Scientists have determined the temperature near the earth’s centre to be 6,000° Celsius, 1,000° C hotter than was reported in an experiment run 20 years ago. These measurements confirm geophysical models that say that the temperature difference between the solid core and the mantle above must be at least 1,500° C to explain why the earth has a magnetic field. The results were published in the April 26 issue of Science. The research team, which was led by Agnes Dewaele from the CEA, a French technological research organisation, used X-rays from the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, as a key investigating tool.
The earth’s core consists mainly of a sphere of liquid iron at temperatures above 4,000° C and pressures of more than 1.3 million atmospheres. Under these conditions, iron is as liquid as the water in the oceans and only at the very centre of the earth, where pressure and temperature rise even higher, does it solidify. Analyses of earthquake-triggered seismic waves tell one the thickness of the solid and liquid cores and even how the pressure in the earth increases with depth, but provide no information on temperature. The temperature difference between the mantle and the core is the main driver of large-scale thermal movements that 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 used a diamond anvil cell to compress speck-sized samples to pressures of several million atmospheres, and powerful laser beams to heat them to 4,000° C or even 5,000° C. “In practice, many experimental challenges have to be met,” explains Agnes Dewaele. “Even if a sample reaches the extreme temperatures and pressures at the centre of the earth, it will only do so for a matter of seconds. In this short time frame, it is extremely difficult to determine whether it has started to melt or is still solid.” The scientists used an intense beam of X-rays from the synchrotron to probe the sample and deduce whether it was solid, liquid or partially molten in as short a time as a second. “This is short enough to keep the temperature and the pressure constant and at the same time avoid any chemical reactions,” says Mohamed Mezouar from the ESRF.
The melting point of iron was measured up to 4,800° C and 2.2 million atmospheres pressure, and then it was extrapolated to determine that at 3.3 million atmospheres, the pressure at the border between liquid and solid core, to give a value of 6,000° +/- 500° C. The experiment 20 years ago used an optical technique to determine whether the samples were solid or molten, and it is highly probable that the observation of recrystallisation at the surface, which begins at about 2,400° C, was interpreted as melting, which is why perhaps Reinhard Boehler had in 1993 published values that were about 1,000° C lower.
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