mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

the formation of lunar mascon basins from impact to contemporary form positive free air gravity anomalies associated with large lunar impact basins represent a superisostatic mass concentration or mascon high resolution lunar gravity data from the gravity recovery and interior laboratory spacecraft reveal that these mascons are part of a bulls eye pattern in which the central positive anomaly is surrounded by an annulus of negative anomalies which in turn is surrounded by an outer annulus of positive anomalies to understand the origin of this gravity pattern we modeled numerically the entire evolution of basin formation from impact to contemporary form with a hydrocode we simulated impact excavation and collapse and show that during the major basin forming era the preimpact crust and mantle were sufficiently weak to enable a crustal cap to flow back over and cover the mantle exposed by the impact within hours with hydrocode results as initial conditions we simulated subsequent cooling and viscoelastic relaxation of topography using a finite element model focusing on the mare free freundlich sharonov and mare infilled humorum basins by constraining these models with measured free air and bouguer gravity anomalies as well as surface topography we show that lunar basins evolve by isostatic adjustment from an initially subisostatic state following the collapse stage the key to the development of a superisostatic inner basin center is its mechanical coupling to the outer basin that rises in response to subisostatic stresses enabling the inner basin to rise above isostatic equilibrium our calculations relate basin size to impactor diameter and velocity and they constrain the preimpact lunar thermal structure crustal thickness viscoelastic rheology and for the humorum basin the thickness of its postimpact mare fill

mercury cratering record viewed from messenger s first flyby morphologies and size frequency distributions of impact craters on mercury imaged during messenger s first flyby elucidate the planet s geological history plains interior to the caloris basin displaying color and albedo contrasts have comparable crater densities and therefore similar ages smooth plains exterior to caloris exhibit a crater density 40 less than on interior plains and are thus volcanic and not caloris impact ejecta the size distribution of smooth plains craters matches that of lunar craters postdating the late heavy bombardment implying that the plains formed no earlier than 3 8 billion years ago ga at diameters less than or equal to 8 to 10 kilometers secondary impact craters on mercury are more abundant than primaries this transition diameter is much larger than that on the moon or mars a low density of craters on the peak ring basin raditladi implies that it may be younger than 1 ga

a rock mechanical assessment of mercury s global tectonic fabric abstract mercury s global tectonic history is thought to have been dominated by two major processes tidal despinning and global contraction each process is expected to have produced a distinctive global stress field and resultant fault pattern thousands of thrust fault related landforms documented on mercury can be attributed to global contraction but no global signature of tidal despinning has been conclusively documented because global contraction operated throughout an extended portion of mercury s geologic history any tidal despinning pattern either would have formed together with global contraction or would have been modified by global contraction after despinning was complete here we reassess global fracture patterns predicted to result from tidal despinning and from a combination of tidal despinning and global contraction we specifically make use of rock strength and deformability parameters appropriate for mercury s fractured lithosphere results indicate that a tidal despinning pattern would consist only of a global set of opening mode fractures joints in the upper part of the lithosphere whereas the combination of tidal despinning and global contraction would have produced a global population of thrust faults with no preferred orientations in the polar regions but with an increasing preference for north south orientations toward the equator if an equatorial bulge from an early state of rapid spin was supported by mercury s lithosphere two end member scenarios for the timing and duration of these two processes may be considered in one tidal despinning predated global contraction in the other tidal despinning and global contraction overlapped in time we test the predictions of both scenarios against the distribution and orientations of mercury s tectonic landforms the global pattern of thrust faults is generally consistent with predictions for the scenario under which tidal despinning and global contraction temporally overlapped

nonuniform cratering of the moon and a revised crater chronology of the inner solar system abstract we model the cratering of the moon and terrestrial planets from the present knowledge of the orbital and size distribution of asteroids and comets in the inner solar system in order to refine the crater chronology method impact occurrences locations velocities and incidence angles are calculated semi analytically and scaling laws are used to convert impactor sizes into crater sizes our approach is generalizable to other moons or planets the lunar cratering rate varies with both latitude and longitude with respect to the global average it is about 25 lower at 65 n 90 e and larger by the same amount at the apex of motion 0 n 90 w for the present earth moon separation the measured size frequency distributions of lunar craters are reconciled with the observed population of near earth objects under the assumption that craters smaller than a few kilometers in diameter form in a porous megaregolith varying depths of this megaregolith between the mare and highlands is a plausible partial explanation for differences in previously reported measured size frequency distributions we give a revised analytical relationship between the number of craters and the age of a lunar surface for the inner planets expected size frequency crater distributions are calculated that account for differences in impact conditions and the age of a few key geologic units is given we estimate the orientale and caloris basins to be 3 73 ga old and the surface of venus to be 240 ma old the terrestrial cratering record is consistent with the revised chronology and a constant impact rate over the last 400 ma better knowledge of the orbital dynamics crater scaling laws and megaregolith properties are needed to confidently assess the net uncertainty of the model ages that result from the combination of numerous steps from the observation of asteroids to the formation of craters our model may be inaccurate for periods prior to 3 5 ga because of a different impactor population or for craters smaller than a few kilometers on mars and mercury due to the presence of subsurface ice and to the abundance of large secondaries respectively standard parameter values allow for the first time to naturally reproduce both the size distribution and absolute number of lunar craters up to 3 5 ga ago and give self consistent estimates of the planetary cratering rates relative to the moon

magnetic signature of upper plate structures and subducting seamounts at the convergent margin off costa rica offshore the pacific margin of costa rica the caribbean plate converges with the subducting cocos plate along the middle america trench the tectonics of both plates have been studied on the basis of reflection seismic gravimetric and magnetic data and swath mapping the magnetic data were acquired by geomar kiel germany during cruise so 76 of r v sonne off costa rica in 1992 two different magnetic anomaly patterns are observed off nicoya peninsula the linear magnetic anomalies trend about n50 e and can be traced from the ocean basin to the landward end of the profiles to the southeast of a major structural break trending parallel to fisher seamount and the associated ridge the strike of the magnetic anomalies is n70 e in the area southeast of fisher seamount the magnetic signatures of numerous seamounts are superimposed upon the linear anomalies we interpret the magnetic lineations as chrons 5c to 6b 16 22 5 ma magnetic modeling demonstrates a significant magnetization 1 a m of the margin wedge in the continental slope a structural unit known from seismic studies this is not compatible with the previously discussed origin of the continental slope as an accretionary wedge a composition of the margin wedge similar to the ophiolitic rocks found onshore on nicoya peninsula appears likely a number of morphological furrows observed in the landward trench slope opposite the seamount covered domain of the oceanic crust coincide with magnetic anomalies three dimensional magnetic modeling demonstrates that these anomalies can be explained well by subducting seamounts this implies that the seamounts remain intact at least magnetically for a long time interval during the subduction process thus they may indeed be important seismogenic asperities

large impact basins on mercury global distribution characteristics and modification history from messenger orbital data 1 the formation of large impact basins diameter d 300 km was an important process in the early geological evolution of mercury and influenced the planet s topography stratigraphy and crustal structure we catalog and characterize this basin population on mercury from global observations by the messenger spacecraft and we use the new data to evaluate basins suggested on the basis of the mariner 10 flybys forty six certain or probable impact basins are recognized a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain the spatial density of large basins d 500 km on mercury is lower than that on the moon morphological characteristics of basins on mercury suggest that on average they are more degraded than lunar basins these observations are consistent with more efficient modification degradation and obliteration of the largest basins on mercury than on the moon this distinction may be a result of differences in the basin formation process producing fewer rings relaxation of topography after basin formation subduing relief or rates of volcanism burying basin rings and interiors during the period of heavy bombardment on mercury from those on the moon

viscoelastic deformation of lunar impact basins implications for heterogeneity in the deep crustal paleo thermal state and radioactive element concentration 1 diverse geological characteristics found for the three major lunar provinces i e the feldspathic highlands terrane fht the south pole aitken terrane spat and the procerallum kreep terrane pkt strongly suggest their distinctly different thermal histories quantitative differences among these provinces in their early thermal histories and crustal radioactive element concentrations however are highly unknown one of the few observables that retain a record of the ancient lunar thermal structure is the viscoelastic state of impact basins this study investigates the long term evolution of basin structures using global lunar gravity field data obtained by kaguya tracking and derives constraints for 1 the paleo thermal state of impact basins and for 2 crustal column averaged radioactive element concentrations for each province our calculation results indicate that impact basins in the central anorthositic region of the fht i e the fht an require a very cold interior dt dr 20 k km 1 on the surface this result strongly suggests that the deep portion of the thick farside highlands crust is highly depleted in radioactive elements th 0 5 ppm indicating that the th rich spa basin floor crust is clearly different from the lower crust underneath the fht an and cannot be accounted for by simple exposure of the lower crust our analysis also indicates that the observed basin structure allows as high as 6 ppm of column averaged th concentration in the crust inside the pkt these results indicate that radioactive element concentrations deep in the crust probably vary greatly region by region similarly to those observed on the surface

shallow basins on mercury evidence of relaxation abstract stereo derived topographic models have shown that the impact basins beethoven and tolstoj on mercury are shallow for their size with depths of 2 5 and 2 0 7 km respectively while caloris basin has been estimated to be 9 3 km deep on the basis of photoclinometric measurements we evaluate the depths of beethoven and tolstoj in the context of comparable basins on other planets and smaller craters on mercury using data from mariner 10 and the first flyby of the messenger spacecraft we consider three scenarios that might explain the anomalous depths of these basins 1 volcanic infilling 2 complete crustal excavation and 3 viscoelastic relaxation none of these can be ruled out but the fill scenario would imply a thick lithosphere early in mercury s history and the crustal excavation scenario a pre impact crustal thickness of 15 55 km depending on the density of the crust in the area of beethoven and tolstoj the potential for viscous relaxation of beethoven tolstoj and caloris is explored with a viscoelastic model results show that relaxation of these basins could occur at plausible heat flux values for a range of crustal thicknesses however the amplitude of current topographic relief points to a crustal thickness of at least 60 km under this hypothesis relaxation of caloris may have occurred if the floor is underlain by crust at least 20 km thick we discuss future observations by messenger that should distinguish among these scenarios

viscoelastic evolution of lunar multiring basins 1 we investigate the evolution of multiring basins on the moon the orbital geophysical and geochemical data collected by lunar prospector allow three distinct groups of basins to be identified group 1 basins show well preserved topography a bull s eye gravity signature and little mare basalt fill group 2 basins contain large amounts of mare basalt and show mare dominated gravity and topography signatures group 3 basins are characterized by low amplitude topographic relief and weak gravity anomalies while the crustal structures of group 1 and 2 basins appear to date from the collapse of the transient cavity group 3 basins are highly degraded and show very little crustal thinning using a viscoelastic model we explore the evolution of the lunar multiring basins with the goal of explaining the degraded condition of group 3 basins our results show that while viscous relaxation is a plausible explanation for the state of group 3 farside basins requiring heat flux 25 40 mw m 2 the thin nearside crust renders subsolidus ductile flow in the crust unlikely the existence of many degraded basins on the nearside points to a weakened and possibly partially molten lower crust that south pole aitken appears not to have relaxed suggests that similar conditions did not exist there and that the nearside was hotter than the farside early on this is likely a consequence of the nearside farside crustal thickness dichotomy which would have caused the farside to reach full crystallization first leaving the nearside with the remaining liquid and an excess of heat producing elements

influence of an insulating megaregolith on heat flow and crustal temperature structure of mercury mercury is covered by a megaregolith layer which constitutes a poor thermally conducting layer that must have an influence on the thermal state and evolution of the planet although most thermal modeling or heat flow studies have overlooked it in this work we have calculated surface heat flows and subsurface temperatures from the depth of thrust faults associated with several prominent lobate scarps on mercury valid for the time of the formation of these scarps by solving the heat equation and taking into account the insulating effects of a megaregolith layer we conclude that megaregolith insulation could have been an important factor limiting heat loss and therefore interior cooling and contraction of mercury as mercurian megaregolith properties are not very well known we also analyze the influence of these properties on the results and discuss the consequences of imposing the condition that the total radioactive heat production must be lower than the total surface heat loss this is the urey ratio ur must be lower than 1 in a cooling and thermally contracting planet such as mercury at the time of scarp emplacement our results show that satisfying the condition of ur 1 implies that the average abundances of heat producing elements silicate layer is 0 4 times or less the average surface value placing an upper bound on the bulk content of heat producing elements in mercury s interior

stratigraphy of the caloris basin mercury implications for volcanic history and basin impact melt abstract caloris basin mercury s youngest large impact basin is filled by volcanic plains that are spectrally distinct from surrounding material post plains impact craters of a variety of sizes populate the basin interior and the spectra of the material they have excavated enable the thickness of the volcanic fill to be estimated and reveal the nature of the subsurface the thickness of the interior volcanic plains is consistently at least 2 5 km reaching 3 5 km in places with thinner fill toward the edge of the basin no systematic variations in fill thickness are observed with long wavelength topography or azimuth the lack of correlation between plains thickness and variations in elevation at large horizontal scales within the basin indicates that plains emplacement must have predated most if not all of the changes in long wavelength topography that affected the basin there are no embayed or unambiguously buried ghost craters with diameters greater than 10 km in the caloris interior plains the absence of such ghost craters indicates that one or more of the following scenarios must hold the plains are sufficiently thick to have buried all evidence of craters that formed between the caloris impact event and the emplacement of the plains the plains were emplaced soon after basin formation or the complex tectonic deformation of the basin interior has disguised wrinkle ridge rings localized by buried craters that low reflectance material lrm was exposed by every impact that penetrated through the surface volcanic plains provides a means to explore near surface stratigraphy if all occurrences of lrm are derived from a single layer the subsurface lrm deposit is at least 7 5 8 5 km thick and its top likely once made up the caloris basin floor the caloris forming impact would have generated a layer of impact melt 3 15 km thick such a layer could account for the entire thickness of lrm this material would have been derived from a combination of lower crust and upper mantle

estimates of martian crustal thickness from viscous relaxation of topography isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients which may cause the lower crust to flow and reduce the relief areas of thicker crust are generally associated with more rapid relaxation of topography on mars topographic features such as impact basins and the hemispheric dichotomy have survived for 4 gyr we use a finite difference representation of depth dependent non newtonian lower crustal flow to investigate how topography decays with time for a dry diabase rheology total radiogenic concentrations 80 of terrestrial values and crustal radiogenic concentrations similar to terrestrial basalts we find that an upper bound on the mean planetary crustal thickness is 100 km in the probably unrealistic case where all the radiogenic elements are in the crust this maximum crustal thickness can be increased to 115 km the main uncertainty in these results is the total radiogenic abundances on mars comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow both hellas and the dichotomy are isostatically compensated if the mechanism is airy isostasy then the lower bound on mean crustal thickness is 30 km crustal thicknesses of 30 100 km on mars can be produced by mid ocean ridge spreading at potential temperatures of 1350 1600 c however for such crustal thicknesses the lithosphere is likely to be positively buoyant making subduction difficult

control of mars global atmospheric loss by the continuous rotation of the crustal magnetic field a time dependent mhd study we present a time dependent mhd study of the controlling effects of the mars crustal field on atmospheric escape we calculate globally integrated planetary ion loss rates under quiet solar conditions considering the continuous rotation of crustal anomalies with the planet it is found that the rotating crustal field plays an important role in controlling atmospheric escape significant time variation of 20 and 50 is observed during the entire rotation period for o and for o2 and co2 respectively the control is exerted mainly through two processes first the crustal magnetic pressure over the subsolar regime controls solar wind penetration and mass loading and therefore the escaping planetary ion source there is a strong negative correlation between the magnetic pressure and ion loss with a time lag of 1 h for o and 2 5 h for o2 and co2 second the crustal magnetic pressure near the terminator region controls the cross section area between the induced magnetospheric boundary and 100 km altitude at the terminator the change in day night connection regulates the extent to which planetary ions created on the dayside can be ultimately carried away by the solar wind and escape mars there is a strong positive correlation between the cross section area and ion loss with no significant time lag as the planet rotates the dayside process and the terminator process work together to control the total amount of escaping planetary ions however their relative importance changes with the local time of the strong crustal field region

the global population of large craters on mercury and comparison with the moon 1 we have compiled a near global catalog of impact craters on mercury for diameters 20 km from images obtained during the flybys of the mariner 10 and messenger spacecraft the observed variations in crater density suggest that the smooth plains within and around the caloris basin are the most prominent contiguous comparatively young regions on mercury no other comparably large area appears as young in more heavily cratered terrain even the most densely cratered regions on mercury are deficient in craters between 20 km and 100 km in diameter compared with the moon a result that extends an observation made from mariner 10 images of a smaller fraction of mercury s surface this deficit is interpreted to reflect crustal resurfacing of mercury early in its history most likely by volcanic processes for craters larger than 100 km in diameter the density of craters on mercury in a given size range is similar to that of the moon because such larger craters are less easily removed by volcanic resurfacing we interpret this similarity to be the result of impact saturation effects on both bodies

melting of the fefeo and the fefes systems at high pressure constraints on core temperatures melting temperatures of feo fes and fes2 have been accurately determined in a hydrostatic inert pressure environment up to about 0 5 mbar using a new yttrium lithium fluoride ylf heating laser the melting curve for feo is in perfect agreement with data obtained from multi anvil experiments at 160 kbar 1 but is in stark disagreement with previous laser heating experiments 2 preliminary measurements show strong melting depression on mixtures of fe and fes whereas mixtures of fe and feo were observed to melt close to the melting curve of iron the kraut kennedy melting relationship 3 in which the melting temperature is a linear function of volume is successfully tested in this study for li na k fe fes fes2 and feo to compressions up to 30 at 1 36 mbar core mantle boundary cmb feo fe and fes are estimated to melt at 3670 3260 and 3060 100 k respectively assuming outer core compositions with about 60 fe and about 40 feo or fes and a solid solution system the melting temperature at the cmb on the core side would be 3300 k 200 k compared to a temperature at the bottom of the mantle of 2650 100 k if these systems exhibit eutectic behaviour the melting gradient through the outer core would have to be substantially higher than the adiabatic gradient in order to maintain a thermal boundary at the cmb the present melting data and experimental constraints on the adiabatic gradient in the outer core suggest a temperature at the inner core outer core boundary of nearly 4200 k

using the viscoelastic relaxation of large impact craters to study the thermal history of mars we simulate the long term deformation of martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography using the finite element method and a viscoelastic rheological model we model the deformation of more than 30 large craters and quasi circular depressions qcds in the diameter range of 200 500 km in both the northern lowlands and southern highlands we determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust mantle boundary ranges from 40 to 90 mw m 2 our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface by applying various viscous creep parameters for hydrous and anhydrous rheologies we demonstrate that mars s interior is wet to a certain degree which is consistent with other estimates since craters and qcds are distributed fairly equally on the surface of the red planet this study provides a less regionally biased picture of the thermal history of early mars than in previous studies based on our results the ancient average background heat flux in the northern lowlands was higher than that of the southern highlands which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle noachian

thermochemical evolution of mercury s interior 1 a number of observations performed by the messenger spacecraft can now be employed to better understand the evolution of mercury s interior using recent constraints on interior structure surface composition volcanic and tectonic histories we modeled the thermal and magmatic evolution of the planet we ran a large set of monte carlo simulations based on one dimensional parametrized models spanning a wide range of parameters we complemented these simulations with selected calculations in 2 d cylindrical and 3 d spherical geometry which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection core radii of 1940 km 2040 km and 2140 km have been considered and while in the first two cases several models satisfy the observational constraints no admissible models were found for a radius of 2140 km a typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt the evolution subsequent to 2 gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today most of the models predict mantle convection to cease after 3 4 gyr indicating that mercury may be no longer dynamically active finally assuming the observed surface abundance of radiogenic elements to be representative for the entire crust we determined bulk silicate concentrations of 35 62 ppb th 20 36 ppb u and 290 515 ppm k similar to those of other terrestrial planets

the relative timing of lunar magma ocean solidification and the late heavy bombardment inferred from highly degraded impact basin structures the solidification of the lunar magma ocean lmo and formation of impact basins are important events that took place on the early moon the relative timing of these events however is poorly constrained the aim of this study is to constrain the formation ages of old impact basins based on inferences of their thermal state most proposed basins formed before pre nectarian pn 5 stage do not exhibit clear concentric features in either topography or gravity suggesting substantial viscous lateral flow in the crust recent geodetic measurements reveal that the lunar crust is thinner than previously estimated indicating that an extremely high crustal temperature is required for lateral flow to occur in this study we calculate lunar thermal evolution and viscoelastic deformation of basins and investigate the thermal state at the

mercury s low degree geoid and topography controlled by insolation driven elastic deformation mercury experiences an uneven insolation that leads to significant latitudinal and longitudinal variations of its surface temperature these variations which are predominantly of spherical harmonic degrees 2 and 4 propagate to depth imposing a long wavelength thermal perturbation throughout the mantle we computed the accompanying density distribution and used it to calculate the mechanical and gravitational response of a spherical elastic shell overlying a quasi hydrostatic mantle we then compared the resulting geoid and surface deformation at degrees 2 and 4 with mercury s geoid and topography derived from the mercury surface space environment geochemistry and ranging spacecraft more than 95 of the data can be accounted for if the thickness of the elastic lithosphere were between 110 and 180 km when the thermal anomaly was imposed the obtained elastic thickness implies that mercury became locked into its present 3 2 spin orbit resonance later than about 1 gyr after planetary formation

widespread effusive volcanism on mercury likely ended by about 3 5 ga crater size frequency analyses have shown that the largest volcanic plains deposits on mercury were emplaced around 3 7 ga as determined with recent model production function chronologies for impact crater formation on that planet to test the hypothesis that all major smooth plains on mercury were emplaced by about that time we determined crater size frequency distributions for the nine next largest deposits which we interpret also as volcanic our crater density measurements are consistent with those of the largest areas of smooth plains on the planet model ages based on recent crater production rate estimates for mercury imply that the main phase of plains volcanism on mercury had ended by 3 5 ga with only small scale volcanism enduring beyond that time cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet

thermal evolution of mercury as constrained by messenger observations 1 orbital observations of mercury by the mercury surface space environment geochemistry and ranging messenger spacecraft provide new constraints on that planet s thermal and interior evolution specifically messenger observations have constrained the rate of radiogenic heat production via measurement of uranium thorium and potassium at the surface and identified a range of surface compositions consistent with high temperature high degree partial melts of the mantle additionally messenger data have placed new limits on the spatial and temporal variation in volcanic and tectonic activity and enabled determination that the planet s core is larger than previously estimated because mercury s mantle layer is also thinner than previously thought this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting we simulate mantle convection and magma generation within mercury s mantle under two dimensional axisymmetry and a broad range of conditions to understand the implications of messenger observations for the thermal evolution of the planet these models demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of mercury s history and often to the present as long as the mantle is thicker than 300 km we also find that magma generation in mercury s convecting mantle is capable of producing widespread magmas by large degree partial melting consistent with messenger observations of the planet s surface chemistry and geology