Thomas S. Kruijer

Protracted core formation and rapid accretion of protoplanets.

The chronology of planetary embryos Protoplanets, or early planetary embryos such as iron meteorite parent bodies, formed in the early protoplanetary disk from dust, debris, and planetesimals. Defining the precise chronology of accretion and differentiation—including core formation—of these planetary embryos will aid in a richer understanding of the chemical evolution of the solar system. Through high-precision tungsten isotope measurements, Kruijer et al. show that the timing of accretion and core formation for iron meteorite groups falls within 0.6 to 2 million years of the age of the solar system (see the Perspective by Elliott). Differences of timing within this group are probably a function of volatile contents of the parent bodies or spatial and chemical heterogeneity within the protoplanetary disk. Science, this issue p. 1150; see also p. 1086 Tungsten isotopes record the chronology of accretion and core formation of iron meteorite groups. [Also see Perspective by Elliott] Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in 182W, resulting from the decay of now-extinct 182Hf, among five magmatic iron meteorite groups. These 182W variations indicate that core formation occurred over an interval of ~1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently ~0.1 to 0.3 million years after the formation of Ca-Al–rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.

Tungsten isotopic constraints on the age and origin of chondrules

Significance The origin of chondrules—millimeter-sized silicate-rich spherules that dominate the most primitive meteorites, the chondrites—is a long-standing puzzle in cosmochemistry. Here, we present isotopic evidence that chondrules and matrix from the Allende chondrite contain different and complementary proportions of presolar matter, indicating that chondrules and matrix formed together from a single reservoir of solar nebula dust. This finding rules out formation of chondrules in protoplanetary impacts and demonstrates that chondrules are the product of localized melting events in the solar protoplanetary disk. Moreover, the isotopic complementarity of chondrules and matrix requires that chondrules formed in a narrow time interval and were rapidly accreted to a parent body, implying that chondrule formation was a critical step toward forming planetesimals. Chondrules may have played a critical role in the earliest stages of planet formation by mediating the accumulation of dust into planetesimals. However, the origin of chondrules and their significance for planetesimal accretion remain enigmatic. Here, we show that chondrules and matrix in the carbonaceous chondrite Allende have complementary 183W anomalies resulting from the uneven distribution of presolar, stellar-derived dust. These data refute an origin of chondrules in protoplanetary collisions and, instead, indicate that chondrules and matrix formed together from a common reservoir of solar nebula dust. Because bulk Allende exhibits no 183W anomaly, chondrules and matrix must have accreted rapidly to their parent body, implying that the majority of chondrules from a given chondrite group formed in a narrow time interval. Based on Hf-W chronometry on Allende chondrules and matrix, this event occurred ∼2 million years after formation of the first solids, about coeval to chondrule formation in ordinary chondrites.

Age of Jupiter inferred from the distinct genetics and formation times of meteorites

Significance Jupiter is the most massive planet of the Solar System and its presence had an immense effect on the dynamics of the solar accretion disk. Knowing the age of Jupiter, therefore, is key for understanding how the Solar System evolved toward its present-day architecture. However, although models predict that Jupiter formed relatively early, until now, its formation has never been dated. Here we show through isotope analyses of meteorites that Jupiter’s solid core formed within only ∼1 My after the start of Solar System history, making it the oldest planet. Through its rapid formation, Jupiter acted as an effective barrier against inward transport of material across the disk, potentially explaining why our Solar System lacks any super-Earths. The age of Jupiter, the largest planet in our Solar System, is still unknown. Gas-giant planet formation likely involved the growth of large solid cores, followed by the accumulation of gas onto these cores. Thus, the gas-giant cores must have formed before dissipation of the solar nebula, which likely occurred within less than 10 My after Solar System formation. Although such rapid accretion of the gas-giant cores has successfully been modeled, until now it has not been possible to date their formation. Here, using molybdenum and tungsten isotope measurements on iron meteorites, we demonstrate that meteorites derive from two genetically distinct nebular reservoirs that coexisted and remained spatially separated between ∼1 My and ∼3–4 My after Solar System formation. The most plausible mechanism for this efficient separation is the formation of Jupiter, opening a gap in the disk and preventing the exchange of material between the two reservoirs. As such, our results indicate that Jupiter’s core grew to ∼20 Earth masses within <1 My, followed by a more protracted growth to ∼50 Earth masses until at least ∼3–4 My after Solar System formation. Thus, Jupiter is the oldest planet of the Solar System, and its solid core formed well before the solar nebula gas dissipated, consistent with the core accretion model for giant planet formation.

Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf‐W Chronometry of Iron Meteorites

Introduction: Magmatic iron meteorites are believed to sample metallic cores of planetesimals and each group represents a separate parent body. The IVB irons are extremely depleted in volatiles and enriched in refractory elements. The W content indicates that the metal-silicate separation occurred at ≈2.9 Ma after the formation of calcium-aluminiumrich inclusions (CAIs). Previous thermal models that attempted reproducing the differentiation of IVB parent body consider heat conduction and fit the Hf-W age by assuming melt percolation at the Fe-Ni melting T≈1760 K. However, at 1760 K most of the silicates would be liquid, causing an earlier phase separation. For reasonable temperature and porosity dependent parameters, the Hf-W data is not reproduced for any formation time t0 and parent body radius R. Intense heating and fast phase separation make an early accretion unlikely, while for a late accretion some melting and phase separation occurs, but the metal remains solid contradicting IVB meteorites being magmatic. Further processes, e.g. depletion of the interior in Al and liquid-state convection need to be considered as they can prevent rapid heating for early accretion and delay the phase separation. We calculated the differentiation of the IVB parent body comparing its evolution to the Hf-W model ages and provide a best fit on its radius R and formation time t0. Model: The numerical model solves energy balance in spherical symmetry considering heating by shortand long-lived radionuclides, temperatureand porosity-dependent parameters, compaction, melting and latent heat, metal-rock differentiation by Darcy flow, redistribution of radionuclides, and convection in a magma ocean and in the metallic liquid core. A typical ordinary chondritic composition is considered.

Hf-W chronology of planetary accretion and differentiation at the dawn of solar system history

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