Martian Core Research Articles

Onset of magma ocean solidification on Mars inferred from Mn-Cr chronometry

The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. Here we applied the short-lived 53Mn-53Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic 53Cr variations, and instead have a uniform +20.2±1.2 (95% conf.) parts-per-million excess in 53Cr/52Cr relative to the terrestrial mantle. The investigated groups of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of 53Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of 53Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within <10 Ma after Solar System formation. Thus, the new Mn-Cr data imply that there was a small, but resolvable, time gap of at least 5 Ma between core formation and magma ocean solidification on Mars. If the age of core segregation is taken at face value, our results imply that the martian magma ocean remained mostly molten over several Ma. This inferred longevity of the magma ocean is inconsistent with thermal models predicting rapid (<1 Ma) solidification of the martian magma ocean. Although there is currently no unique solution to this conundrum, our results can potentially be explained by a protracted history of impact bombardment that delayed differentiation in a shallow magma ocean on Mars, or perhaps more readily, by the presence of an early and dense atmosphere that acted as an insulator and prevented the magma ocean from cooling quickly.

Thermal state and solidification regime of the martian core: Insights from the melting behavior of FeNi-S at 20 GPa

A series of multi-anvil experiments have been conducted to define the iron-rich liquidus of the iron-nickel-sulfur (FeNi-S) system at 20 GPa, the estimated pressure of the martian core-mantle boundary (CMB). The liquidus curve of FeNi-S containing about 9 wt.% Ni has a concave up shape, and is as much as 400 K lower than the liquidi previously applied to the martian core with sparse experimental constraints. Unlike existing liquidi of Fe-S and FeNi-S at 23 GPa, which predict a fully molten core for a narrow range of sulfur content between 14 and 15 wt.% S, our results are consistent with a molten state for all proposed core compositions, and establishes a new minimum CMB temperature of 1500 K for 10 wt.% S and 1250 K for 16 wt.% S. Extrapolating our FeNi-S liquidus to high pressures and comparing it to calculated areotherms, we find that three core crystallization regimes are possible. For a martian core with moderate sulfur content (10 to 13 wt.%) or lower, crystallization takes the form of iron snow near the CMB, while for cores with higher sulfur content (15-16 wt.%), solidification occurs near the center of the planet in the form of solid Fe3S. At an intermediate sulfur content of 14 wt.%, Fe3S would precipitate over a broad depth range and may appear fully molten to surface observations.

The composition of Mars

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.

Hydrogenation of the Martian Core by Hydrated Mantle Minerals With Implications for the Early Dynamo

AbstractMars lacks an internally generated magnetic field today. Crustal remanent magnetism and meteorites indicate that a dynamo existed after accretion but died roughly four billion years ago. Standard models rely on core/mantle heat flow dropping below the adiabatic limit for thermal convection in the core. However, rapid core cooling after the Noachian is favored instead to produce long‐lived mantle plumes and magmatism at volcanic provinces such as Tharsis and Elysium. Hydrogenation of the core could resolve this apparent contradiction by impeding the dynamo while core/mantle heat flow is superadiabatic. Here we present parameterized models for the rate at which mantle convection delivers hydrogen into the core. Our models suggest that most of the water that the mantle initially contained was effectively lost to the core. We predict that the mantle became increasingly ironrich over time and a stratified layer awaits detection in the uppermost core—analogous to the E′ layer atop Earth's core but likely thicker than alternative sources of stratification in the Martian core such as iron snow. Entraining buoyant, hydrogen‐rich fluid downward in the core subtracts gravitational energy from the total dissipation budget for the dynamo. The calculated fluxes of hydrogen are high enough to potentially reduce the lifetime of the dynamo by several hundred million years or longer relative to conventional model predictions. Future work should address the complicated interactions between the stratified, hydrogen‐rich layer and convection in the underlying core.

Core formation and geophysical properties of Mars

The chemical and physical properties of the interiors of terrestrial planets are largely determined during their formation and differentiation. Modeling a planet's formation provides important insights into the properties of its core and mantle, and conversely, knowledge of those properties may constrain formational narratives. Here, we present a multi-stage model of Martian core formation in which we calculate core–mantle equilibration using parameterizations from high pressure–temperature metal–silicate partitioning experiments. We account for changing core–mantle boundary (CMB) conditions, composition-dependent partitioning, and partial equilibration of metal and silicate, and we evolve oxygen fugacity (fO2) self-consistently. The model successfully reproduces published meteorite-based estimates of most elemental abundances in the bulk silicate Mars, which can be used to estimate core formation conditions and core composition. This composition implies that the primordial material that formed Mars was significantly more oxidized (0.9–1.4 log units below the iron–wüstite buffer) than that of the Earth, and that core–mantle equilibration in Mars occurred at 42–60% of the evolving CMB pressure. On average, at least 84% of accreted metal and at least 40% of the mantle were equilibrated in each impact, a significantly higher degree of metal equilibration than previously reported for the Earth. In agreement with previous studies, the modeled Martian core is rich in sulfur (18–19 wt%), with less than one weight percent O and negligible Si.We have used these core and mantle compositions to produce physical models of the present-day Martian interior and evaluate the sensitivity of core radius to crustal thickness, mantle temperature, core composition, core temperature, and density of the core alloy. Trade-offs in how these properties affect observable physical parameters like planetary mass, radius, moment of inertia, and tidal Love number k2 define a range of likely core radii: 1620–1870 km. Seismic velocity profiles for several combinations of model parameters have been used to predict seismic body-wave travel times and planetary normal mode frequencies. These results may be compared to forthcoming Martian seismic data to further constrain core formation conditions and geophysical properties.

Partition coefficient of phosphorus between liquid metal and silicate melt with implications for the Martian magma ocean

Phosphorus in Martian mantle is believed to be five to ten times more abundant than in Earth's mantle, and the distribution of this essential ingredient for life between different deep reservoirs is critical for understanding the habitability of the red planet. In this study, we investigated the behavior of phosphorus in a Martian magma ocean scenario, and measured the partition coefficient of phosphorus (DP) between liquid metal and silicate melt within the pressure range of 3–8 GPa, temperatures between 1973 and 2173 K and oxygen fugacity ranging from −1.5 to ~ −2.5 as normalized to the iron-wüstite oxygen buffer. Our results show DP increasing with pressure but decreasing with temperature. A decrease of oxygen fugacity has a negative effect on DP. The moderately siderophile character of phosphorus indicates that the Martian core might be an important reservoir of phosphorous. Based on our experimental results and phosphorus abundance in Martian mantle and bulk Mars, a minimum pressure of 5.8–10.4 GPa is estimated at the base of Martian magma ocean or during the impact melting if a contribution from the late accretion scenario is taken into account. The shallow Martian magma ocean would avail the preservation of volatiles after the rapid solidification of the planet.

Terrestrial single-station analog for constraining the martian core and deep interior: Implications for InSight

We used a terrestrial single-station seismometer to quantify the uncertainty of InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) data for determining Martian core size. To mimic Martian seismicity, we formed a catalog using 917 terrestrial earthquakes, from which we randomly selected events. We stacked ScS amplitudes on modeled arrival times and searched for where ScS produced coherent seismic amplitudes. A core detection was defined by a coherent peak with small offset between predicted and user-selected arrival times. Iterating the detection algorithm with varying signal-to-noise (SNR) ranges and quantity of events determined the selection frequency of each model and quantified core depth uncertainty. Increasing the quantity of events reduced core depth uncertainty while increasing the recovery rate, while increasing event SNR had little effect. Including ScS2 multiples increased the recovery rate and reduced core depth uncertainty when we used low quantities of events. The most-frequent core depths varied by back azimuth, suggesting our method is sensitive to the presence of mantle heterogeneities. When we added 1° in source distance errors, core depth uncertainty increased by up to 11 km and recovery rates decreased by <5%. Altering epicentral distances by 25% added ~35 km of uncertainty and reduced recovery rates to <50% in some cases. From these experiments, we estimate that if InSight can detect five events with high location precision (<10% epicentral distance errors), that there is at least an 88% chance of core depth recovery using ScS alone with uncertainty in core depth approaching 18 km and decreasing as more events are located. Plain language summaryWe used a seismometer on Earth to study how well the NASA mission, InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) could detect the Martian core. We created a catalog of earthquakes from which we randomly selected events to mimic the seismic events (marsquakes) InSight is expected to detect. Several models of Earth's interior were tested to see which best predicted the arrival of seismic waves reflecting off the core. We found utilizing more events allowed us to determine the core size better than using a limited number of events. Using an additional seismic phase also helped us recover a more accurate core depth if there were not a lot of events. We were also able to find variations in the mantle by using events within certain location ranges. Since the InSight team may not be able to locate marsquakes as accurately as we locate earthquakes, we changed the location of the earthquakes. When changes in location were small, we were still able to determine the size of the core. Larger changes in location prevented us from finding an accurate core depth. If InSight detects at least five events with high location accuracy, we will be able to determine the Martian core size.

High Precision SEIS Calibration for the InSight Mission and Its Applications

Part of the InSight mission, the SEIS instrument (Seismic Experiment for Interior Structures), is planned to arrive on Mars in November 2018. In order to prepare its future recordings on the red planet, special attention was directed towards calibrating the seismometer in-situ on the Martian surface. Besides relative calibrations, we studied the possibility of actively calibrating the two kinds of seismometers onboard SEIS, the Very Broad Band seismometers (VBB) and the Short Period seismometers (SP) and extended the analysis towards a possible absolute calibration. For that purpose, we developed additional noise models at low frequency and elaborate on how they will be sensed by the seismic sensors from long-period data recorded by the seismometer. Such work will improve SEIS capabilities to unveil the inner structure of Mars by checking SEIS well-being and with applications such as gravimetry with the main Phobos tide. The current calibration procedure is planned to take one hour to calibrate the VBB sensors using the SP sensors, and determine the VBB gain with an accuracy of 0.4%, good enough to resolve the state of the Martian core.

Compressibility of natural schreibersite up to 50 GPa

Schreibersite-(Fe, Ni)3P is a common accessory of iron meteorites, which carry direct information of planetary cores. Herein, in situ synchrotron-radiation X-ray diffraction (XRD) experiments of a natural single-crystal schreibersite have been carried out up to ~ 50 GPa in a diamond anvil cell (DAC) at room temperature. Experimental results show that schreibersite-Fe2.15Ni0.85P is very stable and remains the tetragonal structure ( $$I\overline {4}$$ ) throughout the investigated pressure range. The pressure–volume (P–V) data were fitted by a third-order Birch–Murnaghan equation of state (B–M EoS), yielding K0 = 184(4) GPa, V0 = 365.9(1), $$K_{0}^{'}$$ = 4.1(2), and also a second-order B–M EoS, yielding K0 = 185(1) GPa, V0 = 365.8(1), $$K_{0}^{'}$$ = 4.0 (fixed). Unlike its isostructural minerals (e.g., Fe3S and Fe3P), schreibersite-Fe2.15Ni0.85P demonstrates an isotropic axial compressibility and an inhibition of magnetic transition under compression. Comparison in densities of Fe2.15Ni0.85P, Fe3S, Fe3P, FeS, and FeP at relative pressure–temperature conditions of the Martian core shows that the doping of Ni and P has strong effects on density reduction of iron sulfides, and schreibersite-(Fe, Ni)3P could provide a needful compensation for iron sulfides in the Martian core.

A synthesis of geochemical constraints on the inventory of light elements in the core of Mars

Accurate constraints on the light element composition of the Martian core are required for models of the Martian core dynamo, the conditions under which the Martian core formed (i.e. the existence and extent of a magma ocean) and the overall volatile inventory of Mars. Here, we present a synthesis of geochemical constraints on the abundances of light elements S, C and O in the Martian core using mass balance calculations combined with published expressions that predict their high-pressure metal–silicate partitioning behaviour. We incorporate recently proposed bulk S Martian mantle abundances and find that the Martian core must be S-rich, virtually independent of the type of bulk composition considered and the P-T conditions during core-mantle differentiation of Mars. The core contains at least 7 wt % S and may be up to stoichiometric FeS in composition, depending on which P-T conditions (and bulk compositions) are assumed. If bulk Mars was formed from chondritic building blocks, the core S content is constrained to 13.5 ± 3.5 wt.%, in good agreement with geophysical models of the Martian interior and with measured siderophile element depletions in SNC meteorites. Our calculations yield O contents for the Martian core of < 4 wt.%, with the highest concentrations for the highest P-T conditions of Martian core formation. Carbon contents in the Martian core are expected to be low (< 1.4 wt.%) given the abundance of C in chondritic meteorite groups. The calculated solubility limit for C in Fe-Ni-S alloys is higher than calculated core C contents in virtually all cases, suggesting the Martian primitive mantle is not graphite saturated if the bulk Mars C budget is (close to) chondritic.The estimated depletions of volatile elements Se and Te in the Martian interior can be reconciled easily with formation of a Martian S-rich core. This implies that these volatile elements may not have been lost from Mars by degassing in Mars early history.The calculated Martian core S contents cannot be used to distinguish between the two different proposed modes of core crystallization, but do suggest the Martian core may still be fully liquid today.

Iron snow in the Martian core?

The decline of Mars' global magnetic field some 3.8–4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth's core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the Fe–S liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars' core could crystallize from the top down. We focus on the “iron snow” regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, ξ0. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5–2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have ξ0≈10% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

Silver contents and Cu/Ag ratios in Martian meteorites and the implications for planetary differentiation

Silver and Cu show very similar partitioning behavior in sulfide melt-silicate melt and metal-silicate systems at low and high pressure-temperature (P-T) experimental conditions, implying that mantle melting, fractional crystallization and core-mantle differentiation have at most modest (within a factor of 3) effects on Cu/Ag ratios. For this reason, it is likely that Cu/Ag ratios in mantle-derived magmatic products of planetary bodies reflect that of the mantle and, in some circumstances, also the bulk planet composition. To test this hypothesis, new Ag mass fractions and Cu/Ag ratios in different groups of Martian meteorites are presented and compared with data from chondrites and samples from the Earth’s mantle.Silver contents in lherzolitic, olivine-phyric and basaltic shergottites and nakhlites range between 1.9 and 12.3ng/g. The data display a negative trend with MgO content and correlate positively with Cu contents. In spite of displaying variable initial Ɛ143Nd values and representing a diverse spectrum of magmatic evolution and physiochemical conditions, shergottites and nakhlites display limited variations of Cu/Ag ratios (1080±320, 1s, n=14). The relatively constant Cu/Ag suggests limited fractionation of Ag from Cu during the formation and evolution of the parent magmas, irrespectively of whether sulfide saturation was attained or not. The mean Cu/Ag ratio of Martian meteorites thus reflects that of the Martian mantle and constrains its Ag content to 1.9±0.7ng/g (1s).Carbonaceous and enstatite chondrites display a limited range of Cu/Ag ratios of mostly 500–2400. Ordinary chondrites show a larger scatter of Cu/Ag up to 4500, which may have been caused by Ag redistribution during parent body metamorphism. The majority of chondrites have Cu/Ag ratios indistinguishable from the Martian mantle value, indicating that Martian core formation strongly depleted Cu and Ag contents, but probably did not significantly change the Cu/Ag ratio of the mantle compared to bulk Mars. Bulk Mars is richer in moderately volatile elements than Earth, however, the Martian mantle displays a much stronger depletion of the moderately volatile elements Cu and Ag, e.g., by a factor of 15 for Cu. This observation is consistent with experimental studies suggesting that core formation at low P-T conditions on Mars led to more siderophile behavior of Cu and Ag than at high P-T conditions as proposed for Earth. In contrast, Cu/Ag ratios of the mantles of Mars and Earth (Cu/AgEarth=3500±1000) display only a difference by a factor of 3, which implies restricted fractionation of Cu and Ag even at high P-T conditions. The concentration data support the notion that siderophile element partitioning during planetary core formation scales with the size of the planetary body, which is particularly important for the differentiation of large terrestrial planets such as Earth. Collectively, the Ag and Cu data on magmatic products from the mantles of Mars and Earth and the data on chondrites confirm experimental predictions and support the limited fractionation of Cu and Ag during planetary core formation and high-temperature magmatic evolution, and probably also in early solar nebular processes.

Chalcophile elements in Martian meteorites indicate low sulfur content in the Martian interior and a volatile element-depleted late veneer

It is generally believed that the Martian mantle and core are rich in sulfur and that shergottites originated from sulfide-saturated magma. However, recent work suggests that the high FeO contents would require very high S concentrations in shergottite parent magmas at sulfide saturation. Here we combine new and published data on chalcophile elements in shergottites, nakhlites and ALH84001 to constrain the sulfide saturation state of the parent magmas and the chalcophile element concentrations in their mantle sources.Regardless of the MgO content and the long-term depletion history of incompatible lithophile elements as indicated by initial ε143Nd, different groups of shergottites display limited variations in ratios of Pt, Pd, Re, Cu, S, Se and Te. The emplacement of most shergottites within the crust and limited variations of ratios of chalcophile elements with substantial differences in volatility during eruption (e.g., Cu/S, Cu/Se and Pt/Re) indicate little degassing losses of S, Se, Te and Re from shergottites. Limited variations in ratios of elements with very different sulfide–silicate melt partition coefficients and negative correlations of chalcophile elements with MgO require a sulfide-undersaturated evolution of the parent magmas from mantle source to emplacement in the crust, consistent with the FeO-based argument. Sulfide petrography and the komatiite-like fractionation of platinum group elements (PGE) in shergottites also support this conclusion. The absence of accumulated sulfides in the ancient Martian cumulate ALH84001 results in very low contents of PGE, Re, Cu, Se and Te in this meteorite, hinting that sulfide-undersaturated magmas may have occurred throughout the Martian geological history. The negative correlation of Cu and MgO contents in shergottites suggests approximately 2±0.4(1s) μg/g Cu in the Martian mantle. The ratios of Cu, S, Se and Te indicate 360±120 μg/g (1s) S, 100±27 ng/g (1s) Se and 0.50±0.25 ng/g (1s) Te in the Martian mantle. At such low S concentrations, all S in Martian mantle sources may dissolve in basaltic melts that form at >5 % partial melting.Assuming equilibrium metal–silicate partitioning, and provided that the compositional model of the Martian mantle based on SNC meteorites is correct, Martian mantle inventories of Cu, S and Se were mostly established by core formation and the Martian core should contain <5–10 wt.% S only (depending on the choice of metal–silicate partition coefficients). The low S content in the Martian interior is consistent with the low Zn content in the Martian mantle, which indicates about 5 wt.% S in the core. In contrast, the highly siderophile PGE, Re and Te were added to the mantle by late accreted material after the Martian core formed. The near chondritic PGE ratios and the very low ratio of volatile Te to refractory PGE reflect a strongly volatile element-depleted late veneer and imply that the delivery of Martian water, presumably from carbonaceous chondrite like materials, must have occurred before accretion of the late veneer, likely within 2–3 million years after formation of the solar system.

Thermal State and Solidification Regime of the Martian Core: Insights from the Fe-Ni-S Liquidus at 20 GPa

Thermal State and Solidification Regime of the Martian Core: Insights from the Fe-Ni-S Liquidus at 20 GPa

Siderophile and chalcophile element abundances in shergottites: Implications for Martian core formation

AbstractElemental abundances for volatile siderophile and chalcophile elements for Mars inform us about processes of accretion and core formation. Such data are few for Martian meteorites, and are often lacking in the growing number of desert finds. In this study, we employed laser ablation inductively coupled plasma–mass spectrometry (LA‐ICP‐MS) to analyze polished slabs of 15 Martian meteorites for the abundances of about 70 elements. This technique has high sensitivity, excellent precision, and is generally accurate as determined by comparisons of elements for which literature abundances are known. However, in some meteorites, the analyzed surface is not representative of the bulk composition due to the over‐ or underrepresentation of a key host mineral, e.g., phosphate for rare earth elements (REE). For other meteorites, the range of variation in bulk rastered analyses of REE is within the range of variation reported among bulk REE analyses in the literature. An unexpected benefit has been the determination of the abundances of Ir and Os with a precision and accuracy comparable to the isotope dilution technique. Overall, the speed and small sample consumption afforded by this technique makes it an important tool widely applicable to small or rare meteorites for which a polished sample was prepared. The new volatile siderophile and chalcophile element abundances have been employed to determine Ge and Sb abundances, and revise Zn, As, and Bi abundances for the Martian mantle. The new estimates of Martian mantle composition support core formation at intermediate pressures (14 ± 3 GPa) in a magma ocean on Mars.

Three‐dimensional simulations of the southern polar giant impact hypothesis for the origin of the Martian dichotomy

AbstractWe 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.

Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water

In order to test accretion simulations as well as planetary differentiation scenarios, we have integrated a multistage core–mantle differentiation model with N-body accretion simulations. Impacts between embryos and planetesimals are considered to result in magma ocean formation and episodes of core formation. The core formation model combines rigorous chemical mass balance with metal–silicate element partitioning data and requires that the bulk compositions of all starting embryos and planetesimals are defined as a function of their heliocentric distances of origin. To do this, we assume that non-volatile elements are present in Solar System (CI) relative abundances in all bodies and that oxygen and H2O contents are the main compositional variables. The primary constraint on the combined model is the composition of the Earth’s primitive mantle. In addition, we aim to reproduce the composition of the martian mantle and the mass fractions of the metallic cores of Earth and Mars. The model is refined by least squares minimization with up to five fitting parameters that consist of the metal–silicate equilibration pressure and 1–4 parameters that define the starting compositions of primitive bodies. This integrated model has been applied to six Grand Tack N-body accretion simulations. Investigations of a broad parameter space indicate that: (1) accretion of Earth was heterogeneous, (2) metal–silicate equilibration pressures increase as accretion progresses and are, on average, 60–70% of core–mantle boundary pressures at the time of each impact, and (3) a large fraction (70–100%) of the metal of impactor cores equilibrates with a small fraction of the silicate mantles of proto-planets during each core formation event. Results are highly sensitive to the compositional model for the primitive starting bodies and several accretion/core-formation models can thus be excluded. Acceptable fits to the Earth’s mantle composition are obtained only when bodies that originated close to the Sun, at <0.9–1.2AU, are highly reduced and those from beyond this distance are increasingly oxidized. Reasonable concentrations of H2O in Earth’s mantle are obtained when bodies originating from beyond 6–7AU contain 20wt% water ice (icy bodies that originated between the snow line and this distance did not contribute to Earth’s accretion because they were swept up by Jupiter and Saturn). In the six models examined, water is added to the Earth mainly after 60–80% of its final mass has accreted. The compositional evolution of the mantles of Venus and Mars are also constrained by the model. The FeO content of the martian mantle depends critically on the heliocentric distance at which the Mars-forming embryo originated. Finally, the Earth’s core is predicted to contain 8–9wt% silicon, 2–4wt% oxygen and 10–60ppm hydrogen, whereas the martian core is predicted to contain low concentrations (<1wt%) of Si and O.

Sulfur-controlled iron isotope fractionation experiments of core formation in planetary bodies

A series of high pressure and temperature experiments were conducted to better constrain the Fe isotope fractionation during core–mantle differentiation in planetesimal and planetary bodies. Synthetic mixtures of oxides and metal having varying amounts of sulfur, approximating terrestrial and Martian compositions, were melted at 1–2GPa and 1650°C. Iron isotopic equilibrium between the resulting metal and glass run products was verified for all experiments using the three-isotope technique. Purified Fe from metal and glass was analyzed by multiple-collector ICP-MS in high resolution mode. Iron alloy and silicate glass show a well-resolved Δ57Femetal–silicate of +0.12±0.04‰ in a sulfur-free system. Isotope fractionation increases with sulfur content to +0.43±0.03‰ at 18wt.% sulfur in the metal. These results cannot be easily interpreted within the context of known Fe isotope ratios in most natural samples of planetary and asteroidal mantles and therefore suggest more complex processes affected the Fe isotope fractionation therein. However, to reconcile Martian meteorite iron isotopic signatures with geophysical models using this new experimental data requires a smaller amount of sulfur in the Martian core than previous estimates, with an upper limit of ∼8wt.%.

Consequences of giant impacts in early Mars: Core merging and Martian dynamo evolution

AbstractA 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.

Composition of Mars constrained using geophysical observations and mineral physics modeling

We use the total mass, possible core radius and the observed mean moment of inertia factor of Mars to constrain mineralogical and compositional structures of Mars. We adopt a liquid Fe–S system for the Martian core and construct density models of the interior of Mars for a series of mantle compositions, core compositions and temperature profiles. The moment of inertia factor of the planet is then calculated and compared to the observation to place constraints on Mars composition. Based on the independent constraints of total mass, possible core radius of 1630–1830km, and the mean moment of inertia factor (0.3645±0.0005) of Mars, we find that Fe content in the Martian mantle is between 9.9 and 11.9mol%, Al content in the Martian mantle smaller than 1.5mol%, S content in the Martian core between 10.6 and 14.9wt%. The inferred Fe content in the bulk Mars lies between 27.3 and 32.0wt%, and the inferred Fe/Si ratio in Mars between 1.55 and 1.95, within a range too broad to make a conclusion whether Mars has the same nonvolatile bulk composition as that of CI chondrite. We also conclude that no perovskite layer exists in the bottom of the Martian mantle. Based on the inferred density models, we estimate the flattening factor and J2 gravitational potential related to the hydrostatic figure of the rotating Mars to be (5.0304±0.0098)×10-3 and (1.8151±0.0065)×10-3, respectively. We also discuss implications of these compositional models to the understanding of formation and evolution of the planet.