Where did the all the carbon responsible for every living thing come from? Scientists from Rice University suggest the answer to this question is a planetary collision.

“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,” says Rajdeep Dasgupta, a petrologist at Rice University.

Researchers struggle to explain how carbon and other volatile elements such as hydrogen and nitrogen stayed outside the Earth’s core and locked in the mantle. Most of these volatile elements should have either boiled away in Earth’s earliest days or got stuck in its metallic core.

Previous studies penned by these same Rice University researchers showed that if the carbon did not get boiled away “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.

Researchers have a mystery on their hands.

One theory most of us know about points the finger at meteorites and comets. It’s reasonable to believe Earth would have constantly been pelted meteorites and comets in the early years of the solar system. And if they impacted Earth more than about 100 million years after the formation of the solar system, they could have avoided the hot, magma ocean covering Earth during its formative years.

There’s just one problem with that theory according to the researchers. “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,” said Yuan Li, a postdoctoral researcher at Rice.

To come up with a possible answer, the researchers needed to think outside the box. They turned to the solar system’s other rocky bodies. Experiments were conducted to see how carbon’s affinity for iron may change if other elements, such as silicon or sulfur, were present.

“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,” said Dasgupta.

Dasgupta’s lab can recreate the conditions seen deep below Earth and other rocky planets. The new experiments showed that carbon could be kept out of the core, and stay in Earth’s mantle – if the iron alloys in the core were rich in silicon or sulfur.

One mechanism that could explain this ratio is a planetary collision. A Mercury-like embryonic planet (with an already formed silicon-rich core and carbon-rich mantle) slams into Earth and is absorbed by our planet.

planetary collision

“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,” said Dasgupta.

This research, like most, isn’t a slam dunk on the subject. The researchers’ paper only focused on carbon and sulfur. More research will be needed on other volatile elements. But this scenario could explain the carbon-sulfur ratio we see on Earth today.

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