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A growing body of evidence indicates that binary neutron-star mergers are the primary origin of heavy elements produced exclusively through rapid neutron capture1-4 (the 'r-process'). As neutron-star mergers occur infrequently, their deposition of radioactive isotopes into the pre-solar nebula could have been dominated by a few nearby events. Although short-lived r-process isotopes-with half-lives shorter than 100 million years-are no longer present in the Solar System, their abundances in the early Solar System are known because their daughter products were preserved in high-temperature condensates found in meteorites5. Here we report that abundances of short-lived r-process isotopes in the early Solar System point to their origin in neutron-star mergers, and indicate substantial deposition by a single nearby merger event. By comparing numerical simulations with the early Solar System abundance ratios of actinides produced exclusively through the r-process, we constrain the rate of occurrence of their Galactic production sites to within about 1-100 per million years. This is consistent with observational estimates of neutron-star merger rates6-8, but rules out supernovae and stellar sources. We further find that there was probably a single nearby merger that produced much of the curium and a substantial fraction of the plutonium present in the early Solar System. Such an event may have occurred about 300 parsecs away from the pre-solar nebula, approximately 80 million years before the formation of the Solar System.

The early evolution of planetesimals and planets can be constrained using variations in the abundance of neodymium-142 ((142)Nd), which arise from the initial distribution of (142)Nd within the protoplanetary disk and the radioactive decay of the short-lived samarium-146 isotope ((146)Sm). The apparent offset in (142)Nd abundance found previously between chondritic meteorites and Earth has been interpreted either as a possible consequence of nucleosynthetic variations within the protoplanetary disk or as a function of the differentiation of Earth very early in its history. Here we report high-precision Sm and Nd stable and radiogenic isotopic compositions of four calcium-aluminium-rich refractory inclusions (CAIs) from three CV-type carbonaceous chondrites, and of three whole-rock samples of unequilibrated enstatite chondrites. The CAIs, which are the first solids formed by condensation from the nebular gas, provide the best constraints for the isotopic evolution of the early Solar System. Using the mineral isochron method for individual CAIs, we find that CAIs without isotopic anomalies in Nd compared to the terrestrial composition share a (146)Sm/(144)Sm-(142)Nd/(144)Nd isotopic evolution with Earth. The average (142)Nd/(144)Nd composition for pristine enstatite chondrites that we calculate coincides with that of the accessible silicate layers of Earth. This relationship between CAIs, enstatite chondrites and Earth can only be a result of Earth having inherited the same initial abundance of (142)Nd and chondritic proportions of Sm and Nd. Consequently, (142)Nd isotopic heterogeneities found in other CAIs and among chondrite groups may arise from extrasolar grains that were present in the disk and incorporated in different proportions into these planetary objects. Our finding supports a chondritic Sm/Nd ratio for the bulk silicate Earth and, as a consequence, chondritic abundances for other refractory elements. It also removes the need for a hidden reservoir or for collisional erosion scenarios to explain the (142)Nd/(144)Nd composition of Earth.

According to the generally accepted scenario, the last giant impact on the Earth formed the Moon and initiated the final phase of core formation by melting the Earth's mantle. A key goal of geochemistry is to date this event, but different ages have been proposed. Some argue for an early Moon-forming event, approximately 30 million years (Myr) after the condensation of the first solids in the Solar System, whereas others claim a date later than 50 Myr (and possibly as late as around 100 My) after condensation. Here we show that a Moon-forming event at 40 Myr after condensation, or earlier, is ruled out at a 99.9 per cent confidence level. We use a large number of N-body simulations to demonstrate a relationship between the time of the last giant impact on an Earth-like planet and the amount of mass subsequently added during the era known as Late Accretion. As the last giant impact is delayed, the late-accreted mass decreases in a predictable fashion. This relationship exists within both the classical scenario and the Grand Tack scenario of terrestrial planet formation, and it holds across a wide range of disk conditions. The concentration of highly siderophile elements (HSEs) in Earth's mantle constrains the mass of chondritic material added to Earth during Late Accretion. Using HSE abundance measurements, we determine a Moon-formation age of 95 +/- 32 Myr since condensation. The possibility exists that some late projectiles were differentiated and left an incomplete HSE record in Earth's mantle. Even in this case, various isotopic constraints strongly suggest that the late-accreted mass did not exceed 1 per cent of Earth's mass, and so the HSE clock still robustly limits the timing of the Moon-forming event to significantly later than 40 My after condensation., Includes Supplemental Material

Whereas space weathering of some airless bodies, such as the Moon, occurs through the accumulation on regolith of nanophase metallic particles, spectroscopic data show that space weathering of the asteroid Vesta occurs through the small-scale mixing of diverse surface components, which gradually generates locally homogenized upper regolith. Between 16 July 2011 and 5 September 2012, NASA's space probe Dawn was orbiting Vesta, a protoplanet thought to have survived virtually intact since an early phase of Solar System formation. In this issue of Nature, two groups report on the encounter. Carle Pieters and co-workers find that space weathering on Vesta has followed a different course from that observed on the Moon and on Itokawa, the asteroid sampled in an Earth-return mission. On Vesta, weathering involved fine-scale regolith (soil) mixing that has removed clear traces of recent impact deposits. There are no signs of the nanophase metallic-particle deposits seen on the Moon and Itokawa. Thomas McCord and co-authors describe two main types of material on Vesta's surface: bright and dark. The bright material may be uncontaminated indigenous Vesta basaltic soil, with the darker material derived from low-albedo impactors. Dawn has now moved on and is due to rendezvous with the protoplanet Ceres in February 2015. The surface of the asteroid Vesta has prominent near-infrared absorption bands characteristic of a range of pyroxenes, confirming a direct link to the basaltic howardite–eucrite–diogenite class of meteorites1,2,3. Processes active in the space environment produce ‘space weathering’ products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere4,5, and it has long been a mystery why Vesta’s absorption bands are so strong. Analyses of soil samples from both the Moon6 and the asteroid Itokawa7 determined that nanophase metallic particles (commonly nanophase iron) accumulate on the rims of regolith grains with time, accounting for an observed optical degradation. These nanophase particles, believed to be related to solar wind and micrometeoroid bombardment processes, leave unique spectroscopic signatures that can be measured remotely8,9,10 but require sufficient spatial resolution to discern the geologic context and history of the surface, which has not been achieved for Vesta until now. Here we report that Vesta shows its own form of space weathering, which is quite different from that of other airless bodies visited. No evidence is detected on Vesta for accumulation of lunar-like nanophase iron on regolith particles, even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age. Instead, spectroscopic data reveal that on Vesta a locally homogenized upper regolith is generated with time through small-scale mixing of diverse surface components.

Noble gas contents of the Iceland mantle plume show that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation. This analysis of noble gas content in Icelandic basaltic rocks — material produced by the melting of a deep upwelling of hot rock in Earth's mantle — indicates that their source is less degassed than that of mid-ocean-ridge basalts and that Earth's mantle has accreted volatiles from at least two separate sources. The author concludes that neither the Moon-forming impact nor billions of years of mantle convection has erased the signature of Earth's heterogeneous accretion and early differentiation. The isotopes 129Xe, produced from the radioactive decay of extinct 129I, and 136Xe, produced from extinct 244Pu and extant 238U, have provided important constraints on early mantle outgassing and volatile loss from Earth1,2. The low ratios of radiogenic to non-radiogenic xenon (129Xe/130Xe) in ocean island basalts (OIBs) compared with mid-ocean-ridge basalts (MORBs) have been used as evidence for the existence of a relatively undegassed primitive deep-mantle reservoir1. However, the low 129Xe/130Xe ratios in OIBs have also been attributed to mixing between subducted atmospheric Xe and MORB Xe, which obviates the need for a less degassed deep-mantle reservoir3,4. Here I present new noble gas (He, Ne, Ar, Xe) measurements from an Icelandic OIB that reveal differences in elemental abundances and 20Ne/22Ne ratios between the Iceland mantle plume and the MORB source. These observations show that the lower 129Xe/130Xe ratios in OIBs are due to a lower I/Xe ratio in the OIB mantle source and cannot be explained solely by mixing atmospheric Xe with MORB-type Xe. Because 129I became extinct about 100 million years after the formation of the Solar System, OIB and MORB mantle sources must have differentiated by 4.45 billion years ago and subsequent mixing must have been limited. The Iceland plume source also has a higher proportion of Pu- to U-derived fission Xe, requiring the plume source to be less degassed than MORBs, a conclusion that is independent of noble gas concentrations and the partitioning behaviour of the noble gases with respect to their radiogenic parents. Overall, these results show that Earth’s mantle accreted volatiles from at least two separate sources and that neither the Moon-forming impact nor 4.45 billion years of mantle convection has erased the signature of Earth’s heterogeneous accretion and early differentiation.

Planets and most asteroids revolve around the Sun in the same direction. But an asteroid that shares Jupiter's orbit has been revolving in the opposite direction for about a million years. See Letter p.687 Recent theoretical work suggests that asteroids may stably orbit the Sun in the opposite direction to the planets in our Solar System as co-orbital bodies. In 2015, astronomers detected asteroid 2015 BZ509, but the observations could not be used to determine its orbital motion. Here Paul Wiegert et al. present and analyse additional observations to confirm that 2015 BZ509 is circling the Sun in a clockwise (retrograde) direction, when looking down on the Solar System, in the region of Jupiter, which is orbiting anticlockwise (prograde motion). They find that 2015 BZ509 could remain for a million years in its current state, and speculate that it may have been a Halley-family comet, pulled into resonance through an interaction with Saturn. Retrograde co-orbital asteroids of Jupiter and other planets may be more common than previously thought.

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.

A new tungsten isotope study presents revised ages for the formation of the Moon. The Moon is thought to have formed from debris ejected by a giant impact with the early Earth. The high energies involved would have caused melting, and the formation of a lunar magma ocean. Previous work on tungsten isotopes had suggested that the Moon solidified within the first 60 million years of the Solar System. The new data from lunar metals based on the hafnium/tungsten clock are consistent with samarium/neodymium chronometry, and point to a later date for solidification, when the Solar System was 50 to 150 million years old. The Moon is thought to have formed from debris ejected by a giant impact with the early ‘proto’-Earth1 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 chronometry2,3,4, but these methods have yielded contradicting results. In earlier studies3,5,6,7, 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 ∼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 surface3,7, 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 ∼60 Myr of the Solar System, which is no longer inconsistent with Sm–Nd chronometry8,9,10,11. 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 rocks8,9,10,11, 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 Moon12.

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.

The abundances of oxygen isotopes in the most refractory mineral phases (calcium-aluminium-rich inclusions, CAIs) in meteorites have hitherto defied explanation. Most processes fractionate isotopes by nuclear mass; that is, 18O is twice as fractionated as 17O, relative to 16O. In CAIs 17O and 18O are nearly equally fractionated, implying a fundamentally different mechanism. The CAI data were originally interpreted as evidence for supernova input of pure 16O into the solar nebula, but the lack of a similar isotope trend in other elements argues against this explanation. A symmetry-dependent fractionation mechanism may have occurred in the inner solar nebula, but experimental evidence is lacking. Isotope-selective photodissociation of CO in the innermost solar nebula might explain the CAI data, but the high temperatures in this region would have rapidly erased the signature. Here we report time-dependent calculations of CO photodissociation in the cooler surface region of a turbulent nebula. If the surface were irradiated by a far-ultraviolet flux approximately 10(3) times that of the local interstellar medium (for example, owing to an O or B star within approximately 1 pc of the protosun), then substantial fractionation of the oxygen isotopes was possible on a timescale of approximately 10(5) years. We predict that similarly irradiated protoplanetary disks will have H2O enriched in 17O and 18O by several tens of per cent relative to CO.

The discovery in primitive components of meteorites of large oxygen isotopic variations that could not be attributed to mass-dependent fractionation effects has raised a fundamental question: what is the composition of the protosolar gas from which the host grains formed? This composition is probably preserved in the outer layers of the Sun, but the resolution of astronomical spectroscopic measurements is still too poor to be useful for comparison with planetary material. Here we report a precise determination of the oxygen isotopic composition of the solar wind from particles implanted in the outer hundreds of nanometres of metallic grains in the lunar regolith. These layers of the grains are enriched in 16O by >20 +/- 4 per thousand relative to the Earth, Mars and bulk meteorites, which implies the existence in the solar accretion disk of reactions--as yet unknown--that were able to change the 17O/16O and 18O/16O ratios in a way that was not dependent strictly on the mass of the isotope. Photochemical self-shielding of the CO gas irradiated by ultraviolet light may be one of these key processes, because it depends on the abundance of the isotopes, rather than their masses.

The abundances of elements and their isotopes in our Galaxy show wide variations, reflecting different nucleosynthetic processes in stars and the effects of Galactic evolution. These variations contrast with the uniformity of stable isotope abundances for many elements in the Solar System, which implies that processes efficiently homogenized dust and gas from different stellar sources within the young solar nebula. However, isotopic heterogeneity has been recognized on the subcentimetre scale in primitive meteorites, indicating that these preserve a compositional memory of their stellar sources. Small differences in the abundance of stable molybdenum isotopes in bulk rocks of some primitive and differentiated meteorites, relative to terrestrial Mo, suggest large-scale Mo isotopic heterogeneity between some inner Solar System bodies, which implies physical conditions that did not permit efficient mixing of gas and dust. Here we report Mo isotopic data for bulk samples of primitive and differentiated meteorites that show no resolvable deviations from terrestrial Mo. This suggests efficient mixing of gas and dust in the solar nebula at least to 3 au from the Sun, possibly induced by magnetohydrodynamic instabilities. These mixing processes must have occurred before isotopic fractionation of gas-phase elements and volatility-controlled chemical fractionations were established.

Comets are remnants from the time when the outer planets formed, approximately 4-4.5 billion years ago. They have been in storage since then in the Oort cloud and Kuiper belt-distant regions that are so cold and sparsely populated that it was long thought that comets approaching the Sun were pristine samples from the time of Solar System formation. It is now recognized, however, that a variety of subtle but important evolutionary mechanisms operate on comets during their long storage, so they can no longer be regarded as wholly pristine.

Chondrules and calcium-aluminium-rich inclusions (CAIs) are preserved materials from the early history of the Solar System, where they resulted from thermal processing of pre-existing solids during various flash heating episodes which lasted for several million years1. CAIs are believed to have formed about two million years before the chondrules2,3,4,5. Here we report the discovery of a chondrule fragment embedded in a CAI. The chondrule's composition is poor in 16O, while the CAI has a 16O-poor melilite (Ca, Mg, Al-Silicate) core surrounded by a 16O-rich igneous mantle. These observations, when combined with the previously reported CAI-bearing chondrules6,7,8,9, strongly suggest that the formation of chondrules and CAIs overlapped in time and space, and that there were large fluctuations in the oxygen isotopic compositions in the solar nebula probably synchronizing astrophysical pulses.

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 System1,2,3. 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 formation4,5. Some hafnium–tungsten isotope data, however, require that Vesta formed later6 (∼16 Myr after the formation of the Solar System) and that the formation of the terrestrial planets took a much longer time7,8,9,10 (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 results7,8,9,10, the bulk of metal–silicate separation in the Solar System was completed within

Chondrules are silicate spherules that are found in abundance in the most primitive class of meteorites, the chondrites. Chondrules are believed to have formed by rapid cooling of silicate melt early in the history of the Solar System1, and their properties should reflect the composition of (and physical conditions in) the solar nebula at the time when the Sun and planets were forming. It is usually believed that chondrules lost all their noble gases at the time of melting2,3,4. Here we report the discovery of significant amounts of trapped noble gases in chondrules in the enstatite chondrite Yamato-791790, which consists of highly reduced minerals. The elemental ratios 36Ar/132Xe and 84Kr/132Xe are similar to those of ‘subsolar’ gas5,6, which has the highest 36Ar/132Xe ratio after that of solar-type noble gases7. The most plausible explanation for the high noble-gas concentration and the characteristic elemental ratios is that solar gases were implanted into the chondrule precursor material, followed by incomplete loss of the implanted gases through diffusion over time.

Throughout the history of the Solar System, Earth has been bombarded by interplanetary dust particles (IDPs), which are asteroid and comet fragments of diameter approximately 1-1,000 microm. The IDP flux is believed to be in quasi-steady state: particles created by episodic main belt collisions or cometary fragmentation replace those removed by comminution, dynamical ejection, and planetary or solar impact. Because IDPs are rich in 3He, seafloor sediment 3He concentrations provide a unique means of probing the major events that have affected the IDP flux and its source bodies over geological timescales. Here we report that collisional disruption of the >150-km-diameter asteroid that created the Veritas family 8.3 +/- 0.5 Myr ago also produced a transient increase in the flux of interplanetary dust-derived 3He. The increase began at 8.2 +/- 0.1 Myr ago, reached a maximum of approximately 4 times pre-event levels, and dissipated over approximately 1.5 Myr. The terrestrial IDP accretion rate was overwhelmingly dominated by Veritas family fragments during the late Miocene. No other event of this magnitude over the past approximately 10(8) yr has been deduced from main belt asteroid orbits. One remarkably similar event is present in the 3He record 35 Myr ago, but its origin by comet shower or asteroid collision remains uncertain.

Recent discoveries of objects orbiting beyond Neptune1,2,3,4,5 have emphasized that our understanding of the distribution and dynamics of material in the outer Solar System is very incomplete. This trans-neptunian population—known as the Kuiper belt—is thought to act as a relatively stable reservoir of objects that could become short-period comets6,7,8,9, although there may be other regions of stability in the outer Solar System that could also supply such comets. Here I use numerical simulations to identify one such long-lived region between the orbits of Uranus and Neptune. I show that in the region 24–27AUfrom the Sun, about 0.3 per cent of an initial population of small bodies moving on low-eccentricity, low-inclination orbits could survive for the age of the Solar System. The actual existence of this hypothetical belt is not precluded by currently available observational limits, and there could be as much as ∼5 × 10−4 Earth masses of material populating this region—comparable to the mass of the asteroid belt between Mars and Jupiter.

Claims that Kuiper belt body 2003 UB313 is the ‘tenth planet’, and the ongoing dispute about the ‘planet-ness’ of Pluto, mean that the question of whether there is a firm line between planets and asteroids could not be more topical. A new study of Hubble Space Telescope images of the largest known asteroid encourages the view that the dividing line is at best blurred. Ceres has many of the characteristics of the much larger planets, and a similar history. It is a lot smaller than our Moon, yet HST images show it is a smooth, ellipsoid, probably shaped by separation of a rocky core from an icy mantle. Unlike the icy satellites of Saturn and Jupiter, Ceres is dark, suggesting that its mantle may underlie a thin crust of rocky material. The accretion of bodies in the asteroid belt was halted nearly 4.6 billion years ago by the gravitational influence of the newly formed giant planet Jupiter. The asteroid belt therefore preserves a record of both this earliest epoch of Solar System formation and variation of conditions within the solar nebula. Spectral features in reflected sunlight indicate that some asteroids have experienced sufficient thermal evolution to differentiate into layered structures1. The second most massive asteroid—4 Vesta—has differentiated to a crust, mantle and core2,3. 1 Ceres, the largest and most massive asteroid, has in contrast been presumed to be homogeneous, in part because of its low density, low albedo and relatively featureless visible reflectance spectrum, similar to carbonaceous meteorites that have suffered minimal thermal processing4. Here we show that Ceres has a shape and smoothness indicative of a gravitationally relaxed object. Its shape is significantly less flattened than that expected for a homogeneous object, but is consistent with a central mass concentration indicative of differentiation. Possible interior configurations include water-ice-rich mantles over a rocky core.

THE accretion of the Earth and Moon within the solar nebula is thought1–3 to have taken 50 to 100 million years. But the timing of formation of the Earth's core has been controversial, with some4,5 proposing that it took place within the first 15 Myr of Earth's accretion history and others6,7 proposing that it occurred after 50 Myr of accretion. Meteorite chronometry based on the 182Hf–182W system has the potential to resolve this debate, as segregation of a metal core from silicates should induce strong fractionation of hafnium from tungsten. Here we report tungsten isotope compositions for two iron meteorites, two carbonaceous chondrites, and a lunar mare basalt. We see clear 182W deficits in both iron meteorites, in agreement with previous results4,5. But the data for chondrites are inconsistent with the hypothesis of early core formation, suggesting that both this event and the formation of the Moon must have occurred at least 62 ± 10 Myr after the iron meteorites formed.

CHONDRULES are millimetre-sized spheroidal bodies composed mainly of olivine and orthopyroxene, which comprise the dominant fraction of most chondritic meteorites. They are the products of partial melting of aggregates of fine-grained silicates with minor contributions from metals, sulphides and oxides. Although the formation conditions of chondrules are not well understood, these are thought to involve a transient melting event in the solar nebula1–3. The ubiquity of reduced carbon in interstellar clouds and primitive meteorites implies that it was also present in the early solar nebula, and may thus have been a potential constituent of chondrule pre-cursor material. We describe here experiments in which carbon and magnesian silicate precursor material of primitive chondrule composition are 'flash-heated' together and then crystallized. The resulting material shows many mineralogical features character-istic of natural chondrules, which are not produced in the absence of carbon4–12. Our results suggest not only that carbon was present in the solar nebula, but also that it played a key role in chondrule formation by creating within the melt a reducing environment that was decoupled from the nebula gas.

Lunar magmatic rocks are shown to be enriched in the heavy isotopes of zinc and to have lower zinc concentrations than terrestrial or Martian igneous rocks; these variations represent the large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming giant impact event. The heavily favoured theory for the origin of the Earth–Moon system is a giant impact between the proto-Earth and a Mars-sized body. Such a cataclysmic event would have left its mark on the isotopic composition of the Moon, because light isotopes evaporate more readily than heavier ones. Zinc in particular is a powerful indicator of the volatile histories of planets — it undergoes strong isotopic fractionation in planetary rocks, but is hardly fractionated following volcanic activity on Earth. This study compares high-precision zinc isotopic data for lunar basalts, Martian meteorites and terrestrial igneous rocks, and finds that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian samples. The authors conclude that these variations are the result of large-scale evaporation of zinc in the aftermath of the Moon-forming giant-impact event. Volatile elements have a fundamental role in the evolution of planets. But how budgets of volatiles were set in planets, and the nature and extent of volatile-depletion of planetary bodies during the earliest stages of Solar System formation remain poorly understood1,2. The Moon is considered to be volatile-depleted and so it has been predicted that volatile loss should have fractionated stable isotopes of moderately volatile elements3. One such element, zinc, exhibits strong isotopic fractionation during volatilization in planetary rocks4,5, but is hardly fractionated during terrestrial igneous processes6, making it a powerful tracer of the volatile histories of planets. Here we present high-precision zinc isotopic and abundance data which show that lunar magmatic rocks are enriched in the heavy isotopes of zinc and have lower zinc concentrations than terrestrial or Martian igneous rocks. Conversely, Earth and Mars have broadly chondritic zinc isotopic compositions. We show that these variations represent large-scale evaporation of zinc, most probably in the aftermath of the Moon-forming event, rather than small-scale evaporation processes during volcanism. Our results therefore represent evidence for volatile depletion of the Moon through evaporation, and are consistent with a giant impact origin for the Earth and Moon.

Primitive meteorites contain grains of silicon carbide, graphite and diamond formed outside the Solar System and probably before its birth. The isotopic compositions of these grains provide a record of stellar nucleosynthesis and of condensation processes near carbon stars; the fact of their survival places constraints on conditions in the solar nebula and early Solar System. The search is now on for other surviving stellar condensates, such as nitrides and oxides.

Simulations of Earth's growth show a correlation between the timing of the Moon's formation and the amount of mass that Earth accreted afterwards. This relationship provides a way of measuring the age of our planet. See Letter p.84 The age of the Moon has been a focus for geochemists for at least the past three decades. A number of chronometers have been used to address the question but the results differ from method to method, in part because of the varying assumptions required in the calculation of the so-called model ages. Seth Jacobson et al. have used an alternative approach. They run a large number of numerical simulations, some based on early Moon-forming events, others later events. They then arrive at a model-independent correlation between the formation age of the Moon and the amount of mass accreted by the Earth since then, the so-called Late Veneer. The concentration of highly-siderophile (iron-loving) elements observed in the Earth's mantle provides a constraint on the timing and rules out an early Moon-forming event. Instead, the authors calculate that the Moon-forming impact must have occurred at least 40 million years after formation of the Solar System.

The discovery of a second resident in a region of the Solar System called the inner Oort cloud prompts fresh thinking about this no-man's-land between the giant planets and the reservoir of comets of long orbital period. See Letter p.471 Solar System formation models indicate that the remote 1,000-km-diameter dwarf planet Sedna — 76 astronomical units (AU) from the Sun at its closest (perihelion) point — could be a link between the Kuiper belt objects orbiting at 30 to 50 AU and the as-yet unseen outer Oort cloud, some 10,000 AU from the Sun. Chadwick Trujillo and Scott Sheppard now report the presence of a second Sedna-like object, 2012 VP113, with a perihelion of 80 AU. This discovery confirms that Sedna is not an isolated object. Both bodies may be members of a population of inner Oort cloud objects that could outnumber all other dynamically stable populations in the Solar System.

Diamond grains are the most abundant presolar grains found in primitive meteorites. They formed before the Solar System, and therefore provide a record of nuclear and chemical processes in stars and in the interstellar medium. Their origins are inferred from the unusual isotopic compositions of trace elements-mainly xenon-which suggest that they came from supernovae. But the exact nature of the sources has been enigmatic, as has the method by which noble gases were incorporated into the grains. One observation is that different isotopic components are released at different temperatures when the grains are heated, and it has been suggested that these components have different origins. Here we report results of a laboratory study that shows that ion implantation (previously suggested on other grounds) is a viable mechanism for trapping noble gases. Moreover, we find that ion implantation of a single isotopic composition can produce both low- and high-temperature release peaks from the same grains. We conclude that both isotopically normal and anomalous gases may have been implanted by multiple events separated in space and/or time, with thermal processing producing an apparent enrichment of the anomalous component in the high-temperature release peak. The previous assumption that the low- and high-temperature components were not correlated may therefore have led to an overestimate of the abundance of anomalous argon and krypton, while obscuring an enhancement of the light-in addition to the heavy-krypton isotopes.

The major mineral phases contained in individual carbonaceous chondrite meteorites exhibit both positive and negative anomalies in their 54Cr abundances. No significant chromium-bearing phase has a terrestrial (solar) chromium isotopic composition. These observations provide evidence that the chromium isotopic composition of the Solar System results from the mixing of several major components with distinct isotopic compositions. The formation of the solar nebula from material carrying different isotopic signatures may provide constraints on nucleosynthetic models.

It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarede 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 Albarede's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.

Although the outermost planet, Pluto, is 6 x 10 to the 9th km from the sun, the sun's gravitational sphere of influence extends much further, out to about 2 x 10 to the 13th km. This space is occupied by the Oort cloud, comprising 10 to the 12th-10 to the 13th cometary nuclei, formed in the primordial solar nebula. Observations and computer modeling have contributed to a detailed understanding of the structure and dynamics of the cloud, which is thought to be the source of the long-period comets and possibly comet showers.

Primitive meteorites (such as the unequilibrated ordinary chondrites) have undergone only minor thermal processing on their parent asteroids, and thus provide relatively unaltered isotopic records from the early Solar System. For terrestrial materials, oxygen isotope compositions form a linear array called the terrestrial fractionation line. In meteorites the oxygen isotopic composition commonly deviates from this line, the magnitude of the deviation being expressed by the quantity Δ^(17)O. Such deviations, which cannot be explained by mass-dependent fractionation processes, are probably caused by the mixing of two or more nebular components having different nucleosynthetic histories, for example, solids and gas. But no direct evidence for the oxygen isotopic composition of the latter (which is the dominant oxygen reservoir) has hitherto been available. Here we report in situ oxygen-isotope measurements of magnetite grains in unequilibrated ordinary chondrites. Magnetite (which formed by aqueous alteration of metal in the parent asteroid) may serve as a proxy for nebular H_2O. We measured a value of Δ^(17)O ≈5‰, much higher than typical values of 0–2‰ in ordinary-chondrite silicate grains. Our results imply that a nebular component of high- Δ^(17)O H_2O was incorporated into the parent asteroid of the unequilibrated ordinary chondrites.

Primitive meteorites contain isotopes that are the decay products of short-lived nuclides in the early Solar System1,2. The relative abundances of these isotopes provide a means to determine timescales for the formation and accretion of primitive Solar System objects, the abundances of the parent nuclides being fixed when these objects solidified. The abundances can also be used to investigate the source of the nuclides (such as 41Ca, 26Al, 60Fe, 53Mn and 107Pd), although this is an area of controversy. The nuclides could have originated from a single stellar object2,3,4,5,6, such as a nearby red-giant or a supernova. But observations of enhanced ion fluxes in a molecular cloud7 have led to other models8,9,10 in which these nuclides are formed by energetic particle irradiation of gas and dust in the protosolar molecular cloud; alternatively, irradiation by energetic particles from the active early Sun may have occurred within the solar nebula itself11,12,13,14,15,16,17,18. Here we show that there is a correlation between the initial abundances of 41Ca and 26Al in samples of primitive meteorite (as inferred from their respective decay products, 41K and 26Mg), implying a common origin for the short-lived nuclides. We can therefore rule out the mechanisms based onenergetic particle irradiation, as they cannot produce simultaneously the inferred initial abundances of both nuclides. If, as our results suggest, a single stellar source is responsible for generating these nuclides, we can constrain to less than one million years the timescale for the collapse of the protosolar cloud to form the Sun.

Since the discovery of the trans-neptunian objects (TNOs) in 1992, nearly one thousand new members have been added to our Solar System, several of which are as big as--or even larger than--Pluto. The properties of the population of TNOs, such as the size distribution and the total number, are valuable information for understanding the formation of the Solar System, but direct observation is only possible for larger objects with diameters above several tens of kilometres. Smaller objects, which are expected to be more abundant, might be found when they occult background stars, but hitherto there have been no definite detections. Here we report the discovery of such occultation events at millisecond timescales in the X-ray light curve of Scorpius X-1. The estimated sizes of these occulting TNOs are < or =100 m. Their abundance is in line with an extrapolation of the distribution of sizes of larger TNOs.

Chondrules are millimetre-sized spherules (mostly silicate) that dominate the texture of primitive meteorites. Their formation mechanism is debated, but their sheer abundance suggests that the mechanism was both energetic and ubiquitous in the early inner Solar System. The processes suggested--such as shock waves, solar flares or nebula lightning--operate on different length scales that have been hard to relate directly to chondrule properties. Chondrules are depleted in volatile elements, but surprisingly they show little evidence for the associated loss of lighter isotopes one would expect. Here we report a model in which molten chondrules come to equilibrium with the gas that was evaporated from other chondrules, and which explains the observations in a natural way. The regions within which the chondrules formed must have been larger than 150-6,000 km in radius, and must have had a precursor number density of at least 10 m(-3). These constraints probably exclude nebula lightning, and also make formation far from the nebula midplane problematic. The wide range of chondrule compositions may be the result of different combinations of the local concentrations of precursors and the local abundance of water ice or vapour.

Some three dozen objects have now been discovered1,2,3,4,5 beyond the orbit of Neptune and classified as members of the Kuiper belt—a remnant population of icy planetesimals that failed to be incorporated into planets. At still greater distances is believed to lie the Oort cloud—a massive population of cometary objects distributed approximately in a sphere of characteristic dimension 50,000au(ref. 6). Here we report the discovery of an object, 1996TL66, that appears to be representative of a population of scattered bodies located between the Kuiper belt and the Oort cloud. 1996TL66has an orbital semimajor axis of 84au, and is in an extremely eccentric and highly inclined orbit (e = 0.58, i = 24°). With a red magnitude ∼20.9, it is the brightest trans-neptunian object yet found since Pluto and Charon. Its discovery suggests that the Kuiper belt extends substantially beyond the 30–50auregion sampled by previous surveys, and may contain much more mass than previously suspected.

There has been a long-running debate about the origins of the eight (at last count) metal-rich meteorites called CB chondrites, named after Bencubbin in Australia where the first one was found in 1930. Their origins are important as an indication of the conditions prevailing in the early Solar System. New lead-207/lead-206 ages of chondrules (small round inclusions) in two CB chondrites are inconsistent with an origin in shockwave heating in the solar nebula, the usual source of chondrules in other chondritic meteorites. Instead they seem to have formed 5 million years later from material generated by collisions between planet-sized objects in the early Solar System. Chondrules, which are the major constituent of chondritic meteorites, are believed to have formed during brief, localized, repetitive melting of dust (probably caused by shock waves1,2) in the protoplanetary disk around the early Sun. The ages of primitive chondrules3,4,5,6 in chondritic meteorites indicate that their formation started shortly after that of the calcium-aluminium-rich inclusions (4,567.2 ± 0.7 Myr ago) and lasted for about 3 Myr, which is consistent with the dissipation timescale for protoplanetary disks around young solar-mass stars7. Here we report the 207Pb–206Pb ages of chondrules in the metal-rich CB (Bencubbin-like) carbonaceous chondrites Gujba (4,562.7 ± 0.5 Myr) and Hammadah al Hamra 237 (4,562.8 ± 0.9 Myr), which formed during a single-stage, highly energetic event8,9,10,11. Both the relatively young ages and the single-stage formation of the CB chondrules are inconsistent with formation during a nebular shock wave2. We conclude that chondrules and metal grains in the CB chondrites formed from a vapour–melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated. These findings therefore provide evidence for planet-sized objects in the earliest asteroid belt, as required by current numerical simulations of planet formation in the inner Solar System12.

The highest oxygen isotope ratios ever measured in the Galaxy have been recorded in silica-rich grains from the Murchison meteorite. The excess oxygen-18 and oxygen-17 could be the result of nucleosynthesis in a single exotic star in the early Solar System or evidence of irradiation by highly accelerated particles produced during an active early phase of the Sun. The origins of the building blocks of the Solar System can be studied using the isotopic composition of early planetary and meteoritic material. Oxygen isotopes in planetary materials show variations at the per cent level that are not related to the mass of the isotopes1,2; rather, they result from the mixture of components having different nucleosynthetic or chemical origins1,2,3. Isotopic variations reaching orders of magnitude in minute meteoritic grains are usually attributed to stellar nucleosynthesis before the birth of the Solar System, whereby different grains were contributed by different stars4,5. Here we report the discovery of abundant silica-rich grains embedded in meteoritic organic matter, having the most extreme 18O/16O and 17O/16O ratios observed (both ∼10-1) together with a solar silicon isotopic composition. Both O and Si isotopes indicate a single nucleosynthetic process. These compositions can be accounted for by one of two processes: a single exotic evolved star seeding the young Solar System6, or irradiation of the circumsolar gas by high energy particles accelerated during an active phase of the young Sun. We favour the latter interpretation, because the observed compositions are usually not expected from nucleosynthetic processes in evolved stars, whereas they are predicted by the selective trapping of irradiation products.

ALL known asteroids and comets are believed to have been gravitationally bound to the Sun since they formed (together with the Sun and planets) from the solar nebula. This is because no such object has been observed with a speed exceeding the solar escape velocity, although some comets have been close to this limit1. As comets are occasionally ejected from the Solar System, interstellar comets might be expected to arrive every few centuries, having been ejected from similar systems around other stars2. The flux of interstellar dust into the Solar System should be much higher, but its detection poses significant technological challenges. Recently, the Ulysses spacecraft detected a population of dust particles near Jupiter, identified as being of interstellar origin on the basis of their speeds and trajectories3,4. Here we report the radar detection of interstellar particles in the Earth's atmosphere. From intra-annual variations in particle flux, we infer the existence of two discrete sources, one associated with nearby A-type stars, and the other with the Sun's motion about the Galactic Centre. The data also suggest the presence of a third source, possibly associated with local B-type stars and young stellar clusters.

THE interstellar material from which the Solar System formed has been modified by many processes1: evaporation and condensation in the solar nebula, accretion into protoplanetary bodies and post-accretion processes within these bodies. Meteorites provide a record of these events and their chronology2. Carbonaceous CI chondrites are among the most primitive, undifferen-tiated meteorites3–6, but nevertheless show evidence of post-accretion alteration7; they contain carbonates that are believed to have formed by reactions between anhydrous CI precursor materials and circulating fluids in the meteorite parent body (or bodies), yet little is known about the nature of these reactions or the timescale on which they occurred. Here we report measurements of excess 53Cr—formed by the decay of short-lived 53Mn— in five carbonate fragments from the CI chondrites Orgueil and Ivuna. Our results show that aqueous alteration on small protoplanetary bodies must have begun less than 20 Myr after the time of formation of the oldest known solar-nebula condensates2 (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies8, and clearly establishes aqueous alteration as one of the earliest processes in the chemical evolution of the Solar System.

Our Solar System formed about 4.6 billion years ago, and within 4 million years small planetary bodies had formed, some melting to form volcanic and related rocks. Two families of meteorites (the HEDs and angrites) are thought to have originated from asteroids that melted at this time. New oxygen isotope measurements confirm that these meteorites are from two distinct asteroids that underwent large-scale melting in the early Solar System. These new results show that early, global-scale melting was a feature of all the differentiated planets (Earth, Moon and Mars) and asteroids so far sampled. Immediately following the formation of the Solar System, small planetary bodies accreted1, some of which melted to produce igneous rocks2,3. Over a longer timescale (15–33 Myr), the inner planets grew by incorporation of these smaller objects4,5 through collisions. Processes operating on such asteroids strongly influenced the final composition of these planets4, including Earth5. Currently there is little agreement about the nature of asteroidal igneous activity: proposals range from small-scale melting, to near total fusion and the formation of deep magma oceans2. Here we report a study of oxygen isotopes in two basaltic meteorite suites, the HEDs (howardites, eucrites and diogenites, which are thought to sample the asteroid 4 Vesta6) and the angrites (from an unidentified asteroidal source). Our results demonstrate that these meteorite suites formed during early, global-scale melting (≥ 50 per cent) events. We show that magma oceans were present on all the differentiated Solar System bodies so far sampled. Magma oceans produced compositionally layered planetesimals; the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features, such as Earth's high Mg/Si ratio.

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.

Although stars form from clouds of gas and dust, there are insignificant amounts of gas around ordinary (Sun-like) stars. This suggests that hydrogen and helium, the primary constituents of planets such as Jupiter and Saturn, are not easily retained in orbit as a star matures. The gas-giant planets in the Solar System must therefore have formed rapidly. Models of their formation generally suggest that a solid core formed in < or = 10(6) yr, followed by the accretion of the massive gaseous envelope in approximately 10(7) yr (refs 1-5). But how and when the gas of the solar nebula dissipated, and how this compares with the predicted timescale of gas-giant formation, remains unclear, in part because direct observations of circumstellar gas have been made only for stars either younger or older than the critical range of 10(6)-10(7) yr (refs 8-15). Here we report observations of the molecular gas surrounding 20 stars whose ages are likely to be in this range. The gas dissipates rapidly; after a few million years the mass remaining is typically much less than the mass of Jupiter. Thus, if gas-giant planets are common in the Galaxy, they must form even more quickly than present models suggest.

The latest studies of asteroid impacts on Earth and the Moon beginning about 450 million years after the formation of the Solar System provide insight into the duration, number and size of these events. The Late Heavy Bombardment was a period of time, generally put at about 4.1 billion to 3.8 billion years ago, when the inner planets of the Solar System were subjected to a high-frequency barrage of asteroids. This left its mark on the Moon, but on Earth the craters quickly disappeared owing to tectonic processes and erosion. In the first of two papers on the bombardment, Brandon Johnson and Jay Melosh determine the properties of the asteroids by looking at spherule beds: layers of debris ejected during the impacts. The thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit Earth. This historical record of impacts indicates that the number of projectiles colliding with Earth was substantially higher 3.5 billion years ago than it is today, with a gradual decline in the number of strikes after the Late Heavy Bombardment. Bottke et al. modelled the evolution of an asteroid belt that extended farther towards Mars than the present one. They find that most of the impactors traced by the spherule beds probably originated in this 'E-belt', which was disrupted during migrations of some of the giant planets.

COMETS are extremely fragile objects. Observations of outbursts and splitting have inspired suggestions that their nuclei are 'rubble piles', consisting of components that are weakly bonded, perhaps held together by mutual gravity1,2. This structure was confirmed in spectacular fashion by comet Shoemaker-Levy 9, which disintegrated into about 20 fragments after passing near Jupiter; the tidal stresses induced by the planet were only of the order of 10-4 bar (ref. 3). Attempts to explain the formation of comets in the outer Solar System have emphasized either collisional coagulation1 or gravitational collapse of a layer of dust particles4,5. Here I argue that the observed sizes and structure of comet nuclei are better explained by a two-stage process, involving elements of both models-collisional coagulation in the solar nebula, followed by gravitational instability of a layer of macroscopic bodies.

THE high abundance of radioactive 26A1 in meteoritic materials1 presents a puzzle in attempts to describe the formation of the Solar System. It has been suggested11 that collapse of the solar nebula may have been accompanied by an injection of 26A1 from nucleosynthesis in a neighbouring star. The relatively high level of 26A1 in the interstellar medium2,3 is also unexplained. Here I suggest that this isotope may be formed in nuclear reactions between hydrogen and heavy cosmic rays. This suggestion is prompted by the recent discovery4 of γ-ray line emission from 12C and 16O in the Orion cloud complex, thought to be caused by the interaction of these cosmic-ray ions with interstellar hydrogen. I show that these observations also imply enhanced production of 26A1 in the Orion molecular clouds, and that such nuclear reactions can account for the meteoritic abundances. Reactions involving heavy cosmic rays might also account for the presence of other extinct radioactive nuclides in meteorites.

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.

Grains of dust that pre-date the Sun provide insights into their formation around other stars and into the early evolution of the Solar System1,2,3,4. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain3,4. If that is true, then nanodiamonds should be at least as abundant in comets, because they are thought to have formed further out in the early Solar System than the asteroid parent bodies, and because they should be more pristine5,6,7. Here we report that nanodiamonds are absent or very depleted in fragile, carbon-rich interplanetary dust particles, some of which enter the atmosphere at speeds within the range of cometary meteors8,9. One interpretation of the results is that some (perhaps most) nanodiamonds formed within the inner Solar System and are not presolar at all, consistent with the recent detection of nanodiamonds within the accretion discs of other young stars10. An alternative explanation is that all meteoritic nanodiamonds are indeed presolar, but that their abundance decreases with heliocentric distance, in which case our understanding of large-scale transport and circulation within the early Solar System is incomplete11.

A study of cerium in zircon minerals has allowed an assessment of the redox conditions that prevailed when Earth's earliest magmas formed. The results suggest that the mantle became oxidized sooner than had been thought. See Letter p.79 The composition of Earth's earliest atmosphere, which accumulated more than four billion years ago during the Hadean eon, may have been influenced by magmatic outgassing of volatiles from Earth's interior. This paper reports a redox-sensitive calibration to determine the oxidation state of Hadean magmatic melts based on the incorporation of cerium into zircon crystals. The authors find that the melts have oxygen fugacities that are consistent with the idea that Earth's mantle reached its present-day oxidation state as early as 4.35 billion years ago. The findings suggest that outgassing of Earth's interior about 200 million years into the history of Solar System formation would not have resulted in a reducing atmosphere.

The chemical and isotopic homogeneity of the early solar nebula, and the processes producing fractionation during its evolution, are central issues of cosmochemistry. Studies of the relative abundance variations of three or more isotopes of an element can in principle determine if the initial reservoir of material was a homogeneous mixture or if it contained several distinct sources of precursor material. For example, widespread anomalies1,2,3,4 observed in the oxygen isotopes of meteorites have been interpreted as resulting from the mixing of a solid phase that was enriched in 16O with a gas phase in which 16O was depleted1,2,3, or as an isotopic ‘memory’ of Galactic evolution5. In either case, these anomalies are regarded as strong evidence that the early solar nebula was not initially homogeneous. Here we present measurements of the relative abundances of three iron isotopes in meteoritic and terrestrial samples. We show that significant variations of iron isotopes exist in both terrestrial and extraterrestrial materials. But when plotted in a three-isotope diagram, all of the data for these Solar System materials fall on a single mass-fractionation line, showing that homogenization of iron isotopes occurred in the solar nebula before both planetesimal accretion and chondrule formation.

The Oort cloud1 of comets was formed by the ejection of icy planetesimals from the region of giant planets—Jupiter, Saturn, Uranus and Neptune—during their formation2. Dynamical simulations3,4 have previously shown that comets reach the Oort cloud only after being perturbed into eccentric orbits that result in close encounters with the giant planets, which then eject them to distant orbits about 104 to 105 AU from the Sun (1 AU is the average Earth–Sun distance). All of the models constructed until now simulate formation of the Oort cloud using only gravitational effects; these include the influence of the Sun, the planets and external perturbers such as passing stars and Galactic tides. Here we show that physical collisions between comets and small debris play a fundamental and hitherto unexplored role throughout most of the ejection process. For standard models of the protosolar nebula (starting with a minimum-mass nebula) we find that collisional evolution of comets is so severe that their erosional lifetimes are much shorter than the timescale for dynamical ejection. It therefore appears that collisions will prevent most comets escaping from most locations in the region of the giant planets until the disk mass there declines sufficiently that the dynamical ejection timescale is shorter than the collisional lifetime. One consequence is that the total mass of comets in the Oort cloud may be less than currently believed.

Chondrules1,2,3,4,5 are millimetre-sized, solidified melt spherules formed in the solar nebula by an early widespread heating event of uncertain nature6,7,8. They were accreted into chondritic asteroids, which formed about 4.56 billion years ago and have not experienced melting or differentiation since that time. Chondrules have diverse chemical compositions, corresponding to liquidus temperatures1,4,9 in the range 1,350–1,800 °C. Most chondrules, however, show porphyritic textures (consisting of large crystals in a distinctly finer grained or glassy matrix), indicative of melting within the narrow range 0–50 °C below the liquidus9,10. This suggests an unusual heating mechanism for chondrule precursors, which would raise each individual chondrule to just the right temperature (particular to individual bulk composition) in order to form porphyritic textures. Here we report the results of isothermal melting of a chondritic composition at nebular pressures. Our results suggest that evaporation stabilizes porphyritic textures over a wider range of temperatures below the liquidus (about 200 °C) than previously believed, thus removing the need for individual chondrule temperature buffering. In addition, we show that evaporation explains many chondrule bulk and mineral compositions that have hitherto been difficult to understand.