Earth's Inner Core Might Be Weaker Than We Thought

Earth scientists have modeled that at the very centre of our planet is a solid sphere made largely of iron and nickel. But the Earth's core doesn't act like we'd expect a Moon-sized iron ball to. A new study suggests why: the core might not be as solid as previously thought.

In trying to explain a disparity between observational and predicted data about the Earth’s core, a team of scientists have come up with a novel idea: the Earth’s core might be weaker than we think. It’s a strange thing to wrap your head around—that the core of our planet could have any weakness—but this idea might be the answer to a lot of questions.

One of the Earth’s neatest features might be its magnetic field, the unseen bubble that protects our atmosphere from the charged particles streaming away from the Sun, preventing our planet from turning into a Mars-like barren world. It exists thanks to the Earth’s inner structure. Surrounding the solid inner core is a molten outer core made of liquid iron that generates currents, and these currents generate our magnetic fields.

Of course, scientists can’t study the Earth’s inner workings directly. Even this upper layer of the Earth’s core, which lies more than 3,720 miles beneath the surface, is too far beneath our feat for direct observations. So scientists use seismic waves as a proxy to study the inaccessible core. Measuring pulses of energy that are generated during earthquakes, scientists can study what’s happening inside the Earth, both in the outer molten core and the inner solid core.

This method has uncovered as many questions as it’s answered, including one major problem: results show that seismic waves move through the Earth’s solid inner core much more slowly than models predict. Specifically, the weirdness appears with a type of seismic wave called shear waves, which move through the Earth in a transverse (side to side rather than up and down) motion.

These waves move much more slowly through the Earth’s iron core compared to how fast scientists predict they should move through iron. It’s been a quandary, a mismatch in data that has given rise to a variety of complex theories about the state, genesis, and evolution of the Earth’s core.

But now, a team of Earth scientists from the University College London are offering an explanation. In a new paper published in Science Express, they’ve suggested that the iron in the Earth’s core might weaken just before melting, leaving it much less stiff.

They looked at how the temperature of the Earth’s core affects seismic waves. As the core heats to about 95 percent of what is needed to melt iron, the speed of seismic waves decreases linearly. As soon as the core heats above 95 percent, seismic wave speed drops dramatically. With the core at 99 percent of the temperature needed to melt iron, the team’s calculated seismic wave velocity agrees with their observations: the seismic waves move much more slowly.

It’s a finding that’s consistent with existing data. Other geophysical research groups have found evidence to suggest that Earth’s inner core is around 99 or 100 percent the melting temperature of iron. In other words, the inner core is more malleable than expect. If these two studies are right, taken together they explain the strange behaviour of seismic waves compared to modeled data.

The UCL team’s result might bring scientists a step further in their understanding of the Earth’s inner workings, but it’s not the end of the story. There are other factors that need to be worked into this idea. The Earth’s core contains nickel and other light elements like silicon and sulphur. The team will have to see how this weakened core model works with other elements before they can create a definitive model of the Earth’s core.

In any case, a weak core doesn’t mean the planet is going to implode anytime soon. The inner solid core may be hotter than the liquid outer core, but it stays solid because of the immense pressure it’s under.