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Earth's Mysteriously Light Core Contains Brimstone
By Elizabeth Goldbaum, Staff Writer | June 18, 2015 07:22am ET
Biblical views of the center of the Earth as a hellish pit raging with fire and brimstone have some support
from new research. Scientists have found that the vast majority of brimstone — reverently referred to in
biblical times as "burning stone," but now known more commonly as sulfur — dwells deep in the Earth's
core.
"In a way, we can also say that we have life imitating art," study lead author Paul Savage, a research
scientist in the Department of Earth Sciences at Durham University in the United Kingdom, said in a
statement." For millennia, tales have been told of the underworld being awash with fire and brimstone.
Now at least, we can be sure of the brimstone."
The researchers estimate that the Earth's core contains 10 times the amount of sulfur than in the rest of
the world, or comparable to about 10 percent of the mass of the moon. [Religion and Science: 6 Visions
of Earth's Core]
Inside Earth
Scientists have generally understood that at the time of Earth's formation, heavy metals such as iron and
nickel sunk to the planet's core, and light elements, like oxygen, silicon, aluminum, potassium, sodium,
and calcium, mostly concentrated in the outer layers of the Earth, in the mantle and crust.
However, the mass of the Earth's solid inner core, which is too light to be composed solely of metal, has
been an enduring inconsistency in our understanding of the planet's distribution of elements. To explain
the core's lighter-than-expected weight, scientists assumed that the core had to contain some lighter
elements, such as oxygen, carbon, silicon and sulfur.
"Scientists have suspected that there is sulphur in the core for some time, but this is the first time we
have solid geochemical evidence to support the idea," Savage said.
Confirming the presence of lighter elements, like sulfur, in the core, provides information about the
temperatures, pressures and oxygen content in the Earth's mantle, which surrounds the core and
separates it from the crust on which we walk. "It'd be nice to know what the Earth is formed of, as a
fundamental aspect of understanding the Earth," Savage told Live Science.
Peeling back the layers
Without the technology to dig 1,800 miles (2,900 kilometers, or the equivalent of around 3,000 Eiffel
Towers stacked on top of one another), scientists looked for clues created by a 4.47 billion-year-old
impact— the moon-forming collision between Earth and a large, planet-size body called Theia.
"The giant impact wouldn't have just formed the moon; it wouldn't have just sort of sliced a bit of
material off and end up becoming the moon," Savage said. "The amount of energy involved in this sort
of impact would have, if not completely, it would have partially melted the Earth's mantle to a certain
depth." When the mantle melted, some of its sulfur-rich liquid seeped into the core, and some of it
evaporated into space, he added.
"You could lose a lot of it during evaporation," Savage said. "Just by looking at the sulfur, we can't really
tell much about how much is in the core versus how much has been lost to space," making sulfur
virtually impossible to directly measure. [Photo Timeline: How the Earth Formed]
To track and quantify the elusive sulfur, the researchers looked to copper isotopes (atoms of the same
element with different numbers of neutrons). "We chose copper, because it is a chalcophile element,
which means it prefers to be in sulphide-rich material — so it is a good element to trace the fate of
sulphur on Earth," Frédéric Moynier, the study's senior author and a professor at the Institut de
Physique du Globe in Paris, said in a statement. "Generally, where there is copper, there is sulphur;
copper gives us a proxy measurement for sulphur."
Searching for sulfur
The researchers measured the copper isotope values from both the mantle and core to discover where
they would find sulfur. Meteorites were used to represent the "bulk Earth," which includes the core,
mantle and crust. Meteorites are jumbles of extraterrestrial matter that have been orbiting the sun
since even before planets formed. "They're like cosmic sediments," Savage said. "If we got a planet and
milled it down, if we sort of crushed it up and mixed it around, that's what we assume would be in
meteorites."
Samples formed from lava eruptions, as well as from tectonic events, which pushed the mantle onto the
surface of the Earth, were used to represent so-called "bulk silicate Earth" values, which include the
copper content in the mantle and crust. Researchers can then figure out the copper content in the
Earth's core by subtracting the "bulk silicate Earth" value from the "bulk Earth" value.
The scientists measured a heavy "bulk silicate Earth" copper isotope value compared with the "bulk
Earth" value, which could indicate that the mantle has a lot of heavy copper and the core does not.
However, through experiments, they found that the "copper in the core should be slightly heavy
compared to the mantle — so the core cannot balance out the heavy mantle compared to meteorites,
because it is also heavy," Savage said. If there are a lot of heavy copper isotopes in one part of the Earth,
another part will have a lot of light copper isotopes.
To explain copper's "heaviness" in both the mantle and core, the researchers predicted that a sulfur-rich
liquid with "light" copper formed after the impact that created the moon. "So the [melted mantle] is
light, the mantle is heavy, and the two, when mixed together, would equal bulk Earth (meteorites),"
Savage said.
After the Earth formed from meteorites and other extraterrestrial matter like dust and rock, it started to
melt, forming its core. During core formation, some "heavy" copper left the melting mantle and entered
the core, leaving the mantle with "lighter" copper, Savage said. Then, following the giant moon-forming
impact, the Earth's mantle re-melted, forming a sulfur-rich liquid. "Light" copper attached itself to the
liquid, leaving the mantle with the "heavier" copper, reflected in the compositions measured in presentday lava and rocks, the researchers said.
"This study is the first to show clear geochemical evidence that a sulphide liquid must have separated
from the mantle early on in Earth's history — which most likely entered the core," Savage said.
The researchers detailed their findings yesterday (June 16) in the journal Geochemical Perspectives
Letters.
Editor's Note: This story was updated to reflect the accurate number of Eiffel Towers it would take to get
to the Earth's core.
Elizabeth Goldbaum is on Twitter. Follow Live Science @livescience, Facebook & Google+. Original article
on Live Science
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