Where Did Earth’s Water Originate? Solar Nebula, Study Suggests

Scientists have long posited that water arrived on our planet via ice-filled comets and asteroids, but new research identifies an additional point of origin for the life-sustaining liquid: solar nebula, or clouds of gas and dust lingering in the universe following the sun’s formation.

The chemical formula behind water is deceptively simple. Take two parts hydrogen and one part oxygen, then combine into a molecule bearing a distinct resemblance to Mickey Mouse. “Because … oxygen is abundant,” Steven Desch, study co-author and astrophysicist at Arizona State University, explains in a statement, “any source of hydrogen could have served as the origins of Earth’s water.”

As Chelsea Gohd writes for Discover magazine, hydrogen gas held within the solar nebula was incorporated into planets’ interiors during their formation. Although much of this hydrogen remains trapped in our planet’s core, the team’s analysis suggests that a small portion managed to escape, eventually contributing the building blocks of one out of every 100 water molecules found on Earth, according to the new study published in the Journal of Geophysical Research: Planets.

Until now, researchers citing the two most commonly accepted sources of water—asteroids and comets—based their assessments on ocean water and asteroids’ chemical signatures, which contain similar ratios of deuterium, a heavy hydrogen isotope, and normal hydrogen. But as Nick Carne reports for Cosmos, samples collected from deep within the Earth’s interior, near the boundary between the core and mantle, exhibit lower levels of deuterium, pointing toward the gas’ non-asteroidal origins.

“Earth must have started with some extra source of hydrogen that has lower deuterium-to-hydrogen than asteroids,” Desch tells Popular Science’s Neel V. Patel. “The only possible source is solar nebula gas.”

According to Patel, the scientists’ leading theory surrounds early interactions between water-logged asteroids, which crashed into each other to form planetary embryos complete with an external layer of magma, and hydrogen-heavy solar nebula gas. When the solar nebula encountered these burgeoning planets’ magma, it began creating an atmosphere, sending dissolved hydrogen beyond the magma to the embryos’ interior. Thanks to a process known as isotopic fractionation, normal hydrogen kept moving deep into the core, while the deuterium isotopes remained in the mantle. Continued merging with smaller embryos and other celestial bodies eventually enabled Earth to gain enough water and mass to reach its final size.

These asteroid impacts generated the majority of the planet’s water, Mindy Weisberger reports for Live Science, but a small portion—relatively speaking, as the quantity of water nestled within Earth is actually high enough to form several oceans—found close to the core appears to originate from the hydrogen produced by the solar nebula.

Desch tells Popular Science that the team’s findings could help scientists better explore the habitability of other worlds.

“Even planets that form far away from sources of water-rich asteroids may still have water,” he says. “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.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's associate digital editor, history.