In 2007, something strange happened over the eastern Atlantic Ocean. According to satellites orbiting Earth, our planet’s gravity field developed a continent-scale anomaly before subsiding to its original state.
The odd event, undetectable to humans on the surface, has only just been discovered in data collected by gravity-monitoring satellites – and the cause was a tremendous redistribution of mass far beneath us, a new analysis suggests.
According to a team led by geophysicist Charlotte Gaugne Gouranton of Paris City University in France, that redistribution was likely the result of a phase change of material some 3,000 kilometers (1,864 miles) down, close to the lower boundary of the mantle.
Related: NASA Is Watching a Huge Anomaly Growing in Earth’s Magnetic Field
The anomaly was detected by a joint DLR and NASA mission called Gravity Recovery and Climate Experiment (GRACE), a pair of satellites that orbited Earth from 2002 to 2017.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>As they whirl around in low-Earth orbit, satellite configurations are far more sensitive to changes in Earth’s gravity field than sensors on the surface. They stay within a detectable proximity to each other in a precise formation; the distance between them changes as the strength of the gravity field changes.
These changes can then be linked to changes in mass distribution on the surface, such as changes in groundwater reservoirs, glacial melt, and sea level changes, all of which manifest as tiny changes in the planetary gravity field.
Gaugne Gouranton and her colleagues thought it possible that these satellites might have recorded some mass shifts from deeper within the planet, so they scoured the data collected during the GRACE mission’s tenure, looking for signs of an anomaly that didn’t fit with surface activity.
Between 2006 and 2008, with a peak in January 2007, they found a subtle but vast dipole pattern: stronger gravity in one band and weaker just next to it, spanning about 7,000 kilometers over the eastern Atlantic. This pattern indicates mass being redistributed, rather than simply added or removed.
What made this signal even more intriguing was that it coincided with a jerk in Earth’s magnetic field in the same region, recorded by other satellites, known as a geomagnetic jerk. These changes are thought to be the result of changes deep inside the planet, close to or even in the liquid outer core.
The researchers modeled various scenarios to determine if the gravitational anomaly could be linked to surface processes, with a specific focus on shifting water, both groundwater and oceanic.
Not only were the models unable to match the location, scale, or timing of the anomaly, but the sheer amount of moving water required to produce an anomaly of the observed scale would be physically impossible.
Once surface processes had been ruled out, it remained for the researchers to work out what processes deep under the surface could have been responsible.
The most abundant mineral in Earth’s mantle is bridgmanite ((Mg,Fe)SiO₃). Under certain conditions, bridgmanite can undergo a phase transition, shifting from a perovskite crystalline structure to a post-perovskite structure.
This means that the atoms inside the mineral shift into a different lattice configuration, and it happens under very high pressures and temperatures that are only naturally found at the core-mantle boundary.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>This change in the structure of bridgmanite also involves a significant change in the material’s density, which would redistribute the mass in that region quite rapidly, producing dramatic shifts in the gravity field above. This can also explain the geomagnetic jerk that occurred at the same time.
The team’s work is yet to be confirmed, but there are other implications. Seismic data has revealed strange blobs of material near Earth’s core that seem to have different properties from the surrounding material. The proposed location of the 2007 gravitational anomaly phase shift was very close to one of these blobs, which could mean the two phenomena are linked.
“By analyzing time series of GRACE-derived gravity gradients, we have identified an anomalous large-scale gravity gradient signal in the eastern Atlantic Ocean, maximum at the beginning of 2007, which cannot be fully explained by surface water sources nor core fluid flows,” the researchers write.
“This leads us to suggest that at least part of this signal could reflect rapid mass redistributions deep in the mantle.”
How common these mass redistributions could be, and how they fit in with the broader landscape of Earth’s interior dynamics, remains to be investigated.
The research has been published in Geophysical Research Letters.
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