NASA promised some “exciting” news about the Moon today, and we’re pleased to report the space agency has delivered. As many of you correctly guessed, the discovery has to do with frozen 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: water. 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 lunar surface. The new research also identifies a slew of shadowy pockets, known as cold traps, in which much of this 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 tantalising 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 2.74 m-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.
Incredibly, much of this water was detected in areas exposed to direct sunlight. According to the new Nature Astronomy paper, 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 it’s the first “full-fledged” detection of water molecules on the lunar surface, as opposed to the previously observed 3-micron feature that could be confused for hydroxide compounds.
“Hydroxide can form from solar wind protons crashing into any oxygen hanging out on the Moon, such as those contained in rocks,” wrote Siegler, who wasn’t involved in the new research. “It’s a bit harder for water to form that way,” he said, so it probably came about through different processes, such as ancient Moon volcanoes or through impacting objects like asteroids and comets. 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.
“These water molecules appear to be trapped within glasses, rather than free and bouncing on the surface, so they could be fairly global and not only in the ultra-cold polar regions,” explained Siegler. “Free water molecules need to be chilled to less than around 110 degrees Kelvin [-163 degrees Celsius, -261 degrees Fahrenheit] to stick around very long, and those conditions only happen in cold traps.”
Which nicely leads us to the second paper, also published today in Nature Astronomy. This research, led by Paul Hayne from the University of Colorado at Boulder, suggests 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.
“If you can imagine standing on the surface of the Moon near one of its poles, you would see shadows all over the place,” explained Hayne in a UC-B press release. “Many of those tiny shadows could be full of ice.”
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 -184 degrees Celsius.
Using NASA’s Lunar Lunar Reconnaissance Orbiter, 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 40,000 square kilometres 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 18,100 square kilometres of lunar real estate, with the new estimate significantly raising this figure.
“Our results suggest that water trapped at the lunar poles may be more widely distributed and accessible as a resource for future missions than previously thought,” wrote the researchers in the study.
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, “basically introduce a potential new global reservoir,” said Siegler.
The Moon, as these papers suggest, is a better place for storing water than we realised. 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.
“If water is really widespread, even if very surficial and trapped in glasses, you could imagine ‘strip mining’ the lunar surface for water could be a more viable thing — not that I am advocating that we strip mine the Moon,” he explained. “But water is very useful if you are on the Moon and want a martini, or drinking water, or if you want to refuel your standard liquid hydrogen, liquid oxygen rocket.”
To which he added: “It’s really expensive to bring water from Earth out of our gravity well, so having water on the Moon makes lunar exploration much cheaper and makes refuelling in space feasible.”
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!