Planetary core

A planetary core consists of the innermost layers of a planet. Cores may be entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% (the Moon) to 85% of a planet's radius (Mercury). Gas giants also have cores, though the composition of these are still a matter of debate and range in possible composition from traditional stony/iron, to ice or to fluid metallic hydrogen. Gas giant cores are proportionally much smaller than those of terrestrial planets, though they can be considerably larger than the Earth's nevertheless; Jupiter's is 10–30 times heavier than Earth, and exoplanet HD149026 b may have a core 100 times the mass of the Earth. Planetary cores are challenging to study because they are impossible to reach by drill and there are almost no samples that are definitively from the core. Thus, they are studied via indirect techniques such as seismology, mineral physics, and planetary dynamics.

Discovery

Earth's core

In 1797, Henry Cavendish calculated the average density of the Earth to be 5.48 times the density of water (later refined to 5.53), which led to the accepted belief that the Earth was much denser in its interior. Following the discovery of iron meteorites, Emil Wiechert in 1898 postulated that the Earth had a similar bulk composition to iron meteorites, but the iron had settled to the interior of the Earth, and later represented this by integrating the bulk density of the Earth with the missing iron and nickel as a core. The first detection of Earth's core occurred in 1906 by Richard Dixon Oldham upon discovery of the P-wave shadow zone; the liquid outer core. By 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core.

Moon's core The internal structure of the Moon was characterized in 1974 using seismic data collected by the Apollo missions of moonquakes. The Moon's core has a radius of 300 km. The Moon's iron core has a liquid outer layer that makes up 60% of the volume of the core, with a solid inner core.

Cores of the rocky planets The cores of the rocky planets were initially characterized by analyzing data from spacecraft, such as NASA's Mariner 10 that flew by Mercury and Venus to observe their surface characteristics. The cores of other planets cannot be measured using seismometers on their surface, so instead they have to be inferred based on calculations from these fly-by observation. Mass and size can provide a first-order calculation of the components that make up the interior of a planetary body. The structure of rocky planets is constrained by the average density of a planet and its moment of inertia. The moment of inertia for a differentiated planet is less than 0.4, because the density of the planet is concentrated in the center. Mercury has a moment of inertia of 0.346, which is evidence for a core. Conservation of energy calculations as well as magnetic field measurements can also constrain composition, and surface geology of the planets can characterize differentiation of the body since its accretion. Mercury, Venus, and Mars’ cores are about 75%, 50%, and 40% of their radius respectively.

Formation

Accretion Planetary systems form from flattened disks of dust and gas that accrete rapidly (within thousands of years) into planetesimals around 10 km in diameter. From here gravity takes over to produce Moon- to Mars-sized planetary embryos over 0.1–1 million years, and these develop into planetary bodies over an additional 10–100 million years. Jupiter and Saturn most likely formed around previously existing rocky and/or icy bodies, rendering these previous primordial planets into gas-giant cores. This is the planetary core accretion model of planet formation.

Differentiation Planetary differentiation is broadly defined as the development from one thing to many things; homogeneous body to several heterogeneous components. The hafnium-182/tungsten-182 isotopic system has a half-life of 9 million years, and is approximated as an extinct system after 45 million years. Hafnium is a lithophile element and tungsten is siderophile element. Thus if metal segregation (between the Earth's core and mantle) occurred in under 45 million years, silicate reservoirs develop positive Hf/W anomalies, and metal reservoirs acquire negative anomalies relative to undifferentiated chondrite material. The observed Hf/W ratios in iron meteorites constrain metal segregation to under 5 million years, the Earth's mantle Hf/W ratio places Earth's core as having segregated within 25 million years. Several factors control segregation of a metal core including the crystallization of perovskite. Crystallization of perovskite in an early magma ocean is an oxidation process and may drive the production and extraction of iron metal from an original silicate melt.

Core merging and impacts Impacts between planet-sized bodies in the early Solar System are important aspects in the formation and growth of planets and planetary cores.

Earth–Moon system The giant impact hypothesis states that an impact between a theoretical Mars-sized planet Theia and the early Earth formed the modern Earth and Moon. During this impact the majority of the iron from Theia and the Earth became incorporated into the Earth's core.

Mars Core merging between the proto-Mars and another differentiated protoplanet could have been as fast as 1000 years or as slow as 300,000 years (depending on viscosity of both cores).

Chemistry

Determining primary composition – Earth Using the chondritic reference model and combining known compositions of the crust and mantle, the unknown component, the composition of the inner and outer core, can be determined: 85% Fe, 5% Ni, 0.9% Cr, 0.25% Co, and all other refractory metals at very low concentration. This leaves Earth's core with a 5–10% weight deficit for the outer core, and a 4–5% weight deficit for the inner core; which is attributed to lighter elements that should be cosmically abundant and are iron-soluble; H, O, C, S, P, and Si. Earth's core contains half the Earth's vanadium and chromium, and may contain considerable niobium and tantalum. Earth's core is depleted in germanium and gallium.

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