Earth’s Core May Not Just Be Made Of Iron

Studying earthquakes allows us to gauge what the interior of our planet looks likes. As our measurements improve, so does our understanding of what lies underneath our feet. Now, a new study has suggested that the composition of the core might be more complex than previously thought.

The structure of Earth’s core was first described in 1936, and for a long time we thought it was made exclusively of metal-loving heavy elements, mainly iron. But Tatsuya Sakamaki of Tohoku University in Japan and his colleagues have demonstrated Earth’s core cannot be solely made of pure iron (or heavy elements) as it is 4 to 5 percent less dense than expected. They suggest that the core contains a large reservoir of lighter elements like hydrogen, silicon, or sulfur, which were trapped there when Earth was still young. Their findings are published in Science Advances.

Studying the interior of the planet using earthquakes is similar to having an ultrasound. The seismic waves’ velocity changes when it goes from one layer to the next, and those differences can be measured.

The wave velocity at the core is consistent with a large (about 3,400 kilometers or 2,100 miles in radius) sphere, with a liquid outer shell and a solid inner part. The heat and pressure in the inner core are so high that iron is crystallized in a way that cannot be seen on the surface, forming the hexagonal close-pack iron (hpc-Fe).

Testing the properties of this peculiar state is very difficult, so scientists need to put a small sample of pure iron between two microscopic diamond anvils. The anvils generate temperatures up to 3,000 K (2,730°C, 4,940°F) and a pressure of over 1.6 million atmospheres.

They looked at the velocity of waves going through the sample and compared it to the value observed in nature, and they saw that the core has a velocity between 4 and 10 percent smaller than the pure sample. This is an indication that the core has a mixed composition.

“We believe that this is a great improvement. Obviously, our new data can provide better extrapolation, and constrain the core composition by comparison of hcp-Fe with Earth’s inner core,” Dr. Sakamaki told IFLScience.

The presence of a large fraction of lighter elements in the core complicates our understanding of its formation. Earth’s core is supposed to have formed when most of the heavy material migrated towards the center. Lighter materials should have floated upwards, but somehow it seems some of them were trapped in the nucleus.

The next step for the group will be to create a mixed sample to work out the best composition for the core. “We have already started the experiment. So, we’d like to publish new data on hcp-Fe + light elements system as soon as possible,” added Dr. Sakamaki.