A research team at the Université de Strasbourg is using a new mathematical approach to sift through nearly 2 decades of measurements from GRACE satellites (Gravity Recovery and Climate Experiment) of NASA. The fluid outer core of the Earth 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 and creates a dynamo that generates the magnetic field of the Earth. However, these fluid density differences should also show up as subtle variations in Earth’s gravitational field. The key challenge is that there are so many other processes at or near Earth’s surface that bend and warp the planet’s gravity more dramatically, measured by sensitive satellites orbiting the planet.
But, a French research group now thinks they are on the verge of teasing out gravitational disturbances arising from the core for the first time. Their technique identifies and removes expected gravity field variations near Earth’s surface. It is a significant step toward creating a new model to study Earth’s fluid core. 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. Differences in mass and density all-cause Earth’s gravity field to vary. GRACE is a pair of low-flying satellites separated by about 220 kilometers, one traveling right after the other. The variations in gravitational pull make the satellites move faster or slower as they pass overhead. The distance between the two lengthens or shortens if one satellite changes its speed relative to the other as a result.
The twin GRACE vessels continually measure their separation to detect this with an accuracy of less than 1 micrometer (roughly 100 times thinner than a human hair). Point to be noted that 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. GRACE-FO project manager at the German Research Centre for Geosciences in Potsdam, Germany, Frank Flechtner said, “According to NASA, GRACE, and GRACE-FO are one of their most important missions in Earth system sciences”. 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.