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Image of an airplane with a dark spot near the tail.
enlarge The instrument used to detect the water flies on a 747.

Despite its proximity to a very blue planet, Earth’s moon appeared to be completely dry, and the samples returned by the Apollo missions were nearly devoid of water. But in recent years, a number of studies have shown that there appears to be water in some locations on the moon, although the evidence has not always been conclusive.

Today, NASA announces that it has used an airborne observatory to detect clear indications of water in unexpected places. But the water can be in a form that makes it much more difficult to access. Separately, an analysis of places where water is easier to reach indicates that there are more potential reservoirs than we previously suspected.

In the air

Without an atmosphere and low gravity, the moon cannot hold onto water on its surface. The first time sunlight heats up lunar water, it will form a vapor and eventually escape into space. But there are areas on the moon, mainly near the poles, that are permanently in shadow. There temperatures remain perpetually low and ice can survive indefinitely. And to test this possibility, NASA crashed some hardware in a shaded area near the moon’s south pole and found water vapor among the debris.

In fact, water released from elsewhere on the moon can condense there before escaping into space, potentially creating a growing iceberg. Since water will be supplied by impacts with asteroids and cometary material, it is likely that this will be a continuous process.

But we wouldn’t expect this to happen in areas exposed to sunlight. There, all the water would have to be heated enough to propel 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 shown that some water-like material was present, but could not distinguish between water and a hydroxyl group (OH), which could be found in some minerals. So we didn’t really know what we were seeing there.

To find out, NASA turned to an infrared observatory that inserted it into the back of a 747 with a hole cut out the side. Known as the Stratospheric Observatory for Infrared Astronomy or SOFIA, the 747 lifts the hardware above much of the atmosphere. From there, there are far fewer molecules that would happily absorb some of the infrared light that the telescopes at SOFIA are designed for.

One of SOFIA’s instruments is sensitive to wavelengths in the region 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 can’t. So anything detected here is definitely 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 location, which gets more sunlight and therefore less chance of water, as a control. The Arctic, which is more likely to contain water, was the experimental one. And it had a clear, strong signal that matches water. Nearly all the areas imaged saw the water signal with a significance somewhere between the two sigma, and 20 percent of them exceeded the four sigma. (For the moon, the instrument can solve surface areas of 1.5 x 5 km.)

The authors of the new paper estimate the abundance of water at 100 to 400 micrograms per gram of lunar material. However, in a press conference, NASA decided to give an approximate value by mixing units: It’s the equivalent of every cubic meter of lunar material with an average of a 12-ounce bottle of water in it.

And this is weird. The sunlight that sees the area should be enough to cause all the water to boil away quickly. How is the water still there?

The authors’ proposal – and at the moment it is only a hypothesis – is that the water is enclosed in glass. Instead of imagining a literal 12 ounce glass bottle, think of the disordered material formed by shocks. Some impacts on the moon will come from hydrous materials and that water will evaporate from the impact. Just like some of the rock and other materials, though they quickly condense back to liquid. As that rocky liquid cools to form a disordered, glassy solid, it will trap some of the water vapor.

Once trapped in a glassy rock, the water will be impermeable 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 it will be a lot harder to get to the water. Many ideas about future lunar activities involve collecting water on the surface. But if getting the water involves grinding small glass granules, it can cause considerably more problems.

In the shadow

But again, the focus on lunar water was not in the sunny regions. Instead, the focus was on the places where the shadow could cause the water to condense and form ice. And that’s where the second paper comes in; it basically catalogs all the potential locations on the moon that are cold enough to keep ice stable. And we mean alleven by considering rough surfaces that can create shady areas as small as an inch.

The researchers determine the location of large sites by looking through the images of the lunar surface and creating a 3D model that shows them the areas that would be in shadow under all conditions. For areas of lower volume, they examine images of the moon’s surface and determine what percentage of that surface would be in shadow. They then model the diffusion of heat from the sun-exposed sections and figure out which areas remain cool enough to trap 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 many areas with cold traps ranging from one meter to 10 kilometers. But the southern polar region has many more that are more than 10 km in size. All told, this equates to about 40,000 square miles of lunar surface that could contain water ice.

This does not mean that all that water is there. Some of it is clearly based on NASA’s earlier “experiment” where the probe crashed, releasing some water vapor from the moon’s surface. But how much remains completely unclear. And whether it resides in large, easily accessible ice deposits remains an unknown until we get hardware in one of the locations that we expect to host a large deposit.

Natural Astronomy, 2020. DOI: 10.1038/s41550-020-01222-x, 10.1038/s41550-020-1198-9x (About DOIs).

By akfire1

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