The evolution of our Earth is the story of its cooling: 4.5 billion years ago, the young Earth’s surface experienced extreme temperatures and was covered by a deep ocean of magma. Over millions of years, the planet’s surface cooled and formed a brittle crust. However, the enormous thermal energy emanating from the Earth’s interior initiates dynamic processes such as mantle convection, plate tectonics & volcanism.
However, unanswered questions remain about how quickly the Earth has been cooled and how long this sustained cooling might take to halt the aforementioned heat-driven processes.
One possible answer could lie in the thermal conductivity of minerals that form the boundary between Earth’s core & mantle.
This boundary layer is relevant because it is here that viscous rocks of the Earth’s mantle are in direct contact with the hot molten iron and nickel of the planet’s outer core. The temperature gradient between the 2 layers is very steep, a lot of heat potentially flows in here. The boundary layer is mainly formed by the mineral bridgmanite. However, researchers have difficulty estimating how much heat this mineral conducts from core to the mantle because experimental verification is so difficult.
Now, ETH professor Motohiko Murakami & colleagues from the Carnegie Institution for Science have now developed a sophisticated measuring system which enables them measure the thermal conductivity of bridgmanite in the laboratory under the pressure & temperature conditions prevailing in the Earth’s interior. For the measurements, they used a newly developed optical absorption measuring system in a diamond unit heated with a pulsed laser.
“With this measurement system, we can show that the thermal conductivity of bridgmanite is about 1.5 times higher than expected,” says Murakami. This suggests that the heat flow from core to the mantle is also higher than previously thought. Increased heat flow in turn. , increases mantle convection & accelerates the cooling of the Earth. This may cause plate tectonics, which is kept-going by convective motions of the mantle, to decelerate faster than expected by the researchers based on previous heat conduction values.
Murakami and colleagues have also shown that rapid mantle cooling will alter stable mineral phases at the core mantle boundary. On cooling, bridgmanite transforms into the mineral post-perovskite. But once post-perovskite emerges at the core-mantle interface and begins to dominate, the researchers estimate that mantle cooling could actually be accelerated even more, as this mineral conducts heat even more efficiently than bridgmanite.
“Our results could give us a new perspective on evolution of Earth dynamics. They suggest that Earth, like the other rocky planets Mercury & Mars, is cooling and becoming inactive much faster than expected,” Murakami explains.
However, one cannot say how long it will take, for example, for convection currents in the Earth’s mantle to stop. “We still don’t know enough about these types of events to pin-down their timing.” This requires a better understanding of how mantle convection works in spatial & temporal terms. Scientists also need to understand how the decay of radioactive elements in the Earth’s interior, one of the main sources of heat, affects mantle dynamics.
The findings published in Earth and Planetary Science Letters.
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