Among the many distinctive characteristics of our planet, there's one very important, life-related fact: Earth is surprisingly carbon-rich. This abundance has been quite the mystery, but now researchers think the carbon might have been brought to Earth by a collision with a Mercury-sized object.

The very young Earth was a molten world, which means volatiles like carbon and sulfur had two options: Either evaporate into space or sink in the core by reacting with heavy metals. An international team of researchers has simulated those extreme conditions and discovered that to enrich the planet, these elements must have made it to Earth at a later date.  

“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, from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, in a statement. “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.”

This idea has a flaw. No known meteorite has the right chemical abundance to explain the element ratio we observe in the crust and mantle. For this reason, the team decided to explore different core-composition scenarios. The Earth’s core is believed to be composed of iron and nickel, but the researchers suggest it could also be rich in sulfur or silicon.

When the team simulated this scenario under realistic conditions, they discovered that carbon remained within the outer layer of the planet. They concluded that, given the right conditions, a large carbon-rich object could explain the abundance of carbon on Earth.

^{This figure shows the proto-Earth’s merger with a potentially Mercury-like object. Rajdeep Dasgupta} 

“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,” co-author Rajdeep 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."

The research is published in Nature Geophysics, and although it doesn’t explain the complete composition of the planet, it may solve the mystery of where the carbon, and later life, came from.