These traces were in fact believed to be due to contamination on Earth.
But over the past two decades, re-analyses of lunar samples, observations by spacecraft missions, and theoretical modeling have proved this initial assessment to be wrong.
"Water" has since been detected inside the minerals in lunar rocks. Water ice has also been discovered to be mixed in with lunar dust grains in cold, permanently shadowed regions near the lunar poles.
But scientists haven't been sure how much of this water is present as "molecular water" - made up of two parts hydrogen and one part oxygen (H2O).
Water and more water
The term water isn't just used for molecular water, but also also for detections of hydrogen (H) and hydroxyl (OH).
Although H and OH could be combined by astronauts to form molecular water at the lunar surface, it is important to know in what form these compounds are present initially.
That's because this will have an impact on their stability and location under lunar surface conditions, and the effort required to extract them. Molecular water, if present as water ice, would be easier to extract than hydroxyl locked in rocks.
The presence of water on the Moon is scientifically interesting:
Knowing more about the specific compound could help us find out.
The lunar surface,
seen by Apollo 11.
Understanding how much water is present, and its location, is also incredibly useful for planning human missions to the Moon and beyond.
Water represents a key resource that can be used for life-support purposes - but it can also be split apart into its constituent elements and put to other uses.
Oxygen could replenish air supplies, or be used in simple chemical reactions at the lunar surface to extract other useful resources from the regolith (soil composed of small grains).
Water could also be used as rocket fuel in the form of liquid hydrogen and liquid oxygen.
This means that the Moon has great potential to become a refueling base for space missions further into the Solar System or beyond. Its lower gravity and lack of atmosphere means it would require less fuel to launch from there than from Earth.
So when space agencies talk of in-situ resource utilization at the Moon, water is front and centre of their plans, making the new papers extremely exciting.
Instruments on board various spacecraft have previously measured "reflectance spectra" (light broken down by wavelength) from the Moon.
These detect light coming from a surface to measure how much energy it reflects at a specific wavelength. This will differ based on what the surface consists of. Because it has water, the Moon's surface absorbs light at 3µm wavelengths (0.000003 meters).
However, absorptions at this wavelength cannot distinguish between molecular water and hydroxyl compounds.
Using the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) telescope, flown at 43,000 feet, the team behind one of the new papers observed sunlit sections of the Moon's surface in wavelengths of 5-8µm.
H2O results in a characteristic peak in the spectrum at 6µm, and by comparing a near-equatorial area as a baseline (thought to have almost no water) with an area near the south pole, this study reports the first unequivocal observations of molecular water under ambient conditions at the lunar surface at an abundance of 100-400 parts per million.
This is several orders of magnitude too large for most of the water to be adsorbed onto regolith grain surfaces.
Instead, the authors suggest that,
Where exactly this water is sited would be of extreme interest for future explorers as it would dictate the processes and energy required to extract it.
Luckily, the other paper used new theoretical models, based on temperature data and higher resolution images from the Lunar Reconnaissance Orbiter, to refine predictions of where conditions are right for molecular water to be trapped as ice.
Previous research has shown already that there are such kilometers-wide "cold traps" in permanently shadowed areas near the poles, where water ice may be present.
Evidence from orbiting spacecraft, however, was inconclusive about this being molecular water or hydroxyl.
The new study finds that there are also abundant small cold traps where conditions permit water ice to accumulate - on the scale of centimeters or decimeters. In fact, such traps should be hundreds to thousands of times more numerous than larger cold traps.
The team calculates that 0.1% of the total lunar surface is cold enough to trap water as ice, and that the majority of these icy cold traps are at high latitudes (> 80°).
This is particularly near to the lunar south pole, narrowing down the choice of future landing sites with the highest chance of finding trapped water ice.
However, it is important to realize that the two studies investigated areas at different latitudes (55°-75°S vs. >80°S) and therefore cannot be compared directly.
Nevertheless, these latest discoveries further enhance our understanding of the history of water on our nearest neighbor. They will undoubtedly strengthen plans for a return to the Moon.
Instruments such as the European Space Agency's (PROSPECT payload on Luna 27) will be able to make measurements on the Moon to "ground-truth" these tantalizing glimpses of the wealth of information yet to be discovered.
There's Water All Over
As many of you correctly guessed, the discovery has to do with water on the lunar surface.
The new research, in addition to providing the best evidence to date of water ice on the Moon, suggests this valuable resource is scattered across the lunar surface, including in areas exposed to direct sunlight and in tiny pockets cast in permanent shade.
Two papers published today in Nature Astronomy are redefining what we know about the Moon and its ability to stock a precious natural resource:
Scientists have long suspected that frozen water exists on the Moon, particularly at the poles, but the new research provides the most definitive evidence yet, owing to the detection of actual water molecules on the Moon's sunlit surface.
The new research also identifies a slew of shadowy pockets, known as cold traps, in which frozen water could be hiding.
That water ice exists on the Moon is significant from a purely scientific perspective, but it's also important in terms of how it will influence future missions to the lunar surface.
An important goal for the upcoming NASA Artemis missions will be to collect and retrieve water ice from the southern polar regions, which now seems more possible than ever.
What's more, the apparent abundance of water on the Moon means it can be sourced locally, which is excellent news for future explorers or colonists.
Previously, the detection of bright splotches at the lunar south pole hinted at this possibility.
Data from a NASA radar attached to India's Chandrayaan-1 spacecraft suggested the same, revealing dozens of small craters that appeared to be filled with water ice. And in 2016, residual traces of water ice were considered evidence of the Moon's tilting axis.
Yet despite these and other tantalizing discoveries, actual proof of water molecules on the Moon was limited to spectral signatures spotted at 3 microns.
That's a problem, because, at this wavelength, scientists cannot distinguish between water and hydroxyl bound minerals (hydroxide contains oxygen bonded to hydrogen).
To overcome this limitation, scientists took new measurements of the lunar surface from the Stratospheric Observatory for Infrared Astronomy (SOFIA) - a modified, high-flying Boeing 747 jumbo jet equipped with a 9-foot-long telescope.
With SOFIA, the researchers detected a spectral water signature at 6 microns, which is not shared with other hydroxyl groups.
This water signature was spotted at the high southern latitudes, at amounts ranging between 100 and 400 parts per million.
To put that into perspective,
Incredibly, this water was detected in areas exposed to direct sunlight.
According to the new Nature Astronomy paper (Molecular water detected on the sunlit Moon by SOFIA), co-authored by planetary scientist Casey Honniball from the University of Hawaiʻi at Mānoa, this water could be packed within fine bits of glass strewn across the lunar surface, or sandwiched between grains of dust that shield the water from the Sun's rays.
In an email, Matthew Siegler, a research scientist with the Planetary Science Institute in Dallas, Texas, said,
Looking ahead to future work, Siegler would like to see more SOPHIA data, or data from any instrument capable of scanning at 6 microns, to map the full extent of the water on the lunar surface.
Indeed, the team's observations were limited to the gigantic Clavius Crater, a sunny region in the Moon's southern hemisphere. Future observations are needed to determine if similar concentrations of water are located in other sunlit regions.
Ultimately, NASA would like to create a kind of hydrological map, showing the relative abundance of water across the Moon.
Speaking at a press briefing today, Jacob Bleacher, chief exploration scientist for the Human Exploration and Operations Mission Directorate at NASA Headquarters, said scientists will need to determine how stable this water is and how much of it might be lost to space over time.
Paul Hertz, astrophysics division director at NASA Headquarters in Washington, chimed in, saying this finding,
The scientists would like to know how this water might be accessible to astronauts, how it might be affecting the chemical makeup of the lunar dust, and if it might have an impact on equipment, such as drills.
In addition to water potentially trapped in glass or shielded by dust, today's revelations also include the possibility of frozen water scattered in small hiding places across the Moon.
In the second paper (Micro cold traps on the Moon) published today in Nature Astronomy, researchers led by Paul Hayne from the University of Colorado at Boulder report that hidden pockets of water are more common on the Moon than previously suspected.
Known as cold traps, these are special spots on the surface that are perpetually cast in darkness, potentially allowing for the long-term preservation of water ice.
Examples of cold traps on the moon,
in which areas are
perpetually cast in shade.
Take Shackleton crater, for example, a depression measuring 13 miles (21 km) across and several miles deep.
Portions of the crater are angled such that interior parts are always in shade, with temperatures holding steady at around -300º F (-184º C).
Using NASA's Lunar Reconnaissance Orbiter (LRO), Hayne and his colleagues studied a wide range of possible cold traps, some no larger than a penny.
Incredibly, some of these cold traps have remained in complete darkness for billions of years.
Small-scale cold traps near the Moon's polar regions are surprisingly abundant, amounting to 15,400 square miles (40,000 square kilometers) of the lunar surface, according to the study.
Around 60% of the cold traps are located near the southern polar regions.
Previous research suggested cold traps account for 7,000 square miles (18,100 square kilometers) of lunar real estate, with the new estimate significantly raising this figure.
Importantly, these places are capable of storing water, with capable being the key word; the new paper doesn't actually provide evidence of water ice existing inside these dark pockets.
Siegler said any potential water inside of these cold traps came from sources like comets, asteroids, lunar volcanoes, or chemical reactions caused by solar wind, but they could have also originated from the release of molecules found inside glasses (which were mentioned as a possibility in the Honniball paper).
These glasses, produced by the heat from impacts,
The Moon, as these papers suggest, is a better place for storing water than we realized.
This has huge implications for missions to the Moon, as lunar explorers could potentially source water locally (it's quite burdensome to transport lots of the stuff from Earth).
It's not clear how easy it would be to actually collect water ice from the lunar surface, but the new research should give us cause for optimism.
This ice could produce water for drinking, but it could also be used to produce rocket fuel, Siegler pointed out.
To which he added:
For the Artemis III mission, which could see a woman and man land on the Moon in 2024, this is good news, as it means they have a decent chance of accomplishing their goal of finding water in the southern polar regions.
Let the treasure hunt begin...!