Enlarge /. The water fly detection instrument on a 747.
Despite its proximity to a very blue planet, Earth's moon appeared completely dry, and the samples returned from the Apollo missions contained almost no water. However, in recent years, a number of studies have found that there appears to be water in some places on the moon, although the evidence has not always been decisive.
Today NASA announces that it has used an airborne observatory to see clear evidence of water in unexpected locations. However, the water can be in a form that makes access considerably more difficult. Regardless, an analysis of places where water might be easier to reach shows that there are more potential reservoirs than we previously suspected.
Without an atmosphere and low gravity, the moon cannot hold onto water on its surface. When sunlight first warms the moon water, it forms a vapor and eventually escapes into space. However, there are regions on the moon, mainly near the poles, that are permanently shadowed. There the temperatures remain constantly low and the ice can survive indefinitely. To test this possibility, NASA hardware crashed into a shady area near the south pole of the moon and found water vapor amid the debris.
In fact, water released from elsewhere on the moon can condense there before escaping into space, potentially creating a growing ice pile. Since water is supplied by collisions with asteroids and cometary material, it is likely that this will be an ongoing process.
However, we wouldn't expect this to happen in areas exposed to sunlight. There, any water there should be warmed enough to be driven into the atmosphere, which would explain why samples returned from the Apollo missions show little water.
However, there was some ambiguity in the data. Studies had shown that some water-like material was present but failed to differentiate between water and a hydroxyl group (OH) that might be present in some minerals. So we weren't sure what we were seeing there.
To find out, NASA turned to an infrared observatory tucked into the back of a 747 with a hole cut out on the side. Known as the Stratospheric Infrared Astronomy Observatory or SOFIA, the 747 brings the hardware over much of the atmosphere. From there there are far fewer molecules that would like to absorb some of the infrared light that the telescopes on SOFIA are supposed to observe.
One of the SOFIA instruments is sensitive to wavelengths in the range 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. Everything that is discovered here is definitely water.
Where is the water?
The researchers examined two regions of the moon, one equatorial and one near a pole. This allowed them to use the equatorial spot, which receives more sunlight and therefore less likely to have water, as a control. The polar region, which is more likely to contain water, was the experimental one. And it had a clear, strong signal that corresponded to water. In almost all of the areas shown, the water signal was seen to be two sigmas significant, and 20 percent of them exceeded four sigmas. (For the moon, the instrument could resolve surface patches of 1.5 x 5 km.)
The authors of the new paper estimate the amount of water to be 100 to 400 micrograms per gram of lunar material. At a press conference, however, NASA decided to specify an addressable value by mixing units: this is the average of every cubic meter of lunar material with a 12-ounce bottle of water.
And that's weird. The sunlight the area sees should be enough for the water to boil quickly. How is the water still there?
The authors' suggestion – and it is just a hypothesis at this point – is that the water was enclosed in glass. Instead of imagining a literal 12-ounce glass bottle, think about the disorganized material created by bumps. Some of the effects on the moon will come from water-containing materials, and this water will be vaporized from the impact. As do some stones and other materials, although they quickly condense back into liquid. As this rocky liquid cools to form a disordered, glassy solid, it traps some of the water vapor.
Once the water is trapped in a glassy rock, it is insensitive to the heating and cooling cycles that would normally drive the water back from the lunar surface, which is why it remains in a sunny spot on the lunar surface.
It also means that getting to the water will be a lot harder. Many ideas for future lunar activities involve collecting water on the surface. However, when the water is needed to grind tiny glass pellets, it can create significantly bigger problems.
In the shade
But here, too, the focus on moon water was not in the sunny regions. Instead, the focus was on the places where water can condense in the shade and form ice. And this is where the second paper comes into play; Basically, a catalog is made of all potential locations on the moon that are cold enough for the ice to remain stable. And we mean anything, even when we're dealing with rough surfaces that can create shaded areas as little as an inch.