Thousands of kilometers below your feet, there lies a red-hot ball of solid iron. Simply known as Earth’s inner core, the metallic heart of our planet remains relatively enigmatic despite the advances of modern science.
The innermost depths of our planet are too distant, not to mention too hot, to probe and collect samples from. The deepest hole ever dug by humans is the 12,263 meters (40,230 feet) deep Kola Superdeep Borehole, built in northwestern Russia during the Cold War. So, in other words, we’ve barely scratched the surface of Earth’s depths.
However, scientists can gain some sturdy insights into the nature of the inner core by measuring things like seismic waves and magnetic field studies.
The inner core starts at approximately 5,150 kilometers (about 3,200 miles) below the Earth's surface. It consists of a metallic sphere that has a radius of about 1,216 kilometers (755 miles), roughly the size of Pluto.
In 1936, Danish seismologist Inge Lehmann suggested that Earth’s center consisted of a solid inner core surrounded by a liquid outer core. Her bold theory was proven correct decades later when seismographs detected waves deflecting off the solid inner core. Just as Lehmann predicted, the solid inner core is then surrounded by a liquid outer core, then the rocky mantle, and finally the crust.
Scientists still don’t really know what the inner core is made of, although the current consensus says it’s primarily an iron alloy with significant quantities of nickel and very small amounts of elements like chromium, manganese, phosphorus, and cobalt.
It’s no exaggeration to say it’s like another planet down there. Temperatures soar at around 5,430°C (9,800°F) – about the temperature at the surface of the Sun – and pressure clocks in at nearly 3.6 million atmospheres. These temperatures would usually be hot enough to melt iron and nickel into a liquid, but the immense pressure keeps them in a solid state.
Weirdly, the inner core doesn't seem to rotate uniformly with the rest of the Earth. In the 1990s, research suggested that Earth’s inner corner undergoes a super-rotation, in which the inner core rotates slightly faster than the rest of the planet. While the reason for this super-rotation is not fully understood, it might have something to do with Earth's magnetic field and the flow of molten iron in the outer core.
Earth is estimated to be 4.54 billion years old, but it’s believed the inner core only formed 1 billion years ago when a super-hot iron nugget spontaneously began to crystallize inside the core’s liquid center.
The inner core has managed to remain red-hot for a billion years thanks to several processes. One of the most important is left-over heat from planetary formation when early Earth was an inhospitable ball of molten lava. Further heat is likely to be added to the inner core thanks to the decay of radioactive elements that have been naturally laced throughout Earth’s inner structure.
New discoveries about the inner core are constantly arising from the deep. In October 2023, a team of Chinese scientists reported findings that might indicate the supposedly solid inner core is slightly squishy due to its wandering iron atoms.
Who knows what other findings might emerge in the coming future, but Earth's inner core is sure to be full of surprises.