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Collision with Mars-sized object could explain Earth’s unusual element ratio

Impact could have forced more carbon out of the early Earth’s core

by Sam Lemonick
February 3, 2019 | A version of this story appeared in Volume 97, Issue 5


Artist's conception of a Mars-sized planetary embryo impacting proto-Earth.
Credit: NASA/JPL-Caltech/T. Pyle
The ratio of carbon to nitrogen on Earth may stem from a collision with a Mars-sized planetary embryo during our planet's formation.

Carbon, nitrogen, and sulfur are crucial to life on Earth. But the relative amounts of those elements are different from those of other rocky objects in our solar system, so scientists have been puzzled about how this ratio came to be. Now researchers have shown that a collision with a Mars-sized impactor could explain Earth’s chemical composition (Sci. Adv. 2019, DOI: 10.1126/sciadv.aau3669).

The ratio of carbon to nitrogen in Earth’s mantle is about twice what’s observed in rocky meteorites that formed from the same starting materials. Experiments have so far failed to explain the mismatch, in part because they focused on only one or two elements at a time.

Rice University geochemist Rajdeep Dasgupta, his graduate student Damanveer S. Grewal, and colleagues combined more sophisticated experiments with computer simulations to figure out what happened, taking into account carbon, nitrogen, and sulfur.

Their group looked at how changing the amount of sulfur in Earth’s core affects the ratio of carbon to nitrogen in the rest of the planet. From assumptions about the early Earth’s composition, they made materials to represent the ancient planet and subjected them to extreme temperatures and pressures.

As they increased the amount of sulfur in the simulated cores to represent a collision with a sulfur-rich impactor, the element ratios in the mantle began to look more Earth-like. Interactions between carbon and sulfur force the former out of the core and into the mantle.

Computer simulations of billions of collisions between a proto-Earth and objects of varying mass and composition showed a Mars-sized planetary embryo with a sulfur-rich core would create the right conditions to produce the C:N ratio we see in Earth’s mantle today.


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