Lunar water is water that is present on the Moon. Diffuse water molecules in low concentrations can persist at the Moon's sunlit surface, as discovered by the SOFIA observatory (an 80/20 joint project of NASA and the German Aerospace Centre, DLR) in 2020. Gradually, water vapor is decomposed by sunlight, leaving hydrogen and oxygen lost to outer space. Scientists have found water ice in the cold, permanently shadowed craters at the Moon's poles. Water molecules are also present in the extremely thin lunar atmosphere.
NASA's Ice-Mining Experiment-1 (set to launch on the PRIME-1 mission no earlier than early 2024) is intended to answer whether or not water ice is present in usable quantities in the southern polar region.
Water (H2O) and the related hydroxyl group (-OH) exist in forms chemically bounded as hydrates and hydroxides to lunar minerals (rather than free water), and evidence strongly suggests that this is the case in low concentrations as for much of the Moon's surface. In fact, of surface matter, adsorbed water is calculated to exist at trace concentrations of 10 to 1000 parts per million. Inconclusive evidence of free water ice at the lunar poles had accumulated during the second half of the 20th century from a variety of observations suggesting the presence of bound hydrogen.
On 18 August 1976, the Soviet Luna 24 probe landed at Mare Crisium, took samples from the depths of 118, 143, and 184 cm of the lunar regolith, and returned them to Earth. In February 1978, laboratory analysis of these samples showed that they contained 0.1% (1,000 ppm) water by mass. Spectral measurements showed minima near 3, 5, and 6 µm, distinctive valence-vibration bands for water molecules, with intensities two or three times larger than the noise level.
On 24 September 2009, the Indian Space Research Organisation's Chandra's Altitudinal Composition Explorer (CHACE) and NASA's Moon Mineralogy Mapper (M3) spectrometer on board the Chandrayaan-1 probe had detected absorption features near 2.8–3.0 μm on the surface of the Moon. On 14 November 2008, Chandrayaan-1 released the Moon Impact Probe to impact the Shackleton crater, which helped confirm the presence of water ice. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. In August 2018, NASA confirmed that M3 showed water ice is present on the surface at the Moon poles. Water in concentrations of 100 to 412 parts per million (0.01%-.042%) was confirmed to be on the sunlit surface of the Moon by the SOFIA observatory on October 26, 2020.
Water may have been delivered to the Moon over geological timescales by the regular bombardment of water-bearing comets, asteroids, and meteoroids or continuously produced in situ by the hydrogen ions (protons) of the solar wind impacting oxygen-bearing minerals.
The search for the presence of lunar water has attracted considerable attention and motivated several recent lunar missions, largely because of water's usefulness in rendering long-term lunar habitation feasible.
History of observations
20th century
Apollo Program
The possibility of ice in the floors of polar lunar craters was first suggested in 1961 by Caltech researchers Kenneth Watson, Bruce C. Murray, and Harrison Brown. Although trace amounts of water were found in lunar rock samples collected by Apollo astronauts, this was assumed to be a result of contamination, and the majority of the lunar surface was generally assumed to be completely dry. However, a 2008 study of lunar rock samples revealed evidence of water molecules trapped in volcanic glass beads.
The first direct evidence of water vapor near the Moon was obtained by the Apollo 14 ALSEP Suprathermal Ion Detector Experiment, SIDE, on March 7, 1971. A series of bursts of water vapor ions were observed by the instrument mass spectrometer at the lunar surface near the Apollo 14 landing site.
Luna 24
In February 1978 Soviet scientists M. Akhmanova, B. Dement'ev, and M. Markov of the Vernadsky Institute of Geochemistry and Analytical Chemistry published a paper claiming a detection of water fairly definitively. Their study showed that the samples returned to Earth by the 1976 Soviet probe Luna 24 contained about 0.1% water by mass, as seen in infrared absorption spectroscopy (at about wavelength), at a detection level about 10 times above the threshold.