introduction to the special issue the formation and evolution of ceres occator crater abstract ceres the largest body in the asteroid belt was explored from orbit for the first time by the dawn mission the 92 km diameter occator crater contains bright regions or faculae which are one of the most remarkable discoveries of dawn s exploration of ceres a pit within the center of the crater contains both the cerealia facula and a central dome the vinalia faculae are located in the eastern crater floor occator s faculae are composed mostly of sodium carbonate and their single scattering albedo is the highest of any material on ceres surface the faculae do not occur in other impact craters on ceres and occator s extensive interior lobate materials are unusual understanding the driving forces behind the formation of occator crater and its faculae has the potential to lead us to a new understanding of the processes and conditions in ceres past and to possibly provide constraints about the present day internal state in this special issue we present a variety of investigations which use dawn data theoretical modeling and laboratory experiments to investigate occator crater and its faculae the results of these investigations are summarized and synthesized in the final paper of this collection herein we introduce ceres and the dawn mission and summarize the geophysical geological mineralogical and geochemical properties of ceres as derived from dawn data we also discuss the regional setting of occator crater and introduce occator crater itself

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

the last interglacial glacial cycle in ne fennoscandia a nearly continuous record from sokli finnish lapland abstract boreholes from the sokli basin in finnish lapland have recovered a unique sequence of diamicts interlayered with fine grained sediments that are rich in fossils the diamicts are interpreted as till beds deposited by the fennoscandian ice sheet the fine grained intercalations seem to represent a series of warm stages that include one interglacial and three interstadials the interglacial is characterized by a mixed taiga pollen assemblage and corresponds to a distinct diatom gyttja bed the interstadials show progressively colder floras from open birch forest to arctic forest limit to shrub tundra their sediments comprise gyttja interlayered with wood and sand interstadial 1 sands grading into sandy gyttja interstadial 2 and sands overlain by laminated silt and clay interstadial 3 the interstadial 2 and 3 deposits are underlain by diamict and are interpreted as deglaciation sequences silts with a tundra pollen assemblage separate the interglacial and interstadial 1 deposits luminescence dating evidence indicates that the interglacial bed most probably represents the nw european mainland eemian interglacial marine isotope stage mis 5e the interstadial 1 2 and 3 deposits are tentatively correlated with the early weichselian brorup and odderade interstadials and an interstadial of middle weichselian age respectively mis 5c 5a and 3 respectively the correlation implies that the easternmost part of the ice divide zone of the fennoscandian continental ice sheet was deglaciated during part of mis 3

the geology of the kerwan quadrangle of dwarf planet ceres investigating ceres oldest largest impact basin abstract we conducted a geologic mapping investigation of dawn spacecraft data to determine the geologic history of the kerwan impact basin region of dwarf planet ceres which is mostly located in the ac 7 kerwan quadrangle geological mapping was applied to dawn framing camera images from the low altitude mapping orbit lamo 35 m pixel and supplemented by digital terrain models and color images from the high altitude mapping orbit hamo 135 m pixel as well as preliminary visible and infrared spectrometer vir and gravity data the 284 km diameter kerwan impact basin is the oldest unequivocal impact crater on ceres and has a highly discontinuous polygonal degraded rim and contains a smooth unit that both fills the basin floor and surrounds the degraded rim to the west south and east although there are some subtle topographic features in the kerwan basin that could be interpreted as flow boundaries there is no indisputable evidence of cryovolcanic features in or around the basin however if such features existed they could be buried nevertheless all data point to impact induced melting of a cerean crust enriched in a volatile likely water ice to produce the kerwan smooth material subsequent geologic activity in this region includes emplacement of impact craters such as dantu which produced a variety of colorful deposits and rayed craters such as rao and cacaguat based on the crater size frequency distribution absolute model ages of the kerwan smooth material in and around the basin marking a minimum age for the kerwan basin our mapping defines this as the oldest boundary within the cerean geologic timescale separating the pre kerwanan and kerwanan periods at 1 3 ga lunar derived chronology model or 230 850 ma asteroid derived chronology model depending on strength of target material

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

an improved gravity model for mars goddard mars model 1 doppler tracking data of three orbiting spacecraft have been reanalyzed to develop a new gravitational field model for the planet mars goddard mars model 1 gmm 1 this model employs nearly all available data consisting of approximately 1100 days of s band tracking data collected by nasa s deep space network from the mariner 9 and viking 1 and viking 2 spacecraft in seven different orbits between 1971 and 1979 gmm 1 is complete to spherical harmonic degree and order 50 which corresponds to a half wavelength spatial resolution of 200 300 km where the data permit gmm 1 represents satellite orbits with considerably better accuracy than previous mars gravity models and shows greater resolution of identifiable geological structures the notable improvement in gmm 1 over previous models is a consequence of several factors improved computational capabilities the use of optimum weighting and least squares collocation solution techniques which stabilized the behavior of the solution at high degree and order and the use of longer satellite arcs than employed in previous solutions that were made possible by improved force and measurement models the inclusion of x band tracking data from the 379 km altitude near polar orbiting mars observer spacecraft should provide a significant improvement over gmm 1 particularly at high latitudes where current data poorly resolve the gravitational signature of the planet

composition and structure of the shallow subsurface of ceres revealed by crater morphology the dwarf planet ceres is thought to have an ice rich layer in its shallow subsurface the morphologies of craters however suggest little relaxation by viscous flow has occurred and instead indicate a subsurface that is less than 40 ice

an analysis of eemian climate in western and central europe on the basis of 31 pollen diagrams and additional data for botanical macrofossils an analysis is made of the last interglacial eemian climatic history in western and central europe the main tool for this analysis is the climatic indicator species method only selected woody species are used for the quantification of data partial climatic range diagrams are presented for abies alba acer monspessulanum acer tataricum buxus sempervirens tilia tomentosa the problem of time correlation and pollen zonation of the eemian is discussed the climatic analysis itself is based on an improved version of the indicator species method in this version not every site is analysed for its climatic values instead maps and tables on the migrational history of hedera ilex buxus abies and species of acer tilia and abies are the basis for climatic maps showing respectively january and july isotherms for the periods of the corylus zone e4a and the carpinus zone e5 it is concluded that mean january temperatures were as much as 3 c higher at amsterdam the netherlands than at present and mean temperatures in july were 2 c higher however the thermal maximum in winter was later zone e5 than the summer thermal maximum zone e4a winter temperatures changed parallel to rise and fall of global sea level precipitation changes are more difficult to estimate in the first part of the eemian precipitation must have been relatively low but from zone e4b onward it increased to higher values reaching 800 mm and probably substantially more in zones e5 and e6 hence the eemian climate was in its beginning relatively more contintental and later from e4b onward more oceanic however as compared with the holocene the eemian climate was generally speaking more oceanic

gravity field of the orientale basin from the gravity recovery and interior laboratory mission the orientale basin is the youngest and best preserved major impact structure on the moon we used the gravity recovery and interior laboratory grail spacecraft to investigate the gravitational field of orientale at 3 to 5 kilometer km horizontal resolution a volume of at least 3 4 0 2 106 km3 of crustal material was removed and redistributed during basin formation there is no preserved evidence of the transient crater that would reveal the basin s maximum volume but its diameter may now be inferred to be between 320 and 460 km the gravity field resolves distinctive structures of orientale s three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle the crustal structure of orientale provides constraints on the formation of multiring basins

palaeomagnetism of caledonian intrusive suites in the northern highlands of scotland constraints to tectonic movements within the caledonian orogenic belt abstract to evaluate the scale of tectonic movements within the northern sector of the 500 400 ma caledonian orogenic belt and its precambrian foreland zone between the great glen fault ggf zone to the southeast and the laurentian block to the northwest we have studied the palaeomagnetism of minor intrusive rocks within the northern highlands terrain these rocks include 1 1 amphibolites and other metamorphic rocks predating deformation 2 2 microdiorities dolentes and related suites emplaced and probably magnetised between 450 and 420 ma and 3 3 lower middle devonian lamprophyres a range of predominantly nne negative and ssw positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority wnw ese axis isolated southerly negative directed hematite held components suggests limited but no widespread remagnetisation in devonian carboniferous times comparison is made with data from other tectonic divisions in the caledonian orogenic belt and the bordering forelands palaeopoles from the northern highlands closely conform in part with north american ordovician poles and in part with the post ordovician palaeopoles from britain south of the ggf the definitive motions of the british caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the caledonian terrain in early ordovician times small relative motions during the remainder of ordovician times followed by large clockwise and then anticlockwise rotations during late ordovician to early silurian times contemporary with the last major movements on the moine thrust ca 430 ma late silurian devonian movements along the ggf were probably below the limits of palaeomagnetic detectability the collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by north american studies and the path for lower palaeozoic times is more complex than recognised hitherto

slurry extrusion on ceres from a convective mud bearing mantle ceres is a 940 km diameter dwarf planet that is predominantly composed of silicates and water ice in ceres partially differentiated interior extrusive processes have led to the emplacement on its surface of domes with heights of kilometres here we report the analysis of a gravity anomaly detected by the dawn spacecraft which is associated with the geologically recent dome ahuna mons by modelling the anomaly with a mass concentration method we determine that the subsurface structure includes a regional mantle uplift which we interpret as a plume this structure is the probable source of fluids forming ahuna mons and together with constraints from the dome s morphology indicates a rheological regime corresponding to a slurry of brine and solid particles we propose that the properties of such a solid liquid mixture can explain the viscous relaxation and the mineralogy of the dome the presence of a plume and of slurry material indicate recent convection in a mud bearing mantle the inferred slurry extrusion on ceres differs from the water dominated cryovolcanism of icy satellites and so reveals compositional and rheological diversity in extrusive phenomena on planetary surfaces ahuna mons dome on ceres formed by extrusion of a mixture of brine and solids sourced from a muddy mantle plume according to numerical modelling of slurry rheology and a gravity anomaly found by the dawn mission

on the origin of mascon basins on the moon and beyond 1 mascon basins on the moon are large craters that display significant positive free air and bouguer gravity anomalies an important question is why is not every large crater a mascon as less than half have been previously determined to be we detrend the free air topographic and bouguer gravity anomalies and find that most large basins 28 of 41 display mascon characteristics e g strong positive bouguer anomalies narrower than the surface rims negative free air gravity annuli surrounding the central highs generally are absent in the bouguer gravity implicating surface topography we propose that beneath a forming large basin the relatively narrow transient crater drives mantle uplift while upward and inward collapse forms the surface topography furthermore the nonmascon basins are all ancient and heavily degraded indicating a postimpact evolutionary process our results suggest that mascon formation is the standard for large impacts on the moon and by extension on other terrestrial planets

high velocity collisions from the lunar cataclysm recorded in asteroidal meteorites lunar samples suggest that the inner solar system was bombarded by asteroids about 4 gyr ago radiometric ages of meteorites suggest an unusual number of high velocity asteroids at this time consistent with a dynamical origin of the bombardment in which the asteroids were pushed by outer planet migration onto highly eccentric orbits

mobility of icy sand packs with application to martian permafrost 1 the physical state of water on mars has fundamental ramifications for both climatology and astrobiology the widespread presence of softened martian landforms such as impact craters can be attributed to viscous creep of subsurface ground ice we present laboratory experiments designed to determine the minimum amount of ice necessary to mobilize topography within martian permafrost our results show that the jammed to mobile transition of icy sand packs neither occurs at fixed ice content nor is dependent on temperature or stress but instead correlates strongly with the maximum dry packing density of the sand component viscosity also changes rapidly near the mobility transition the results suggest a potentially lower minimum volatile inventory for the impact pulverized megaregolith of mars furthermore the long term preservation of partially relaxed craters implies that the ice content of martian permafrost has remained close to that at the mobility transition throughout martian history

predictions of depth to ice on asteroids based on an asynchronous model of temperature impact stirring and ice loss abstract water ice near the surface of main belt asteroids is gradually lost to space a mantle of low thermal conductivity causes large surface temperature amplitudes and thus increased cooling by thermal re radiation lowering temperatures well below the fast rotator limit a computational barrier for modeling this ice loss is the multi scale character of the problem accurate temperatures require many time steps within a solar day but ice retreats slowly over billions of years this barrier is overcome with asynchronous coupling models of temperature ice loss and impact stirring each use their own time steps and are coupled with one another the model is applied to 1 ceres and 7968 elst pizarro on ceres ice can be expected in the top half meter poleward of 60 latitude on both hemispheres even if excursions of the axis tilt took place and even in the presence of impact gardening at the poles ice can be expected within a centimeter of the surface the retreating ice crust leads to emission of water from the surface mainly at the equator the gradually retreating ice supplies a water exosphere less dense than has been observed by the herschel telescope for main belt comet elst pizarro depths to ice depend on the properties of the surface mantle for a dust mantle estimated depths are on the order of a decimeter for a rocky surface the depth at the pole is on the order of one meter hence it could have been activated by a small impact that exposed buried ice

distribution of phyllosilicates on the surface of ceres the dwarf planet ceres is known to host phyllosilicate minerals at its surface but their distribution and origin have not previously been determined we used the spectrometer onboard the dawn spacecraft to map their spatial distribution on the basis of diagnostic absorption features in the visible and near infrared spectral range 0 25 to 5 0 micrometers we found that magnesium and ammonium bearing minerals are ubiquitous across the surface variations in the strength of the absorption features are spatially correlated and indicate considerable variability in the relative abundance of the phyllosilicates although their composition is fairly uniform these data along with the distinctive spectral properties of ceres relative to other asteroids and carbonaceous meteorites indicate that the phyllosilicates were formed endogenously by a globally widespread and extensive alteration process

the last interglacial glacial cycle in fennoscandia abstract the climate and glacier fluctuations during the last interglacial glacial cycle in fennoscandia seem to correspond roughly in time with the climate fluctuations recorded in france germany holland however the climate in fennoscandia was considerably colder and the coldest phases are recorded as periods with glacier fluctuations admittedly the exact age and amplitudes of some fluctuations are still much debated

viscosity of rock ice mixtures and applications to the evolution of icy satellites theory and experiments are used to establish lower and upper bounds on the ratio of actual viscosity to pure ice viscosity for a suspension of rock particles in a water ice matrix for typical conditions encountered in icy satellites this ratio is of order ten or possibly larger depending on unknown factors such as the particle size distribution it is shown that even this modest increase in viscosity may be enough to have caused a failure of solid state convective self regulation early in the evolution of a homogeneous rock water ice satellite provided the satellite is large enough and sufficiently silicate rich the criteria for this failure are satisfied by ganymede and are marginal for callisto if the silicates are hydrated failure of self regulation means that the viscosity is too high for the interior to remain completely solid and eliminate the heat production of long lived radioisotopes by solid state convection partial melting of the ice then occurs it is further shown that satellites of this size may then undergo runaway differentiation into a rock core and almost pure ice mantle because the gravitational energy release is sufficient to melt nearly all the ice and the rayleigh taylor instability time scale is short although the high pressure phases of ice melt the resulting water quickly refreezes at a higher level it is conjectured that these results explain the striking surface dissimilarity of ganymede and callisto if these satellites accreted cold and undifferentiated ganymede may have gone supercritical melted and differentiated because of a failure of self regulation whereas callisto remained undifferentiated to the present day like all proposed explanations for the ganymede callisto dichotomy this conjecture cannot be quantified with confidence because of inadequate or incomplete observations theory and experimental data

pitted terrains on 1 ceres and implications for shallow subsurface volatile distribution prior to the arrival of the dawn spacecraft at ceres the dwarf planet was anticipated to be ice rich searches for morphological features related to ice have been ongoing during dawn s mission at ceres here we report the identification of pitted terrains associated with fresh cerean impact craters the cerean pitted terrains exhibit strong morphological similarities to pitted materials previously identified on mars where ice is implicated in pit development and vesta where the presence of ice is debated we employ numerical models to investigate the formation of pitted materials on ceres and discuss the relative importance of water ice and other volatiles in pit development there we conclude that water ice likely plays an important role in pit development on ceres similar pitted terrains may be common in the asteroid belt and may be of interest to future missions motivated by both astrobiology and in situ resource utilization

the origin of lunar mascon basins high resolution gravity data from the gravity recovery and interior laboratory spacecraft have clarified the origin of lunar mass concentrations mascons free air gravity anomalies over lunar impact basins display bull s eye patterns consisting of a central positive mascon anomaly a surrounding negative collar and a positive outer annulus we show that this pattern results from impact basin excavation and collapse followed by isostatic adjustment and cooling and contraction of a voluminous melt pool we used a hydrocode to simulate the impact and a self consistent finite element model to simulate the subsequent viscoelastic relaxation and cooling the primary parameters controlling the modeled gravity signatures of mascon basins are the impactor energy the lunar thermal gradient at the time of impact the crustal thickness and the extent of volcanic fill

does strong tropospheric forcing cause large amplitude mesospheric gravity waves a deepwave case study the deepwave deep propagating wave experiment campaign was designed for an airborne and ground based exploration of gravity waves from their tropospheric sources up to their dissipation at high altitudes it was performed in and around new zealand from 24 may till 27 july 2014 being the first comprehensive field campaign of this kind a variety of airborne instruments was deployed onboard the research aircraft nsf ncar gulfstream v gv and the dlr falcon additionally ground based measurements were conducted at different sites across the southern island of new zealand including the dlr rayleigh lidar located at lauder 45 04 s 169 68 e we focus on the intensive observing period iop 10 on the 4 july 2014 when strong wsw winds of about 40 m s at 700 hpa provided intense forcing conditions for mountain waves at tropopause level the horizontal wind exceeded 50 m s and favored the vertical propagation of gravity waves into the stratosphere the dlr rayleigh lidar measured temperature fluctuations with peak to peak amplitudes of about 20 k in the mesosphere 60 km to 80 km msl over a period of more than 10 hours two research flights were conducted by the dlr falcon falcon flight 04 and 05 during this period with straight transects mt aspiring 2a over new zealand s alps at three different flight levels around the tropopause approx 11 km msl these research flights were coordinated with the gv research flight 16 where the largest mountain wave amplitudes at flight level approx 13 km msl were measured during deepwave additionally a first analysis of falcon s in situ flight level data revealed amplitudes in the vertical wind larger than 4 m s at all altitudes in the vicinity of the highest peaks of the southern alps here we present a comprehensive picture of the gravity wave characteristics and propagation properties during this interesting gravity wave event we use the airborne observations combined with a comprehensive set of ground based measurements consisting of 13 radiosoundings 1 5 hourly interval together with the dlr rayleigh lidar to cover the altitude range from the troposphere to the mesosphere high resolution 1 hourly ecmwf analyses and forecasts are used to estimate the propagation conditions of the excited mountain waves the goal of our investigation is to find out whether the large amplitude mesospheric gravity waves are caused by the strong tropospheric forcing

thermal stability of ice on ceres with rough topography the dwarf planet ceres may have an ice rich crust and subsurface ice exposed by impacts or endogenic activity would be subject to sublimation we model surface and subsurface temperatures on ceres to assess lifetimes of water ice and other volatiles topographic shadowing allows a small but non negligible fraction 0 4 of ceres surface to be perennially below the 110 k criterion for 1 gyr of stability these areas are found above 60 latitude other molecules ch3oh nh3 so2 co2 may be cold trapped in smaller abundances a model for the transport gravitational escape and photoionization of h2o molecules suggests net accumulation in the cold traps buried ice is stable within a meter for 1 gyr at latitudes higher than 50 an illuminated polar cap of water ice would be stable within a few degrees of the poles only if it maintained a high albedo 0 5 at present obliquity if the obliquity exceeded 5 in the geologically recent past then a putative polar cap would have been erased at latitudes 0 30 ice is stable under solar illumination only briefly 10 100 yr unless it has high albedo and thermal inertia in which case lifetimes of 104 yr are possible finally a small hemispheric asymmetry exists due to the timing of ceres perihelion passage which would lead to a detectable enhancement of ice in the northern hemisphere if the orbital elements vary slowly relative to the ice accumulation rate our model results are potentially testable during the dawn science mission

cratering on ceres implications for its crust and evolution introduction thermochemical models have predicted that the dwarf planet ceres has to some extent formed a mantle moreover due to viscous relaxation these models indicate that ceres should have an icy crust with few or no impact craters however the dawn spacecraft has shown that ceres has elevation excursions of 15 km cliffs graben steep sided mountains and a heavily cratered surface rationale we used dawn s framing camera to study the morphology size frequency and spatial distribution of the craters on ceres these data allow us to infer the structure and evolution of ceres outer shell results a large variety of crater morphologies are present on ceres including bowl shaped craters polygonal craters floor fractured craters terraces central peaks smooth floors flowlike features bright spots secondary craters and crater chains the morphology of some impact craters is consistent with water ice in the subsurface although this might have favored relaxation there are also large unrelaxed craters the transition from bowl shaped simple craters to modified complex craters occurs at diameters of about 7 5 to 12 km craters larger than 300 km are absent but low pass filtering of the digital elevation model suggests the existence of two quasi circular depressions with diameters of 570 km 125 56 e and 19 60 n and 830 km 24 76 w and 0 5 n craters are heterogeneously distributed across ceres surface with more craters in the northern versus the southern hemisphere the lowest crater densities are associated with large well preserved southern hemisphere impact craters such as urvara and yalode because the low crater density lcd terrain extends across a large latitude range in some cases e g urvara and yalode 18 n and 75 s kerwan 30 n and 46 s its spatial distribution is inconsistent with simple relaxation driven by warmer equatorial temperatures we instead propose that impact driven resurfacing is the more likely lcd formation process although we cannot completely rule out an internal endogenic origin we applied two different methodologies to derive absolute model ages from observed crater size frequency distributions the lunar derived model adapts the lunar production and chronology functions to impact conditions on ceres taking into account impact velocities projectile densities current collision probabilities and surface gravity the asteroid derived model derives a production function by scaling the directly observed object size frequency distribution from the main asteroid belt extended to sizes 5 km by a collisional model to the resulting size frequency distribution of cerean craters using similar cerean target parameters as the lunar derived model by dating a smooth region associated with the kerwan crater we determined absolute model ages of 550 million and 720 million years depending on which chronology model is applied conclusion crater morphology and the simple to complex crater transition indicate that ceres outer shell is likely neither pure ice nor pure rock but an ice rock mixture that allows for limited relaxation the heterogeneous crater distribution across the surface indicates crustal heterogeneities and a complex geologic evolution of ceres there is evidence for at least some geologic activity occurring in ceres recent history figure 1 spatial density of craters larger than 20 km on ceres crater rims are shown as black solid circles blue indicates areas with lcds yellow and red represent more highly cratered areas the smallest dashed ellipse denotes the idealized former rim of an extremely degraded impact crater at 48 9 e and 44 9 s which is barely recognizable in imagery but apparent from the global digital elevation model also shown as dashed circles are the outlines of two large putative basins unambiguously recognized basins 300 km in diameter are missing and there are several areas with lcds associated with large impact craters e g yalode urvara kerwan ezinu vinotonus dantu and two unnamed craters northeast and southeast of oxo areas a and b are topographic rises with central depressions that also show lcds thermochemical models have predicted that ceres is to some extent differentiated and should have an icy crust with few or no impact craters we present observations by the dawn spacecraft that reveal a heavily cratered surface a heterogeneous crater distribution and an apparent absence of large craters the morphology of some impact craters is consistent with ice in the subsurface which might have favored relaxation yet large unrelaxed craters are also present numerous craters exhibit polygonal shapes terraces flowlike features slumping smooth deposits and bright spots crater morphology and simple to complex crater transition diameters indicate that the crust of ceres is neither purely icy nor rocky by dating a smooth region associated with the kerwan crater we determined absolute model ages amas of 550 million and 720 million years depending on the applied chronology model 1 pending yes

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

predicted crater morphologies on ceres probing internal structure and evolution the detailed internal structure of the dwarf planet ceres target of nasa s dawn mission has not been unequivocally determined from ground based data whereas ceres is most likely differentiated with a near surface ice layer tens to one hundred kilometers thick the possibility of a homogenous ice poor interior structure cannot be completely eliminated these two internal structural end members have profoundly different implications for ceres origin and evolution here we demonstrate that observations of ceres impact craters by the dawn spacecraft will permit unambiguous distinction between the two internal structural models using finite element simulations of crater relaxation we show that if ceres does contain a water ice layer its relatively warm diurnally averaged surface temperature ensures extensive viscous relaxation of even small impact craters at likely equatorial temperatures craters as small as 4km in diameter can be relaxed to the point where complete crater erasure is plausible thus decreasing the overall crater density in the equatorial region at mid latitudes crater relaxation is less extensive but still sufficient to completely relax all craters older than 10 ma and larger than 16 km in diameter as well as smaller ancient craters only in ceres cold polar regions are some crater morphologies expected to be pristine in contrast if ceres is primarily a rocky body we expect crater relaxation to be negligible these basic conclusions are generally independent of ice grain size salt dust contamination of the ice the presence of a thin undifferentiated ice rock crust and the total thickness of the ice layer all of which produce second order modifications to the relaxation process that can be used to better constrain such ice layer properties thus the morphology of impact craters on ceres as revealed by the dawn spacecraft will provide direct insight into the internal structure and evolution of the dwarf planet

geomorphological evidence for ground ice on dwarf planet ceres despite evidence for an ice rich outer shell little water ice has been observed on the surface of ceres lobate morphologies observed on ceres that are increasingly prevalent towards the dwarf planet s poles are consistent with ice rich flows

geologic mapping of the urvara and yalode quadrangles of ceres abstract we conducted geologic mapping of the urvara ac 13 and yalode ac 14 quadrangles 21 66 s 180 360 e of the dwarf planet ceres utilizing morphologic topographic and compositional information acquired by nasa s dawn mission the geologic characteristics of the two large impact basins urvara 170 km diameter and yalode 260 km diameter and their surroundings were investigated using dawn framing camera datasets including survey 415 m pixel hamo 140 m pixel and lamo 35 m pixel images and mosaics color and color ratio images and dtms derived from stereo photogrammetry geologic mapping demonstrates that impact cratering has dominated the geologic history of the urvara and yalode quadrangles with early cratered terrain formation followed by formation of the large basins and widespread emplacement of basin related smooth material impact craters display a wide range of preservation states from nearly completely buried degraded forms to more recent pristine craters with terraced inner walls and lobate ejecta deposits cross cutting relationships and morphologic signatures show that the urvara impact followed the yalode impact consistent with ages derived from crater size frequency distributions 580 40 ma for yalode and 550 50 ma for urvara observed differences in basin materials and rim morphology suggest heterogeneities in the substrate excavated by impact smooth deposits that cover large areas of the quadrangles including the basin floors rims and exterior zones are interpreted to be dominated by urvara ejecta but yalode ejecta and localized ice rich flow material may be minor components geologic mapping results and simulations of ejecta emplacement suggest that urvara and yalode ejecta deposits extend for large distances more than two crater diameters from the basin centers and may serve as important stratigraphic markers for the geologic record of ceres

constraints on ceres internal structure and evolution from its shape and gravity measured by the dawn spacecraft ceres is the largest body in the asteroid belt with a radius of approximately 470 km in part due to its large mass ceres more closely approaches hydrostatic equilibrium than major asteroids pre dawn mission shape observations of ceres revealed a shape consistent with a hydrostatic ellipsoid of revolution the dawn spacecraft framing camera has been imaging ceres since march 2015 which has led to high resolution shape models of the dwarf planet while the gravity field has been globally determined to a spherical harmonic degree 14 equivalent to a spatial wavelength of 211 km and locally to 18 a wavelength of 164 km we use these shape and gravity models to constrain ceres internal structure we find a negative correlation and admittance between topography and gravity at degree 2 and order 2 low admittances between spherical harmonic degrees 3 and 16 are well explained by airy isostatic compensation mechanism different models of isostasy give crustal densities between 1 200 and 1 400 kg m3 with our preferred model giving a crustal density of 1 287 70 87 kg m3 the mantle density is constrained to be 2 434 5 8 kg m3 we compute isostatic gravity anomaly and find evidence for mascon like structures in the two biggest basins the topographic power spectrum of ceres and its latitude dependence suggest that viscous relaxation occurred at the long wavelengths 246 km our density constraints combined with finite element modeling of viscous relaxation suggests that the rheology and density of the shallow surface are most consistent with a rock ice salt and clathrate mixture

dome formation on ceres by solid state flow analogous to terrestrial salt tectonics the dwarf planet ceres s outer crust is a complex heterogeneous mixture of ice clathrates salts and silicates numerous large domes on ceres s surface indicate a degree of geological activity these domes have been attributed to cryovolcanism but that is difficult to reconcile with ceres s small size and lack of long lived heat sources here we alternatively propose that ceres s domes form by solid state flow within the compositionally heterogeneous crust a mechanism directly analogous to salt tectonics on earth we use numerical simulations to illustrate that differential loading of a crust with compositional heterogeneity on a scale of tens of kilometres can produce dome like features of scale similar to those observed the mechanism requires the presence of low viscosity and low density possibly ice rich material in the upper 1 10 km of the subsurface such substantial regional heterogeneity in ceres s crustal composition is consistent with observations from the national aeronautics and space administration s dawn mission we conclude that deformation analogous to that in terrestrial salt tectonics is a viable alternative explanation for the observed surface morphologies and is consistent with ceres being both cold and geologically active domes on the dwarf planet ceres could form by solid state flow of low density ice rich parts of its crust a process analogous to salt doming on earth according to numerical simulations

the lunar crust global structure and signature of major basins new lunar gravity and topography data from the clementine mission provide a global bouguer anomaly map corrected for the gravitational attraction of mare fill in mascon basins most of the gravity signal remaining after corrections for the attraction of topography and mare fill can be attributed to variations in depth to the lunar moho and therefore crustal thickness the large range of global crustal thickness 20 120 km is indicative of major spatial variations in melting of the lunar exterior and or significant impact related redistribution the 61 km average crustal thickness constrained by a depth to moho measured during the apollo 12 and 14 missions is preferentially distributed toward the farside accounting for much of the offset in center of figure from the center of mass while the average farside thickness is 12 km greater than the nearside the distribution is nonuniform with dramatic thinning beneath the farside south pole aitken basin with the global crustal thickness map as a constraint regional inversions of gravity and topography resolve the crustal structure of major mascon basins to half wavelengths of 150 km in order to yield crustal thickness maps with the maximum horizontal resolution permitted by the data the downward continuation of the bouguer gravity is stabilized by a three dimensional minimum slope and curvature algorithm both mare and non mare basins are characterized by a central upwarped moho that is surrounded by rings of thickened crust lying mainly within the basin rims the inferred relief at this density interface suggests a deep structural component to the surficial features of multiring lunar impact basins for large 300 km diameter basins moho relief appears uncorrelated with diameter but is negatively correlated with basin age in several cases it appears that the multiring structures were out of isostatic equilibrium prior to mare emplacement suggesting that the lithosphere was strong enough to maintain their state of stress to the present

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

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

gravity field and internal structure of mercury from messenger radio tracking of the messenger spacecraft has provided a model of mercury s gravity field in the northern hemisphere several large gravity anomalies including candidate mass concentrations mascons exceed 100 milli galileos mgal mercury s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins the low degree gravity field combined with planetary spin parameters yields the moment of inertia c mr2 0 353 0 017 where m and r are mercury s mass and radius and a ratio of the moment of inertia of mercury s solid outer shell to that of the planet of cm c 0 452 0 035 a model for mercury s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron sulfide layer and an iron rich liquid outer core and perhaps a solid inner core

new radiocarbon dates from finnish mammoths indicating large ice free areas in fennoscandia during the middle weichselian new radiocarbon dates from finnish subfossil mammoth material mammuthus sp transported by glacial ice range in age from ca 32000 to ca 22500 yr bp these results suggest that there was a larger ice free area in fennoscandia during the middle weichselian than previously assumed in addition two dates are also presented for bones found in clay with a different transport history copyright 1999 john wiley sons ltd

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

enceladus extreme heat flux as revealed by its relaxed craters 1 enceladus cratered terrains contain large numbers of unusually shallow craters consistent with deformation by viscous relaxation of water ice under conditions of elevated heat flow here we use high resolution topography to measure the relaxation fraction of craters on enceladus far from the active south pole we find that many craters are shallower than expected with craters as small as 2 km in diameter having relaxation fractions in excess of 90 these measurements are compared with numerical simulations of crater relaxation to constrain the minimum heat flux required to reproduce these observations we find that enceladus nominal cold surface temperature 70 k and low surface gravity strongly inhibit viscous relaxation under such conditions less than 3 relaxation occurs over 2 ga even for relatively large craters diameter 24 km and high constant heat fluxes 150 mw m 2 greater viscous relaxation occurs if the effective temperature at the top of the lithosphere is greater than the surface temperature due to insulating regolith and or plume material even for an effective temperature of 120 k however heat fluxes in excess of 150 mw m 2are required to produce the degree of relaxation observed simulations of viscous relaxation of enceladus largest craters suggest that relaxation is best explained by a relatively short lived period of intense heating that decayed quickly we show that infilling of craters by plume material cannot explain the extremely shallow craters at equatorial and higher northern latitudes thus like enceladus tectonic terrains the cratered regions of enceladus have experienced periods of extreme heat flux

the ceres gravity field spin pole rotation period and orbit from the dawn radiometric tracking and optical data abstract ceres gravity field and rotational parameters have been precisely measured using 1 5 years of radiometric doppler and range data and optical landmark tracking from the dawn spacecraft in orbit about the dwarf planet as was the case with dawn at vesta the gravity field orientation parameters landmark locations and ceres orbit are jointly estimated in a global solution even though dawn s radio science investigation at ceres was complicated by additional thrusting for attitude control the resulting spherical harmonic gravity field has a half wavelength resolution of up to 82 km degree 18 near the equator which is similar harmonic resolution to that of vesta the gravity field is consistent with airy isostatic compensation and this model assumption limits ceres crustal density to be between 1200 and 1600 kg m 3 for two layer and three layer models with mean crustal thickness between 27 and 43 km the compensation depth is determined using admittance between gravity and gravity from topography and is superior to admittance between gravity and topography the gravitational mass of ceres is determined to better than 0 002 gm ceres 62 62736 0 00040 km 3 s 2 the spin pole location is improved by 10 over previous results with right ascension 291 427 and declination 66 760 uncertainty less than 0 001 and the rotation rate is improved by 100 over previous determinations from hubble space telescope hst images ceres heliocentric orbit has also been improved with about 17 months of precision range measurements reducing ephemeris uncertainties to about 10 m during the dawn timeframe