resurfacing history of europa from pole to pole geological mapping abstract we produced geologic maps from two regional mosaics of galileo images across the leading and trailing hemispheres of europa in order to investigate the temporal distribution of units in the visible geologic record five principal terrain types were identified plains bands ridges chaos and crater materials which are interpreted to result from 1 tectonic fracturing and lineament building 2 cryovolcanic reworking of surface units with possible emplacement of sub surface materials and 3 impact cratering the geologic histories of both mapped areas are essentially similar and reflect some common trends tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments the opening of wide bands with infilling of inter plate gaps and the buildup of ridges and ridge complexes along prominent fractures in the ice it also appears that lineaments are narrower and more widely spaced with time the lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation in contrast the degree of cryovolcanic resurfacing appears to increase with time as chaos formation dominates the later parts of the geologic record these trends and the transition from tectonic to cryovolcanic dominated resurfacing could be attributed to the gradual thickening of europa s cryosphere during the visible geologic history that comprises the last 2 or 30 80 myr of europa s history an originally thin brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes the degree of fracturing and plate displacements decreased with time in a thickening shell and lineaments became narrower and more widely spaced formation of chaos regions could have occurred where the thickness threshold for solid state convection was exceeded and can be aided by preferential tidal heating of more ductile ice in a long term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite or cyclic or irregular episodes of tectonic and cryovolcanic activity

cryovolcanic features on titan s surface as revealed by the cassini titan radar mapper abstract the cassini titan radar mapper obtained synthetic aperture radar images of titan s surface during four fly bys during the mission s first year these images show that titan s surface is very complex geologically showing evidence of major planetary geologic processes including cryovolcanism this paper discusses the variety of cryovolcanic features identified from sar images their possible origin and their geologic context the features which we identify as cryovolcanic in origin include a large 180 km diameter volcanic construct dome or shield several extensive flows and three calderas which appear to be the source of flows the composition of the cryomagma on titan is still unknown but constraints on rheological properties can be estimated using flow thickness rheological properties of one flow were estimated and appear inconsistent with ammonia water slurries and possibly more consistent with ammonia water methanol slurries the extent of cryovolcanism on titan is still not known as only a small fraction of the surface has been imaged at sufficient resolution energetic considerations suggest that cryovolcanism may have been a dominant process in the resurfacing of titan

deep earth structure major developments in earth structure in the last four years have been concentrated in the description of the earth s lateral heterogeneity the regions that are heterogenous and the per cent variation of velocity and density in each region most studies find that lateral variation is concentrated in the upper 400 and lower 200 km of the mantle a radially symmetric earth model has been defined that represents the best average fit to seismic data in a broad frequency band sampling many regions p and s velocity is found to increase in zones of 50 km or less at 400 and 650 to 700 km depth the model is transversely anisotropic in the upper mantle it possesses a vertical axis of symmetry such that the elastic constants are different for vertical propagation than for horizontal and intermediate angles of propagation the real earth generally exhibits azimuthal anisotropy as well but the azimuthal anisotropy cannot be resolved by a global average of data the nature and magnitude of the anisotropy agrees with that found in ultramafic samples of the upper mantle in attenuation models of intrinsic attenuation have included the dispersive properties of intrinsic anelasticity and constructed relaxation models consistent with an observed frequency dependence of q in the body wave band there has been progress in mapping the scattering properties of the lithosphere attenuation due to scattering in the crust and lithosphere has been shown to have strong effects on the amplitudes of seismic waves at local and teleseismic distances

origins of high frequency scattered waves near pkkp from large aperture seismic array data this article identifies the likely origin of 1 hz scattered waves in the vicinity of pkkp by comparing measurements of slowness and onset time to ray theoretical predictions the measurements are obtained from slant stacks of large aperture seismic array lasa data from 36 earthquakes and six explosions in the range 30 116 three types of scattered waves explain the main features seen in the stacks including p scattered to pkp near the earth9s surface p pkp pkkp scattered near its core mantle boundary cmb reflection point pk kp and skkp scattered near its cmb reflection point sk kp the lasa stacks image the amplitude and slowness variations of the scattered waves with time they also show where these waves can be detected and where they are free from contaminating arrivals sk kp waves rise above the noise approximately 100 sec before the onset time of the main skkp arrival near 113 observations of pk kp span 30 100 however at distances greater than 50 they suffer from p pkp contamination at distances less than 40 the pk kp last for about 280 sec this is approximately 130 sec longer than the maximum ray theoretical prediction for waves scattered at the cmb indicating a possible combination of near surface scattering and contributions from the overlying mantle

fluid volcanism on miranda and ariel flow morphology and composition several types of volcanic units have been recognized on the icy uranian satellites miranda and ariel on miranda ridges characterized by crest grooves are up to 10 km wide and 500 m high a continuous flat topped flow band also 10 km wide and 500 m high forms the outer southern margin of elsinore corona elsinore coronae at least appears to comprise coalesced flow bands and ridges on ariel in addition to at least one ridge unit similar to these on miranda flood plains material has covered the floors of deep chasmata grabens and an irregular depression flows on both satellites are characterized by linear vent geometries and distinct topographic margins which indicate extrusion of a relatively viscous material the topography of the flows can be used to estimate flow viscosity or yield strength using a bingham plastic model extrusion viscosity estimates incorporating plausible volcanologically based emplacement time scales and a rigid crust correction range from 107 to 109 p 1013 p in the unlikely absence of a chilled crust viscosity estimates are dependent on the assumed emplacement time scale however and could be as high as 1016 p if a solid stale based time scale is assumed bingham models indicate that it is yield strength that controls the final flow width and thickness that we see yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale yield strength estimates range from 10 1 to 10 2 bars consistent with yield strengths of partially crystallized silicate melts on other planets nevertheless both the viscosities and yield strengths are well within the range of and in some cases lower than silicate flows silicate compositions are not possible for these flows because of the ice rich composition of these satellites measured properties of ammonia water liquid crystal mixtures are consistent with the observed and estimated flow rheologic properties however and extrusion of such material s is a viable alternative to solid state extrusion for the origin of these flow features

metasomatic clathrate xenoliths as a possible source for the south polar plumes of enceladus abstract the composition and abundance of volatile gases observed in the jets emanating from fissures near the south pole of saturn s moon enceladus are strongly indicative of outgassing from clathrate hydrates which formed as a result of hydrothermal activity rather than nebula condensation i suggest that fluids must be able to permeate the ice shell extensively metasomatising the mantle by emplacement of clathrates along fractures and grain boundaries which subsequently are entrained in rising cryomagmas as xenoliths these are carried upwards to the point where they dissociate releasing their gas load into the magma and promoting the vigorous ice fountaining observed a direct analogue of terrestrial basaltic fire fountains caused by volatile exsolution this clathrate xenolith model can explain the measured volatile abundances eruption velocities the ice to vapour ratio in the plumes and the vent temperatures

identifying regions of strong scattering at the core mantle boundary from analysis of pkkp precursor energy abstract we detect seismic scattering from the core mantle boundary related to the phase pkkp pk kp in data from small aperture seismic arrays in india and canada the detection of these scattered waves in data from small aperture arrays is new and allows a better characterization of the fine scale structure of the deep earth especially in the southern hemisphere their slowness vector is determined from array processing allowing location of the heterogeneities at the core mantle boundary using back projection techniques through 1d earth models we identify strong scattering at the core mantle boundary cmb beneath the caribbean patagonia and the antarctic peninsula as well as beneath southern africa an analysis of the scattering regions relative to sources and receivers indicates that these regions represent areas of increased scattering likely due to increased heterogeneities close to the cmb the 1 hz array data used in this study is most sensitive to heterogeneity with scale lengths of about 10 km given the small size of the scatterers a chemical origin of the heterogeneities is likely by comparing the location of the fine scale heterogeneity to geodynamical models and tomographic images we identify different scattering mechanisms in regions related to subduction caribbean and patagonia and dense thermo chemical piles southern africa

scattered p p waves observed at short distances we detect previously unreported 1 hz scattered waves at epicentral dis tances between 30 and 50 and at times between 2300 and 2450 s after the earthquake origin these waves likely result from off azimuth scattering of pkpbc to pkpbc in the upper mantle and crust and provide a new tool for mapping variations in fine scale 10 km mantle heterogeneity array beams from the large aperture seismic array lasa clearly image the scattered energy gradually emerging from the noise and reaching its peak amplitude about 80 s later and returning to the noise level after 150 s stacks of transverse versus radial slowness t r show two peaks at about 2 2 and 2 2 s indicating the waves arrive along the major arc path 180 to 360 and significantly off azimuth we propose a mantle and surface pkpbc to pkpbc scattering mechanism for these observations because 1 it agrees with the initiation time and distinctive slowness signature of the scattered waves and 2 it follows a scattering path analogous to previously observed deep mantle pkkpscat tering chang and cleary 1981 the observed upper mantle scattered waves and pkkp waves fit into a broader set of scattered waves that we call p 0 dp 0 which can scatter from any depth d in the mantle

seismic modelling of lateral heterogeneity at the base of the mantle summary results from several recent studies suggest that there are lateral heterogeneities of up to a few per cent in the lowermost 150 200 km of the mantle bullen s d region inferred anomaly sizes span the range from less than 50 km to greater than 1000 km in this study differences in the velocity structure among regions at the base of the mantle were inferred from an analysis of amplitude ratios of pkpab and pkpdf for given earthquake station pairs at distances greater than 155 sacks snoke beach we distinguish two kinds of regions a anomalous regions in which the mean median and spread in ab df amplitude ratios are significantly higher 50 per cent than for a reference radial earth model and n normal regions in which the distribution of the amplitude ratios is as expected the ab branch has near grazing incidence to the core and therefore maximum sensitivity to velocity structure compared to the near normal incident df phases using an iterative forward modelling approach we have determined general characteristics of the velocity structure for regions at the base of the mantle which can produce amplitude ratio distributions similar to those for an a region agreement between model and data is obtained over the period range from 0 5 s to greater than 10 s using a laterally heterogeneous model for the d region the model consists of cells which are 200 km in lateral extent with velocity variations of up to 1 per cent this structure is modulated by a region wide 1000km perturbation which increases smoothly from zero at the edges of the region to a negative 1 per cent at the centre small cells 40 km cannot produce anomalously large amplitude long period ab arrivals and larger cells 1000km cannot match the observed scatter the 200 km scale anomalies could be small scale convection cells confined to the d region

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

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

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

ammonium sulfate on titan possible origin and role in cryovolcanism abstract we model the chemical evolution of titan wherein primordial nh3 reacts with sulfate rich brines leached from the silicate core during its hydration the resulting differentiated body consists of a serpentinite core overlain by a high pressure ice vi mantle a liquid layer of aqueous ammonium sulfate and a heterogeneous shell of methane clathrate low pressure ice ih and solid ammonium sulfate cooling of the subsurface ocean results in underplating of the outer shell with ice ih this gravitationally unstable system can produce compositional plumes as ice ih ascends buoyantly ice plumes may aid in advection of melt pockets through the shell and in combination with surface topography provide the necessary hydraulic pressure gradients to drive such melts to the surface moreover contact between the magma and wall rock methane clathrate will allow some methane to dissolve in the magma as well as eroding fragments of wall rock that can be transported as xenoliths upon rising to the clathrate decomposition depth 2 mpa or 1700 m the entrained xenoliths will break down to ice methane gas powering highly explosive eruptions with lava fountains up to several kilometers high hence we predict that titan is being resurfaced by cryoclastic ash consisting of ice and ammonium sulfate or its tetrahydrate providing an abundance of sedimentary grains a potential source of bedload for fluvial transport and erosion and of sand sized material for aeolian transport and dune building the infrared reflectance spectrum of ammonium sulfate makes it a plausible candidate for the 5 m bright material on titan s surface

a clathrate reservoir hypothesis for enceladus south polar plume we hypothesize that active tectonic processes in the south polar terrain of enceladus the 500 kilometer diameter moon of saturn are creating fractures that cause degassing of a clathrate reservoir to produce the plume documented by the instruments on the cassini spacecraft advection of gas and ice transports energy supplied at depth as latent heat of clathrate decomposition to shallower levels where it reappears as latent heat of condensation of ice the plume itself which has a discharge rate comparable to old faithful geyser in yellowstone national park probably represents small leaks from this massive advective system

the influence of thermal inertia on temperatures and frost stability on triton abstract seasonal subsurface heat conduction can have a large influence on triton s n2 frost distribution increasing surface thermal inertia reduces the thickness and extent of seasonal n2 frosts if the thermal inertia of the nonvolatile substrate is greater than about 30 of the value for nonporous h2o or co2 or if nonporous h2o or co2 is overlaid by a porous regolith of low thermal inertia but less than a few meters thick the northernmost latitudes visible to voyager should have been frost free at the time of the encounter possibly accounting for their relatively low albedo if the substrate in the northern hemisphere has sufficiently low albedo and or emissivity and also has a thermal inertia comparable to that of nonporous h2o or co2 there may be no seasonal or permanent n2 deposits in the northern hemisphere at all because this model like previous ones predicts a monotonic recession of permanent n2 deposits toward the poles and very limited seasonal n2 frost in the southern hemisphere at voyager time and because of new spectroscopic evidence for nonvolatile co2 on triton s bright southern hemisphere we consider it possible that much of the bright material on triton s southern hemisphere is not n2 bright nonvolatiles in the southern hemisphere may allow seasonal n2 frosts to form there during the southern summer possibly helping to explain triton s spectroscopic changes during the past decade all the models considered here predict 10 fold or greater seasonal variations in atmospheric pressure with pressure currently increasing in high thermal inertia models and decreasing in models with low thermal inertia

thermal evolution of kuiper belt objects with implications for cryovolcanism abstract we investigate the internal thermal evolution of kuiper belt objects kbos small radii 2500 kg m 3 bodies orbiting beyond neptune focusing on pluto s moon charon in particular our calculations are time dependent and account for differentiation we review evidence for ammonia hydrates in the ices of kbos and include their effects on the thermal evolution a key finding is that the production of the first melt at the melting point of ammonia dihydrate 176 k triggers differentiation of rock and ice the resulting structure comprises a rocky core surrounded by liquids and ice enclosed within a 100 km thick undifferentiated crust of rock and ice this structure is especially conducive to the retention of subsurface liquid and bodies the size of charon or larger radii 600 km are predicted to retain some subsurface liquid to the present day we discuss the possibility that this liquid can be brought to the surface rapidly via self propagating cracks we conclude that cryovolcanism is a viable process expected to affect the surfaces of large kbos including charon

impact of the global warming on the fluvial thermal erosion over the lena river in central siberia 1 the hydrology of the lena and its tributaries is characterized by an extremely episodic flow regime here we report recent climatic change in central siberia and its impact on the fluvial thermal erosion we point out three major changes since the 1980s a marked reduction of the river ice thickness in winter a pronounced increase of the water stream temperature in spring and a slight increase of the discharge during the break up may june a gis analysis based on aerial pictures and satellite images highlights the impact of the water warming on the frozen banks the vegetated islands appear to be very sensitive to the water temperature increase showing an acceleration of their head retreat 21 29 this suggests that recent global warming directly affects the fluvial dynamics and the erosional process of one of the largest arctic fluvial system

dynamical links between small and large scale mantle heterogeneity seismological evidence abstract we identify pkp pkp scattered waves also known as p p from earthquakes recorded at small aperture seismic arrays at distances less than 65 p p energy travels as a pkp wave through the core up into the mantle then scatters back down through the core to the receiver as a second pkp p p waves are unique in that they allow scattering heterogeneities throughout the mantle to be imaged we use array processing methods to amplify low amplitude coherent scattered energy signals and resolve their incoming direction we deterministically map scattering heterogeneity locations from the core mantle boundary to the surface we use an extensive dataset with sensitivity to a large volume of the mantle and a location method allowing us to resolve and map more heterogeneities than have previously been possible representing a significant increase in our understanding of small scale structure within the mantle our results demonstrate that the distribution of scattering heterogeneities varies both radially and laterally scattering is most abundant in the uppermost and lowermost mantle and a minimum in the mid mantle resembling the radial distribution of tomographically derived whole mantle velocity heterogeneity we investigate the spatial correlation of scattering heterogeneities with large scale tomographic velocities lateral velocity gradients the locations of deep seated hotspots and subducted slabs in the lowermost 1500 km of the mantle small scale heterogeneities correlate with regions of low seismic velocity high lateral seismic gradient and proximity to hotspots in the upper 1000 km of the mantle there is no significant correlation between scattering heterogeneity location and subducted slabs between 600 and 900 km depth scattering heterogeneities are more common in the regions most remote from slabs and close to hotspots scattering heterogeneities show an affinity for regions close to slabs within the upper 200 km of the mantle the similarity between the distribution of large scale and small scale mantle structures suggests a dynamic connection across scales whereby mantle heterogeneities of all sizes may be directed in similar ways by large scale convective currents

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

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

heat flows through the ice lithosphere of europa some of the geological features on the surface of europa s ice shell suggest the existence of a brittle lithosphere that in some regions is no more than 2 km thick the surface heat flow needed to put the brittle ductile transition in ice at a depth of 2 km is at least 100 mw m 2 much higher than predicted by tidal heating models for a purely conductive ice shell a possible explanation lies in the probable existence of a convective layer beneath the brittle lithosphere which owing to tidal dissipation in warm ice could contribute to higher heat flows if a convective subsurface layer and grain size dependent flow behavior of ice are admitted the grain size at brittle ductile transition depth cannot be 1 mm which is consistent with the required value for the onset of convection in an ice shell thinner than 20 km

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

physical properties of ordinary chondrites physical properties intrinsic and bulk densities porosity compressional and shear wave velocities thermal diffusivity and conductivity are measured on 11 ordinary chondrites one carbonaceous chondrite and two achondritic clasts of a mesosiderite with the previously reported results from antarctic ordinary chondrites the data are useful in clarifying the difference between h and l chondrites for example the intrinsic density of h chondrites 3800 kg m3 is generally higher than that of l chondrites 3600 kg m3 the sample porosity less than 20 strongly controls its elastic wave velocities thermal diffusivity and conductivity the variations of elastic wave velocities and thermal properties with porosity show that as with lunar rocks chondrites contain many cracks these cracks are thought to be created on their parent bodies by the cumulative effects of many mutual collisions and impacts thermal properties are linear functions of elastic wave velocities for both h and l chondrites the difference between h and l chondrites can be attributed to the content of metallic fe ni correlations between petrologic types which roughly represent metamorphic temperature and porosity are less obvious for l chondrites than for h chondrites since porosity is not changed significantly by impact events it appears that the sintering process for l chondrites is independent of the metamorphic events represented by petrologic type

resurfacing saturn s satellites models of partial differentiation and expansion abstract the faulting seen on the surfaces of saturn s icy moons may have been caused either by external events such as large impacts or internal stresses caused by the expansion of the moons as long lived radionuclides produced internal heating and phase changes we estimate the stress as a function of radius expansion is 44 r r kbar the extensional stress needed for fracture is probably something less than 40 bar so extensional fracture is likely to occur when r r is greater than one part in a thousand the radius change for these moons can be calculated analytically given suitable assumptions in addition detailed time dependent computer models of the thermal and physical evolution of tethys dione rhea and iapetus were carried out from these calculations we conclude that the most reasonable cause for rifting on dione and rhea is the refreezing of an ammonia water eutectic melt inside these moons roughly two billion years after their formation while the rift on tethys was caused by a large impact and little rifting should be expected on iapetus

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

on the observation of high frequency pkikp and its coda in australia the seismic phase pkikp reflected from the inner core and its coda give information on the short scale heterogeneities at and below the inner core boundary icb of the earth we have collected pkikp recorded at short distances 45 at broadband stations in australia and at the warramunga seismic array wra despite potential complications from a dual passage through d pkikp is frequently observed on single traces in the frequency band 1 5 hz pkikp usually has a sharp onset but sometimes its waveforms have multiple pulses separated by times of the order of 1 s at wra the coda of pkikp initially decays very rapidly after the main pulse thereafter its amplitude is nearly constant and more than three times smaller than pkikp with duration longer than 200 s our observations differ from those of nature 404 2000 273 275 at rather greater distances who found that the coda is larger than pkikp and increases slowly we suggest that the pkikp coda is generated near the inner core boundary by complex reverberation effects rather than scattering through the volume of the inner core 2004 elsevier b v all rights reserved

geological terrains and crater frequencies on ariel the southern hemisphere of ariel a satellite of uranus can be divided into several terrain types data on the size frequency distribution of craters for those different terrain types indicate that these terrains formed over a relatively short period of time much information on ariel s geological history can be gained from these data

gas driven water volcanism and the resurfacing of europa we have analyzed the problem of how pathways are created for the resurfacing by water or volatiles for a europa model in which there is an ocean underlying a thin ice shell 10 km thickness open cracks propagating downward cannot reach the ocean unless the relative tension in the ice is both very high 100 bar and applied very rapidly 10 4 10 5sec there is no known way to satisfy these criteria water filled cracks propagating upward can approach the surface for relative tensions of 5 10 bar comparable to the stresses induced by the eccentricity tide exsolution of plausible gas species co 2 co ch 4 so 2 can then occur at the crack tip the application of linear elastic fracture mechanics shows that the gas filled portion of the crack pinches off from the water filled portion and may rise rapidly to the surface entraining some water many of these cracks may end up as intrusions rather than extrusions because of partial freeze out of co 2 and water on the walls of the crack the resulting eruption is initially gas dominated but may include a subsequent less extended eruption of foam there may not be a direct relationship between the resurfacing process described here and the currently known geologic features observed on the surface the main problem with this or any model for tapping the ocean is the propagation of cracks through the soft ice immediately overlying the ocean

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

internal structure of rhea 1 we model the interior of rhea on the basis of observational constraints and the results from geodynamical models available in the literature ten main types of models are defined depending on the presence or absence of a high pressure ice layer ice ii and the extent of separation of the rock component from the volatiles the degree two gravity coefficients are computed for each of these models in order to assess which properties of the interior are likely to be inferred from cassini radio science measurements scheduled on 26 november 2005 c22 greater than 2 5 10 4 indicates that the satellite is undifferentiated except for a slight increase in density with depth resulting from material self compression c22 between 1 67 10 4 lower bound and 1 90 10 4 indicates the presence of a rocky core whose radius can be determined from the satellite s mass and ices densities for a given temperature profile for other values most of the ten models cannot be distinguished from each other however assumptions on the density of the rock phase presence or absence of ice ii and the degree of differentiation could allow a unique model to be determined in many cases while the calculation presented in this work assumes that rhea is in hydrostatic equilibrium it is likely that rhea gravity field is partly affected by nonhydrostatic anomalies

inhomogeneities near the core mantle boundary evidenced from scattered waves a review statistical properties of small scale inhomogeneities wavelengths between 20 and 70 km near the core mantle boundary are inferred from scattered core waves observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics however there may exist a few regions having markedly stronger or weaker strengths scattering by volumetric inhomogeneities of about 1 inp wave velocity in the lower mantle or by about 300 m of topographic relief of the core mantle boundary can explain the observations at present it is not possible to rule out either of these two alternatives or a combination of both

scattered pkkp further evidence for scattering at a rough core mantle boundary abstract pkkp signals from novaya zemlya recorded at lasa at distances around 60 show consistent anomalies in both slowness and azimuth the observed anomaly suggests that the signal is a bc branch arrival although the arrival time corresponds to the df branch the bc branch however does not extend back to this distance the azimuth of approach is in the range 229 245 instead of the expected 186 these anomalies are associated only with pkkp analysis of the core phases pkikp and p p bc from the same events show that they arrived at lasa with the appropriate slownesses and azimuths the pkkp signals can be interpreted as scattered pkkp the scattering occurs on underside reflection at the core mantle boundary and is probably caused by topographic irregularities on the boundary itself the calculated scattering region has a surface projection at about 60 s 134 e which is outside the diametral plane through source and receiver and about 21 from the expected pkkp reflection point at 76 s 95 e both the direct and scattered arms of the pkkp signal have a pk path close to that of the c end of the bc branch the unexpectedly large amplitude of the arrival suggests that there may be a focusing of energy at c which would indicate a change in velocity gradient just above the inner core boundary the observations nevertheless require on the scattering interpretation lateral variations in the topography of the core mantle boundary and a region of relatively large topography responsible for the anomalous pkkp observations

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