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The Stratospheric Observatory for Infrared Astronomy (SOFIA) has provided the first direct and unambiguous evidence of water molecules on the Moon outside the permanent shadow at the lunar poles. The infrared observatory, which is operated by US space agency NASA and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) was able to detect the molecules in the Moon’s southern hemisphere using SOFIA’s FORCAST instrument (Faint Object InfraRed CAmera for the SOFIA Telescope). The findings from the scientific research were published on 26 October, 2020 in the journal Nature Astronomy.

“We have been looking for water on the Moon since the first lunar rocks were brought to Earth in the 1960s,” says Alessandra Roy, SOFIA project scientist at the DLR Space Administration. “However, most of these samples showed no evidence of its presence.” The scientists only obtained confirmation of its existence in 2008, from NASA's Moon Mineralogy Mapper on board the Indian Chandrayaan-1 mission, and this demonstrated the presence of water only in the Moon’s dark polar regions. SOFIA has now been able to demonstrate that water also exists in areas of the Moon’s surface that are illuminated by the Sun. The airborne infrared observatory had already used the FORCAST instrument to observe the Moon on 30 August 2018. This allowed scientists to identify the unmistakable fingerprint of water molecules in the mid-infrared range (at a wavelength of six micrometres) in the vicinity of the Clavius Crater in the Moon’s southern hemisphere.

Where does the Moon's water come from?

"The sunlit parts of the Moon can reach temperatures of approximately 120 degrees Celsius. Having practically no atmosphere, there is no protection for its water which at this temperature evaporates under the heat of the light from the Sun. There is no way of catching any of it," says Roy. "However, water is present on the surface." Currently, there are two theories to explain the presence of water on the surface. Some scientists believe that micrometeorites falling onto the Moon's surface and carrying small quantities of water could deposit the liquid within the rock as they collide with it. In the process, the water becomes enclosed in tiny, glass, bead-like structures in the ground. However, there is also the possibility that a two-stage process may occur, in which hydrogen from the solar wind reaches the Moon's surface, where it combines with hydroxyl – a hydrogen atom bound to an oxygen atom – to form water. The data acquired by SOFIA indicates that most of the water that has been detected lies within the substrate covering the lunar surface.

Water as the basis for future space missions

One of the objectives of the ESA Space Resources Strategy, which focuses on the Moon, is to confirm whether resources such as water could enable sustainable space exploration. "The amount of water discovered by SOFIA is roughly equivalent to the contents of a 300-millilitre can of drink, spread over the same surface area as a football pitch," explains Roy. "The Moon remains drier than deserts on Earth, but the quantity of water that has been discovered could still prove important for future crewed missions in space."

SOFIA will now observe the Moon's sunlit surface during different lunar phases to investigate the water phenomenon in greater detail. The scientists hope that this will open up new insight into where water on the Moon comes from, how it is stored, and how it is distributed across the lunar surface. Data from SOFIA will complement the findings from future Moon missions.