Even if you breezed through a few geology classes in your day, it's easy to think of the Earth's interior like a Cadbury Egg: solid on the outside and molten in the center. Yet we've known for more than 60 years that the very center of the Earth is actually solid.
Danish seismologist Inge Lehmann made the discovery in 1936 when she noticed seismic waves bouncing off a boundary point deep within what was believed to be a liquid center. With her finding, the world learned that Earth's core is solid at the center and liquid on the outside.
"The Earth has a radius of 6,371 kilometers (3,959 miles)," explains seismology professor Xiaodong Song of the University of Illinois at Urbana-Champaign. "The radius of the outer core is 3,400 kilometers (2,113 miles), and that of the inner core is 1221 kilometers (763 miles). So the size of the inner core is just slightly smaller than the Earth's moon, but the outer core is more than half the radius of the Earth."
The core is composed mostly of an iron-nickel alloy and, as Princeton geosciences professor Jeroen Tromp explains, it didn't always possess a solid center.
"The inner core is basically the result of the slow cooling of the outer core," Tromp says. "The temperature drops below the melting point at the inner core boundary so over time, slowly, the inner core has crystallized within the liquid outer core. That will continue and eventually there won't be a liquid outer core anymore. It will be gone."
The solidification of the outer core will take billions of years, but future inhabitants of Earth certainly will notice the difference. The liquid portion of the core is crucial to the processes that produce Earth's magnetic field.
Without that magnetic field, the planet would be much more exposed to solar wind, a deadly stream of highly charged particles.
"The Earth's magnetic field is generated as a result of hydrodynamic convection," Tromp explains, "It's driven by thermal and compositional variations. As the inner core freezes, lighter elements get left out in the outer core, and those lighter elements help to drive the dynamo."
Of course there's no call to panic. The inner core began to form billions of years ago, and it will be billions more before the outer core disappears.
Another interesting feature of the Earth's core is that many seismologists think the inner core spins faster than the rest of the Earth.
"We made the discovery more than 10 years ago that the inner core is actually spinning faster than the rotation of the Earth, by a fraction of a degree per year, relative to the spinning of the surface," says Song.
Although this might not seem like much time, it would certainly add up over the centuries. The theory, however, is not without its detractors.
"It's highly debated within the community," says Tromp. "Initially the difference in rate was large from a seismological perspective, but now we're down to a very subtle effect and I'm no longer convinced that we can actually see this."
So the center of the Earth contains neither liquid iron nor sugary cream, but there is a solid, dense, mostly iron sphere roughly the size of the moon.