numerical models of martian mantle evolution induced by magmatism and solid state convection beneath stagnant lithosphere 1 numerical models of magmatism in a convecting mantle are presented to understand martian mantle evolution the viscosity is strongly temperature dependent and the lithosphere is stagnant magmatism is modeled as a permeable flow of basaltic magma generated by decompression melting when the initial mantle temperature is sufficiently high a magma ocean develops to generate a thick basaltic crust and create a compositionally layered mantle the upper layer of the mantle consists of compositionally buoyant residue of the basaltic crust while the lower layer consists of compositionally denser materials that are not as depleted in the basaltic component hot plumes grow from the lower layer making it thinner with time by erosion the hot plumes also induce magmatism which extracts heat producing elements hpes from the mantle the hpe extraction regulates plume magmatism activity and controls the duration of plume magmatism plume magmatism also keeps the mantle temperature below the solidus by efficiently extracting heat as soon as the mantle temperature exceeds the solidus the martian mantle is likely to have evolved as a relaxation from a compositionally layered state formed by the magma ocean and plume magmatism has probably played a crucial role in the relaxation process

two stage plume melting a possible mechanism for the origin of martian magmatism abstract the origin of martian magmatism is investigated by geochemical modeling based on the isotopic and rare earth element ree compositions of the shergottite nakhlite and chassignite snc meteorites the model proposed in this study consists of three processes early mantle differentiation in the deepest mantle to produce a plume source region composed of hydrated ringwoodite and majorite first stage dehydration melting of the plume to produce nakhlite and chassignite magmas in the deep mantle and finally second stage decompression melting of the residual plume to produce shergottite magmas in the uppermost mantle the production of light ree enriched nakhlite and chassignite magmas can be explained by the persistence of residual garnet through first stage melting and the depleted isotopic composition of the nakhlite and chassignite meteorites can be reasonably attributed to the presence of majorite in the plume source region the strongly depleted ree and isotopic composition of the primitive shergottites can similarly be explained by the existence of majorite in the plume source region coupled with extraction of the nakhlite melt from the depleted source the proposed two stage plume melting model therefore reproduces the chemical and isotopic variations of the martian magmas well suggesting a genetic relationship among the snc magmas

water storage and early hydrous melting of the martian mantle abstract we report an experimental investigation of the near solidus phase equilibria of a water saturated analog of the martian mantle experiments were performed at low temperatures 700 920 c and high pressure 4 7 gpa using multi anvil apparatus and piston cylinder device 4 gpa the results of this study are used to explore the role of water during early melting and chemical differentiation of mars and to further our understanding of the near solidus behavior in planetary mantle compositions at high pressure water has a significant effect on the temperature of melting and therefore on accretion and subsequent differentiation processes experiments locate the wet solidus at 800 c and is isothermal between 4 gpa and 7 gpa the martian primitive mantle can store significant amounts of water in hydrous minerals stable near the solidus humite group minerals and phase e represent the most abundant hydrous minerals stable in the 4 7 gpa pressure range the amount of water that can be stored in the mantle and mobilized during melting ranges from 1 to up to 4 wt near the wet solidus we discuss thermal models of mars accretion where the planet formed very rapidly and early on in solar system history we incorporate the time constraint of dauphas and pourmand 2011 that mars had accreted to 50 of its present mass in 1 8 myr and include the effects of 26al radioactive decay and heat supplied by rapid accretion when accretion has reached 30 of mars current mass 70 of its present size melting starts and extends from 100 to 720 km depth below this melt layer water can still be bound in crystalline solids the critical stage is at 50 accretion 80 of its size where mars is above the wet and dry solidi with most of its interior melted this is earlier in the accretion process than what would be predicted from dry melting we suggest that water may have promoted early core formation on mars and rapidly extended melting over a large portion of mars interior

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

metal silicate fractionation in the growing earth energy source for the terrestrial magma ocean based on the homogeneous accretion model of planets thermal evolution of the growing earth has been studied after the surface of the growing earth starts melting owing to the blanketing effect of the primordial atmosphere gravitational differentiation between metallic and silicate materials takes place in its upper region the differentiation would give rise to a large amount of gravitational energy release as well as layered structure inside the proto earth a part of released energy would be transported outward by convection considering the effects of gravitational fractionation and convection we have pursued numerically the evolution of the growing earth our results show that gravitational differentiation can supply energy large enough to keep the whole silicate layer in the molten state and that half of the released energy is conveyed outward by convective heat transport in the growing stage the temperature distribution in the silicate layer always lies between liquidus and solidus temperatures except in an uppermost thin zone the thermal structure obtained favors a notion of the primordial magma ocean and of chemical differentiation of mantle the structure of the growing earth which is composed of the central protocore the metallic layer and the silicate layer magma ocean would become gravitationally unstable and overturn to form a metallic core energy released by this overturn would not affect the structure of the upper magma ocean too much but undifferentiated primordial materials which come from the protocore would add to mantle materials through the overturn and this would help explain the isotopic mantle heterogeneity presently observed

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

isotopic outcomes of n body accretion simulations constraints on equilibration processes during large impacts from hf w observations abstract most estimates of planetary core formation timescales using hafnium tungsten hf w isotopes employ analytical expressions assuming either continuous planetary growth or instantaneous core formation in contrast dynamical modelling of planetary accretion suggests that the final stage of terrestrial planet formation is punctuated by multiple large and stochastic impacts such giant impacts have significant thermal and isotopic consequences we present a framework for calculating the hf w isotope evolution of individual bodies based on the results of an n body accretion simulation and assuming constant partition coefficients the results show that smaller bodies exhibit a larger range in isotopic values than larger bodies because the latter have suffered more impacts the analytical core formation timescales calculated using these isotopic values can differ very significantly from the timing of the final giant impact each planet actually experiences simulations in which 1 even the largest impactors undergo re equilibration with the target s mantle rather than the cores merging directly and 2 the original planetary embryos possessed radially variable iron silicate ratios produce results which are consistent with the observed physical and isotopic characteristics of inner solar system bodies varying w partition coefficients due to changing mantle oxidation state or initial planetesimal hf w ratios might produce similar isotopic outcomes and potentially permit core mergers without violating the isotopic constraints the style of re equilibration required suggests that magma oceans were present on mars sized and larger bodies an alternative for bodies of mars size and smaller is that the bulk of the mass was delivered as impactors much smaller than the target for mars we conclude that a prolonged 10 myr accretion process is both dynamically and isotopically plausible we also predict likely pd ag isotopic anomalies for vesta mars and earth size bodies

structural and tidal models of titan and inferences on cryovolcanism titan saturn s largest satellite is subject to solid body tides exerted by saturn on the timescale of its orbital period the tide induced internal redistribution of mass results in tidal stress variations which could play a major role for titan s geologic surface record we construct models of titan s interior that are consistent with the satellite s mean density polar moment of inertia factor obliquity and tidal potential love number k2 as derived from cassini observations of titan s low degree gravity field and rotational state in the presence of a global liquid reservoir the tidal gravity field is found to be consistent with a subsurface water ammonia ocean more than 180 km thick and overlain by an outer ice shell of less than 110 km thickness the model calculations suggest comparatively low ocean ammonia contents of less than 5 wt and ocean temperatures in excess of 255 k i e higher than previously thought thereby substantially increasing titan s potential for habitable locations the calculated diurnal tidal stresses at titan s surface amount to 20 kpa almost comparable to those expected at enceladus and europa tidal shear stresses are concentrated in the polar areas while tensile stresses predominate in the near equatorial midlatitude areas of the sub and anti saturnian hemispheres the characteristic pattern of maximum diurnal tidal stresses is largely compliant with the distribution of active regions such as cryovolcanic candidate areas the latter could be important for titan s habitability since those may provide possible pathways for liquid water ammonia outbursts on the surface and the release of methane in the satellite s atmosphere

a multi phase model of runaway core mantle segregation in planetary embryos abstract a classic scenario of core formation suggests that growing proto planets are heated by the impacts of accreting planetesimals at their surface until their shallow layers reach the melting temperature of their metallic components or even of the silicates in this partially molten shell metal and silicates differentiate and the metallic phase ponds on top of the still undifferentiated inner planet later a gravitational instability brings dense metallic diapirs to the center of the planet we test this multi phase scenario by using a formalism that self consistently accounts for the presence of solid silicates solid and liquid iron at each point of the mixture an average velocity and a separation velocity of the solid and liquid phases are defined the energy balance accounts from the changes in potential energy associated with the segregation we show that core formation starts before a significant melting of the silicates as soon as impact heating is large enough to reach the melting temperature of the metallic component segregation proceeds in a few thousand years by a runaway process due to the conversion of gravitational energy into heat that occurs necessarily in all undifferentiated embryos of moon to mars sizes the first metallic diapirs leave behind them a trailing conduit along which most of the further melting occurs the cores of large planets do not form at the end of accretion but must result from the merging of the already differentiated hot cores of embryos

earth and mars early thermal profiles abstract the extent of formation heating for the earth and mars has been evaluated assuming that the terrestrial planets accumulated from planetesimals the main result is that even if a long accumulation time is assumed 100 ma it is possible to obtain a planetary structure with a large melted shell taking into account the role played by massive projectiles which upon reaching depths of several kilometres are able to deposit heat significantly below the planetary surface internal temperatures sufficient for the downward migration of the liquid iron alloy have been obtained

consequences of giant impacts in early mars core merging and martian dynamo evolution a giant impact is an increasingly popular explanation for the formation of the northern lowland on mars it is plausible that at the impact time both mars and the impactor were differentiated with solid silicate mantles and liquid iron cores such a large impact likely resulted in merging of the cores of both bodies a process which will have implications on the thermal state of the planet we model the evolution of the martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor s core as a single diapir the impact heating and the viscous heating induced during the core merging may affect the early thermal state of mars during several tens of million years our results show that large viscosity contrasts between the impactor s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature when the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000 the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core mantle boundary finally the direct impact heating of martian core leads to thermal stratification of the core and kills the core dynamo it takes on the order of 150 200 myr to reinitiate a strong dynamo anew the merging of the impactor s core with the martian core only delays the reinitiation of the dynamo for a very short time

magnetic lineations in the ancient crust of mars the mars global surveyor spacecraft in a highly elliptical polar orbit obtained vector magnetic field measurements above the surface of mars altitudes 100 kilometers crustal magnetization mainly confined to the most ancient heavily cratered martian highlands is frequently organized in east west trending linear features the longest extending over 2000 kilometers crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude groups of quasi parallel linear features of alternating magnetic polarity were found they are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on earth but with a much larger spatial scale they may be a relic of an era of plate tectonics on mars

along axis variation in crustal thickness at the ultraslow spreading southwest indian ridge 50 e from a wide angle seismic experiment the southwest indian ridge swir is characterized by an ultraslow spreading rate thin crust and extensive outcrops of serpentinized peridotite previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the swir from the prince edward 35 30 e to the gallieni 52 20 e fracture zones here we present a new analysis of wide angle seismic data along the ridge 49 17 e 50 49 e our main conclusions are as follows 1 we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient underlain by a layer 3 with variable thickness and low velocity gradient 2 the crustal thickness varies from 5 km beneath nontransform discontinuities ntds up to 10 km beneath a segment center 3 the melt supply is focused in segment centers despite a small ntd between adjacent segments 4 the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate and that mantle temperature or chemistry or both must vary significantly along axis

the early thermal evolution of mars hf w isotopic systematics of martian meteorites have provided evidence for the early accretion and rapid core formation of mars we present the results of numerical simulations performed to study the early thermal evolution and planetary scale differentiation of mars the simulations are confined to the initial 50 myr ma of the formation of solar system the accretion energy produced during the growth of mars and the decay energy due to the short lived radio nuclides 26al 60fe and the long lived nuclides 40k 235u 238u and 232th are incorporated as the heat sources for the thermal evolution of mars during the core mantle differentiation of mars the molten metallic blobs were numerically moved using stoke s law toward the center with descent velocity that depends on the local acceleration due to gravity apart from the accretion and the radioactive heat energies the gravitational energy produced during the differentiation of mars and the associated heat transfer is also parametrically incorporated in the present work to make an assessment of its contribution to the early thermal evolution of mars we conclude that the accretion energy alone cannot produce widespread melting and differentiation of mars even with an efficient consumption of the accretion energy this makes 26al the prime source for the heating and planetary scale differentiation of mars we demonstrate a rapid accretion and core mantle differentiation of mars within the initial 1 5 myr this is consistent with the chronological records of martian meteorites

volatiles in the atmosphere of mars the effects of volcanism and escape constrained by isotopic data abstract we study the long term evolution of the conditions on the surface of mars through the modeling of the effects of volcanic degassing and atmospheric non thermal escape during the last four billion yr we propose to use the recent advances due to observation and modeling to constrain possible evolutions of the atmosphere of mars with the help of isotopic data from carbon nitrogen and argon the history of argon is studied through direct calculation of its degassing and escape whereas for other species the analysis is an integration backwards in time from present day situation in our calculation we do not consider early impact erosion hydrodynamic escape or carbonate formation volcanic degassing is obtained from crust production models observation of the surface and realistic volatile contents of the lavas aspera analyzer of space plasma and energetic atoms measurements and modeling of the escape rates produced by ionic escape sputtering and dissociative recombination constitute the sink of volatiles we constrain the maximum escape flux of co 2 with the evolution of argon and the 40 ar 36 ar ratio in the atmosphere and measurements of the present day situation this imposes restricted escape flux consistent with the recent lowering of the expected escape efficiency on mars our model is able to reproduce present day 36 ar abundance and 40 ar 36 ar ratio we also show that the present day atmosphere of mars is likely to be constituted by a large part of volcanic gases with a low co 2 concentration in the magma 150 ppm present atmosphere is constructed of 50 of volcanic gases emitted since 3 7 billion yr ago we oppose this late volcanic atmosphere to the early atmosphere in place during noachian and composed of primitive volatile brought during accretion magma ocean phase and pre 3 7 ga volcanism likewise the mean age of the atmosphere is estimated to be no more than 1 9 to 2 3 billion yr atmospheric pressures and variations on mars are predicted to be low 50 mbar as the result of degassing and non thermal escape this seems in line with the assumption of a big loss of volatiles during the first 500 myr isotopic ratios lead us to propose that nitrogen is probably old in the martian atmosphere and has been subjected to the fractionation of atmospheric escape the 12 c 13 c on the other hand is more stable and indicates that carbon is younger water could have existed on mars through the last 4 billion yr evolution within a factor of 1 6 times greater to 3 times less than the present day inventory including polar caps depending on the volcanic degassing it is however unlikely to reside in the atmosphere or in liquid form unless large scale perturbations occur changes in obliquity and large input of greenhouse gases due to a short burst of volcanism

early evolution of the earth accretion atmosphere formation and thermal history atmospheric and thermal evolution of the earth growing by planetesimal impacts was modeled by taking into account the blanketing effect of an impact induced h2o atmosphere and the temperature dependence of h2o degassing when the water content of planetesimals is larger than 0 1 by weight and the accretion time of the earth is less than 5 107 years the surface of the accreting earth melts and thus a magma ocean forms and covers the surface the formation of a magma ocean will result in the initiation of core mantle separation and mantle differentiation during accretion once a magma ocean is formed the surface temperature the degree of melting in the magma ocean and the mass of the h2o atmosphere are nearly constant as the protoplanet grows further the final mass of the h2o atmosphere is about 1021 kg a value which is insensitive to variations in the model parameter values such as the accretion time and the water content of planetesimals that the final mass of the h2o atmosphere is close to the mass of the present oceans suggests an impact origin for the earth s hydrosphere on the other hand most of the h2o retained in planetesimals will be deposited in the solid earth free water within the proto earth may affect differentiation of the proto mantle in particular the mantle feo abundance and the incorporation of a light element in the outer core

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

two dimensional numerical studies on the effects of water on martian mantle evolution induced by magmatism and solid state mantle convection 1 numerical models of magmatism and mantle convection beneath a stagnant lithosphere are presented to understand how water affects martian mantle evolution magmatism is modeled as an upward permeable flow of the basaltic magma generated by decompression melting first a global partially molten layer gpml develops and generates a basaltic crust a layer of compositionally buoyant residue of melt extraction in the uppermost mantle and a denser layer with a higher content of the basaltic component in the deep mantle the gpml extracts most of the water initially contained in the upper layer but some water remains in the lower layer of the mantle subsequently hot plumes ascend from the lower layer to induce magmatism the water allows plume magmatism to continue for a long duration up to 5 gyr depending on the initial water content and the detail of the initial temperature distribution in the mantle provided that the mantle is initially not too hot just after planetary formation the plume magmatism is sufficiently active to cause significant crustal growth and dehydration of the crust and mantle in the early evolutionary stage when the internal heating is strong the amount of extracted water is equivalent to a water layer of up to several hundred meters in depth water can also enhance the extraction of heat producing elements hpes from the mantle reduce the heat flux from the mantle to the crust and make the lithosphere thicker both crustal growth and dehydration eventually subside as the hpes decay

can a sinking metallic diapir generate a dynamo metallic diapirs may have strongly contributed to core formations during the first million years of planetary evolutions the aim of this study is to determine whether the dynamics induced by the diapir sinking can drive a dynamo and to characterize the required conditions on the size of the diapir the mantle viscosity and the planetary latitude at which the diapir sinks we impose a classical hadamard flow solution for the motion at the interface between a spherical sinking diapir and a viscous mantle on dynamical simulations that account for rotational and inertial effects in order to model the flow within the diapir the flows are confined to a velocity layer with a thickness that decreases with increasing rotation rate these 3d flows are is then used as input for kinematic dynamo simulations to determine the critical magnetic reynolds number for dynamo onset our results demonstrate that the flow pattern inside a diapir sinking into a rotating planet can generate a magnetic field large diapirs r 10 km sinking in a mantle with a viscosity ranging from 10 9 to 10 14 pa s provide plausible conditions for a dynamo equatorial sinking diapirs are confined to a thicker velocity layer and are thus possibly more favorable for dynamo generation than polar sinking diapirs in addition equatorial sinking diapirs produce stronger saturated magnetic fields however for the range of parameters studied here estimation of the intensity of diapir driven magnetic fields suggests that they could not have contributed to the lunar or martian crustal paleomagnetic fields

magnetic field and plasma observations at mars initial results of the mars global surveyor mission the magnetometer and electron reflectometer investigation mag er on the mars global surveyor spacecraft has obtained magnetic field and plasma observations throughout the near mars environment from beyond the influence of mars to just above the surface at an altitude of 100 kilometers the solar wind interaction with mars is in many ways similar to that at venus and at an active comet that is primarily an ionospheric atmospheric interaction no significant planetary magnetic field of global scale has been detected to date 2 1021 gauss cubic centimeter but here the discovery of multiple magnetic anomalies of small spatial scale in the crust of mars is reported

formation of a solid inner core during the accretion of earth the formation of an inner core during the accretion of earth is investigated using self gravitating and compressible earth models formed by accreting a total of 25 or 50 moon to mars size planetary embryos the impact of an embryo heats the proto earth s interior differentially more below the impact site than elsewhere the rotating core dynamically overturns and stratifies shortly after each impact creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile merging of an embryo to the proto earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification a total of 36 thermal evolution models of the growing proto earth s core are calculated to investigate effects of major physical parameters no solidification is considered in the first 21 models where modified two body escape velocities are used as the impact velocities of the embryos at the end of accretion temperatures in the upper part of the core are significantly different among these models whereas temperatures in the deeper parts are similar the core solidification considered in the remaining 15 models where impact velocities higher than the modified two body escape velocities are adopted drastically changes the temperature distribution in the deeper parts of the core all of the models produce partially solidified stiff inner cores 1000 2100 km in radius at the end of accretion where the solid fraction is larger than 50 the innermost of the stiff inner cores are completely solidified to radii 250 1500 km

thermal evolution of a growing mars 1 we developed a new numerical model which can simulate the thermal history and metal silicate separation of a growing mars in this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating crater excavation and isostatic rebound a metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature sinking of the metal blobs is traced assuming stokes velocity their coalescence during sinking is treated by a monte carlo approach a series of simulations is carried out assuming that mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process our numerical results show that 1 no global magma ocean is formed during accretion 2 metal silicate separation takes place without global scale melting and 3 instead of a metallic core a metal rich layer is formed at the late stage of accretion

the interior structure of mars implications from snc meteorites two end member models of mars present interior structure are presented the first model is optimized to satisfy the geochemical data derived from the snc meteorites in terms of the bulk chondritic ratio fe si 1 71 while the second model is optimized to satisfy the most probable maximum value c 0 366 x m p r p 2 of the polar moment of inertia factor hydrostatic equilibrium and stationary heat transfer are assumed and the basic differential equations for the mechanical and thermal structure are solved numerically together with an isothermal murnaghan birch type equation of state truncated in eulerian strain at forth order we obtain the radial distribution of mass hydrostatic pressure gravity temperature and heat flow density along with the corresponding density stratification viscosity profiles and the global seismic velocity structure of model mars the first model being consistent with the geochemical requirement produces c 0 357 x m p r p 2 whereas the second model commensurate with the geophysical constraint gives fe si 1 35 the calculated central pressure is about 40 gpa in both models and the central temperature is in the 2000 to 2200 k range the model calculations suggest a fe ni fes core a little less than one half of the planetary radius in size surrounded by a silicate mantle subdivided into lower spinel and upper olivine layers and overlain by a 100 to 250 km thick basaltic crust and a surface heatflow density of 25 to 30 mw m 2 in both models the pressure in the mantle is not sufficient for the spinel to perovskite transition to occur the present thermal lithosphere is estimated to be about 500 km thick and to be subdivided into a 300 km thick outermost rheological lithosphere and an underlying thermal boundary layer of mantle convection given the core sulfur content of 14 wt as derived from snc meteorites the martian core is found to be entirely molten implying the nonoperation of a self sustained dynamo due to the absence of sufficiently vigorous convection

three dimensional simulations of the southern polar giant impact hypothesis for the origin of the martian dichotomy we demonstrate via numerical simulations that the impact of a lunar sized body with mars is capable of creating a hemispherical magma ocean that upon cooling and solidification resulted in the formation of the southern highlands and thus the martian dichotomy the giant impact may have contributed a significant amount of iron to the martian core and generated a deep thermal anomaly that led to the onset and development of the volcanism in the southern highlands our model predicts several mantle plumes converging to the south pole from the equatorial regions as well as new plumes forming in the equatorial region and also an absence of significant large scale volcanism in the northern lowlands the core heat flux evolution obtained from our numerical models is consistent with the decline of the magnetic field we argue that such a scenario is more consistent with a range of observations than a northern giant impact excavating the borealis basin for the formation of the martian dichotomy

heat partitioning in terrestrial planets during core formation by negative diapirism article i nfo we investigate the gravitational heat partitioning between silicate and metallic phases during core formation by negative diapirism in terrestrial planets we model the dynamic evolution of an iron diapir sinking through a solid silicate proto mantle our calculations include viscous heating and strong temperature and composition dependence of viscosity we show that the diapir mean temperature evolves towards an asymptotic value td which depends on the efficiency of viscous heating and heat transfer between the diapir and its surroundings and on the viscous rheology we derived a simple analytic model that captures the thermal evolution observed in our numerical experiments this model can be applied to determine the heat distribution within terrestrial planets with a large number of diapirs during their growth and early differentiation we show that the partitioning of core formation heating during negative diapirism depends strongly on the size distribution of the iron diapirs a large number of small sinking iron diapirs favors metal silicate heat exchange and leads to a relatively hot core which allows a sustainable dynamo as on earth a small number of large diapirs have the opposite effect and could correspond to a mars like planet with an early dynamo that is no longer active in all cases negative diapirism tends to leave the lowermost mantle significantly hotter than its upper part which could trigger large amounts of melting in the early deep mantle

formation ages and evolution of shergotty and its parent planet from u th pb systematics the isotopic composition of lead was determined in different phases from shergotty zagami and eeta 79001 using phases with low 238 u 204 pb the initial leads of these meteorites could be well defined and are distinct from each other and have high 204 pb 206 pb from 0 0652 to 0 0739 these leads evolved in different reservoirs over most of solar system history in a low 238 u 204 pb 5 environment it follows that the parent planet of the shergottites has a high 204 pb concentration relative to u and must also be high in other volatiles if the parent planet of the shergottites is mars then this planet must be volatile rich compared to the earth by a factor of 1 8 this implies that mars lost its original atmosphere and that the present atmosphere was derived from a previously outgassed martian upper mantle the u th pb isotopic relationships involving phases with high 238 u 204 pb are quite regular and give ages for three shergottites which indicate an event of u th pb fractionation at 200 m y the initial leads at that time were highly evolved relative to the primordial lead the absence of substantial additions of radiogenic lead above the initial lead in high u phases precludes an older age for the u th pb fractionation however details of the data arrays are complex the young age is in general agreement with some of the ages obtained by other methods but precise concordance between the different methods is not established it is suggested that the shock event that produced maskelynite may possibly be associated with the late stage breakup at 2 5 m y and not necessarily related to the 180 m y record the major event of chronometer resetting is taken to be 180 m y this event does not appear to be an endogenic igneous event but an almost complete equilibration during a major impact on more ancient igneous rocks which were contained in the target problems of the polymetamorphic history of meteorites appear as complicated as those of polymetamorphic terrestrial terranes

is the primordial crust of mars magnetized abstract a meteorite impact capable of creating a 200 km diameter crater can demagnetize the entire crust beneath and produce an appreciable magnetic anomaly at satellite altitudes of 400 km in case the pre existing crust is magnetized in this study we examine the magnetic field over all of the craters and impact related quasi circular depressions qcds with diameters larger than 200 km that are located on the highlands of mars excluding the tharsis bulge in order to estimate the mean magnetization of the highland crust using the surface topography and the gravity of mars we first identify those qcds that are likely produced by impacts the magnetic map of a given crater or impact related qcd is derived using the mars global surveyor high altitude nighttime radial magnetic data two extended ancient areas are identified on the highlands the south province and the tempe terra which have large number of craters and impact related qcds but none of them has an appreciable magnetic signature the primordial crust of these areas is not magnetized or is very weakly magnetized at most we examine some plausible scenarios to explain the weak magnetization of these areas and conclude that no strong dynamo existed in the first 100 myr of mars history when the newly formed primordial crust was cooling below the magnetic blocking temperatures of its minerals