The Earth’s core is cooling surprisingly fast, scientists say – and it could tell us how our planet dies

‘Our results could give us a new perspective on the evolution of the Earth’s dynamics’

Andrew Griffin
Monday 24 January 2022 15:02
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The Earth’s core is cooling more quickly than we realised, scientists have found – and the discovery could have important implications for the future of our planet.

The history of Earth has been one of gradually getting colder. Some 4.5 billion years ago, when it was young, its surface was just a deep ocean of violently hot magma; over time, that cooled down to form the crust that we walk on top of today.

That process left behind a range of processes that keep our Earth active today, such as volcanoes and plate tectonics.

But it remains a mystery exactly how fast it is cooling – and when it might end. That question is key because it could tell us where our planet is headed, and how and when it might meet a kind of death that will leave it more like inactive planets such as Mars.

One of the ways of answering that question is to delve into the minerals that make up the boundary between the Earth’s core and its mantle. That is where rock meets the molten core, and so is one of the key parts where the rate of that cooling will be decided.

Actually examining that layer is difficult however, since it is so deep beneath our feet and hard to experiment on. In a new study, scientists instead looked in a laboratory at how the materials making that boundary might conduct heat, and used those to understand what might be happening inside our planet.

They looked at the mineral bridgmanite, which makes up most of that layer. And they were shocked to find that it is actually more conductive than they thought, suggesting that the heat is likely flowing out of the core more quickly too.

“This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed,” said Motohiko Murakami, an ETH Zurich professor who works on the study.

If the Earth is cooling more quickly, then the various effects of that process would be altered too. Plate tectonics – which happen as the mantle moves around in the heat – could slow down more quickly than realised too, for instance.

What’s more, that process could accelerate even more quickly in future. As bridgmanite cools, it changes into post-perovskite – and as that more conductive material comes to dominate, it could further speed up.

“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 said.

But the full story remains a mystery. Scientists hope to do yet more research to understand the Earth’s interior and better pin down a more detailed understanding of the timing.

The research is described in a paper, ‘Radiative thermal conductivity of single-crystal bridgmanite at the core-mantle boundary with implications for thermal evolution of the Earth’, recently published in the journal Earth and Planetary Science Letters.

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