Earth is cooling faster than expected – measurement shows higher thermal conductivity for minerals in the core-mantle boundary –

Creeping heat loss: More heat escapes from the hot core of the earth than expected – as a result, our planet cools down faster. Evidence of this is provided by the thermal conductivity of the mineral bridgmanite, which dominates at the core-mantle boundary. According to new measurements, it is 1.5 times higher than previously assumed, which is why the mineral allows more heat to penetrate from the earth’s core into the earth’s mantle.

The warmth of the Earth’s interior is an important engine of our planet. Because it drives geological processes such as plate tectonics, mountain formation and volcanism, but also keeps the geodynamo of the earth’s magnetic field running. Without these processes, Earth would be far less hospitable and possibly as dead today as the Moon or Mars.

heat loss from the earth’s core

But the enormous heat reservoir, which is mostly located in the Earth’s core, is not inexhaustible: Heat constantly penetrates from the Earth’s core into the Earth’s mantle and further outwards – as a result, the Earth gradually cools down. “This raises the question of how quickly the earth loses heat and how long it can remain dynamically active,” explain Motohiko Murakami from ETH Zurich and his colleagues.

Crucial to the answer is the core-mantle boundary deep within our planet. This is where the hot, liquid iron-nickel melt of the outer core of the earth meets the viscous mantle rock, which is around a thousand degrees cooler. “Due to its steep temperature gradient, this is the largest thermal boundary on Earth,” the researchers explain. This limit therefore largely determines how much heat can escape from the Earth’s core – and how much heat the Earth loses overall.

How much heat does bridgmanite let through?

The problem, however, is that how well bridgmanite, the dominant mineral at the core-mantle interface, conducts heat has been a matter of debate. Because in order to measure its thermal conductivity, pressure and temperatures must correspond to those of the lower limit of the mantle – and the measuring systems must withstand these extreme conditions. However, Murakami’s team has now succeeded in developing such a measuring system.

The measurement was carried out in a diamond anvil cell to generate the necessary pressure. © Murakami et al., 2021

For their measurements, the researchers first produced high-purity, monocrystalline bridgmanite crystals under high pressure and heat. They placed one of each in a diamond anvil cell and subjected it to a pressure of 80 gigapascals. With the help of a laser, they gradually heated the crystal sample up to around 2,200 degrees. Meanwhile, a special spectroscope recorded the radiation emanating from the crystal – it reveals heat, heat flow and the state of the crystal lattice.

Border more permeable than expected

The result: The radiative thermal conductivity of bridgemanite – the transfer of radiant heat – is around 5.3 watts per millikelvin (W/mK. Together with the thermal conductivity dependent on the crystal lattice, this results in a total thermal conductivity of the mineral of approximately 15.2 watts per millikelvin), as Murakami and his team determined. Accordingly, the bridgmanite conducts heat 1.5 times better than previously assumed based on geophysical models.

However, this means that the core-mantle boundary lets significantly more heat through than expected. However, if the earth’s core transfers more heat to the earth’s mantle, the interior of the earth also cools down faster. Because in the mantle, strong convection currents ensure that the heat is quickly transported to the surface and lost there. Our planet could therefore cool down faster than expected.

What does this mean for the future of the earth?

“Our results could give us a new perspective on the evolution of Earth’s dynamics,” says Murakami. “They indicate that the earth, like the other rocky planets Mercury and Mars, is cooling down and becoming inactive much faster than expected.” Because these processes are driven by the thermal gradient of the earth’s mantle and its convection currents.

However, the scientists cannot predict how long it will take for the convection currents in the mantle to come to a standstill, for example. “With the current state of knowledge, it is not possible to limit such events in time,” says Murakami. Because the decay of radioactive elements in the earth’s interior and the exact mechanism of mantle convection also play a role. So far, however, both have only been partially elucidated. (Earth and Planetary Science Letters, 2022; doi: 10.1016 / j.epsl.2021.117329)

Source: Swiss Federal Institute of Technology Zurich (ETH Zurich)

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