All life on Earth is carbon-based, but where did that carbon come from? Models of the planet's early stages suggest that most of the initial reserves of carbon should either have escaped into space, or become locked away in the planet's core. Now a study by Earth scientists at Rice University suggests a new answer: a collision with a Mercury-like protoplanet some 4.4 billion years ago may have brought with it a fresh supply of the vital element.

In the very early stages of its development, it's generally accepted that the surface of the Earth was covered with a planet-wide ocean of magma. Over time, metals segregated down into the core and the iron-rich alloys there, which have a strong affinity for carbon, would have dragged much of that key element down with them. Of the remaining carbon, much more would have vaporized in the intense heat of the magma ocean and escaped into space. If this was indeed what occurred, as the planet cooled and became more hospitable, how did it retain enough carbon for life to begin development?

"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, co-author of the study.

One of the main theories suggests that volatile elements like carbon, sulfur, nitrogen and hydrogen were brought to Earth by meteorites after the core had finished forming and the magma had cooled.

"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," says Yuan Li, lead author of the study. "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 (mantle) of our planet."

The team looked beyond Earth to explore how different core compositions may affect how much carbon remains in the mantle of a planet.

"We thought we definitely needed to break away from the conventional core composition of just iron and nickel and carbon," says 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."

Using lab equipment that recreates the high-temperature, high-pressure conditions deep within a planet, the team experimented with different levels of sulfur and silicon, and mapped out how carbon levels in the mantle would be affected by those levels, as well as by variations in temperature and pressure.

The results showed that if a planet's iron-alloy core was rich in sulfur or silicon, carbon would be excluded from it, leaving more behind in the mantle. From there, the team compared those figures to the concentration of volatile elements we see in Earth's mantle, and suggest that a collision with a protoplanet with a sulfur- or silicon-rich core – and as a result, a carbon-rich mantle – could be responsible for the unusual abundance here.

"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," says 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."

While this scenario helps to explain Earth's current supplies of carbon and sulfur, the researchers acknowledge that more work needs to be done in order to reconcile the origins of the other volatile elements.

The research was published in the journal Nature Geoscience.

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