Extract from ABC News
Water is an essential ingredient for life on Earth. It is all around us in our oceans, rivers, clouds and our bodies. But where did it come from?
Key points:
- About 71 per cent of the Earth's surface is covered in water, but we don't know where all this water came from
- A new analysis shows it's possible that some of it could have come from the solar wind transforming dust particles
- While the dust on the debate about the origins of Earth's water is far from settled, the discovery has important implications for the Moon
Some scientists argue that it was bound up in the rocks that glommed together as Earth was born out of a spinning disk of debris 4.5 billion years ago.
Others believe Earth was dry for its first million years or so until it was bombarded by comets and asteroids bearing water.
But neither hypothesis can completely account for the amount or composition of water we see today on Earth.
Now an international team of scientists believe they may have come up with a missing source of Earth's water: the Sun.
Their analysis of tiny fragments of an asteroid and experiments at the atomic scale suggest that the solar wind produces water on the surface of dust grains, they report in Nature Astronomy.
The idea is that water covering specks of dust floating around in the protoplanetary disk was incorporated into planets and asteroids as they formed.
"On the outermost parts of mineral grains attached to asteroids or as dust particles, we find a very thin layer of water that is associated with a very thin layer of hydrogen that has a solar contribution," said study co-author Steve Reddy of Curtin University.
Light vs heavy water
Most of the water on Earth, known as light water, is made up of two atoms of hydrogen bound to one atom of oxygen (H2O).
But a small amount of water on Earth is made out of heavier atoms of deuterium — an isotope of hydrogen with an added neutron — bound to an atom of oxygen instead (D2O).
The ratio of deuterium to hydrogen in water tells us about where in the Solar System it came from.
Most comets and asteroids contain water with a slightly higher ratio of deuterium to hydrogen than on Earth.
"So there's always been a bit of an ongoing debate about how that light component of water gets to Earth," Professor Reddy said.
Only one type of asteroid, known as an enstatite chondrite, contains a similar ratio to water on Earth, but they are very rare, so they can't account for the total amount of light water.
Getting down to an atomic level
To find the missing source of water, the team led by Luke Daly turned to a technique known as atom probe tomography to study the chemical structure of grains of dust from an asteroid called Itokawa.
Dust particles of a mineral called olivine from this stony asteroid between Mars and Jupiter were returned to Earth in 2010 by the Japanese mission Hayabusa.
"The set of samples we analysed are incredibly small — they are thousandths of the width of a human hair," Professor Reddy said.
They also blasted samples of olivine collected on Earth with hydrogen and deuterium to simulate the effect of the solar wind.
"Over time we can build up a three-dimensional image of the chemistry and also the composition of the material," Professor Reddy said.
Analysis of dust from the asteroid and the lab samples showed an increase in hydrogen and deuterium in the outer layers of the particles.
"So what will happen is that the hydrogen will hit the mineral and will produce some sort of defect … and some of those defects will capture the hydrogen."
In other words, the hydrogen breaks the bonds of silicon and oxygen in the olivine, and combines with oxygen to make hydroxl (OH) and water (H2O).
The team said the discovery also meant there could be untapped reservoirs of water on the Moon, as well as asteroids.
'Really neat' for the Moon …
The ability to trace water in the crust of a speck of dust is "really neat", said geochronologist Trevor Ireland of the Australian National University, who was not involved in the study.
"It's an amazing result in terms of being able to resolve differences in the content in the last 50 microns to be able to establish that it's water," said Professor Ireland, who previously analysed samples from Itokawa.
He said the potential of the solar wind to generate water had important implications for establishing a base on the Moon.
"If you can establish that there is actually water being generated in the lunar soil by proton bombardment by solar wind, then that gives a potential resource in the future for water supply."
But what about Earth?
Whether or not the solar wind was a major contributor of water to the dust that made up Earth is still up for debate.
"Going back 4.5 billion years and trying to assemble players and the timing becomes rather complicated," Professor Ireland said.
But, he said, the finding opened up the possibility that light water was delivered to the Earth on asteroids later on.
Different types of asteroids have different levels of water content, while some, like Itokawa, are only wet on their surface, others can contain up to 10 per cent water.
"There are lots of potential sources of water in the Solar System and they all have distinct signatures, both in hydrogen, as well as oxygen isotopes.
"So going forward, we'd like to see some of those predictions tested by looking at oxygen isotopes and potentially nailing down the proportion of these sources in the early Solar System," he said.
There will be more opportunities to dive deeper into the mystery of Earth's water soon.
Along with Itokawa, Professor Ireland is also involved in missions retrieving samples from two other asteroids: Ryugu and Bennu.
The Ryugu mission returned to Earth last year in the South Australian desert, and the first results from the sample return are expected soon.
The Bennu mission is expected to land in 2023.
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