Life May Stem From|Earth-Planet Smashup

(CN) - Scientists may finally know how Earth's life-giving carbon managed to avoid being boiled away or trapped in the planet's core: It arrived through a planetary collision 4.4 billion years ago.

A team of researchers from Rice University believes that a collision between our planet and an embryonic planet similar to Mercury roughly 4.4 billion years ago may be why carbon-based life continued to develop, despite circumstances that otherwise would have left them extinct.

"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 study co-author Rajdeep Dasgupta, whose work was published Monday in the journal Nature Geoscience.

Using tools that enabled the team to recreate the high-pressure and high-temperature conditions that existed deep inside Earth and other rocky planets, the researchers squeezed rocks in hydraulic presses to simulate conditions roughly 250 miles below our planet's surface, or at the core-mantle boundary of smaller planets like Mercury.

Earth's core, which is mostly iron, represents roughly 1/3 of the planet's mass, while the mantle accounts for the rest of the mass, extending more than 1,500 miles beneath Earth's surface. The planet's crust and atmosphere are so thin that they account for less than 1 percent of Earth's mass.

"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.

Scientists believe that Earth's initial supply of carbon boiled away into space or got stuck in the core, which begged the question: What was the new source of carbon in the planet's biosphere?

"One popular idea has been that volatile elements like carbon, sulfur, nitrogen and hydrogen were added after Earth's core finished forming," said lead author Yuan Li.

"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."

After conducting experiments to address issues of volatiles and core composition, the team realized that carbon could be excluded from the core and relegated to the mantle if the iron alloys — a mixture of metals, or a mixture of metal and another element — in the core were rich in either sulfur or silicon.

"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.

So the team mapped out the relative concentrations of carbon that would come from sulfur and silicon enrichment and compared the concentrations to 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.

While the findings are promising, Dasgupta says more research is needed to confirm the team's theory.

"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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