gravity field of the moon from the gravity recovery and interior laboratory grail mission spacecraft to spacecraft tracking observations from the gravity recovery and interior laboratory grail have been used to construct a gravitational field of the moon to spherical harmonic degree and order 420 the grail field reveals features not previously resolved including tectonic structures volcanic landforms basin rings crater central peaks and numerous simple craters from degrees 80 through 300 over 98 of the gravitational signature is associated with topography a result that reflects the preservation of crater relief in highly fractured crust the remaining 2 represents fine details of subsurface structure not previously resolved grail elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies

lunar magnetism polar displacements and primeval satellites in the earth moon system the discovery that rocks of the lunar crust possess remanent magnetism is a surprising result in view of the fact that the present moon does not exhibit a magnetic field nor was there once evidence of an iron core in which one could be generated this review looks at the concept of lunar palaeomagnetism and discusses its implications for lunar science as a whole

the geomorphology of rhea implications for geologic history and surface processes analyses of landforms on rhea are used to define three physiographic provinces cratered terrain 1 undifferentiated cratered terrain 1 lineated cratered terrain 2 for which stratigraphic relationships are derived and examined in relation to major impact basins and tectonic features the formation of multiringed basins may have caused or at least controlled the locations of major resurfacing and mantling events the diameters of central peaks relative to impact crater diameters are significantly larger than those within craters on the moon and mercury rim segments of polygonal craters have preferential orientations both crater and noncrater lineaments show regional orientations that do not fit current global or regional stress models superposition relationships of provinces and associated features imply the following postaccretional sequence for the geological evolution of rhea s surface cratered terrain 1 undifferentiated formed first then linear troughs and chains of coalescing pits developed extensively on cratered terrain 1 lineated a resurfacing event emplaced cratered terrain 2 on which small craters scarps and ridges formed small grooves on cratered terrain 1 undifferentiated are tentatively assigned a young age based on their susceptibility to impact erosion this geologic history is generally consistent with theoretical models for rhea s evolution

albedo and color maps of the saturnian satellites abstract maps of the normal reflectances and color ratios 0 58 0 41 m have been prepared from voyager images for the following saturian satellites mimas enceladus tethys dione and rhea albedo and color variegations on these objects are in general much less than those of the galilean satellites enceladus and mimas have no albedo or color variegations greater than 10 in the observed regions of their surfaces with the exception of a small high albedo area in the southern polar region of enceladus which may be similar to the bright wispy streaks linea on rhea and dione we find an albedo dichotomy of 20 between the leading bright and trailing sides of dione and confirm a similar dichotomy found by verbiscer and veverka icarus 1989 82 336 for rhea this dichotomy is not as marked in the color maps differences of 5 10 were found for the hemispheres of these two satellites unlike the galilean satellites there is little general correlation between color and albedo the only clear example we could find of the expected result that higher albedo surfaces are bluer is for the bright wispy streaks linea on the trailing sides of rhea and dione on tethys we see two lower albedo features the one on the trailing side is redder than the ambient terrain whereas the one on the leading side is bluer there is little correlation between mapped geologic units and features and the color albedo patterns rhea and the trailing side of dione are the only examples where correlations exist the higher albedo photometrically bland leading sides of tethys dione and rhea suggest enhanced erosion of this hemisphere by meteoritic and cometary impact in addition the optical properties of these satellites may be affected by two exogenic processes not considered extensively in previous studies alteration of the surfaces by ultraviolet radiation and accretion of particles from saturn s e ring

east west faults due to planetary contraction abstract contraction expansion and despinning have been common in the past evolution of solar system bodies these processes deform the lithosphere until it breaks along faults their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface while the search for despinning tectonics has not been conclusive there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion though the pattern is seldom isotropic as predicted the cause of the non random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction expansion with another deformation despinning tidal deformation reorientation another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation using thin elastic shells with variable thickness i show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction expansion into a pattern of faults striking east west preferably formed in the equatorial region by contrast lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike slip faults and polar normal faults if contraction is added to despinning the despinning pattern first shifts to thrust faults striking north south and then to thrust faults striking east west if the lithosphere is thinner at the poles the tectonic pattern caused by contraction expansion consists of faults striking north south i start by predicting the main characteristics of the stress pattern with symmetry arguments i further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two making it easy to determine fault orientation for combined contraction and despinning i give two methods for solving the equations of elasticity one numerical and the other semi analytical the latter method yields explicit formulas for stresses as expansions in legendre polynomials about the solution for constant shell thickness though i only discuss the cases of a lithosphere thinner at the equator or at the poles the method is applicable for any latitudinal variation of the lithospheric thickness on iapetus contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge on mercury the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation north south or east west which may be relevant to the orientation of lobate scarps

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

albedo dichotomy of rhea hapke analysis of voyager photometry abstract the photometric behavior of rhea s surface has been characterized by fitting hapke s model to both full disk and disk resolved voyager observations the two solutions agree well demonstrating that accurate determinations of hapke parameters are possible from whole disk phase curves provided that extensive phase coverage is available the voyager clear filter eff 0 48 m phase coverage for rhea extends form 1 8 to 135 3 the low phase angle data contain evidence of a measurable opposition effect the high phase angle data allow a good estimate of hapke s large scale roughness parameter 13 and a better determination of the phase integral q 0 78 0 05 and bond albedo ab 0 49 0 06 the hapke analysis shows that the regolith compaction parameter h for rhea is definitely larger more compact regolith than that for titania but is comparable to that of the moon we find evidence of photometric differences other than albedo between the satellite s trailing and leading hemispheres our analysis suggests that the roughness parameter may be smaller for the brighter leading hemisphere than for the darker trailing half of the satellite our final product an albedo map of rhea closely reproduces the satellite s observed lightcurve and indicates that normal reflectances on rhea range from 0 52 near 310 w on the trailing hemisphere to 0 89 at 20 s 80 w on the leading hemisphere the albedo map shows that no terrain or feature in the trailing hemisphere is as bright as any in the leading hemisphere it also demonstrates a striking quasi circular low albedo region centered close to the satellite s anti apex of motion

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

the formation of terrains antipodal to major impacts abstract in 1975 schultz and gault suggested that seismic energy from a major impact could extensive could cause crystal fracturing and surface disruption on the part of the planet directly opposite the impact disrupted terrains of this type have been identified on mercury antipodal to the caloris impact basin and on the moon antipodal to imbrium large impact basins are found on many icy satellites herschel on mimas valhalla on callisto odysseus on tethys and others antipodal disruption from these impacts might appear as a fractured surface or as a resurfaced area the simplified arbitrary lagrangian eulerian sale code was used to calculate the antipodal pressures generated by large impacts into stylized icy satellites and planets that have different compositions and core sizes it was found that in a planet with a high density core sorrounded by a lower density mantle the incident pressure wave is refracted away from the antipode by the core and the generated antipodal pressure will be low in a planet with a low velocity core sorrounded by a mantle with a higher wave speed the core will focus the wavefront and the antipodal pressure will be high in both cases for equal impacts the magnitude is dependent upon the size of the core relative to the planet we also modeled specific impact basins on the moon mercury and several icy satellites to determine whether the impacts were capable of producing the observed antipodal surface features we found surface accelerations and pressures which exceed the strength of surface materials for the caloris impact on mercury imbrium on the moon odysseus on tethys and the herschel impact on mimas images of the surface antipodal to all of these impacts except herschel show a morphology which could be interpreted as impact induced seismically disrupted terrain

the tidal rotational shape of the moon and evidence for polar wander analysis of the moon s topography reveals that when its largest basins are removed the lunar shape is consistent with processes controlled by early earth tides and implies a reorientation of the moon s principal shape axes attempts to assess the various theories for the formation of the moon and to determine its true shape are complicated by the presence of several large basins on its surface that formed after the crust solidified here ian garrick bethell et al concentrate on the topography and gravity outside these basins and propose that the long wavelength shape of the moon has evolved through a combination of two early tidal processes first a crust building process controlled by early tidal heating throughout the moon and second the later development of a frozen tidal rotational bulge the authors also infer that an uneven distribution of internal density eventually reoriented the moon s polar axis by about 36 to the configuration that we see today the origin of the moon s large scale topography is important for understanding lunar geology1 lunar orbital evolution2 and the moon s orientation in the sky3 previous hypotheses for its origin have included late accretion events4 large impacts5 tidal effects6 and convection processes7 however testing these hypotheses and quantifying the moon s topography is complicated by the large basins that have formed since the crust crystallized here we estimate the large scale lunar topography and gravity spherical harmonics outside these basins and show that the bulk of the spherical harmonic degree 2 topography is consistent with a crust building process controlled by early tidal heating throughout the moon the remainder of the degree 2 topography is consistent with a frozen tidal rotational bulge that formed later at a semi major axis of about 32 earth radii the probability of the degree 2 shape having both tidal heating and frozen shape characteristics by chance is less than 1 we also infer that internal density contrasts eventually reoriented the moon s polar axis by 36 4 to the configuration we observe today together these results link the geology of the near and far sides and resolve long standing questions about the moon s large scale shape gravity and history of polar wander

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

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

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

the lunar mascons revisited the entire spectra of the surface topography and gravity field of lunar mascons are well presented by the high resolution clementine data the negative correlation between the topography and gravitational potential of the mascons suggests that they are dynamically supported both elastic support and viscous decay models are examined for the elastic support models the spectral characteristics and the lateral variations of the thickness of the elastic layer are determined on the basis of the thin spherical shell flexure formulation and using only the antivarying harmonics of the topography and potential an elastic layer thickness of about 50 km is required to support imbrium serenitatis and nectaris mascons about 35 km for crisium mascon and about 30 km for smythii and humorum mascons a layer of about 20 km thickness can support orientale mascon the strength envelopes of the upper 100 km of the moon within 4 3 gyr ago show that the elastic layer was not thick enough to support the mascons in the early history they decayed through viscous deformation of the lunar interior a lower limit of 6 1024 pa s is estimated for the lunar viscosity within 3 6 3 gyr ago we also determine the thicknesses of the crust and mare flows of the mascon basins on the basis of the assumption that the basins were isostatically compensated prior to the mascon formation the crust beneath the mascon basins is about 30 40 km except for crisium and orientale where the crust is about 20 km the mare flows of about 3 6 km are obtained for almost all of the mascon basins the topography and gravitational potential of aitken basin shows that the basin is compensated at 52 54 km depth the mantle beneath the basin has rebounded upward by about 20 30 km and is overlain by about 20 25 km of a mixture of the excavated crustal and mantle material the lack of a mascon and pervasive mare flows in the basin brings into question the current models of the mascon formation

coupled thermal orbital evolution of the early moon abstract coupled thermal orbital histories of early lunar evolution are considered in a simple model we consider a plagioclase lid overlying a magma ocean overlying a solid mantle tidal dissipation occurs in the plagioclase lid and heat transport is by conduction and melt migration we find that large orbital eccentricities can be obtained in this model we discuss possible consequences of this phase of large eccentricities for the shape of the moon and geochronology of lunar samples we find that the orbit can pass through the shape solution of garrick bethell et al garrick bethell i wisdom j zuber m 2006 science 313 652 but we argue that the shape cannot be maintained against elastic deformation as the orbit continues to evolve

tectonics on iapetus despinning respinning or something completely different abstract saturn s moon iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge stresses arising from such a non hydrostatic shape should in principle cause surface deformation tectonics as part of a search for such a tectonic signature lineaments linear surface features on iapetus were mapped on both its bright and dark hemispheres lineament orientations were then compared to model stress patterns predicted for spin down from a rotation period of 16 5 h or less to its present synchronous period and for a range of lithospheric thicknesses many lineaments are straight segments of crater rimwalls which may be faults or joints reactivated during complex crater collapse most striking are several large troughs on the bright trailing hemisphere these troughs appear to be extensional and are distinctive on that hemisphere because the interior floors and walls of the troughs contain dark material globally no specific evidence of strike slip or thrust offsets are seen but this could be due to the age and degraded nature of any such features we find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere the equatorial ridge was specifically excluded from this analysis and is considered separately modest evidence for preferred orientations 40 from north could be construed as consistent with respinning which is not necessarily far fetched assuming the rigidity of unfractured ice predicted maximum lithospheric differential stresses from despinning range from 1 mpa to 160 mpa for the elastic spheroid and thin lithosphere limits respectively although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure the tectonic signature of despinning may have been obscured over time because the surface of iapetus is very ancient iapetus thick lithosphere may have inhibited the full tectonic expression of despinning or both several prominent lineaments strike e w and are thus parallel to the equatorial ridge though not physically close to it but a tectonic or volcanic origin for the ridge is highly problematic

the effect of target properties on crater morphology comparison of central peak craters on the moon and ganymede we examine the morphology of central peak craters on the moon and ganymede in order to investigate differences in the near surface properties of these bodies we have extracted topographic profiles across craters on ganymede using galileo images and use these data to compile scaling trends comparisons between lunar and ganymede craters show that crater depth wall slope and amount of central uplift are all affected by material properties we observe no major differences between similar sized craters in the dark and bright terrain of ganymede suggesting that dark terrain does not contain enough silicate material to significantly increase the strength of the surface ice below crater diameters of 12 km central peak craters on ganymede and simple craters on the moon have similar rim heights indicating comparable amounts of rim collapse this suggests that the formation of central peaks at smaller crater diameters on ganymede than the moon is dominated by enhanced central floor uplift rather than rim collapse crater wall slope trends are similar on the moon and ganymede indicating that there is a similar trend in material weakening with increasing crater size and possibly that the mechanism of weakening during impact is analogous in icy and rocky targets we have run a suite of numerical models to simulate the formation of central peak craters on ganymede and the moon our modeling shows that the same styles of strength model can be applied to ice and rock and that the strength model parameters do not differ significantly between materials

measurements of the elastic thickness under ancient lunar terrain abstract the part of a planet s gravity that is coherent with its topography provides information about the deflection of its crust after loading and hence the resistance of its lithosphere to bending at the time the load was emplaced we used observed line of sight accelerations from lunar prospector together with the accelerations we would have expected if anomalies in the gravity field were only caused by topography to estimate the admittance and coherence between topography and gravity over several regions of the lunar nearside we then compared our estimates to what we would expect if the lithosphere behaved as a floating elastic plate or shell assuming a linear relationship between topography and gravity we found in the region surrounding the southern highland crater clavius that the data can be modeled using a thin plate with t e 12 5 km and uncorrelated loads at the surface and base of the upper crust a spherical shell model with surface loading is less satisfactory to fit the admittance adequately requires topography with wavelengths over 400 km to be formed when t e 1 km and the remainder when t e 7 5 km by contrast the apparent lack of compensation around the youngest giant impact basins requires a plate with t e 80 km or a shell with t e 25 km our results indicate the thickness of the lunar lithosphere increased from 12 km in the pre nectarian to 25 km in the nectarian

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

ice cap melting and low viscosity crustal root explain the narrow geodetic uplift of the western alps more than 10 years of geodetic measurements demonstrate an uplift rate of 1 3 mm yr of the high topography region of the western alps by contrast no significant horizontal motion has been detected two uplift mechanisms have been proposed 1 the isostatic response to denudation responsible for only a fraction of the observed uplift and 2 the rebound induced by the wurmian ice cap melting which predicts a broader uplifting region than the one evidenced by geodetic observations using a numerical model to fit the geodetic data we show that a crustal viscosity contrast between the foreland and the central part of the alps the latter being weaker with a viscosity of 1021 pa s is needed the vertical rates are enhanced if the strong uppermost mantle beneath the moho is interrupted across the alps therefore allowing a weak vertical rheological anomaly over the entire lithosphere

the tectonic and volcanic history of rhea as inferred from studies of scarps ridges troughs and other lineaments abstract analyses of landforms on rhea are used to define 13 types of geomorphic features interpreted to be of tectonic and volcano tectonic origin all types except one seem to be extensional in origin they occurred after the formation of population i craters and are contemporaneous with a major resurfacing event the troughs grabens grooves pit chains scarps and other lineaments vare pure extensional features while the ridges are volcanic features formed in an extensional stress field this global surface extension was followed by an era of global compression which produced megaridges and megascarps these compressional features are not numerous enough to fit the theoretical stresses predicted by the typical thermal models of rhea all the extensional landforms appear to form a global grid pattern this planetary wide grid is similar in direction to the theoretical pattern of a tidally distorted planet that suggests either an orbital variation very early in the history of rhea or the existence of an unknown process which has oriented the stress field during the extensional period

the geology of tethys the surface of the saturnian satellite tethys has been recognized to consist of four major landforms or physiographic provinces hilly cratered terrain the odysseus impact structure rifts and lineaments including ithaca chasma and plains terrain two alternate scenarios for the geological evolution of the satellite are developed the first model assumes that ithaca chasma and the other rifts and lineaments were created by the odysseus impact event the second model assumes that rifting was a result of the freeze expansion of tethys liquid water interior in both models the hilly cratered terrain is postulated to be the oldest unit and the plains terrain the youngest

proteus geology shape and catastrophic destruction abstract proteus with a mean radius of 209 8 km is the largest irregular shaped icy body in the solar system it has a primitive heavily cratered surface its most prominent feature is an impact crater 255 km in diameter and 10 to 15 km deep relatively one of the largest craters in the solar system this prominent crater is similar in relative size to giant impacts on several other planets and satellites all of which are larger than a recent theoretical limit for catastrophic disruption consistency is obtained by inclusion of reaccumulation in the disruption analysis based on this reanalysis proteus was not brought to the brink of catastrophic destruction by the giant impact but only to the limit beyond which the surface of the impacted body and the form of the disrupting crater are modified beyond recognition a network of large streaks interpreted as troughs are also discernable this network too extensive to be explained by strains produced in traditional thermal history calculations may represent the onset of significant internal disruption in near catastrophic impacts proteus sphericity is near unity but it has one of the roughest surfaces known thus proteus is a transitional object in the irregular spherical shape spectrum for icy satellites its global figure is relaxed but its surface features are unrelaxed proteus intermediate relaxation state requires relatively high internal temperatures the required temperatures and the observed radius of the shape transition for icy bodies is consistent with radiogenic heating in bodies with deep porous regoliths

evidence for lunar true polar wander and a past low eccentricity synchronous lunar orbit as first noted 200 years ago by laplace the moon s rotational and tidal bulges are significantly larger than expected given the moon s present orbital and rotational state this excess deformation has been ascribed to a fossil figure frozen in when the moon was closer to the earth however the observed figure is only consistent with an eccentric and non synchronous orbit contrary to our understanding of the moon s formation and evolution here we show that lunar mascons and impact basins have a significant contribution to the observed lunar figure removing their contribution reveals a misaligned fossil figure consistent with an early epoch of true polar wander driven by the formation of the south pole aitken basin and an early low eccentricity synchronous lunar orbit this new self consistent model solves a long standing problem in planetary science and will inform future studies of the moon s dynamical evolution and early dynamo

fossil figure contribution to the lunar figure abstract the unusual shape of the moon given its present rotational and orbital state has been explained as due to a fossil figure preserving a record of remnant rotational and tidal deformation jeffreys h 1915 mem r astron soc 60 187 217 lambeck k pullan s 1980 phys earth planet interiors 22 29 35 garrick bethell i wisdom j zuber m t 2006 science 313 652 655 however previous studies assume infinite rigidity and ignore deformation due to changes in the rotational and orbital potentials as the moon evolves to the present state we interpret the global lunar figure with a physical model that takes into account this deformation although the moon deforms in response to rotational and orbital changes a fossil figure capable of explaining the observed figure can be preserved by an elastic lithosphere

sources of shape variation in lunar impact craters fourier shape analysis r mode factor analysis of fourier harmonics that describe the shape in plan view of 716 large diameter 15 km nearside lunar craters shows that two factors explain 84 3 of shape variance observed in the sample factor i accounts for 68 2 of the sample variance and describes moderate scale roughness defined by harmonics 7 through 10 shape variation described by these harmonics is related to surficial lunar processes of degradation that modify crater shape in plan dominant among these processes are ejecta scour from large impact events and ongoing aging factor ii accounts for 16 1 of the observed shape variance and describes polygonal shape elements related to harmonics 2 3 4 and 6 variation in these harmonics is tied to variables that distort the spherical symmetry of crater forming processes the dominant contributor among these variables is the nature of geologic structural patterns in impacted material unlike transient features described by factor i polygonal shape elements described by factor ii do not change appreciably with time the permanence of these features and their relation to lunar geologic structure suggest that the shape of old craters carries the imprint of geologic structural relationships present in early lunar crust

shallow normal fault slopes on saturnian icy satellites fault dips are a function of the coefficient of internal friction i of the lithospheric material laboratory deformation experiments of h2o ice at conditions applicable to icy bodies yield 0 i 0 55 such that normal faults dip between 45 and 59 we tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three saturnian satellites analyzed faults within ithaca chasma on tethys and avaiki chasmata on rhea all exhibit shallower than predicted topographic slopes across the fault scarp which we term fault slopes a scarp of padua chasmata within dione s wispy terrain also has a shallow fault slope although three others that make up palatine chasmata exhibit steeper slopes as predicted we infer that viscous relaxation is the most viable explanation for these shallow fault slopes and we model the potential role of viscous relaxation in creating shallow fault slopes our modeling results support formation of these normal faults with steep dips consistent with deformation experiments followed by their relaxation due to lithospheric heating events related to radionuclide decay the steepest fault slopes in this terrain yield 0 i 0 73 for dione s lithospheric ice which overlaps the dip range predicted from experiments results of this work suggest that viscous relaxation substantially affected fault slopes on tethys rhea and dione by implication these processes may have also affected fault geometries on other icy satellites

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

the tectonic and volcanic history of dione abstract the tectonic and volcanic modifications of dione are described and interpreted it is proposed that after the formation of a brittle outer shell but before the end of heavy meteoritic bombardment global expansion due to radionuclide heating and perhaps a loss of oblateness due to tidal despinning and orbital recession produced a global system of lineaments an nh 3 h 2 o melt was produced and erupted on the surface to form plains units cooling of the interior or a phase change led to horizontal compression in the surface compression of a thick deposit of plains material possibly overlying a decollement surface is proposed to explain cratered plains ridges developed as thrust or high angle reverse faults following formation of ridges and smooth plains the surface experienced light cratering

saturn s icy satellites and rings investigated by cassini vims iii radial compositional variability in the last few years cassini vims the visible and infrared mapping spectrometer returned to us a comprehensive view of the saturn s icy satellites and rings after having analyzed the satellites spectral properties filacchione g capaccioni f mccord t b coradini a cerroni p bellucci g tosi f d aversa e formisano v brown r h baines k h bibring j p buratti b j clark r n combes m cruikshank d p drossart p jaumann r langevin y matson d l mennella v nelson r m nicholson p d sicardy b sotin c hansen g hibbitts k showalter m newman s 2007 icarus 186 259 290 paper i and their distribution across the satellites hemispheres filacchione g capaccioni f clark r n cuzzi j n cruikshank d p coradini a cerroni p nicholson p d mccord t b brown r h buratti b j tosi f nelson r m jaumann r stephan k 2010 icarus 206 507 523 paper ii we proceed in this paper to investigate the radial variability of icy satellites principal and minor and main rings average spectral properties this analysis is done by using 2264 disk integrated observations of the satellites and a 12 700 pixels wide rings radial mosaic acquired with a spatial resolution of about 125 km pixel using different vis and ir spectral indicators e g spectral slopes and band depths we perform a comparative analysis of these data aimed to measure the distribution of water ice and red contaminant materials across saturn s system the average surface regolith grain sizes are estimated with different indicators through comparison with laboratory and synthetic spectra these measurements highlight very striking differences in the population here analyzed which vary from the almost uncontaminated and water ice rich surfaces of enceladus and calypso to the metal organic rich and red surfaces of iapetus leading hemisphere and phoebe rings spectra appear more red than the icy satellites in the visible range but show more intense 1 5 2 0 m band depths although their orbits are close to the f ring prometheus and pandora are different in surface composition prometheus in fact appears very water ice rich but at the same time very red at vis wavelengths these properties make it very similar to a b ring particles while pandora is bluer moving outwards we see the effects of e ring particles generated by enceladus plumes which contaminate satellites surfaces from mimas out to rhea we found some differences between tethys lagrangian moons calypso being much more water ice rich and bluer than telesto among outer satellites hyperion iapetus and phoebe we observe a linear trend in both water ice decrease and in reddening hyperion being the reddest object of the population the correlations among spectral slopes band depths visual albedo and phase permit us to cluster the saturnian population in different spectral classes which are detected not only among the principal satellites and rings but among co orbital minor moons as well these bodies are effectively the connection elements both in term of composition and evolution between the principal satellites and main rings finally we have applied hapke s theory to retrieve the best spectral fits to saturn s inner regular satellites from mimas to dione using the same methodology applied previously for rhea data discussed in ciarniello et al ciarniello m capaccioni f filacchione g clark r n cruikshank d p cerroni p coradini a brown r h buratti b j tosi f stephan k 2011 icarus 214 541 555

pristine impact crater morphology on pluto expectations for new horizons abstract this paper combines previous cratering studies and numerical modeling of the impact process at different impact velocities to predict crater morphology on pluto as an icy body pluto s craters are expected to be similar in morphology to those on the icy satellites lesser depth diameter ratios d d shallower wall slopes and the development of central uplifts in craters of smaller rim to rim diameter than craters on rocky bodies of similar gravity the low impact velocity of the pluto system 2 km s 1 might cause deviation from this generalization as the simulations presented in this work suggest that decreasing impact velocity from 10 km s 1 to 2 km s 1 results in deeper craters larger d d and a simple to complex transition diameter at larger crater sizes than predicted based on gravity scaling alone d 6 km conversely decreasing impact velocity from 2 km s 1 to 300 m s 1 produced smaller d d akin to the lower d d noted for secondary craters this complex relationship between impact velocity and d d suggests that there might be a larger range of pristine simple crater depths on pluto than on bodies with higher mean impact velocity the low impact velocities and correspondingly low volumes of impact melt generated at pluto might prevent the occurrence or limit the size of floor pits if their formation involves impact melt water the presence or not of central floor pit craters on pluto will thus provide a valuable test of floor pit formation theories the presence of summit pits or concentric craters on pluto is plausible and would indicate the presence of layering in the near sub surface palimpsests multi ring basins and other crater morphologies associated with high heat flow are not expected and would have important implications for pluto s thermal history if observed by new horizons