All of Earth’s carbon came from planetary collision 4.4 billion years ago

The carbon and sulfur that helped give rise to life on our planet may have been the result of a collision between the Earth and an embryonic world similar to Mercury, according to research led by scientists at Rice University and published this week in Nature Geosciences.

Said collisions would have occurred approximately 4.4 billion years ago and would explain how carbon-based life developed on Earth despite the fact that most of the element should have either been sealed in the planet’s core or boiled away shortly after its formation, Rajdeep Dasgupta, co-author of the new study, and his colleagues said in a statement Monday.

“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,” explained Dasgupta, a petrologist at the university. “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.”

The results of those earlier studies left the researchers with a mystery: if that life-giving carbon should have either been vaporized or sucked into the planet’s core, how was there still enough of it left in the mantle and the biosphere, and where did those volatile elements come from?

Collision would explain Earth’s carbon and sulfur budgets

Dasgupta, postdoctoral researcher Yuan Li, Rice research scientist Kyusei Tsuno, and a pair of scientists from the Woods Hole Oceanographic Institute in Massachusetts (Nobumichi Shimizu and Brian Monteleone) conducted a series of simulations designed to measure how either sulfur or silicon could have altered iron’s affinity for carbon.

“We thought we definitely needed to break away from the conventional core composition of just iron and nickel and carbon,” said Dasgupta. “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 added. As it turns out, the experiments revealed that if the iron alloys in the core were rich in either one of those elements, then carbon would have been relegated to the Earth’s mantle.

The researchers then determined what the different concentrations of carbon would have been under different levels of silicon or sulfur enrichment, and compared those to the amount of the known volatiles in the planet’s silicate mantle. The scenario that best explained Earth’s current elemental makeup involved an embryonic world with a silicon-rich core similar to Mercury.

That planet “collided with and was absorbed by Earth,” said Dasgupta. “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… 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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Image credit: NASA/JPL Caltech

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