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

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

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

experiments on granular flows to predict avalanche runup design of deflectors or barriers to slow or stop snow avalanche debris in the runout zone requires estimates of runup height in this paper experimental data on runup of dense dry granular flows in a flume are presented the data are then compared with two one dimensional theoretical equations for runup estimation 1 a formulation based on following the leading edge of the flow up the barrier and 2 the traditional method adapted from equations presented by a voellmy for describing the centre of mass of the avalanche the results show that the leading edge model more closely matches the experimental results and in field applications the leading edge model is expected to provide higher and therefore more conservative runup estimates key words avalanche run up granular flows avalanche defences runout zone

three dimensional displacement field of the 2015 mw8 3 illapel earthquake chile from across and along track sentinel 1 tops interferometry wide swath imaging has become a standard acquisition mode for radar missions aiming at applying synthetic aperture radar interferometry insar at global scale with enhanced revisit frequency increased swath width compared to classical stripmap imaging mode is achieved at the expense of azimuthal resolution this makes along track displacements and subsequently north south displacements difficult to measure using conventional split beam multiple aperture insar or cross correlation techniques alternatively we show here that the along track component of ground motion can be deduced from the double difference between backward and forward looking interferograms within regions of burst overlap burst overlap interferometry takes advantage of the large squint angle diversity of sentinel 1 1 to achieve subdecimetric accuracy on the along track component of ground motion we demonstrate the efficiency of this method using sentinel 1 data covering the 2015 mw8 3 illapel earthquake chile for which we retrieve the full 3 d displacement field and validate it against observations from a dense network of gps sensors

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

saturn s satellite rhea is a homogeneous mix of rock and ice 1 a close flyby of saturn s satellite rhea by the cassini spacecraft on 26 november 2005 can be used to infer its mass and quadrupole gravity moments from these data we derive a mean density of 1233 5 kg m 3 for rhea and an axial moment of inertia consistent with an undifferentiated interior the small density implies an interior made up of about 25 rock metal and 75 water ice by mass the moment of inertia implies a homogeneous mix of these constituents with some compression of the ice and the transition from ice i to ice ii at depth

rhea s gravitational field and interior structure inferred from archival data files of the 2005 cassini flyby abstract there are three published gravitational fields for rhea and they all differ significantly all three published fields fit the data equally well although not to the noise level there are obvious trends in the doppler residuals however by restricting the data in the fit to a shorter interval surrounding the closest approach to rhea an unbiased estimate for rhea s quadrupole gravitational field is obtained this paper also investigates the effects of thermal emission which are shown to be quite important and may explain part of the discrepancy between this study and previous works with this approach and under the assumption that rhea is in hydrostatic equilibrium he our earlier conclusions about rhea s internal structure are confirmed rhea is an undifferentiated satellite made up of about 25 rock metal and 75 water ice by mass the best fit value of the quadrupole gravitational coefficient c 22 from the present analysis is 267 6 4 9 1 0 6 further any assertions that rhea is not in he are not supported by the data and only an additional highly inclined flyby to rhea s equator could resolve this issue provided rhea were not in he no useful constraint on rhea s interior structure could be imposed non he contributions to the first order deviations from sphericity considered here are not readily available

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 tidal history of iapetus spin dynamics in the light of a refined dissipation model 1 we study the tidal history of an icy moon basing our approach on a dissipation model which combines viscoelasticity with anelasticity and takes into account the microphysics of attenuation we apply this approach to iapetus the most remote large icy moon in the saturnian system different authors provide very different estimates for iapetus s despinning timescale by several orders of magnitude one reason for these differences is the choice of the dissipation model used for computing the spin evolution as laboratory data on viscoelastic properties of planetary ices are sparse many studies relied on dissipation models that turned out to be inconsistent with experiment a pure water ice composition generally assumed in the previous studies of the kind yields despinning times of the order of 3 7 gyr for most initial conditions we demonstrate that through accounting for the complexity of the material like second phase impurities one arrives at despinning times as short as 0 9 gyr a more exact estimate will remain unavailable until we learn more about the influence of impurities on ice dissipation by including the triaxial shape caused torque we encounter a chaotic behavior at the final stage of despinning with the possibility of entrapments in the intermediate resonances the duration of these entrapments turns out to be sensitive to the dissipation model no long entrapments have been found for iapetus described with our laboratory based dissipation model

gravity driven granular free surface flow around a circular cylinder snow avalanches and other hazardous geophysical granular flows such as debris flows lahars and pyroclastic flows often impact on obstacles as they flow down a slope generating rapid changes in the flow height and velocity in their vicinity it is important to understand how a granular material flows around such obstacles to improve the design of deflecting and catching dams and to correctly interpret field observations in this paper small scale experiments and numerical simulations are used to investigate the supercritical gravity driven free surface flow of a granular avalanche around a circular cylinder our experiments show that a very sharp bow shock wave and a stagnation point are generated in front of the cylinder the shock standoff distance is accurately reproduced by shock capturing numerical simulations and is approximately equal to the reciprocal of the froude number consistent with previous approximate results for shallow water flows as the grains move around the cylinder the flow expands and the pressure gradients rapidly accelerate the particles up to supercritical speeds again the internal pressure is not strong enough to immediately push the grains into the space behind the cylinder and instead a grain free region or granular vacuum forms on the lee side for moderate upstream froude numbers and slope inclinations the granular vacuum closes up rapidly to form a triangular region but on steeper slopes both experiments and numerical simulations show that the pinch off distance moves far downstream

a simple analytical model for pressure on obstacles induced by snow avalanches the forces snow avalanches are able to exert on protection dams or buildings are of crucial interest in order to improve avalanche mitigation measures and to quantify the mechanical vulnerability of structures likely to be damaged by snow avalanches this paper presents an analytical model that is able to calculate these forces taking into account dead zone mechanisms first we present a 2 d analytical hydrodynamic model describing the forces on a wall overflown by gravity driven flows down an inclined plane second the 2 d model is successfully validated on discrete simulations of granular flows third we provide ingredients to extend the 2 d model to flows of dry and cold snow fourth we propose a simplified 3 d analytical model taking into account lateral fluxes finally the predictions from the simplified 3 d analytical model are successfully compared to recent measurements on two full scale snow avalanches released at the lautaret site in france

deflecting dams and the formation of oblique shocks in snow avalanches at flateyri iceland snow avalanches are a threat in many populated mountainous regions and deflecting dams are often built to divert them away from people and infrastructure into less harmful areas when an avalanche is deflected by a dam or wedge it often generates rapid changes in the flow thickness and velocity which can be modeled as an oblique shock wave this paper reviews classical oblique shock theory which was originally developed for shallow water flows and uses it to make predictions of the maximum runup height on a deflecting dam the downstream flow velocity and the width of the channelized stream the theory is used to investigate field observations of snow avalanches at flateyri in iceland where a dam has deflected two avalanches away from the town and produced a channelized stream that flowed parallel to the dam the results indicate that there is no one single set of upstream flow conditions that parameterizes the flow behavior but the solution evolves as the avalanche propagates along the dam in response to the deceleration imposed by the slope fully time dependent shock capturing numerical simulations of the skollahvilft avalanche which hit the dam on 21 february 1999 are used to show how the channelized stream widens as the avalanche slows down and thickens toward the end of the runout zone the oblique shock relations nevertheless provide useful local order of magnitude estimates for the flow conditions immediately upstream of the shock

viscosity of the lithosphere of enceladus abstract high resolution voyager ii images of enceladus reveal that some regions on its surface are highly cratered the most heavily cratered surfaces probably date back to a period of heavy bombardment the forms of many of the craters on enceladus are similar to those of fresh lunar craters but many of the craters are much shallower in depth and the floors of some craters are bowed up the flattering of craters and bowing up of the floors are indicative of viscous relaxation of the topography analysis of the forms of the flattened craters suggests that the viscosity at the top of the lithosphere in the most heavily cratered regions is between 10 24 and 10 25 p the exact time scale for the collapse of the craters is not known but probably was between 100 my and 4 gy the flattened craters are located in distinct zones that are adjacent to zones of similar age where craters have not flattened the zones where flattened craters occur possibly are regions in which the heat flow was or is higher than that in the adjacent terrains because the temperature at the top of the lithosphere of enceladus would be less than or equal to that of ganymede and callisto if it is covered by a thick regolith and because the required viscosity on enceladus is one to two orders of magnitude less than that for ganymede and callisto it can be concluded that the lithospheric material on enceladus is different from that of ganymede and callisto enceladus probably has a mixture of ammonia ice and water ice in the lithosphere whereas the lithospheres of ganymede and callisto are composed primarily of water ice

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

shock waves dead zones and particle free regions in rapid granular free surface flows shock waves dead zones and particle free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography understanding and modelling these flows is of considerable practical interest for industrial processes as well as for the design of defences to protect buildings structures and people from snow avalanches debris flows and rockfalls these flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models in this paper a simple hydraulic theory first suggested in the russian literature is generalized to model quasi two dimensional flows around obstacles exact and numerical solutions are then compared with laboratory experiments these indicate that the theory is adequate to quantitatively describe the formation of normal shocks oblique shocks dead zones and granular vacua such features are generated by the flow around a pyramidal obstacle which is typical of some of the defensive structures in use today

a potential thermal erosion lava channel on io 1 we have discovered a prominent 190 km long 0 5 6 km wide lava channel on io the channel is sinuous with interior islands and may be associated with the active tawhaki patera hotspot photoclinometric analysis of the galileo images indicates that this channel provisionally named tawhaki vallis is 40 65 m deep although a constructional contribution cannot be ruled out the depth morphology and sinuosity of the channel is consistent with erosion by lava erosion of flowing silicate over silicate substrate or flowing sulfur over sulfur substrate likely requires eruption durations of days to months to form an 50 m deep channel whereas flowing silicate over a sulfur substrate or flowing sulfur over a frozen so2 substrate would likely require only hours to days future spacecraft observation of actively forming lava channels on io are possible and desirable

the topography of iapetus leading side abstract we have used cassini stereo images to study the topography of iapetus leading side a terrain model derived at resolutions of 4 8 km reveals that iapetus has substantial topography with heights in the range of 10 km to 13 km much more than observed on the other middle sized satellites of saturn so far most of the topography is older than 4 ga neukum g wagner r denk t porco c c 2005 lunar planet sci xxxvi abstract 2034 which implies that iapetus must have had a thick lithosphere early in its history to support this topography models of lithospheric deflection by topographic loads provide an estimate of the required elastic thickness in the range of 50 100 km iapetus prominent equatorial ridge porco c c and 34 colleagues 2005 science 307 1237 1242 reaches widths of 70 km and heights of up to 13 km from their base within the modeled area the morphology of the ridge suggests an endogenous origin rather than a formation by collisional accretion of a ring remnant ip w h 2006 geophys res lett 33 doi 10 1029 2005gl025386 l16203 the transition from simple to complex central peak craters on iapetus occurs at diameters of 11 3 km the central peaks have pronounced conical shapes with flanking slopes of typically 11 and heights that can rise above the surrounding plains crater depths seem to be systematically lower on iapetus than on similarly sized rhea which if true may be related to more pronounced crater wall slumping which widens the craters on iapetus than on rhea there are seven large impact basins with complex morphologies including central peak massifs and terraced walls the largest one reaches 800 km in diameter and has rim topography of up to 10 km generally no rings are observed with the basins consistent with a thick lithosphere but still thin enough to allow for viscous relaxation of the basin floors which is inferred from crater depth to diameter measurements in particular a 400 km basin shows up domed floor topography which is suggestive of viscous relaxation a model of complex crater formation with a viscoplastic bingham rheology melosh h j 1989 impact cratering oxford univ press new york of the impact shocked icy material provides an estimate of the effective cohesion viscosity at 0 04 0 01 mpa 0 6 0 2 gpa s the local distribution of bright and dark material on the surface of iapetus is largely controlled by topography and consistent with the dark material being a sublimation lag deposit originating from a bright icy substrate mixed with the dark components but frost deposits are possible as well

iapetus geophysics rotation rate shape and equatorial ridge iapetus has preserved evidence that constrains the modeling of its geophysical history from the time of its accretion until now the evidence is a its present 79 33 day rotation or spin rate b its shape that corresponds to the equilibrium figure for a hydrostatic body rotating with a period of 16 h and c its high equatorial ridge which is unique in the solar system this paper reports the results of an investigation into the coupling between iapetus thermal and orbital evolution for a wide range of conditions including the spatial distributions with time of composition porosity short lived radioactive isotopes slri and temperature the thermal model uses conductive heat transfer with temperature dependent conductivity only models with a thick lithosphere and an interior viscosity in the range of about the water ice melting point can explain the observed shape short lived radioactive isotopes provide the heat needed to decrease porosity in iapetus early history this increases thermal conductivity and allows the development of the strong lithosphere that is required to preserve the 16 h rotational shape and the high vertical relief of the topography long lived radioactive isotopes and slri raise internal temperatures high enough that significant tidal dissipation can start and despin iapetus to synchronous rotation this occurred several hundred million years after iapetus formed the models also constrain the time when iapetus formed because the successful models are critically dependent upon having just the right amount of heat added by slri decay in this early period the amount of heat available from short lived radioactivity is not a free parameter but is fixed by the time when iapetus accreted by the canonical concentration of 26al and to a lesser extent by the concentration of 60fe the needed amount of heat is available only if iapetus accreted between 2 5 and 5 0 myr after the formation of the calcium aluminum inclusions as found in meteorites models with these features allow us to explain iapetus present synchronous rotation its fossil 16 h shape and the context within which the equatorial ridge arose

dry flowing avalanche run up and run out the prediction of run up and run out of dry flowing avalanches is very important for land use planning design of run out zone defenses and construction of risk maps in avalanche terrain in this paper we present a numerical dynamics model to predict the stop position of the tip of these avalanches when friction coefficients internal and external initial flow depth and incoming speed are specified for known path geometry in the run out zone we also compare our model to that in the swiss guidelines and to field examples the results of these calculations clearly define model differences and the implications of different choices of friction coefficients

geophysical implications of the long wavelength topography of rhea 1 we use limb profiles to investigate the long wavelength topography and topographic variance spectrum of rhea one dimensional variance spectra show a break in slope at a wavelength of 300 km a similar effect is seen on the moon and may be a signature of an elastic lithosphere having a thickness te 10 km the implied heat flux is 15 mw m 2 much higher than can be explained by radiogenic heating we use the 1 d spectral behavior to constrain our solution for the long wavelength global topography of rhea the degree 3 topography is large enough if uncompensated to contaminate estimates of the degree 2 gravity using existing flyby data current models of rhea internal structures which rely on these degree 2 estimates may thus be inaccurate illustrating the need to acquire further rhea gravity data

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

measurement and analysis of lunar basin depths from clementine altimetry altimetric profiles from the clementine lidar are used to calculate the depths of 29 large craters and basins on the moon plotting the depths of the best preserved structures together with values for simple and complex craters measured in pre clementine studies reveals an inflection in the depth diameter d d curve in addition to the one revealed by pre clementine data this inflection occurs in the diameter range that corresponds to the morphologic transition from complex crater to basin the best empirical power law fit for basin depths is log10 d 0 41 log10 d 0 57 this relationship is characterized by a lower slope than that for complex craters demonstrating that this morphologic transition corresponds to a further decrease in the depth of an impact structure relative to its diameter with increasing size qualitative consideration of possible causes for the second inflection leads to the conclusion that it is most likely a consequence of a short term modification mechanism that influences fundamental crater morphology such as the increasing influence of gravity with diameter thicknesses of maria in the major basins are calculated by assuming that their unfilled depths would follow thed drelation results are compared with previous estimates and yield thicknesses that are generally greater than those determined by studies of flooded craters and less than those obtained from analysis of gravity

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

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

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

origin of mountains on io by thrust faulting and large scale mass movements voyager stereoimages of euboea montes io indicate that this mountain formed when a large crustal block was uplifted 10 5 kilometers and tilted by approximately 6 degrees uplift triggered a massive slope failure on the northwest flank forming one of the largest debris aprons in the solar system this slope failure probably involved relatively unconsolidated layers totaling approximately 2 kilometers in thickness overlying a rigid crust or lithosphere at least 11 kilometers thick mountain formation on io may involve localized deep rooted thrust faulting and block rotation due to compression at depth induced during vertical recycling of io s crust

geology and topography of ra patera io in the voyager era prelude to eruption voyager era stereo images are used to map the geology and topography of ra patera a major active volcanic center and possible site of sulfur eruptions on io the summit of ra patera reaches only 1 km above the surrounding plains pre voyager era lava flows occur on slopes of 0 1 0 3 comparable to the lunar mare these flows were emplaced at either low viscosities high eruption rates or both a 600 km long ridged mountain unit rising to 8 km near carancho patera forms a 60 by 90 km wide plateau 0 5 km high 50 km east of ra patera the new lava flows observed by galileo flowed around the southern edge of this plateau

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