There really is water on the Moon — and it might be much more widespread than previously suspected.
The findings of two separate studies, published today in the journal Nature Astronomy, are a major boost for plans to send humans back to the Moon.
In the first study, a team of scientists led by Casey Honniball of NASA's Goddard Space Flight Center, discovered the signature of water that's either trapped in glass or between grains of sand on the Moon's surface.
"For the first time we have unambiguously detected molecular water on the sunlit Moon," Dr Honniball said.
Scientists have long suspected that large amounts of frozen water lurk in deep, polar craters that never see the Sun.
But Dr Honniball and colleagues detected water molecules in a pockmarked, sunlit region near the Moon's south pole.
"Prior to this it was believed water could not survive on the sunlit Moon," she said.
"Our detection shows that water may be more widespread on the surface of the Moon than previously thought and not constrained to only the poles."
The hunt for water
The two new papers are the high point in a decade of increasingly tantalising hints about water on the Moon.
Spacecraft like NASA's Lunar Reconnaissance Orbiter detected hydrogen — one of water's molecular components — in permanently shady areas at the north and south pole.
The case strengthened when data from India's Chandrayaan-1 spacecraft revealed tiny patches of exposed ice in some of those same shadowy craters.
But Dr Honniball's study reports the signature of water that's not ice.
"Water ice at the poles is a different detection than the water we detect in glass on the sunlit moon," she explained.
It is something that has been hinted at in the past. In high-latitude, sunlit areas of the Moon, scientists have detected the presence of hydrogen bound to oxygen — but it was impossible to tell if it was molecular water (H2O) or hydroxyl groups (OH), which are common in minerals.
To find out, Dr Honniball and her colleagues booked a flight on NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA) in 2018.
SOFIA is a souped-up 747 aeroplane with a telescope inside it that can collect infrared light from above the clouds; in this case, it used a camera that focuses on wavelengths in the 5- to 8-micron range.
Water molecules reflect light at a wavelength of 6 microns.
"This is unique to molecular water because it requires two hydrogen atoms and one oxygen," Dr Honniball explained.
"Hydroxyl, with only one hydrogen atom, cannot make a 6 micron spectral fingerprint.
"With SOFIA we can say it's not just hydrogen we see, it's molecular water."
In fact, water appears to be present in Clavius Crater — a huge basin in the rugged high-latitude highlands — in abundances of around 100 to 400 parts per million.
Water in glass beads — in the sunshine
Craig O'Neill, a planetary scientist at Macquarie University, said the detection of the water signature was a big result.
"We know it's not just a little bit of water bound up in another mineral, its actually there as a water molecule in and of itself," he said.
"It means there's lots of bound water in places on the Moon where you wouldn't necessarily expect it."
Dr Honniball said the detection of water in a sunlit area indicated there were processes occurring on the Moon that were creating and storing water.
She and her colleagues proposed the water could have been trapped in melted rocks, transformed into crystals by the impact of micrometeorites slamming into the surface of the Moon.
The micrometeorites either brought water with them, or the shock of the collision converted existing hydrogen and oxygen in minerals to water as the rocks melted.
Dr O'Neill said it was very common for water bubbles to be trapped in rocks transformed into glass by extreme heat and pressure.
"We see that all the time in geology," he said.
Gathering samples of the glass beads could help answer long-standing questions about how the Moon — and Earth — got their water.
"We've got this entire record of micrometeorite bombardment through time locked up in there, just waiting for us to access it," he said.
But, he added, water stored in glass beads is not as easily accessible for people to use as water stored as ice.
Ice could be widespread in 'cold traps' at the poles
Luckily, the second new study indicates that areas where water could be trapped as ice around the poles are a lot more abundant and accessible than previously thought.
"What they've shown in that paper is once you get above 80 degrees north or south, towards the poles, there's an enormous potential reservoir of ice," Dr O'Neill said.
A team led by Paul Hayne from the University of Colorado modelled the Moon's surface and identified billions of tiny "cold traps": freezing shadows where ice could be stable for billions of years.
Their research, based on data from NASA's Lunar Reconnaissance Orbiter, suggests approximately 40,000 square kilometres of the lunar surface at the poles has the capacity to trap water.
"We find that there are tens of billions of cold traps about a centimetre in size on the Moon," Dr Hayne said.
Because they are so ubiquitous, these small cold traps could be much easier to access than large craters that don't see the light of day and can't be accessed with solar-powered landers or rovers.
"An astronaut or robotic rover or lander could access these smaller shadows and any ice deposits inside them, simply by reaching in — as opposed to venturing into the deep, dark shadows of the larger craters," Dr Hayne said.
"This presents an opportunity to rethink technologies needed to extract and utilise lunar water for scientific and exploration purposes."
Mining water on the Moon
A number of nations are eyeing the south pole of the Moon.
The US recently announced plans to put humans on the Moon in 2024 and have a permanent presence at the south pole by 2028.
This "Artemis" mission, to which Australia is a signatory, will also hunt for water.
If present, it could be used to supply drinking water, as well as produce rocket fuel to sustain space exploration.
Andrew Dempster, head of space engineering at the University of New South Wales, has long argued that Australia should play a role in space mining.
He said the findings of the two papers confirmed assumptions about the Moon and reduced uncertainty for the mining industry.
"We are now much more certain that what we can go and look for is real," Professor Dempster said.
"If [water ice] is more widespread then maybe we don't have to concentrate on these big craters, maybe we can look at these smaller things that are easier to deal with."