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Earth's core is a billion years old

Earth's layers shown in this modified NASA image.
(Image: © Shutterstock)

The solid inner core of Earth is a mere billion years old, new research finds.

Modern Earth is like a layer cake, with a solid outer crust, a hot, viscous mantle, a liquid outer core and a solid inner core. That solid inner core is growing slowly as the liquid iron in the core cools and crystallizes. This process helps power the churning motion of the liquid outer core, which in turn creates the magnetic field that surrounds Earth and helps protect the planet from harmful cosmic radiation.

In other words, the inner core is pretty important.

But not much is known about the history of this 1,500-mile-wide (2,442 kilometers) iron ball. Estimates of its age have ranged from half a billion years to more than 4 billion years, almost as old as 4.5-billion-year-old Earth itself. Now, researchers have squeezed a miniscule piece of iron between two diamonds and blasted it with lasers to arrive at a new estimate of 1 billion to 1.3 billion years old — a date range that coincides with a measurable strengthening of the Earth's magnetic field that happened around the same time.

Related: 10 ways Earth revealed its weirdness

"Earth is unique in our solar system in that it has a magnetic field, and that it's habitable," study author Jung-Fu Lin, a geoscientist at the University of Texas at Austin, told Live Science. "Eventually our results could be used to think about why other planets in our solar system don't have magnetic fields."

The geodynamo

Earth's magnetic field is powered by what scientists call the "geodynamo." That's the movement of the iron-rich outer core, which turns the planet into a giant, if somewhat messy, magnet. The geodynamo is responsible for Earth's North Pole and South Pole and the invisible shield of magnetism that deflects and traps charged particles flowing from the sun. These particles would otherwise slowly strip Earth of its atmosphere.

Part of the movement of the inner core is powered by heat, known as its thermal energy source. As Earth's core gradually cools, it crystallizes from the inside out. This crystallization process releases energy that can further power the movement of the still-liquid outer core. This energy release from crystallization is called the compositional energy source of the geodynamo, Lin said.

Lin and his team wanted to use experimental evidence to pin down the energy from each of these sources. Knowing the amount of energy would allow them to estimate the age of the inner core.

To do this, the researchers recreated the conditions of the core on a tiny scale. They heated a piece of iron a mere 6 microns thick (about the same as the length of a red blood cell) to temperatures up to 4,940 degrees Fahrenheit (2,727 degrees Celsius), and squeezed the sample between two diamonds to match the extreme pressures at Earth's core. They then measured the conductivity of the iron under these conditions.

A young core

This conductivity measurement allowed the researchers to calculate the thermal cooling of the core that is available to power the geodynamo. They found that the geodynamo drew on about 10 terawatts of energy from the cooling core — just over a fifth of the amount of heat the Earth dissipates into space from its surface (46 terawatts, Live Science previously reported).

One they calculated the amount of energy loss, the researchers could calculate the age of the Earth's inner core, Lin said. Knowing the rate of energy loss allowed the researchers to calculate how long it would take to get a solid mass the size of today’s core from a blob of molten iron.

The 1 billion to 1.3 billion year result suggests that Earth's core is "actually relatively young," Lin said.

This estimate isn't as young as some estimates, such as one published in 2016 in the journal Nature that used similar methods but found the core was a mere 700 million years old. Lin said the new experiment used more reliable ways of handling the pressures and temperatures generated on the core, making that younger estimate unlikely.

Ancient magnetic rocks revealed that the magnetic field suddenly strengthened between 1 billion and 1.5 billion years ago, a 2015 study in the journal Nature found. The new age lines up nicely with that evidence, as the crystallization of the inner core would have provided a "boost" to the magnetic field, Lin said.

There are still questions about the way heat moves around in the core, Lin said. Unlike the sample they tested, the core isn't just iron — it also contains lighter elements such as carbon, hydrogen, oxygen, silicon and sulfur. But the proportions of these light elements are unknown, making it difficult to know how they change the conductivity of the inner core. That's what Lin and his team are working on now.

"We are trying to understand how the existence of those light elements would actually affect the thermal transport properties of iron at such high-pressure, high temperature conditions," Lin said.

The researchers reported their findings Aug. 13 in the journal Physical Review Letters.

Originally published in Live Science.

  • Broadlands
    " the core isn't just iron — it also contains lighter elements such as carbon, hydrogen, oxygen, silicon and sulfur."

    How do they know that these other elements are in the Earth's core?
    Reply
  • Valentine Michael Smith
    Yes, my interest ran similar, my main interest in such things - how do they know, or figure out such?
    Reply
  • Chem721
    Broadlands said:
    " the core isn't just iron — it also contains lighter elements such as carbon, hydrogen, oxygen, silicon and sulfur."

    How do they know that these other elements are in the Earth's core?


    Hello again, Broadlands.

    The composition is largely based on pure estimates, as one might imagine. This is a direct quote from Wiki on the earth's core*:

    "There is still no direct evidence about the composition of the inner core. However, based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of an iron–nickel alloy.

    At the known pressures and estimated temperatures of the core, it is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That result implies the presence of lighter elements in the core, such as silicon, oxygen, or sulfur, in addition to the probable presence of nickel. Recent estimates (2007) allow for up to 10% nickel and 2–3% of unidentified lighter elements.
    According to computations by D. Alfè and others, the liquid outer core contains 8–13% of oxygen, but as the iron crystallizes out to form the inner core the oxygen is mostly left in the liquid.
    Laboratory experiments and analysis of seismic wave velocities seem to indicate that the inner core consists specifically of ε-iron, a crystalline form of the metal with the hexagonal close-packed (hcp) structure. That structure can still admit the inclusion of small amounts of nickel and other elements.
    Also, if the inner core grows by precipitation of frozen particles falling onto its surface, then some liquid can also be trapped in the pore spaces. In that case, some of this residual fluid may still persist to some small degree in much of its interior.. "

    end quote.


    It is likely these lighter elements were estimated as they simply represent most of the elements that exist in the universe higher than hydrogen, helium and lithium (except nickel). Note that D. Alfè et al. suggest the presence of 8-13% of that pesky oxygen in the outer core.

    It does seem like a bit of a WAG.


    * https://en.wikipedia.org/wiki/Earth%27s_inner_core#Composition
    Reply
  • Broadlands
    Yes Chem721... It is clear that it is a SWAG... a big scientific WAG. If the Earth's magnetic field is strengthened then a SWAG is about the only thing humans can do about it. Guess.
    Reply
  • Chem721
    Broadlands said:
    It is clear that it is a SWAG


    Yes, it is clearly a SWAG. But I had to chase down the oxygen aspect of the outer core. And here is what I found in the abstract * :

    "The investigation is based on the application of the implementation of quantum mechanics known as density functional theory. We shall show that these techniques are very accurate at predicting the properties of iron, and therefore can be usefully used to study the properties of the core."

    end quote

    From this they somehow managed to guess that the outer core contained a very light element, oxygen, at 8%-13% of the outer core content.

    It certainly appears to be a bit of a SWAG, perhaps even a tad more.


    "Temperature and composition of the Earth's core" :

    https://www.tandfonline.com/doi/abs/10.1080/00107510701529653
    Reply
  • TorbjornLarsson
    My response to the same question elsewhere - and congratulate to asking questions, even if it is tainted by personal opinion on the response - was much the same as Chem 721 but add some detail.

    Iron is the most common siderophile element (well, it would be) https://en.wikipedia.org/wiki/Goldschmidt_classification#Siderophile_elements ].

    "There is still no direct evidence about the composition of the inner core. However, based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of an iron–nickel alloy.

    At the known pressures and estimated temperatures of the core, it is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That result implies the presence of lighter elements in the core, such as silicon, oxygen, or sulfur, in addition to the probable presence of nickel. Recent estimates (2007) allow for up to 10% nickel and 2–3% of unidentified lighter elements."

    https://en.wikipedia.org/wiki/Earth's_inner_core#Composition ]

    No, it is not a personal opinion "SWAG", as can be seen from the quote. The density and chemistry fits perfectly as well as the formation theory of the differentiated planet, and we have no other alternative. The term "direct evidence" is an opinion - because how do you define "direct/indirect" in a testable way? - but in this case it means we have no example of core material transported to the surface which annoys some. The rest accepts it is an iron core and move on with questions that we have no clear answer to.

    I should also add that the mission to Psyche intends to sample what we think is a core remnant asteroid. https://en.wikipedia.org/wiki/Psyche_(spacecraft) ]. With a bit of luck we will test the core composition in more detail by 2026-2031. Meanwhile, we have the pallasites which formation constraint now has forced people to conclude that they sample the core/mantle interface of partially differentiated bodies https://www.sciencedirect.com/science/article/abs/pii/S0012821X20303630?via=ihub ]. . Yes, they contain lots of iron!
    Reply
  • Edward
    Metallic meteorites have repeatedly struck the earth and they are iron-nickel. Since they purportedly represent the former interiors of a much larger body (proto-planet say) which was smashed probably by collision in the asteroid belt, why not iron-nickel in the interior of the earth also? The denser iron, nickel and maybe uranium would differentiate in a liquid state during earth's violent formation. This is fundamental extractive metallurgy where the slag (the earth's crust) floats on the dense underlying metal (iron). Same as happens in the manufacture of iron in a blast furnace. The earth is too heavy to have slag all the way to the centre so why not a metallic core such as some meteorites and asteroids exhibit.
    Reply
  • Chem721
    Edward said:
    why not iron-nickel in the interior of the earth also?

    There is no doubt about an iron-nickel core (1). We would not have a magnetic field without it. The issue was really about the lighter elements. The concept still is intriguing, especially about oxygen. I wonder if it is true. If there really is a lot of oxygen trapped in planetary cores, it could be released periodically over time to provide a substantial oxygenated atmosphere, if it were made gaseous. Not likely lasting long, but it could explain unique iron formations in earth's geological record.

    I wonder if Broadlands is still out there and can shoot down any notion of primordial oxygen in or near the core, and the potential for periodic mass releases as a gas over geological time. He is an expert on this stuff. Stranger things have been known to happen.


    (1) https://en.wikipedia.org/wiki/Planetary_differentiation
    Reply
  • Chem721
    Here is some additional information on the potential for oxygen inside the earth :

    https://www.livescience.com/64940-pressurized-oxygen-earth-core.html
    Reply