Imagine reconstructing the itinerary of a globe-trotting, Carmen-Sandiego-like character using nothing but the beach sand accumulated in the bottom of her suitcase. That’s a task that even a CSI writer would consider implausible, but it’s not far off from the work of reconstructing the Earth’s formation.

In the earliest years of the Solar System, matter was clumping together from a diffuse disk of gas and dust spinning around an infant Sun. As the clumps grew, the largest of them could transform, partitioning elements and minerals between inner cores and outer mantles. Clumps from different areas of the disk formed from different starting elements, and collisions mixed this with that. From that chaotic picture, we somehow want to work out how the Earth got the exact mix of elements that helped make it what it is today.

The puzzle of how the Earth got its water is perhaps familiar, but this isn’t the only fleeting substance researchers have pondered. Carbon, nitrogen, and sulfur were also critical “volatile” elements, meaning that they could easily have been boiled off by impacts. But impacts would have also been needed to deliver these materials to a growing Earth. How could both of those be true?

Building a planet

From analyzing elements and isotopes on the Earth and in meteorites, scientists have identified a class of asteroids that most likely provided most of the early building blocks for a growing Earth. But this class is distinctly light in volatile elements. So what added the carbon, nitrogen, and sulfur?

Potential explanations all have their problems. There is another class of asteroid that carries much more of these elements, but their ratio is off. If they had arrived after the Earth’s core had formed (perhaps because the movement of the maturing gas giant planets mixed with the asteroids closer to the Sun), they would have left the Earth short on carbon. And if Earth’s metallic core was still separating from the rocky exterior, carbon would be preferentially pulled into the core, leaving the exterior even shorter.

To test another explanation, a group at Rice University led by Damanveer Grewal and Rajdeep Rasgupta needed to melt stuff in the lab. In a special setup, they mixed iron-nickel alloys with varying amounts of carbon, nitrogen, and sulfur along with basalt rock. The mix was put under extreme pressure and melted. After it cooled off, they analyzed the blobs of metal encased in the glass that formed from the molten basalt to see how the elements moved around.

They found that, when lots of sulfur was present, the amount of nitrogen stuck in the metallic blobs was a little smaller. More notably, the amount of carbon in the metallic blobs dropped off sharply as more sulfur was added. Compared to previous experiments without the nitrogen, this actually pushes much more carbon into the glass, so this experiment suggests a new possibility that might work.

Instead of different classes of asteroids delivering these volatile elements to the Earth, these researchers considered whether another planetary “embryo” might be responsible. While much smaller than the Earth, another planet-in-the-making would also have separated into a metallic core and rocky mantle.

At the core

The experiment was meant to get at the chemistry of a planetary embryo with a sulfur-rich core smashing into the Earth. Even if that embryo was basically built of the same materials as the Earth, the fact that it had a separated core could change how much carbon or nitrogen was left in the rocky mantle instead of getting tied up in the core.

Using the data from their experiment, the researchers looked at the range of plausible element concentrations and mass of the planetary embryo, calculating thousands of possible combinations. They found that a body around the size of Mars with a reasonably high amount of sulfur could actually make the mixing math work out, producing the right ratio of carbon, nitrogen, and sulfur.

A Mars-sized impactor, you say? As it happens, that is precisely how we think the Moon formed—through the collision of the Earth and a Mars-sized planet a few tens of millions of years after the Earth started forming. That timeline seems to work here, as well. It seems that the Earth and the Moon had the same source of volatile elements, and the violent event that formed the Moon is the last obvious chance to share a source.

It could be there are other possible solutions to this puzzle yet to be identified, but for now there’s a pretty good working hypothesis to poke at.

Open Access at Science Advances, 2019. DOI: 10.1126/sciadv.aau3669 (About DOIs).