Protoplanets growing by pebble accretion capture massive hydrogen-helium atmospheres from the surrounding nebula. Pebbles settling through such atmospheres continuously release gravitational potential energy, heating both the atmosphere and the pebbles. Under these conditions, atmosphere temperatures above large protoplanets are sufficiently high to melt silicate pebbles, support long-lived magma oceans, and drive evaporation of volatile species. Because these atmospheres are open to the nebula, some amount of volatile loss is inevitable. Here we analyze the depletion of moderately volatile elements from terrestrial protoplanets undergoing pebble accretion. We consider chondrule-size silicate pebbles enriched in Si, Na, K, and Zn relative to Earth, settling through a hydrogen-helium-rich atmosphere containing these same volatiles. We show that volatile depletion depends critically on protoplanet mass, the timescale of atmosphere exhaust, and the pebble composition. The protoplanetary mass effect is especially strong. For exhaust timescales of a few centuries, we find that substantial depletion of Zn begins around 0.4 Earth mass, and for Na and K around 0.6 Earth mass. Shorter exhaust timescales deplete these elements at somewhat smaller protoplanet masses. Using a pebble composition that matches Earth’s major element abundances, best agreement with Earth’s depletion trend for moderately volatile elements is found by merging a large (approximately 0.7 Earth mass) volatile-depleted target protoplanet with one or more smaller, less-depleted impactors.

Pebble accretion is an efficient mechanism for early terrestrial protoplanet growth and differentiation. Metal-silicate partitioning of moderately siderophile elements offers constraints on the role of pebble accretion in Earth's formation and the segregation of its core. Here, we determine pebble accretion properties of the proto-Earth that are consistent with metal-silicate partitioning measurements and siderophile abundances in the mantle and core. We combine a pebble accretion model that includes mass balances for siderophile abundances in the mantle and core of a growing terrestrial protoplanet with experimentally-determined partition functions for seven moderately siderophile elements: Ni, Co, V, Cr, Mo, Mn, and W. Mantle and core abundances of these elements during pebble accretion are calculated, as well as changes to their abundances following the addition of large and giant impactors built with pebbles. Model results are compared to the estimated abundances of these elements in Earth's primitive mantle and core. We find that metal-silicate partitioning of these elements is especially sensitive to the total mass of accreted pebbles. Best fits to primitive mantle and core siderophile abundances are found in cases where the proto-Earth accreted with pebbles to approximately 0.6 times its present mass under slightly reducing conditions, then added the remaining mass via one or more impactors with the same composition. We also find that pebbles consisting of chondritic components (chondrules, metal grains, AOAs, and CAIs) generally yield better partitioning results compared to pebbles made from chondrites.

Pebble accretion provides new insights into Earth's building blocks and early protoplanetary disk conditions. Here, we show that mixtures of chondritic components: metal grains, chondrules, calcium-aluminum-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs) match Earth's major element composition (Fe, Ni, Si, Mg, Ca, Al, O) within uncertainties, whereas no combination of chondrites and iron meteorites does. Our best fits also match the epsilon 54Cr and epsilon 50Ti values of Earth precisely, whereas the best fits for chondrites, or components with a high proportion of E chondrules, fails to match Earth. In contrast to some previous studies, our best-fitting component mixture is predominantly carbonaceous, rather than enstatite chondrules. It also includes 15 wt% of early-formed refractory inclusions (CAIs + AOAs), which is similar to that found in some C chondrites (CO, CV, CK), but notably higher than NC chondrites. High abundances of refractory materials is lacking in NC chondrites, because they formed after the majority of refractory grains were either drawn into the Sun or incorporated into terrestrial protoplanets via pebble accretion. We show that combinations of Stokes numbers of chondritic components build 0.35-0.7 Earth masses in 2 My in the Hill regime accretion, for a typical pebble column density of 1.2 kg/m2 at 1 au. However, a larger or smaller column density leads to super-Earth or moon-mass bodies, respectively. Our calculations also demonstrate that a few My of pebble accretion with these components yields a total protoplanet mass inside 1 au exceeding the combined masses of Earth, Moon, Venus, and Mercury. Accordingly, we conclude that pebble accretion is a viable mechanism to build Earth and its major element composition from primitive chondritic components within the solar nebula lifetime.

About 40% of Earth's surface is covered by continental crust. It has been found that the global detrital zircon age distribution is dominated by multiple periodicities from <100 Myr to ∼800 Myr, suggesting that continental crust may have been produced periodically. However, what causes the periodic growth of continental crust remains unclear. Continental crust is produced mainly by arc magmatism at subduction zones and plume-induced magmatism. With 3D global mantle convection models, we investigate the long-term evolution of heat flux of mantle plumes and surface flow velocity, which can be respectively related to plume-induced magmatism and arc magmatism. About 10-30 plumes are present in our models, with significant spatial and temporal variations of heat flux for individual plumes. Both the temporal variations of the total plume heat flux and the root-mean-square (rms) of the surface velocity in our models have periodicities that decrease with the vigor of mantle convection. When extrapolated to conditions with Earth-like vigor of convection, the total plume heat flux and the rms of the surface velocity vary with periodic cycles of ∼90-200 Myr and ∼300 Myr, respectively. Therefore, the ∼90-200 Myr and the ∼300 Myr periodic cycles in the growth of continental crust as suggested by the global detrital zircon age distribution may be respectively caused by plume-induced magmatism and arc magmatism.

Supplementary Figure S1. B cells in human PDAC. Supplementary Figure S2. B cells and FcRgamma-positive cells regulate murine PDAC. Supplementary Figure S3. Leukocytes and BTK in human and murine PDAC. Supplementary Figure S4. Macrophage PI3Kgamma activates BTK to promote PDAC progression. Supplementary Figure S5. BTK and PI3Kgamma-inhibition decrease PDAC growth. Supplementary Figure S6. BTK inhibition improves gemcitabine response in late-stage PDAC.

We combine calculations of pebble accretion and accretion by large and giant impacts to quantify the effects of pebbles on the hafnium-tungsten system during Earth formation. Our models include an early pebble accretion phase lasting 4–6 Myr with a global magma ocean and core segregation, a 20–50 Myr phase of large impacts, and a late giant impact representing the Moon-forming event. We consider various mass additions during each accretion phase, vary the metal-silicate partition coefficient for tungsten over a wide range, and track Hf180, Hf182, W182 and W184 in proto-Earth and impactor models over time using standard chondritic values for these isotopes in the pebbles. We find that an early phase of pebble accretion is compatible with the tungsten anomaly of Earth's early mantle as well as the present-day Hf/W ratio, but under restricted conditions. In particular, the pebble mass of proto-Earth is limited to 0.7 Earth masses or less, the average metal-silicate partition coefficient for tungsten is 30–50, and because the metal-silicate equilibration efficiency for giant impacts is low, the equilibration efficiency must be high for the large impactors.

We first review the composition of the core: primarily Fe, Ni, and the case for light elements. With the assumption that the entire outer core is convecting so that the geotherm lies along the adiabat, and with the inner core boundary temperature fixed at the Fe alloy melting temperature, we derive the core geotherm in terms of reasonably constrained geophysical parameters. We then examine the physics of the important, but not well-constrained, core transport properties. Thermodynamics allows us to estimate the age of the inner core, and the power available for the geodynamo, as well as to frame a discussion of stably stratified layers in the core. We conclude the chapter with a discussion of inner core mineralogy: its stable phase, and elastic and anelastic properties.

We begin this chapter with a description of the physical environment of the outer core, focusing on its most distinctive attributes. We then characterize the outer core environment using dimensionless parameters defined in terms of the physical and chemical properties that govern its dynamics. Special emphasis is given to thermochemical buoyancy, the primary driver for outer core convection. Next, we describe the multiple types of laminar and wave-like flows in the outer core, and what their observations imply about the structure and state of the core. We then apply the results of laboratory and numerical experiments to infer the properties of rotating convection in the outer core. Finally, we show how numerical dynamo models are used to link outer core convection to the state of the core described in Chapter 2, the seismic structure of the core described in Chapters 1, 5, and 6, the dynamics of the inner core described in Chapter 7, and the evolution of the core and the geodynamo described in Chapters 3 and 8.

We present a model for terrestrial planet formation by pebble accretion, focusing on core segregation in the early Earth. Our results indicate that if the proto-Earth and the Moon-forming impactor Theia grew by pebble accretion, core-forming metals in each body segregated from mantle-forming silicates within the first few million years of solar system history, while both were enveloped in atmospheres composed of nebular gas. Thermal blanketing by their energy-absorbing atmospheres, heat produced by radioactive decay of aluminum-26, and gravitational energy released by metal segregation resulted in very high internal temperatures, such that the mantle and core of both bodies experienced partial or total melting during accretion. We calculate pressure-temperature conditions where the core-forming metals are predicted to have segregated from magma ocean silicates under pebble accretion. Twobody combinations of these conditions, representing the merger of proto-Earth and Theia, yield average segregation pressures and temperatures that are similar to core segregation conditions previously inferred for impact-driven Earth accretion constrained by metal-silicate partitioning of siderophile elements. (c) 2022 Elsevier B.V. All rights reserved.

The physical properties and their lateral variation in 200–400 km thick regions surrounding Earth's core-mantle boundary and inner core boundary are important to the functioning and evolution of Earth's dynamo. The complexity of structure in the lowermost mantle rivals that near its tectonically active surface, but improvements in its imaging may allow better estimates of heat transport across the core-mantle boundary. Some, but not all, seismic observations suggest the existence of 100–200 km thick stably stratified regions at both the top and bottom of the liquid outer core, both of which may affect magnetic secular variations. Seismically resolvable large-scale topography on the core-mantle and inner core boundaries can affect fluid flow in the outer core, inducing signals in the magnetic field and length of day. Lateral variations in seismic velocities in the lowermost outer core and solid inner core, including variations in the anisotropy of the inner core, may reflect lateral variations in the solidification and melting of the inner core.

A number of studies have used one or several isotopic systems to estimate the origin of Earth's volatiles. Several volatile sources and subsequent volatile loss are generally considered in these models. In this communication, we use a forward model based on the presumed formation history of Earth to simultaneously constrain the sources for seven volatile elements (H, He, N, Ne, Ar, Kr and Xe). We consider the three potential volatile sources: nebular ingassing, chondrites, and comets. Sinks include loss by early hydrodynamic escape and long-term loss of ionized Xe. 10,000 Monte Carlo simulations generate several hundred solutions that match the abundance of all these elements to within a factor of -2 of the present-day Earth values, as well as critical isotope ratios (N-515, Ne-20/Ne-22, Ar-36/Ar-38, Kr and Xe). The source of volatiles is distinctly different for different elements. Our results indicate that there was a large excess of H, He and Ne supplied by nebular ingassing, with sub-sequent massive loss (> 99% He and Ne) by early hydrodynamic escape. Kr and Xe were supplied primarily by comets, and N was supplied almost entirely (> 98%) by chondrites. The source of Ar is mixed, with 50-90% chondrites and the remainder from ingassing. Solutions with nitrogen isotope ratios that match Earth values require a > 92% E chondrite source. 515N values are far too high using a C chondrite source (> 20 &parts per thousand vs AIR). Our results suggest late addition of 7.5 +/- 0.7 x 1021 g comets, 8.3 +/- 5.6 x 1024 g C chondrites and 1.2 +/- 0.5 x 1026 g E chondrites. The Kr isotope pattern should follow that of cometary input, given that > 90% of all Kr comes from a comet source. Our results fit the measured values of Comet 67P/C-G within error. Xe isotope data can be matched to Earth values using solar isotope values as an assumed cometary source if we assume a large mass-dependent enrichment factor during loss of ionized Xe to space. The measured isotope data for Comet 67P/C-G have both light and heavy Xe isotope ratios that do not match the Earth atmosphere data, suggesting that this comet is not, in our model, representative of the Earth cometary Xe source. The amount of ingassed H is critically dependent on oxygen fugacity, ranging from 11 to 22 times the present day ocean amount for presumed low f(O2) of the early magma ocean. Even at low f(O2) values, most of the water is dissolved as H2O rather than H. A large hydrogen isotope fractionation during hydrodynamic escape (a = 1.6 to 1.7) is required to explain the present-day D/H values. This a value corresponds to equilibrium between H2 and H2O at-300 & DEG;C or loss of atomic H to space. Loss of hydrogen early in Earth's history easily accounts the relatively high f(O2) of Earth's present-day mantle.(c) 2022 Elsevier Ltd. All rights reserved.

Earth's gross stratification into a silicate mantle and denser, fluid, iron core are apparent from its bulk density, moment of inertia, and spatial and temporal variations of its magnetic field. The discovery of a solid inner core and the resolution of its structure at spatial scales as small as 10 km have primarily come from the observation and interpretation of seismic body waves and free oscillations. From these, the average radial physical properties of Earth's core can be described by its elastic moduli, densities, viscoelasticity and rheology, fluctuations about these average properties, and their vertical gradients in the lower mantle, outer core, and inner core.

The solid inner core represents only 0.7% of our planet's total volume, and its surface area only 3.7% of Earth's surface area. Although its elastic structure is relatively poorly sampled by seismic body waves, it is of great interest to understanding the evolution of the core. Compositional convection in the outer core driven by inner core solidification is thought to be the most important driving force for sustaining the geodynamo. Challenges remain in understanding how the complex structure of the inner core revealed from seismology is related to the freezing, melting, and deformation of the inner core. Yet to be well understood are depth and laterally varying elastic anisotropy, high attenuation, a hemispherical dichotomy in its structure, variations in its rotation relative to the mantle, and an anomalously low shear modulus.

The materials processes of solidification, deformation, and annealing, along with the geodynamics and material properties of the core Fe alloy, determine the inner core microstructure (including grain size) and texture. We first review solidification, including nucleation, translation, and the F layer. We then examine geodynamic sources of stress/strain and review dislocation and diffusion creep. At core temperatures, deformed materials can easily anneal, which includes the competing processes of recovery, and recrystallization and grain growth. The stress and grain size, along with the stable phase, self-diffusivity, shear modulus, and available slip systems of Fe, determine the deformation mechanism and hence the solid-state viscosity. The viscosity in part controls the geodynamics however, and material properties are still uncertain. Modeling that includes the relevant materials processes, correct geodynamics, and accurate material properties offers the best prospect for reproducing the seismically inferred inner core elastic and anelastic properties.

This chapter summarizes the properties of the geodynamo and the geomagnetic field it produces. It includes discussions of the physical ingredients necessary for planetary dynamos in general, the magnetic induction equation and its application to magnetic field generation by fluid motions in the outer core, the magnetic Reynolds number, the key dimensionless parameter controlling geodynamo behavior, energy pathways in the core that maintain the geodynamo, as well as descriptions of the present-day geomagnetic field and the ancient paleomagnetic field as they pertain to the geodynamo process. We begin with a few basic definitions, some nomenclature, and the rationale for the dynamo mechanism. Next, we examine the geomagnetic field on the core-mantle boundary and its variation over time scales ranging from decades to hundreds of million years. The final parts of this chapter summarize the basics of dynamo theory, including a conceptual model of a fluid dynamo, a simple kinematic dynamo, a laboratory dynamo experiment, estimates of the energy dissipated by the geodynamo, and images of the fluid motions in the outer core derived from measurements of the secular variation of the geomagnetic field.

Mars lacks ongoing tectonic activities such as volcanism and mountain-building processes. Modern plate tectonic movements on the Earth's surface are driven primarily by the descent of subducted slabs into the mantle. Slab crust made of dense eclogite metamorphosed from the Mid-Ocean Ridge Basalt provides an important driving force for slab subduction. Thus, mantle convection inside Mars can be hindered if the density contrast between Martian slab crust and the ambient Martian mantle is sufficiently smaller than that of Earth. To evaluate this hypothesis, we carried out high pressure-temperature phase equilibrium experiments on three different Martian basalts: Yamato 980459, NWA 8159, and GUSEV basalt (Humphrey). The GUSEV basalt and NWA 8159 undergo partial or complete melting along the Martian areotherm due to their high Fe content, suggesting that both compositions are geochemically evolved. Yamato 980459, the nearly primitive Martian basalt, on the other hand, would transform to a low-density eclogite at a depth of ∼250 km. The density contrast between a Martian crustal slab made of Yamato 980459, and the ambient Martian mantle is much smaller than that for Earth's mantle. Calculated slab sinking torques and velocities further suggest that sustained buoyancy-driven subduction of thin slabs was unlikely early in Martian history. Additional experiments exploring wider composition and pressure-temperature ranges are needed to fully understand the consequences of Martian mantle compositions and cooling history for the tectonic history of Mars.

Volatiles from the solar nebula are known to be present in Earth's deep mantle. The core also may contain solar nebula‐derived volatiles, but in unknown amounts. Here we use calculations of volatile ingassing and degassing to estimate the abundance of primordial 3 He now in the core and track the rate of 3 He exchange between the core and mantle through Earth history. We apply an ingassing model that includes a silicate magma ocean and an iron‐rich proto‐core coupled to a nebular atmosphere of solar composition to calculate the amounts of 3 He acquired by the mantle and core during accretion and core formation. Using experimentally determined partitioning between core‐forming metals and silicate magma, we find that dissolution from the nebular atmosphere deposits one or more petagrams of 3 He into the proto‐core. Following accretion, 3 He exchange depends on the convective history of the coupled core‐mantle system. We combine determinations of the present‐day surface 3 He flux with estimates of the present‐day mantle 3 He abundance, mantle and core heat fluxes, and our ingassed 3 He abundances in a convective degassing model. According to this model, the mantle 3 He abundance is evolving toward a statistical steady state, in which surface losses are compensated by enrichments from the core.

In this chapter, we present current theories on how the Earth and its core were formed, and how the core evolved between the time of formation and the present day. We begin by summarizing the enormous amounts of energy expended in Earth's formation. We briefly discuss the genesis of core-forming elements and the origin and early evolution of the solar system, setting the stage for a discussion of Earth's accretion, core formation models, and the geochemical evidence behind them. We argue that the earliest history of the core was largely over written by the catastrophic Moon-forming event. The next part deals with evolution of the core after its formation, with a focus on the timing of inner core nucleation. We then consider the long-term history of the geodynamo, including dynamo initiation.