Earth’s internal 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 the planet is a story of a cooler: 4.5 billion years ago, the young Earth’s surface was at extreme temperatures, and it was covered in 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 determines dynamic motion processes, such as mantle convection, plate tectonics, and volcanoes.
Unanswered questions about how quickly Earth would cool and how long it might take for this continuous cooling to stop the above thermal processes, however, remain unanswered.
One possible answer may lie in the thermal conductivity of the minerals that form the boundary between Earth’s core and mantle.
This boundary layer is relevant because it is here where the sticky rock of the Earth’s mantle is in direct contact with hot molten iron and nickel in the planet’s outer core. The temperature gradient between the two layers is quite steep, so a lot of heat is likely to flow here. The boundary layer is mainly composed of the mineral bridgemanite. However, researchers find it difficult to estimate how much heat this mineral passes from the Earth’s core to the mantle because experimental verification is very difficult.
Now, ETH Professor in Zurich 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 bridgemanite in the laboratory, under conditions of pressure and temperature prevailing inside the Earth. For the measurements, they used a newly developed optical absorbance measurement system in a pulsed laser heated diamond unit.
“This measurement system allowed us to show that the thermal conductivity of bridgemanite is about 1.5 times higher than previously assumed,” Murakami says in a statement. This indicates that the heat flow from the core into the mantle is also greater than previously thought. Increased heat flow, in turn, increases the mantle convection and accelerates the cooling of the Earth. This may cause the movement of plate tectonics, which is sustained by convective motions in the mantle, to slow faster than the researchers expected based on previous values of thermal conductivity.
Murakami and colleagues also show that rapid mantle cooling will alter the stable mineral phases at the core-mantle boundary. When it cools, bridgemanite turns into the mineral post-perovskite. But once post-perovskite appears at the core-mantle boundary and begins to dominate, the cooling of the mantle could be accelerated even further, the researchers estimate, since this mineral conducts heat more efficiently than pargmentamite.
“Our results can give us a new perspective on the evolution of Earth’s dynamics. They indicate that Earth, like the other rocky planets Mercury and Mars, is cooling and sleeping much faster than expected,” Murakami explains.
However, you cannot say how long it would take, for example, for convective currents in the mantle to stop. “We still don’t know enough about these types of events to determine their timing.”
Doing so first requires a better understanding of how convection works in the mantle of spatio-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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