Scientists suspect that Earth is cooling from the inside faster than they thought.

The tale of our planet’s cooling is the story of its evolution: Extreme temperatures existed on the surface of the young Earth 4.5 billion years ago, and it was covered by a deep ocean of magma. The planet’s surface cooled over millions of years, forming a fragile crust. The vast thermal energy emitted from the Earth’s interior, on the other hand, set in motion dynamic processes such as mantle convection, plate tectonics, and volcanism.

The concerns of how quickly the Earth cooled and how long it might take for this continued cooling to bring the aforementioned heat-driven processes to an end remain unanswered.

The thermal conductivity of the minerals that constitute the border between the Earth’s core and mantle could be one answer.

This boundary layer is crucial because it is here that the Earth’s mantle’s viscous rock comes into direct touch with the planet’s outer core’s boiling iron-nickel melt. Because the temperature difference between the two levels is so great, there might be a lot of heat going here. The mineral bridgmanite makes up the majority of the border layer. However, because experimental verification is difficult, researchers are having difficulty determining how much heat this mineral transmits from the Earth’s core to the mantle.

Now, ETH Professor Motohiko Murakami and his colleagues from the Carnegie Institution for Science have developed a sophisticated measuring system that allows them to assess the thermal conductivity of bridgmanite in the laboratory, under the pressure and temperature conditions that exist inside the Earth. They used a newly built optical absorption measurement method in a diamond unit heated with a pulsed laser for the tests.

“This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed,” Murakami said.

This implies that the heat transfer from the core to the mantle is also greater than previously assumed. As a result of the increased heat transfer, mantle convection increases, speeding up the cooling of the Earth. This could lead plate tectonics, which is propelled by the mantle’s convective motions, to slow down faster than experts anticipated based on past heat conduction values.

Murakami and colleagues also discovered that fast cooling of the mantle alters the stable mineral phases at the core-mantle interface. When bridgmanite cools, it transforms into the mineral post-perovskite. However, the researchers believe that after post-perovskite forms at the core-mantle border and takes over, the cooling of the mantle would speed much more, because this mineral transfers heat even more efficiently than bridgmanite.

“Our results could give us a new perspective on the evolution of the Earth’s dynamics. They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inactive much faster than expected,” Murakami explained.

He can’t estimate how long it will take for convection currents in the mantle, for example, to end.

“We still don’t know enough about these kinds of events to pin down their timing.”

To do so, scientists must first have a better grasp of how mantle convection operates in terms of space and time. Furthermore, scientists must determine how the decay of radioactive materials in the Earth’s interior – one of the primary sources of heat – influences the mantle’s dynamics.

Source: 10.1016/j.epsl.2021.117329

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