Laboratory researchers have found a way to liberate water from moon dust, a result that may someday be used to supply water for future lunar bases.
Scientists have long known that the moon is mostly dry, but recent space missions have found traces of water vapour in its hyper-tenuous exosphere – a layer of gas so thin that it can’t really be called an atmosphere.
They have also found ice in permanently shaded “cold traps” at the bottom of deep craters near its south pole.
Theories for where this water comes from are numerous, ranging from vapour leaking from deep in the moon’s interior to ice delivered to its surface by meteorites or comets.
One of the leading theories is that the water might be produced by the solar wind, which is composed of high-energy particles streaming out from the sun.
Among these particles are protons, which are simply hydrogen atoms stripped of their electrons. In other words, they are half of what you need in order to form H2O.
The other half, oxygen, is plentiful in the minerals of moon rocks. That means that when the high-energy protons bash into the lunar rocks, they may react with oxygen atoms in them to form water, which is later released into the exosphere.
To see if this might actually work, a team led by Ralf Kaiser, a physical chemist at The University of Hawaii at Manoa, US, conducted two laboratory experiments.
First, they placed powdered olivine crystals — a mineral known to be common on the moon — in a vacuum chamber and cooled them to 10 degrees Kelvin (minus-263 degrees Celsius), a temperature at which any water formed in the crystals would accumulate to levels at which it could later be measured.
Because protons don’t penetrate very far into rocks, powdered olivine was used in order to make the process more efficient.
The scientists then bathed their simulated moon dust in enough radiation to mimic 300 years of solar wind.
Afterward, they heated the rocks up to 300 degrees Kelvin (27 degrees Celsius) and looked to see if they emitted water. They didn’t.
Other researchers had tried the same thing, Kaiser says, with some of them finding water emissions, and some not.
Most likely, he explains, those who did find water emissions were simply finding water from the Earth’s atmosphere, never fully removed from their vacuum chambers. That water had then deposited on the cold dust particles, like dew on autumn grass.
“That’s what happens in a refrigerator,” Kaiser says. “If you have a cold surface, all kinds of junk is building up on [it].”
To beat this problem, his team began by using a vacuum chamber capable of holding an ultra-thin vacuum. Then, because no vacuum chamber is perfect, they irradiated their simulated moon dust with deuterium ions instead of hydrogen.
Deuterium is a form of heavy hydrogen, the nucleus of which is composed of a proton plus a neutron, rather than just a proton.
Chemically, it is identical to hydrogen, but on Earth it’s only 0.01% as abundant. That means that any deuterated water emitted by the irradiated rocks would have to have been formed from the deuterium bombardment, rather than being a laboratory contaminant.
The fact that no water was released in this experiment didn’t mean it wasn’t there, however. It might just be too tightly bound to the mineral particles to be released at moderate temperatures.
For example, Kaiser says, the simulated solar wind particles might have reacted not only with oxygen in the olivine but also with silicon to form a substance known as silanol, which has the chemical notation Si-O-H (although, in this case, it should be notated as Si-O-D, in which the ‘D’ is deuterium).
But solar heating isn’t the only energy source that might release water from such a chemical complex.
In addition to being warmed by the midday sun, Kaiser says, moon rocks are constantly being bombarded by micro-meteorites, in impacts that can briefly flash localised temperatures up to 1400 degrees Kelvin (or approximately 1100 degrees Celsius).
Kaiser’s team couldn’t simply heat the dust up to temperatures even remotely that high. “We cannot go higher than 320 degrees Kelvin or we will destroy our machine,” he says.
Instead, they zapped the irradiated dust with micro-bursts from a laser designed to simulate the brief pinpricks of heat created by micro-meteorite impacts. And this time, they got water.
Exactly how much water can be formed this way is still unclear, Kaiser says, but, “under our conditions, we know that a few per cent of the protons can be converted”.
It’s an important finding for two reasons.
First, it is a laboratory demonstration (Kaiser calls it proof-of-concept) of the theory that the moon’s water could indeed have at least partially originated from protons in the solar wind.
But it also means that water trapped in moon dust might be a valuable resource for future astronauts.
“If colonisation of the moon can proceed, humans need drinking water,” Kaiser says. And not only that, he adds, but water can also be turned into hydrogen and oxygen gases, which can then be burned as a much-needed fuel.
The research is published in the journal Proceedings of the National Academy of Sciences.