Earth's interior is cooling faster than expected

Date:
January 14, 2022
Source:
ETH Zurich
Summary:
Researchers have demonstrated in the lab how well a mineral
common at the boundary between the Earth's core and mantle conducts
heat. This leads them to suspect that the Earth's heat may dissipate
sooner than previously thought.



FULL STORY ==========================================================================
The evolution of our Earth is the story of its cooling: 4.5 billion years
ago, extreme temperatures prevailed on the surface of the young Earth,
and it was covered by a deep ocean of magma. Over millions of years, the planet's surface cooled to form a brittle crust. However, the enormous
thermal energy emanating from the Earth's interior set dynamic processes
in motion, such as mantle convection, plate tectonics and volcanism.


========================================================================== Still unanswered, though, are the questions of how fast the Earth
cooled and how long it might take for this ongoing cooling to bring the aforementioned heat-driven processes to a halt.

One possible answer may lie in the thermal conductivity of the minerals
that form the boundary between the Earth's core and mantle.

This boundary layer is relevant because it is here that the viscous rock
of the Earth's mantle is in direct contact with the hot iron-nickel
melt of the planet's outer core. The temperature gradient between
the two layers is very steep, so there is potentially a lot of heat
flowing here. The boundary layer is formed mainly of the mineral
bridgmanite. However, researchers have a hard time estimating how much
heat this mineral conducts from the Earth's core to the mantle because experimental verification is very difficult.

Now, ETH Professor Motohiko Murakami and his colleagues from Carnegie Institution for Science have developed a sophisticated measuring system
that enables them to measure the thermal conductivity of bridgmanite
in the laboratory, under the pressure and temperature conditions that
prevail inside the Earth. For the measurements, they used a recently
developed optical absorption measurement system in a diamond unit heated
with a pulsed laser.

"This measurement system let us 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 into the mantle is also higher
than previously thought. Greater heat flow, in turn, increases mantle convection and accelerates the cooling of the Earth. This may cause plate tectonics, which is kept going by the convective motions of the mantle,
to decelerate faster than researchers were expecting based on previous
heat conduction values.

Murakami and his colleagues have also shown that rapid cooling of
the mantle will change the stable mineral phases at the core-mantle
boundary. When it cools, bridgmanite turns into the mineral
post-perovskite. But as soon as post- perovskite appears at the
core-mantle boundary and begins to dominate, the cooling of the mantle
might indeed accelerate even further, the researchers estimate, since
this mineral conducts heat even more efficiently than bridgmanite.

"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 explains.

However, he cannot say how long it will take, for example, for convection currents in the mantle to stop. "We still don't know enough about these
kinds of events to pin down their timing." To do that calls first for
a better understanding of how mantle convection works in spatial and
temporal terms.

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.

========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Peter Rueegg. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Motohiko Murakami, Alexander F. Goncharov, Nobuyoshi Miyajima,
Daisuke
Yamazaki, Nicholas Holtgrewe. Radiative thermal conductivity
of single- crystal bridgmanite at the core-mantle boundary with
implications for thermal evolution of the Earth. Earth and Planetary
Science Letters, 2022; 578: 117329 DOI: 10.1016/j.epsl.2021.117329 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220114115640.htm

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