Water on the Moon

After the Apollo astronauts returned to Earth in 1969, the lunar surface was believed to be completely dry, with no indication of any source of hydration let alone water. But that belief has changed, because on October 26, 2020, NASA shared results published in Nature Astronomy that confirm the presence of water on the Moon.

This discovery was made possible by the Stratospheric Observatory for Infrared Astronomy, SOFIA. SOFIA was a joint project of NASA and the German Aerospace Center, in service from 2014 to 2022, which consisted of a Boeing 747SP airliner modified to carry a 106-inch diameter telescope.

A test with great results

SOFIA was originally used to observe distant objects, such as galaxies and black holes, using stars as guides to keep the telescope fixed on its target. But since the Moon is such a close and bright celestial object, it was not certain that SOFIA could stay fixed on it, since it would fill almost the entire field of view used for the guide stars.

In August 2018, operators decided to try a test observation of the Moon, and in that simple test they found much more than expected: they found H₂O molecules on the Moon.

How was it discovered?

Prior to SOFIA's discoveries, in 2009 there was already an indication of the presence of some hydration on the Moon, thanks to information collected from 3 independent spacecraft: Chandrayaan-1 (with the Moon Mineralogy Mapper, M₃), Deep Impact, and Cassini (with the Visual and Infrared Mapping Spectrometer, VIMS). They observed a characteristic absorption feature of 3 μm on the Moon, which is typical of water (H₂O) and hydroxyl (OH), but they were unable to differentiate between these compounds.

Using the 3 µm band, water and hydroxyl look the same; however, taking into account that a fundamental vibration of water molecules emits in a spectral line at 6 μm, and that hydroxyl does not have this characteristic, the 6 μm band was used with SOFIA to confirm the presence of H₂O on the Moon.

The results

The water molecules were found in the sunlit Clavius Crater, located in the Moon's southern hemisphere, which is one of the largest craters visible from Earth.

An abundance of 100 to 400 µg of water per gram of soil (parts per million) was estimated; for comparison, the Sahara desert contains 100 times more water than SOFIA detected in the lunar soil.

Indeed, the amounts of water found on the Moon have been very small, but they turn out to be far from insignificant, since they raise new questions that demand answers. It is not yet known exactly how these water molecules were generated or how they reached the Moon and, even more surprising, how they managed to be preserved on the Moon despite the fact that it does not have an atmosphere and the water is in a sunlit area of the Moon.

Until now, there are only estimates and grounded theories to try to answer these questions, such as that they may be in the form of ice in permanently shadowed areas _[1] , or the water could be trapped in tiny bead-like structures that form from the high heat created by micrometeorite impacts on the lunar surface _[2] .

Undoubtedly, the presence of water, be it a lot or a little, already represents a factor of great impact for future missions to explore the universe. NASA's Artemis program now has a new factor to consider as it returns astronauts to the Moon: its first crewed flight, Artemis II, flew around the Moon from April 1 to 10, 2026; in 2027, Artemis III will test in Earth orbit the systems needed to land, and Artemis IV is scheduled to put astronauts on the lunar surface in 2028.