The giant structures that sit where Earth’s core meets the mantle and slow seismic waves as they pass through could be key to the planet hosting life, at least in the abundance we see. If the newly presented idea is right, it would provide yet another way in which the world we take for granted is the product of events that were far from inevitable, and might be rare in the universe.
Earthquakes create seismic waves that travel through the planet, bending, slowing, or reflecting as they encounter different compositions or phases of matter. It’s how we know there is a solid inner and liquid outer core. Expanding seismic waves data from different locations has alerted geologists to two continent-sized regions at the base of the mantle known as large-low-shear-velocity provinces (LLVP), which contain within them ultralow velocity zones.
After decades of representing near-complete enigmas, in recent years a rush of ideas have been proposed about the causes of these provinces, and their effects on the Earth’s outer layers, as well as exploring the differences between the two. Not all these papers may stand the test of time, but the latest could be the most dramatic, making the case that without these provinces, humans might not be here to study them.
“These are not random oddities,” study author Dr Yoshinori Miyazaki of Rutgers University said in a statement. “They are fingerprints of Earth’s earliest history. If we can understand why they exist, we can understand how our planet formed and why it became habitable.”
The reason Miyazaki thinks the LLVPs are so important lies in the paradox of magma ocean cooling. The Earth was tremendously hot after the impact that created the Moon. That heat turned the planet into a magma ocean, which gradually cooled. Models of this process anticipate that the mantle would differentiate into layers with different chemistry and density. Yet, the LLVPs aside, that’s not the picture seismic waves provide, instead indicating a well-mixed mantle.
“Something was missing,” Miyazaki said. The geodynamicist and collaborators came to suspect the missing piece was leakage from the core. If the core/mantle boundary is not impenetrable, but instead is crossed by silicon, tungsten, and magnesium mixing into the mantle, iron-oxide concentrations would fall in what would otherwise become a dense layer at the bottom of the mantle. The LLVPs represent a mix of the original magma ocean and escaped core material, richer in iron than the mantle as a whole, but not as enriched as predicted by models that do not allow for core-escape.
The Pacific Low-Shear Velocity Province, dotted with ultra-low velocity regions, is less famous than its South Atlantic counterpart, but just as significant
As interesting as that may be to those whose special subject is the center of the Earth, the idea doesn’t have an obvious relevance for surface dwellers. However, Miyazaki and colleagues think the provinces create volcanic hotspots like Hawaii. The chemistry of lava at these sites is more consistent with their model than alternatives, they claim. Besides creating environmental wonderlands and great places to place telescopes, these hotspots release mantle material into the oceans and atmosphere, including chemicals essential for life.
“Earth has water, life and a relatively stable atmosphere,” Miyazaki said. “Venus’ atmosphere is 100 times thicker than Earth’s and is mostly carbon dioxide, and Mars has a very thin atmosphere. We don’t fully understand why that is. But what happens inside a planet, that is, how it cools, how its layers evolve, could be a big part of the answer.”
At this stage, the path the authors have proposed from the edge of the core to a stable atmosphere and rich surface chemistry is still very tentative. Several steps have yet to be proven, or even fully explained. Nevertheless, the authors argue their work offers potential aspects to study to see if their suggestion stands up.
“This work is a great example of how combining planetary science, geodynamics and mineral physics can help us solve some of Earth’s oldest mysteries,” said first author Dr Jie Deng of Princeton University. “The idea that the deep mantle could still carry the chemical memory of early core–mantle interactions opens up new ways to understand Earth’s unique evolution.”
“Even with very few clues, we’re starting to build a story that makes sense,” Miyazaki said. “This study gives us a little more certainty about how Earth evolved, and why it’s so special.”
The study is published in Nature Geoscience.
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