The mantle of the planet, which makes up around 84 percent of its volume, is a thick layer of silicate rock that lies between its crust and its molten core. Although the mantle is mostly made of solid material, it acts like a viscous fluid over geologic time scales, making it just as challenging to mix and stir as a pot of caramel.

But if we’re comparing candies, perhaps consider malt balls rather than gooey caramels. According to a study from Washington University in St. Louis, the deep part of the ancient mantle closest to the Earth’s core began significantly drier than the area of the mantle closest to the early planet’s surface.

Assistant professor of earth and planetary sciences in Arts & Sciences Rita Parai using data on noble gas isotopes found that the water concentration in the old plume mantle (the deep section) was between four and two hundred and fifty times lower than in the upper mantle.

The viscosity contrast may have prevented mantle mixing, helping to explain Earth’s creation and history. The findings of the study were presented in a paper published in the Proceedings of the National Academy of Sciences today.

According to Parai, “a primordial viscosity contrast may explain why the giant impacts that triggered whole-mantle magma oceans did not homogenize the growing planet.” 

It could also explain why the plume mantle has gone through less partial melting over the course of Earth’s history.

People in Parai’s field used to think that the Earth’s mantle was always the same, but her research calls that idea into question. Around 4.5 billion years ago, when the solar system began to take on its current shape, the third planet from the sun—Earth—was created when gravity drew spinning gas and dust in. While the Earth was filled with volatiles during its formation, such as water, carbon, nitrogen, and noble gases, Parai’s research indicates that the early material that accreted was a drier sort of rock than the later material.

She found that the isotopes of helium, neon, and xenon (Xe) in the mantle indicate that the plume mantle had less Xe and water than the upper mantle at the end of that period of accretion. It’s possible that the upper mantle benefited from a greater mass contribution from volatile-rich substances akin to the carbonaceous chondrite meteorite class.

Parai uses multiple methods to determine a planet’s history. Parai also conducts her own experimental work with rock materials in her high-temperature isotope geochemistry lab at Washington University, but this article in PNAS shows a model she created. She examines noble gas isotopes, particularly those from the element Xe, in volcanic rocks to comprehend how the makeup of the Earth’s mantle has changed through time and in terrestrial rocks on the surface to see how the atmosphere has changed.

“In my lab,” Parai added, “we take natural rock samples — mostly modern volcanic rocks, but also some ancient rocks — and we try to understand different things about Earth history. Specifically, we want to know how Earth got its atmosphere, its oceans and other features related to habitability.”

Image Credit: Getty

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