"How hot it is there, but can not be determined by these measurements, " explain Simone Anzellini from the French Commissariat of Energy Research in Arpajon and her colleagues. But it is precisely this temperature that is crucial for almost all models of the geophysical processes in which the Earth's core is involved. Among other things, numerous research groups have already tried to calculate this value theoretically, for example by thermodynamic modeling.
Another approach is to replicate the conditions in the inner core of the earth in the laboratory and to check when iron melts at a pressure like in the inner core. However, it has been difficult to accurately determine the exact melting point, making the measurements inaccurate. Among other things, therefore, both theoretical and practical studies have yielded highly divergent results. "The estimated values vary between 4850 and 7, 600 Kelvin, " the researchers said.
Iron lumps in the diamond press display
To provide more clarity, Anzellini and her colleagues have now carried out a laboratory experiment, using a new technique to more accurately determine the melting point of iron. Instead, they placed a tiny lump of iron just between the tips of two diamond punches - only this material is hard enough to withstand the tremendous pressures. Both diamond tips were then slowly pressed together more and more until the pressure between them corresponded to more than two million atmospheres. Meanwhile, a laser beam heated the ensemble installed in an isolated sample chamber to temperatures of 3, 000 to 5, 000 Kelvin. The goal was to find exactly the heat at which the iron lump began to melt under the pressure.
And it was at this point that the innovation came into play: The researchers used an ultrafine X-ray beam to observe the structure of the iron. Using the diffraction pattern of the rays, they were able to determine, almost to the second, exactly when the arrangement of the iron atoms changed and they changed from the crystalline to the liquid state. The result: At 2.2 million atmospheric pressure, the melting point of the iron lump was around 4, 800 Kelvin - the apparatuses did not get higher. But using the measured curves for different pressures and temperatures, the researchers were able to extrapolate to the conditions of the boundary between the inner and outer core of the Earth. They came to 6, 230 Kelvin - with possible deviations of 500 K up or down.
"Our value is about 1, 000 Kelvin higher than the previous experiment, " say Anzellini and her colleagues. He thus fits much better to the previous theoretical calculations and narrow the range of possible core temperatures further. Also with the models for the heat transport from the core to the lower mantle the new value agrees well and therefore provides a good basis for further investigations.Simone Anzellini (Commissariat à l'Energie Atomique, Arpajon) et al., Science, doi: 10.1126 / science.1233514 © wissenschaft.de - === Nadja Podbregar