How zapping squeezed iron recalculates Earth core temperature

Squeezing iron between gemstones

How did Earth create a shield of magnetism long before its solid inner core froze into being? The answer links closely to Earth core temperature and how well iron passes heat. No drill can reach thousands of kilometres deep to sample the centre. To overcome this obstacle, a science team recreated those deep conditions inside a room.

The team trapped tiny specks of pure iron inside a diamond anvil cell, a high-pressure clamp that squashes samples between two hard gem tips. They reached extreme loads up to 200 gigapascals, matching deep mantle forces. Next came pulsed Joule heating, a technique that fires microsecond bursts of electric current into the metal. These quick pulses melt the iron in an instant before any heat can leak away into the surrounding rig. More than 300 reliable melting tests tracked the exact moment the solid iron turned into a liquid.

A cooler deep border

The tests revealed a gentle melting slope. Under 200 gigapascals of crush, pure iron melted at 4160 kelvin. The team also took the first direct reading of the liquid metal's electrical resistivity, which measures how strongly a substance fights electric flow, clocking it at 76 micro-ohm centimetres. This value let the team work out how rapidly heat escapes the core.

By scaling their numbers to deep pressures and metallic blends, the team estimated that the core-mantle boundary sits at a relatively cool 3430 kelvin. This border separates the rocky mantle from the liquid outer core. Heat flow along this edge was measured at 9.8 terawatts. That means natural thermal convection—warm fluid floating up while cool fluid sinks—was strong enough on its own to drive the ancient geodynamo, the liquid-metal engine that powers Earth's magnetic field. This modest heat flow also tells us that the modern lowermost mantle is completely solid today.

Remaining mysteries

These findings depend on extending results from pure iron rather than directly sampling deep alien interiors. Natural planetary cores hold lighter elements mixed into the molten metal. Future work must test specific light alloys under full inner-core forces and test liquid iron above 200 gigapascals.