A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites

Determining the chronology for the assembly of planetary bodies in the early Solar System is essential for a complete understanding of star- and planet-formation processes. Various radionuclide chronometers (applied to meteorites) have been used to determine that basaltic lava flows on the surface of the asteroid Vesta formed within 3 million years (3 Myr) of the origin of the Solar System. Such rapid formation is broadly consistent with astronomical observations of young stellar objects, which suggest that formation of planetary systems occurs within a few million years after star formation. Some hafnium-tungsten isotope data, however, require that Vesta formed later (approximately 16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time (62(-14)(+4504) Myr). Here we report measurements of tungsten isotope compositions and hafnium-tungsten ratios of several meteorites. Our measurements indicate that, contrary to previous results, the bulk of metal-silicate separation in the Solar System was completed within <30 Myr. These results are completely consistent with other evidence for rapid planetary formation, and are also in agreement with dynamic accretion models that predict a relatively short time (approximately 10 Myr) for the main growth stage of terrestrial planet formation.     

Hf-W-Th evidence for rapid growth of Mars and its status as a planetary embryo

Terrestrial planets are thought to have formed through collisions between large planetary embryos of diameter ～1,000-5,000?km. For Earth, the last of these collisions involved an impact by a Mars-size embryo that formed the Moon 50-150?million years (Myr) after the birth of the Solar System. Although model simulations of the growth of terrestrial planets can reproduce the mass and dynamical parameters of the Earth and Venus, they fall short of explaining the small size of Mars. One possibility is that Mars was a planetary embryo that escaped collision and merging with other embryos. To assess this idea, it is crucial to know Mars' accretion timescale, which can be investigated using the (182)Hf-(182)W decay system in shergottite-nakhlite-chassignite meteorites. Nevertheless, this timescale remains poorly constrained owing to a large uncertainty associated with the Hf/W ratio of the Martian mantle and as a result, contradicting timescales have been reported that range between 0 and 15?Myr (refs 6-10). Here we show that Mars accreted very rapidly and reached about half of its present size in only 1.8(+0.9)(-1.0) Myr or less, which is consistent with a stranded planetary embryo origin. We have found a well-defined correlation between the Th/Hf and (176)Hf/(177)Hf ratios in chondrites that reflects remobilization of Lu and Th during parent-body processes. Using this relationship, we estimate the Hf/W ratio in Mars' mantle to be 3.51?±?0.45. This value is much more precise than previous estimates, which ranged between 2.6 and 5.0 (ref. 6), and lifts the large uncertainty that plagued previous estimates of the age of Mars. Our results also demonstrate that Mars grew before dissipation of the nebular gas when ～100-km planetesimals, such as the parent bodies of chondrites, were still being formed. Mars' accretion occurred early enough to allow establishment of a magma ocean powered by decay of (26)Al.     

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.     

Core formation in planetesimals triggered by permeable flow

The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation. But core formation requires a mechanism for segregating metal, and the 'wetting' properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron-sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron-sulphur melting point (approximately 1,000 degrees C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of pre-formed cores.     

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry

Our Solar System formed approximately 4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of &<10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar-39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed approximately 160 Myr to reach 390K.     

Cooling of the Earth and core formation after the giant impact

Kelvin calculated the age of the Earth to be about 24 million years by assuming conductive cooling from being fully molten to its current state. Although simplistic, his result is interesting in the context of the dramatic cooling that took place after the putative Moon-forming giant impact, which contributed the final approximately 10 per cent of the Earth's mass. The rate of accretion and core segregation on Earth as deduced from the U-Pb system is much slower than that obtained from Hf-W systematics, and implies substantial accretion after the Moon-forming impact, which occurred 45 +/- 5 Myr after the beginning of the Solar System. Here we propose an explanation for the two timescales. We suggest that the Hf-W timescale reflects the principal phase of core-formation before the giant impact. Crystallization of silicate perovskite in the lower mantle during this phase produced Fe(3+), which was released during the giant impact, and this oxidation resulted in late segregation of sulphur-rich metal into which Pb dissolved readily, setting the younger U-Pb age of the Earth. Separation of the latter metal then occurred 30 +/- 10 Myr after the Moon-forming impact. Over this time span, in surprising agreement with Kelvin's result, the Earth cooled by about 4,000 K in returning from a fully molten to a partially crystalline state.     

Mg isotope evidence for contemporaneous formation of chondrules and refractory inclusions

Primitive or undifferentiated meteorites (chondrites) date back to the origin of the Solar System, and thus preserve a record of the physical and chemical processes that occurred during the earliest evolution of the accretion disk surrounding the young Sun. The oldest Solar System materials present within these meteorites are millimetre- to centimetre-sized calcium-aluminium-rich inclusions (CAIs) and ferromagnesian silicate spherules (chondrules), which probably originated by thermal processing of pre-existing nebula solids. Chondrules are currently believed to have formed approximately 2-3 million years (Myr) after CAIs (refs 5-10)--a timescale inconsistent with the dynamical lifespan of small particles in the early Solar System. Here, we report the presence of excess (26)Mg resulting from in situ decay of the short-lived (26)Al nuclide in CAIs and chondrules from the Allende meteorite. Six CAIs define an isochron corresponding to an initial (26)Al/(27)Al ratio of (5.25 +/- 0.10) x 10(-5), and individual model ages with uncertainties as low as +/- 30,000 years, suggesting that these objects possibly formed over a period as short as 50,000 years. In contrast, the chondrules record a range of initial (26)Al/(27)Al ratios from (5.66 +/- 0.80) to (1.36 +/- 0.52) x 10(-5), indicating that Allende chondrule formation began contemporaneously with the formation of CAIs, and continued for at least 1.4 Myr. Chondrule formation processes recorded by Allende and other chondrites may have persisted for at least 2-3 Myr in the young Solar System.     

Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites

Long- and short-lived radioactive isotopes and their daughter products in meteorites are chronometers that can test models for Solar System formation. Differentiated meteorites come from parent bodies that were once molten and separated into metal cores and silicate mantles. Mineral ages for these meteorites, however, are typically younger than age constraints for planetesimal differentiation. Such young ages indicate that the energy required to melt their parent bodies could not have come from the most likely heat source-radioactive decay of short-lived nuclides ((26)Al and (60)Fe) injected from a nearby supernova-because these would have largely decayed by the time of melting. Here we report an age of 4.5662 +/- 0.0001 billion years (based on Pb-Pb dating) for basaltic angrites, which is only 1 Myr younger than the currently accepted minimum age of the Solar System and corresponds to a time when (26)Al and (60)Fe decay could have triggered planetesimal melting. Small (26)Mg excesses in bulk angrite samples confirm that (26)Al decay contributed to the melting of their parent body. These results indicate that the accretion of differentiated planetesimals pre-dated that of undifferentiated planetesimals, and reveals the minimum Solar System age to be 4.5695 +/- 0.0002 billion years.     

Geochemistry: does U-Pb date Earth's core formation?

Constraining the timing of the formation of Earth's core, which defines the birth of our planet, is essential for understanding the early evolution of Earth-like planets. Wood and Halliday and Halliday discuss the apparent discrepancy between the U-Pb (60-80 Myr) and Hf-W clocks (30 Myr) in determining the timescale of Earth's accretion and core formation. We find that the information the authors present is at times contradictory (for example, compare Fig. 1 in ref. 1 with Fig. 1 in ref. 2) and confusing and could suggest that the U-Pb clock constrains core formation better than the Hf-W system. Here we point out the limitations of the U-Pb system and show that the U-Pb age cannot be used to argue for protracted accretion and/or core formation (>50 Myr) because this clock only records the processes that occurred during the last 1% of Earth's accretion and core formation in the Wood and Halliday mechanism.     

Stardust silicates from primitive meteorites

Primitive chondritic meteorites contain material (presolar grains), at the level of a few parts per million, that predates the formation of our Solar System. Astronomical observations and the chemical composition of the Sun both suggest that silicates must have been the dominant solids in the protoplanetary disk from which the planets of the Solar System formed, but no presolar silicates have been identified in chondrites. Here we report the in situ discovery of presolar silicate grains 0.1-1 microm in size in the matrices of two primitive carbonaceous chondrites. These grains are highly enriched in 17O (delta17O(SMOW) > 100-400 per thousand ), but have solar silicon isotopic compositions within analytical uncertainties, suggesting an origin in an oxygen-rich red giant or an asymptotic giant branch star. The estimated abundance of these presolar silicates (3-30 parts per million) is higher than reported for other types of presolar grains in meteorites, consistent with their ubiquity in the early Solar System, but is about two orders of magnitude lower than their abundance in anhydrous interplanetary dust particles. This result is best explained by the destruction of silicates during high-temperature processing in the solar nebula.     

The tungsten isotopic composition of the Earth's mantle before the terminal bombardment

Many precious, 'iron-loving' metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet's deep interior. One explanation of their over-abundance is a 'late veneer'--a flux of meteorites added to the Earth after core formation as a 'terminal' bombardment that culminated in the cratering of the Moon. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower. These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio (182)W/(184)W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon (more than four billion years ago) or core-mantle interaction, do not explain the W isotope data well. The decrease in mantle (182)W/(184)W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in (142)Nd/(144)Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.     

Iron meteorite evidence for early formation and catastrophic disruption of protoplanets

In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.1-1 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5-200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr(-1), increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed approximately 4,500 Myr ago in a 'hit-and-run' collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of approximately 10(3) km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.     

Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon

Small isotopic differences in the atomic abundance of neodymium-142 (142Nd) in silicate rocks represent the time-averaged effect of decay of formerly live samarium-146 (146Sm) and provide constraints on the timescales and mechanisms by which planetary mantles first differentiated. This chronology, however, assumes that the composition of the total planet is identical to that of primitive undifferentiated meteorites called chondrites. The difference in the 142Nd/144Nd ratio between chondrites and terrestrial samples may therefore indicate very early isolation (<30 Myr from the formation of the Solar System) of the upper mantle or a slightly non-chondritic bulk Earth composition. Here we present high-precision 142Nd data for 16 martian meteorites and show that Mars also has a non-chondritic composition. Meteorites belonging to the shergottite subgroup define a planetary isochron yielding an age of differentiation of 40 +/- 18 Myr for the martian mantle. This isochron does not pass through the chondritic reference value (100 x epsilon(142)Nd = -21 +/- 3; 147Sm/144Nd = 0.1966). The Earth, Moon and Mars all seem to have accreted in a portion of the inner Solar System with approximately 5 per cent higher Sm/Nd ratios than material accreted in the asteroid belt. Such chemical heterogeneities may have arisen from sorting of nebular solids or from impact erosion of crustal reservoirs in planetary precursors. The 143Nd composition of the primitive mantle so defined by 142Nd is strikingly similar to the putative endmember component 'FOZO' characterized by high 3He/4He ratios.     

The building blocks of planets within the 'terrestrial' region of protoplanetary disks

Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System.     

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (ref. 1) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ～100,000 years (ref. 2). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration. The terrestrial planets finished accreting much later, and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (1 au is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 au, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 au; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 au and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.     

Timescales of shock processes in chondritic and martian meteorites

The accretion of the terrestrial planets from asteroid collisions and the delivery to the Earth of martian and lunar meteorites has been modelled extensively. Meteorites that have experienced shock waves from such collisions can potentially be used to reveal the accretion process at different stages of evolution within the Solar System. Here we have determined the peak pressure experienced and the duration of impact in a chondrite and a martian meteorite, and have combined the data with impact scaling laws to infer the sizes of the impactors and the associated craters on the meteorite parent bodies. The duration of shock events is inferred from trace element distributions between coexisting high-pressure minerals in the shear melt veins of the meteorites. The shock duration and the associated sizes of the impactor are found to be much greater in the chondrite (approximately 1 s and 5 km, respectively) than in the martian meteorite (approximately 10 ms and 100 m). The latter result compares well with numerical modelling studies of cratering on Mars, and we suggest that martian meteorites with similar, recent ejection ages (10(5) to 10(7) years ago) may have originated from the same few square kilometres on Mars.     

Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets

The petrology record on the Moon suggests that a cataclysmic spike in the cratering rate occurred approximately 700 million years after the planets formed; this event is known as the Late Heavy Bombardment (LHB). Planetary formation theories cannot naturally account for an intense period of planetesimal bombardment so late in Solar System history. Several models have been proposed to explain a late impact spike, but none of them has been set within a self-consistent framework of Solar System evolution. Here we propose that the LHB was triggered by the rapid migration of the giant planets, which occurred after a long quiescent period. During this burst of migration, the planetesimal disk outside the orbits of the planets was destabilized, causing a sudden massive delivery of planetesimals to the inner Solar System. The asteroid belt was also strongly perturbed, with these objects supplying a significant fraction of the LHB impactors in accordance with recent geochemical evidence. Our model not only naturally explains the LHB, but also reproduces the observational constraints of the outer Solar System.     

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.     

Iron meteorites as remnants of planetesimals formed in the terrestrial planet region

Iron meteorites are core fragments from differentiated and subsequently disrupted planetesimals. The parent bodies are usually assumed to have formed in the main asteroid belt, which is the source of most meteorites. Observational evidence, however, does not indicate that differentiated bodies or their fragments were ever common there. This view is also difficult to reconcile with the fact that the parent bodies of iron meteorites were as small as 20 km in diameter and that they formed 1-2 Myr earlier than the parent bodies of the ordinary chondrites. Here we show that the iron-meteorite parent bodies most probably formed in the terrestrial planet region. Fast accretion times there allowed small planetesimals to melt early in Solar System history by the decay of short-lived radionuclides (such as 26Al, 60Fe). The protoplanets emerging from this population not only induced collisional evolution among the remaining planetesimals but also scattered some of the survivors into the main belt, where they stayed for billions of years before escaping via a combination of collisions, Yarkovsky thermal forces, and resonances. We predict that some asteroids are main-belt interlopers (such as (4) Vesta). A select few may even be remnants of the long-lost precursor material that formed the Earth.     

Identification of molecular-cloud material in interplanetary dust particles

Interplanetary dust particles (IDPs) collected in the Earth's stratosphere and meteorites are fragments of comets and asteroids. These are 'primitive' meteorites in part because they have preserved materials which predate the formation of the Solar System. The most primitive (least altered) meteorites contain a few parts per million of micrometre-sized dust which formed in the atmospheres of giant stars. Some meteorites have elevated D/H and 15N/14N ratios that are attributed to surviving interstellar organic molecules which have probably been strongly diluted and altered by parent-body processes. Most IDPs are chemically, mineralogically, and texturally primitive in comparison to meteorites. Here I show that H and N isotopic anomalies among fragile 'cluster' IDPs are far larger, more common, and less equilibrated than those previously observed in other IDPs or meteorites. In some cases, the D/H ratios that we measure reach the values of interstellar molecules, suggesting that molecular-cloud material has survived intact. These observations indicate that cluster IDPs are the most primitive class of Solar System materials currently available for laboratory analysis.