The origins of Earth’s water are a big mystery—but we may have one more piece of the puzzle

An old theory gets a new assist from the sun.

If Earth didn’t have water, it wouldn’t have life, and the universe would be sorely lacking in dank memes. But exactly how water sprung up on this planet has always been a perpetual mystery. Earth didn’t just magically appear out of the ether covered in oceans upon oceans. Water has an origin story, and scientists have never completely agreed on what that story is.

In a new paper published in the Journal of Geophysical Research: Planets, Arizona State University researchers suggest that water on Earth originated from material brought by asteroids, assisted by some leftover gas strewn about after the sun’s formation.

This is certainly far from the first time people have suggested water as we know it (and drink) it has an extraterrestrial origin. Historically, the easiest explanation has been that all of Earth’s water came from asteroids that impacted the Earth during the early days of its 4.6 billion year life. Why? Water from Earth shares the same chemical signatures as water found in asteroids—specifically, the ratio of deuterium (a heavy hydrogen isotope) to normal hydrogen. And previous experiments have shown that, in spite of all the heat and energy created by these massively powerful collisions, that water could have been preserved as it found itself on the yet-to-be-blue planet.

Still, those theories have never been quite enough to fill in some of the other blind spots we have about water’s origin. The hydrogen found in Earth’s oceans isn’t necessarily the same sort of hydrogen present throughout the rest of the planet—samples collected closer to the Earth’s core possess exceedingly low amounts of deuterium, which seems to suggest this hydrogen didn’t come from asteroid impacts.

“While a lot of models consider it likely that the Earth has hydrogen in the core, none considered how much that would shift the isotopic ratios of hydrogen [deuterium versus hydrogen] in the mantle,” says Steven Desch, a researcher at Arizona State and a co-author of the new study. “The Earth must have started with some extra source of hydrogen that has lower deuterium-to-hydrogen than asteroids. The only possible source is solar nebula gas.”

The group started to take this idea more seriously, thanks to research in recent years that began to illustrate how proto-planetary bodies could have coexisted with solar nebula gas (which was once thought to disappear too soon before planet formation would begin) and created more opportunities for hydrogen to incorporate itself into deeper parts of a growing planet.

Ultimately, the researchers used the new framework they developed to land on the most likely scenario for Earth’s water history: asteroids holding heaps of water started to coalesce with one-another billions of years ago while the sun still retained a solar nebula. Those asteroids created what we might call planetary embryos, colliding and merging, and eventually crashing with enough energy to form a magma layer outside the embryo.

At the same time, solar nebula gas, which includes hydrogen and other noble gas elements, started interacting with the magma to create an atmosphere. Hydrogen from the solar nebula dissolved into the iron of the magma layer. A chemical process called isotopic fractionation pulled normal hydrogen even further toward embryo’s core, while the heavier (and more rare) deuterium stayed in the mantle. Smaller embryos made from other water-filled asteroids impacted the growing Earth fetus until the solar system finally found itself with the full-sized Earth it knows and loves, teeming with water.

All together, that history confirms most of Earth’s water came from asteroidal sources, but it also demonstrates that a sliver— 0.1 to 0.2 percent—of the oceans on the planet were formed by hydrogen that came from solar nebula gas.

In addition, the researchers used the model to predict Earth has enough hydrogen to make about eight oceans worth of water: one residing on the surface, two oceans’ worth of hydrogen residing in the mantle, and enough hydrogen for five oceans sitting in the Earth’s core.

Ultimately, the biggest limitation to these findings is that we’re working with models. There’s no real way to prove any of this happened.

Still, there are some things scientists can do to test out some of the possibilities pitched by this new theory. We have no idea what isotopic fractionation looks like at the sort of depths and pressures that would have been found in an early Earth embryo, but the team plans to move forward with lab experiments that can characterize this process in greater detail so the model can better reflect what actually happens. The team is also looking to collect and analyze more mantle samples exhibiting very low deuterium-to-hydrogen ratios, which would add more support to this origin story.

Beyond Earth, the biggest implications of the new theory have to do with habitability on other worlds. “Even planets that form far away from sources of water-rich asteroids may still have water,” says Desch. “Not as much as Earth, perhaps, but there is a floor of about 0.1 to 0.2 oceans’ worth of hydrogen,” applicable to Venus and many other exoplanets. “To the extent the model is verified, it strongly supports the idea of rapid planetary growth,” and creates excitement about the possibility for habitable worlds to form more quickly than we think. “This changes a lot of our understanding about planets.”

David P. O’Brien, a researcher at the Planetary Science Institute in Tucson, Arizona, who was not involved with the study, thinks the new model is pretty interesting for incorporating several different mechanisms for water. “Most models in the past have looked at these different mechanisms in isolation, trying to show how they could individually explain all of earth’s water,” he says. “This new study looks at them together, and shows that they were likely both in operation… and the end result is consistent with measured deuterium-to-hydrogen values and noble gas abundances on Earth.” While the new model doesn’t exactly upend what we thought we knew about the origin of water on Earth, O’Brien finds it to be a good demonstration that these sorts of processes are complex and multifaceted.

At the very least, the new paper is a good reminder that there a bunch of rocks in space that are probably home to troves of water, and they might make for decent sites to one day drill, baby, drill.