Did the Earth’s Water Come From the Sun?

Where did Earth’s water come from? Comets may have brought some of it. Asteroids may have brought some. Icy planetesimals may have played a role by crashing into the young Earth and depositing their water. Hydrogen from inside the Earth may have contributed, too. Another hypothesis states the collision that formed the Moon gave Earth its water.

There’s evidence to back up all of these hypotheses.

But new research suggests that the Sun and its Solar Wind may have helped delivered some water, too.

Earth’s oceans contain about 1.37 × 1021 kg of water by mass. (That’s one sextillion three hundred seventy quintillion kilograms.) Lakes, rivers, ice, groundwater, and water vapour amount to another 5.0 × 1020kg, which is five hundred quintillion kilograms. All that water covers about 70% of the Earth’s surface. A few moons in our Solar System have sub-surface oceans, but among planets, Earth is unique. And without all that water, of course, we wouldn’t be here and there’d be no life. So the source of that water is an enduring mystery in science.

Earth formed out of the protoplanetary disk, the material swirling around the Sun after it became a star. It’s the same material that all the other planets and moons formed from. Earth formed close to the Sun, where temperatures were relatively higher, so Earth’s water didn’t form at the same time the planet did. 90% of Earth’s mass is iron, oxygen, silicon, sulphur, and magnesium. But water has a much lower condensation temperature than those materials so any water available at the time would’ve been vapour.

But further from the Sun, water would’ve condensed into icy asteroids, planetesimals, and comets. That’s why the idea of an extraplanetary source for Earth’s water endures. One line of evidence shows that after the planets formed, Jupiter migrated inward toward the Sun, sending icy asteroids from the outer Solar System toward the inner Solar System where some of them crashed into Earth. After a long period of time, enough water arrived by asteroids and comets to account for Earth’s water.

2004 EW95, seen in this artist’s view, may be a primordial asteroid. Its optical spectrum shows that it contains phyllosilicates, which means its rocky components were altered by the presence of water. Evidence like this reinforces the idea that asteroids delivered water to Earth. Credit: M. Kornmesser/European Southern Observatory.

There’s a problem with the asteroid water-delivery theory, though. The idea is centred on C-type asteroids like 2004 EW95 which contain hydrated minerals. Research has shown a distinct difference between the isotopic composition of water on C-type asteroids and Earth’s water. There must be a missing source of water that is isotopically different than C-type asteroids.

This new paper suggests that there’s another mechanism that can help explain Earth’s water: the Sun and its solar wind.

The new paper is titled “Solar wind contributions to Earth’s oceans.” It’s published in the journal Nature Astronomy and the first author is Dr. Luke Daly, from the School of Geographical and Earth Sciences at the University of Glasgow.

Professor Phil Bland, from Curtin University’s Space Science and Technology Center, was part of the study. In a press release, Dr. Bland explained one of the problems with the hypothesis that icy asteroids delivered Earth’s water. “An existing theory is that water was carried to Earth in the final stages of its formation on C-type asteroids, however, previous testing of the isotopic ‘fingerprint’ of these asteroids found they, on average, didn’t match with the water found on Earth meaning there was at least one other unaccounted for source,” Professor Bland said.

“Our research suggests the solar wind created water on the surface of tiny dust grains and this isotopically lighter water likely provided the remainder of the Earth’s water,” Bland said.

These results are based on samples from asteroid Itokawa. JAXA’s Hayabusa mission returned samples from near-Earth asteroid Itokawa to Earth in 2010. Those samples showed that Itokawa contained abundant water. (That discovery fortified the idea that Earth got its water from asteroids.) Itokawa is an S-type asteroid, which means it formed much further from the Sun than Earth did, out in the cold reaches of the Solar System where water would freeze rather than vapourize.

The asteroid Itokawa, visited by Hayabusa in 2005. Credit: JAXA

The researchers examined the tiny samples from Itokawa with Atom Probe Tomography. What they found is that H+ (Hydron, or hydrogen ions) in the solar wind irradiates silicate minerals on the surface of the asteroid and creates water molecules.

“This new solar wind theory is based on meticulous atom-by-atom analysis of miniscule fragments of an S-type near-Earth asteroid known as Itokawa, samples of which were collected by the Japanese space probe Hayabusa and returned to Earth in 2010,” Professor Bland said.

“Our world-class atom probe tomography system here at Curtin University allowed us to take an incredibly detailed look inside the first 50 nanometres or so of the surface of Itokawa dust grains, which we found contained enough water that, if scaled up, would amount to about 20 litres for every cubic metre of rock,” Bland explained.

The surface patterns on one of the microscopic dust particles from asteroid Itokawa. Image: JAXA

In their paper, the authors write that “We used atom probe tomography to directly observe an average ~1?mol% enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H+ irradiation of silicate mineral surfaces produces water molecules.”

These results lead the team to hypothesize that the mechanism is widespread in the Solar System. “These results suggest that the Itokawa regolith could contain ~20?l?m?3 of solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy.”

This discovery helps explain how Earth got all its water. If the researchers are correct, we have a much more complete picture of the early Earth and how it came to be life-sustaining. “The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios,” they write.

An artist’s image of the solar wind. Though Earth is protected by its magnetosphere, asteroids have no protection. H+ can strike the surface of asteroids and produce water from silicate minerals. Image Credit: ESO/K. Endo.

The discovery may mean something for future astronauts, too.

As humanity continues to explore space, the presence or lack of water will enable and constrain our space-faring activities. But if this process of creating water by the solar wind is widespread, it means that asteroid and lunar regolith contains water, there for the taking.

“How astronauts would get sufficient water, without carrying supplies, is one of the barriers of future space exploration,” first author Daly said.

“Our research shows that the same space weathering process which created water on Itokawa likely occurred on other airless planets, meaning astronauts may be able to process fresh supplies of water straight from the dust on a planet’s surface, such as the Moon.”