Italian researchers have determined that the Earth's core may be much cooler than previous estimates suggested, possibly confirming what Jules Verne anticipated nearly 150 years ago in his Journey to the Centre of the Earth.

Core Values

While we know with great accuracy the temperature of the Sun and, in fact, of many celestial bodies several light years away, we have yet to accurately determine the temperature of the Earth's core, a 'bare' 3,000 kilometres (km) beneath our feet.

With remarkable foresight, Jules Verne, in Journey to the Centre of the Earth (1864), maintained that the Earth's core is solid--an opinion that contrasted sharply with those of most scientists in his day. The latter contended that the Earth's core is comprised of hot volcanic liquid.

In the early twentieth century, more than 50 years after Verne published his classic science fiction travel story, seismic data gathered by scientists indicated that the Earth's nucleus consists mainly of iron in two states--a liquid layer about 2,200 km thick, surrounded by a solid core of about 1,200 km in radius. This solid core, foreshadowed by Verne, guarantees that the core's inner temperature cannot exceed iron's melting temperature.

The melting temperature of iron under ordinary conditions is well known--about 1,500°C. But the temperature at pressures found in the Earth's core, of the order of 3 million atmospheres, is much more open to question.

To reproduce in the laboratory such extreme pressure and temperature, researchers at Lawrence Livermore National Laboratory, in California, created impacts with special ultra-high-speed 'gunshots' containing bullets travelling at 10 km per second. Others, including Soviet scientists in the 1960s, conducted experiments in the immediate vicinity of nuclear explosions. These violent methods led to estimates that the core's melting temperature was most likely below 7,000°C. But the results were never free of uncertainties.

In an effort to remove these uncertainties, Alessandro Laio, Guido L. Chiarotti, Sandro Scandolo and Erio Tosatti, of the International School for Advanced Studies (SISSA), Italian National Institute for the Physics of Matter (INFM), and Abdus Salam International Centre for Theoretical Physics (ICTP), and Stéphane Bernard of the French Commisariat à l'Energie Atomique, chose an alternative approach that is perhaps more elegant and surely less dangerous.

Instead of trying to reproduce for real the Earth's core conditions in the laboratory, they sought to reproduce the conditions--or, more precisely, simulate them--in the computer by solving in real time the fundamental equations governing the motion of individual iron atoms. The simulations have provided surprising results, as the authors of the research revealed in the 11 February edition of Science.

The conclusions suggest that the Earth's core is somewhat cooler than previous estimates--a 'mere' 4500-5000°C or 2000°C cooler than prevailing projections. The heat liberated during crystallisation is also more modest, about one-half the previously accepted value.

This conclusion implies, among other things, an increase in the speed at which the Earth's solid core grows at the expense of the liquid layer above it. That, in turn, may reduce the time expected for the entire core to solidify and begin to cool down. Moreover, the simulations reveal that the theoretical speed of transverse sound in solid iron at Earth's core is anomalously low, and essentially identical to that accurately measured for real seismic waves.

As a result, we could hypothesise, as Ronald Cohen of the Geophysical Laboratory in Washington, D.C., and Lars Stixrude of the Georgia Institute of Technology in Atlanta, Georgia, have done, that the Earth's solid core may be compared to a single iron crystal of cyclopic dimensions, a unique gigantic gem confined forever by geological history to the deepest recesses of our planet.

Erio Tosatti

Erio Tosatti is a professor of physics at the International School for Advanced Studies (SISSA) and a long-time consultant with ICTP's Condensed Matter Group.

For more detailed information, see Physics of Iron at Earth's Core Conditions in Science 287 (11 February 2000), pp. 1027-1030.

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