Did Earth get its water from… the Sun?

Where did the Earth’s oceans come from? For the past few years, astronomers have theorised that asteroids and comets deposited water on the Earth’s surface during the early days of our planet, but many questions remain.

Now, a paper in Nature Astronomy has shown that the Sun itself could have been an additional source, with solar winds reacting with minerals in space dust to produce water molecules.

The international team of researchers were examining samples from Japan’s Hayabusa spacecraft mission, which brought part of an asteroid to Earth in 2010.

“We started off wanting to measure the effects of space weathering on the particles that were brought back from this asteroid,” says co-author Associate Professor Nick Timms, from Curtin University.

But the researchers found that near the surface, the ‘isotope composition’ of hydrogen (the ratio of heavier to lighter hydrogen atoms) in the sample wasn’t what they were expecting.

They determined that the ratio had been thrown off by the solar wind – which distributes protons, which are the same thing as hydrogen (H⁺) ions.

“It’s only the surfaces of the particles in this asteroid that have been exposed to the Sun that have this implanted layer of hydrogen,” says Timms.

The researchers believe that this additional source of hydrogen can make sense of the isotope composition of the Earth’s oceans. The hydrogen ions (H⁺) from the solar wind reacted with oxygen atoms (O) in silicates in the rock, creating water (H₂O).

“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,” says co-author Professor Phil Bland, also of Curtin.

“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.”

Timms says the discovery also carries exciting outcomes for space exploration.

“Another implication is that if the surfaces of other asteroids or other airless bodies have the same implanted water on the surface particles, that’s quite a neat reservoir to think about trying to release that water as a resource – for fuel, for life, or whatever it might be that we want for interplanetary exploration missions.”