Thanks to the seismic waves produced by earthquakes, scientists have been able to probe the interior of our planet. Layers and composition, as well as changes and motions, are the targets of these investigations. But nothing in the universe is simple. Our planet has weird things buried deep within itself, and even when we expect something to be simple it turns out that it is not.

New work makes this point about the Earth’s core. At its edge, scientists have found evidence of a “ceiling.” The core has a hard edge. However, new research suggests that this ceiling is not a sphere covering the entire core – it is only present at certain latitudes. It is basically a donut, or a large belt, wrapped around the outer core of the planet.

The Earth’s core is 6,970 kilometers (4,330 miles) across and it is divided into an inner solid core and a liquid outer core. The liquid core is responsible for the magnetic field of our planet in a process called the geodynamo. Previous research found evidence of a thin layer around the outer core. Seismic waves travel 2 percent more slowly through that layer than elsewhere in the core.

Previous work argued that water from the surface of the planet slowly migrates to the core and chemical reactions create this layer. The new work doesn’t discuss its formation from above, but focuses on below. The team found that the region likely had lighter elements, as in previous work; but they argue that its shape is not spherical.

The evidence for that is in an approach called the coda-correlation wavefield. Basically, the researchers, Xiaolong Ma and Hrvoje Tkalčić, looked at the coda – the tail – of seismic waves, searching for similarities. Putting them together they saw the reverberation signals from the deeper layers of the Earth, but also found that the signals from stations at the equator are different from those at higher latitudes. Hence the idea that the layer is a donut.

It is possible to create such a shape if the outer core is transferring more heat in this region than elsewhere, which is an interesting hypothesis. And if indeed the (relatively) thin layer is only present in a specific band, it might have implications for how the magnetic field of our planet behaves, something that it is not fully understood.

Clearly, there is a lot more work to be done to understand the deep interior of our world, especially at such depths. The layer is 2,900 kilometers (1,800 miles) beneath our feet but, if this study is correct, only if you are not too distant from the equator.

The study is published in the journal Science Advances.