These measurements confirm geophysical models that the temperature difference between the solid core and the mantle above, must be at least 1500 degrees to explain why the Earth has a magnetic field.
The study was led by Agnes Dewaele from the French national technological research organisation CEA, and members of the French National Center for Scientific Research CNRS and the European Synchrotron Radiation Facility ESRF in Grenoble.
The Earth's core consists mainly of a sphere of liquid iron at temperatures above 4000 degrees and pressures of more than 1.3 million atmospheres. Under these conditions, iron is as liquid as the water in the oceans.
The temperature difference between the mantle and the core is the main driver of large-scale thermal movements, which together with the Earth's rotation, act like a dynamo generating the Earth's magnetic field.
To generate an accurate picture of the temperature profile within the Earth's centre, scientists looked at the melting point of iron at different pressures in the laboratory, using a diamond anvil cell to compress speck-sized samples to pressures of several million atmospheres, and powerful laser beams to heat them to 4000 or even 5000 degrees Celsius.
"We have developed a new technique where an intense beam of X-rays from the synchrotron can probe a sample and deduce whether it is solid, liquid or partially molten within as little as a second, using a process known diffraction," said Mohamed Mezouar from the ESRF.
"And this is short enough to keep temperature and pressure constant, and at the same time avoid any chemical reactions," said Mezouar in the study published in journal Science.
The scientists determined experimentally the melting point of iron up to 4800 degrees Celsius and 2.2 million atmospheres pressure.
They then used an extrapolation method to determine that at 3.3 million atmospheres, the pressure at the border between liquid and solid core, the temperature would be 6000 degrees.
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