Listening to the Pulse of the Planet

-Karthik Gurumurthy

Imagine standing on the surface of the Earth, knowing that nearly 6,400 kilometers (about 4,000 miles) below your feet lies the planet’s core — a place we’ve never even come close to reaching. The deepest anyone has ever drilled into the Earth is only about 10 kilometers (around 6 miles), which is barely scratching the surface compared to the distance to the core.

Because we can’t drill anywhere near deep enough, scientists have had to get creative. They can’t directly measure the temperature down there, so instead, they study the way seismic waves — the vibrations caused by earthquakes — travel through the Earth. By observing how fast these waves move and how they bend and bounce, geophysicists can figure out details about the density and stiffness of the rocks they pass through.

It’s a little like trying to guess what’s inside a wrapped gift just by shaking it and listening carefully.

From there, scientists compare the properties they’ve inferred to what they know about materials under extreme conditions of heat and pressure. If the seismic properties match those of certain known materials — like iron — heated to very high temperatures in the lab, then they can make educated guesses about what’s happening deep inside the Earth.

But here’s the catch: recreating the intense pressure and heat of the Earth’s core in a lab is extremely challenging. The core is under more than three million times the pressure of the atmosphere at the surface! And the temperatures? Well, current estimates vary widely. Some suggest the core could be around 4,000 kelvins (over 7,000 degrees Fahrenheit), while others place it above 7,000 kelvins (around 12,000 degrees Fahrenheit).

If we could measure exactly how molten iron behaves under those crushing pressures, we’d have a much better idea of the core’s true temperature — especially since the core is mostly made of molten iron. But for now, because lab experiments at those extremes are so tricky, there’s still a lot of uncertainty. Until we get more precise measurements under the right conditions, the true temperature at the heart of our planet remains a bit of a mystery.