Despite its proximity to a very blue planet, the Earth's Moon appeared to be completely dry, with samples returned by the Apollo missions being nearly devoid of water. But in recent years, a number of studies have turned up what appears to be water in some locations on the Moon, although the evidence wasn't always decisive.
Today, NASA is announcing that it has used an airborne observatory to spot clear indications of water in unexpected places. But the water may be in a form that makes accessing it much harder. Separately, an analysis of spots where water could be easier to reach indicates that there are more potential reservoirs than we'd previously suspected.
Up in the air
With no atmosphere and low gravity, the Moon can't hang on to water on its surface. The first time that sunlight heats lunar water up, it will form a vapor and eventually escape into space. But there are regions on the Moon, primarily near the poles, that are permanently shadowed. There, temperatures remain perpetually low, and ice can survive indefinitely. And, to test this possibility, NASA crashed some hardware into a shady area near the Moon's south pole and found water vapor amidst the debris.
In fact, water liberated from elsewhere on the Moon can condense there before it escapes into space, potentially creating a growing pile of ice. Since water is going to be delivered by impacts with asteroids and cometary material, it's likely that this is an ongoing process.
But we wouldn't expect this to be happening in any areas exposed to sunlight. There, any water should be heated enough to drive it into the atmosphere, which would explain why samples returned from the Apollo missions show little water.
But there was a certain ambiguity in the data. Studies had indicated that some water-like material was present but couldn't differentiate between water and a hydroxyl group (OH), which could exist in some minerals. So, we weren't really sure what we were seeing there.
To figure this out, NASA turned to an infrared observatory that it has stuck in the back-end of a 747 with a hole cut out of the side. Known as the Stratospheric Observatory for Infrared Astronomy or SOFIA, the 747 brings the hardware up above much of the atmosphere. From there, there are far fewer molecules that would happily absorb some of the infrared light that the telescopes on SOFIA are designed to observe.
One of SOFIA's instruments is sensitive to wavelengths in the area of six micrometers (the Faint Object infraRed CAmera for the SOFIA Telescope, or FORCAST). And that's critical because, while water can absorb and emit at this wavelength, hydroxyl groups cannot. So, anything detected here is definitively water.
Where’s the water?
The researchers looked at two regions of the Moon, one equatorial and one near a pole. This allowed them to use the equatorial site, which gets more sunlight and is therefore less likely to have water, as a control. The polar region, more likely to contain water, was the experimental. And it had a clear, strong signal corresponding to water. Nearly all of the areas imaged saw the water signal with a significance of anywhere from two sigma, and 20 percent of them exceeded four sigma. (For the Moon, the instrument could resolve patches of surface that are 1.5 x 5km.)
The authors of the new paper estimate the abundance of water as ranging from 100 to 400 micrograms per gram of lunar material. In a press conference, however, NASA decided to give an approachable value by mixing units: it's the equivalent of each cubic meter of Moon material having a 12 ounce bottle of water in it, on average.
And this is weird. The sunlight the area sees should be enough to cause any water to be cooked off rapidly. How is the water still there?
The authors' proposal—and it's just a hypothesis at this point—is that the water has been encased in glass. Rather than envisioning a literal 12 ounce glass bottle, you should be thinking of the disordered material that's formed by impacts. Some of the impacts on the Moon will come from water-containing materials, and that water will be vaporized by the impact. As will some of the rock and other materials, although they'll condense back to liquid quickly. As that rocky liquid cools off to form a disordered, glassy solid, it'll trap some of the water vapor.
Once trapped inside some glassy rock, the water will be impervious to the heating and cooling cycles that would normally drive the water back off the Moon's surface, which is why it's persisting at a sunny site on the lunar surface.
It also means that getting at the water will be a lot harder. Plenty of ideas about future lunar activities involve gathering water on the surface. But, if getting the water involves grinding down tiny pellets of glass, it may be significantly more trouble.
In the shade
But again, the focus on lunar water hasn't been in the sunny regions. Instead, the focus has been on the sites where shade might allow water to condense and form ice. And that's where the second paper comes in; it basically makes a catalog of all the potential sites on the Moon that are cold enough for ice to remain stable. And we mean all, even going down to considering rough surfaces that may create shady regions as small as one centimeter.
The researchers figure out the location of large sites by looking through the images of the lunar surface and creating a 3D model that gets them the areas that would be shaded under all circumstances. For lower-volume areas, they examine images of the Moon's surface and figure out what percentage of that surface would end up shaded. They then model the diffusion of heat from the Sun-exposed sections and figure out which areas will remain cool enough to retain ice in the vacuum.
And, well, there's no shortage of potential places where water could exist without being encased in glass. The northern polar region has lots of areas with cold traps ranging from a meter up to 10 kilometers. But the southern polar region has far more that are over 10km in size. All told, this adds up to about 40,000 square kilometers of the Moon's surface that could hold water ice.
This doesn't mean that all that water is there. Some of it clearly is, based on NASA's earlier probe-crashing "experiment" that liberated some water vapor from the Moon's surface. But how much remains completely unclear. And whether it's in large, easily accessible ice deposits will remain an unknown until we get hardware to one of the locations we expect to host a large deposit.
Nature Astronomy, 2020. DOI: 10.1038/s41550-020-01222-x, 10.1038/s41550-020-1198-9x (About DOIs).
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