Nearly twenty years ago, satellites detected an unusual gravitational signal off the coast of Africa, revealing a deep disturbance in Earth's gravitational field.
Researchers identified this anomaly by examining data collected by the GRACE (Gravity Recovery and Climate Experiment) satellites, which operated between 2002 and 2017. The signal, which extended over approximately 7,000 kilometers (about 4,350 miles), reached its peak intensity in January 2007. This period coincides with a "geomagnetic jerk," a sudden change in Earth's magnetic field, suggesting a link between these two phenomena.
The study, published in
Geophysical Research Letters, proposes that mineral transformations in the lower mantle caused a rapid redistribution of mass, affecting both gravity and magnetism.
The GRACE satellites, a collaboration between NASA and the German Aerospace Center, operated as a pair to measure tiny variations in Earth's gravity. By flying one behind the other, they detected distance deviations caused by differences in gravitational attraction, often related to water or mass movements. In this case, the team filtered out surface signals to isolate those coming from the depths, thus confirming the internal origin of the anomaly. Mioara Mandea, a geophysicist at the French National Center for Space Studies, initially doubted the validity of the signal before participating in its interpretation.
The region in question is located in the lower mantle, a rocky zone primarily composed of magnesium silicate, situated just above the liquid outer core. Scientists estimate that the transformation of perovskite into post-perovskite, a change in crystal structure under high pressure, led to a significant mass displacement. This process, rarely observed, could explain both the gravitational distortion and the magnetic jerk, shedding light on the interactions between Earth's layers.
The GRACE satellites detected the gravitational anomaly by measuring variations in distance between them, revealing deep mass changes.
Credit: NASA/JPL
Mandea emphasizes that this discovery illustrates the need to cross-reference various data and methods to probe Earth's interior. By combining gravimetry, magnetism, and other techniques, researchers can uncover hidden mechanisms, like those behind this anomaly. This synergistic approach promises new advances in understanding Earth's deep dynamics, a multifaceted system still largely unknown.
Mineral transformation in the lower mantle
The lower mantle, located between 660 and 2,900 kilometers (410-1,800 miles) deep, is primarily composed of magnesium silicate, a mineral that can change structure under the effects of pressure and temperature. One such transformation is the transition from perovskite to post-perovskite, where atoms rearrange to form a denser crystal.
This phenomenon, occurring near the boundary with the outer core, can release or absorb energy, leading to large-scale mass movements. In the case of the African anomaly, this change likely redistributed rock, altering the gravitational field and interacting with the core to affect magnetism.
Such transformations are rare and difficult to observe directly, but they play a key role in mantle convection, which influences plate tectonics and volcanism. Laboratory studies simulate these extreme conditions to better understand their impact on Earth's dynamics.
GRACE satellites and gravity measurement
The GRACE (Gravity Recovery and Climate Experiment) satellites revolutionized the study of Earth's gravity by using a pair of identical spacecraft flying in formation. By continuously measuring the distance between them, they detect tiny variations caused by mass changes on Earth, such as ocean currents or deep movements.
This technology allows mapping gravitational anomalies with unprecedented precision, revealing otherwise invisible internal processes. For example, a displacement of rocks in the mantle can locally alter gravity, which GRACE was able to identify near Africa.
GRACE data are essential for understanding not only Earth's structure but also climate, by tracking water masses. The mission ended in 2017, but its successors, such as GRACE-Follow On, continue this crucial work for Earth sciences.