A theory that flies in the face of conventional scientific wisdom claims the Earth's inner core doesn't consist of a nickel-iron alloy known as NiFe.

Instead, the hypothesis put forward by Bin Chen of the University of Michigan and principal investigator Jie Li, an associate professor in U-M's Department of Earth and Environmental Sciences, is the inner core consists of iron carbide or Fe7C3.

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Iron carbide, also known as Cementite, is a chemical compound of iron and carbon with the formula Fe3C. By weight, iron carbide is 6.67 percent carbon and 93.3 percent iron.

The new model put forward by both men is that some two-thirds of Earth's carbon is to be found in the inner core. This will make the inner core the Earth's largest carbon reservoir.

The U-M researchers suggest iron carbide provides a good match for the density and sound velocities of Earth's inner core under relevant conditions.

The current theory which holds the inner core consists of crystalline iron alloyed with some lighter elements is unable to explain why seismic waves called S waves travel through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures.

Some scientists say the S-wave velocities could be due to the presence of a liquid. Others propose the existence of light elements (carbon, sulfur, silicon, oxygen, for example) could explain the density deficit of the Earth's core.

In putting forward their theory of an iron carbide inner core, Michigan university researchers claim the presence of iron carbide could explain anomalously slow S waves.

"This model challenges the conventional view that the Earth is highly depleted in carbon, and therefore bears on our understanding of Earth's accretion and early differentiation," said the authors of the study.

Li claims their model of a carbide inner core is compatible with existing cosmochemical, geochemical and petrological constraints, "but this provocative and speculative hypothesis still requires further testing.

"Should it hold up to various tests, the model would imply that as much as two-thirds of the planet's carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth."

If correct, the Michigan university model could help resolve observations that have perplexed scientists for decades,