Earth’s fluid outer core lies about 2,900 kilometers under our feet, sandwiched between the mantle and the solid inner core. This free-flowing layer of liquid metal has the viscosity of water, but its density changes over space and time as the planet spins—creating a dynamo that generates Earth’s magnetic field. Studying variations in that field traditionally has been the only way for researchers to infer the dynamic physics of motions in the outer core.
However, these fluid density differences should also show up as subtle variations in Earth’s gravitational field. The challenge is that so many other processes at or near Earth’s surface bend and warp the planet’s gravity more dramatically, as measured by sensitive satellites orbiting the planet. But now, a French research group thinks they are on the verge of teasing out gravitational disturbances arising from the core for the first time.
A research team at the Université de Strasbourg is using a new mathematical approach to sift through nearly 2 decades of measurements from the Gravity Recovery and Climate Experiment (GRACE) satellites, a partnership between NASA and the German Research Centre for Geosciences (GFZ). Their technique, which identifies and removes expected gravity field variations near Earth’s surface, is a significant step toward creating a new model to study Earth’s fluid core, as will be reported by lead author Hugo Lecomte on 15 December at AGU’s Fall Meeting 2021.
A New Purpose for GRACE
If Earth was a perfectly smooth sphere with uniform density, its gravity field would also be uniform. Fortunately for life on the planet, this is far from true. Differences in mass and density (such as snowpacks in mountain ranges, waxing and waning ice sheets, massive flows of ocean currents, and fluctuating groundwater basins) all cause Earth’s gravity field to vary. GRACE satellites are sensitive enough to detect these variations—not by directly sensing what’s happening below them, but by constantly measuring changes between the satellites themselves.
GRACE is a pair of low-flying satellites separated by about 220 kilometers, one traveling right after the other. Variations in gravitational pull make the satellites move faster or slower as they pass overhead. If one satellite changes its speed relative to the other as a result, the distance between the two lengthens or shortens. To detect this, the twin GRACE vessels continually measure their separation with an accuracy of less than 1 micrometer—roughly 100 times thinner than a human hair.
NASA launched GRACE in 2002 and its follow-on mission, GRACE-FO, in 2018, primarily to monitor how climate change affects Earth’s natural systems. “According to NASA, GRACE and GRACE-FO are one of their most important missions in Earth system sciences,” said Frank Flechtner, GRACE-FO project manager at the German Research Centre for Geosciences in Potsdam, Germany. Their data can have broader applications, though.
The French team analyzed the complete set of GRACE measurements to date using low spherical harmonics coefficients, up to degree and order 8—in essence, revealing changes in the gravity field at the largest wavelengths, which the core itself would produce. This analysis led to a time-varying pattern arising from the motion of Earth’s fluid core, explained poster coauthor Mioara Mandea of the Centre National d’Études Spatiales, the French space agency.
Then, the team attempted to find correlations between measurements of the gravity field and the planet’s magnetic field over that time. “We apply some mathematical methods in order to tell us where and how these two quantities are correlated,” Mandea said. Using their results, the team hopes to develop a new model to explain the correlations and infer the core’s fluid motions over the 2-decade period.
Challenges at the Surface
The multifaceted nature of GRACE’s data poses a steep and ongoing challenge. To isolate the gravity signal coming from the liquid core, the French team must first identify and subtract mountains of noise arising from constant motions in the atmosphere, oceans, and subsurface water basins and other seasonal changes. Mathieu Dumberry of the University of Alberta in Edmonton, Canada, who researches the physics of planetary interiors, noted that at least 90% of the gravity signal picked up by the satellites is due to Earth’s surface processes, rather than core dynamics. Dumberry was not involved in the new research.
Dumberry explained that because the fluid core contributes to such a small proportion of the detected gravity signal, drawing correlations between the planet’s magnetic and gravity field signals is a statistical hurdle. “The level of the change of the gravity signal is at least a factor of 10 larger than what we expect the core can produce,” he said. “So then the question is, Why are these signals correlated at all? It could be a coincidence, or there is something we don’t understand.”
Still, Lecomte and his colleagues are forging onward. Their approach, they say, provides an alternate method to studying fluid core dynamics. This method, in turn, may allow for a better understanding of movement within the liquid core and the boundary between the core and mantle—both of which are poorly understood.
—Megan Kalomiris, Science Writer