Earth's water may have come from the Sun

Scientists have been attempting to decipher the origin of water on Earth for decades. A new study in the Journal of Geophysical Research: Planets of the American Geophysical Union puts forward a new theory towards resolving this mystery, suggesting that the hydrogen in Earth’s water originated from clouds of dust and gas remaining after the formation of the Sun.

Historically, it has been widely accepted that most, if not all, of Earth’s water came from asteroids, largely due to a similar deuterium-to-hydrogen ratio between oceanic water and that found on asteroids in our solar systems (the deuterium-to-hydrogen ratio is a unique chemical signature of water sources, since deuterium is a heavier isotope of ordinary hydrogen).

Researchers for the study, conducted at Arizona State University, chose to study hydrogen since it is much more common in our solar system than oxygen, the other chemical component of water. According to the authors, the hydrogen in oceans does not represent all hydrogen on Earth, since hydrogen found near the boundary between the core and mantle of the planet have significantly less deuterium than ocean water.

In a statement, Steven Desch, a professor of astrophysics in the School of Earth and Space Exploration at Arizona State University, said of the use of the deuterium-to-hydrogen ratio, “it’s a bit of a blind spot in the community. When people measure the [deuterium-to-hydrogen] ratio in ocean water and they see that it is pretty close to what we see in asteroids, it was always easy to believe it all came from asteroids.”

To disprove this assumption, Desch and co-author Peter Buseck, a Regents' Professor in the School of Earth and Space Exploration and School of Molecular Sciences, developed a novel theoretical model of Earth’s foundation several billion years ago where large waterlogged asteroids developed into planets while clouds of dust and gas still spun around the Sun. These planetary embryos rapidly collided and grew, the largest of which eventually became Earth. Gases from the cloud around the Sun became Earth’s atmosphere, and hydrogen from the Sun’s gas cloud dissolved into the molten iron of the still-forming Earth’s magma ocean (this hydrogen was lighter and had less deuterium than the hydrogen in other asteroids at the time). As more asteroids impacted Earth, their hydrogen was pulled towards the core of the planet due to the attraction between molten iron and hydrogen with higher amounts of deuterium. Once Earth cooled, much of the hydrogen from asteroids was left in the mantle while atmospheric hydrogen originating from the gas cloud around the Sun settled into Earth’s oceans.

The research team used their model to estimate the quantities of hydrogen originating from each source, concluding that most was indeed from asteroids but a significant portion came from the gas cloud around the sun. Lead author Jun Wu, assistant research professor in the School of Molecular Sciences and School of Earth and Space Exploration at Arizona State University, said in a statement, “For every 100 molecules of Earth's water, there are one or two coming from solar nebula.”

A major application of this study could be new models about the development of other planets and their ability to support life. Other planets could have developed in a similar fashion to the theoretical model put forth by the authors. “This model suggests that the inevitable formation of water would likely occur on any sufficiently large rocky exoplanets in extrasolar systems,” Wu said. “I think this is very exciting.”