Researchers from Rice University say that around 4.4 billion years ago, a Mercury-like planet smashed into Earth, seeding our primordial planet with life-giving carbon. Had this never occurred, it's an open question as to whether or not life could have ever emerged. Image by A. Passwaters/Rice University/NASA/JPL-Caltech
Geoscientists have struggled to explain how life was able to arise on Earth given that most of the planet's carbon — an important prerequisite for life — should have either boiled away during the planet's earliest stages or become trapped within the Earth's core. By conducting high-pressure and high-temperature experiments in the lab, researchers from Rice University have concluded that virtually all of our planet's carbon likely arrived when a Mercury-like planet smashed into the young Earth some 4.4 billion years ago.
Scientists aren't entirely sure how Earth's volatile elements, such as carbon, hydrogen, nitrogen and sulphur, were able to remain outside the Earth's core and stay locked within the mantle. Models show that most of our planet's carbon should have vaporised into space, or ended up in the metallic core of our planet, sucked up by its iron-rich alloys.
Prior to the new study, many scientists speculated that these volatile elements came to Earth after our planet's core finished forming. As Rice University geoscientist and study co-author Yuan Li pointed out 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." Trouble is, there are no known meteorites capable of producing the required ratio of volatile elements.
Three years ago, Li and his colleagues began to take a different approach to the problem. They conducted a series of experiments to assess how carbon's affinity for iron may have been altered by other compounds present in the Earth's early environment. Importantly, they considered the potential role of other celestial bodies with characteristically different chemical compositions.
"We thought we definitely needed to break away from the conventional core composition of just iron and nickel and carbon," noted study co-author Rajdeep Dasgupta. "So 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."
The proposed collision was so intense that Earth essentially absorbed the incoming proto-planet. (Image: Rajdeep Dasgupta)
Their experiments recreated the high-pressure and high-temperature conditions found deep inside the Earth and other rocky planets. Results showed that carbon could be excluded from the core and relegated to the Earth's mantle, provided that the iron alloys in the core were rich in either silicon or sulphur. One scenario that explains this particular ratio is that an embryonic planet — one that already formed a silicon-rich core — slammed into Earth, and was absorbed by Earth.
"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," said Gupta.
The researchers say this collision likely happened about 4.4 billion years ago, which is only about 150 to 200 million years after the Earth formed. With carbon locked within the crust, and with the planet settling down to produce habitable conditions, life soon emerged. Indeed, the most recent estimates suggest that microbial life formed approximately 4.1 billion years ago.
It's important to point out that evidence for this primordial collision is circumstantial as best. The researchers agree that more work is needed to support this theory, including analyses of abundant elements other than carbon. If true, however, it could mean that Earth only became a habitable oasis only after this tremendous cosmic smashup. Carbon forms a key component of all known life on Earth; complex molecules are comprised of carbon bonded with other elements, such as oxygen, hydrogen and nitrogen.
Regardless, the theory makes you wonder about life on other planets, and how specific the conditions need to be for life to finally emerge on dead, rocky worlds.