Earth’s core is known to be metallic, with a density of about 9.90 Mg·m⁻³ at the core-mantle boundary and as such is substantially denser than the surrounding mantle (5.56 Mg·m⁻³ at the core-mantle boundary; Dziewonski and Anderson 1981). Comparison with cosmic abundances suggests that the core is predominantly Fe with around 5% Ni (Allègre et al. 1995; McDonough 2003) and 8–12% of one or more light elements (Birch 1952). The latter conclusion comes from the observation that the core is appreciably less dense than pure Fe or Fe-Ni alloys under any plausible...

Significance: Seismic studies revealed that shear wave ( S wave) travels through the inner core at an anomalously low speed, thus challenging the notion of its solidity. Here we show that for the candidate inner core component Fe ₇ C ₃ , shear softening associated with a pressure-induced spin-pairing transition leads to exceptionally low S -wave velocity ( v _S ) in its low-spin and nonmagnetic phase. An Fe ₇ C ₃ -dominant inner core would match seismic observations and imply a major carbon reservoir in Earth’s deepest interior.

Earth’s precursor material lost most carbon through sublimation in the nebular disk shortly after the Sun’s birth.

Several features of Earth owe their origin to processes occurring during and shortly following Earth formation. Collisions with planetary embryos caused substantial melting of the growing Earth, leading to prolonged core formation, atmosphere outgassing, and deepening of the magma ocean as Earth grew. Mantle noble gas isotopic compositions and the mantle abundance of elements that partition into the core record this very early Earth differentiation. In contrast, the elements that are not involved in either core or atmosphere formation show surprisingly muted evidence of the fractionation expected during magma ocean crystallization, and even this minimal evidence for early intramantle differentiation appears to have been erased by mantle convection within ∼1.5 billion years of Earth formation. By 4.36 Ga, Earth's surface and shallow interior had reached temperatures similar to those of the present Earth, and mantle melting, and perhaps plate subduction, was producing crustal rock types similar to those seen today. Remnants of early Earth differentiation may still exist in the deep mantle and continue to influence patterns of large-scale mantle convection, sequestration of some trace elements, geomagnetic reversals, vertical motions of continents, and hot-spot volcanism.

Significance: Habitable rocky worlds require a supply of essential volatile elements (C, H, N, and S). These are plentiful in early solar systems but depleted during processes leading to planet formation. Here, evidence for loss during differentiation of small precursor bodies (planetesimals) is derived from iron meteorites, which are samples of planetesimal cores. Reconstruction of the C and S contents of planetesimal cores indicates severe C depletions compared to inferred original planetesimal compositions. Modeling of depletion processes shows that preferential loss of C compared to S is transferred to cores during differentiation. Iron meteorites preserve evidence of a key devolatilization stage in the formation of habitable planets and suggest pervasive carbon loss is likely associated with the birth of terrestrial worlds.

The Leibfried-Schlömann (LS) equation, a commonly assumed model for the pressure dependence of thermal conductivity $\Lambda$, is tested by measurements on compressed ${\mathrm{H}}_2$O using a combination of the time-domain thermoreflectance method with the diamond anvil cell technique. The thermal conductivity of ice VII increases by an order of magnitude between 2 and 22 GPa, reaching $\Lambda \approx 25$ W m${}^{-1}$ K${}^{-1}$. Over a large compression range of $\approx$4%–33%, the LS equation describes the pressure dependence of $\Lambda$ of ice VII to better than 20%.