Crystal at the center of the earth

How Earth's inner core remains solid despite heat

Even though it is hotter than the surface of the Sun, the crystallized iron core of the Earth remains solid. A new study from KTH Royal Institute of Technology in Sweden may finally settle a longstanding debate over how that’s possible, as well as why seismic waves travel at higher speeds between the planet’s poles than through the equator.


What's it Like at the Centre of the Earth?

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http://www.bbc.co.uk/programmes/b0148vph The earth's inner core is made of iron nickel alloy. But what happens to it under the immense temperatures and pressures found there? Professor Kei Hirose spent 10 years devising an experiment to recreate these extreme conditions. Here he reveals the results...


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A Seismic Adventure

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There's a giant crystal buried deep within the Earth, at the very center, more than 3,000 miles down. It may sound like the latest fantasy adventure game or a new Indiana Jones movie, but it happens to be what scientists discovered in 1995 with a sophisticated computer model of Earth's inner core. This remarkable finding, which offers plausible solutions to some perplexing geophysical puzzles, is transforming what Earth scientists think about the most remote part of our planet.

"To understand what's deep in the Earth is a great challenge," says geophysicist Lars Stixrude. "Drill holes go down only 12 kilometers, about 0.2 percent of the Earth's radius. Most of the planet is totally inaccessible to direct observation." What scientists have pieced together comes primarily from seismic data. When shock waves from earthquakes ripple through the planet, they are detected by sensitive instruments at many locations on the surface. The record of these vibrations reveals variations in their path and speed to scientists who can then draw inferences about the planet's inner structure. This work has added much knowledge over the last ten years, including a puzzling observation: Seismic waves travel faster north-south than east-west, about four seconds faster pole-to-pole than through the equator.

This finding, confirmed only within the past two years, quickly led to the conclusion that Earth's solid-iron inner core is "anisotropic" -- it has a directional quality, a texture similar to the grain in wood, that allows sound waves to go faster when they travel in a certain direction. What, exactly, is the nature of this inner-core texture? To this question, the seismic data responds with sphinx-like silence. "The problem," says Ronald Cohen of the Carnegie Institution of Washington, "is then we're stymied. We know there's some kind of structure, the data tells us that, but we don't know what it is. If we knew the sound velocities in iron at the pressure and temperature of the inner core, we could get somewhere." To remedy this lack of information, Stixrude and Cohen turned to the CRAY C90 at Pittsburgh Supercomputing Center.

Getting to the Core

Earth's layered structure -- a relatively thin crust of mobile plates, a solid mantle with gradual overturning movement, and the outer and inner core of molten and solid iron.

Earth's layered structure -- a relatively thin crust of mobile plates, a solid mantle with gradual overturning movement, and the outer and inner core of molten and solid iron.

Don't believe Jules Verne. The center of the Earth is not a nice place to visit, unless you like hanging out in a blast furnace. The outer core of the Earth, about two-thirds of the way to the center, is molten iron. Deeper yet, at the inner core, the pressure is so great -- 3.5 million times surface pressure -- that iron solidifies, even though the temperature is believed to exceed 11,000 degrees Fahrenheit, hotter than the surface of the sun.

Despite rapid advances in high-pressure laboratory techniques, it's not yet possible to duplicate these conditions experimentally, and until Stixrude and Cohen's work, scientists could at best make educated guesses about iron's atom-to-atom architecture -- its crystal structure -- at the extremes that prevail in the inner core. Using a quantum-based approach called density-functional theory, Stixrude and Cohen set out to do better than an educated guess. With recent improvements in numerical techniques, density-functional theory had predicted iron's properties at low pressure with high accuracy, leading the researchers to believe that with supercomputing they could, in effect, reach 3,000 miles down into the inner core and pull out what they needed.

Rethinking Inner Earth

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On Earth's surface, iron comes in three flavors, standard crystalline forms known to scientists as body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed (hcp). Working with these three structures as their only input, Stixrude and Cohen carried out an extensive study -- more than 200 separate calculations over two years -- to determine iron's quantum-mechanical properties over a range of high pressures. "Without access to the C90," says Stixrude, "this work would have taken so long it wouldn't have been done."

Prevalent opinion before these calculations held that iron's crystal structure in the inner core was bcc. To the contrary, the calculations showed, bcc iron is unstable at high pressure and not likely to exist in the inner core. For the other two candidates, fcc and hcp, Stixrude and Cohen found that both can exist at high pressure and both would be directional (anisotropic) in how they transmit sound. Hcp iron, however, gives a better fit with the seismic data. All this was new information, but even more surprising was this: To fit the observed anisotropy, the grain-like texture of the inner core had to be much more pronounced than previously thought.

"Hexagonal crystals have a unique directionality," says Stixrude, "which must be aligned and oriented with Earth's spin axis for every crystal in the inner core." This led Stixrude and Cohen to try a computational experiment. If all the crystals must point in the same direction, why not one big crystal? The results, published in Science, offer the simplest, most convincing explanation yet put forward for the observed seismic data and have stirred new thinking about the inner core. 

Could an iron ball 1,500 miles across be a single crystal? Unheard of until this work, the idea has prompted realization that the temperature-pressure extremes of the inner core offer ideal conditions for crystal growth. Several high-pressure laboratories have experiments planned to test these results. A strongly oriented inner core could also explain anomalies of Earth's magnetic field, such as tilted field lines near the equator. "To do these esoteric quantum calculations," says Stixrude, "solutions which you can get only with a supercomputer, and get results you can compare directly with messy observations of nature and help explain them -- this has been very exciting." 


New theory explains how Earth’s inner core remains solid despite extreme heat

Published Feb 13, 2017

Seismic waves traveling in between the Earth's poles travel faster than those between the equator - one sign of the textured nature of Earth's solid iron inner core. A recent study from KTH explains how the core retains its unique solid form. (Image: Dixon Rohr)

Seismic waves traveling in between the Earth's poles travel faster than those between the equator - one sign of the textured nature of Earth's solid iron inner core. A recent study from KTH explains how the core retains its unique solid form. (Image: Dixon Rohr)

Even though it is hotter than the surface of the Sun, the crystallized iron core of the Earth remains solid. A new study from KTH Royal Institute of Technology may finally settle a longstanding debate over how that’s possible, as well as why seismic waves travel at higher speeds between the planet’s poles than through the equator. 

Spinning within Earth’s molten core is a crystal ball – actually a mass formation of almost pure crystallized iron – nearly the size of the moon. Understanding this strange, unobservable feature of our planet depends on knowing the atomic structure of these crystals – something scientists have been trying to do for years.

As with all metals, the atomic-scale crystal structures of iron change depending on the temperature and pressure the metal is exposed to. Atoms are packed into variations of cubic, as well as hexagonal formations. At room temperatures and normal atmospheric pressure, iron is in what is known as a body-centered cubic (BCC) phase, which is a crystal architecture with eight corner points and a center point. But at extremely high pressure the crystalline structures transform into 12-point hexagonal forms, or a close packed (HCP) phase.

At Earth’s core, where pressure is 3.5 million times higher than surface pressure – and temperatures are some 6,000 degrees higher – scientists have proposed that the atomic architecture of iron must be hexagonal. Whether BCC iron exists in the center of the Earth has been debated for the last 30 years, and a recent 2014 study ruled it out, arguing that BCC would be unstable under such conditions.

However, in a recent study published in Nature Geosciences, researchers at KTH found that iron at Earth’s core is indeed in the BCC phase. Anatoly Belonoshko, a researcher in the Department of Physics at KTH, says that when the researchers looked into larger computational samples of iron than studied previously, characteristics of the BCC iron that were thought to render it unstable wound up doing just the opposite.

Anatoly Belonoshko

Anatoly Belonoshko

“Under conditions in Earth’s core, BCC iron exhibits a pattern of atomic diffusion never before observed,” Belonoshko says.

Belonoshko says the data also shows that pure iron likely accounts for 96 percent of the inner core’s composition, along with nickel and possibly light elements.

Their conclusions are drawn from laborious computer simulations performed using Triolith, one of the largest Swedish supercomputers. These simulations allowed them to reinterpret observations collected three years ago at Livermore Lawrence National Laboratory in California. “It appears that the experimental data confirming the stability of BCC iron in the Core were in front of us – we just did not know what that really meant,” he says.

At low temperature BCC is unstable and crystalline planes slide out of the ideal BCC structure. But at high temperatures, the stabilization of these structures begins much like a card game – with the shuffling of a “deck”. Belonoshko says that in the extreme heat of the core, atoms no longer belong to planes because of the high amplitude of atomic motion.

“The sliding of these planes is a bit like shuffling a deck of cards,” he explains. “Even though the cards are put in different positions, the deck is still a deck. Likewise, the BCC iron retains its cubic structure.”

Such a shuffling leads to an enormous increase in the distribution of molecules and energy – which leads to increasing entropy, or the distribution of energy states. That, in turn, makes the BCC stable.

Normally, diffusion destroys crystal structures turning them into liquid. In this case, diffusion allows iron to preserve the BCC structure. “The BCC phase goes by the motto: 'What does not kill me makes me stronger',” Belonoshko says. “The instability kills the BCC phase at low temperature, but makes the BCC phase stable at high temperature.”

He says that this diffusion also explains why the Earth’s core is anisotropic – that is, it has a texture that is directional – like the grain of wood. Anisotropy explains why seismic waves travel faster between the Earth’s poles, than through the equator.

"The unique features of the Fe BCC phase, such as high-temperature self-diffusion even in a pure solid iron, might be responsible for the formation of large-scale anisotropic structures needed to explain the Earth inner core anisotropy,” he says. “The diffusion allows easy texturing of iron in response to any stress.”

The prediction opens the path to understanding the interior of the Earth and eventually to predicting Earth’s future, Belonoshko says. “The ultimate goal of Earth Sciences is to understand the past, present and future of the Earth - and our prediction allows us to do just that.”

David Callahan

Read the paper in Nature Geosciences

Contact Anatoly Belonoshko at anatoly@kth.se or +46 8 790 82 88

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