Carbon from a Cosmic Collision?

Scientists have long wondered how carbon-based life developed on Earth since most of the carbon on our planet should have either vaporized into space during the early molten days of Earth or have become locked in the Earth’s core. Now, research by scientists at Rice University suggests a cosmic explanation: that almost all of the carbon on our planet could have come from a collision between Earth and an embryonic planet some 4.4 billion years ago. One popular theory was that volatile elements such as carbon, sulfur, nitrogen, and hydrogen were added after Earth finished forming, and that any of these elements that fell to Earth in meteorites and comets could have then avoided the intense heat in previous eras of the planet. But while that theory could account for the abundance of volatile elements, there are no known meteorites that would produce the ratio of these elements in the silicate portion of Earth. The researchers turned their attention to our planetary neighbors and conducted experiments to determine how sulfur or silicon might change the affinity of iron for carbon. They began examining alloys rich in sulfur and silicon, in part because Mars is thought to be sulfur-rich and the core of Mercury is believed to be abundant in silicon. Their investigation revealed that carbon could be excluded from the core and relegated to the silicate mantle if the iron alloys within the core were rich in either silicon or sulfur. Thus, if an embryonic planet resembling Mercury that had already formed a silicon-rich core had collided with Earth and had been absorbed by our planet, the interaction could have led to the core of that planet going directly into the core of Earth, and the carbon-rich mantle mixing with the mantle of Earth. Researchers will continue to explore the sources of all the volatile elements, but this theory is a strong explanation for the abundance of Earth’s carbon and sulfur.