The Earth’s core consists of a solid inner and a fluid outer region and extends from a depth of about 2900 km (1800 mi), the lower edge of the mantle, to about 6400 km (3800 mi), the center of the planet. At about 5100 km (3100 mi) down, the core is solid iron by virtue of the enormous pressure (3.3 million atmospheres or 3.3 gigapascals) at that depth. The fluid outer core consists of about 90% iron and 10% other elements. Electrical currents and fluid motions in the outer core generate the Earth's magnetic field by a mechanism known as the geodynamo. See also: Earth interior; Geodynamo; Geoelectricity; Geomagnetism
It is not possible to probe the Earth’s core directly to determine its temperature. However, it is possible to estimate its temperature in the laboratory with a high-pressure apparatus known as a diamond-anvil cell. In April 2013, scientists using a refined technique for observing melting iron under extreme pressure reported in the journal Science that the temperature at the outer boundary of the solid core is about 6000°C, which is considered to be a more accurate estimate and about 1000°C higher than previously determined. Those results suggest that the temperature of the Earth’s core is roughly the same as that of the surface of the Sun. See also: High-pressure mineral synthesis
In the diamond-anvil apparatus, a sample very small in area (μm2) is placed between two anvil-shaped diamonds. Because pressure is equal to the force per unit area, a force applied to this small area can result in very high pressure. In the experiment, a sample of iron powder placed between the anvils was compressed to the target pressure and then heated to the melting point using two laser beams. The novel aspect of the experiment was that the melting was detected by x-ray diffraction, a highly accurate method whereby changes in the diffraction patterns of incidental x-rays indicate when iron becomes molten. An experiment of this type takes about 1 second. Using this method, the researchers determined the melting point of iron at 2.2 million atm (220 GPa) and then extrapolated to the pressure at the molten–solid core boundary (that is, 3.3 million atm), where the melting temperature was calculated to be 6230 ± 500 kelvin (6000°C). See also: Laser; Pressure; X-ray diffraction