2023-12-30 13:52:13
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Researchers in China recently discovered that every 8.5 years, the Earth’s inner core oscillates around its axis of rotation — which might challenge the standard geophysical model. During this oscillation, the axis would tilt up to 0.17 degrees relative to that of the mantle, which might induce important changes in the internal dynamics of the planet, such as variations in the magnetic field.
The Earth has 4 layers: the crust, the mantle, the outer core (liquid) and the inner core (solid), the latter being located at a depth of approximately 2896 kilometers. Spanning a radius of approximately 1,200 kilometers, the inner core (primarily composed of iron and nickel) plays a critical role in the planet’s geophysical processes, such as maintaining the magnetic field and rotation.
According to the conventional geophysics model, the axis of rotation of the Earth’s core aligns with that of the mantle. This would imply a uniform density distribution at both structures. However, the researchers in the new study detected periodic signals of deviation from the rotation axis of the inner core, which are believed to occur every 8.5 years.
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According to Hao Ding of Wuhan University (China), one of the authors of the study, “ The results of the free oscillation of the Earth (natural oscillations of the Earth as a whole) indicate that the density distribution in its interior is very heterogeneous. The hypothesis [traditionnelle] should therefore not be realistic ».
These oscillation signals from the internal core were also brought to light in the context of a previous study, when the same researchers collected measurements over several decades of the polar motion of the Earth’s rotation axis and changes in its rotation speed. The new study, published in the journal Nature Communicationsaimed to confirm these observations, potentially leading to a paradigm shift in our understanding of terrestrial geophysical dynamics.
Diagram showing the inner core wobbling around its axis of rotation (tilted relative to the mantle) in a small circular orbit (in blue). Ωm is the axis of rotation of the mantle, Ω′m and Ωic are respectively the axis of rotation and the axis of the dynamic figure of the inner core, with the static inclination angle between Ωm and Ω′m, θ . © Yachong An et al.
Changes in the density distribution of the core
To support their previous observations, Chinese researchers took measurements of subtle variations in day length in several regions of the world. These variations are in fact the main indicators of changes in the inclination of the Earth’s axis of rotation. The measurements were then compared with those made for polar movements.
Calculations suggest that the periodic oscillation of the inner core is caused by a 0.17 degree tilt of its axis of rotation, relative to that of the mantle. “ This implies a potential eastward differential rotation angle of the inner core of less than 1 degree and a misalignment of the boundary layer symmetry axes between the lower mantle and the core. “, explains Ding.
These results contradict conventional geophysical theory, suggesting not only an alignment between the axes of rotation of the mantle and the core, but also a perfectly spherical shape of the latter. Indeed, this inclination would be likely to modify the shape and movement of the liquid core – which might potentially lead to changes in the Earth’s magnetic field. In addition, this might also lead to a difference in density at the level of the “layer” separating the outer core from the inner core of the order of 0.52 g/cm3, according to the researchers. The northwest pole of the inner core would also be a little denser than the rest of the structure.
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These results suggest that the oscillation of the inner core induced by the tilt of its axis might have important implications in the overall internal dynamics of the Earth. However, they were obtained by excluding other potential sources of variations that might influence polar movement, including in particular atmospheric, oceanic and hydrological parameters. It is in fact difficult to assert that these parameters played no role in the periodic oscillation noted by the study. Nevertheless, this discovery contributes to significantly improving the understanding of the internal dynamics of the planet.
Next, Ding and his colleagues plan to further explore the influence of the inner core’s oscillation on its density distribution. “ The stratified structure and density of the Earth’s core have always been a problem for geoscientific research. Our goal is to delve deeper into the periodic oscillation and differential rotation of the Earth’s core, seeking to clarify conceptual theories that are different and whose coexistence may be difficult », concludes Ding.
Source : Nature Communications
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