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Illustration credit: NASA/JPL-Caltech/T. Pyle (SSC)

I. Accretion Process

Accretion from dust to gas-giant planet occurred within 3 m.y. ('Pebble Accretion and the Diversity of Planetary Systems', J. E. Chambers, The Astrophysical Journal, vol. 825, 2016). In the beginning, µm-sized dust particles are embedded in a gaseous protoplanetary disk. By 0.02 m.y., mutual collisions between dust grains result in the formation of mm- to cm-sized pebbles. By 0.15 m.y., pebbles inside the ice line (~2.5–4.5 AU) have aggregated into planetesimals with diameters of 30–300 km. Just outside the ice line, aggregation of the larger ice-rich pebbles is more efficient, and larger planetesimals with diameters of ~1,500 km are formed during runaway growth. By 0.5 m.y., some of the larger planetesimals located within ~5 AU enter an oligarchic growth stage and become planetary embryos with diameters of 2,000 km. The largest embryos located just beyond the ice line begin to grow by "pebble accretion" due to the inward drift of pebbles, reaching sizes of a few Earth masses (M). By 3 m.y., the largest of these embryos exceed a critical mass (3 × M) and undergo runaway gas accretion to form gas-giant planets (Levison et al., 2015). A large protoplanetary disk (radius = ~100 AU) and a small turbulence strength (α = 0.0005) help promote the formation of these gas giants, which ultimately clear their orbits. Inside the ice line, the growth of terrestrial planets (0.02–1.4 M) ceases due to pebble depletion in the disk.

Matsumoto et al. (2019) employed advanced techniques including synchrotron radiation–based x-ray computed nanotomography in a study of the accretionary formation of the carbonaceous parent body of the Acfer 094 chondrite. They presented a formation scenario as follows (see also schematic illustration below):

  1. During migration from the outer to the inner protoplanetary disk, the Acfer 094 parent body grew through the accretion of fluffy ice-silicate aggregates (FISA) and fluffy silicate aggregates (FSA), respectively, forming a stratified planetesimal
  2. As the planetesimal passed through the H2O snow line, an additional ice-bearing ultraporous lithology (UPL), which was formed through sintering of fluffy icy dust aggregates, was incorporated into the growing mantle
  3. Subsequent heating of the planetesimal caused melting of ice, especially that composing the core, which led to widespread aqueous alteration
  4. The parent meteoroid of Acfer 094 was ultimately ejected from the mantle of the planetesimal in an impact event
Schematic illustration of the Acfer 094 Parent Body Formation Model
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Schematic illustration credit: Matsumoto et al., Science Advances, vol. 5, #11 (20 Nov 2019, open access link)
'Discovery of fossil asteroidal ice in primitive meteorite Acfer 094'

II. Insights From Radiogenic Dating

Improvements in the early chronology of the Solar System were presented by P. Koefoed (2017) based on more precise U–Pb data for the meteorites NWA 6704, NWA 10132, NWA 7325, and A-881394. The formation of crustal material of these planetesimals can be dated to as early as ~2 m.y. after CAIs, indicating that accretion of planetesimals was occurring contemporaneously with CAI formation.

Crystallization Ages for Early Solar System Meteorites
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Diagram credit: Piers P. Koefoed, PhD thesis of The Australian National University (2017 open access link)
'Sequencing Planetary Accretion Using Chronology Of Ungrouped Achondrites'

III. Insights From Nucleosynthetic Isotope Anomalies

Based on O, Cr, Ti, and Ni stable-isotopic data, Warren (2011) recognized the existence of two distinct taxonomic super-groups: those which accreted inside the orbit of Jupiter where thermal processing occurred under reducing conditions, termed 'non-carbonaceous' (NC), and those which accreted outside of its orbit where thermal processing occurred under oxidizing conditions, termed 'carbonaceous' (CC); the difference in redox conditions is attributed to differences in the ice, dust, and gas abundances (see also open access papers in EPSL by Budde et al. [2016], LPSC by Worsham et al. [2018 #2720], and Space Science Reviews by Kleine et al. [2020]). These two reservoirs were segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution (i.e., susceptibility to thermal processing) of proton (p-) process, rapid neutron capture (r-) process, and slow neutron capture (s-) process isotopes inherited as dust ejecta from explosive stellar nucleosynthesis—core-collapse supernovae, neutron star mergers, and low mass (1.5–4 M) AGB stars, respectively (Poole et al., 2017; Bermingham et al., 2018; Ek et al., 2019). In a study of W, Mo, and Ru isotopes in iron meteorites, Kruijer et al. (2017) recognized that both of these reservoirs were coeval and remained spatially separated within the protoplanetary disk for a prolonged period (~ 3.6–4.8 m.y., inferred from timing of CR and CB parent body accretion, respectively) associated with the rapid growth of proto-Jupiter (~30 M core at 2.9–3.2 AU within the first 0.6 m.y.; Desch et al., 2018).

Model Plot Predicting When and Where Meteorite Types Formed
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Data Key

Diagram credit: Desch et al., The Astrophysical Journal Supplement Series, vol. 238, #1, p. 23 (2018 open access version link)
'The Effect of Jupiter's Formation on the Distribution of Refractory Elements and Inclusions in Meteorites'

It has been posited that after Jupiter had grown to a massive size (>50 M) by ~4 m.y. at an initial heliocentric distance of ~3 AU, it underwent a chaotic migration in a 3:2 (or 2:1) resonance with Saturn—first inward for ~100 t.y. to ~1.5–2.0 AU while clearing the inner disk of planetesimals, and then outward for 500 t.y. ultimately ending at its current location near 5.2 AU (and Saturn near 7.1 AU); this is known as the 'Grand Tack' scenario (Walsh et al. 2011, #2585; Johnson et al., 2016; Brasser et al., 2016). In accord with the 'Nice Model' (Gomes et al., 2005; Tsiganis et al., 2005; Morbidelli et al., 2005; named after the French city where proposed), an orbital instability caused the final outward migration of the four (or initially five to six before their ejection; Nesvorný and Morbidelli, 2012) gas planets within a few m.y. of the dissipation of gas and dust in the protoplanetary disk, ultimately settling into their current orbits. This led to the scattering of existing planetesimals from the carbonaceous reservoir into the inner Solar System where some of this material acquired orbital stability in the outer asteroid belt (Pierens et al., 2014). This instability event can also explain other characteristics of the current Solar System such as the relatively small mass of both Mars and the asteroid belt, as well as the dynamically cold orbits of the current terrestrial planets (Clement et al., 2018, Clement et al., 2019).

“The Grand Tack” Model of Jupiter and Saturn
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Diagram credit: Walsh et al., MAPS, vol. 47, #12, p. 3 (2012 open access link)
'Populating the asteroid belt from two parent source regions due to the migration of giant planets—“The Grand Tack”'

Kruijer et al. (2017) determined the ε182W values for representative iron meteorites, which reflect the timing of metal–silicate differentiation on their respective parent bodies. Based on coupled Mo- and W-isotopic diagrams (see below), they demonstrated that irons from groups IAB, IC, IIAB, IIE, IIG, IIIAB, IIIE, and IVA (and IIG; Wasson and Choe, 2009; A. Rubin, 2018) are associated with the non-carbonaceous reservoir (NC), in which iron parent bodies experienced earlier accretion (<0.4 m.y. after CAIs) and core formation (~ 0.3–1.8 m.y. after CAIs). By contrast, irons from groups IIC, IID, IIF, IIIF, and IVB are associated with the carbonaceous reservoir (CC), and their parent bodies experienced a later accretion (~1 m.y. after CAIs) and core formation (~ 2.2–2.8 m.y. after CAIs).

Alan Rubin (2018) recognized that the CC irons are enriched in refractory siderophile elements and attributed this to the inheritance of refractory metal nuggets initially present in CAIs. The enrichment in Ni observed in CC irons compared to NC irons was considered to reflect in part the higher oxidizing conditions that existed in the outer Solar System where CC irons accreted. However, in an expanded sampling of ungrouped irons conducted by Spitzer et al. (2019, #2592), no significant difference in Ni content was found to exist between irons from the two reservoirs. It was also demonstrated by A. Rubin (2018) that CC irons generally have significantly higher CRE ages than NC irons, consistent with their location in the outer asteroid belt and thus greater distance traveled prior to intersection with the Earth. He also found that there are generally fewer members in the CC iron groups compared to the NC iron groups, which is likely a factor of 1) a smaller core volume for CC iron asteroids due to later accretion, less differentiation, and perhaps most importantly, a higher oxidation state resulting in less production of metal, and 2) greater fragmentation experienced by CC iron meteoroids given their longer transit times in space. Furthermore, he posited that most ungrouped magmatic irons would derive from the CC reservoir, as are the two which had been adequately analyzed up to then, Mbosi and Grand Rapids. In their analysis of Mo, W, and Pt isotopes for ungrouped irons, Spitzer et al. (2019) found support for this latter hypothesis of Rubin (2018), as they determined that more of the ungrouped irons originated from the CC region compared to the NC region reflecting a ratio of 8:5. In addition, they recognized that those originating from the CC region generally have negative Δ17O values while those from the NC region have positive values.

Nucleosynthetic Mo and W Isotope Dichotomy
standby for carbonaceous vs. non-carbonaceous irons mo diagram standby for carbonaceous vs. non-carbonaceous irons w diagram
Diagrams credit: Kruijer et al., PNAS, vol. 114, #26, p. 6713 (2017)
'Age of Jupiter inferred from the distinct genetics and formation times of meteorites'

A subsequent Mo isotope study conducted by Budde et al. (2019) and utilizing a more comprehensive sampling of meteorite groups led to a refinement in the slopes and intercepts for the NC and CC groups (see revised coupled Mo isotope diagram below). The mixing lines of both groups have identical slopes with a value of 0.596, while the offset in the y-axis intercept between the CC and the NC groups reflects an excess of r- and possibly p-process Mo in the CC region of the protoplanetary disk. Further details about their study to constrain the origin of the Moon-forming impactor 'Theia' can be found on the NWA 032 page.

Nucleosynthetic Mo Isotope Dichotomy (Revised I)
(ε notation denotes deviation from terrestrial standards in parts per ten thousand)
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Diagram credit: Budde et al., Nature Astronomy, vol. 3, pp. 736–741 (May 2019)
'Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth'

The Mo-isotopic dichotomy that exists between the CC and NC meteorites was also investigated by Yokoyama et al. (2019) employing an extremely precise measurement technique. Their study included ten irons (IIAB [4], IVB [4], IVB-like, IIE-like), two carbonaceous chondrites (CK4, CK5), five ordinary chondrites (H4, H5, LL5 [2], LL6), and two rumuruti chondrites (R3.9, R4). While the previous study by Budde et al. (2019) utilized a broader sampling of meteorite groups for their revision of the 95Mo–94Mo isotope diagram after Kruijer et al. (2017), the new study by Yokoyama et al. (2019) used improved values for irons that are corrected for neutron-capture modification due to galactic cosmic ray (GCR) exposure during prolonged transit times in space. Based on these more accurate values for iron meteorites, along with new values for other meteorite groups combined with recent literature data (e.g., Kruijer et al., 2017; Budde et al., 2019), they derived new slopes and intercepts for the coupled Mo isotope diagram as shown below.

Nucleosynthetic Mo Isotope Dichotomy (Revised II)
(µ notation denotes deviation from terrestrial standards in parts per million)
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Diagram credit: Yokoyama et al., The Astrophysical Journal, vol. 883, #1, article 62 (2019)
'Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs'

The new slope derived by Yokoyama et al. (2019) for the CC meteorites is consistent with that obtained in previous studies, with the Mo-isotopic variability among samples being attributed either to differences in the abundances of s-process-enriched presolar grains or to parent body processing (e.g., oxidation, thermal metamorphism). The new slope derived for the NC meteorites can be explained by the existence of two end-member components, NC-A and NC-B, which are resolved from each other by variations in s-process and r-process Mo isotope depletions. The NC-B component is represented most closely by the R chondrite and ACA–LOD groups, which exhibit the highest depletion in r-process Mo with no depletion in s-process Mo. Yokoyama et al. (2019) posit that the NC-B component could be a residual fine-grained phase that is isotopically complementary to the the type B CAI nebular reservoir, as established by a thermally-induced destruction of an r-process-rich carrier in the early protoplanetary disk (see schematic diagram by Yokoyama et al., 2019). Thereafter, the NC-A component was formed either by 1) another thermal processing event that destroyed an s-process-depleted carrier phase, or 2) the removal of a metal component from (thus increasing the ηs and ηr values), or the addition of a martix component to (thus decreasing the ηs and ηr values) an initial NC-B-like reservoir (see schematic illustration below).

Schematic Illustration of the Origin of the NC-A and NC-B Reservoirs
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Diagram credit: Yokoyama et al., The Astrophysical Journal, vol. 883, #1, article 62 (2019)
'Origin and Evolution of Distinct Molybdenum Isotopic Variabilities within Carbonaceous and Noncarbonaceous Reservoirs'

Nucleosynthetic isotope anomalies created during supernovae events were heterogeneously distributed within the molecular cloud, and this heterogeneity was imparted to the protoplanetary accretion disk throughout the early and late infall stages (Kruijer et al., 2019). Nucleosynthetic anomalies for many elements support the dichotomy between the NC and CC regions of the protoplanetary disk, including Cr, Ti, Mo, W, Hf, Sr, Nd, Ca, Ni, Sm, Ru, Ba, Zr, and Os (Burkhardt, et al., 2019). The isotopic offset between the CC and the NC groups demonstrated by most of these elements is attributed by Burkhardt et al. (2019) to primordial variability in the admixture of CAIs and CAI-like material on a molecular cloud scale. This isotopic heterogeneity was on the order of only ~0.1% (Kruijer et al., 2019). Burkhardt et al. (2019) suggest that during the cloud collapse phase the isotopic composition of the infalling dust evolved from CAI-like (termed 'IC' for Inclusion-like Chondritic component) to NC-like (more depleted in neutron-rich isotopes). Consequently, this led to an NC-like isotopic dilution of the inner nebular region, while at the same time, the outward expansion of the accretionary radius led to an enrichment of the outer nebular region in unprocessed, primitive, refractory- and volatile-rich material. The rapid formation of Jupiter to ~10–20 M within <0.5 m.y. created a barrier to further mixing between the NC and CC reservoirs (Kruijer et al., 2019). A possible inward-then-outward migration of Jupiter and Saturn ('Grand Tack') and/or the creation of an instability due to a mean motion resonance configuration between Jupiter and Saturn ('Nice Model') can explain many characteristics of the Solar System, including the distribution of NC and CC material in the asteroid belt, the volatile budget of Earth, and the small size of Mars. See two schematic illustrations of this scenario below.

1. Schematic Illustration of the Origin of the NC and CC Dichotomy
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Diagram credit: Burkhardt et al., GCA, vol. 261, p. 165 (2019 open access link)
'Elemental and isotopic variability in solar system materials by mixing and processing of primordial disk reservoirs'

2. Schematic Illustration of the Origin of the NC and CC Dichotomy
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Diagram credit: Kruijer et al., Nature Astronomy, vol. 4, p. 36 (2020)
'The great isotopic dichotomy of the early Solar System'

In a related study, Nanne et al. (2019) analyzed the Ni isotopes in irons from several CC-region chemical groups and found an ~60 ppm 58Ni excess for these irons compared to those from the NC-region chemical groups (see top diagrams below). They also assert that the NC–CC dichotomy reflects the late infall to the inner protoplanetary disk of material depleted in supernova-derived nuclides such as 54Cr, 50Ti, 58Ni, and r-process Mo at a time when the Jupiter gap prevented efficient exchange between the two isotopically-distinct disk regions. Thus, the present CC region in the outer protoplanetary disk preserves for the most part the initial disk isotopic composition that was established through an enrichment in supernova-derived nuclides which were hosted by the earliest formed nebular solids, CAIs.

Ni Isotope Compositions for NC and CC Meteorites
(µ notation denotes deviation from terrestrial standards in parts per million)
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Diagrams credit: Nanne et al., Earth and Planetary Science Letters, vol. 511, p. 47 (2019 open access link)
'Origin of the non-carbonaceous—carbonaceous meteorite dichotomy'

IV. Insights From Nearby Protoplanetary Disks

Insights into the earliest stages of planetary accretion can also be made through studies of nearby protoplanetary disks. The Disk Substructures at High Angular Resolution Project (DSHARP) survey was instituted to characterize the substructures of a select group of 20 protoplanetary disks associated with young stars (Andrews et al., 2018). These stars have median ages of ~1 m.y., with some as young as a few hundred thousand years, and they have masses of ~0.2–2 M. Utilizing the very high resolution data (spatial resolution of ~5 AU) from the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, the team investigated small-scale disk features, including concentric bright rings, dark gaps, spiral patterns, and asymmetries (e.g., crescent/arc structures [vortex scenario of Pérez et al., 2018]; Isella et al., 2018), which are associated with the earliest stages of planet formation. The rings are likely formed through dust trapping at the outer edge of a pressure bump produced by one or more embedded planets (Dullemond et al., 2018; Guzmán et al., 2018). In-depth analyses of the disk features enabled the identification of potential planets with Neptune- to Jupiter-scale masses (Zhang et al., 2018; see diagram below).

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Diagram credit: Zhang et al., The Astrophysical Journal Letters, vol. 869, #2, article L47, p. 26 (2018 open access link)
'The Disk Substructures at High Angular Resolution Project (DSHARP): VII. The Planet-Disk Interactions Interpretation'

Images credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

High-resolution images of 20 protoplanetary disks obtained with the Atacama Large Millimeter/submillimeter Array (ALMA)
that are included in the Disk Substructures at High Angular Resolution Project (DSHARP) survey

The compositional dichotomy between the NC and CC regions of the protoplanetary disk has been attributed to the rapid growth of Jupiter to a size of ~20 M in ~1 m.y., at which point it became a sustained barrier to the inward streaming of dust and pebbles due to gas drag. An alternative model for the creation of this barrier was proposed by Brasser and Mojzsis (2020). Based on data from their dynamical simulations, they contend that the accretion of Jupiter to the minimum size necessary to establish an effective barrier would have taken a longer time, and therefore would not have prevented proto-Mars from growing into a much more massive body than it did. They posit instead that the protoplanetary disk had a ringed structure with coherent pressure maxima and minima (bright and dark annuli, respectively) which enabled the accretion of Jupiter and other gas giants and which provided an effective barrier near 5 AU separating the NC and CC regions.

Protoplanetary Disk Structure of the Early Solar System
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Schematic illustration credit: Brasser and Mojzsis, Nature Astronomy, in press (Jan 2020)
'The partitioning of the inner and outer Solar System by a structured protoplanetary disk'

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