Earth cools faster

Earth is hot: up to 3500 degrees Celsius (6300 °F) in the mantle, 5000 degrees Celsius (9000 °F) in the outer core and 6000 °C (10,800 °F) in the (solid) inner core. This brings us some advantages. Not only us, but all life on Earth. There is, for example, the magnetic field, which is fueled by iron currents in the outer core and protects us from cosmic radiation. But also plate tectonics, which not only gives us mountains, volcanoes and earthquakes, but has also favored the emergence of complex life forms. If it stops at some point, erosion will erode away all differences in elevation over time, and the sea will flood much of the Earth.

Like all hot objects, Earth is cooling. Some geologists estimate that in 1.5 billion years plate tectonics will grind to a halt, while others say that while it is not yet possible to predict the timing, it will in any case come sooner than the destruction of Earth by the inevitable inflation of the Sun into a red giant in 4.5 billion years. But perhaps it happens even clearly earlier than thought. At least that’s what a recent research paper in Earth and Planetary Science Letters says. It deals with the thermal conductivity of the minerals that form the boundary between Earth’s core and mantle.

This boundary layer is relevant because here the viscous rock of Earth’s mantle is in direct contact with the hot iron-nickel melt of the outer core. The temperature gradient between the two layers is very steep, so potentially a lot of heat flows here. The boundary layer is formed mainly from the mineral bridgmanite. However, it is difficult for researchers to estimate how much heat this mineral conducts from the Earth’s core into the mantle because experimental detection is very difficult. Now ETH professor Motohiko Murakami and his colleagues at the Carnegie Institution for Science have developed a sophisticated measurement system that allows them to measure the thermal conductivity of bridgmanite in the laboratory under the pressure and temperature conditions that prevail in Earth’s interior.

“With this measurement system, we were able to show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed,” Murakami says. This suggests that the heat flow from the core to the mantle is also higher than previously thought. Greater heat flux, in turn, increases convection in the mantle and accelerates the cooling of the Earth. Murakami and his colleagues have also shown that rapid cooling of the mantle alters the stable mineral phases at the core-mantle boundary. During cooling, bridgmanite transforms into the mineral post-perovskite. But once post-perovskite appears at the core-mantle boundary and begins to dominate, cooling of the mantle could accelerate even further, the researchers estimate, because this mineral conducts heat even more efficiently than bridgmanite.

“Our results could give us a new perspective on the evolution of Earth dynamics. They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inert much faster than expected,” Murakami explains. However, the researcher cannot say approximately how long it will take for the convection currents in the Earth’s mantle to stop. “We still don’t know enough about it to pinpoint that timing. To do that, we first need to better understand how mantle convection works. Scientists also need to clarify how the decay of radioactive elements in Earth’s interior – one of the main sources of heat – affects the dynamics of the mantle.”

So we don’t need to worry about that yet, and projects to reheat Earth would be well within the realm of science fiction at best.

The structure of the earth (not to scale, image:

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  • BrandonQMorris
  • Brandon Q. Morris is a physicist and space specialist. He has long been concerned with space issues, both professionally and privately and while he wanted to become an astronaut, he had to stay on Earth for a variety of reasons. He is particularly fascinated by the “what if” and through his books he aims to share compelling hard science fiction stories that could actually happen, and someday may happen. Morris is the author of several best-selling science fiction novels, including The Enceladus Series.

    Brandon is a proud member of the Science Fiction and Fantasy Writers of America and of the Mars Society.