Space dust could solve mystery of Earth's water

Artist’s impression of asteroids and space dust raining down on the Earth, carrying some of the water that formed its oceans. Image: University of Glasgow

An international team of scientists may have solved a key mystery about the origins of the Earth’s water, after uncovering evidence pointing to an unlikely culprit - the Sun.

In a new paper published in the journal Nature Astronomy, a team of researchers from the UK, Australia and America describe how new analysis of an ancient asteroid suggests that grains of extraterrestrial dust carried water to Earth as the planet formed. The water in the grains was produced by space weathering, a process by which charged particles from the Sun known as solar wind altered the chemical composition of the grains to produce water molecules.

The finding could answer the longstanding question of just where the unusually water-rich Earth got the oceans which cover 70 percent of its surface – far more than any other rocky planet in our solar system. It could also help future space missions find sources of water in airless worlds.

Dr Luke Daly, of the University of Glasgow’s School of Geographical & Earth Sciences, Scotland, is the paper’s lead author. He said: “We’re very excited by the evidence that we’ve collected. It could open the door to a much better understanding of what the early Solar System looked like and how the Earth and its oceans were formed.”

Planetary scientists have puzzled for decades over the source of Earth’s oceans. One theory suggests that one type of water-carrying space rock known as C-type asteroids could have brought water to the planet in the final stages of its formation 4.6 billion years ago.

To test that theory, scientists have previously analysed the isotopic ‘fingerprint’ of chunks of C-type asteroids which have fallen to Earth as water-rich meteorites. If the properties of the meteorite water matched that of terrestrial water, scientists could conclude that C-type meteorites were the likely source.

The results were not clear-cut. While some meteorites’ fingerprints did indeed match Earth’s water, many did not. In other words, while some of Earth’s water must have come from C-type meteorites, the forming Earth must have received water from at least one more isotopically-light source which originated somewhere else in Earth's solar system.

"The evidence could open the door to a much better understanding of how Earth and its oceans were formed.”

Dr Luke Daly, University of Glasgow

The University of Glasgow led team used a cutting-edge analytical process called atom probe tomography to scrutinise samples from a different type of space rock known as an S-type asteroid, which orbit closer to the sun than C-types. The samples they analysed came from an asteroid called Itokawa, which were collected by the Japanese space probe Hayabusa and returned to Earth in 2010.

Atom probe tomography enabled the team to measure the atomic structure of the grains one atom at a time and detect individual water molecules. Their findings demonstrate that a significant amount of water was produced just below the surface of dust grains from Itokawa by space weathering.

The early solar system was a very dusty place, providing a plenty of opportunity for water to be produced under the surface of space-borne dust particles. This water-rich dust, the researchers suggest, would have rained down on early Earth alongside C-type asteroids as part of the delivery of the oceans.

Dr Daly said: “The solar winds are streams of mostly hydrogen and helium ions which flow constantly from the Sun out into space. When those hydrogen ions hit an airless surface like an asteroid or a spaceborne dust particle, they penetrate a few tens of nanometres below the surface, where they can affect the chemical composition of the rock.

"Over time, the ‘space weathering’ effect of the hydrogen ions can eject enough oxygen atoms from materials in the rock to create H2O – water – trapped within minerals on the asteroid.

“Crucially, this solar wind derived water produced by the early solar system is isotopically light. That strongly suggests that fine-grained dust, buffeted by the solar wind and drawn into the forming Earth billions of years ago, could be the source of the missing reservoir of the planet’s water.”

Professor Phil Bland, from the School of Earth and Planetary Sciences at Curtin University, Perth, Australia, and co-author of the paper said, “Atom probe tomography lets us take an incredibly detailed look inside the first 50 nanometres or so of the surface of dust grains on Itokawa, which orbits the sun in 18-month cycles. It allowed us to see that this fragment of space-weathered rim contained enough water that, if we scaled it up, would amount to about 20 litres for every cubic metre of rock.”

Co-author Professor Michelle Thompson of the Department of Earth, Atmospheric & Planetary Sciences at Purdue University, Indiana, US, added, “It’s the kind of measurement that simply would not have been possible without this remarkable technology. It gives us an extraordinary insight into how tiny dust particles floating in space might help us balance the books on the isotopic composition of the Earth’s water, and give us new clues to help solve the mystery of its origins.”

Their estimates of just how much water might be contained in space-weathered surfaces also suggest a way future space explorers could manufacture supplies of water on even the most seemingly arid planets.

Co-author Professor Hope Ishii of the University of Hawai’i at Mānoa said: “One of the problems of future human space exploration is how astronauts will find enough water to keep them alive and accomplish their tasks without carrying it with them on their journey.

“We think it’s reasonable to assume that the same space weathering process which created the water on Itokawa will have occurred to one degree or another on many airless worlds like the Moon or the asteroid Vesta.

"It’s exciting to think that the processes which formed the planets could help to support human life as we reach out beyond Earth.”

Professor Hope Ishii, University of Hawai’i

"That could mean that space explorers may well be able to process fresh supplies of water straight from the dust on the planet’s surface. It’s exciting to think that the processes which formed the planets could help to support human life as we reach out beyond Earth.”

The team’s paper, Solar, Wind Contributions to the Earth’s Oceans, is published in Nature Astronomy.

Researchers from the University of Glasgow, Curtin University, the University of Sydney, the University of Oxford, the University of Hawai‘i at Mānoa, the Natural History Museum, Idha National Laboratory, Lockheed Martin, Sandia National Laboratories, NASA Johnson Space Center, the University of Virginia, Northern Arizona University and Purdue University all contributed.

Dr Luke Daly, lead author. Image: University of Glasgow