Why the heck is Earth wet?

Our planet started off bone dry. Then space sent ice balls and 'water balloons.'
A gray asteroid against the black background of our solar system.
This illustration depicts an asteroid that has been detected by a team of European astronomers using NASA’s James Webb Space Telescope. N. Bartmann (ESA/Webb), ESO/M. Kornmesser and S. Brunier, N. Risinger

Each time you take a sip of water, you’re imbibing liquid that, in all likelihood, is up to 4.5 billion years old. Earth is awash in a life-sustaining substance about as ancient as the planet itself. Astrophysicists don’t completely know where the stuff came from, but circumstantial evidence suggests that water-containing meteorites might have pummeled an infant Earth. Those rocky showers would have helped transform a bone-dry place into a unique wet world. 

Or, at least, a damper one. Although our planet is covered by an estimated 326 quintillion gallons of H2O, it’s drier than you’d imagine. Sean N. Raymond, an astronomer at France’s Laboratory of Astrophysics of Bordeaux, has compared Earth, which could be as little as 0.023 percent water, to crackers, which are around 2 percent water. That’s still a lot more moisture than we had at the beginning.

A very dry start

When the solar system first came together, some of the young planets were too hot for water. “Earth and Mars should have formed extremely dry,” says Humberto Campins, an asteroid expert at the University of Central Florida—due to their locations in the solar system’s frost line.

When the sun was coalescing out of a collapsing cloud of gas and dust 4.6 billion years ago, its tremendous heat made a boundary. Outside of it, space was cool enough for ice grains to solidify. (This helps explain why far-out Jupiter and Saturn have ocean moons.) Inside of it, heat vaporized water. Earth and the other inner planets clumped together from the dry rock and dense metal that remained. Something must have happened, some millions of years later, to nourish those planets with water. Astronomers have explored several possible scenarios. 

A white, snowy ring encircles a dry, dusty center of a forming solar system.
An artist’s impression of the frost line around a young star, with water concentrated in the snowy outer rim. A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Craters on the surface of our moon indicate that our side of the frost line was constantly hit with space rocks, including a particularly violent shower known as the Late Heavy Bombardment. Some experts think those projectiles—specifically meteorites, the bits of asteroids that fall to Earth—might have been more like cosmic water balloons. The hypothesis is supported by the 2010 discovery of a thin crust of frost on asteroid 24 Themis. More recently, NASA found water-bearing clay minerals in the near-Earth asteroid Bennu during a ground-breaking sample-retrieval mission.

Still, it’s possible that other processes were involved in delivering water to Earth, such as gas from the cloudy solar nebula that dissolved hydrogen into the planet’s magma layer. It’s also possible that there were multiple sources and steps.

“The pieces of the puzzle are not clear,” says Campins, who is a member of the team that probed Bennu’s contents. But he points to one major clue that “gives us an idea of where the water may be coming from”: the type of hydrogen that flows through our aquatic systems.

Matching elements

The most common form of hydrogen in the universe has a lone proton orbited by an electron. But there’s a slightly different version called deuterium with a proton and a neutron squished into the center. Astronomers have measured the proportion of deuterium to regular hydrogen in Earth’s water and looked for that “D-H ratio” in other objects around the solar system.

Turns out, carbonaceous chondrites, a kind of meteorite, are a pretty good match. If our solar system was once an ancient construction site, think of the chondrites as the unmelted rubble. They hail from the asteroid belt’s outer section, closer to Jupiter than Mars, which means they probably formed on the wet side of the frost line. Raymond estimates that about a single ton of carbonaceous space rocks, rich in ice and watery minerals, could have delivered 110 to 220 pounds of water to Earth. When Jupiter and Saturn’s masses “grew big really fast,” he says, the gas giant kicked those rocks toward the sun and the inner planets.

Comet 67p image taken by European Space Agency's Rosetta lander
Comet 67p documented by the European Space Agency’s Rosetta spacecraft during the first mission to pull off a comet landing. This image was taken on January 31, 2015. ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The meteorites “contain a lot of organic goop” like carbon and other molecules associated with life, Raymond explains. They also hold volatile materials—substances that evaporate easily when heated—like water, zinc, and hydrogen from the early days of the solar system. While those can be found on our planet today, a few volatile materials are still missing. “If the carbonaceous chondrites contributed Earth’s water, they would have also contributed Earth’s noble gasses,” Campins says. But they don’t support those elements, so something else must have filled the gap. Comet 67P, closely studied in the mid-2010s by the European Space Agency’s Rosetta probe and Philae lander, has the right noble gas content, Campins notes. 

This lends to the idea that a bunch of space bodies hit Earth with noble gasses, H2O, and who knows what else. “If most of the water gets contributed by asteroid impacts and most of the noble gasses are contributed by comets,” the elemental math seems to add up, Campins says. “But I think that nature is a little bit more complicated than that…it could be that the timing of those two was not the same.” 

In fact, newer evidence emphasizes a different kind of space rock from closer to home.

Local rocks

Enstatite chondrites are meteorites with a similar composition to the original building blocks of Earth. Because they formed within the inner solar system—on our side of the asteroid belt—astronomers classify them as “non-carbonaceous.” While they don’t have as much water as their distant counterparts, they could pack some punch. A 2020 paper in the journal Science concluded that past astrophysics models vastly underestimated the amount of hydrogen in them, killing off “the old idea that the rocks in Earth’s vicinity were dry,” Raymond says. Even cooler, they have a promising D-H ratio, too.

As Raymond wrote this summer in Nautilus, a suite of more recent studies have linked nitrogen and other volatile elements on Earth to enstatite chondrites. He also highlights an analysis of Martian zinc, indicating that debris from the inner solar system transported the metal to our neighbor. If zinc existed within those meteorites, they probably carried other volatile materials—specifically, water. Mars had liquid water at one point and may have some still lurking under an ice cap.

If space rocks brought water to the Red Planet, could they have done so elsewhere? “What we’re learning here may not only be applicable to our understanding of what we should expect on Mars,” Campins says, “but about the possibility of water and organic molecules being delivered to planets around other stars, which would give you an environment that could be conducive to the formation of life.”

Putting these lines of evidence together gives us a recipe that would have involved lots of damp local rocks and a few of the more distant ice balls. Hydrogen, nitrogen, and zinc isotopes “all tell the same story” of a wet Earth, Raymond says: Previously overlooked non-carbonaceous meteorites probably supplied about 70 percent of the planet’s water, and just a dash of carbonaceous meteorites touched up its vast blue surface.