Did a Mars-size Body Bring Life-essential Elements to Earth?

Collision with a Mars-size body — the same body thought to have formed the Moon — might also have brought life-supporting volatile elements to Earth.

Scientists have long hypothesized about the origins of Earth’s volatiles. These low-boiling-point elements, such as carbon, nitrogen, and sulfur, are necessary for life, but they weren’t present at Earth’s beginnings. Now, researchers at Rice University propose that volatiles could have arrived via a single collision with a Mars-sized planetary object — the same event that created the Moon. Their findings are published in the January 23rd issue of Science Advances.

Core vs. Crust

When Earth started forming about 4.5 billion years ago, it accreted gas, dust, and debris from its surroundings into a molten soup. Any volatiles would have boiled out of this hot, early mixture. Nevertheless, the existence of life suggests that something delivered volatiles later on.

Scientists have long proposed that pristine carbonaceous chondrites — meteorites rich in volatiles — flew in from the outer part of the solar system and collided with Earth in the very late stages of our planet’s formation. The problem with this explanation, says the paper’s lead author, grad student Damanveer Grewal, is that the carbon to nitrogen ratio in carbonaceous chondrites is 20:1. But the bulk of Earth’s silicates (its mantle, crust, oceans, and atmosphere combined) has twice the abundance of carbon relative to nitrogen, leading the researchers to think that something else delivered the volatiles.

To start, the team set out to recreate the high-temperature and high-pressure conditions of a primordial planetary body in the lab. They added volatiles to a mixture of silicates — representing the body’s crust and mantle — and iron-nickel alloy, representing the body’s core. Then the scientists watched to see how the volatiles evolved as the metal alloy separated from the silicates.

The scientists paid particular attention to carbon’s behavior: if it were drawn into the metal alloy, then this would result in a low carbon-to-nitrogen ratio in the silicates. If, however, carbon were drawn into the silicates, they would have a higher carbon-to-nitrogen ratio.

In the experiment carbon’s behavior depended on the amount of sulfur in the system, explains coauthor Rajdeep Dasgupta. With no sulfur present, most carbon was drawn into the metal alloy, but if the system contained 25% sulfur, most carbon floated to the silicate-rich portion, giving it the high carbon-to-nitrogen ratio found in Earth’s mantle.

Volatiles from Theia

With these results in hand, the team was ready to run simulations to see what kind of body had brought volatiles to Earth. Coauthor Chenguang Sun says that the team ran about a billion scenarios with impactors of different sizes and compositions, looking for ones that could explain present-day carbon, nitrogen, and sulfur abundances. The most likely culprit? A Mars-sized planetary body — just like what’s predicted in Moon formation scenarios.

In the simulations, this scenario works out if the volatile-rich body collided with Earth when our planet had about 90% of its current mass, says Grewal. Earth would have absorbed the small planet upon impact — the body’s core would have fused with Earth’s core, giving Earth’s center a boost in sulfur content. Meanwhile, the two mantles would have fused as well. While the merger would have diluted the total abundance of carbon and nitrogen, the carbon-to-nitrogen ratio would have stayed the same. As a result, Earth would have gained the remainder of its mass and the volatiles necessary for life. Grewal points out that the simulations of such an impact give the 40:1 ratio of carbon to nitrogen that’s seen in Earth’s silicate-based crust and mantle.

But not all agree that the origin of Earth’s volatiles is settled. “The idea works better for carbon, nitrogen, and sulfur than it does for H₂O, which probably has a more complex origin,” cautions Marc Hirschmann (University of Minnesota), who was not involved in the study. He adds that volatiles might not have been delivered in a single event: “There were lots of other opportunities for Earth to gain some of its volatiles both earlier and later.”