differential cratering of synchronously rotating satellites by ecliptic comets abstract we use monte carlo methods to simulate impacts of ecliptic comets on the synchronously rotating satellites of giant planets we reconfirm the long standing prediction that the cratering rate should be much higher on the leading hemispheres than on the trailing hemisphere indeed we find that previously published analytical formulations modestly underestimate the degree of apex antapex asymmetry to be expected we then compare our results to new mapping of impact craters on ganymede callisto and triton ganymede reveals a pronounced apex antapex asymmetry that is nonetheless much less than predicted all of triton s confirmed impact craters are clustered toward the apex of motion far exceeding the predicted asymmetry no asymmetry is observed on callisto in each case at least one of our basic assumptions must be wrong likely candidates include the following i the surfaces of all but the most sparsely cratered satellites are saturated or nearly saturated with impact craters ii these satellites have rotated nonsynchronously over geological time iii most of the craters are made not by heliocentric sun orbiting comets and asteroids but rather by planetocentric planet orbiting debris of indeterminate origin or iv pathological endogenic resurfacing has created illusions of structure callisto s surface is readily classified as nearly saturated ganymede s bright terrains although less heavily cratered than those of callisto can also be explained by crater densities approaching saturation on a world where endogenic processes were active the leading alternative is nonsynchronous rotation an explanation supported by the distribution of catenae crater chains produced by impact of tidally disrupted comets triton s craters can be explained by planetocentric debris or by capricious resurfacing but both hypotheses are inherently improbable

cassini imaging science initial results on phoebe and iapetus the cassini imaging science subsystem acquired high resolution imaging data on the outer saturnian moon phoebe during cassini s close flyby on 11 june 2004 and on iapetus during a flyby on 31 december 2004 phoebe has a heavily cratered and ancient surface shows evidence of ice near the surface has distinct layering of different materials and has a mean density that is indicative of an ice rock mixture iapetus s dark leading side cassini regio is ancient heavily cratered terrain bisected by an equatorial ridge system that reaches 20 kilometers relief local albedo variations within and bordering cassini regio suggest mass wasting of ballistically deposited material the origin of which remains unknown

hidden tectonism on miranda s elsinore corona revealed by polygonal impact craters abstract we mapped the spatial distribution and orientations of polygonal impact craters pics on the classical uranian satellite miranda to gain insight into the complex tectonic history of this moon pic straight rim segments in regions without visible fractures can be used to infer sub regolith and or sub resolution fracture orientations we identified and analyzed 14 pics on miranda nine of which are located on its trailing hemisphere overprinting elsinore corona and the nearby cratered terrain these pics do not follow the e w orientations of the ridges within elsinore s bounding terrain indicating that these ridges are not underlain by deep rooted at least as deep as the crater bowl sub vertical normal or strike slip faults instead the ridges may be contractional features consisting of folds and possibly thrust faults or cryovolcanic features without fault structures furthermore the pics in this region indicate that a previously undetected nw oriented fracture system is present over much of miranda s trailing hemisphere another pic somewhat distal from elsinore on miranda s anti uranus hemisphere might be associated with the nw fractures as well additionally three pics are present proximal to verona rupes on miranda s uranus facing hemisphere these pics exhibit n s orientations consistent with the surrounding fractures that make up miranda s global rift system one other pic is proximal to arden and might be associated with the arden bounding terrain identification of the nw fracture system provides new information on miranda s complex geologic history based on cross cutting relationships with miranda s ancient subdued crater population the nw fracture system is older than elsinore however this fracture system also overprints elsinore suggesting reactivation of the nw fractures after this corona had formed along with discussion of miranda s geologic history we consider the possibility that miranda has undergone true polar wander and we compare its terrains and geologic features to similar features on the saturnian satellite enceladus

windy mars a dynamic planet as seen by the hirise camera 1 with a dynamic atmosphere and a large supply of particulate material the surface of mars is heavily influenced by wind driven or aeolian processes the high resolution imaging science experiment hirise camera on the mars reconnaissance orbiter mro provides a new view of martian geology with the ability to see decimeter size features current sand movement and evidence for recent bedform development is observed dunes and ripples generally exhibit complex surfaces down to the limits of resolution yardangs have diverse textures with some being massive at hirise scale others having horizontal and cross cutting layers of variable character and some exhibiting blocky and polygonal morphologies reticulate fine polygonal texture bedforms are ubiquitous in the thick mantle at the highest elevations

large impact features on middle sized icy satellites abstract all of the large impact features of the middle sized icy satellites of saturn and uranus that were clearly observed by the voyager spacecraft are described new image mosaics and stereo and photoclinometrically derived digital elevation models are presented landforms related to large impact features such as secondary craters and possible antipodal effects are examined and evaluated of the large impacts odysseus on tethys appears to have had the most profound effect on its target satellite of any of the impact features we examined our modeling suggests that the odysseus impact may have caused the prompt formation of ithaca chasma a belt of tectonic troughs that roughly follow a great circle normal to the center of odysseus although other hypotheses remain viable we identify probable secondary cratering from tirawa on rhea we attribute a number of converging coalescing crater chains on rhea to a putative possibly relatively fresh 350 km diameter impact feature we examine the antipodes of odysseus the putative 350 km diameter rhean impact feature and tirawa and conclude that evidence from voyager data for damage from seismic focusing is equivocal although our modeling results indicate that such damage may have occurred we propose a number of observations and tests for cassini that offer the opportunity to differentiate among the various explanations and speculations reviewed and evaluated in this study

crustal failure on icy moons from a strong tidal encounter abstract close tidal encounters among large planetesimals and moons should have been more common than grazing or normal impacts using a mass spring model within an n body simulation we simulate the deformation of the surface of an elastic spherical body caused by a close parabolic tidal encounter with a body that has similar mass as that of the primary body such an encounter can induce sufficient stress on the surface to cause brittle failure of an icy crust and simulated fractures can extend a large fraction of the radius of body strong tidal encounters may be responsible for the formation of long graben complexes and chasmata in ancient terrain of icy moons such as dione tethys ariel and charon

is there evidence for polar wander on europa abstract the orientations of the albedo lineaments bands and lineations on europa s surface have been compared in previous studies with the global stress fields set up by orbital eccentricity orbital recession and nonsynchronous rotation of these orbital and rotational effects nonsynchronous rotation combined with an offsetting of the tidal bulge comes closest to providing agreement between the stress field generated and the lineation orientations if the lineations trace tension or extension fractures mcewen 1986 nature321 49 51 however inferred minimum principal stress directions for a broad region of wedge shaped bands near the anti jove point cannot satisfactorily be accounted for by any of the stress fields above but are consistent with the stresses resulting from a rotation of europa s ice shell about an axis through the sub and anti jove points clockwise as seen from the anti jove hemisphere p m schenk and w b mckinnon 1989 icarus79 75 100 calculations by ojakangas and stevenson 1989 icarus81 220 241 of the thermal state of europa s ice shell indicate that spatial variations in the thickness of the shell may cause it to undergo such a reorientation we have investigated whether any reorientation of the shell about an axis through the sub and anti jove points produces a stress field consistent with the full global set of prominent lineations on europa s surface we find that no such reorientation provides a good fit between the lineations and plausible fracture orientations derived from the principal stress trajectories topographic ridges identified in a limited zone south of the anti jove point are roughly consistent with compression due to clockwise polar wander but the orientations of these ridges may be strongly biased by illumination direction within the limitations of the presently available imagery nonsynchronous rotation is still the most likely cause of the prominant fractures on europa s surface and the best specific albeit regionally limited tectonic evidence consistent with recent polar wander remains the wedge shaped bands

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

transverse aeolian ridges tars on mars ii distributions orientations and ages transverse aeolian ridges tars 10 m scale ripple like aeolian bedforms with simple morphology are widespread on mars but it is unknown what role they play in mars wider sediment cycle we present the results of a survey of all mars global surveyor narrow angle images in a pole to pole study area 45 longitude wide following on from the classification scheme and preliminary surveys of balme et al balme m r berman d c bourke m c zimbelman j r 2008a geomorphology 101 703 720 and wilson and zimbelman wilson s a zimbelman j r 2004 j geophys res 109 e10 doi 10 1029 2004je002247 we searched more than 10 000 images and found that over 2000 reveal at least 5 areal cover by tars the mean tar areal cover in the study area is about 7 3 in the northern hemisphere and 11 in the southern hemisphere but tars are not homogenously distributed they are concentrated in the mid low latitudes and almost absent poleward of 35 n and 55 s we found no clear correlation between tar distribution and any of thermal inertia kilometer scale roughness or elevation we did find that tars are less common at extremes of elevation we found that tars are most common near the equator especially in the vicinity of meridiani planum in which area they have a distinctive barchan like morphology and in large southern hemisphere impact craters tars in the equatorial band are usually associated with outcrops of layered terrain or steep slopes hence their relative absence in the northern hemisphere tars in the southern hemisphere are most commonly associated with low albedo intercrater dune fields we speculate that the mid latitude mantling terrain e g mustard j f cooper c d rifkin m k 2001 nature 412 411 414 kreslavsky m a head j w 2002 j geophys res 29 15 doi 10 1029 2002gl015392 could also play a role in covering tars or inhibiting saltation we compared tar distribution with general circulation model gcm climate data for both surface wind shear stress and wind direction we performed gcm runs at various obliquity values to simulate the effects of changing obliquity on recent mars climate we found good general agreement between tar orientation and gcm wind directions from present day obliquity conditions in many cases but found no good correlation between wind shear stress and tar distribution we performed preliminary high resolution crater count studies of tars in both equatorial and southern intracrater dunefield settings and compared these to superposition relationships between tars and large dark dunes our results show that tars near dunefield appear to be younger than tars in the equatorial regions we infer that active saltation from the large dunes keeps tars active but that tars are not active under present day condition when distal to large dunes perhaps supporting the interpretation that tars are granule ripples we conclude that local geology rather than wind strength controls tar distribution but that their orientation matches present day regional wind patterns in most cases we suggest that tars are likely most perhaps only active today when they are proximal to large dark dune fields

long term rotation and mantle dynamics of the earth mars and venus in response to internal and surface tectonic processes the terrestrial planets have the ability to displace the axis of rotation with respect to the mantle this behavior is quantified by means of a nondimensional number defined here as the rotational number ro that allows classification of the planets into two categories the first containing mars and the earth where true polar wander is a feasible mechanism and the second to which venus belongs where rotation equilibrium is attained by means of mega wobbles driven by internal mass anomalies the number ro is related to the timescale characterizing the readjustment of the equatorial bulge during long term polar motion and to the length of the sidereal day if these two timescales are well separated the planet experiences true polar wander nonlinear liouville equations for stratified viscoelastic bodies with linear maxwell rheology are solved in order to assess the relevance of surface and mantle processes in driving long term rotation instabilities in the terrestrial planets the amount of true polar wander estimated for the earth and mars is reproduced correctly by our modeling with mantle viscosities and lithospheric thickness consistent with other studies the major difference between the earth and mars is the driving mechanism subduction for the earth and lithospheric loading for mars when mantle viscosities similar to those of the earth are considered for venus the most updated estimate for the offset of about 0 5 between the rotation axis and the axis of maximum inertia is well reproduced during the mega wobbles induced by internal mass redistribution we show that the degree 2 topography of these three planets can be affected by their rotation which is responsible for the dominance of the sectorial component on the earth and mars and for the zonal component on venus

a progression of induration in medusae fossae formation transverse aeolian ridges evidence for ancient aeolian bedforms and extensive reworking a progression of induration erosion and redeposition of transverse and networked transverse aeolian ridges tars has been documented in the medusae fossae formation mff mars cratered and eroded aeolian bedforms are rarely observed on mars indicating that those found in the mff have been inactive for much longer than those found elsewhere indurated tars are observed to grade into faceted mff terrain indicating a genetic relationship between the two we propose that tar deposition induration and erosion have played a larger role in the surface morphology and evolution of the mff than previously recognized the deposition induration and erosion of tars indicate that the mff has undergone multiple cycles of reworking and that much of its current surface morphology does not reflect the circumstances of its primary emplacement copyright 2011 john wiley sons ltd

reorientation of the early lunar pole an active core dynamo may have operated on the early moon extraction of palaeomagnetic pole positions on the moon from magnetic anomalies measured by the lunar prospector and kaguya orbiters suggests that the ancient lunar dynamo experienced reversals and an ancient reorientation of the moon rotated the geographic locations of the poles

reorientation of icy satellites by impact basins 1 large impact basins are present on many of the icy satellites of the outer solar system assuming that their present day topography is uncompensated such basins can cause significant poleward reorientations for slow rotating satellites this reorientation may have been accompanied by transient large amplitude wobble the largest basins on tethys rhea and titania are predicted to have caused reorientations of roughly 4 7 and 12 respectively resulting in global tectonic stresses up to 0 5 mpa the potential anomalies associated with the basins can be up to one third of those expected for a hydrostatic tidally and rotationally deformed body and may complicate interpretation of the satellite interior structure pluto and charon because of their slow rotation are also likely to have undergone reorientation of 10 20 if they possess impact basins of comparable sizes to those of the saturnian satellites

impact basin relaxation as a probe for the thermal history of pluto we investigate viscoelastic impact basin relaxation on pluto for a variety of thermal evolution scenarios encompassing both convective and conductive ice shells basins smaller than 200 km in diameter do not relax appreciably while relaxation fractions can be up to 60 for large impact basins the main control on basin relaxation is the amount of radiogenic heat produced in the rocky core our results are insensitive to the formation time of the basin the ice reference viscosity adopted and the presence absence of a subsurface ocean other volatiles such as co2 or nh3 if present in the ice shell in sufficient quantities could increase the predicted relaxation fraction of basins relaxation causes extensional stresses interior to the basin the orientation of the resulting tectonic features is controlled by the effective elastic thickness beneath the basin future observations of the relaxation states and tectonics of impact basins are therefore likely to provide a key constraint on pluto s thermal history

global reorientation and its effect on tectonic patterns on ganymede the basins of ganymede are studied in order to examine the effects of global reorientation on the grooves and furrows of the satellite an impact basin and the global reorientation of ganymede were modeled the damping time the time for relaxation of tidal bulge and the time for isostatic adjustment of craters are significant in calculating the duration and amount of global reorientation the regional global control of groove orientation is examined the data reveal that the tidal despinning and furrow formation created the reactivated zones of weakness in which the groove sets were formed it is observed that the grooved terrain emplacement caused a 15 deg shift in the paleopole

variable wind ripple migration at great sand dunes national park and preserve observed by timelapse imaging abstract granule ripples at great sand dunes national park and preserve gsdnpp were observed using inexpensive digital timelapse cameras over a 70 day period in winter 2010 2011 the ripples migrated during a handful of discrete events visible ripple movement occurred on only 11 days during the observation period the movement conditions are documented with hourly and 15 minute records from two nearby weather stations and by a cup anemometer at the site itself during the most prominent movement episode when local winds averaged 10 m s ripples of several sizes were observed simultaneously and a reciprocal relationship of ripple size and propagation speed was seen with small 10 cm ripples moving at 1 4 cm min and larger 80 cm ripples at 0 15 cm min ripple sizes and morphologies evolve throughout the observation period

aeolian processes in proctor crater on mars mesoscale modeling of dune forming winds 1 both atmospheric modeling and spacecraft imagery of mars are now of sufficient quality that the two can be used in conjunction to acquire an understanding of regional and local scale aeolian processes on mars we apply a mesoscale atmospheric model adapted for use on mars the mars mm5 to proctor crater a 150 km diameter crater in the southern highlands proctor crater contains numerous aeolian features that indicate wind direction including a large dark dune field with reversing transverse and star dunes containing three different slipface orientations small and older bright bedforms that are most likely transverse granule ripples and seasonally erased dust devil tracks results from model runs spanning a martian year with a horizontal grid spacing of 10 km predict winds aligned with two of the three dune slipfaces as well as spring and summer winds matching the dust devil track orientations the primary most prevalent dune slipface orientation corresponds to a fall and winter westerly wind created by geostrophic forces the tertiary dune slipface orientation is caused by spring and summer evening katabatic flows down the eastern rim of the crater influencing only the eastern portion of the crater floor the dunes are trapped in the crater because the tertiary winds enhanced by topography counter transport from the oppositely oriented primary winds which may have originally carried sand into the crater the dust devil tracks are caused by light spring and summer westerly winds during the early afternoon caused by planetary rotation the secondary dune slipface orientation is not predicted by model results from either the mars mm5 or the geophysical fluid dynamics laboratory mars general circulation model the reason for this is not clear and the wind circulation pattern that creates this dune slipface is not well constrained the mars mm5 model runs do not predict stresses above the saltation threshold for dune sand of the appropriate size and composition as with previous work the calculated wind velocities are too low which may be caused by too large of a grid spacing

present day aeolian activity in herschel crater mars abstract in this report we show evidence for ripple and dune migration in herschel crater on mars we estimate an average dune migration of 0 8 m and a minimum ripple migration of 1 1 m in a time span of 3 7 earth years these dunes and ripples are mainly shaped by prevailing winds coming from the north however we also report the presence of secondary winds which elongate the barchans horns such a complex wind scenario is likely caused by the influence of winds blowing off the western crater rim as suggested by the mars regional atmospheric modeling system mrams an atmospheric mesoscale model a multi directional wind regime at the local scale is also supported by the observed bimodal distribution of the ripple trends for the first time a survey integrating the assessment of dune and ripple migration is presented showing how dune topography can influence the migration patterns of ripples and how underlying topography appears to control the rates of dune migration

theoretical constraints on true polar wander 1 for the present geologic epoch true polar wander tpw is relatively small but simple theoretical considerations suggest that it could have been larger in other epochs in this work we use scaling arguments to assess the qualitative behavior of tpw and a simple maxwell model to analytically describe how changes in mass anomalies translate into tpw unlike previous work we derive simple analytical estimates of tpw based on the characteristic amplitudes and timescales for changes in the moment of inertia we find estimates for both the amplitude and speed of tpw as a function of earth properties the following four main factors influence how large the maximum tpw can be the geological timescale over which the desired tpw occurs tpw the viscosity structure of the mantle which yields a weighted average viscosity the characteristic amplitude of the nonhydrostatic changes in the moment of inertia c and the added moment of inertia due to the equatorial bulge c a for the nominal values 3 1022 pa s and c c a 0 003 the maximum tpw is 61 over 100 myr and 8 over 10 myr the maximum tpw speed is only sensitive to c and c a and is 2 4 myr 1 for the nominal values tpw is shown to act as a low pass filter rapid changes in moment of inertia produce smaller and delayed tpw a consequence of this is that inertial interchange tpw does not have a different character than tpw tpw can have an important contribution to plate motions over relatively long timescales but not over shorter timescales our simple approach allows us to assess whether multiple tpw events are possible but the major uncertainty continues to be the mantle viscosity structure

antipodal terrains created by the rheasilvia basin forming impact on asteroid 4 vesta 1 the rheasilvia impact on asteroid 4 vesta may have been sufficiently large to create disrupted terrains at the impact antipode this paper investigates the amount of deformation expected at the rheasilvia antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses we find that the magnitude and mode of deformation expected at the impact antipode is strongly dependent on both the sound speed and porosity of vesta s mantle as well as the strength of the vestan core in the case of low mantle porosities and high core strengths we predict the existence of a topographic high a peak caused by the collection of spalled and uplifted material at the antipode observations by nasa s dawn spacecraft cannot provide definite evidence that large amounts of deformation occurred at the rheasilvia antipode largely due to the presence of younger large impact craters in the region however a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur

analysis of transverse aeolian ridge profiles derived from hirise images of mars photoclinometry was used to analyze selected high resolution imaging science experiment hirise images of transverse aeolian ridges tars on mars sixty mars profiles have been assessed and a summary of their morphologic characteristics compiled measurements collected quantified the symmetry of the feature the curvature of the crest flank slopes width height and several comparative ratios results show that small tars have physical characteristics generally similar to measurements obtained from lower resolution images of mars the hirise image data allow for much improved sampling along each feature the improved resolution reveals relatively few features not seen in earlier studies but is well suited for topographic sampling measured tar profiles when scaled by the width of the feature can be compared to similarly scaled profiles for terrestrial dunes and megaripples published 2012 this article is a u s government work and is in the public domain in the usa

active aeolian processes on mars a regional study in arabia and meridiani terrae 1 we present evidence of widespread aeolian activity in the arabia terra meridiani region mars where different kinds of aeolian modifications have been detected and classified passing from the regional to the local scale we describe one particular dune field in meridiani planum where two ripple populations are distinguished by means of different migration rates moreover a consistent change in the ripple pattern is accompanied by significant dune advancement between 0 4 1 meter in one martian year that is locally triggered by large avalanche features this suggests that dune advancement may be common throughout the martian tropics

sinker tectonics an approach to the surface of miranda images of the uranian moon miranda show three areas of complex terrain trapezoidal to ovoid in shape with no previously known counterpart these features called coronae are characterized by an inner core of intersecting ridges and troughs surrounded by a series of subparallel and concentric bands two of the proposed explanations for these tectonic features involve mantle convection driven by density anomalies the present study determines the surface stresses induced by such density anomalies and their expected surface expression miranda is modeled as having a rigid core surrounded by a mantle capable of viscous flow which is overlain by a thin elastic lithosphere the density anomaly is modeled as a point mass representing the sinker or riser the model is developed in a spherical coordinate system having axial symmetry about a 0 axis passing through the sinker the flow field set up by the sinking or rising mass is expressed in terms of a stream function using legendre sums streamlines are functions of the position of the mass anomaly and the radii of the core mantle and mantle lithosphere boundaries only magnitudes of the stream function depend upon the size of the mass anomaly its local gravitational field and the mantle lithosphere radius the stresses on the base of the lithosphere are then determined from the equations of motion the viscosity of the mantle material has no effect on the stresses produced at any given time but does control the velocity of the anomaly expressions for displacements and stresses in the elastic shell are developed as legendre sums and coupled to the viscous mantle solution with appropriate boundary conditions the stresses derived yield a pattern of expected tectonic expression this tectonic style varies with the radius of the body for a small body surface stresses are dominant in the area above the mass anomaly while farther away stresses at the base of the lithosphere control the tectonics directly over a sinker and extending outward 15 is a zone of both latitudinal and longitudinal surface compression which produces folding and thrust faults with no preferred orientation beyond 15 longitudinal stresses at the base of the lithosphere are extensional while latitudinal stresses are compressional producing a zone of folding and thrusts concentric about the sinker stresses induced by a riser are opposite in sign and predict extensional features with the same orientation as the compressional features predicted for a sinker stress magnitudes generally decrease with distance from the sinker and also decrease with both increasing sinker depth and lithosphere thickness the predictions of the sinker model are in good agreement with the tectonic styles of the banded and ridged terrains of miranda as determined from the topography as imaged by voyager 2 this agreement supports the idea that the coronae on miranda were produced by dense masses originally located near the surface that sank into miranda s interior for larger bodies the stresses at the surface and base are nearly equal longitudinal stresses are greater than latitudinal stresses resulting in compressional features with a preferred orientation radial to the sinker location the changeover in tectonic styles occurs at a planetary radius of approximately 1500 km

reorientation of vesta gravity and tectonic predictions 1 vesta s large southern hemisphere impact basin is likely to have caused reorientation however because the basin is not centred at the south pole vesta likely also has a remnant rotational figure reorientation of 6 is predicted to have occurred based on the dimensions of the basin existing measurements of vesta s shape are consistent with 20 or less reorientation and 20 or less despinning both the remnant rotational figure and the basin contribute to the degree 2 gravity coefficients which will be measured by the dawn mission and will provide a test of the reorientation hypothesis reorientation and despinning also give rise to stresses vesta s stress state is likely to be dominated by isotropic contraction due to cooling which does not affect the gravity coefficients however the orientation of the resulting thrust features will be controlled by the amount of reorientation and despinning providing another observational test

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

megaripple migration on mars aeolian megaripples with 5 to 50 m spacing are abundant on the surface of mars these features were repeatedly targeted by high resolution orbital images but they have never been observed to move thus aeolian megaripples especially the bright toned ones often referred as transverse aeolian ridges tars have been interpreted as relict features of a past climate in this report we show evidence for the migration of bright toned megaripples spaced 1 to 35 m 5 m on average in two equatorial areas on mars indicating that megaripples and small tars can be active today the moving megaripples display sand fluxes that are 2 orders of magnitudes lower than the surrounding dunes on average and unlike similar bedforms on earth can migrate obliquely and longitudinally in addition the active megaripples in the two study areas of syrtis major and mawrth vallis show very similar flux distributions echoing the similarities between dune crest fluxes in the two study areas and suggesting the existence of a relationship between dune and megaripple fluxes that can be explored elsewhere active megaripples together with high sand flux dunes represent a key indicator of strong winds at the surface of mars a past climate with a denser atmosphere is not necessary to explain their accumulation and migration

multiscale seismic attributes a wavelet based method and its application to high resolution seismic and ground truth data we propose a wavelet based method to characterize acoustic impedance discontinuities from a multiscale analysis of seismic reflected waves our approach relies on the analysis of ridge functions which contain most of the information of the wavelet transform in a sparse support this method falls in the framework of the wavelet response wr introduced by le gonidec et al which analyses the impedance multiscale behaviour by propagating dilated wavelets into the medium we further extend the wr by considering its application to broad band seismic data we take into account the bandpass filter effect related to the limited frequency range of the seismic source we apply the method to a deep water seismic experiment performed in 2008 during the erig3d cruise to demonstrate the potential of ridge functions as multiscale seismic attributes in conjunction to the analysis of seismic data acquired by the deep towed sysif system 200 2200 hz we use ground truth data to characterize the fine scale structure of superficial sediments by using the continuous wavelet transform cwt the availability of in situ measurements allows to validate the relationship between cwt andwr and to estimate the attenuation of seismic waves into the sediments once validated the method is applied on a whole seismic profile and wr ridge functions are computed for two particular reflectors the reflector thicknesses fall below the resolution limit of the seismic experiment making the wr seismic attributes a super resolution method

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

object based dune analysis automated dune mapping and pattern characterization for ganges chasma and gale crater mars abstract a method that enables the automated mapping and characterization of dune fields on mars is described using ctx image mosaics the introduced object based dune analysis obda technique produces an objective and reproducible mapping of dune morphologies over extensive areas the data set thus obtained integrates a large variety of data allowing a simple cross analysis of dune patterns spectral and morphometric information and mesoscale wind models two dune fields located in gale crater and ganges chasma were used to test and validate the methodology the segmentation of dune related morphologies is highly efficient reaching overall accuracies of 95 in addition we show that the automated segmentation of slipface traces is also possible with expected accuracies of 85 90 a qualitative and quantitative comparison of the final outputs with photointerpretations is performed and the precision of the directional characterization of the dune patterns is evaluated we demonstrate a good agreement between the obda outputs and the photointerpreted dune morphologies with local trend deviations below 45 for 80 95 of the mapped areas because the developed algorithm is tuned for the recognition of linear features from the imagery the slipfaces of small barchans can be preferentially overlooked owing to their small extent at the spatial resolution of the ctx mosaics dune types composed of longer linear morphologies are much better represented including correct mapping of secondary structures having proved the effectiveness and accuracy of the mapping procedure we discuss its future applications for the improvement of dune catalogs on mars

lunar multiring basins and the cratering process abstract numerous studies of the lunar gravity field have concluded that the lunar moho is substantially uplifted beneath the young multiring basins this uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor using a new dual layered crustal thickness model of the moon the excavation cavities of some nearside multiring basins grimaldi and larger and younger than tranquillitatis were reconstructed by restoring the uplifted moho to its preimpact location the farside south pole aitken spa basin was also considered due to its importance in deciphering lunar evolution restoring the moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined using these reconstructed excavation cavities the basin forming process was investigated excavation cavity diameters were generally found to be on the small side of most previous estimates for orientale the modeled excavation cavity lies within the inner rook ring additionally with the exception of the three largest basins serenitatis imbrium and south pole aitken the depth diameter ratios of the excavation cavities were found to be 0 115 0 005 a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size the three largest basins however appear to have significantly shallower depths of excavation compared to this trend it is possible that this may reflect a different physical process of crater formation e g nonproportional scaling special impact conditions or postimpact modification processes the crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust we interpret this thickened crust as representing thick basin ejecta deposits and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small scale experimental studies if multiring basins ever possessed a terraced main crater rim this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim we also show that the interiors of many basins were superisostatic before mare volcanism commenced those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat producing elements

tectonic patterns on reoriented and despun planetary bodies abstract several processes may produce global tectonic patterns on the surface of a planetary body the stresses associated with distortions of biaxial figures due to despinning or reorientation were first calculated by vening meinesz vening meinesz f a 1947 trans am geophys union 28 1 1 23 we adopt a mathematically equivalent but physically more meaningful treatment for distortions associated with rotation the new approach allows us to find analytic solutions for the general case of stresses associated with distortions of biaxial or triaxial planetary figures distortions of biaxial figures may be driven by variations in rotation rate rotation axis orientation or the combination of both distortions of triaxial figures may be driven by the same mechanisms and or variations in tidal axis orientation for tidally deformed satellites while the magnitude of the resulting stresses depends on the adopted elastic and physical parameters the expected tectonic pattern is independent of these parameters for these mechanisms reorientation of the rotation tidal axis alone is expected to produce normal thrust faulting provinces enclosing the initial rotation tidal poles and thrust normal faulting provinces enclosing the final rotation tidal poles reorientation of both the rotation and tidal axis results in a wide variety of tectonic patterns for different reorientation geometries on europa the tidal axis reorientation which generally accompanies rotation axis reorientations may provide an alternative explanation for tectonic features that have been interpreted as evidence for non synchronous rotation the observed tectonic pattern on enceladus is more easily explained by a large reorientation 90 of the rotation axis than by rotation rate variations

cassini observes the active south pole of enceladus cassini has identified a geologically active province at the south pole of saturn s moon enceladus in images acquired by the imaging science subsystem iss this region is circumscribed by a chain of folded ridges and troughs at 55 s latitude the terrain southward of this boundary is distinguished by its albedo and color contrasts elevated temperatures extreme geologic youth and narrow tectonic rifts that exhibit coarse grained ice and coincide with the hottest temperatures measured in the region jets of fine icy particles that supply saturn s e ring emanate from this province carried aloft by water vapor probably venting from subsurface reservoirs of liquid water the shape of enceladus suggests a possible intense heating epoch in the past by capture into a 1 4 secondary spin orbit resonance

polar wander of an ice shell on europa an ice shell on europa that is decoupled from the silicate core by a layer of liquid water has a thermal equilibrium thickness profile that varies with position over its surface because of spatial variations in the surface temperature and tidal dissipation within the ice g w ojakangas and d j stevenson 1989 icarus 81 220 241 the second spherical harmonic degree components of these thickness variations and of any fossil rotational and tidal bulges present on the shell contribute to the inertia tensor of the body the problem is that of a planetary elastic lithosphere that is topographically loaded from below following r j willemann and d l turcotte 1981 proc lunar planet sci b 12 837 851 we develop equations describing the variations in the inertia tensor of a body when second harmonic degree topography is added to the base of the crust for an ice shell on europa it is found that a state of thermal equilibrium may involve an unusual orientation of the principal axes of inertia when hydrostatic bulges are ignored in which the intermediate and maximum principal moments are interchanged to reach the preferred orientation for synchronous satellites a thermal equilibrium shell must execute a net reorientation of 90 about the satellite planet direction we present a simple model of rigid synchronously rotating satellite in a circular orbit for which the principal moment difference b c increases with time becoming positive for t 0 the model demonstrates that the expected reorientation is indeed dynamically favored we then consider a more realistic model in which europa s shell and ocean are assumed to reorient as a single entity independently of the core hindered only by viscous dissipation within the shell such coupling of the shell with the ocean and lack of coupling with the core is suggested by g w ojakangas 1988 coupled thermal and dynamical evolution of planetary bodies ph d thesis california institute of technology the model suggests that friction in the shell eliminates the possibility of polar wander unless a low conductivity regolith increases the near surface temperature by a few tens of degrees so the ice just below the regolith behaves viscously on the polar wander time scale however if the equator to pole near surface temperature difference is decreased by more than a critical model dependent amount the shell s inertia tensor no longer has an unstable form in thermal equilibrium alternatively polar wander may occur if preexisting surface fractures e g due to tidal stresses extend to a depth where the ice behaves viscously these fractures must be lubricated perhaps by liquid water from below if the temperature t f at the base of the regolith or the surface fractures is 125 k the model suggests that polar wander occurs on a time scale 2 x 10 6 years decreasing as t f increases after b c becomes postive in the absence of dissipation polar wander would occur in few x 10 3 years large scale polar wander may occur episodically separated by periods on the order of the thermal diffusion time for the shell 10 7 years although a state of slow continuous drifting of the pole is also possible the time scales of viscous flow of topography at the base of the ice is also near 10 7 years polar wander is a very effective means for fracturing the ice and may have contributed to the observed global fracture systems in europa s ice

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

reorientation of sputnik planitia implies a subsurface ocean on pluto to explain the position of the sputnik planitia basin on pluto the feature would need to have formed via impact and pluto would need to have a subsurface ocean the present location of sputnik planitia the prominent deep icy basin on pluto is close to one of the longitudes of the dwarf planet s tidal axis by analogy with other large basins in the solar system it is thought to be an impact feature although reorientation arising from tidal and rotational torques can explain the present day location of the basin it requires the feature to be a positive gravity anomaly despite its negative topography francis nimmo et al argue that if sputnik planitia formed via impact and if pluto possesses a subsurface ocean a positive gravity anomaly would naturally result because of shell thinning and ocean uplift followed by later modest nitrogen deposition this is one of four papers on the geology of sputnik planitia in this issue of nature in news views amy barr puts these latest contributions into context the deep nitrogen covered basin on pluto informally named sputnik planitia is located very close to the longitude of pluto s tidal axis1 and may be an impact feature2 by analogy with other large basins in the solar system3 4 reorientation5 6 7 of sputnik planitia arising from tidal and rotational torques can explain the basin s present day location but requires the feature to be a positive gravity anomaly7 despite its negative topography here we argue that if sputnik planitia did indeed form as a result of an impact and if pluto possesses a subsurface ocean the required positive gravity anomaly would naturally result because of shell thinning and ocean uplift followed by later modest nitrogen deposition without a subsurface ocean a positive gravity anomaly requires an implausibly thick nitrogen layer exceeding 40 kilometres to prolong the lifetime of such a subsurface ocean to the present day8 and to maintain ocean uplift a rigid conductive water ice shell is required because nitrogen deposition is latitude dependent9 nitrogen loading and reorientation may have exhibited complex feedbacks7

the birth and death of transverse aeolian ridges on mars transverse aeolian ridges tars are small bright windblown deposits found throughout the martian tropics that stand a few meters tall and are spaced a few tens of meters apart the origin of these features remains mysterious more than 20 years after their discovery on mars this paper presents a new hypothesis that some of the tars could be indurated dust deposits emplaced millions of years ago during periods of higher axial obliquity it suggests that these tars are primary depositional bed forms that accumulated in place from dust carried by the winds in suspension perhaps in a manner comparable to antidunes on earth and were subsequently indurated and eroded to their current states by eons of sandblasting it points out examples of modern dust drifts and dune like features that appear to have been recently formed by dust accumulating directly onto the surface from atmospheric suspension it shows how these pristine dust deposits could evolve to explain the range of morphologies of the tars finally it explains how the known properties of many tars are consistent with this hypothesis including their composition thermal behavior and distribution

ripple migration and dune activity on mars evidence for dynamic wind processes 1 in this report we show evidence of widespread ripple migration over the stoss side of dark barchan dunes in nili patera on mars the measured average migration of 1 7 meters in less than 4 terrestrial months clearly indicates that active sand saltation is occurring in the study area in addition we document widespread changes in the dune base ground surface contact and in the slip face structures showing that not only the ripples but the whole dunes are actually migrating in the present day atmospheric setting these results provide unequivocal evidence of recent aeolian activity and suggest that other dunes and ripples on mars may also be active

reorientation of planets with elastic lithospheres abstract the orientation of a planet is controlled by the positions of the principal axes of the inertia tensor relative to the planetary surface using the theory for the deflection of thin elastic shells the principal axes are computed after emplacement of an arbitrary axisymmetrical load the partial compensation of the load and the partial relaxation of rotational flattening are included in the computation it is found that the amount of reorientation is independent of lithosphere thickness the parameters controlling the amount of reorientation are the location of the load and the size of the load compared to the rotational flattening the results indicate that the tharsis rise has probably reoriented mars by only 3 to 9 and certainly less that 18 the position of the caloris basin on mercury indicates that if the surrounding lava sheet controls the planetary orientation then the lava sheet is probably less than 2000 m thick

diapir induced reorientation of saturn s moon enceladus the recent discovery of active water venting from the surface of saturn s icy moon enceladus has caused a ripple of excitement as it suggests the possibility of subsurface liquid water geologically enceladus poses problems one the reason why the hotspot of geological activity is centred precisely on the south pole has been tackled by francis nimmo and robert t pappalardo they identify reorientation of enceladus s rotation axis as the likely cause and an upwelling of low density material to the surface as the cause of that reorientation comparison of the distribution of impact craters on the leading and trailing hemispheres will be a good test of the reorientation part of the theory enceladus is a small icy satellite of saturn its south polar region consists of young tectonically deformed terrain and has an anomalously high heat flux1 2 this heat flux is probably due to localized tidal dissipation within either the ice shell3 or the underlying silicate core4 the surface deformation is plausibly due to upwelling of low density material diapirism5 as a result of this tidal heating here we show that the current polar location of the hotspot can be explained by reorientation of the satellite s rotation axis because of the presence of a low density diapir if the diapir is in the ice shell then the shell must be relatively thick and maintain significant rigidity elastic thickness greater than 0 5 km if the diapir is in the silicate core then enceladus cannot possess a global subsurface ocean because the core must be coupled to the overlying ice for reorientation to occur the reorientation generates large 10 mpa tectonic stress patterns6 that are compatible with the observed deformation of the south polar region2 we predict that the distribution of impact craters on the surface will not show the usual leading hemisphere trailing hemisphere asymmetry a low density diapir also yields a potentially observable negative gravity anomaly

constraints on gas giant satellite formation from the interior states of partially differentiated satellites an icy satellite whose interior is composed of a homogeneous ice rock mixture must avoid melting during its entire history including during its formation when it was heated by deposition of accretional energy and short lived radioisotopes estimates of the temperature rise associated with radiogenic and accretional heating coupled with limits on satellite melting can be used to constrain the timing of formation of a partially differentiated satellite relative to the origin of the calcium aluminum rich inclusions cai s as a function of its accretion timescale and the protosatellite disk temperature td geological characterization and spacecraft radio tracking data suggest that callisto the outermost regular satellite of jupiter and saturn s mid sized satellite rhea are partially differentiated if their interiors are in hydrostatic equilibrium because the specific conditions during the satellites formation are uncertain we determine accretional temperature profiles for a range of values for td and accretion time scales with the limiting assumption that impactors deposit all their energy close to the surface leading to maximally effective radiative cooling we find that callisto can remain unmelted during formation if it accreted on a time scale longer than 0 6 myr considering both radiogenic and accretional heating callisto must have finished accreting no earlier than 4 myr after formation of cai s for td 100 k warmer disks or larger impactors that deposit their energy at depth in the satellite would require longer and or later formation times if rhea accreted slowly in 10 5