Challenging core belief: Have we misunderstood how Earth's solid center formed?

February 7, 2018, Case Western Reserve University
A composite image of the Western hemisphere of the Earth. Credit: NASA

It is widely accepted that the Earth's inner core formed about a billion years ago when a solid, super-hot iron nugget spontaneously began to crystallize inside a 4,200-mile-wide ball of liquid metal at the planet's center.

One problem: That's not possible-or, at least, has never been easily explained-according to a new paper published in Earth and Planetary Science Letters from a team of scientists at Case Western Reserve University.

The research team-comprised of post-doctoral student Ludovic Huguet; Earth, Environmental, and Planetary Sciences professors James Van Orman and Steven Hauck II; and Materials Science and Engineering Professor Matthew Willard-refer to this enigma as the "inner-core nucleation paradox."

That paradox goes like this: Scientists have known for more than 80 years that a crystallized inner core exists. But the Case Western Reserve team asserts that this widely accepted idea neglects one critical point-one that, once added, would suggest the inner core shouldn't exist.

The inner core contradiction

Here's why: While it is well known that a material must be at or below its freezing temperature to be solid, it turns out that making the first crystal from a liquid takes extra energy. That extra energy-the nucleation barrier-is the ingredient that models of Earth's deepest interior have not included until now.

To overcome the nucleation barrier and start to solidify, however, the liquid has to be cooled well below its freezing point-what scientists call "supercooling."

Alternatively, something different has to be added to the of the core-at the center of the planet-that substantially reduces the amount of required supercooling.

But the nucleation barrier for -at the extraordinary pressures at the center of the Earth-is enormous.

"Everyone, ourselves included, seemed to be missing this big problem-that metals don't start crystallizing instantly unless something is there that lowers the energy barrier a lot," Hauck said.

The Case Western Reserve team contends the most obvious solutions are suspect:

" That the inner core was somehow subjected to a massive supercooling of about 1,800 degrees Fahrenheit (1,000 Kelvin)-well beyond the amount of cooling scientists have concluded. If the Earth's center had reached this temperature, nearly the entire core should be crystallizing rapidly, but the evidence indicates that it is not.

"That something happened to lower the nucleation barrier, allowing crystallization to occur at a higher temperature. Scientists do this in the lab by adding a piece of solid metal to a slightly supercooled liquid metal, causing the now-heterogeneous material to quickly solidify. But it's difficult to figure on an earth-sized scale how this could have happened, how a nucleation enhancing solid could have found its way to the center of the planet to allow for the hardening (and expansion) of the inner core, Huguet said.

"So, if the core is a pure (homogenous) liquid, the inner core shouldn't exist at all because it could not have been supercooled to that extent," Van Orman said. "And if it's not homogeneous, how did it become so?

"That's the inner-core nucleation paradox."

Possible answers

Then how did the form?

At the moment, the team's favored idea is akin to the second solution above: that large bodies of solid metal slowly dropped from the rocky mantle and into the core to lower the .

But that would require a massive nugget-maybe the size of a large city-to be heavy enough to drop through the mantle and then large enough to make it the core without entirely dissolving.

If that's the case, "we need to figure out how that could actually happen," Van Orman said.

"On the other hand," he said, "is there some ordinary feature of planetary cores that we have not thought of before-something that allows them to overcome that ?

"It's time for the whole community to think about this problem and how to test it. The inner core exists, and now we have to figure out how it got there."

Explore further: New study indicates Earth's inner core was formed 1 - 1.5 billion years ago

Related Stories

Just what sustains Earth's magnetic field anyway?

June 1, 2016

Earth's magnetic field shields us from deadly cosmic radiation, and without it, life as we know it could not exist here. The motion of liquid iron in the planet's outer core, a phenomenon called a "geodynamo," generates the ...

Why doesn't Venus have a magnetosphere?

December 11, 2017

For many reasons, Venus is sometimes referred to as "Earth's twin" (or "sister planet," depending on who you ask). Like Earth, it is terrestrial (i.e. rocky) in nature, composed of silicate minerals and metals that are differentiated ...

How ice in clouds is born

November 8, 2017

Something almost magical happens when you put a tray full of sloshing, liquid water into a freezer and it comes out later as a rigid, solid crystal of ice. Chemists at the University of Utah have pulled back the curtain a ...

Gradients in the Earth's outermost core

December 8, 2010

Evidence that the outermost portion of the Earth’s core is stratified is provided by earthquake data reported by scientists at the University of Bristol this week in Nature.

Recommended for you

Life cycle of sulphur predicts location of valuable minerals

October 23, 2018

A team of researchers from The University of Western Australia and two Canadian universities has applied a first-of-its-kind technique that measures the long-term life cycle of sulphur, helping to explain the preferential ...

4 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

24volts
5 / 5 (3) Feb 07, 2018
"So, if the core is a pure (homogenous) liquid, the inner core shouldn't exist at all because it could not have been supercooled to that extent," Van Orman said. "And if it's not homogeneous, how did it become so?"

It shouldn't be. There are a lot of metals that are denser than iron that have to also be mixed up in it. It's probably layered like an onion to some extent.
Nik_2213
not rated yet Feb 07, 2018
The Moon-forming mega-impact was the only one we're confident about, but there must have been many more. They've been proposed, IIRC, as triggering formation of the first cratons. Below those, cooler, denser 'blobs' must have rained onto the core, perhaps seeding it ?
tallenglish
not rated yet Feb 09, 2018
Personally I think dark matter has to be involed to compress the rock/metal and give it the extra kick it needs. The other option to have metals like Uranium and Plutonium in the mix which would decay to iron.
HenryE
not rated yet Feb 09, 2018
Why is there no mention of the intense pressure that is found at the core? It is highly likely that it plays a fundamental role in this. Temperature is only part of the picture.

After all, the pressure at the core supposedly ranges from 3,300,000 to 3,600,000 atmospheres. Have laboratory experiments been performed to see how molten metal behaves under such an enormous pressure gradient?

It is good to theorize and crunch numbers but, until an experiment is performed that shows what actually happens at that heat and pressure, we honestly won't be any closer to a real understanding of core dynamics.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.