Molecular water "unambiguously" detected on the Moon for first time

Astronomers have detected water on the Moon. While that statement might sound all too familiar in recent years, previous reports were based on spectral signatures that could have been other related compounds – this time, the detection is unambiguously water, in the molecular form we need.

In 2009, NASA announced that three separate spacecraft had detected what looked like water on the surface of the Moon. The observations were done by bouncing light beams off the surface and studying how it reflected back – measuring which wavelengths of light were absorbed can reveal much about the composition of the lunar dust, or regolith.

Using this method the spacecraft detected, at wavelengths of three micrometers, the spectral signature of hydrogen and oxygen molecules. The most famous of these of course is water, containing two hydrogen atoms bound to an oxygen atom (H2O), but it’s not the only one. The absorption pattern was also consistent with related hydroxyl compounds, containing one hydrogen and one oxygen atom.

But in a pair of new studies, researchers have been able to definitively say that molecular water is present on the Moon. The Stratospheric Observatory for Infrared Astronomy (SOFIA), a telescope that operates from a 747 plane, observed the lunar surface at a longer wavelength of six micrometers and detected a spectral signature that can only be explained by H2O – that wavelength is not shared by any other hydroxyl compounds.

The scientists estimated that the water is present in the regolith in abundances of between 100 to 400 parts per million. Higher concentrations can be found at the poles, particularly the south pole, which is in agreement with previous studies.

Of course this water isn’t in the form of lakes or even puddles. The team suggest that the water they detected is most likely stored between the grains of regolith or in glasses created from micrometeor impacts.

“The Moon was once thought to be a desert, devoid of water, but in just the last few years we’ve seen hints of it either locked away inside minerals and impact glasses or as hydroxyls, but now this measurement … of molecular water seems to be solid enough to drink,” comments Alan Duffy, Lead Scientist of the Royal Institution of Australia, who wasn’t involved with the study. “It might be a bit of a dusty drink as the water molecules are likely sheltered within or between tiny lunar grains. To fill a liter bottle with Moon water you’d need about 10 tonnes of lunar soil, so it still makes the Sahara look waterlogged by comparison.”

A second study looked at how this water may remain trapped at the surface. Previous studies have pointed towards areas where sunlight never reaches, such as at the bottom of craters, as places where water ice could form and avoid melting away. The team found that these so-called cold traps could get much smaller than expected, down to 1 cm (0.4 in) wide, and could be far more widespread than larger ones. All up, the researchers estimate that cold traps could cover around 40,000 km² (15,400 miles²) of the lunar surface.

With humanity set to return to the Moon in the next few years, these findings could be crucial to how we set up a permanent presence. Already, NASA has its eyes set on the lunar south pole as a likely location for its Artemis Base Camp. The new study backs up that choice as a wise one, but the next major puzzle is how exactly astronauts would extract it.

“We haven’t found a fountain or lake on the moon, the water density is very low, it is confined to the poles, and is likely trapped in glasses or rocks on the surface,” says Ben Montet of the University of New South Wales, who wasn’t involved in the study. “This would provide challenges for a short-term crewed mission like the Apollo missions to access this water quickly and in sufficient quantities to meet the demands of a human crew. Future extended lunar missions, like a permanent moon base, may be able to bring equipment to access this water, but they will need to be careful to conserve this precious resource, even more so than on Earth given its likely limited extent.”

Both studies were published in the journal Nature Astronomy.

Sources: NASA, University of Colorado Boulder