The study has been published in the 'Earth and Planetary
Science Letters Journal'.
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," ETH
Professor Motohiko Murakami said.
This suggested 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 estimated, since this mineral
conducted 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 explained.
However, he could not 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," he said.
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-affected the dynamics of the mantle.