You, me, and every known life-form are children of an ancient encounter between Earth and a Mars-sized object. Congratulations: You are part-alien.
That’s the takeaway from a study out Wednesday in Science Advances. Led by planetary scientist Damanveer Grewal, a graduate student at Rice University, the authors present evidence that the elements that led to life on Earth were seeded by a giant impactor that smashed into our planet during the solar system’s infancy.
Scientists have long theorized that the Moon was formed from debris created by a Mars-sized planetary embryo—sometimes called “Theia”—crashing into Earth some 4.4 billion years ago.
By combining high-pressure laboratory experiments with sophisticated simulations of Earth’s evolution, Grewal and his colleagues tested out the idea that this impactor also altered the chemical makeup of our planet and made it conducive to life.
“Our study shows that the giant impact of a rocky planet was required to deliver the unique composition of life-essential elements to Earth—the only habitable planet that we know of,” Grewal told Motherboard in an email.
It’s wild to imagine that life on Earth may have derived from two parent bodies that merged their unique chemical makeup to create a habitable planet. Beyond its implications for understanding ourselves and our world, the research can help scientists search for extraterrestrial life elsewhere in the universe. This is the goal of the CLEVER Planets project, led by Rice University planetary scientist and study co-author Rajdeep Dasgupta, which seeks to understand how life-essential volatile elements end up on rocky planets.
One of the team’s biggest clues about volatile delivery was Earth’s 40:1 carbon-to-nitrogen ratio, which does not match the 20:1 ratio seen in primitive space rocks called carbonaceous chondrites. Given that carbon is a fundamental ingredient for life as we know it, it’s imperative to understand why Earth ended up with twice as much of it as other solar system objects such as chondrites.
Dasgupta runs a laboratory at Rice that simulates the extreme conditions deep inside planets. This allowed the team to model what would happen if various amounts of the volatile element sulfur were introduced to early Earth.
The results revealed that nitrogen mixes with sulfur-rich alloys in subsurface planetary conditions, while carbon was far more insoluble to materials with high sulfur-concentrations.
“Our laboratory experiments showed that if the metallic core of a rocky planet is rich in sulfur, then carbon is expelled from the core,” at a higher rate than nitrogen, Grewal explained. It follows that if a sulfur-rich object slammed into Earth, it would leave behind a high carbon-to-nitrogen ratio.
The team applied this data to computer simulations and ran roughly a billion potential variations of Earth’s early history, looking for scenarios that match what we know about the origins of our planet and its biosphere. The simulations also supported the likelihood that a planetary collision enriched Earth with volatile elements that proved essential to the emergence of life.
“If we want to identify the potential habitability of exoplanets, it is critical to understand the origin of the life-essential volatiles on Earth,” Grewal said. “Are giant impactors necessary to bring these special elements?”
“When people look at giant impactors, they look at them as completely destructive events,” he added. “But they could actually be life-giving events as well.”
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