Earth’s interior is cooling faster than expected

A measuring device for determining the thermal conductivity of bridgemanite under high pressure and maximum temperature. Credit: Murakami M et al, DOI: 10.1016/j.epsl.2021.117329

Researchers at ETH Zurich have shown in the lab how well a common metal at the boundary between Earth’s core and mantle conducts heat. This leads them to suspect that the Earth’s heat may be dissipating sooner than previously thought.

The evolution of our planet is a story of coolness: 4.5 billion years ago, extreme temperatures prevailed on the surface of the young Earth, and it was covered with a deep ocean of magma. Over millions of years, the planet’s surface cooled to form a brittle crust. However, the enormous thermal energy released from Earth’s interior drives dynamic processes, such as mantle convection, plate tectonics and volcanoes.

However, questions regarding how quickly the Earth will cool and how long it may take for this continuous cooling to stop the above thermal processes 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 rocks of the Earth’s mantle are in direct contact with the hot melting of iron and nickel in the planet’s outer core. The temperature gradient between the two layers is quite steep, so there’s likely to be a lot of heat flowing here. The boundary layer is mainly composed of the mineral bridgemanite. However, researchers have difficulty estimating how much heat this mineral passes from the Earth’s core to the mantle because experimental verification is so difficult.

Now, ETH Professor Motohiko Murakami and colleagues from the Carnegie Institution for Science have developed a sophisticated measurement system that enables 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 allows us to show that the thermal conductivity of bridgemanite is about 1.5 times higher than assumed,” Murakami says. This indicates that the heat flux from the core to the mantle is also higher than previously thought. The greater heat flow, in turn, increases convection in the mantle and accelerates the cooling of the Earth. This may cause the movement of tectonic plates, which is sustained by thermal motions of the mantle, to slow faster than the researchers had expected based on previous thermal conductivity values.

Murakami and colleagues also show that rapid cooling of the mantle 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 may actually accelerate, the researchers estimate, because this mineral conducts heat more efficiently than bridgemanite.

“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 becoming inactive much faster than expected,” Murakami explains.

However, he 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. To do so first requires a better understanding of how convection works in the mantle of space and time. Moreover, 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.

Heat transfer feature at the bottom of the earth mantle

more information:
Motohiko Murakami et al, The radiative thermal conductivity of monocrystalline bridgemanite at the core-mantle boundary with implications for geothermal evolution, Earth and Planetary Science Letters (2021). DOI: 10.1016 / j.epsl.2021.117329

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