Astronomers have pondered the source of Earth’s water at least since Newton proposed it arrived on comets. The debate has shifted many times over the centuries, but one idea has barely been contemplated: that it came from the Sun. However, improbable as it may seem, this is what some scientists propose in a new paper published in Nature Astronomy. 

Although there was plenty of hydrogen and oxygen in the material Earth formed from, it is thought most escaped quite early in Earth’s evolution, while the oxygen became trapped in rocks. Any water must have arrived later, after the processes that drove off the early hydrogen had passed. 

“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,” said Professor Phil Bland of Curtin University in a statement. 

C-type asteroids have more of a hydrogen isotope called deuterium per water molecule than the Earth’s oceans, so a low deuterium source is required to balance them. 

The solar wind pushed out by the Sun contains much ordinary hydrogen, but very little deuterium. Dr Nick Timms of Curtin University told IFLScience the same low deuterium water was found on the asteroid Itokawa, visited by the Hayabusa mission – but only in a very thin layer at the surface. Deeper in, deuterium concentrations are closer to those of asteroids from further out from the Sun.

Timms and co-authors propose this hydrogen reacted with oxygen at the surface of inner solar system asteroids, producing an exceptionally thin but water-rich layer. For large asteroids like Itokawa, this had a negligible effect on the object as a whole – but smaller objects, particularly dust particles, have very different surface-to-volume ratios. When these materials arrived on Earth, they brought their water with them, diluting the amount of deuterium in our oceans.  

The authors calculate between 56 and 72 percent of the Earth’s water is from this previously overlooked source. Without it, Timms told IFLScience; “The Earth would not be the water-rich world we know.” 

Other objects in the inner Solar System must have been exposed to the same rain of water-rich dust, the authors say. The fact Mars once had oceans but lost them indicates most of the dust accreted onto planets early on, having long been insufficient to compensate Mars rate of water loss. This fits with our models which propose a dusty early Solar System.  

Nevertheless, said lead author Dr Luke Daly; “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.” 

Timms told IFLScience some of the paper’s authors were originally skeptical of an idea so different from those previously proposed, but they; “Jumped on board and confirmed our numbers are right.” 

Meteorites have their outer layers burned off in the descent through the atmosphere, so can’t be used to verify the Itokawa measurements. However, the team has gained access to the Hayabusa-2 samples from Ryugu, and will also seek to study rocks recently collected from the surface of Bennu to see if these also contain a surface layer of low-deuterium water.