Earth's outer core

Light elements

Composition

Earth's outer core cannot be entirely constituted of iron or iron-nickel alloy because their densities are higher than geophysical measurements of the density of Earth's outer core. The outer core is approximately 5 to 10 percent lower density than iron at Earth's core temperatures and pressures. Hence it has been proposed that light elements with low atomic numbers compose part of Earth's outer core, as the only feasible way to lower its density.

Although Earth's outer core is inaccessible to direct sampling, the composition of light elements can be meaningfully constrained by high-pressure experiments, calculations based on seismic measurements, models of Earth's accretion, and carbonaceous chondrite meteorite comparisons with bulk silicate Earth (BSE). As of 2023, estimates are that Earth's outer core is composed of iron along with 0 to 0.26 percent hydrogen, 0.2 percent carbon, 0.8 to 5.3 percent oxygen, 0 to 4.0 percent silicon, 1.7 percent sulfur, and 5 percent nickel by weight, and the temperature of the core-mantle boundary and the inner core boundary ranges from 4,137 to 4,300 K and from 5,400 to 6,300 K respectively.

Constraints

Accretion

The variety of light elements present in Earth's outer core is constrained in part by Earth's accretion. Namely, the light elements contained must have been abundant during Earth's formation, must be able to partition into liquid iron at low pressures, and must not volatilize and escape during Earth's accretionary process.

CI chondrites

CI chondritic meteorites are believed to contain the same planet-forming elements in the same proportions as in the early Solar System, so differences between CI meteorites and BSE can provide insights into the light element composition of Earth's outer core. For instance, the depletion of silicon in Earth's primitive mantle compared to CI meteorites may indicate that silicon was absorbed into Earth's core; however, a wide range of silicon concentrations in Earth's outer and inner core is still possible.

Implications for Earth's accretion and core formation history

Tighter constraints on the concentrations of light elements in Earth's outer core would provide a better understanding of Earth's accretion and core formation history.

Consequences for Earth's accretion

Models of Earth's accretion could be better tested if we had better constraints on light element concentrations in Earth's outer core. For example, accretionary models based on core-mantle element partitioning tend to support proto-Earths constructed from reduced, condensed, and volatile-free material, despite the possibility that oxidized material from the outer Solar System was accreted towards the conclusion of Earth's accretion. If we could better constrain the concentrations of hydrogen, oxygen, and silicon in Earth's outer core, models of Earth's accretion that match these concentrations would presumably better constrain Earth's formation.

Consequences for Earth's core formation

The depletion of siderophile elements in Earth's mantle compared to chondritic meteorites is attributed to metal-silicate reactions during formation of Earth's core. These reactions are dependent on oxygen, silicon, and sulfur, so better constraints on concentrations of these elements in Earth's outer core will help elucidate the conditions of formation of Earth's core.

In another example, the possible presence of hydrogen in Earth's outer core suggests that the accretion of Earth's water was not limited to the final stages of Earth's accretion and that water may have been absorbed into core-forming metals through a hydrous magma ocean.

Implications for Earth's magnetic field

Earth's magnetic field is driven by thermal convection and also by chemical convection, the exclusion of light elements from the inner core, which float upward within the fluid outer core while denser elements sink. This chemical convection releases gravitational energy that is then available to power the geodynamo that produces Earth's magnetic field. Carnot efficiencies with large uncertainties suggest that compositional and thermal convection contribute about 80 percent and 20 percent respectively to the power of Earth's geodynamo.

Traditionally it was thought that prior to the formation of Earth's inner core, Earth's geodynamo was mainly driven by thermal convection. However, as of 2020, claims that the thermal conductivity of iron at core temperatures and pressures is much higher than previously thought imply that core cooling was largely by conduction not convection, limiting the ability of thermal convection to drive the geodynamo. This conundrum is known as the new "core paradox." An alternative process that could have sustained Earth's geodynamo requires Earth's core to have initially been hot enough to dissolve oxygen, magnesium, silicon, and other light elements. As the Earth's core began to cool, it would become supersaturated in these light elements that would then precipitate into the lower mantle forming oxides leading to a different variant of chemical convection.

The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the solar wind. The rate of cooling by conduction and convection is uncertain, but one estimate is that the core would not be expected to freeze up for approximately 91 billion years, which is well after the Sun is expected to expand, sterilize the surface of the planet, and then burn out.