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The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario., Comment: In press as a Frontier paper in Earth and Planetary Science Letters

The nucleosynthetic isotope signatures of meteorites and the bulk silicate Earth (BSE) indicate that Earth consists of a mixture of "carbonaceous" (CC) and "non-carbonaceous" (NC) materials. We show that the fration of CC material recorded in the isotopic composition of the BSE varies for different elements, and depends on the element's tendency to partition into metal and its volatility. The observed behaviour indicates that the majority of material accreted to the Earth was NC-dominated, but that CC-dominated material enriched in moderately-volatile elements by a factor of ~10 was delivered during the last ~2-10% of Earth's acccretion. The late delivery of CC material to Earth contrasts with dynamical evidence for the early implantation of CC objects into the inner solar system during the growth and migration of the giant planets. This, together with the NC-dominated nature of both Earth's late veneer and bulk Mars, suggests that material scattered inwards had the bulk of its mass concentrated in a few, large CC embryos rather than in smaller planetesimals. We propose that Earth accreted a few of these CC embryos while Mars did not, and that at least one of the CC embryos impacted Earth relatively late (when accretion was 90-90% complete). This scenario is consistent with the subsequent Moon-formign impact of a large NC body, as long as this impact did not re-homogenize the entire Earth's mantle.

The isotopic compositions of samples returned from Cb-type asteroid Ryugu and Ivuna-type (CI) chondrites are distinct from other carbonaceous chondrites, which has led to the suggestion that Ryugu and CI chondrites formed in a different region of the accretion disk, possibly around the orbits of Uranus and Neptune. We show that, like for Fe, Ryugu and CI chondrites also have indistinguishable Ni isotope anomalies, which differ from those of other carbonaceous chondrites. We propose that this unique Fe and Ni isotopic composition reflects different accretion efficiencies of small FeNi metal grains among the carbonaceous chondrite parent bodies. The CI chondrites incorporated these grains more efficiently, possibly because they formed at the end of the disk's lifetime, when planetesimal formation was also triggered by photoevaporation of the disk. Isotopic variations among carbonaceous chondrites may thus reflect fractionation of distinct dust components from a common reservoir, implying CI chondrites and Ryugu may have formed in the same region of the accretion disk as other carbonaceous chondrites., Comment: Published open access in Science Advances

Refractory inclusions in chondritic meteorites, namely amoeboid olivine aggregates (AOAs) and Ca-Al-rich inclusions (CAIs), are among the first solids to have formed in the solar system. The isotopic composition of CAIs is distinct from bulk meteorites, which either results from extreme processing of presolar carriers in the CAI-forming region, or reflects an inherited heterogeneity from the Sun's parental molecular cloud. Amoeboid olivine aggregates are less refractory than CAIs and provide a record of how the isotopic composition of solid material in the disk may have changed in time and space. However, the isotopic composition of AOAs and how this composition relates to that of CAIs and later-formed solids is unknown. Here, using new O, Ti, and Cr isotopic data for eight AOAs from the Allende CV3 chondrite, we show that CAIs and AOAs share a common isotopic composition, indicating a close genetic link and formation from the same isotopic reservoir. Because AOAs are less refractory than CAIs, this observation is difficult to reconcile with a thermal processing origin of the isotope anomalies. Instead, the common isotopic composition of CAIs and AOAs is readily accounted for in a model in which the isotopic composition of infalling material from the Sun's parental molecular cloud changed over time. In this model, CAIs and AOAs record the isotopic composition of the early infall, while later-formed solids contain a larger fraction of the later, isotopically distinct infall. This model implies that CAIs and AOAs record the isotopic composition of the Sun and suggests that the nucleosynthetic isotope heterogeneity of the solar system is predominantly produced by mixing of solar nebula condensates, which acquired their distinct isotopic compositions as a result of time-varied infall from the protosolar cloud., Comment: Published gold open access in Earth and Planetary Science Letters

The carbonaceous Bencubbin-like (CB), high-metal (CH), and Renazzo-like (CR) chondrites are metal-rich chondrites that have been suggested to be genetically linked and are sometimes grouped together as the CR chondrite clan. Of these, the CB and CH chondrites are thought to have formed in an impact-generated vapor-melt plume from material that may be isotopically akin to CR chondrites. We report Mo, Ti, Cr, and Hf-W isotopic data for CB and CH chondrites in order to determine their formation time, to assess whether these chondrites are genetically related, and to evaluate their potential link to CR chondrites. An internal Hf-W isochron for the CH chondrite Acfer 182 yields an age of 3.8 +- 1.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which is indistinguishable from the mean Hf-W model age for CB metal of 3.8 +- 1.3 Ma. The Mo isotopic data for CB and CH chondrites indicate that both contain some of the same metal and silicate components, which themselves are isotopically distinct. As such, the different Mo isotopic compositions of bulk CB and CH chondrites reflect their distinct metal-to-silicate ratios. CR metal exhibits the same Mo isotopic composition as CB and CH metal, but CR silicates have distinct Mo and Ti isotopic compositions compared to CB and CH silicates, indicating that CB/CH chondrites may be genetically related to CR metal, but not to CR silicates. Together, the new isotopic data are consistent with formation of CB and CH chondrites in different regions of a common impact-generated vapor- melt plume and suggest that the CB and CH metal may derive from a metal-rich precursor genetically linked to CR chondrites. The Hf-W systematics of CH and CB chondrites indicate that the impact occurred at 3.8 +- 0.8 Ma after the formation of Ca-Al-rich inclusions and, hence, up to ~1 Ma earlier than previously inferred based on Pb- Pb chronology., Comment: 54 pages, 8 figures, 5 tables

Complex interelement trends among magmatic IIIF iron meteorites are difficult to explain by fractional crystallization and have raised uncertainty about their genetic relationships. Nucleosynthetic Mo isotope anomalies provide a powerful tool to assess if individual IIIF irons are related to each other. However, while trace-element data are available for all nine IIIF irons, Mo isotopic data are limited to three samples. We present Mo isotopic data for all but one IIIF irons that help assess the genetic relationships among these irons, together with new Mo and W isotopic data for Fitzwater Pass (classified IIIF), and the Zinder pallasite (for which a cogenetic link with IIIF irons has been proposed). After correction for cosmic-ray exposure, the Mo isotopic compositions of the IIIF irons are identical within uncertainty and confirm their belonging to carbonaceous chondrite-type (CC) meteorites. The mean Mo isotopic composition of Group IIIF overlaps those Groups IIF and IID, but a common parent body for these groups is ruled out based on distinct trace element systematics. The new Mo isotopic data do not argue against a single parent body for the IIIF irons, and suggest a close genetic link among these samples. By contrast, Fitzwater Pass has distinct Mo and W isotopic compositions, identical to those of some non-magmatic IAB irons. The Mo and W isotope data for Zinder indicate that this meteorite is not related to IIIF irons, but belongs to the non-carbonaceous (NC) type and has the same Mo and W isotopic composition as main-group pallasites., Comment: Accepted for publication in Meteoritics & Planetary Science

Nucleosynthetic isotope anomalies in meteorites allow distinguishing between the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs and show that correlated isotope anomalies exist in both reservoirs. It is debated, however, whether these anomalies reflect thermal processing of presolar dust in the disk or are primordial heterogeneities inherited from the Solar System's parental molecular cloud. Here, using new high-precision 84Sr isotope data, we show that NC meteorites, Mars, and the Earth and Moon are characterized by the same 84Sr isotopic composition. This 84Sr homogeneity of the inner Solar System contrasts with the well-resolved and correlated isotope anomalies among NC meteorites observed for other elements, and most likely reflects correlated s- and (r-, p-)-process heterogeneities leading to 84Sr excess and deficits of similar magnitude which cancel each other. For the same reason there is no clearly resolved 84Sr difference between NC and CC meteorites, because in some carbonaceous chondrites the characteristic 84Sr excess of the CC reservoir is counterbalanced by an 84Sr deficit resulting from s-process variations. Nevertheless, most carbonaceous chondrites exhibit 84Sr excesses, which reflect admixture of refractory inclusions and more pronounced s-process heterogeneities in these samples. Together, the correlated variations of s-, (r-, p-)-process nuclides revealed by the 84Sr data of this study refute an origin of these isotope anomalies solely by processing of presolar dust grains, but points to primordial mixing of isotopically distinct dust reservoirs as the dominant process producing the isotopic heterogeneity of the Solar System., Comment: 18 pages 4 Figures

Carbonaceous chondrites are some of the most primitive meteorites and derive from planetesimals that formed a few million years after the beginning of the solar system. Here, using new and previously published Cr, Ti, and Te isotopic data, we show that carbonaceous chondrites exhibit correlated isotopic variations that can be accounted for by mixing among three major constituents having distinct isotopic compositions, namely refractory inclusions, chondrules, and CI chondrite-like matrix. The abundances of refractory inclusions and chondrules are coupled and systematically decrease with increasing amount of matrix. We propose that these correlated abundance variations reflect trapping of chondrule precursors, including refractory inclusions, in a pressure maximum in the disk, which is likely related to the water ice line and the ultimate formation location of Jupiter. The variable abundance of refractory inclusions/chondrules relative to matrix is the result of their distinct aerodynamical properties resulting in differential delivery rates and their preferential incorporation into chondrite parent bodies during the streaming instability, consistent with the early formation of matrix-poor and the later accretion of matrix-rich carbonaceous chondrites. Our results suggest that chondrules formed locally from isotopically heterogeneous dust aggregates which themselves derive from a wide area of the disk, implying that dust enrichment in a pressure trap was an important step to facilitate the accretion of carbonaceous chondrite parent bodies or, more generally, planetesimals in the outer solar system., Comment: 12 pages, 4 figures, 1 table. Accepted for publication in ApJL

We have conducted a NanoSIMS-based search for presolar material in samples recently returned from C-type asteroid Ryugu as part of JAXA's Hayabusa2 mission. We report the detection of all major presolar grain types with O- and C-anomalous isotopic compositions typically identified in carbonaceous chondrite meteorites: 1 silicate, 1 oxide, 1 O-anomalous supernova grain of ambiguous phase, 38 SiC, and 16 carbonaceous grains. At least two of the carbonaceous grains are presolar graphites, whereas several grains with moderate C isotopic anomalies are probably organics. The presolar silicate was located in a clast with a less altered lithology than the typical extensively aqueously altered Ryugu matrix. The matrix-normalized presolar grain abundances in Ryugu are 4.8$^{+4.7}_{-2.6}$ ppm for O-anomalous grains, 25$^{+6}_{-5}$ ppm for SiC grains and 11$^{+5}_{-3}$ ppm for carbonaceous grains. Ryugu is isotopically and petrologically similar to carbonaceous Ivuna-type (CI) chondrites. To compare the in situ presolar grain abundances of Ryugu with CI chondrites, we also mapped Ivuna and Orgueil samples and found a total of SiC grains and 6 carbonaceous grains. No O-anomalous grains were detected. The matrix-normalized presolar grain abundances in the CI chondrites are similar to those in Ryugu: 23 $^{+7}_{-6}$ ppm SiC and 9.0$^{+5.3}_{-4.6}$ ppm carbonaceous grains. Thus, our results provide further evidence in support of the Ryugu-CI connection. They also reveal intriguing hints of small-scale heterogeneities in the Ryugu samples, such as locally distinct degrees of alteration that allowed the preservation of delicate presolar material., Comment: 12 pages, 3 figures, 2 tables. Published in ApJL

Two fundamentally different processes of rocky planet formation exist, but it is unclear which one built the terrestrial planets of the solar system. Either they formed by collisions among planetary embryos from the inner solar system, or by accreting sunward-drifting millimeter-sized 'pebbles' from the outer solar system. We show that the isotopic compositions of Earth and Mars are governed by two-component mixing among inner solar system materials, including material from the innermost disk unsampled by meteorites, whereas the contribution of outer solar system material is limited to a few percent by mass. This refutes a pebble accretion origin of the terrestrial planets, but is consistent with collisional growth from inner solar system embryos. The low fraction of outer solar system material in Earth and Mars indicates the presence of a persistent dust-drift barrier in the disk, highlighting the specific pathway of rocky planet formation in the solar system., Comment: combined main test + supplement of published article

The formation of planetesimals is expected to occur via particle-gas instabilities that concentrate dust into self-gravitating clumps. Triggering these instabilities requires the prior pileup of dust in the protoplanetary disk. Until now, this has been successfully modeled exclusively at the disk's snowline, whereas rocky planetesimals in the inner disk were obtained only by assuming either unrealistically large particle sizes or an enhanced global disk metallicity. However, planetesimal formation solely at the snowline is difficult to reconcile with the early and contemporaneous formation of iron meteorite parent bodies with distinct oxidation states and isotopic compositions, indicating formation at different radial locations in the disk. Here, by modeling the evolution of a disk with ongoing accretion of material from the collapsing molecular cloud, we show that planetesimal formation may have been triggered within the first 0.5 million years by dust pileup at both the snowline (at approximately 5 au) and the silicate sublimation line (at approximately 1 au), provided turbulent diffusion was low. Particle concentration at approximately 1 au is due to the early outward radial motion of gas and is assisted by the sublimation and recondensation of silicates. Our results indicate that, although the planetesimals at the two locations formed about contemporaneously, those at the snowline accreted a large fraction of their mass (approximately 60 percent) from materials delivered to the disk in the first few 10,000 yr, whereas this fraction is only 30 percent for the planetesimals formed at the silicate line. Thus, provided that the isotopic composition of the delivered material changed with time, these two planetesimal populations should have distinct isotopic compositions, consistent with observations., Comment: Published online in Nature Astronomy on Dec. 22, 2021

Nucleosynthetic Fe isotopic anomalies in meteorites may be used to reconstruct the early dynamical evolution of the solar system and to identify the origin and nature of the material that built planets. Using high-precision iron isotopic data of 23 iron meteorites from nine major chemical groups we show that all iron meteorites show the same fundamental dichotomy between non-carbonaceous (NC) and a carbonaceous (CC) meteorites previously observed for other elements. The Fe isotopic anomalies are predominantly produced by variation in 54Fe, where all CC iron meteorites are characterized by an excess in 54Fe relative to NC iron meteorites. This excess in 54Fe is accompanied by an excess in 58Ni observed in the same CC meteorite groups. Together, these overabundances of 54Fe and 58Ni are produced by nuclear statistical equilibrium either in type Ia supernovae or in the Si/S shell of core-collapse supernovae. The new Fe isotopic data reveal that Earth's mantle plots on or close to correlations defined by Fe, Mo, and Ru isotopic anomalies in iron meteorites, indicating that throughout Earth's accretion, the isotopic composition of its building blocks did not drastically change. While Earth's mantle has a similar Fe isotopic composition to CI chondrites, the latter are clearly distinct from Earth's mantle for other elements (e.g., Cr and Ni) whose delivery to Earth coincided with Fe. The fact that CI chondrites exhibit large Cr and Ni isotopic anomalies relative to Earth's mantle, therefore, demonstrates that CI chondrites are unlikely to have contributed significant Fe to Earth., Comment: 1 Table, 9 Figures

The 182Hf-182W chronology of iron meteorites provides crucial information on the timescales of accretion and differentiation of some of the oldest planetesimals of the Solar System. Determining accurate Hf-W model ages of iron meteorites requires correction for cosmic ray expo-sure (CRE) induced modifications of W isotope compositions, which can be achieved using in-situ neutron dosimeters such as Pt isotopes. Until now it has been assumed that all Pt isotope variations in meteorites reflect CRE, but here we show that some ungrouped iron meteorites display small nucleosynthetic Pt isotope anomalies. These provide the most appropriate starting composition for the correction of CRE-induced W isotope variations in iron meteorites from all major chemical groups, which leads to a ~1 Ma upward revision of previously reported Hf-W model ages. The revised ages indicate that core formation in non-carbonaceous (NC) iron meteorite parent bodies occurred at ~1-2 Ma after CAI formation, whereas most carbonaceous (CC) iron meteorite parent bodies underwent core formation ~2 Ma later. We show that the younger CC cores have lower Fe/Ni ratios than the earlier-formed NC cores, indicating that core formation under more oxidizing conditions occurred over a more protracted timescale. Thermal modeling of planetesimals heated by 26Al-decay reveals that this protracted core formation timescale is consistent with a higher fraction of water ice in CC compared to NC planetesimals, implying that in spite of distinct core formation timescales, NC and CC iron meteorite parent bodies accreted about contemporaneously within ~1 Ma after CAI formation, but at different radial locations in the disk., Comment: Accepted for publication in Earth and Planetary Science Letters

The ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the non-carbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf-W chronology of Nedagolla indicates that this mixed NC-CC isotopic composition was established relatively late, more than 7 million years after Solar System formation. The mixed NC-CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re-equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit-and-run collision between two differentiated bodies, which also provides a possible pathway for Nedagolla's extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiter's growth., Comment: Accepted for publication in Meteoritics & Planetary Science

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30-50 km and on top a more mafic-rich layer ~10 km thick; and (2) the nearside exhibits a large area of low-Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside-farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega-basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside's mafic-rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep-seated KREEP (potassium, rare-earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry-forming impact, the Moon may have had an 182W anomaly compared to the immediate post-giant impact Earth's mantle, as predicted if the Moon was created through a giant collision with the proto-Earth., Comment: 45 pages, 12 figures

Isotopic anomalies in chondrules hold important clues about the dynamics of mixing and transport processes in the solar accretion disk. These anomalies have been interpreted to indicate either disk-wide transport of chondrules or local heterogeneities of chondrule precursors. However, all previous studies relied on isotopic data for a single element (either Cr, Ti, or O), which does not allow distinguishing between source and precursor signatures as the cause of the chondrules isotope anomalies. Here we obtained the first combined O, Ti, and Cr isotope data for individual chondrules from enstatite, ordinary, and carbonaceous chondrites. We find that chondrules from non-carbonaceous (NC) chondrites have relatively homogeneous {\Delta}17O, {\epsilon}50Ti, and {\epsilon}54Cr, which are similar to the compositions of their host chondrites. By contrast, chondrules from carbonaceous chondrites (CC) have more variable compositions. Although the compositions of the analyzed CC and NC chondrules may overlap for either {\epsilon}50Ti, {\epsilon}54Cr, or {\Delta}17O, in multi-isotope space none of the CC chondrules plot in the compositional field of NC chondrites, and no NC chondrule plots within the field of CC chondrites. As such, our data reveal a fundamental isotopic difference between NC and CC chondrules, which is inconsistent with a disk-wide transport of chondrules across and between the NC and CC reservoirs. Instead, the isotopic variations among CC chondrules reflect local precursor heterogeneities, which most likely result from mixing between NC-like dust and a chemically diverse dust component that was isotopically similar to CAIs and AOAs.The same mixing processes, but on a larger, disk-wide scale, were likely responsible for establishing the distinct isotopic compositions of the NC and CC reservoirs, which represent in inner and outer disk, respectively., Comment: 1 Table 5 Figures Supplementary Materials: 1 Table, 1Figure

The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca-Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun's parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals., Comment: Accepted for publication in Astrophysical Journal Letters, June 17, 2020

The Mo and Ru isotopic compositions of meteorites and the bulk silicate Earth (BSE) hold important clues about the provenance of Earth's building material. Prior studies have argued that non-carbonaceous (NC) and carbonaceous (CC) meteorite groups together define a Mo-Ru 'cosmic' correlation, and that the BSE plots on the extension of this correlation. These observations were taken as evidence that the final 10-15% of Earth's accreted material derived from a homogeneous inner disk reservoir with an enstatite chondrite-like isotopic composition. Here, using new Mo and Ru isotopic data for previously uninvestigated meteorite groups, we show that the Mo-Ru correlation only exists for NC meteorites, and that both the BSE and CC meteorites fall off this Mo-Ru correlation. These observations indicate that the final stages of Earth's accretion were heterogeneous and consisted of a mixture of NC and CC materials. The Mo-Ru isotope systematics are best accounted for by either an NC heritage of the late veneer combined with a CC heritage of the Moon-forming giant impactor, or by mixed NC-CC compositions for both components. The involvement of CC bodies in the late-stage accretionary assemblage of Earth is consistent with chemical models for core-mantle differentiation, which argue for the addition of more oxidized and volatile-rich material toward the end of Earth's formation. As such, this study resolves the inconsistencies between homogeneous accretion models based on prior interpretations of the Mo-Ru systematics of meteorites and the chemical evidence for heterogeneous accretion of Earth.

Isotope anomalies among planetary bodies provide key constraints on planetary genetics and the Solar System's dynamical evolution. However, to unlock the full potential of these anomalies for constraining the processing, mixing, and transport of material in the disk it is essential to identify the main components responsible for producing planetary-scale isotope variations, and to investigate how they relate to the isotopic heterogeneity inherited from the Solar System's parental molecular cloud. To address these issues we measured the Ti and Sr isotopic compositions of Ca,Al-rich inclusions (CAIs) from the Allende CV3 chondrite, as well as acid leachates and an insoluble residue from the Murchison CM2 chondrite, and combine these results with literature data for presolar grains, hibonites, chondrules, and bulk meteorites. Our analysis reveals that the mineral-scale nebular isotopic heterogeneity as sampled by leachates and presolar grains is largely decoupled from the planetary-scale isotope anomalies. Combining isotope anomaly data for a large number of elements reveals that the difference between non-carbonaceous (NC) and carbonaceous (CC) meteorites is the product of mixing between NC material and a nebular reservoir (termed IC) whose isotopic composition is similar to that of CAIs, but whose chemical composition is similar to bulk chondrites. In our preferred model, the distinct isotopic compositions of these two nebular reservoirs reflect an inherited heterogeneity of the solar system's parental molecular cloud core, which therefore has never been fully homogenized during collapse. Planetary-scale isotopic anomalies are thus caused by variable mixing of isotopically distinct primordial disk reservoirs, the selective processing of these reservoirs in different nebular environments, and the heterogeneous distribution of the thereby forming nebular products., Comment: Accepted for publication in Geochimica et Cosmochimica Acta

Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here we show that chondrules from enstatite and ordinary chondrites show only small 50Ti variations and scatter closely around the 50Ti composition of their host chondrites. By contrast, chondrules from carbonaceous chondrites have highly variable 50Ti compositions, which, relative to the terrestrial standard, range from the small 50Ti deficits measured for enstatite and ordinary chondrite chondrules to the large 50Ti excesses known from Ca-Al-rich inclusions (CAIs). These 50Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous nuggets into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Lastly, the ubiquitous presence of 50Ti-enriched material in carbonaceous chondrites, and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other through the formation of Jupiter., Comment: accepted for publication in the Astrophysical Journal Letters, May 10, 2017

The cooling histories of individual meteorites can be empirically reconstructed by using ages from different radioisotopic chronometers with distinct closure temperatures. For a group of meteorites derived from a single parent body such data permit the reconstruction of the cooling history and properties of that body. Particularly suited are H chondrites because precise radiometric ages over a wide range of closure temperatures are available. A thermal evolution model for the H chondrite parent body is constructed by using all H chondrites for which at least three different radiometric ages are available. Several key parameters determining the thermal evolution of the H chondrite parent body and the unknown burial depths of the H chondrites are varied until an optimal fit is obtained. The fit is performed by an 'evolution algorithm'. Empirical data for eight samples are used for which radiometric ages are available for at least three different closure temperatures. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data of six of the examined eight chondrites. The new thermal model constrains the radius and formation time of the H chondrite parent body (possibly (6) Hebe), the initial burial depths of the individual H chondrites, the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial 60Fe content of the H chondrite parent body., Comment: 16 pages, 7 figures

The major aim of this study is to assess the effects of sintering of initially porous material on the thermal evolution of planetesimals, and to constrain the values of basic parameters that determined the structure and evolution of the H chondrite parent body. A new code is presented for modeling the thermal evolution of ordinary chondrite parent bodies that initially are highly porous and undergo sintering by hot pressing as they are heated by decay of radioactive nuclei. The pressure and temperature stratification in the interior of the bodies is calculated by solving the equations of hydrostatic equilibrium and energy transport. The decrease of porosity of the granular material by hot pressing due to self-gravity is followed by solving a set of equations for the sintering of powder materials. For the heat conductivity of granular material we combine recently measured data for highly porous powder materials, relevant for the surface layers of planetesimals, with data for heat conductivity of chondrite material, relevant for the strongly sintered material in deeper layers. To demonstrate the capability of our new model, the thermal evolution of the H chondrite parent body was reconstructed. The model starts with a porous body that is later compacted first by 'cold pressing' at low temperatures and then by 'hot pressing' for temperatures above \approx 700 K, i.e., the threshold temperature for sintering of silicates. The thermal model was fitted to the well constrained cooling histories of the two H chondrites Kernouve (H6) and Richardton (H5)., Comment: 19 pages 12 figures submitted to A&A