Glass beads on the surface of the Moon could provide a readily accessible source of water for future missions, a new study suggests.
An international team analysed lunar glass in soil samples brought to Earth by the Chinese Chang'e-5 mission.
The researchers, led by Huicun He of the Chinese Academy of Sciences, estimate between 300 billion and nearly 300 trillion kilograms of water is held in tiny glass beads strewn across the Moon.
According to their analysis, published today in Nature Geoscience, water forms from hydrogen delivered in the stream of charged particles flowing from the Sun called the solar wind.
"These findings indicate that lunar soils contain a much higher amount of solar wind-derived water than previously thought," they wrote.
This suggested the beads played a central role in the water cycle of the Moon and could be an important reservoir for space bases to tap.
"Impact glass beads have the capacity to store significant quantities of solar wind-derived water at the surface ... in addition to the possible presence of water ice trapped in permanently shadowed areas in polar regions."
Finding water in the Ocean of Storms
It was once thought that the Moon was dry, but over the past 20 years orbiting spacecraft have detected hydrogen and ice in deep craters at the lunar poles.
In 2020, a team of NASA scientists detected molecules of water in sunlit areas of the Moon for the first time.
They proposed the water could be trapped in glass beads, which would explain why it hadn't evaporated.
Hints of water had previously been found in glass from samples collected by the Apollo missions 40 years ago.
In late 2020, China's Chang'e-5 became the first spacecraft since the Apollo era to return to Earth with rocks from the Moon.
Change-5 landed in the Ocean of Storms, a geologically unique area that hadn't previously been sampled, said Craig O'Neill, a planetary scientist at the Queensland University of Technology, who was not involved in the study.
"It's an extraordinarily radioactive part of the Moon.
"It's also close enough to the equator that it's getting a decent amount of sun ... so [water] is going to have to be bound in a mineral to be stable."
Glass beads are created when micro-meteorites slam into the surface melting rock around them.
Previous analysis dating the age of the beads scooped up by Chang'e-5 spacecraft showed the area had been bombarded over the past 2 billion years in a process known as impact gardening.
How does the solar wind create water?
While any water is blasted out of the beads by the initial force of impact, they are porous enough to absorb hydrogen delivered by the solar wind.
Hydrogen interacts with oxygen trapped in the glass to create hydroxyl (OH), an ion that can link up with more hydrogen to form water.
"[Hydrogen] is like uninvited guests at a party," Dr O'Neill said.
"They reach their way in and then they hook up with the oxygen atoms that are in the mineral."
For this study, the team blasted 32 beads with a laser looking for signs of hydrogen isotopes.
Higher concentrations of hydroxyl/water were detected in the Chang'e-5 beads than found in samples collected by the Apollo missions.
The water builds up as the beads are constantly bombarded by the solar wind.
Cross sections of five beads revealed more hydroxyl/water was present on the outside of the glass than in the centre.
"The solar wind irradiates the outside of these beads, so you're basically forming water on the outside and it diffuses in towards the centre."
The analysis showed that water formation was relatively quick, only taking about 15 years to reach the levels detected in the beads.
The Chang'e-5 team suggested the water could then be released into space as vapour by heat from the Sun or by impacts from micro-meteorites.
Marc Norman of the Australian National University said the idea that beads played a key role in the Moon's water cycle was very feasible.
"Geophysical models have suggested a very similar sort of process, this is just one of the first demonstrations of it using samples and measured data," said Dr Norman, who has previously studied beads from both the Apollo and Chang'e-5 missions.
Dr Norman said the higher water concentration in the Chang'e-5 sample compared with the Apollo samples may be due to the different types of rocks.
While Apollo beads were volcanic glass, Chang'e-5 beads are impact beads.
"A lot of the Apollo missions went to impact craters, [and drilled] into the deeper sub-surface, so a lot of the samples would probably not have been exposed for as long as the Chang'e-5 samples were," he said.
'Big deal' for settlement?
If the water in the beads turns out to be easy to get to, it would be a "really big deal" for space exploration, said Phil Bland, a planetary scientist at Curtin University, who was not involved in the study.
"The more accessible resources you can find at the Moon, the greater the likelihood that we can develop a permanent settlement," Professor Bland said.
The question is: How much water is in the beads? And how easy will it be to extract?
Previous analysis of Moon dirt or regolith by the Chang'e-5 team reveals the beads contain more water than the surrounding soil.
Dr O'Neill said water in the beads would be easier to release than molecules in soil, which are tightly bound to a mineral called apatite.
"You've got a limited amount of apatite in the regolith," he said, adding that it would require a lot of energy to extract.
"If you go to the Moon and you're looking to utilise in-situ resources, it's hard to make the business case for [extracting water from apatite] because the costs versus what you're going to get is not terribly high."
Experiments by the Chang'e-5 team showed it was relatively easy to liberate water from the beads at lower temperatures using very low tech.
"You heat this up to a few hundred degrees, and with a condenser on top, you could basically extract water from these beads with a fairly simple operation," Dr O'Neill said.
"So for future in-situ resource planning, this is a big step forward that points the way to a resource that has fairly high contents of water.
"We're not talking about [quantities] anything like Earth, but we're talking about enough to use."
But Dr Norman was not convinced.
"The beads are small and there's typically a modest amount of them in the soil ... so extracting them on an industrial scale for in-situ resource utilisation — at least at this point in time — may be overstating their potential significance," he said.
