Over 70 percent of Earth’s surface is covered in water, almost all of which is in the oceans. But where did all this water come from? Several hypotheses seek to explain how water arrived on our planet in the early days of its formation, including the idea that molecular water leaked out of hydrated minerals on Earth, and the possibility that asteroids and comets delivered water to Earth.

New research in the journal Science Advances lends support to the asteroid hypothesis, suggesting that these rocky visitors could have delivered up to half of Earth’s water billions of years ago.

"We find ourselves in this ‘pale blue dot,’ a planet full of water, rich in organics and supportive of life."

In a paper published Wednesday, a pair of cosmochemists at Arizona State University showed that the first asteroid ever sampled by humans contains hydrogen isotopes in levels that are almost identical to the concentrations found in rocks on Earth. Even though their study examined samples from just one asteroid, this evidence suggests that asteroids could have very well been the source of a significant portion of water in Earth’s oceans.

Maitrayee Bose, Ph.D., an assistant professor in the ASU School of Earth and Space Exploration and the principal investigator on the project, co-authored the study with postdoctoral scholar Ziliang Jin, Ph.D..

“We find ourselves in this ‘pale blue dot,’ a planet full of water, rich in organics and supportive of life,” Bose tells Inverse. “We know of no other such planet. My aim is to find out how.”

Bose (left) and Jin (right) load asteroid samples into a secondary ion mass spectrometer. M. Bose

Almost a decade before Hayabusa2 landed on the asteroid Ryugu and shot it with a bullet (for science, of course), the first Hayabusa spacecraft delivered microscopic particles of the asteroid Itokawa back to Earth.

The Japanese Aerospace Exploration Agency (JAXA) provided Bose and Jin with five particles brought back to Earth from Itokawa, an S-type (stony) asteroid, in 2010. By analyzing the particles in a secondary ion mass spectrometer, Bose and Jin identified that Itokawa contained water and hydrogen isotopes in levels that are “indistinguishable” from rocks found on Earth.

“Ziliang and I study small bodies and processes within them to understand asteroids, which are considered the building blocks of our solar system,” says Bose, explaining that the source of Earth’s water is a matter that the scientific community has hotly debated.

Two particles harvested from the asteroid Itokawa. Japan Aerospace Exploration Agency (JAXA), edited by Z. Jin

“The most popular scenario is that water on Earth was delivered by water-rich asteroids from the outer solar system during different periods of planetary formation, i.e., early, when Earth was not as big as it is now (proto-Earth) or late, after the Earth had completely formed,” she says. “Thus, we wanted to study water in this asteroid Itokawa (first asteroid that we have visited!) and see if it had enough water to account for water on Earth.”

Based on their analysis, Bose and Jin write in the paper that “S-type asteroids formed in the inner solar system were hydrous despite high temperatures and could have been a potential source for Earth’s water.”

This is just the first bulk water estimate of the water-content in an S-type asteroid, so Bose is careful to point out that this study is the launching point for future investigations into the water conditions of early Earth.

“There are currently several theories about water delivery to Earth, and we are suggesting another source,” says Bose. “But this is not the end-point but the beginning of trying to gauge through other laboratory investigations of other isotopes and elements, if the arguments hold true.”

Bose hopes the picture will become even clearer once the newer spacecraft Hayabusa2 and OSIRIS-REx return their carbonaceous asteroid samples to Earth — she plans to analyze these samples, of course.

Abstract: We performed the first measurements of hydrogen isotopic composition and water content in nominally anhydrous minerals collected by the Hayabusa mission from the S-type asteroid Itokawa. The hydrogen isotopic composition (dD) of the measured pyroxene grains is −79 to −53‰, which is indistinguishable from that in chondritic meteorites, achondrites, and terrestrial rocks. Itokawa minerals contain water contents of 698 to 988 parts per million (ppm) weight, after correcting for water loss during parent body processes and impact events that elevated the tempera- ture of the parent body. We infer that the Bulk Silicate Itokawa parent body originally had 160 to 510 ppm water. Asteroids like Itokawa that formed interior to the snow line could therefore have been a potential source of water (up to 0.5 Earth’s oceans) during the formation of Earth and other terrestrial planets.