Earth’s interior heat dissipates sooner than previously thought, according to laboratory evidence of how a common mineral conducts heat at the boundary between Earth’s mantle and core.
The evolution of our Earth is the story of its cooling: 4.5 billion years ago, the surface of the young Earth was at extreme temperatures, and it was covered by a deep ocean of magma. Over millions of years, the planet’s surface cooled to form a brittle crust. However, the enormous thermal energy emanating from the Earth’s interior sets in motion dynamic processes, such as mantle convection, plate tectonics, and volcanism.
However, unanswered questions remain about how fast the Earth cooled and how long this continued cooling might take to stop the aforementioned heat-driven processes.
One possible answer may lie in the thermal conductivity of minerals that form the boundary between Earth’s core and mantle.
This boundary layer is relevant because it is here that the viscous rock of the Earth’s mantle is in direct contact with the hot molten iron and nickel of the planet’s outer core. The temperature gradient between the two layers is very steep, so potentially a lot of heat flows here. The boundary layer is formed mainly by the mineral bridgmanite. However, researchers have difficulty estimating how much heat this mineral conducts from Earth’s core to the mantle because experimental verification is so difficult.
Now, ETH Zurich Professor Motohiko Murakami and colleagues at the Carnegie Institution for Science have developed a sophisticated measurement system that allows them to measure the thermal conductivity of bridgmanite in the laboratory, under the pressure and temperature conditions that prevail within the Earth. Earth. For the measurements, they used a newly developed optical absorption measurement system in a diamond unit heated with a pulsed laser.
"This measurement system allowed us to show that the thermal conductivity of bridgmanite is about 1.5 times higher than previously assumed."Murakami says in a statement. This suggests that the heat flux from the core to the mantle is also greater than previously thought. Increased heat flux, in turn, increases mantle convection and accelerates Earth’s cooling. This may cause plate tectonics, which is kept going by convective motions of the mantle, to slow faster than the researchers expected based on past values ​​of heat conduction.
Murakami and colleagues have also shown that rapid mantle cooling will change stable mineral phases at the core-mantle boundary. When cooled, bridgmanite converts to the mineral post-perovskite. But as soon as post-perovskite appears at the core-mantle boundary and begins to dominate, mantle cooling could accelerate further, the researchers estimate, since this mineral conducts 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 going dormant much faster than expected."Murakami explains.
However, you can’t say how long it will take, for example, for convection currents in the mantle to stop. "We still don’t know enough about these kinds of events to pinpoint their timing.".
Doing that first requires a better understanding of how mantle convection works in spatial and temporal terms. In addition, scientists need to clarify how the decay of radioactive elements in the Earth’s interior, one of the main sources of heat, affects the dynamics of the mantle.
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