lunar global shape and polar topography derived from kaguya lalt laser altimetry a global lunar topographic map with a spatial resolution of finer than 0 5 degree has been derived using data from the laser altimeter lalt on board the japanese lunar explorer selenological and engineering explorer selene or kaguya in comparison with the previous unified lunar control network ulcn 2005 model the new map reveals unbiased lunar topography for scales finer than a few hundred kilometers spherical harmonic analysis of global topographic data for the moon earth mars and venus suggests that isostatic compensation is the prevailing lithospheric support mechanism at large scales however simple rigid support is suggested to dominate for the moon venus and mars for smaller scales which may indicate a drier lithosphere than on earth especially for the moon and venus

interferences encountered in the determination of ad of mollusc samples inflexion points interfere with the radiation growth curve of the dating signal at g 2 0014 for mollusc samples several experiments give evidence that the definition of the dating signal at g 2 0014 has to be reconsidered a reliable ad determination very often seems questionable for esr dating of mollusc samples

timing of water plume eruptions on enceladus explained by interior viscosity structure water plume eruptions on saturn s icy moon enceladus are delayed relative to the peak tidal stresses simulations suggest the delay can be explained by the moon s interior structure and the presence of a subsurface ocean at the south pole of saturn s icy moon enceladus eruptions of water vapour and ice emanate from warm tectonic ridges1 2 3 4 observations in the infrared5 and visible6 spectra have shown an orbital modulation of the plume brightness which suggests that the eruption activity is influenced by tidal forces however the observed activity seems to be delayed by several hours with respect to predictions based on simple tidal models6 7 here we simulate the viscoelastic tidal response of enceladus with a full three dimensional numerical model8 9 and show that the delay in eruption activity may be a natural consequence of the viscosity structure in the south polar region and the size of the putative subsurface ocean by systematically comparing simulations of variations in normal stress along faults with plume brightness data we show that the observed activity is reproduced for two classes of interior models with contrasting thermal histories a low viscosity convective region above a polar sea extending about 45 60 from the south pole at a depth below the surface as small as 30 km or a convecting ice shell of 60 70 km in thickness above a global ocean our analysis further shows that the eruption activity is controlled by the average normal stress applied across the cracks thus providing a constraint on the eruption mechanism

strength of the lithosphere of the galilean satellites abstract several approaches have been used to estimate the ice shell thickness on callisto ganymede and europa here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell lithosphere using measurements of topography the minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to byerlee s law and the shell has relatively uniform density and thickness under these conditions the topography of the ice provides a direct measure of the bending moment in the lithosphere this topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from byerlee s law the model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength this explains why europa is rugged at shorter wavelengths 10 km but extremely smooth and perhaps conforming to an equipotential surface at longer wavelengths 100 km previously compiled data on impact crater depth and diameter schenk p m 2002 nature 417 419 421 on europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2 5 km a more realistic model including a ductile lower lithosphere requires a thickness greater than 3 5 km future measurements of topography in the 10 100 km wavelength band will provide tight constraints on lithospheric strength

estimates of europa s ice shell thickness from elastically supported topography 1 the thickness of europa s solid ice shell is uncertain and has important implications for europa s habitability and thermal history and the design of future spacecraft missions here we obtain an estimate of the ice shell thickness from observations of a plateau sw of cilix impact crater stereo topographic profiles suggest that the plateau is flexurally supported with an effective elastic thickness te of 6 2 5 km for a conductive temperature profile this te value implies a solid ice shell thickness of 15 9 20 km if the shell is convecting this estimate is a lower bound combined with independent estimates we infer a probable shell thickness of 25 km the shell thickness is likely to be uniform over the entire satellite

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

mass anomalies on ganymede abstract radio doppler data generated with nasa s galileo spacecraft during its second encounter with jupiter s moon ganymede are used to infer the locations and magnitudes of mass anomalies on ganymede we construct models for both surface and buried anomalies with only one flyby and no global coverage a solution for mass anomalies cannot be uniquely determined however we are able to constrain acceptable solutions for mass anomalies to four broad regions a near polar region and three that are roughly equatorial if the mass anomalies are constrained to lie at the surface the centers of the regions are located near the coordinates 77 n 333 w 36 n 0 w 33 n 130 w and 7 n 194 w if the mass anomalies are located at the deep ice rock interface 800 km below the surface the regions centers are approximately 65 n 17 w 32 n 30 w 37 n 175 w and 15 n 211 w for both models the regions are up to a few thousand kilometers across the magnitude of mass anomalies on the surface is on the order of 10 17 kg mass anomalies at the ice rock interface are on average no more than an order of magnitude larger 10 18 kg there are two positive and two negative mass anomalies in both the surface and ice rock interface models one of the positive mass anomalies at the surface is associated with galileo regio the other positive surface mass anomaly is located at high northern latitudes with no obvious geological association negative surface mass anomalies lie near uruk sulcus and between perrine regio and barnard regio near sicyan sulcus and phrygia sulcus the locations of the ice rock interface mass anomalies lie approximately radially below the surface anomalies positive mass anomalies at the surface could be associated with the silicate rich ice or accumulated silicate layers of the dark regions negative mass anomalies at the surface could be associated with the relatively clean low lying ice of sulci alternatively ganymede s mass anomalies could be associated with the topography or other mass concentrations at the deep ice rock interface

unstable extension of enceladus lithosphere abstract regions near enceladus equator sarandib and diyar planitia contain extensive sets of parallel ridges and troughs that may be diagnostic of the region s formation conditions we present photoclinometry profiles across these ridges and troughs which indicate that they are periodic low slope features with dominant wavelengths of 3 to 4 km and amplitudes between 100 and 400 m the morphology of these terrains is consistent with formation via unstable extension of the lithosphere our numerical modeling demonstrates that unstable extension can generate large scale topography under enceladus like conditions comparison of our photoclinometry profiles with the dominant wavelengths produced by our numerical model permits estimation of the background heat flow at the time the sarandib diyar province formed we estimate heat flows of 110 to 220 mw m 2 suggesting that resurfacing of the planitiae was accompanied by strong localized heating the extension necessary to produce the ridges and troughs may have been caused by now inactive diapirs internal phase changes or other mechanisms our heat flux estimates imply elastic thickness at the time of resurfacing of 0 4 to 1 4 km which are sufficient to have allowed satellite reorientation if the province was underlain by a low density region it is therefore plausible that enceladus has experienced multiple heating events each leading to localized resurfacing and global reorientation

on the recovery of effective elastic thickness using spectral methods examples from synthetic data and from the fennoscandian shield there is considerable controversy regarding the long term strength of continents te while some authors obtain both low and high te estimates from the bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength others obtain estimates of only 25 km using the free air admittance and suggest that the mantle is weak at the root of this controversy is how accurately te can be recovered from coherence and admittance we investigate this question by using synthetic topography and gravity anomaly data for which te is known we show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free air admittance we reformulate the admittance method and show that it can recover synthetic te estimates similar to those recovered using coherence in light of these results we estimate te in fennoscandia and obtain similar results using both techniques te is 20 40 km in the caledonides 40 60 km in the swedish svecofennides 40 60 km in the kola peninsula and 70 100 km in southern karelia and svecofennian central finland independent rheological modeling using a xenolith controlled geotherm predicts similar high te in central finland because te exceeds crustal thickness in this area the mantle must contribute significantly to the total strength te in fennoscandia increases with tectonic age seismic lithosphere thickness and decreasing heat flow and low te correlates with frequent seismicity however in proterozoic and archean lithosphere the relationship of te to age is ambiguous suggesting that compositional variations may influence the strength of continents copyright 2004 by the american geophysical union

near surface heating on enceladus and the south polar thermal anomaly 1 strike slip motion is predicted to be a consequence of diurnal tidal stresses in most satellites of the outer solar system such motion can lead to near surface heating through friction or viscous dissipation here we discuss the effect of near surface shear heating on convection in the underlying ice shells of icy satellites we present models of convection in spherical shells including tidal and shear heating and show that localized near surface heating enhances convective upwelling beneath it the near surface heating promotes regional melting of the ice shell which likely results in subsidence of the surface topography the long wavelength geoid resulting from the subsidence plume buoyancy and dynamic topography may lead to small 4 per ma amounts of true polar wander potentially contributing to the south polar location of the observed thermal anomaly however compositional effects are likely required in addition to thermal effects to generate larger degrees of reorientation

the roughness of natural terrain a planetary and remote sensing perspective we examine the various methods and parameters in common use for quantifying and reporting surface topographic roughness it is shown that scale dependent roughness parameters are almost always required though not widely used we suggest a method of standardizing the parameters that are computed and reported so that topographic data gathered by different workers using different field techniques can be directly and easily intercompared we illustrate the proposed method by analyzing topographic data from 60 different surfaces gathered by five different groups and examine the information for common features we briefly discuss the implications of our analysis for studies of planetary surface roughness lander safety and radar remote sensing modeling and analysis

a new stereo topographic map of io implications for geology from global to local scales we use voyager and galileo stereo pairs to construct the most complete stereo digital elevation model dem of io assembled to date controlled using galileo limb profiles given the difficulty of applying these two techniques to io due to its anomalous surface albedo properties we have experimented extensively with the relevant procedures in order to generate what we consider to be the most reliable dems our final stereo dem covers 75 of the globe and we have identified a partial system of longitudinally arranged alternating basins and swells that correlates well to the distribution of mountain and volcano concentrations we consider the correlation of swells to volcano concentrations and basins to mountain concentrations to imply a heat flow distribution across io that is consistent with the asthenospheric tidal heating model of tackley et al 2001 the stereo dem reveals topographic signatures of regional scale features including loki patera ra patera and the tvashtar paterae complex in addition to previously unrecognized features including an 1000 km diameter depression and a 2000 km long topographic arc comprising mountainous and layered plains material

enceladus an active ice world in the saturn system enceladus one of the mid sized icy moons of saturn has an importance to planetary science far greater than its modest 504 km diameter would suggest intensive exploration of enceladus by the cassini saturn orbiter has revealed that it is the only known icy world in the solar system with ongoing deep seated geological activity active tectonic fractures at enceladus s south pole dubbed tiger stripes warmed by internal tidally generated heat spew supersonic jets of water vapor other gases and ice particles into circum saturnian space a subsurface saltwater sea probably exists under the south pole between the ice shell and the silicate core because of evidence that liquid water is probably present at the jet sources enceladus is also of great astrobiological interest as a potential habitat for life

one hundred km scale basins on enceladus evidence for an active ice shell 1 stereo derived topographic mapping of 50 of enceladus reveals at least 6 large scale ovoid depressions basins 90 175 km across and 800 to 1500 m deep and uncorrelated with geologic boundaries in contrast the south polar depression is larger and apparently shallower and correlates with active resurfacing the shape and scale of the basins is inconsistent with impact geoid surface deflections or with dynamically supported topography isostatic thinning of enceladus ice shell associated with upwellings and tidally driven ice melting can plausibly account for these basins thinning implies upwarping of the base of the shell of 10 20 km beneath the depressions depending on total shell thickness loss of near surface porosity due to enhanced heat flow may also contribute to basin lows alternatively the basins may overly cold inactive and hence denser ice but thermal isostasy alone requires thermal expansion more consistent with clathrate hydrate than water ice

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

tethys lithospheric thickness and heat flux from flexurally supported topography at ithaca chasma 16 20 km surface heat fluxes are 18 30 mw m 2 for adopted strain rates of 10 17 10 14 s 1 these heat fluxes are more than 4 times higher than those calculated from thermal history models of tethys and indicate a strongly enhanced local heat flux associated with the formation of ithaca chasma we use crater size frequency counts and a lunar like impact chronology to fix the time of formation of ithaca chasma at 4 ga from present day ithaca chasma predates the odysseus basin and thus could not have been formed by this impact feature citation giese b r wagner g neukum p helfenstein and p c thomas 2007 tethys lithospheric thickness and heat flux from flexurally supported topography at ithaca chasma geophys res lett 34 l21203

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

shapes of the saturnian icy satellites and their significance the sizes and shapes of six icy saturnian satellites have been measured from cassini imaging science subsystem iss data employing limb coordinates and stereogrammetric control points mimas enceladus tethys dione and rhea are well described by triaxial ellipsoids iapetus is best represented by an oblate spheroid all satellites appear to have approached relaxed equilibrium shapes at some point in their evolution but all support at least 300 m of global wavelength topography the shape of enceladus is most consistent with a homogeneous interior if enceladus is differentiated its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and or an irregularly shaped core iapetus supports a fossil bulge of over 30 km and provides a benchmark for impact modification of shapes after global relaxation satellites such as mimas that have smoother limbs than iapetus and are expected to have higher impact rates must have relaxed after the shape of iapetus was frozen

geology and origin of europa s mitten feature murias chaos 1 the mitten provisionally named murias chaos by the international astronomical union is a region of elevated chaos like terrain in the leading hemisphere of europa its origin had been explained under the currently debated theories of melting through a thin lithosphere or convection within a thick one galileo observations reveal several characteristics that suggest that the mitten is distinct from typical chaos terrain and point to a different formational process photoclinometric elevation estimates suggest that the mitten is slightly elevated with respect to the surrounding terrain geologic relations indicate that it must have raised significantly from the plains in its past resembling disrupted domes on europa s trailing hemisphere moreover the mitten material appears to have extruded onto the plains and flowed for tens of kilometers the area subsequently subsided as a result of isostatic adjustment viscous relaxation and or plains loading using plate flexure models we estimated the elastic lithosphere in the area to be several kilometers thick we propose that the mitten originated by the ascent and extrusion of a large thermal diapir thermal mechanical modeling shows that a mitten sized plume would remain sufficiently warm and buoyant to pierce through the crust and flow unconfined on the surface such a diapir probably had an initial radius between 5 and 8 km and an initial depth of 20 40 km consistent with a thick lithosphere model in this scenario the mitten appears to represent the surface expression of the rare ascent of a large diapir in contrast to lenticulae and chaos terrain which may form by isolated and clustered small diapirs respectively

the local topography of uruk sulcus and galileo regio obtained from stereo images high resolution 100 m pixel stereo images obtained by the galileo ssi camera are analyzed using photogrammetric techniques to derive local digital terrain models dtms of ganymede s surface the models cover areas of 22 56 km in uruk sulcus and 63 102 km in galileo regio and have spatial resolutions of 500 m pixel and 1000 m pixel respectively for uruk sulcus the dtm features a wave like topography with wavelengths of 2 6 km and amplitudes of up to 500 m individual topographic highs show asymmetric shapes with slopes of up to 20 and terracing the terrain model in galileo regio has isolated knobs furrows of up to 10 km in width and ridges of up to 1 km in height surface roughness in galileo regio at dtm scales is markedly higher than in uruk sulcus bowl shaped craters are identified in the models with depth to diameter ratios of about 1 12 uruk sulcus and 1 9 galileo regio this is in approximate agreement with data for simple craters measured in voyager images the large complex 20 km crater ea is found to have a central dome 300 m above the crater floor reaching more than half of the crater rim s height these unique topographic data at far higher resolution than what could be derived from voyager images will provide new insights into the formation of ganymede s surface

ore forming mechanism of quartz vein type w sn deposits of the xitian district in se china implications from the trace element analysis of wolframite and investigation of fluid inclusions abstract the xitian w sn district is located in eastern hunan province of se china and comprises the heshuxia goudalan and longshang w sn deposits our study on the occurrence mineral assemblage and geochemistry of the orebodies shows that quartz vein orebodies are dominant mode of mineralization and can be divided into three stages stage i quartz molybdenite wolframite i stage ii quartz wolframite ii polymetallic sulfides and stage iii quartz pyrite fluorite the elements nb ta sc sn zn cr v mo cu and zr are abundant in both wolframite i and wolframite ii wolframite i contains higher concentrations of nb 1408 9245 ppm ta 241 2407 ppm and sc 123 521 ppm than wolframite ii nb 144 1080 ppm ta 3 76 315 ppm and sc 1 86 11 44 ppm these results indicate that wolframite i was formed closer to the host rock and at a greater ore forming depth than wolframite ii the total ree concentrations of wolframite i and ii range from 34 96 to 133 36 ppm and 21 30 to 43 73 ppm respectively both wolframite i and wolframite ii have low lree concentrations 0 00 0 05 and 0 08 8 18 ppm respectively and high hree concentrations 34 96 133 31 and 21 23 41 87 ppm respectively the enrichment of hrees in wolframite may be due to the fact that the sizes of the hrees3 0 94 1 02 a in combination with nb5 or ta5 0 72 a are closer to that of w6 0 68 a in combination with ca2 1 08 a or mg2 0 80 a than that of the lrees3 1 03 1 13 a making the coupled substitution easier a study of fluid inclusions in the coexisting gangue minerals shows that the homogenization temperatures of the fluid inclusions decrease from stage i to stage iii stage i 187 382 c average 275 c stage ii 122 278 c average 175 c stage iii 92 172 c average 139 c whereas the salinities of the fluid inclusions increase from stages i to ii and decrease from stages ii to iii stage i 1 8 18 2 wt nacleqv average 10 3 wt nacleqv stage ii 7 3 24 4 wt nacleqv average 14 9 wt nacleqv stage iii 0 2 5 3 wt nacleqv average 1 5 wt nacleqv raman spectroscopy reveals that the fluid inclusions mainly contain h2o co2 is minor and h2s ch4 and n2 are rare we conclude that the formation of wolframite was driven by post magmatic thermodynamic processes the ore forming fluid flowed through a multi fractured low pressure zone in the fault system in the district and a temperature pressure decrease led to fluid immiscibility characterized by co2 escaping in a low ph and high eh environment in stage i crust mantle mixed fluid and meteoric water mixing in stage ii and natural cooling of the fluid system in stage iii

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

shell thickness variations and the long wavelength topography of titan the long wavelength topography of titan has an amplitude larger than that expected from tidal and rotational distortions at its current distance from saturn this topography is associated with small gravity anomalies indicating a high degree of compensation both observations can be explained if titan has a floating isostatically compensated ice shell with a spatially varying thickness the spatial variations arise because of laterally variable tidal heating within the ice shell models incorporating shell thickness variations result in an improved fit to the observations and a degree two tidal love number h2t consistent with expectations without requiring titan to have moved away from saturn our preferred models have a mean shell thickness of 100 km in agreement with the observed gravity anomalies and a heat flux appropriate to a chondritic titan shell thickness variations are eliminated by convection we therefore conclude that titan s ice shell is not convecting at the present day

tidal dissipation in enceladus uneven fractured ice shell abstract analysis of enceladus gravity topography and libration data suggests that the thickness of the ice crust significantly varies which may have important consequences for the heat transfer across and the tidal dissipation within the ice shell understanding these processes is a critical prerequisite for analyzing enceladus thermal evolution and assessing the long term stability of its subsurface ocean here we investigate the impact of ice shell thickness variations on the tidal deformation of the moon and the associated heat production using a finite element model that includes faults tiger stripes in the south polar region spr since the tidal deformation and the thermal structure of the ice shell are coupled through temperature and deviatoric stress we simultaneously solve the equations governing the anelastic deformation of ice and also equations which model conductive heat transfer in the ice shell we find that tidal heating is concentrated in a narrow low viscosity zone near the base of the ice shell and along faults outside the spr the thickness of this zone is about 1 10 of the local ice thickness and the associated volumetric heating is 1 0 6 w m 3 corresponding to less than 1 gw of dissipated power in the spr the tidal effects are enhanced by the combined action of faults and ice shell thinning although the volumetric heating in this relatively small region may be larger than 10 4 w m 3 the total heat production in this region does not exceed 1 1 gw our computations show that tidal heating in the ice shell can explain only a small fraction of enceladus heat production derived from astrometric observations implying that enceladus heat engine is powered by dissipation in the core or in the ocean

enceladus an estimate of heat flux and lithospheric thickness from flexurally supported topography fluxes of 200 270 mw m 2 at the time of formation the heat fluxes are comparable to average heat flux values measured in enceladus active south polar region and they are consistent with estimates derived via models of unstable extension of the lithosphere at this location surface porosity can reduce the obtained heat fluxes to an estimated minimum of 45 60 mw m 2 c rater size frequency counts fix the time of formation from presentday of the rift zone at either 3 5 0 4 0 1 ga assuming an

true polar wander of enceladus from topographic data many objects in the solar system are suspected to have experienced reorientation of their spin axes as their rotation rates are slow and their shapes are nearly spherical the formation of mass anomalies by either endogenic or exogenic processes can change objects moments of inertia therefore the objects reorient to align their largest moment of inertia with their spin axis such a phenomenon is called true polar wander tpw here we report the discovery of a global series of topographic lows on saturn s satellite enceladus that we interpret to show that this synchronously locked moon has undergone tpw by 55 about the tidal axis we use improved topographic data from the spherical harmonic expansion of cassini limb and stereogrammetric measurements to characterize regional topography over the surface of enceladus we identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non equatorial circumglobal belt across enceladus surface we argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles respectively we argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell our hypothesis is consistent with a variety of geological features visible in cassini images

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

volcanic constructs on ganymede and enceladus topographic evidence from stereo images and photoclinometry paul m schenklunar and planetary institute houston texasjeffrey m moorespace sciences division and the seti institute nasa ames research center moffett field californiaabstract the morphology of volcanic features on ganymede differs significantly from that onthe terrestrial planets few if any major volcanic landforms such as thick flows or shieldvolcanoes have been identified to date using new stereo voyager images we have searchedganymede for relief generating volcanic constructs we observed seven major types of volcanicstructures including several not previously recognized the oldest are broad flat topped domespartially filling many older craters in dark terrain similar domes occur on enceladus togetherwith smooth dark deposits these domes indicate that the volcanic history of the dark terrain iscomplex bright terrain covers vast areas although the style of emplacement remains unclear smooth bright materials embay and flood older terrains and may have been emplaced as low viscosity fluids associated with smooth bright material are a number of scalloped shaped semi enclosed scarps that cut into preexisting terrain in planform these structures resemble terrestrialcalderas the youngest volcanic materials identified are a series of small flows that may haveflooded the floor of the multiring impact structure gilgamesh forming a broad dome theidentification of volcanic constructs up to 1 km thick is the first evidence for extrusion ofmoderate to high viscosity material on ganymede viscosity and yield strength estimates for thesematerials span several orders of magnitude indicating that volcanic materials on ganymede have arange of compositions and or were extruded under a wide range of conditions and or eruptive styles introductionthe voyager survey of the outer solar system has revealedan astonishing variety of endogenic landforms on the surfacesof icy satellites despite extensive endogenic resurfacing ofsomc satellites for example ganymede enceladus andtriton volcanism on these surfaces has little topographicexpression we do not see massive volcanic edifices similarto olympus mons or mauna kea even the best putativeexamples of volcanic landforms seen on ariel miranda triton and enceladus exhibit rarely more than 1 2 km ofrelief jankowski and squyres 1988 pozio and kargel 1989 schenk 1991a croft et al 1995 this may berelated to the soft theology of ice see poirier 1982 durham et al 1983 and kirby et al 1987 or torheology and volcanic styles peculiar to icy petrologicsystems e g kargel 1991 kargel et al 1991 well over half of the surface of ganymede radius 2634 kin has been covered by endogenic effusions it is now consideredlikely that ganymede experienced one or more episodes ofchaotic orbital behavior malhotra 1991 possiblyresulting in tidal heating volcanic resurfacing constitutes arecord of ganymede s compositional and thermal evolution andmay record some of these putative tidal heating events yetthis satellite has been the subject of morphological study for15 years without the unambiguous detection of relief copyright 1995 by the american geophysical union paper number 95je01854 0148 0227 95 95je 01854505 00generating volcanic constructions on its landscape the styleof emplacement and composition of those volcanic depositsthat have been identified remains unclear on any planetary surface volcanic and other nonimpactconstructs can be difficult to recognize in spacecraft images particularly those obtained under high sun conditions e g malin 1978 topography is especially low on the galileansatellites and imaging resolution is at best 500 m pixel major local albedo variations on ganymede further complicategeologic and topographic mapping with these problems inmind we have undertaken the production of stereo pairs ofvoyager images where coverage permits to aid in the searchfor and characterization of volcanic landforms on ganymede ssurface in addition to an examination of previously identifiedvolcanic materials we report the discovery of severalpreviously unrecognized volcanic landforms and at least twotypes with substantial relief and describe their morphologyand geologic setting morphology of volcanic depositsindicates that ganymede extrusions probably had a range oftheologies and may be compositionally variable we concludewith speculations and predictions regarding their mode ofemplacement and the implications of volcanic landforms forthe geologic evolution of ganymede stereo imaging with voyagerthis study was motivated by the recognition that asignificant fraction of the surfaces of most of the outer planetsatellites were imaged by voyager in stereo e g croft andsoderblom 1991 schenk and moore 1993 we can usestereo imaging to detect subtle topographic features that were19 009

geophysical implications of the long wavelength topography of the saturnian satellites 1 we use limb profiles to quantify the long wavelength topography of the saturnian satellites the degree 2 shapes of mimas enceladus and tethys are not consistent with hydrostatic equilibrium we derive 2 d topographic maps out to spherical harmonic degree 8 there is a good correlation with topography derived from stereo techniques if uncompensated topography at degree 3 and higher is large enough to be detectable during close spacecraft flybys if not properly accounted for this topography may bias estimates of a satellite s degree 2 gravity coefficients which are used to determine the moment of inertia we also derive a one dimensional variance spectrum a measure of how roughness varies with wavelength for each body the short wavelength spectral slope is 2t o 2 5 similar to silicate bodies however unlike the terrestrial planets each satellite spectrum shows a reduction in slope at longer wavelengths if this break in slope is due to a transition from flexural to isostatic support the globally averaged elastic thickness te of each satellite may be derived we obtain te values of 5 km 1 5 5 km 5 km and 5 km for tethys dione rhea and iapetus respectively for europa we obtain te 1 5 km these estimates are generally consistent with estimates made using other techniques for enceladus intermediate wavelengths imply te 0 5 km but the variance spectrum at wavelengths greater than 150 km is probably influenced by long wavelength processes such as convection or shell thickness variations impact cratering may also play a role in determining the variance spectra of some bodies citation nimmo f b g bills and p c thomas 2011 geophysical implications of the long wavelength topography of the saturnian satellites j geophys res 116 e11001 doi 10 1029 2011je003835

mars topography lessons learned from spatial and spectral domain comparisons of mars orbiter laser altimeter and u s geological survey data the topography of mars has been mapped several times by a variety of techniques we compare the recent mars orbiter laser altimeter version with the earlier u s geological survey compilation of photogrammetry and earth based radar we compare them in terms of their respective representations on 1 1 grids though both data sets contain substantially higher spatial resolution we find that the differences between the two mars topography models though locally quite large are generally quite small the difference variance is 4 of the data variance the difference between the two models is also quite notably anisotropic with substantially more variation north south than east west the variance in the difference between the models is a very small fraction of the data variance at the longest wavelengths and approaches a constant value of 30 at harmonic degrees above 20

effect of enceladus s rapid synchronous spin on interpretation of cassini gravity enceladus s degree 2 gravity determined by cassini is nominally nonhydrostatic to 3 j2 c22 3 38 3 63 as opposed to 10 3 iess et al 2014 interpret this in terms of a hydrostatic interior core and isostatic not hydrostatic floating ice shell enceladus s rapid 1 37 d synchronous spin and tide distorts its shape substantially though enough that the predicted hydrostatic j2 c22 is not 10 3 but closer to 3 25 this leads to the following revision to the internal picture of enceladus compared with iess et al 1 the satellite s core is somewhat smaller and slightly denser 190 km radius and 2450 kg m3 2 the compensation depth shell thickness of the global degree 2 ice shell is 50 km rather close to the base of the modeled ice water layer and 3 the compensation depth shell thickness beneath the south polar terrain from j3 remains shallower thinner at 30 km independent of but influenced by the degree 2 solution

the interior structure of ceres as revealed by surface topography abstract ceres the largest body in the asteroid belt 940 km diameter provides a unique opportunity to study the interior structure of a volatile rich dwarf planet variations in a planetary body s subsurface rheology and density affect the rate of topographic relaxation preferential attenuation of long wavelength topography 150 km on ceres suggests that the viscosity of its crust decreases with increasing depth we present finite element fe geodynamical simulations of ceres to identify the internal structures and compositions that best reproduce its topography as observed by the nasa dawn mission we infer that ceres has a mechanically strong crust with maximum effective viscosity 10 25 pa s combined with density constraints this rheology suggests a crustal composition of carbonates or phyllosilicates water ice and at least 30 volume percent vol low density high strength phases most consistent with salt and or clathrate hydrates the inference of these crustal materials supports the past existence of a global ocean consistent with the observed surface composition meanwhile we infer that the uppermost 60 km of the silicate rich mantle is mechanically weak with viscosity 21 pa s suggesting the presence of liquid pore fluids in this region and a low temperature history that avoided igneous differentiation due to late accretion or efficient heat loss through hydrothermal processes

wavelet analyses of mars polar topography we have analyzed topography tracks over the mars northern polar cap composed of h2o ice co2 ice and sediments and the antarctic ice sheet on earth using a mexican hat wavelet transform the great utility of the wavelet transform is that it gives both spatial and spectral resolution we have used the variance of the wavelet output in order to quantify the spectral content of the topography tracks at short wavelengths for both the mars northern polar cap and the antarctic ice sheet we find a power law dependence of the wavelet transform variance on wavelength with a power law exponent of 3 5 3 7 this compares with a power law exponent of 2 0 brownian motion typical of topography on both mars and earth there is a power law smoothing of the ice topography at short wavelengths on both planetary bodies at long wavelengths we infer a similar power law dependence with 1 5 2 0 typical of planetary topography a transition is observed between the smooth short wavelength behavior high to rougher long wavelength behavior low this fairly sharp transition occurs at a wavelength of 24 km for mars and 11 km for antarctica the transitions appear to scale with ice thicknesses and suggest a relaxation of short wavelength topography by ice flows or surface processes