global spherical 3d finite element models are currently not capable of providing high resolution predictions which we expect due to the shallowness of the lvzs we therefore use a regional flat 3d model and compute geoid heights from the predicted displacements at density boundaries by solving laplace s equation in the fourier transformed domain the finite element model is not self gravitating but we compare the results with a self gravitating spectral model under the assumption that the lack of self gravitation is partly compensated by the lack of sphericity and that long wavelength differences largely cancel out when using perturbations which are the difference between a model with and without an lvz we show that geoid height perturbations due to an lvz can be computed accurately though the accuracy deteriorates somewhat with the depth of the lvz moreover we show that horizontal rates of displacement though not accurate for total displacements are accurate for perturbations in the near field we show the effect of lateral variations in the properties of the lvz and in lithospheric thickness and compute geoid height perturbations for northern europe based on a simple laterally heterogeneous model the model is forced with a realistic ice load history and a eustatic ocean load history the errors introduced by using a eustatic ocean load history instead of realistic oceans are generally smaller than 10 but might be critical for perturbations due to crustal lvzs

investigating superswells and sea level changes caused by superplumes via normal mode relaxation theory 1 the superplume hypothesis particularly for one in the mid cretaceous has stimulated the search for evidence for or against such an event and the associated superswell an approach based on normal mode relaxation theory makes it possible to simulate the surface deformation caused by the rising plume using a spherical self gravitating and stratified viscoelastic earth model the results for the superswell are consistent with those of previous analyses using convection and experimental modeling the self consistent coupling of elastic and viscous properties based on maxwell rheology reveals that elasticity plays an important role contributing up to 30 to surface swelling when the plume head crosses the upper mantle we show that the deformation at great distances from the superswell is relatively small of the order of meters at most and opposite in sign with respect to the large uplift over the plume of the order of kilometers the global subsidence over a wide region of the planet surrounding the superswell counteracts its effects on sea level changes and triggers a eustatic signal of a few meters at most a negligible magnitude with respect to 250 m characteristic for the mid cretaceous previous arguments against the mid cretaceous superplume hypothesis which assume that the large amount of water displaced by the superswell causes a eustatic sea level rise of about 200 m thus competing with the displacement due to oceanic crust production are no longer tenable since such estimates did not account for the global behavior of the planet under internal loads

effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced relative sea levels and long wavelength gravity field in a spherical self gravitating maxwell earth the effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced relative sea level rsl changes and the secular rate of change of the earth s long wavelength gravity field in a spherical self gravitating incompressible visco elastic earth are investigated using the coupled laplace finite element method the ice 4g deglaciation model is used with gravitationally self consistent sea levels in realistic oceans to describe the load lateral variations in mantle viscosity and lithospheric thickness are inferred from seismic tomography model s20a the full 3 d earth model which includes all the lateral heterogeneities in the lithosphere and mantle gives a better fit to the global rsl data than the related laterally homogeneous model however the situation is less clear for the observed secular drift of the low degree geopotential coefficients j l because of the uncertain contribution of recent melting but the full 3 d model can fit the j 2 observation if recent melting contributes about 1 0 mm a of equivalent sea level rise it predicts that the gia induced secular gravity rate of change to be detected by the grace mission in the southern part of hudson bay is about 1 2 to 1 6 gal a moreover the contributions of lateral heterogeneities from individual layers in the mantle or in the lithosphere are studied the contribution from the transition zone 420 670 km is generally opposite to that from its neighboring layers and thus can mask their effects as a consequence the effects from the deep lower mantle become dominant for rsl and secular rate of change of gravity over laurentide for the secular rates of change for degrees 2 4 geopotential coefficients the contribution is mostly from lateral heterogeneities in the deeper mantle the effects of background viscosity profiles are also investigated and are found to be significant for all these observables

haze and cloud structure of saturn s north pole and hexagon wave from cassini iss imaging abstract in this paper we present a study of the vertical haze and cloud structure in the upper two bars of saturn s northern polar atmosphere using the imaging science subsystem iss instrument onboard the cassini spacecraft we focus on the characterization of latitudes from 53 to 90 n the observations were taken during june 2013 with five different filters vio bl1 mt2 cb2 and mt3 covering spectral range from the 420 nm to 890 nm in a deep methane absorption band absolute reflectivity measurements of seven selected regions at all wavelengths and several illumination and observation geometries are compared with the values produced by a radiative transfer model the changes in reflectivity at these latitudes are mostly attributed to changes in the tropospheric haze this includes the haze base height from 600 200 mbar at the lowest latitudes to 1000 300 mbar in the pole its particle number density from 20 2 particles cm3 to 2 0 5 particles cm3 at the haze base and its scale height from 18 0 1 km to 50 0 1 km we also report variability in the retrieved particle size distribution and refractive indices we find that the hexagonal wave dichotomizes the studied stratospheric and tropospheric hazes between the outer equatorward regions and the inner polar regions this suggests that the wave or the jet isolates the particle distribution at least at tropospheric levels

cloud features and zonal wind measurements of saturn s atmosphere as observed by cassini vims 1 we present an analysis of data about saturn s atmosphere from cassini s visual and infrared mapping spectrometer vims focusing on the meteorology of the features seen in the 5 m spectral window we present vims mosaics and discuss the morphology and general characteristics of the features backlit by saturn s thermal emission we have also constructed a zonal wind profile from vims feature tracking observation sequences using an automated cloud feature tracker comparison with previously constructed profiles from voyager and cassini imaging data reveals broad similarities suggesting minimal vertical shear of the zonal wind however areas of apparent wind shear are present in the vims zonal wind profile at jet stream cores in particular our analysis shows that the equatorial jet reaches speeds exceeding 450 m s 1 similar to speeds obtained during the voyager era this suggests that recent inferences of relatively slower jet speeds of 275 375 m s 1 are confined to the upper troposphere and that the deep 1 bar jet has not experienced a significant slowdown our measurements of the numerous dark spotted features seen in the vims mosaics reveals that most of these features have diameters less than 1000 km and reside in confined zonal bands between jet stream cores we propose that these spot features are vortices and that vims and imaging science subsystem are sensing the same vortices at two different pressure levels the local structure at the zonal jet streams remains complex as vims may be sensing cloud features that are deeper than the nh3 cloud deck

glacio isostatic adjustment in the po plain and in the northern adriatic region vertical movements in the po plain northern italy are controlled by natural and anthropogenic effects since italy is located in the far field of the former late pleistocene ice sheets isostatic deformations are primarily driven by melt water loading and represent a major component of long term natural movements across the entire mediterranean in addition to far field sources here we consider the isostatic effects of melting of the nearby wurm alpine ice sheet suggesting that it is possible to put bounds on its maximum thickness extent and chronology by holocene relative sea level observations from the northern adriatic using various plausible ice models and adopting a viscosity profile that matches holocene relative sea level observations in the mediterranean we find that melting of the alpine ice sheet is always responsible for upward movements in the po plain currently at rates of 0 5 mm yr when both far and near field sources are considered the rate of sea level change in the venetian lagoon for the most reasonable mantle rheology and melting chronology is negative i e opposite to that attributed to human activity and recent climatic variations however its amplitude fractions of mm yr is small compared to the secular signal observed by tide gauges 2 mm yr which makes glacial isostasy a second order mechanism of sea level variation in this region

glacial isostatic adjustment on 3 d earth models a finite volume formulation summary we describe and present results from a finite volume fv parallel computer code for forward modelling the maxwell viscoelastic response of a 3 d self gravitating elastically compressible earth to an arbitrary surface load we implement a conservative control volume discretization of the governing equations using a tetrahedral grid in cartesian geometry and a low order linear interpolation the basic starting grid honours all major radial discontinuities in the preliminary reference earth model prem and the models are permitted arbitrary spatial variations in viscosity and elastic parameters these variations may be either continuous or discontinuous at a set of grid nodes forming a 3 d surface within the regional or global modelling domain in the second part of the paper we adopt the fv methodology and a spherically symmetric earth model to generate a suite of predictions sampling a broad class of glacial isostatic adjustment gia data types 3 d crustal motions long wavelength gravity anomalies these calculations based on either a simple disc load history or a global late pleistocene ice load reconstruction ice 3g are benchmarked against predictions generated using the traditional normal mode approach to gia the detailed comparison provides a guide for future analyses e g what grid resolution is required to obtain a specific accuracy and it indicates that discrepancies in predictions of 3 d crustal velocities less than 0 1 mm yr 1 are generally obtainable for global grids with 3 106 nodes however grids of higher resolution are required to predict large amplitude 1 cm yr 1 radial velocities in zones of peak post glacial uplift e g james bay to the same level of absolute accuracy we conclude the paper with a first application of the new formulation to a 3 d problem specifically we consider the impact of mantle viscosity heterogeneity on predictions of present day 3 d crustal motions in north america in these tests the lateral viscosity variation is constructed with suitable scaling from tomographic images of seismic s wave heterogeneity and it is characterized by approximately 2 orders of magnitude peak to peak lateral variations within the lower mantle and 1 order of magnitude variations in the bulk of the upper mantle below the asthenosphere we find that the introduction of 3 d viscosity structure has a profound impact on horizontal velocities indeed the magnitude of the perturbation of order 1 mm yr 1 is as large as the prediction generated from the underlying 1 d radial reference model and it far exceeds observational uncertainties currently being obtained from space geodetic surveying the relative impact of lateral viscosity variations on predicted radial motions is significantly smaller

models of active glacial isostasy roofing warm subduction case of the south patagonian ice field 1 modern geodetic techniques such as precise global positioning system gps and high resolution space gravity mapping gravity recovery and climate experiment grace make it possible to measure the present day rate of viscoelastic gravitational earth response to present and past glacier mass changes the andes of patagonia contain glacial environments of dramatic mass change these mass load changes occur near a tectonically active boundary between the antarctic and south american plates the mechanical strength of the continental side of this boundary is influenced by neogene ridge subduction and by the subduction of a youthful oceanic slab a ridge of young volcanos parallels the pacific coastline release of volatiles such as water at depth along this ridge creates a unique rheological environment to assess the influence of this rheological ridge structure on the observational land uplift rate we apply a two dimensional viscoelastic earth model a numerical study is presented which examines the sensitivity of the glacial loading unloading response to the complex structure at depth related to the subducting slab the viscous wedge between slab and continental lithosphere and the increase of elastic thickness from oceanic to continental lithosphere a key feature revealed by our numerical experiments is a continuum flow wherein the slab subdues the material transport toward oceanic mantle and crust the restricted flow is sensitive to the details of slab mechanical strength and penetration into the upper mantle the reduced viscosity within the mantle wedge however enhances the load induced material transport everywhere within the asthenosphere

the saturnian ribbon feature a baroclinically unstable model images of saturn from the two voyager spacecraft showed few discrete atmospheric features in the low contrast latitudinal banding over the planet one unusual feature however was a narrow dark oscillatory structure encircling the planet in the light zone at 45 to 50 north this paper presents a description of this dark structure which we call the ribbon feature a wavelength of 5710 260 km is obtained from moderate resolution images which were used to produce a 360 image of the feature higher resolution image pairs were used to examine the local circulation which shows counterclockwise rotation north of the ribbon and clockwise rotation south of it the zonal circulation measurements show that it is located in a 120 m sec 1 westerly jet but with a small 5 m sec 1 meridional velocity the infrared spectrometer results from voyager showed the greatest thermal wind sheer in this latitude region the regions adjacent to the ribbon are of similar brightness except for the uv 3400 a filter images where the northern part of this zone is lighter than the southern section from this observation we have estimated that the disturbance associated with the ribbon feature extends to at least the 200 mbar level a study of a 120 section of the ribbon feature and the associated latitudinal thermal gradient by l a sromovsky h e revercomb r j krauss and v e suomi 1983 j geophys res 88 8650 8666 led them to suggest that it was produced by some form of baroclinic instability this proposal is considered here with other alternative types of instability we conclude that the ribbon feature could arise from a baroclinic instability we then develop a numerical model of such an instability that is consistent with the observed wavelength the vertical extent of the ribbon and the local latitudinal thermal profile

postglacial rebound in a non newtonian spherical earth using finite elements we study the postglacial rebound in an axisymmetric earth with a non newtonian rheology our approach allows for the first time a self consistent study of the effect of a power law creep on the long wavelength deformations which cannot be performed using flat models we show that for a non newtonian upper mantle the time variations of the low degree stokes coefficients become largely insensitive to the viscosity of the lower mantle this is in contrast with previous evidence based on newtonian models which emphasized the importance of the low degree signatures for constraining the lower mantle viscosity

secular variation of the geocenter the degree 1 deformations of earth induced by a surface load are computed in a reference frame related to the center of mass of the planet using a theoretical approach love number formalism at secular timescale where earth has a viscoelastic behavior the temporal variation of the coordinates of the geocenter induced by the pleistocenic deglaciation is investigated for two loading histories three ice sheets with the same time dependence or ice 3g model testing different viscosity profiles within the mantle the present day secular variation of earth s geocenter induced by postglacial rebound is found to be from 0 2 up to 0 5 mm yr and consequently we suggest that observations from space geodesy of this rate could be used to constrain the viscosity contrast between upper and lower mantle

postglacial rebound with a non newtonian upper mantle and a newtonian lower mantle rheology we have employed a composite rheology consisting of both linear and nonlinear creep mechanisms which are connected by a transition stress background stress due to geodynamical processes is included for models with a non newtonian upper mantle overlying a newtonian lower mantle the temporal responses of the displacements can reproduce those of newtonian models the average effective viscosity profile under the ice load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation this can explain why simple newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions

polar wander caused by the quaternary glacial cycles and fluid love number abstract perturbations of the earth s rotation caused by the quaternary glacial cycles provide an important constraint on the viscosity of the deep mantle because they represent a long wavelength response of the earth to surface load redistribution the predicted present day polar wander speed pws is however sensitive to both the lower mantle viscosity lm the density jump at 670 km depth and the lithospheric thickness and viscosity e g sabadini and peltier geophys j r astron soc 66 1981 553 578 yuen et al j geophys res 87 1982 10745 10762 peltier and wu geophys res lett 10 1983 181 184 wu and peltier geophys j r astron soc 76 1984 753 791 peltier j geophys res 89 1984 11303 11316 vermeersen et al j geophys res 102 1997 27689 27702 mitrovica and milne j geophys res 103 1998 985 1005 johnston and lambeck geophys j int 136 1999 537 558 nakada geophys j int 143 2000 230 238 for earth models with lm 21 pa s and an elastic lithosphere the present day pws is very sensitive to the m1 mode buoyancy mode related to the density jump at 670 km depth mitrovica and milne j geophys res 103 1998 985 1005 the contribution of the m1 mode however is less significant for earth models with a viscoelastic lithosphere nakada geophys j int 143 2000 230 238 this is due to the fact that this contribution depends on the relative strength of the m1 mode k 2 t m1 k f t where k 2 t m1 is the magnitude of tidal love number k 2 t of the m1 mode and k f t is the value of k 2 t in the fluid limit fluid love number the magnitude of k f t for earth models with a viscoelastic lithosphere is larger than that for an elastic lithosphere and it is smaller for a thicker elastic lithosphere than for a thinner one thus for earth models with a viscoelastic lithosphere the pws is mainly sensitive to the lower mantle viscosity regardless of the behavior of the 670 km density discontinuity this relation also explains why the predicted pws increases with increasing thickness of an elastic lithosphere that is since the value of k 2 t m1 k f t with a thicker elastic lithosphere is larger than that with a thinner elastic lithosphere the m1 mode will have a higher contribution in the case of a thicker elastic lithosphere

mode coupling in a viscoelastic self gravitating spherical earth induced by axisymmetric loads and lateral viscosity variations the importance of mode coupling in a viscoelastic self gravitating spherical earth with lateral variations in linear or nonlinear rheology is investigated with a finite element model coupled to poisson s equation both the lateral viscosity variations and the harmonic load are axisymmetric the effects of self gravitation viscosity contrast and the location of the abrupt lateral change in viscosity in a linear mantle are studied and are found to be significant in determining the strength of mode coupling it is demonstrated that a larger number of harmonics than generally assumed is required to give accurate description of the induced deformation mode coupling is also found to be important in a nonlinear mantle especially when the harmonic degree l is large

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

an estimate of motion between the spin axis and the hotspots over the past century 1 estimates of the motion of the spin axis over the past century either neglect plate motion or are relative to the mean lithosphere to understand the response of the spin axis to mass changes it is more useful to estimate spin axis motion relative to the hotspots because the hotspots well represent the solid earth the hotspots originate in the lower mantle which comprises 2 3 of the earth s volume relative to the lower mantle the plates and the upper mantle move in opposite directions to a large extent canceling each other out the mean lithosphere moves quickly relative to the hotspots and poorly represents the solid earth the spin axis has moved relative to the hotspots over the past century along the 68 4 w meridian at 4 03 milliarcsecond yr this motion is 11 counterclockwise of 0 5 milliarcsecond yr faster than and 25 different than spin axis motion relative to the mean lithosphere

eustatic sea level fluctuations induced by polar wander the observation of apparently simultaneous episodes of deposition or non deposition of marine sediments in different parts of the world has led to the proposal that at least some sea level rises and falls must be global or eustatic in character1 3 here we show that polar wander of a viscoelastic stratified earth can induce global sea level fluctuations comparable to the short term component in the eustatic sea level curves the sign of these fluctuations which are very sensitive to the rheological stratification depends on the geographical location of the observation point rises and falls in sea level can thus be coeval in different parts of the world this finding is in distinct contrast to the main assumption underlying the reconstruction of eustatic curves namely that global sea level events produce the same depositional sequence everywhere this apparent contradiction is due to the poor time resolution of the stratigraphie records in the distant past which is comparable to the timescale of polar motion4 6 and to non uniform data coverage we propose that polar wander should be added to the list of geophysical mechanisms the others are glacial instabilities plate tectonic mechanisms sea floor spreading and thermal and compaction induced subsidence that can control the third order cycles in sea level

on the origins of earth rotation anomalies new insights on the basis of both paleogeodetic data and gravity recovery and climate experiment grace data 1 the theory previously developed to predict the impact on earth s rotational state of the late pleistocene glaciation cycle is extended in particular we examine the extent to which a departure of the infinite time asymptote of the viscoelastic tidal love number of degree 2 k2t from the observed fluid love number kf impacts the theory a number of tests of the influence of the difference in these love numbers on theoretical predictions of the model of the glacial isostatic adjustment gia process are explored relative sea level history predictions are shown not to be sensitive to the difference even though they are highly sensitive to the influence of the changing rotational state itself we also explore in detail the accuracy with which the gravity recovery and climate experiment grace satellite system is able to observe the global gia process including the time dependent amplitude of the degree 2 and order 1 spherical harmonic components of the gravitational field the only components that are significantly influenced by rotational effects it is explicitly shown that the grace observation of these properties of the time varying gravitational field is sufficiently accurate to rule out the values predicted by the ice 5g vm2 model of peltier 2004 however we also note that this model is constrained only by data from an epoch during which modern greenhouse gas induced melting of both the great polar ice sheets and small ice sheets and glaciers was not occurring such modern loss of grounded continental ice strongly influences the evolving rotational state of the planet and thus the values of the degree 2 and order 1 stokes coefficients as they are currently being measured by the grace satellite system a series of sensitivity tests are employed to demonstrate this fact we suggest that the accuracy of scenarios for modern land ice melting may be tested by ensuring that such scenarios conform to the grace observations of these crucial time dependent stokes coefficients

dynamics of saturn s polar regions we analyze data retrieved by the imaging science system onboard the cassini spacecraft to study the horizontal velocity and vorticity fields of saturn s polar regions latitudes 60 90 n in june december 2013 and 60 90 s in october 2006 and july december 2008 including the northern region where the hexagonal wave is prominent with the aid of an automated two dimensional correlation algorithm we determine two dimensional maps of zonal and meridional winds and deduce vorticity maps we extract zonal averages of zonal winds providing wind profiles that reach latitudes as high as 89 5 in the south and 89 9 in the north wind measurements cover the intense polar cyclonic vortices that reach similar peak velocities of 150 m s 1 at 88 5 the hexagonal wave lies in the core of an intense eastward jet at planetocentric latitude 75 8 n with motions that become nonzonal at the hexagonal feature in the south hemisphere the peak of the eastward jet is located at planetocentric latitude 70 4 s a large anticyclone the south polar spot sps similar to the north polar spot nps observed at the voyager times 1980 1981 has been observed in images from april 2008 to january 2009 in the south polar region at latitude 66 1 close to the eastward jet the sps does not apparently excite a wave on the jet we analyze the stability of the zonal jets finding potential instabilities at the flanks of the eastward jets around 70 and we measure the eddy wind components suggesting momentum transfer from eddy motion to the westward jets closer to the poles

an estimate of global mean sea level rise inferred from tide gauge measurements using glacial isostatic models consistent with the relative sea level record this study is concerned with the influence of the glacial isostatic adjustment caused by the last pleistocene deglaciation on the present day sea level the viscoelastic deformation caused by the time variable ice and ocean loads is simulated by computing the resulting perturbations for a spherical self gravitating incompressible maxwell viscoelastic earth model the associated variation of the earth rotation is described in terms of the liouville equation which is solved by means of the maccullagh formulae this allows the determination of the vertical displacement and geoid height and thus the solution of the sea level equation we test several viscosity and ice models and evaluate them by comparison of the computed response with the holocene relative sea level record using the optimum combination of viscosity and ice models we then estimate the influence of the last pleistocene deglaciation on the tide gauge measurements a comparison between the observational and residual linear trends for the tide gauge measurements shows a significant reduction of the variance and geographical variability for the latter in particular for the formerly ice covered regions of north america and scandinavia the favoured value determined for the global mean sea level rise is 1 46 0 2 mm a 1

glacial isostatic adjustment and earth rotation refined constraints on the viscosity of the deepest mantle we explore the extent to which two properties of earth s present day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle the analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660 km seismic discontinuity appears to be allowed by these data such observations do not constrain the viscosity of the mantle below about 1400 km depth however and here we estimate the average viscosity in this deepest region by invoking observations of the present day rate and direction of polar wander and the present day magnitude of the nontidal acceleration of the rate of planetary rotation since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation this response is expected to provide the most specific sampling possible of deep mantle properties our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial interglacial transition namely the ice 4g model this analysis shows that when earth rotation observations are combined with relative sea level constraints the viscosity of the deepest mantle is required to be 1 order of magnitude higher than the viscosity of the upper part of the lower mantle such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid the analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations

the viscoelastic relaxation of a realistically stratified earth and a further analysis of postglacial rebound summary we discuss the isostatic adjustment of the earth in response to pleistocene deglaciation and derive constraints on the earth s viscosity profile we model the earth as viscoelastic self gravitating realistically stratified and spherically symmetric it is shown that the earth s relaxation eigenspectrum besides possessing a small set of well known discrete modes also includes infinitely dense sets of modes a continuous spectrum which separate into classes that depend on the principal restoring force of the mode methods for estimating the continuous spectrum are discussed and the effects on the predicted uplift are assessed we find that the effects of the continuous spectrum are far less important than those of the discrete modes though the presence of the continuous spectrum can make the discrete modes more difficult to identify the continuous spectrum does become important when computing the rebound due to more recent loading such as might be associated with present day changes in polar ice we use our model to refine predictions of the earth s response to pleistocene glacial loading with particular emphasis on implications for mantle viscosity we have used time histories that are consistent with the recently recalibrated 14c time scale a special effort is made to match the geologic records of sea level at sites in the vicinity of hudson bay we find consistent with the results of other authors that the curvatures of the records are sensitive indicators of lower mantle viscosity but that there are inconsistencies between the records at different sites however we find we are also able to change the predicted curvatures by modifying the temporal and spatial distribution of the ice loads if we assume a factor of 50 increase in viscosity between the upper and lower mantles and if we assume that the ice on the eastern side of hudson bay melted a few thousand years later than the ice on the western side a result consistent with recent geologic evidence we are able to match the observed sea level data about as well as we are able to if we use instead a uniform mantle viscosity profile a model with a factor of 50 jump in viscosity predicts a large free air gravity anomaly over northern canada that is reasonably consistent with what is observed whereas a uniform viscosity model does not though some authors have argued that the observed north american gravity anomaly may be mostly a consequence of mantle convection and largely unrelated to postglacial rebound our objective here is not to present a preferred ice model or viscosity profile rather we conclude only that the interpretation of the data is ambiguous and that the mantle viscosity profile can not yet be uniquely inferred from them

changes in the earth inertia tensor the role of boundary conditions at the core mantle interface in the early seventies a broad scientific debate began upon the nature of the conditions to be applied at the core mantle interface in order to study the static deformations of the earth this controversy which first arose in the context of post seismic deformations has also affected later investigations on inertia perturbations driven by surface or internal density contrasts the aim of this communication is not to readdress the long standing question about the appropriate set of boundary conditions to be imposed at the core mantle boundary but rather to show how the choice of these conditions may affect the calculation of inertia changes when applied to glacially induced inertia perturbations our results demonstrate that the pitfall of the core mantle boundary conditions has been acting for a long time after the scientific discussion mentioned above came to an end

glacier shrinkage and modeled uplift of the alps 1 present day glacier reduction in the alps estimated from glacier inventories and induced viscoelastic response of a stratified earth s model is responsible for sizable uplift rates patches of 0 4 0 5 mm yr due to ice mass loss of largest ice complexes overprint a characteristic area of slower uplift of 0 1 0 2 mm yr signature of the phenomenon in the whole alpine chain viscous stress relaxation in the lower crust due to glacier mass loss after the end of the little ice age is expected to produce uplift rates of 0 32 mm yr leading to a total viscoelastic response up to 0 8 mm yr our predictions in the western alps show that viscoelastic response to present day glacier shrinkage forms a substantial fraction half of the observed uplift data attempts to constrain the contributions arising from active alpine tectonics and drainage must account for this uplift signal citation barletta v r c ferrari g diolaiuti t carnielli r sabadini and c smiraglia 2006 glacier shrinkage and modeled uplift of the alps geophys res lett 33 l14307 doi 10 1029 2006gl026490

contribution of glacial isostatic adjustment to the geocenter motion abstract the geocenter motion describes the surface net displacement of the entire solid earth with respect to the center of mass of the entire earth including surface masses therefore it resembles an integrative quantity of surface displacement and mass redistribution inside the earth as well as at its surface seasonal variations of this quantity are understood to originate mainly from mass redistribution in the water cycle in contrast a secular trend of the geocenter motion is possible to result also from the dynamics of the earth s interior one mechanism inducing a secular geocenter motion is the glacial isostatic adjustment describing the deformation and mass redistribution in the earth s interior due to glaciations during the pleistocene focusing on this contribution we compute the geocenter motion from the displacement and gravity potential fields calculated for a spherical self gravitating incompressible and viscoelastic earth model loaded by the last pleistocene glacial cycle we discuss the fluid core approximation usually adopted and assess the influence of a list of modelling parameters which are the upper and lower mantle viscosity lithosphere thickness and glaciation history we find a rather robust geocenter motion with respect to parameter variations which is directed towards northeast canada and shows velocities that vary between 0 1 and 1 mm yr depending on the adopted earth model and glaciation history parameterizations

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

shallow upper mantle rheology and secular ice sheet fluctuations on time scales from decades to centuries continental cryospheric forcing in response to climate change constitutes a major source of isostatic disequilibrium that may influence future regional sea level variations current vertical displacements and gravity field variations are often estimated neglecting rheological effects and thus assuming a fully elastic response of the earth in this study we adopt a more general point of view aiming at describing ongoing surface movements resulting from recent glacial instabilities also taking into account the effects associated with shallow upper mantle and crustal rheologies our computations are based on the post widder laplace inversion formula which permits the straightforward computation of load deformation coefficients for steady state and transient rheologies up to very high harmonic degrees using a surface load with a simple geometry and time history we compare the classical elastic solutions to those obtained considering the rheological response of the shallow upper mantle while at the center of the ice sheet rheology only magnifies the elastic response the pattern and time history of vertical displacement at the ice sheet margins show a greater complexity mainly due to the development of lateral forebulges whose shape and amplitude are particularly sensitive to the rheology of the shallow upper mantle in this region assuming an elastic rheology is generally appropriate on a century time scale but significant deviations from a purely elastic response both sign and amplitude are observed at longer time scales or when a low viscosity zone with maxwell rheology is taken into account

ice age cycles earth s rotation instabilities and sea level changes a new class of multilayered viscoelastic earth models based on prem is applied to the modeling of earth s rotation instabilities and associated sea level changes induced by the occurrence of pleistocene ice age cycles that match the oxygen isotope records over the last 0 8 myr the novelty of our approach stands on the usage for the first time in post glacial rebound induced sea level studies of a fully analytical scheme based on normal mode theory that allows to deal with complexities of the real earth such as lithospheric and mantle layering sphericity and self gravitation the ice models are based on ice 3g our results show that differences in true polar wander tpw and tpw induced sea level changes between earth models with a few layers and models having enough layers so that saturated continuum limits are reached can amount factors 2 to 3 this may change conclusions derived from some earlier studies on polar wander induced changes in climate that were based on earth models with a limited amount of layers the results indicate that models containing about 15 layers with a stratified lower mantle and transition zone have reached saturated continuum responses for both tpw and tpw induced sea level changes stratification of the lithosphere is not important this is in contrast with the sensitivity of post seismic deformation models this difference in sensitivity on the radial profile of the earth model can be explained with simple arguments on the dependence on zonal degree

ice 3g a new global model of late pleistocene deglaciation based upon geophysical predictions of post glacial relative sea level change a new high resolution global model of late pleistocene deglaciation is inferred on the basis of geophysical predictions of postglacial relative sea level variations in which the ice ocean solid earth interaction is treated in a gravitationally self consistent fashion for the purpose of these analyses the radial viscoelastic structure of the planet is assumed known on the basis of previously published sensitivity tests on solutions of the forward problem only radiocarbon controlled relative sea level histories from sites that were actually ice covered with one or two additions are employed to constrain the model leaving relative sea level rsl data from sites that were not ice covered to be employed to confirm its consistency results for these confirmatory analyses are reported elsewhere here the new deglaciation model referred to as ice 3g is compared to previous models derived by several independent means and tested against a number of additional observations other than sea level histories including geologically controlled retreat isochrones oxygen isotope data from deep sea sedimentary cores and coral terrace elevations the latter two observations strongly constrain the net sea level rise that has occurred since the onset of deglaciation and therefore the mass of ice that melted during the last glacial interglacial transition

a benchmark study for glacial isostatic adjustment codes the study of glacial isostatic adjustment gia is gaining an increasingly im portant role within the geophysical community understanding the response of the earth to loading is crucial in various contexts ranging from the the interpretation of modern satellite geodetic measurements e g grace and goce to the projections of future sea level trends in response to climate change modern modeling approaches to gia are based on various techniques that range from purely analytical formulations to fully numerical methods despite various teams independently investigating gia we do not have a suitably large set of explicitly validated numerical results through which they may be validated a community benchmark data set would clearly be valuable following the example of the mantle convection community here we present for the first time the results of a benchmark study of codes designed to model gia the approaches benchmarked are based on significantly different codes and different techniques this effort is performed within the working group 4 wg4 of the esf cost action es0701 improved constraints on models of glacial isostatic adjustment our aims are i testing the codes currently in use by the various teams ii establish a sufficiently large set of agreed results iii correct possible systematic errors embedded in the various physical formulations or computer implementations and iv facilitate the dissemination of numerical tools for surface loading studies to the community and to young scientists the test computations are mainly based on models with spherical symmetry and maxwell rheology and include inputs from various different methods and solution techniques viscoelastic normal modes spectral finite elements and finite elements the tests involve the loading and tidal love numbers and their relaxation spectra the deformation and gravity variations driven by surface loads characterized by simple geometry and time history and the rotational fluctuations in response to glacial unloading in spite of the significant differences in the numerical methods employed the test computations show a satisfactory agreement between the results provided by the participants most of the existing misfits have been addressed during the preparation of the manuscript some others are currently the subject of analysis within the wg4 community

clearing observed pgr in grace data aimed at global viscosity inversion weighted mass trends technique 1 the time variable gravity field shows the effects of large present day mass variations associated with hydrological phenomena and melting ice sheets resulting in a shaded post glacial rebound pgr signal a meaningful viscosity inversion based on a global scale comparison of grace data and pgr predictions is thus hard to obtain we derive a weighted surface mass distribution in water equivalent starting from an initial guess which portrays the secular effects of present day phenomena the gravity field it generates is then carefully removed from grace data resulting in a gravity pattern where the effects of pgr are clearer and ready to be compared with the predictions on the basis of viscoelastic stratified earth models and different pleistocene deglaciation models we show that the quality of a global preliminary viscosity inversion greatly improves

postglacial sealevels on a spherical self gravitating viscoelastic earth effects of lateral viscosity variations in the upper mantle on the inference of viscosity contrasts in the lower mantle abstract previous studies of lateral viscosity variations on postglacial rebound often neglect the spherical shape of the earth and the self gravity of the solid earth and oceans in this paper the consistent sealevel equation is solved by the coupled laplace finite element method for a spherical self gravitating incompressible maxwell earth it confirms the importance of self gravity in the oceans on sealevel computation near the ice margin where the data are most sensitive to the existence of lateral variations in lithospheric thickness and asthenospheric viscosity the effects of lateral variations in lower mantle viscosity asthenospheric viscosity and lithospheric thickness are investigated this paper confirms the finding of earlier investigations the combined effect of lateral viscosity variations in the lower mantle with reverse viscosity contrasts in the upper mantle is also investigated for sealevel data near the center of rebound the effect of lateral viscosity contrast in the lower mantle can be masked by the existence of a reverse lateral viscosity contrasts in the upper mantle however this is not the case for sealevel data just outside the ice margin

deformation of an incompressible viscoelastic flat earth with powerlaw creep a finite element approach summary extrapolating experimental creep data to mantle conditions indicates that the upper mantle may deform as a power law medium in all of the previous models of loading unloading in a power law medium some simplifying assumptions were made to keep the calculation tractable unfortunately some of these assumptions are invalid while others are too restrictive in this paper the finite element method is used to study the deformation of incompressible linear and non linear rheological earth models that are subject to constant gravitational acceleration the advantage of this new approach is that none of these simplifying assumptions has to be made thus the solution is much more rigorous the deformation of a non linear half space due to heaviside loading and unloading of a boxcar load is shown to be characterized by i ubiquitous updoming sinking outside the load ii a very high initial rate of deformation followed by a much slower rate inside the load and iii the load history is fully coupled with the rheology of the earth because the state of stress caused by the magnitude of the load and the sequence of loading or unloading events determines the effective viscosity and thus the deformation it is also demonstrated that the usual assumption of self similarity and proportional relaxation are invalid moreover the relaxation time is a function of both space and time the effect of an elastic lithosphere on a linear or non linear channel is shown to be much larger than that for a half space even at small t the effects of a non linear channel a parabolic load and a depth dependent creep parameter a are also studied when the observed uplift data near the rebound centre in fennoscandia are compared to the predicted uplift curves it is demonstrated that for the instantaneous removal of a 15 mpa boxcar load power law rheological models cannot fit the observations as well as the linear models can however more work has to be done before one can decide whether the rebound process actually involves a mantle with linear rheology or power law rheology

saturn s northern hemisphere ribbon simulations and comparison with the meandering gulf stream voyager observations of saturn in 1980 81 discovered a wavy feature engirdling the planet at 47 n planetographic latitude its latitude coincides with that of an eastward jet stream which is the second fastest on saturn after the equatorial jet the 47 n jet s wavy morphology is unique among the known atmospheric jets on the gas giant planets since the voyagers it has been seen in every high resolution image of this latitude for over 25 years and has been termed the ribbon the ribbon has been interpreted as a dynamic instability in the jet stream this study tests this interpretation and uses forward modeling to explore the observed zonal wind profile s stability properties unforced initial value numerical experiments are performed to examine the nonlinear evolution of the jet stream parameter variations show that an instability occurs when the 47 n jet causes reversals in the potential vorticity pv gradient which constitutes a violation of the charney stern stability criterion after the initial instability development the simulations demonstrate that the instability s amplitude nonlinearly saturates to a constant when the eddy generation by the instability is balanced by the destruction of the eddies when the instability saturates the zonal wind profile approaches neutral stability according to arnol d s second criterion and the jet s path meanders in a ribbon like manner it is demonstrated that the meandering of the 47 n jet occurs over a range of tropospheric static stability and background wind speed the results here show that a nonlinearly saturated shear instability in the 47 n jet is a viable mechanism to produce the ribbon morphology observations do not yet have the temporal coverage to confirm the creation and destruction of eddies but these simulations predict that this is actively occurring in the ribbon region similarities exist between the behaviors found in this model and the dynamics of pv fronts studied in the context of meandering western boundary currents in earth s oceans in addition the simulations capture the nonlinear aspects of a new feature discovered by the cassini visual and infrared mapping spectrometer vims the string of pearls which resides in the equatorward tip of the 47 n jet the explicit planetary isentropic coordinate epic model is used herein

changes in rotation induced by pleistocene ice masses with stratified analytical earth models the rotational response of the earth to pleistocene deglaciation is studied by means of a multilayered viscoelastic earth model based on the preliminary reference earth model prem incompressible viscoelastic deformation is evaluated from a self compressed initial state the novelty of our approach stands on the application of a fully analytical normal mode theory to the response of the earth to surface loads and variations in the centrifugal potential for large numbers of viscoelastic layers as requested by prem assuming that both present day true polar wander tpw and changes in the second degree component of the geopotential are solely due to pleistocene postglacial rebound we obtain for a two layer viscosity model that the upper mantle viscosity must be lowered to about 1 5 1020 pa s with respect to the classical value of 1021 pa s this upper mantle viscosity is accompanied by an increase of the lower mantle viscosity by a factor of 25 in agreement with some recent relative sealevel rsl data analyses and convectively supported long wavelength geoid anomalies when the viscosity contrast is located at 1470 km depth tpw and require a viscosity of 1021 pa s in the upper part of the mantle above 1470 km depth with a moderate viscosity increase in the lowermost portion of the mantle this result indicates that a viscosity of 1021 pa s is appropriate for a wider portion of the mantle than the upper mantle in agreement with haskell s 1935 estimate that was not limited to the seismically inferred 670 km boundary

mantle viscosity from monte carlo inversion of very long baseline interferometry data using a monte carlo approach we determine the mantle viscosity profiles which best fit the currently observed very long baseline interferometry vlbi baseline lengths in north america our inversion which is based on the deglaciation model ice 1 and on the most recent vlbi global solution we employ the goddard space flight center vlbi solution 1102 which contains data through july 1998 ma and ryan 1998 demonstrates that the observations are best fit for a shallow upper mantle with viscosity below the traditional reference value of 1021 pa s the lower mantle viscosity cannot be unequivocally determined by means of vlbi data alone in fact our inversion indicates that two distinct solutions are possible the first is characterized by a lower mantle with viscosity of a few times 1021 pa s a value in agreement with that derived by the global set of relative sea level curves the second admissible solution corresponds to a lower mantle with viscosity close to 1023 pa s we have found that the viscosity of the transition zone may vary between 5 1021 and 5 1022 pa s i e larger than the lowest admissible values found for the lower mantle viscosity the results of the monte carlo inversion are essentially stable with respect to time since similar viscosity profiles are suggested if the vlbi global solution 1014 is employed which contained data through july 1995 however the increased precision of vlbi data since that time has somewhat narrowed the set of admissible viscosity profiles above the 670 km depth discontinuity choosing various subsets of solution 1102 we have also tested the robustness of the results presented with reference to possible data contaminations due to tectonic deformations

viscous gravitational relaxation summary this paper is concerned with a detailed examination of the response of maxwell models of the planet to surface mass loads particular attention is devoted to an examination of the factors which determine the isostatic response since the understanding of this response is crucial in a number of different geodynamic problems one particular example which we discuss in detail is concerned with the prediction of free air gravity anomalies produced by large scale deglaciation events using the methods developed here we are able to provide the first direct assessment of the importance of initial isostatic disequilibrium on the observed relative sea level variations and free air gravity anomalies forced by the melting of the laurentide ice sheet we are therefore able to estimate the extent to which such initial disequilibrium might influence the inference of mantle viscosity from isostatic adjustment data our calculations establish that free air gravity data although they are sensitive to the degree of initial disequilibrium provide an extremely high quality constraint upon the viscosity of the lower mantle

temporal gravity variations near shrinking vatnajokull ice cap iceland repeated gravity measurements were carried out from 1991 until 1999 at sites se of vatnajokull iceland to estimate the mass flow and deformation accompanying the shrinking of the ice cap published gps data show an uplift of about 13 5 mm a near the ice margin a gravity decrease of 2 1 gal a relative to the hofn base station was observed for the same sites control measurements at the hofn station showed a gravity decrease of 2 0 5 gal a relative to the station rvik 5473 at reykjavik about 250 km from hofn this is compatible as a bouguer effect with a 10 3 mm a uplift rate of the igs point at hofn and an uplift rate of 20 mm a near the ice margin although the derived gravity change rates at individual sites have large uncertainties the ensemble of the rates varies systematically and significantly with distance from the ice the relationship between gravity and elevation changes and the shrinking ice mass is modelled as response to the loading history the gps data can be explained by 1 d modelling i e an earth model with a 15 km thick elastic lithosphere and a 7 1017 pa s asthenosphere viscosity but not the gravity data based on 2 d modelling the gravity data favour a low viscosity plume in the form of a cylinder of 80 km radius and 1017 to 1018 pa s viscosity below a 6 km thick elastic lid embedded in a layered prem type earth although the elevation data are less well explained by this model strain porosity hydrology effects are likely to enhance the magnitude of the gravity changes but need verification by drilling more accurate data may resolve the discrepancies or suggest improved models

saturn s zonal wind profile in 2004 2009 from cassini iss images and its long term variability abstract five years of cassini imaging science subsystem images from 2004 to 2009 are analyzed in this work to retrieve global zonal wind profiles of saturn s northern and southern hemispheres in the methane absorbing bands at 890 and 727 nm and in their respective adjacent continuum wavelengths of 939 and 752 nm a complete view of saturn s global circulation including the equator at two pressure levels in the tropopause 60 mbar to 250 mbar with the mt filters and in the upper troposphere from 350 mbar to 500 mbar with the cb filter set is presented both zonal wind profiles available at the supplementary material section show the same structure but with significant differences in the peak of the eastward jets and the equatorial region including a region of positive vertical shear symmetrically located around the equator between the 10 c

dynamics of saturn s great storm of 2010 2011 from cassini iss and rpws saturn s quasi periodic planet encircling storms are the largest convecting cumulus outbursts in the solar system the last eruption was in 1990 sanchez lavega a 1994 chaos 4 341 353 a new eruption started in december 2010 and presented the first ever opportunity to observe such episodic storms from a spacecraft in orbit around saturn fischer g et al 2011 nature 475 75 77 sanchez lavega a et al 2011 nature 475 71 74 fletcher l n et al 2011 science 332 1413 here we analyze images acquired with the cassini imaging science subsystem iss which captured the storm s birth evolution and demise in studying the end of the convective activity we also analyze the saturn electrostatic discharge sed signals detected by the radio and plasma wave science rpws instrument the storm s initial position coincided with that of a previously known feature called the string of pearls sops at 33 n planetocentric latitude intense cumulus convection at the westernmost point of the storm formed a particularly bright head that drifted at 26 9 0 8 m s 1 negative denotes westward motion on january 11 2011 the size of the head was 9200 km and up to 34 000 km in the north south and east west dimensions respectively rpws measurements show that the longitudinal extent of the lightning source expanded with the storm s growth the storm spawned the largest tropospheric vortex ever seen on saturn on january 11 2011 the anticyclone was sized 11 000 km by 12 000 km in the north south and east west directions respectively between january and september 2011 the vortex drifted at an average speed of 8 4 m s 1 we detect anticyclonic circulation in the new vortex the vortex s size gradually decreased after its formation and its central latitude shifted to the north the storm s head moved westward and encountered the new anticyclone from the east in june 2011 after the head vortex collision the rpws instrument detected that the sed activities became intermittent and declined over 40 days until the signals became undetectable in early august in late august the sed radio signals resurged for 9 days the storm left a vast dark area between 32 n and 38 n latitudes surrounded by a highly disturbed region that resembles the mid latitudes of jupiter using iss images we also made cloud tracking wind measurements that reveal differences in the cloud level zonal wind profiles before and after the storm

saturn atmospheric dynamics one year after cassini long lived features and time variations in the drift of the hexagon abstract we examine saturn s atmospheric dynamics with observations in the visible range from ground based telescopes and hubble space telescope hst we present a detailed analysis of observations acquired during 2018 obtaining drift rates of major meteorological systems from the equator to the north polar hexagon a system of polar storms that appeared in the planet in march 2018 and remained active with a complex phenomenology at least until september is analyzed elsewhere sanchez lavega et al 2019 many of the regular cloud features visible in 2018 are long lived and can be identified in saturn images in 2017 and in some cases for up to a decade using also cassini iss images without considering the polar storms the most interesting long lived cloud systems are i a bright white spot in the equatorial zone that can be tracked continuously since 2014 with minimal changes in its zonal velocity which was 444 3 3 1 m s 1 in 2014 and 452 4 1 7 m s 1 in 2018 this velocity is remarkably different from the zonal winds at the cloud level at its latitude during the cassini mission and is closer to zonal winds obtained at the time of the voyagers flybys and to zonal winds from cassini vims infrared images of the lower atmosphere ii a large long lived anticyclone vortex here av that formed after the great white spot of 2010 2011 this vortex has changed significantly in visual contrast drift rate and latitude with minor changes in size over the last years iii a system of subpolar vortices at latitudes 60 65 n present at least since 2011 these vortices and additional atmospheric features here studied follow drift rates consistent with zonal winds obtained by cassini we also present a study of the positions of the vertices of saturn s north polar hexagon from 2015 to 2018 these measurements are compared with previous analyses during the cassini mission 2007 2014 observations with hst in the 90s and data from the voyagers in 1980 1981 to explore the long term variability of the hexagon s drift rate we find variations in the drift rate of the hexagon through these epochs that can not be fit by seasonal changes in the polar area instead the different drift rates reinforce the role of the north polar spot that was present in the voyager epoch and in the early 90s to cause a faster drift rate of the hexagon at that time compared with the current slower one

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

saturn s cloud structure inferred from cassini iss abstract using high resolution cassini iss images with wavelengths ranging from the ultraviolet to the near infrared we have retrieved saturn s atmospheric aerosol structure and properties for a broad range of latitudes in the southern hemisphere the observations are consistent with two distinct layers of haze above a scattered cloud each layer is characterized by a vertical location an optical thickness and an effective mean particle size all of which vary with latitude the tropospheric haze is optically thickest and extends to the greatest heights 40 mbar over the equator its top surface is at significantly greater depths 150 mbar at mid latitudes the height of the haze correlates well with position of the tropopause as indicated by the temperature field beneath this haze we find a scattered denser cloud responsible for small scale contrasts at an average depth of 1 75 0 4 bar with some features as deep as 2 5 bar or greater

thermal structure and dynamics of saturn s northern springtime disturbance saturn s slow seasonal evolution was disrupted in 2010 2011 by the eruption of a bright storm in its northern spring hemisphere thermal infrared spectroscopy showed that within a month the resulting planetary scale disturbance had generated intense perturbations of atmospheric temperatures winds and composition between 20 and 50 n over an entire hemisphere 140 000 kilometers the tropospheric storm cell produced effects that penetrated hundreds of kilometers into saturn s stratosphere to the 1 millibar region stratospheric subsidence at the edges of the disturbance produced beacons of infrared emission and longitudinal temperature contrasts of 16 kelvin the disturbance substantially altered atmospheric circulation transporting material vertically over great distances modifying stratospheric zonal jets exciting wave activity and turbulence and generating a new cold anticyclonic oval in the center of the disturbance at 41 n