Earth Manufactured Its Own Oceans During Formation, New Experiments Show

Earth's oceans may have been forged in the hellish atmosphere of the young planet itself, rather than delivered by icy comets billions of years later. A new experimental study demonstrates that dry rocky pebbles falling through a primordial hydrogen envelope can spontaneously generate massive volumes of water.[1][6]

The findings, published in the journal Nature Geoscience, challenge the long-standing assumption that Earth was born dry and required late-stage bombardments to become habitable. Researchers at the University of New Mexico showed that the necessary chemical reactions take only minutes under the right conditions.[1][6]

"Previous ideas about the source of water to Earth rely on an external source, such as comets or meteorites," said Zachary Sharp, director of the university's Center for Stable Isotopes. "Here we show that virtually any Earth-sized planet should have water by this newly identified mechanism."[1][3]

Recreating a primordial atmosphere

The research team focused on a planetary formation model known as pebble accretion. In this scenario, young terrestrial planets grow rapidly by sweeping up millimeter- to centimeter-sized rocky particles from the surrounding solar nebula, rather than relying solely on collisions with larger planetesimals.[1][6]

As a protoplanet reaches sufficient mass, its gravity captures a dense, high-pressure atmosphere composed primarily of hydrogen gas. The research team hypothesized that as iron-rich silicate pebbles descended through this hot envelope, they would undergo a rapid chemical reduction before striking the surface.[1][6]

To test the theory, graduate student Susmita Garai led a series of high-temperature laboratory simulations. The team heated anhydrous silicate rock samples to temperatures between 1,200 and 1,400 degrees Celsius, exposing them to continuous streams of hydrogen gas to mimic the primordial atmosphere.[1][2]

The results were immediate and striking. Within 15 minutes to three hours, the hydrogen stripped oxygen from the iron oxide within the rock, leaving behind metallic iron and releasing the displaced oxygen as water vapor.[1][2]

"Garai's efforts paid off mightily, as she found that water is produced in just minutes in the high-temperature environment of the early Earth's atmosphere," Sharp noted. Control experiments using helium produced no such reaction, confirming hydrogen's essential role in the transformation.[1]

Hundreds of oceans from dry rock

The efficiency of the chemical conversion surprised the research team. In the hydrogen-rich experimental environment, nearly 80 percent of the iron oxide present in the silicate samples was successfully converted into metallic iron, releasing a proportional amount of water vapor.[2][6]

Scaling these laboratory conversion rates to the mass of a growing planet suggests an enormous potential yield. The researchers calculate that this endogenous process could have generated enough water to fill hundreds of Earth-sized oceans during the planet's formative years.[1][3]

"Looking out over a large body of water, it is hard to imagine that that water may have been produced 4.5 billion years ago in a hell-like atmosphere, but it very well may have," Sharp said. The sheer volume of water produced in the models exceeded the team's initial expectations.[1][3]

The metallic iron produced by the reaction would have subsequently sunk to form the planet's core. This mechanism neatly explains why Earth's primitive mantle is relatively depleted of certain elements, resolving a secondary geochemical mystery that has long puzzled planetary scientists.[6]

A shifting consensus on comets

For decades, the dominant paradigm held that the inner solar system was too hot for volatile compounds to survive planetary accretion. Water, the theory went, must have arrived later via a "late veneer" of comets and water-rich meteorites striking the cooled surface.[1][3]

However, recent isotopic analyses have consistently complicated the comet-delivery model. Probes visiting modern comets have found that their water carries a significantly higher ratio of deuterium—a heavy isotope of hydrogen—than the water currently filling Earth's oceans, suggesting a different primary source.[1]

The shift away from comet delivery extends beyond water. Separate studies published this week suggest that comets likely struck the early Earth with too much speed and at too sharp an angle to successfully deliver intact organic molecules, further diminishing their role in seeding life.[4][5]

While meteorites from the outer solar system likely still contributed some fraction of Earth's volatile inventory, the new findings suggest they were not the primary source. The planet was likely born wet, manufacturing its own oceans as it assembled from dry dust.[1][6]

Implications for exoplanet habitability

If rocky planets can synthesize their own water during formation, the implications for the search for extraterrestrial life are profound. The mechanism relies only on common silicate rocks and a primordial hydrogen envelope, both of which are ubiquitous in young star systems.[1][3]

"The idea of pebble accretion is still relatively new," Sharp explained. "But it seems to solve many problems that exist with the existing paradigm for planet formation, and now we see that it also can explain why our planet is habitable."[1]

Astronomers observing distant protoplanetary disks have already detected water vapor around young stars. The new experimental data provides a concrete physical model for how that water becomes permanently incorporated into terrestrial worlds before their primordial atmospheres dissipate into space.[6]

"By extension, it is reasonable to expect that other Earth-sized bodies circling other stars would also have water and possibly life," Sharp concluded. The next generation of space telescopes may soon be able to test whether these homegrown oceans are the galactic standard.[1][3]