Earths core is believed to be mostly an alloy of iron and nickel. The density indicates that it also contains a significant amount of lighter elements. Elements such as hydrogen would be stable in the Earths core, however the conditions at the formation of the core would not be suitable for its inclusion. Carbon is a very likely constituent of the core. Preferential partitioning of the carbon isotopeC into the metallic core, during its formation, may explain why there seems to be more C on the surface and mantle of the Earth compared to other solar system bodies (−5‰ compared to -20‰). The difference can also help to predict the value of the carbon proportion of the core.
The outer core has a density around 11 cm, and a mass of 1.3×10kg. It contains roughly 10 kg of carbon.
Carbon dissolved in liquid iron affect the solution of other elements. Dissolved carbon changes lead from a siderophile to a lithophile. It has the opposite effect on tungsten and molybdenum, causing more tungsten or molybdenum to dissolve in the metallic phase. The measured amounts of these elements in the rocks compared to the Solar System can be explained by a 0.6% carbon composition of the core.
The inner core is about 1221 km in radius. It has a density of 13 g cm, and a total mass of 9×10 kg and a surface area of 18,000,000 square kilometers. Experiments with mixtures under pressure and temperature attempt to reproduce the known properties of the inner and outer core. Carbides are among the first to precipitate from a molten metal mix, and so the inner core may be mostly iron carbides, FeC or FeC. At atmospheric pressure (100 kPa) the iron-FeC eutectic point is at 4.1% carbon. This percentage decreases as pressure increases to around 50 GPa. Above that pressure the percentage of carbon at the eutectic increases. The pressure on the inner core ranges from 330 GPa to 360 GPa at the centre of the Earth. The temperature at the inner core surface is about 6000 K. The material of the inner core must be stable at the pressure and temperature found there, and more dense than that of the outer core liquid. Extrapolations show that either FeC or FeC match the requirements. FeC is 8.4% carbon, and FeC is 6.7% carbon. The inner core is growing by about 1 mm per year, or adding about 18 cubic kilometres per year. This is about 18×10kg of carbon added to the inner core every year. It contains about 8×10 kg of carbon.