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Material deep inside Earth — thousands of kilometres down, near the planet’s core — has undergone a mysterious shift.
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Although the change occurred nearly two decades ago, between 2006 and 2008, scientists discovered it only recently, while analysing data from a pair of satellites that once measured variations in Earth’s gravity. The team thinks it might have happened when the structure of some of the rocks near the boundary between Earth’s core and mantle transformed, becoming denser.
The discovery — possible because the geological shift altered the planet’s gravitational field — is an astonishing testament to Earth-orbiting satellites. “It’s a really new observation,” says Isabelle Panet, a geophysicist at the University Gustave Eiffel in Paris. Along with lead author Charlotte Gaugne Gouranton at Paris City University and other colleagues, Panet reported the findings last month in Geophysical Research Letters1.
The work will help scientists to better understand the connections between Earth’s various layers, from its brittle crust to its solid mantle to its partially liquid core, Panet says. Connections between these layers affect where large earthquakes originate, how the planet maintains a magnetic field that protects it from solar storms and more.
Follow the leader
Panet’s team made the discovery using data from a pair of US–German satellites known as the Gravity Recovery and Climate Experiment (GRACE), which orbited Earth between 2002 and 2017. The satellites flew one in front of the other, separated by a set distance.
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When they encountered a gravitational tug from, for example, the hulking mass of a mountain range, the lead satellite would pull temporarily away from the trailing satellite — a change that could be measured and correlated with the gravitational shift. Researchers have most often used changes in the distance between the GRACE satellites to measure the displacement of masses of water on Earth, such as when groundwater disappears beneath croplands or when glaciers melt.
But, as it turns out, GRACE was also able to spot much deeper changes in Earth’s mass. Panet had already used it to look for mass changes happening hundreds of kilometres below the surface ahead of large earthquakes2,3. Then she realized she could probe even farther down, to a depth of nearly 2,900 kilometres, to the complex boundary between the core and mantle.
Panet and her colleagues spotted a strange signal in the GRACE data that peaked around 2007 and was centred off Africa's Atlantic coast. They were unable to attribute it to water shifting around on Earth’s surface, however. “So at least partially, there has to be an origin within the solid Earth,” Panet says. “It has to come from very deep.”
