Most of the Earth’s life-giving carbon may have come from a collision about 4.4 billion years ago between our planet and an embryonic planet similar to Mercury, scientists, including one of Indian origin, have found.
Rajdeep Dasgupta from Rice University in the US and his colleagues studied how carbon-based life developed on Earth, given that most of the planet’s carbon should have either boiled away in the planet’s earliest days or become locked in Earth’s core.
“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 Dasgupta.
“We had published several studies that showed that even if carbon did not vapourise 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.
“One popular idea has been that volatile elements like carbon, sulphur, nitrogen and hydrogen were added after Earth’s core finished forming,” said Yuan Li, who was a postdoctoral researcher at Rice at the time of the study.
“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,” said Li, who is now at Chinese Academy of Sciences.
“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,” Li said.
In late 2013, Dasgupta’s team decided to conduct experiments to gauge how sulphur or silicon might alter the affinity of iron for carbon.
“We began exploring very sulphur-rich and silicon-rich alloys, in part because the core of Mars is thought to be sulphur-rich and the core of Mercury is thought to be relatively silicon-rich,” Dasgupta said.
Experiments showed that carbon could be excluded from the core – and relegated to the silicate mantle – if the iron alloys in the core were rich in either silicon or sulphur.
The team mapped out the relative concentrations of carbon that would arise under various levels of sulphur and silicon enrichment, and the researchers compared those concentrations to the known volatiles in Earth’s silicate mantle.
“One scenario that explains the carbon-to-sulphur 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,” he said.
The research was published in the journal Nature Geoscience.