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| The ratio of volatile elements in Earth's mantle suggests that virtually all of the planet's life-giving carbon came from a collision with an embryonic planet approximately 100 million years after Earth formed. Credit: A. Passwaters/Rice University based on original courtesy of NASA/JPL-Caltech |
Research by Rice University Earth scientists suggests that virtually
all of Earth's life-giving carbon could have come from a collision about
4.4 billion years ago between Earth and an embryonic planet similar to
Mercury.
In a new study this week in Nature Geoscience, Rice
petrologist Rajdeep Dasgupta and colleagues offer a new answer to a
long-debated geological question: How did carbon-based life develop on
Earth, given that most of the planet's carbon should have either boiled
away in the planet's earliest days or become locked in Earth's core?
"The challenge is to explain the origin of the volatile elements like
carbon that remain outside the core in the mantle portion of our
planet," said Dasgupta, who co-authored the study with lead author and
Rice postdoctoral researcher Yuan Li, Rice research scientist Kyusei
Tsuno and Woods Hole Oceanographic Institute colleagues Brian Monteleone
and Nobumichi Shimizu.
Dasgupta's lab specializes in recreating the high-pressure and
high-temperature conditions that exist deep inside Earth and other rocky
planets.
His team squeezes rocks in hydraulic presses that can simulate
conditions about 250 miles below Earth's surface or at the core-mantle
boundary of smaller planets like Mercury.
"Even before this paper, we had published several studies that showed
that even if carbon did not vaporize into space when the planet was
largely molten, it would end up in the metallic core of our planet,
because the iron-rich alloys there have a strong affinity for carbon,"
Dasgupta said.
Earth's core, which is mostly iron, makes up about one-third of the
planet's mass. Earth's silicate mantle accounts for the other two-thirds
and extends more than 1,500 miles below Earth's surface. Earth's crust
and atmosphere are so thin that they account for less than 1 percent of
the planet's mass. The mantle, atmosphere and crust constantly exchange
elements, including the volatile elements needed for life.
If Earth's initial allotment of carbon boiled away into space or got
stuck in the core, where did the carbon in the mantle and biosphere come
from?
"One popular idea has been that volatile elements like carbon,
sulfur, nitrogen and hydrogen were added after Earth's core finished
forming," said Li, who is now a staff scientist at Guangzhou Institute
of Geochemistry, Chinese Academy of Sciences. "Any of those elements
that fell to Earth in meteorites and comets more than about 100 million
years after the solar system formed could have avoided the intense heat
of the magma ocean that covered Earth up to that point.
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| A schematic depiction of early Earth's merger with an embryonic planet similar to Mercury, a scenario supported by new high-pressure, high-temperature experiments at Rice University. Magma ocean processes could lead planetary embryos to develop silicon- or sulfur-rich metallic cores and carbon-rich outer layers. If Earth merged with such a planet early in its history, it could explain how Earth acquired its carbon and sulfur. Credit: Rajdeep Dasgupta |
"The problem with that idea is that while it can account for the
abundance of many of these elements, there are no known meteorites that
would produce the ratio of volatile elements in the silicate portion of
our planet," Li said.
In late 2013, Dasgupta's team began thinking about unconventional
ways to address the issue of volatiles and core composition, and they
decided to conduct experiments to gauge how sulfur or silicon might
alter the affinity of iron for carbon. The idea didn't come from Earth
studies, but from some of Earth's planetary neighbors.
"We thought we definitely needed to break away from the conventional
core composition of just iron and nickel and carbon," Dasgupta recalled.
"So we began exploring very sulfur-rich and silicon-rich alloys, in
part because the core of Mars is thought to be sulfur-rich and the core
of Mercury is thought to be relatively silicon-rich.
"It was a compositional spectrum that seemed relevant, if not for our
own planet, then definitely in the scheme of all the terrestrial
planetary bodies that we have in our solar system," he said.
The experiments revealed that carbon could be excluded from the
core—and relegated to the silicate mantle—if the iron alloys in the core
were rich in either silicon or sulfur.
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| Rajdeep Dasgupta. Credit: Jeff Fitlow/Rice University |
"The key data revealed how the partitioning of carbon between the
metallic and silicate portions of terrestrial planets varies as a
function of the variables like temperature, pressure and sulfur or
silicon content," Li said.
The team mapped out the relative concentrations of carbon that would
arise under various levels of sulfur and silicon enrichment, and the
researchers compared those concentrations to the known volatiles in
Earth's silicate mantle.
"One scenario that explains the carbon-to-sulfur ratio and carbon
abundance is that an embryonic planet like Mercury, which had already
formed a silicon-rich core, collided with and was absorbed by Earth,"
Dasgupta said. "Because it's a massive body, the dynamics could work in a
way that the core of that planet would go directly to the core of our
planet, and the carbon-rich mantle would mix with Earth's mantle.
"In this paper, we focused on carbon and sulfur," he said. "Much more work will need to be done to reconcile all of the volatile elements,
but at least in terms of the carbon-sulfur abundances and the
carbon-sulfur ratio, we find this scenario could explain Earth's present
carbon and sulfur budgets."
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