The pressure is on some 3000 kilometres beneath our feet, 3.5 Mbar to be precise. Not only that, it is hot, very hot, 7000 kelvin hot. Replicating such conditions in the laboratory in order to study the elemental composition of the Earth's core is impossible. However, understanding how the bulk iron at the core is packed together and with what other lighter elements is important to understanding how the earth formed, the origins of its magnetic field, and how it might change.

Most of our knowledge about the core comes from seismic observations and studies of iron-rich meteorites. This evidence suggests that the core is mainly iron, but is overall less dense than iron, so must also contain lighter elements, such as oxygen, silicon, sulfur, hydrogen and magnesium, with nickel thought to comprise between 5 and 15% of the mass. Igor Abrikosov, a theoretical physicist at Linköping University in Sweden, adds that most studies assumed that the alloy elements were not very important for the structural and elastic properties of the core.

However, new experimental and theoretical evidence could overturn this notion and suggest that iron is not hexagonal close packed despite other structures being less stable.

Colleague Leonid Dubrovinsky of the University of Bayreuth, Germany, is using a diamond anvil cell to recreate extreme pressures. The state of the art in this technology combined with synchrotron radiation and ever more powerful models is taking scientists closer to the core.

Nickel, silicon, oxygen, and magnesium are likely to play a role in iron packing at the core despite the "standard model" of iron packing. Recent experiments have shown that at very high pressures magnesium atoms are compressed to such an extent that they can fit easily into iron structures. Moreover, nickel can pack with iron in a 'face centred cubic' structure.

"At high pressures the magnetism is squeezed out of the other structures and they all have similar stability," explains Abrikosov, face centred cubic and body centred cubic structures cannot be ruled out and that all of these structures are energetically possible. "The standard model of the iron core is dead," he claims.

These studies have implications for studies of how seismic waves travel through the core and the production of the earth's magnetic field. For scientists studying the Earth's core it is time to go back to the drawing board and rethink what lies underneath our feet.
It is not quite correct to conclude that the new results represent something of a circular argument, says Dubrovinsky, "Seismic observations are our "experimental data"," he explains, "We, the geophysical community, could not explain the data based on the properties of a pure iron core, because the density is too high and the ratio of the speed of propagational and longitudinal sound velocities in the inner core is very high. So, we add 'geochemical' and 'cosmochemical'  arguments and come to conclusion that the core should contain some additional element(s) lighter then iron." He points out that geophysicists also know that it has to contain about 5-15% nickel. "Our  group  studied such alloys (both experimentally and theoretically) and found that even a small amount of Ni, Si, O, etc. could significantly change properties and even structure  at such extreme conditions."

Related links:

• Dr Leonid Dubrovinsky's Page
• Professor Igor Abrikosov's Page
• EuroMinScI 

Article by David Bradley