A pooling of lighter elements at the outer edge of Earth's core could help explain how the planet formed. Credit: M. Kulyk/SPL

Seismologists have conducted a planetary ultrasound and found evidence that light elements such as oxygen and sulphur collect at the edge of Earth's core. The finding, if confirmed, could help to explain what keeps Earth's magnetic field going, and might also shed light on how the planet formed billions of years ago. The work appears in this week's issue of Nature1.

Many geologists believe that Earth's core has a solid centre, surrounded by a liquid outer core some 7,000 kilometres across. The idea is supported by studies of seismic waves from earthquakes passing near the Earth's centre. By measuring the speed of the waves as they pass through the core, researchers have estimated that the core's density is similar to that of iron.

Similar but not identical, says George Helffrich, a seismologist at the University of Bristol, UK. "There's been a feeling that there's a light element in addition to iron that makes the core less dense," he says. Many researchers believe that this element, or elements, might be left over from the planet's formation around 4.5 billion years ago.

To take a closer look at the core, Helffrich and his co-author Satoshi Kaneshima, a planetary scientist at Kyushu University in Fukuoka City, Japan, took measurements of three earthquakes from detectors on the opposite side of the planet to where they began. By watching one earthquake in South America using an array of hundreds of detectors in Japan, and two more in Fiji in the South Pacific using a similar array in Europe, the team was able to see in exquisite detail how seismic waves passed through the core.

Measuring multiple reflections of the waves as they bounced along the boundary between core and the mantle — the layer of molten rock around the core — the duo showed that the outermost 300 kilometres of the core are significantly less dense than the rest. The apparent difference is consistent with a level of 3–5% sulphur and oxygen at the core's edge, says Helffrich.

Planetary dynamo

Helffrich says that the lighter elements may be getting concentrated in the outer core as the inner core solidifies. What's more, he speculates that as the light elements rise to the surface of the core, their motion drives a dynamo that powers Earth's protective magnetic field. Researchers have worried in the past about what keeps the dynamo going, says Helffrich. But the rise of light elements "releases a huge amount of gravitational potential energy".

"The observations are really quite impressive," says Bruce Buffett, a planetary scientist at the University of California, Berkeley. "But I had more trouble imagining how the layer he talks about developed."

Buffett says that if oxygen and sulphur were really being squeezed from the planet's inner core, he would expect them to re-dissolve in the liquid outer core, rather than pooling in its outermost 300 kilometres. David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, says that interactions between the core and the mantle might also explain the apparent pooling, although he admits that there aren't enough data to allow researchers to draw a definite conclusion either way.

Models of the Earth's core may need to be modified, says Buffett, and further observations could help to clarify Helffrich's result. "The fact that you can't think of an explanation right away doesn't mean that it's not right," says Buffett.