The extremely hot interior of the Earth is slowly cooling down, but exactly how fast remains unknown. By studying how well a common deep-Earth mineral conducts heat, researchers from Carnegie and ETH Zurich have found that the planet’s interior may be cooling much faster than we thought.
Earth is essentially a giant pie that’s been cooling on a windowsill for 4.5 billion years. At first, it was broiling hot from surface to center, covered in oceans of magma, but as it cooled from the outside in, a solid crust of rock formed. The core is still emanating huge amounts of heat into the mantle, which drives vital processes like plate tectonics and volcanism, but it too will eventually cool off.
Exactly how long this will take is a mystery, but the scientists on the new study sought some answers by investigating a key mineral called bridgmanite. The boundary layer between the outer core and the lower mantle is composed mostly of bridgmanite, so studying how well the mineral conducts heat could have major implications for the planet. The problem is that collecting these measurements is tricky to do in the lab.
So the researchers placed samples of bridgmanite in a diamond anvil cell that was heated using a laser system to simulate the intense pressures and temperatures deep within the Earth. Then they measured the thermal conductivity of the bridgmanite through an optical absorption system.
The team found that bridgmanite was about 1.5 times better at conducting heat than had long been thought. This in turn would mean that heat passes more easily from the core into the mantle, accelerating the rate that the inner Earth cools down.
And it could get even faster over time. As bridgmanite cools, it turns into another mineral called post-perovskite, which is an even more efficient conductor of heat. When this new mineral begins to dominate the boundary, the inner Earth could cool even more quickly.
“Our results could give us a new perspective on the evolution of the Earth’s dynamics,” said Motohiko Murakami, corresponding author of the study. “They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inactive much faster than expected.”
While the researchers can’t be sure how long this process might take, it’s important to note that this acceleration is happening on geological timescales. Earth’s interior may cool down faster than previously thought, but that will still happen over billions of years.
The research was published in the journal Earth and Planetary Science Letters.
Source: ETH Zurich
