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Getting to the depth of the matter

A host of new findings about Earth’s core questions long-standing assumptions

Written by New York Times | Published: June 3, 2012 12:44:47 am


Geologists have long known that Earth’s core,some 1,800 miles beneath our feet,is a dense,chemically doped ball of iron roughly the size of Mars and every bit as alien. It’s a place where pressures bear down with the weight of 3.5 million atmospheres,like 3.5 million skies falling at once on your head,and where temperatures reach 10,000 degrees Fahrenheit—as hot as the surface of the Sun.

Researchers have also known that Earth’s inner Martian makes its outer portions look and feel like home. The core’s heat helps animate the giant jigsaw puzzle of tectonic plates floating far above it,to build up mountains and gouge out seabeds. At the same time,the jostling of core iron generates Earth’s magnetic field,which blocks dangerous cosmic radiation,guides terrestrial wanderers and brightens northern skies with scarves of auroral lights.

Now it turns out that existing models of the core,for all their drama,may not be dramatic enough. Reporting recently in the journal Nature,Dario Alfe of University College London and his colleagues presented evidence that iron in the outer layers of the core is frittering away heat through the wasteful process called conduction at two to three times the rate of previous estimates.

The core accounts for only one-sixth of the volume of the Earth but one-third of its mass,the great bulk of iron maintained in liquid form by the core’s hellish heat. Only in the inner core does pressure win out over temperature and the iron solidify. The core’s thermal bounty is thought to be overwhelmingly primordial,left over from the planet’s gravitational formation and mostly trapped inside by the rocky muffler of the mantle. Yet as the hot Earth orbits relentlessly through frigid space,the core can’t help but obey the second law of thermodynamics and gradually shed some of its stored heat. The heat can be transferred through two basic pathways: conducted straight outward,the way heat travels along a frying pan,or confected out in plumes,the way hot air rises in the atmosphere or soup bubbles in a pot.

In their report in Nature,Alfe and his colleagues used powerful computers and basic considerations of atomic behaviour to calculate the properties of iron and iron alloys under the presumed conditions of the core. They concluded that the core was losing two to three times as much heat to conduction as previously believed,which would leave too little thermal energy to account for the convective forces that power the Earth’s geodynamo.

The theoretical consequences of this discrepancy are far-reaching. The scientists say something else must be going on in Earth’s depths to account for the missing thermal energy in their calculations. They and others offer these possibilities: 1. The core holds a much bigger stash of radioactive material than anyone had suspected,and its decay is giving off heat. 2. The iron of the innermost core is solidifying at a startlingly fast clip and releasing the latent heat of crystallisation in the process. 3. The chemical interactions among the iron alloys of the core and the rocky silicates of the overlying mantle are much fiercer and more energetic than previously believed.

“From what I can tell,people are excited” by the report,Alfe said. “They see there might be a new mechanism going on they didn’t think about before.”

Researchers elsewhere have discovered a host of other anomalies and surprises. They’ve found indications that the inner core is rotating slightly faster than the rest of the planet,although geologists disagree on the size of that rotational difference and on how,exactly,the core manages to resist being gravitationally locked to the surrounding mantle.

Miaki Ishii and her colleagues at Harvard have proposed that not only is there an outer core of liquid iron encircling a moon-size inner core of solidified iron,but seismic data indicate that nested within the inner core is another distinct layer they call the innermost core: a structure some 375 miles in diameter that may well be almost pure iron,with other elements squeezed out.

Some researchers seek to simulate core conditions on a small,fleeting scale: balancing a sample of iron alloy on a diamond tip,for example,and then subjecting it to intense pressure by shooting it with a bullet. Others rely on complex computer models. Everybody cites a famous paper in Nature in 2003 by David J. Stevenson,a planetary scientist at Caltech,who waggishly suggested that a very thin,long crack be propagated in the Earth down to the core,through which a probe in a liquid iron alloy could be sent in.

The core does leave faint but readable marks on the surface,by way of the magnetic field that loops out from the vast chthonic geodynamo of swirling iron,permeating the planet and reaching thousands of miles into space. Magnetic particles trapped in neat alignment in rocks reveal that the field,and presumably the core structures that generate it,has been around for well over 3 billion of Earth’s 4.5 billion years.

For reasons that remain mysterious,the field has a funny habit of flipping. Every 100,000 to a million years or more,the north-south orientation of the magnetosphere reverses,an event often preceded by an overall weakening of the field. As it turns out,the strength of our current north-pointing field,which has been in place for nearly 800,000 years,has dropped by about 10 per cent in the past century,suggesting we may be headed toward a polarity switch.

The portrait of the core thus emerging from recent studies is structured and wild,parts of it riven with more weather than the sky.

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