The evolution of our planet is a story of gradual cooling. While 4.5 billion years ago, extreme temperatures and a deep ocean of magma prevailed on the surface of the Earth, the planet’s surface shifted to a cooling phase over the next millions of years that formed a brittle crust. However, the immense thermal energy emanating from the Earth’s core set various dynamic processes in motion, such as plate tectonics, volcanism, and mantle convection.
There is still no scientific consensus regarding how fast the Earth has cooled, or how long it will take until the heat-related dynamic processes mentioned above will stop due to cooling. By investigating the thermal conductivity of the minerals which form the boundary between the Earth’s core and mantle, a research team led by ETH Zürich has found that the Earth may in fact be cooling much faster than initially estimated.
Studying this boundary layer, formed mainly of a mineral called bridgmanite, is important since it is the place where the viscous rock of the Earth’s mantle is in direct contact with the hot iron-nickel melt of its outer core, causing significant heat flow.
The scientists used a recently developed optical absorption measurement system in a diamond unit heated with a pulse laser in order to measure the thermal conductivity of bridgmanite in the laboratory, under the pressure and temperatures prevailing inside the Earth.
“This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed,” reported study lead author Motohiko Murakami, a specialist in Earth Sciences at ETH Zürich. This finding suggests that the heat flow from the Earth’s core into the mantle is higher than estimated, a phenomenon which increases mantle convection and accelerates the cooling of the Earth.
Moreover, Murakami and his colleagues also discovered that rapid cooling of the mantle will turn bridgmanite into the mineral post-perovskite which, once predominant, will further accelerate the cooling of the mantle due to its higher efficiency in conducting heat.
“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 becoming inactive much faster than expected,” Murakami explained.
Further research is needed to find out how long it will take for convection currents in the mantle to stop. “We still don’t know enough about these kinds of events to pin down their timing,” concluded Murakami.
The study is published in the journal Earth and Planetary Science Letters.