April 26, 2013
Researchers Find Earth Center Is 1800 Degrees Hotter Than Thought
April Flowers for redOrbit.com - Your Universe Online
The temperature near the Earth's core is approximately 10,800 degrees Fahrenheit  a team of scientists has determined. This is 1,800 degrees hotter than in a previous experiment conducted 20 years prior.
The new measurements confirm geophysical models that the temperature difference between the solid core and the mantle above must be at least 1,500 degrees F to explain why the Earth has a magnetic field. The team was even able to determine why the prior experiment produced such a lower temperature figure.
The team consisted of AgnÃ¨s Dewaele from the French national technological research organization CEA, alongside members of the French National Center for Scientific Research (CNRS) and the European Synchrotron Radiation Facility ESRF.
For the most part, the Earth's core is a sphere of liquid iron at temperatures above 7,200 degrees with pressures of more than 1.3 million atmospheres. Iron is as liquid as ocean water in these conditions. The liquid iron solidifies only at the very center of the Earth, where the temperature and pressure are even higher.
Scientists have used analysis of earthquake-triggered seismic waves passing through the Earth to determine the thickness of the solid and liquid cores, and how the pressure in the Earth increases with depth. What these analyses cannot provide information on, however, is temperature. Since temperature affects the movement of material within the liquid core and the solid mantle above, this is a significant gap in our knowledge.
The temperature differential between the mantle and the core is the main driving force of large-scale thermal movements. These movements, together with the Earth's rotation, act like a dynamo generating the Earth's magnetic field. Geophysical models that explain the creation and extreme activity of hot-spot volcanoes like the Hawaiian Islands or La RÃ©union are also informed by the temperature profile of the Earth's interior.
In order to generate an accurate temperature profile, researchers can examine 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. They then heat the samples with a powerful laser beam, raising the temperature to more than 9,000 degrees.
"In practice, many experimental challenges have to be met," explains AgnÃ¨s Dewaele from CEA, "as the iron sample has to be insulated thermally and also must not be allowed to chemically react with its environment. Even if a sample reaches the extreme temperatures and pressures at the center 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."
X-rays become a determining factor at this point. "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," says Mohamed Mezouar from the ESRF. "This is short enough to keep temperature and pressure constant, and at the same time avoid any chemical reactions."
In laboratory experiments, the scientists determined the melting point of iron up to about 8,700 degrees and 2.2 million atmospheres pressure. Using an extrapolation method, they determined at 3.3 million atmospheres, the pressure at the border between liquid and solid core, the temperature would be 10,800 +/- 900 degrees. This value could change slightly if iron undergoes an unknown phase transition between the measured and extrapolated values.
By scanning across the areas of pressure and temperatures, the team was able to determine why Reinhard Boehler, then at the MPI for Chemistry in Mainz (Germany), had, in 1993, published values about 1,800 degrees lower.
Recrystallization effects appear on the surface of the iron samples starting at 4,350 degrees. This leads to dynamic changes of the solid iron's crystalline structure. Boehler's experiment used an optical technique to determine whether the samples were solid or molten, and it is highly probable the observation of recrystallization at the surface was interpreted as melting.
"We are of course very satisfied that our experiment validated today's best theories on heat transfer from the Earth's core and the generation of the Earth's magnetic field. I am hopeful that in the not-so-distant future, we can reproduce in our laboratories, and investigate with synchrotron X-rays, every state of matter inside the Earth," concludes AgnÃ¨s Dewaele.
The results of this study were published in a recent issue of Science.
NOTES:  All temperatures have been converted to Fahrenheit for this feature. The press release from ESRF included measurements in Celsius. Temperature conversions are estimated.