Deep beneath Earth’s surface, at a depth of about 2,890 kilometers, lies the liquid metal core, a critical component of our planet responsible for generating the magnetic field that shields life from harmful solar winds. Scientists use seismic waves produced by earthquakes to probe the structure of this core, functioning much like an ultrasound, to visualize the hidden depths of the planet.
A recent study by Xiaolong Ma and his colleague has revealed an unexpected discovery—a donut-shaped region in the liquid metal core around the Equator, where seismic waves travel about 2% slower than elsewhere in the core. This finding, published in Science Advances, points to the presence of lighter elements like silicon and oxygen in this region, which may be key to understanding the dynamics of the core.
Most previous studies focused on the larger, more easily detectable parts of seismic waves that travel around the globe after earthquakes. However, the researchers realized that they could glean new information by analyzing the later, weaker part of these waves, known as the coda, which occurs hours after the initial shock. This part of the seismic wave, called the “coda-correlation wavefield,” provides insight into the multiple reverberating waves that are otherwise too faint to detect.
By comparing seismic waves detected near the poles with those near the Equator, the researchers found that waves traveled more slowly around the Equator. After running multiple computer models and simulations, they concluded that this must be due to a donut-shaped region in the liquid metal core, where seismic waves experience slower velocities due to a different composition of materials. The presence of this structure, previously undetected, offers a new perspective on the complexity of Earth’s core.
Earth’s outer core has a radius of about 3,480 kilometers, making it slightly larger than the planet Mars. It consists mainly of iron and nickel, with lighter elements such as silicon, oxygen, sulfur, hydrogen, and carbon mixed in. The bottom of the outer core is much hotter than the top, leading to thermal convection—similar to how water boils in a pot on a stove. This constant movement should result in a uniform mixture of materials, and seismic waves should travel at the same speed throughout the core. However, the discovery of the donut-shaped region suggests that the lighter elements, particularly silicon and oxygen, are concentrated in specific areas.
These lighter elements could be accumulating due to processes related to Earth’s rotation and the transfer of heat from the core to the mantle above it. The liquid metal core is also organized into long, vertical vortices that run from the north to the south poles, driven by the Earth’s rotation and the solid inner core. This movement generates the geodynamo, the engine that powers Earth’s magnetic field. The magnetic field is essential for life on Earth as it protects the planet from harmful solar radiation and cosmic particles.
This new discovery of a donut-shaped region of lighter elements in the liquid metal core offers a more detailed understanding of the inner workings of our planet. It could also shed light on how Earth’s magnetic field changes over time and provide clues about the conditions necessary for the magnetic fields of other planets, including exoplanets that may support life.
https://theconversation.com/seismic-echoes-reveal-a-mysterious-donut-inside-earths-core-237489