Spectral data produced by China's Chang'E-5 lander not only confirms the presence of water on the Moon but shows where some of it is, something missing from previous studies.
The lunar rocks returned by the Apollo missions were extremely dry, and the little water that was found was assumed to be from contamination after the return to Earth. Once thoughts turned to establishing a permanent base, however, the question of whether the Moon still has water trapped beneath the surface became more crucial. We can't live without it, and not having to transport huge quantities would lower the cost of future missions dramatically.
It was only in 2009 that NASA found evidence of water on the Moon in the spectra from impact ejecta, something confirmed in 2020 while upping the estimates of its quantity. This work was done at low resolution from orbit, however, leaving uncertainty about where that water would actually be found. Now a paper in Science Advances reveals a volcanic rock with potentially usable water content.
Chang'E-5 landed on Oceanus Procellarum, possibly the youngest basalt “sea” on the Moon at just 2 billion years old, in December 2020. It returned to Earth with 1.7 kilograms (3.7 pounds) of rocks and dust later that month, but before that it used its Lunar Mineralogical Spectrometer to analyze the spectrum of reflected infrared light, looking for the lines at wavelengths of around 3 μm associated with either molecular water (H2O) or hydroxyl (OH-) bonded to other molecules.
After correcting for the emissions from the heat of the lunar surface, the authors concluded they had found an absorption line at 2.85 μm, a location associated with water. The low intensity of the line indicated the concentration in the surface material is no more than 120 parts per million. On that basis, you would need to crush a lot of rock to get a glass of drinking water, let alone enough to irrigate crops.
That might still be cheaper than dragging what is needed out of Earth's gravity well, but better options are at hand. The water in surface rocks is thought to be a result of hydrogen ions in the solar wind reacting with oxygen locked up in lunar soil. A volcanic rock, known as CE5, showed a stronger absorption line at the same wavelength, estimated to be caused by 180 ppm water. That suggests there may be even higher concentrations deeper down if we can find the right locations.
Oceanus Procellarum is at a latitude of 43°N, so it is exposed to the full force of the Sun, presumably boiling off any substantial water that arrives on comets. There is evidence cold traps near the lunar poles (particularly the South Pole) which remain permanently in shadow, may be much richer in water.