Accretion of the Earth and segregation of its core

The Earth took 30-40 million years to accrete from smaller 'planetesimals'. Many of these planetesimals had metallic iron cores and during growth of the Earth this metal re-equilibrated with the Earth's silicate mantle, extracting siderophile ('iron-loving') elements into the Earth's iron-rich core. The current composition of the mantle indicates that much of the re-equilibration took place in a deep (> 400 km) molten silicate layer, or 'magma ocean', and that conditions became more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to the Earth's core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of the smaller planetesimal building blocks.     

The Earth's missing lead may not be in the core

Relative to the CI chondrite class of meteorites (widely thought to be the 'building blocks' of the terrestrial planets), the Earth is depleted in volatile elements. For most elements this depletion is thought to be a solar nebular signature, as chondrites show depletions qualitatively similar to that of the Earth. On the other hand, as lead is a volatile element, some Pb may also have been lost after accretion. The unique (206)Pb/(204)Pb and (207)Pb/(204)Pb ratios of the Earth's mantle suggest that some lead was lost about 50 to 130 Myr after Solar System formation. This has commonly been explained by lead lost via the segregation of a sulphide melt to the Earth's core, which assumes that lead has an affinity towards sulphide. Some models, however, have reconciled the Earth's lead deficit with volatilization. Whichever model is preferred, the broad coincidence of U-Pb model ages with the age of the Moon suggests that lead loss may be related to the Moon-forming impact. Here we report partitioning experiments in metal-sulphide-silicate systems. We show that lead is neither siderophile nor chalcophile enough to explain the high U/Pb ratio of the Earth's mantle as being a result of lead pumping to the core. The Earth may have accreted from initially volatile-depleted material, some lead may have been lost to degassing following the Moon-forming giant impact, or a hidden reservoir exists in the deep mantle with lead isotope compositions complementary to upper-mantle values; it is unlikely though that the missing lead resides in the core.     

Mixing, volatile loss and compositional change during impact-driven accretion of the Earth

The degree to which efficient mixing of new material or losses of earlier accreted material to space characterize the growth of Earth-like planets is poorly constrained and probably changed with time. These processes can be studied by parallel modelling of data from different radiogenic isotope systems. The tungsten isotope composition of the silicate Earth yields a model timescale for accretion that is faster than current estimates based on terrestrial lead and xenon isotope data and strontium, tungsten and lead data for lunar samples. A probable explanation for this is that impacting core material did not always mix efficiently with the silicate portions of the Earth before being added to the Earth's core. Furthermore, tungsten and strontium isotope compositions of lunar samples provide evidence that the Moon-forming impacting protoplanet Theia was probably more like Mars, with a volatile-rich, oxidized mantle. Impact-driven erosion was probably a significant contributor to the variations in moderately volatile element abundance and oxidation found among the terrestrial planets.     

Evidence for an oxygen-depleted liquid outer core of the Earth

On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth's liquid outer core consists of mainly liquid iron alloyed with about ten per cent (by weight) of light elements. Although the concentrations of the light elements are small, they nevertheless affect the Earth's core: its rate of cooling, the growth of the inner core, the dynamics of core convection, and the evolution of the geodynamo. Several light elements-including sulphur, oxygen, silicon, carbon and hydrogen-have been suggested, but the precise identity of the light elements in the Earth's core is still unclear. Oxygen has been proposed as a major light element in the core on the basis of cosmochemical arguments and chemical reactions during accretion. Its presence in the core has direct implications for Earth accretion conditions of oxidation state, pressure and temperature. Here we report new shockwave data in the Fe-S-O system that are directly applicable to the outer core. The data include both density and sound velocity measurements, which we compare with the observed density and velocity profiles of the liquid outer core. The results show that we can rule out oxygen as a major light element in the liquid outer core because adding oxygen into liquid iron would not reproduce simultaneously the observed density and sound velocity profiles of the outer core. An oxygen-depleted core would imply a more reduced environment during early Earth accretion.     

The Earth's 'missing' niobium may be in the core

As the Earth's metallic core segregated from the silicate mantle, some of the moderately siderophile ('iron-loving') elements such as vanadium and chromium are thought to have entered the metal phase, thus causing the observed depletions of these elements in the silicate part of the Earth. In contrast, refractory 'lithophile' elements such as calcium, scandium and the rare-earth elements are known to be present in the same proportions in the silicate portion of the Earth as in the chondritic meteorites-thought to represent primitive planetary material. Hence these lithophile elements apparently did not enter the core. Niobium has always been considered to be lithophile and refractory yet it has been observed to be depleted relative to other elements of the same type in the crust and upper mantle. This observation has been used to infer the existence of hidden niobium-rich reservoirs in the Earth's deep mantle. Here we show, however, that niobium and vanadium partition in virtually identical fashion between liquid metal and liquid silicate at high pressure. Thus, if a significant fraction of the Earth's vanadium entered the core (as is thought), then so has a similar fraction of its niobium, and no hidden reservoir need be sought in the Earth's deep mantle.     

Partitioning of oxygen during core formation on the Earth and Mars

Core formation on the Earth and Mars involved the physical separation of metal and silicate, most probably in deep magma oceans. Although core-formation models explain many aspects of mantle geochemistry, they have not accounted for the large differences observed between the compositions of the mantles of the Earth (approximately 8 wt% FeO) and Mars (approximately 18 wt% FeO) or the smaller mass fraction of the martian core. Here we explain these differences as a consequence of the solubility of oxygen in liquid iron-alloy increasing with increasing temperature. We assume that the Earth and Mars both accreted from oxidized chondritic material. In a terrestrial magma ocean, 1,200-2,000 km deep, high temperatures resulted in the extraction of FeO from the silicate magma ocean owing to high solubility of oxygen in the metal. Lower temperatures of a martian magma ocean resulted in little or no extraction of FeO from the mantle, which thus remains FeO-rich. The FeO extracted from the Earth's magma ocean may have contributed to chemical heterogeneities in the lowermost mantle, a FeO-rich D" layer and the light element budget of the core.     

Volatile accretion history of the Earth

It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ～100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.     

Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry

The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes. Of these, the (182)Hf-to-(182)W decay is ideally suited for dating core formation in planetary bodies. In an earlier study, the W isotope composition of the Earth's mantle was used to infer that core formation was late (> or = 60 million years after the beginning of the Solar System) and that accretion was a protracted process. The correct interpretation of Hf-W data depends, however, on accurate knowledge of the initial abundance of (182)Hf in the Solar System and the W isotope composition of chondritic meteorites. Here we report Hf-W data for carbonaceous and H chondrite meteorites that lead to timescales of accretion and core formation significantly different from those calculated previously. The revised ages for Vesta, Mars and Earth indicate rapid accretion, and show that the timescale for core formation decreases with decreasing size of the planet. We conclude that core formation in the terrestrial planets and the formation of the Moon must have occurred during the first approximately 30 million years of the life of the Solar System.     

Characterization of extrasolar terrestrial planets from diurnal photometric variability

The detection of massive planets orbiting nearby stars has become almost routine, but current techniques are as yet unable to detect terrestrial planets with masses comparable to the Earth's. Future space-based observatories to detect Earth-like planets are being planned. Terrestrial planets orbiting in the habitable zones of stars-where planetary surface conditions are compatible with the presence of liquid water-are of enormous interest because they might have global environments similar to Earth's and even harbour life. The light scattered by such a planet will vary in intensity and colour as the planet rotates; the resulting light curve will contain information about the planet's surface and atmospheric properties. Here we report a model that predicts features that should be discernible in the light curve obtained by low-precision photometry. For extrasolar planets similar to Earth, we expect daily flux variations of up to hundreds of per cent, depending sensitively on ice and cloud cover as well as seasonal variations. This suggests that the meteorological variability, composition of the surface (for example, ocean versus land fraction) and rotation period of an Earth-like planet could be derived from photometric observations. Even signatures of Earth-like plant life could be constrained or possibly, with further study, even uniquely determined.     

Constraints on the composition of the Earth's core from ab initio calculations

Knowledge of the composition of the Earth's core is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron. The outer core, however, is also thought to contain a significant fraction of light elements, because its density--as deduced from seismological data and other measurements--is 6-10 per cent less than that estimated for pure liquid iron. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron-sulphur mixture.     

Two Earth-sized planets orbiting Kepler-20

Since the discovery of the first extrasolar giant planets around Sun-like stars, evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically passes in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered has a radius 1.42 times that of the Earth's radius (R(⊕)), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03R(⊕)) and the other smaller than the Earth (0.87R(⊕)), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets. The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.     

Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum

The high-pressure solubility in silicate liquids of moderately siderophile 'iron-loving' elements (such as nickel and cobalt) has been used to suggest that, in the early Earth, an equilibrium between core-forming metals and the silicate mantle was established at the bottom of a magma ocean. But observed concentrations of the highly siderophile elements--such as the platinum-group elements platinum, palladium, rhenium, iridium, ruthenium and osmium--in the Earth's upper mantle can be explained by such a model only if their metal-silicate partition coefficients at high pressure are orders of magnitude lower than those determined experimentally at one atmosphere (refs 3-8). Here we present an experimental determination of the solubility of palladium and platinum in silicate melts as a function of pressure to 16 GPa (corresponding to about 500 km depth in the Earth). We find that both the palladium and platinum metal-silicate partition coefficients, derived from solubility, do not decrease with pressure--that is, palladium and platinum retain a strong preference for the metal phase even at high pressures. Consequently the observed abundances of palladium and platinum in the upper mantle seem to be best explained by a 'late veneer' addition of chondritic material to the upper mantle following the cessation of core formation.     

Aluminium control of argon solubility in silicate melts under pressure

Understanding of the crystal chemistry of the Earth's deep mantle has evolved rapidly recently with the gradual acceptance of the importance of the effect of minor elements such as aluminium on the properties of major phases such as perovskite. In the early Earth, during its formation and segregation into rocky mantle and iron-rich core, it is likely that silicate liquids played a large part in the transport of volatiles to or from the deep interior. The importance of aluminium on solubility mechanisms at high pressure has so far received little attention, even though aluminium has long been recognized as exerting strong control on liquid structures at ambient conditions. Here we present constraints on the solubility of argon in aluminosilicate melt compositions up to 25 GPa and 3,000 K, using a laser-heated diamond-anvil cell. The argon contents reach a maximum that persists to pressures as high as 17 GPa (up to 500 km deep in an early magma ocean), well above that expected on the basis of Al-free melt experiments. A distinct drop in argon solubility observed over a narrow pressure range correlates well with the expected void loss in the melt structure predicted by recent molecular dynamics simulations. These results provide a process for noble gas sequestration in the mantle at various depths in a cooling magma ocean. The concept of shallow partial melting as a unique process for extracting noble gases from the early Earth, thereby defining the initial atmospheric abundance, may therefore be oversimplified.     

Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals

The Moon is thought to have formed from debris ejected by a giant impact with the early 'proto'-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf-182W and 146Sm-142Nd chronometry, but these methods have yielded contradicting results. In earlier studies, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first approximately 60 million years of the Solar System. However, the dominant 182W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of 181Ta during cosmic-ray exposure of the lunar surface, compromising a reliable interpretation in terms of 182Hf-182W chronometry. Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived 182W. All metals have identical 182W/184W ratios, indicating that the lunar magma ocean did not crystallize within the first approximately 60 Myr of the Solar System, which is no longer inconsistent with Sm-Nd chronometry. Our new data reveal that the lunar and terrestrial mantles have identical 182W/184W. This, in conjunction with 147Sm-143Nd ages for the oldest lunar rocks, constrains the age of the Moon and Earth to Myr after formation of the Solar System. The identical 182W/184W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth's mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon.     

Non-chondritic distribution of the highly siderophile elements in mantle sulphides

The abundances of highly siderophile (iron-loving) elements (HSEs) in the Earth's mantle provide important constraints on models of the Earth's early evolution. It has long been assumed that the relative abundances of HSEs should reflect the composition of chondritic meteorites--which are thought to represent the primordial material from which the Earth was formed. But the non-chondritic abundance ratios recently found in several types of rock derived from the Earth's mantle have been difficult to reconcile with standard models of the Earth's accretion, and have been interpreted as having arisen from the addition to the primitive mantle of either non-chondritic extraterrestrial material or differentiated material from the Earth's core. Here we report in situ laser-ablation analyses of sulphides in mantle-derived rocks which show that these sulphides do not have chondritic HSE patterns, but that different generations of sulphide within single samples show extreme variability in the relative abundances of HSEs. Sulphides enclosed in silicate phases have high osmium and iridium abundances but low Pd/Ir ratios, whereas pentlandite-dominated interstitial sulphides show low osmium and iridium abundances and high Pd/Ir ratios. We interpret the silicate-enclosed sulphides as the residues of melting processes and interstitial sulphides as the crystallization products of sulphide-bearing (metasomatic) fluids. We suggest that non-chondritic HSE patterns directly reflect processes occurring in the upper mantle--that is, melting and sulphide addition via metasomatism--and are not evidence for the addition of core material or of 'exotic' meteoritic components.     

Origin of the Moon in a giant impact near the end of the Earth's formation

The Moon is generally believed to have formed from debris ejected by a large off-centre collision with the early Earth. The impact orientation and size are constrained by the angular momentum contained in both the Earth's spin and the Moon's orbit, a quantity that has been nearly conserved over the past 4.5 billion years. Simulations of potential moon-forming impacts now achieve resolutions sufficient to study the production of bound debris. However, identifying impacts capable of yielding the Earth-Moon system has proved difficult. Previous works found that forming the Moon with an appropriate impact angular momentum required the impact to occur when the Earth was only about half formed, a more restrictive and problematic model than that originally envisaged. Here we report a class of impacts that yield an iron-poor Moon, as well as the current masses and angular momentum of the Earth-Moon system. This class of impacts involves a smaller-and thus more likely-object than previously considered viable, and suggests that the Moon formed near the very end of Earth's accumulation.     

A diffusion mechanism for core-mantle interaction

Understanding the geochemical behaviour of the siderophile elements--those tending to form alloys with iron in natural environments--is important in the search for a deep-mantle chemical 'fingerprint' in upper mantle rocks, and also in the evaluation of models of large-scale differentiation of the Earth and terrestrial planets. These elements are highly concentrated in the core relative to the silicate mantle, but their concentrations in upper mantle rocks are higher than predicted by most core-formation models. It has been suggested that mixing of outer-core material back into the mantle following core formation may be responsible for the siderophile element ratios observed in upper mantle rocks. Such re-mixing has been attributed to an unspecified metal-silicate interaction in the reactive D' layer just above the core-mantle boundary. The siderophile elements are excellent candidates as indicators of an outer-core contribution to the mantle, but the nature and existence of possible core-mantle interactions is controversial. In light of the recent findings that grain-boundary diffusion of oxygen through a dry intergranular medium may be effective over geologically significant length scales and that grain boundaries can be primary storage sites for incompatible lithophile elements, the question arises as to whether siderophile elements might exhibit similar (or greater) grain-boundary mobility. Here we report experimental results from a study of grain-boundary diffusion of siderophile elements through polycrystalline MgO that were obtained by quantifying the extent of alloy formation between initially pure metals separated by approximately 1 mm of polycrystalline MgO. Grain-boundary diffusion resulted in significant alloying of sink and source particles, enabling calculation of grain-boundary fluxes. Our computed diffusivities were high enough to allow transport of a number of siderophile elements over geologically significant length scales (tens of kilometres) over the age of the Earth. This finding establishes grain-boundary diffusion as a potential fast pathway for chemical communication between the core and mantle.     

Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans

The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36Ar as the atmosphere on Earth. The different effects of the solar wind on planet-forming materials for Earth and Venus have been proposed to account for some of this difference in atmospheric composition, but the cause of the compositional difference has not yet been fully resolved. Here we propose that the absence or presence of an ocean at the surface of a protoplanet during the giant impact phase could have determined its subsequent atmospheric amount and composition. Using numerical simulations, we demonstrate that the presence of an ocean significantly enhances the loss of atmosphere during a giant impact owing to two effects: evaporation of the ocean, and lower shock impedance of the ocean compared to the ground. Protoplanets near Earth's orbit are expected to have had oceans, whereas those near Venus' orbit are not, and we therefore suggest that remnants of the noble-gas rich proto-atmosphere survived on Venus, but not on Earth. Our proposed mechanism explains differences in the atmospheric contents of argon, krypton and xenon on Venus and Earth, but most of the neon must have escaped from both planets' atmospheres later to yield the observed ratio of neon to argon.     

Chronological evidence that the Moon is either young or did not have a global magma ocean

Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth. The Earth's Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type. Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems. By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the (207)Pb-(206)Pb, (147)Sm-(143)Nd and (146)Sm-(142)Nd isotopic systems to be 4,360?±?3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism.     

Origin of the Moon's orbital inclination from resonant disk interactions

The Moon is generally believed to have formed from the debris disk created by a large body colliding with the early Earth. Recent models of this process predict that the orbit of the newly formed Moon should be in, or very near, the Earth's equatorial plane. This prediction, however, is at odds with the known history of the lunar orbit: the orbit is currently expanding, but can be traced back in time to reveal that, when the Moon formed, its orbital inclination relative to the Earth's equator was I approximately = 10 degrees. The cause of this initial inclination has been a mystery for over 30 years, as most dynamical processes (such as those that act to flatten Saturn's rings) will tend to decrease orbital inclinations. Here we show that the Moon's substantial orbital inclination is probably a natural result of its formation from an impact-generated disk. The mechanism involves a gravitational resonance between the Moon and accretion-disk material, which can increase orbital inclinations up to approximately 15 degrees.