Earth formed 4.5 billion years or so ago. Ever since then, it's been slowly cooling on the inside.
While the surface and atmosphere temperatures fluctuate over the eons (and yes, those external temperatures are currently warming), the molten interior – the beating heart of our planet – has been cooling this entire time.
That's not a glib metaphor. The rotating, convecting dynamo deep inside Earth is what generates its vast magnetic field, an invisible structure that scientists believe protects our world and allows life to thrive. In addition, mantle convection, tectonic activity and volcanism are thought to help sustain life through the stabilization of global temperatures and the carbon cycle.
Because Earth's interior is still cooling, and will continue to do so, this means that eventually the interior will solidify, and the geological activity will cease, possibly turning Earth into a barren rock, akin to Mars or Mercury. New research has revealed that may happen sooner than previously thought.
The key could be a mineral at the boundary between Earth's outer iron-nickel core and the molten fluid lower mantle above it. This boundary mineral is called bridgmanite, and how quickly it conducts heat will influence how quickly heat seeps through the core and out into the mantle.
Determining that rate is not as simple as testing the conductivity of bridgmanite in ambient atmospheric conditions. Thermal conductivity can vary based on pressure and temperature, which are vastly different deep inside our planet.
To surmount this difficulty, a team of scientists led by planetary scientist Motohiko Murakami of ETH Zurich in Switzerland irradiated a single crystal of bridgmanite with pulsed lasers, simultaneously increasing its temperature to 2,440 Kelvin and pressure to 80 gigapascals, close to what we know to be the conditions in the lower mantle – up to 2,630 Kelvin and 127 gigapascals of pressure.
"This measurement system let us show that the thermal conductivity of bridgmanite is about 1.5 times higher than assumed," Murakami said.
In turn, this means that the heat flow from the core to the mantle is higher than we thought – and, therefore, that the rate at which Earth's interior is cooling is faster than we thought.
And the process could be accelerating. When it cools, bridgmanite transforms into another mineral called post-perovskite, which is even more thermally conductive and would therefore increase the rate of heat loss from the core into the mantle.
"Our results could give us a new perspective on the evolution of Earth's dynamics," Murakami said. "They suggest that Earth, like the other rocky planets Mercury and Mars, is cooling and becoming inactive much faster than expected."
As for exactly how much faster, that's unknown. The cooling of an entire planet isn't something we understand very well. Mars is cooling a bit faster because it's significantly smaller than Earth, but there are other factors that may play a role in how rapidly the planetary interior cools.
For example, the decay of radioactive elements can generate heat, enough to sustain volcanic activity. Such elements are one of the major sources of heat in Earth's mantle, but their contribution isn't well understood.
"We still don't know enough about these kinds of events to pin down their timing," Murakami said.
However, it likely won't be a fast process on human scales, either way it falls. In fact, it's possible that Earth will become uninhabitable by other mechanisms long before then. So we might have a bit of time to work more on the problem to figure it out.
The team's research has been published in Earth and Planetary Science Letters.