Where Did Earth Get Its Oceans? Maybe It Made Them Itself

Introduction

At this moment, a spacecraft is headed from Earth to Europa, an ice-veiled moon of Jupiter thought to contain an ocean similar in some ways to one of our own. NASA engraved a metal plate affixed to the spacecraft with a poem, commissioned from Ada Limón during her time as poet laureate of the United States. It reads, in part:

And it is not darkness that unites us,

not the cold distance of space, but

the offering of water, each drop of rain,

each rivulet, each pulse, each vein.

For decades, NASA’s exploration of the solar system has been dominated by the search for water in places like Europa, because as far as we know, water is essential for life.

It may come as a surprise, then, that scientists don’t really know how water first arrived here on Earth.

For years, the top theory was that water came to our planet via comets — objects made of frozen matter that orbit the sun, often decorated with sparkling tails. In all likelihood, these icy relics, which came into being at the dawn of the solar system, did bring water with them when they rained down on a primeval Earth. But in recent years, several spacecraft caught up to comets to examine them. What they found was that cometary water didn’t match ours; the chemical signatures were different.

After that, “comets sort of fell out of favor,” said Ashley King, a meteoriticist at the Natural History Museum in London. Asteroids — rockier and more metal-rich than comets — then became the most popular choice. Asteroids impact Earth far more frequently than comets do, and their water reserves (while not as voluminous as those of comets) look a lot more like those on our planet.

But asteroids have their own problems, and a radical new idea about planetary water is gaining steam. Through careful observation of worlds orbiting other stars, along with some explosive laboratory experiments involving diamond anvils and lasers, scientists have realized that rocky planets like Earth have a way to make water all by themselves. All you need is an ocean of magma, a whole lot of hydrogen, and a little bit of geological alchemy.

A Showdown Between Comets and Asteroids

Earth formed about 4.54 billion years ago. Through geologic fire and brimstone, much about its earliest eon has been lost to history, but the basics are agreed upon: It began as a ball of mostly molten rock. Then it became a blue marble. How?

Comets provided a well-motivated answer. They often linger far from Earth in a doughnut-shaped highway of icy objects beyond Neptune called the Kuiper Belt, or in the even more distant and nebulous Oort cloud. But when a comet passes close enough to the sun, its ice and frozen gases turn to vapor, creating a tail that can stretch for hundreds of millions of kilometers (in one known case, more than a billion). Compared to asteroids, comets give you “a lot of bang for your buck,” said James Bryson, a meteoriticist at the University of Oxford.

Scientists thought comets could have crashed to the Earth and provided its water. But nobody could prove that comets contained Earth-like water — until the 1980s, when the European Space Agency (ESA) decided to check. Giotto, their first deep-space mission, was truly ambitious: It would be the first spacecraft to get an up-close-and-personal look at a comet’s icy heart.

In 1986, it caught up to Halley’s comet, famous for appearing in Earth’s sky as our paths intersect roughly every 76 years. Giotto managed to send home both dramatic images of the comet’s nucleus and measurements of the cloud of material around it. What raised scientific eyebrows was Giotto’s measurement of something called the D/H ratio.

Almost all the water on Earth is made up of two hydrogen atoms and one oxygen atom: H2O. But there is another form of water, called heavy water, made up of one oxygen atom and two atoms of a heavier form of hydrogen called deuterium.

If comets are responsible for our oceans, one might expect Giotto to have found that the water on Halley’s comet had a similar ratio of deuterium to hydrogen as the water on Earth. That’s not what it found. “It didn’t match at all,” said Karen Meech, a planetary astronomer at the University of Hawai‘i. In fact, Halley’s D/H ratio was twice that of most of the water on Earth.

More cracks appeared in the comet theory during the 1990s and 2000s, when spectroscopic observations of other comets, like Hale-Bopp, also found evidence of heavy water. But the hammer blow arrived in 2014 when the spiritual successor to Giotto, ESA’s Rosetta mission, made history by orbiting and sending a lander to the surface of 67P/Churyumov-Gerasimenko, a comet shaped like a giant rubber duck. During its orbits of 67P, Rosetta made the most precise measurements of a comet’s composition to date — and found that it contained the highest concentration of deuterium of any comet we’ve measured.

If Earth’s water didn’t come from comets, perhaps it came from asteroids. These rocky objects mostly hang out between Mars and Jupiter, and they impact our planet all the time as meteorites, though most of their material burns up in the atmosphere or lands in the ocean. Scientists have collected tens of thousands of meteorites and found that the water molecules contained in a particular group closely resemble those in our world. One meteor that plunged into the sleepy British town of Winchcombe in 2021 — leaving a sizable dent in a family’s driveway — was found to have a D/H ratio that almost perfectly matched that of Earth’s oceans.

Meteorites, though, can be contaminated during their fiery dives and crash landings. That’s why scientists have flown spacecraft out to asteroids and collected material in orbit for forensic analysis. In some cases, they have found that asteroids still moving through space also seem to have Earth-like water. A study published in 2023 revealed that water from the asteroid Ryugu, which Japan’s space agency visited in 2018, had a D/H ratio similar to that of most water on Earth. When it comes to the provenance of Earth’s water, “the community is probably more favorable of asteroids than comets these days,” King said.

But the D/H ratios of asteroids did not close the book on the question of Earth’s water. Asteroids also contain small amounts of noble gases like argon, krypton, and xenon — inert elements that act as tracers of various geologic processes — and scientists have found that those mixtures do not usually correspond to what we find on our planet. In addition, theories based on comets and asteroids have the same fundamental problem: The ability of either type of object to give the planet its oceans relies on luck. Multiple asteroids or comets would have had to impact Earth after its superhot magma ocean phase to produce the inundated world we live on today. This was taken for granted in the past, but the existence of this late-in-the-day bombardment is heavily debated in the scientific community.

There is another possibility, one that relies not on cosmic chance, but on our planet’s own industriousness: Earth made most of its water by itself.

Hydrogen, Meet Magma

When astronomers look at exoplanets — worlds outside our solar system — they see a diversity of atmospheres. But when they simulate the ways the planets took shape, scientists find that many of them could have started out brimming with hydrogen. Could Earth’s formative years have been similar?

Scientists used to think that the early Earth had little hydrogen. They reached this conclusion after examining meteorites called enstatite chondrites that have a suspiciously similar chemical makeup to Earth. Because of this similarity, scientists think the two probably formed from the same material, Bryson said. These meteorites seemed to lack hydrogen, so scientists thought the same went for our planet.

But some studies, including one co-authored by Bryson, found that there was hydrogen in the meteorites all along. It was just hidden in their organic compounds.