7,000-Kilometer Gravity Anomaly Detected By NASA Over The Atlantic. It Came With A “Geomagnetic Jerk”.

There is still plenty to learn about the Earth’s geomagnetic field, the magnetic field that extends from our planet’s core and into space, creating the magnetosphere. One puzzle is the so-called “geomagnetic jerks” that have been detected by observatories, sometimes planet-wide.

“The main part of the geomagnetic field originates in the Earth’s outer core and varies on secular time scales. Geomagnetic jerks are abrupt changes in the trend of this secular variation, i.e., jumps in the second derivative of the main field recorded at magnetic observatories,” a paper on the topic explains, adding, “several jerks are known to have occurred in the 20th and 21[st] centuries, the most recent ones in 1991, 1999 and 2003.”

In 2007, as that paper delved into, a large and “abrupt change” was detected, a “jerk signal” out of line with seasonal variations.

So far, so weird. Now what about the gravity anomaly? NASA’s GRACE is made up of two satellites orbiting the Earth in tandem, and constantly measuring the distance between the two. When one of them passes over an area of higher mass (for ease, picture a mountain range or a giant reservoir), this speeds up the leading satellite slightly due to gravity. When it passes over an area of lower mass, the leading satellite slows slightly, allowing scientists to build up a gravitational map of the Earth below. 

Gravitational anomalies are the somewhat dramatic name we give to the phenomenon when the measured gravity strength differs from what we expect, given what we know of the mass distribution of the Earth below. These are discovered fairly regularly, but can still be pretty puzzling. At the Puerto Rico Trench, gravity has been found to be -380 milliGal, making it the biggest negative gravity anomaly on Earth. Another in the Indian Ocean is where gravity has been found to be lowest compared to what was expected.

Looking through data from GRACE, a team led by geophysicist Charlotte Gaugne Gouranton from Paris Cité University in France found a large anomaly over the Atlantic Ocean, beginning in 2006 and ending in 2008. Spanning around 7,000 kilometers (4,350 miles), the team found that there was a steep gravity gradient, with an area of strong gravity next to an area of weak gravity. At first, the team attempted to explain the anomaly by modeling the shifting of water and groundwater below that, but soon hit a wall.

“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 team explains in their paper.

The team suggests the cause of the anomaly likely took place at the core-mantle boundary (CMB), where enormous pressure and heat caused the mineral bridgmanite, the most abundant mineral on Earth, to undergo a phase transition, altering its crystalline structure and density.

“We propose that rapid mass redistributions at great depths may result from temporal changes in the depth of the perovskite (Pv) to post-perovskite (pPv) phase transition of the main lower mantle mineral bridgmanite, in the presence of heterogeneities in the deep upwellings of the African LLSVP,” the team explains in their paper. “This leads us to suggest that at least part of this signal could reflect rapid mass redistributions deep in the mantle.”

“Because the depth of the Pv-pPv transition is thermally dependent, it may vary temporally as thermal heterogeneities are entrained in the upwelling and downwelling limbs of the convecting mantle,” they add. “When a vertically moving heterogeneity has reached the transition depth corresponding to its temperature, the transformation should take place and create a mass anomaly due to the ~100kg m^-3 density difference between the two phases.”

The team suggests that this mass redistribution within the mantle could be the cause of the geomagnetic kick seen by observatories around the world, though this remains to be shown and thoroughly investigated.

“How the obtained changes in CMB topography may influence core flow dynamics and the geomagnetic field is now to be investigated,” the team concludes, “as well as the existence of other such rapid events in the deep mantle during the GRACE(-FO) observation period and their modeling.”

The study is published in Geophysical Research Letters.