First high-pressure synthesis of iron polymorph found at centre of Earth

High-pressure elemental studies have recreated the phase of iron believed to form up to 90% of the Earth’s core. Subsequent analysis of the elastic properties of this material help to explain why seismic waves from an earthquake travel 4% faster pole-to pole than across the equator.

The material properties of the Earth’s interior have a huge impact on life: movement in the mantle shifts the tectonic plates and the liquid outer core, formed of an alloy of iron and nickel, is responsible for the Earth’s magnetic field. Extreme pressures mean that this molten mixture is compressed into a solid crystal phase called ε-iron within the inner core, but little is known about the properties or wider implications of this pressurised material as neither theory nor experiment have successfully replicated the intense conditions present in the central part of the core. However, experts have theorised that certain physical properties of ε-iron could account for the Earth’s unusual seismic behaviour. ‘The Earth’s inner core behaves anisotropically, meaning it exhibits different properties in different directions,’ explains Jie (Jackie) Li, a mineral physicist at the University of Michigan, US. ‘As a result, seismic waves travel through the inner core at different speeds in the polar and equatorial directions.’