Lunar water is water that is present on the Moon. Liquid water cannot persist at the Moon's surface, and water vapour is quickly decomposed by sunlight and lost to outer space. However, scientists have since the 1960s conjectured that water ice could survive in cold, permanently shadowed craters at the Moon's poles.
Water, and the chemically related hydroxyl group ( · OH), can also exist in forms chemically bound to lunar minerals (rather than as free water), and evidence strongly suggests that this is indeed the case in low concentrations over much of the Moon's surface. In fact, 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 was accumulated from a variety of observations suggesting the presence of bound hydrogen. In September 2009, ISRO's Chandrayaan-1 detected water on the Moon and hydroxyl absorption lines in reflected sunlight. In November 2009, NASA reported that its LCROSS space probe had detected a significant amount of hydroxyl group in the material thrown up from a south polar crater by an impactor; this may be attributed to water-bearing materials – what appears to be "near pure crystalline water-ice". In March 2010, it was reported that the Mini-RF on board the ISRO's Chandrayaan-1 had discovered more than 40 permanently darkened craters near the Moon's north pole which are hypothesized to contain an estimated 600 million metric tonnes(1.3 trillion pounds) of water-ice.
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.
Lunar water discovery 
On 24 September 2009 Science magazine reported that the Moon Mineralogy Mapper (M3) on ISRO's Chandrayaan-1 had detected water on the Moon. M3 detected absorption features near 2.8-3.0 µm on the surface of the Moon. For silicate bodies, such features are typically attributed to hydroxyl- and/or water-bearing materials. On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer H abundance data suggests that the formation and retention of OH and H2O is an ongoing surficial process. OH/H2O production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.
The Moon Mineralogy Mapper (M3), an imaging spectrometer, was one of the 11 instruments on board ISRO's Chandrayaan-I, whose mission came to a premature end on 29 August 2009. M3 was aimed at providing the first mineral map of the entire lunar surface.
Lunar scientists had discussed the possibility of water repositories for decades. They are now increasingly "confident that the decades-long debate is over" a report says. "The Moon, in fact, has water in all sorts of places; not just locked up in minerals, but scattered throughout the broken-up surface, and, potentially, in blocks or sheets of ice at depth." The results from the Chandrayaan mission are also "offering a wide array of watery signals."
History of observations 
20th century 
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.
In February 1978 soviet scientists M. Akhmanova, B. Dement’ev, and M. Markov of the Vernadsky Institute of Geochemistry and Analytic 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 3 microns wavelength), at a detection level about 10 times above the threshold.
A proposed evidence of water ice on the Moon came in 1994 from the United States military Clementine probe. In an investigation known as the 'bistatic radar experiment', Clementine used its transmitter to beam radio waves into the dark regions of the south pole of the Moon. Echoes of these waves were detected by the large dish antennas of the Deep Space Network on Earth. The magnitude and polarisation of these echoes was consistent with an icy rather than rocky surface, but the results were inconclusive, and their significance has been questioned. Resulting computer simulations suggested that an area up to 14,000 km² might be in permanent shadow and hence have the potential to harbour lunar ice.
The Lunar Prospector probe, launched in 1998, employed a neutron spectrometer to measure the amount of hydrogen in the lunar regolith near the polar regions. It was able to determine hydrogen abundance and location to within 50 parts per million and detected enhanced hydrogen concentrations at the lunar north and south poles. These were interpreted as indicating significant amounts of water ice trapped in permanently shadowed craters, but could also be due to the presence of the hydroxyl radical (•OH) chemically bound to minerals. Based on data from Clementine and Lunar Prospector, NASA scientists have estimated that if surface water ice is present, the total quantity could be of the order of 1 to 3 cubic kilometers.
More suspicions about the existence of water on the Moon were generated by inconclusive data produced by Cassini–Huygens mission, which passed the Moon in 1999. In July 1999, at the end of its mission, the Lunar Prospector probe was deliberately crashed into Shoemaker crater, near the Moon's south pole, in the hope that detectable quantities of water would be liberated. However, spectroscopic observations from ground-based telescopes did not reveal the spectral signature of water.
21st century 
- Deep Impact
In 2005, observations of the Moon by the Deep Impact spacecraft produced inconclusive spectroscopic data suggestive of water on the Moon. In 2006, observations with the Arecibo planetary radar showed that some of the near-polar Clementine radar returns, previously claimed to be indicative of ice, might instead be associated with rocks ejected from young craters. If true, this would indicate that the neutron results from Lunar Prospector were primarily from hydrogen in forms other than ice, such as trapped hydrogen molecules or organics. Nevertheless, the interpretation of the Arecibo data do not exclude the possibility of water ice in permanently shadowed craters. In June 2009, NASA's Deep Impact spacecraft, now redesignated EPOXI, made further confirmatory bound hydrogen measurements during another lunar flyby.
As part of its lunar mapping programme, Japan's Kaguya probe, launched in September 2007 for a 19-month mission, carried out gamma ray spectrometry observations from orbit that can measure the abundances of various elements on the Moon's surface. Japan's Kaguya probe's high resolution imaging sensors failed to detect any signs of water ice in permanently shaded craters around the south pole of the Moon, and it ended its mission by crashing into the lunar surface in order to study the ejecta plume content.
- Chang'e 1
On November 14, 2008, the Indian spacecraft Chandrayaan-1 released the Moon Impact Probe (MIP) which impacted Shackleton Crater, of the lunar south pole, at 20:31 on 14 November 2008 releasing subsurface debris that was analyzed for presence of water ice.
On September 25, 2009, NASA declared that data sent from its Moon Mineralogy Mapper (M3) instrument aboard Chandrayaan-1 orbiter confirmed the existence of hydrogen over large areas of the Moon's surface, albeit in low concentrations and in the form of hydroxyl group ( · OH) chemically bound to soil. This supports earlier evidence from spectrometers aboard the Deep Impact and Cassini probes.
On March 2010, it was reported that the Mini-SAR on board the ISRO's Chandrayaan-1 had discovered more than 40 permanently darkened craters near the Moon's north pole which are hypothesized to contain an estimated 600 million metric tonnes of water-ice. The radar's high CPR is not uniquely diagnostic of either roughness or ice; the science team must take into account the environment of the occurrences of high CPR signal to interpret its cause. The ice must be relatively pure and at least a couple of meters thick to give this signature.  The estimated amount of water ice potentially present is comparable to the quantity estimated from the previous mission of Lunar Prospector’s neutron data.
Although the results are consistent with recent findings of other NASA instruments onboard Chandrayaan-1 (the Moon Mineralogy Mapper (MP3) discovered water molecules in the Moon's polar regions, while water vapor was detected by NASA's Lunar Crater Observation and Sensing Satellite, or LCROSS) this observation is not consistent with the presence of thick deposits of nearly pure water ice within a few meters of the lunar surface, but it does not rule out the presence of small (<∼10 cm), discrete pieces of ice mixed in with the regolith.
- Lunar Reconnaissance Orbiter
The search for lunar ice continued with NASA's Lunar Reconnaissance Orbiter (LRO) / LCROSS mission, launched June 18, 2009. LRO's onboard instruments carried out a variety of observations that may provide further evidence of water. On October 9, 2009, the Centaur upper stage of its Atlas V carrier rocket was directed to impact Cabeus crater at 11:31 UTC, followed shortly by the LCROSS spacecraft that flew into the ejecta plume and attempted to detect the presence of water vapor in the debris cloud. Although no immediate spectacular plume was seen, time was needed to analyze the spectrometry data. On November 13, 2009 NASA reported that after analysis of the data obtained from the ejecta plume, the spectral signature of water had been confirmed. However, what was actually detected was the chemical group hydroxyl ( · OH), which is suspected to be from water, but could also be hydrates, which are inorganic salts containing chemically-bound water molecules. The nature, concentration and distribution of this material requires further analysis; chief mission scientist Anthony Colaprete has stated that the ejecta appears to include a range of fine-grained particulates of near pure crystalline water-ice. A later definitive analysis found the concentration of water to be "5.6 ± 2.9% by mass". The Mini-RF instrument on LRO observed the LCROSS landing site and did not detect any evidence of large slabs of water ice, so the water is most likely present as small pieces of ice mixed in with the lunar regolith.
LRO's laser altimeter's examination of the Shackleton crater at the lunar south pole suggests up to 22% of the surface of that crater is covered in ice.
- Melt inclusions in Apollo 17 samples
In May 2011, Erik Hauri et al. reported 615-1410 ppm water in melt inclusions in lunar sample 74220, the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago.
This concentration is comparable with that of magma in Earth's upper mantle. While of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists. The sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to detect them with a state-of-the-art ion microprobe instrument.
Possible water cycle 
Lunar water has two potential origins: water-bearing comets (and other bodies) striking the Moon, and in situ production. It has been theorized that the latter may occur when hydrogen ions (protons) in the solar wind chemically combine with the oxygen atoms present in the lunar minerals (oxides, silicates etc.) to produce small amounts of water trapped in the minerals' crystal lattices or as hydroxyl groups, potential water precursors. (This mineral-bound water, or hydroxylated mineral surface, must not be confused with water ice.)
The hydroxyl surface groups (S–OH) formed by the reaction of protons (H+) with oxygen atoms accessible at oxide surface (S=O) could further be converted in water molecules (H2O) adsorbed onto the oxide mineral's surface. The mass balance of a chemical rearrangement supposed at the oxide surface could be schematically written as follows:
- 2 S-OH —> S=O + S + H2O
- 2 S-OH —> S–O–S + H2O
where S represents the oxide surface.
The formation of one water molecule requires the presence of two adjacent hydroxyl groups, or a cascade of successive reactions of one oxygen atom with two protons. This could constitute a limiting factor and decreases the probability of water production if the proton density per surface unit is too low.
Solar radiation would normally strip any free water or water ice from the lunar surface, splitting it into its constituent elements, hydrogen and oxygen, which then escape to space. However, because of the only very slight axial tilt of the Moon's spin axis to the ecliptic plane (1.5 °), some deep craters near the poles never receive any sunlight, and are permanently shadowed (see, for example, Shackleton crater, and Whipple crater). The temperature in these regions never rises above about 100 K (about −170 ° Celsius), and any water that eventually ended up in these craters could remain frozen and stable for extremely long periods of time — perhaps billions of years, depending on the stability of the orientation of the Moon's axis.
Although free water cannot persist in illuminated regions of the Moon, any such water produced there by the action of the solar wind on lunar minerals might, through a process of evaporation and condensation, migrate to permanently cold polar areas and accumulate there as ice, perhaps in addition to any ice brought by comet impacts.
The hypothetical mechanism of water transport / trapping (if any) remains unknown: indeed lunar surfaces directly exposed to the solar wind where water production occurs are too hot to allow trapping by water condensation (and solar radiation also continuously decomposes water), while no (or much less) water production is expected in the cold areas not directly exposed to the sun. Given the expected short lifetime of water molecules in illuminated regions, a short transport distance would in principle increase the probability of trapping. In other words, water molecules produced close to a cold, dark polar crater should have the highest probability of surviving and being trapped.
To what extent, and at what spatial scale, direct proton exchange (protolysis) and proton surface diffusion directly occurring at the naked surface of oxyhydroxide minerals exposed to space vacuum (see surface diffusion and self-ionization of water) could also play a role in the mechanism of the water transfer towards the coldest point is presently unknown and remains a conjecture.
The presence of large quantities of water on the Moon would be an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. If future investigations find the quantities to be particularly large, water ice could be mined to provide liquid water for drinking and plant propagation, and the water could also be split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator, providing breathable oxygen as well as the components of rocket fuel. The hydrogen component of the water ice could also be used to draw out the oxides in the lunar soil and harvest even more oxygen.
Analysis of lunar ice would also provide scientific information about the impact history of the Moon and the abundance of comets and asteroids in the early inner solar system.
The hypothetical discovery of usable quantities of water on the Moon may raise legal questions about who owns the water and who has the right to exploit it. The United Nations Outer Space Treaty does not prevent the exploitation of lunar resources, but does prevent the appropriation of the Moon by individual nations and is generally interpreted as barring countries from claiming ownership of in-situ resources. However most legal experts agree that the ultimate test of the question will arise through precedents of national or private activity. Some private companies such as Moon Express are already asserting their right to own whatever resources they remove and/or beneficiate from the Moon or asteroids though their own effort, risk and investment. The Moon Treaty specifically stipulates that exploitation of lunar resources is to be governed by an "international regime", but this treaty has not been ratified by any of the major space-faring nations.
See also 
- 24 Themis (detection of water on an asteroid)
- Chandrayaan-1 lunar orbiter
- Chandrayaan-2 lunar orbiter and rovers
- In-Situ Resource Utilization
- Lunar Reconnaissance Orbiter
- Shackleton Energy Company
- Water on Mars
- Lucey, Paul G. (23 October 2009). "A Lunar Waterworld". Science 326 (5952): 531–532. Bibcode:2009Sci...326..531L. doi:10.1126/science.1181471. PMID 19779147. Retrieved 2009-11-18.
- Clark, Roger N. (23 October 2009). "Detection of Adsorbed Water and Hydroxyl on the Moon". Science 326 (5952): 562–564. Bibcode:2009Sci...326..562C. doi:10.1126/science.1178105. PMID 19779152. Retrieved 2009-11-20.
- Lakdawalla, Emily (13 November 2009). "LCROSS Lunar Impactor Mission: "Yes, We Found Water!"". The Planetary Society. Retrieved 13 April 2010.
- Pieters, C. M.; Goswami, J. N.; Clark, R. N.; Annadurai, M.; Boardman, J.; Buratti, B.; Combe, J.-P.; Dyar, M. D.; Green, R.; Head, J. W.; Hibbitts, C.; Hicks, M.; Isaacson, P.; Klima, R.; Kramer, G.; Kumar, S.; Livo, E.; Lundeen, S.; Malaret, E.; McCord, T.; Mustard, J.; Nettles, J.; Petro, N.; Runyon, C.; Staid, M.; Sunshine, J.; Taylor, L. A.; Tompkins, S.; Varanasi, P. (2009). "Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1". Science 326 (5952): 568–72. Bibcode:2009Sci...326..568P. doi:10.1126/science.1178658. PMID 19779151.
- Dino, Jonas; Lunar Crater Observation and Sensing Satellite Team (November 13, 2009). "LCROSS Impact Data Indicates Water on Moon". NASA. Retrieved 2009-11-14.
- Pieters, C. M. Pieters; J. N. Goswami, R. N. Clark, et al. (23 October 2009). "Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on Chandrayaan-1". Science 326 (5952): 568–572. Bibcode:2009Sci...326..568P. doi:10.1126/science.1178658. PMID 19779151. Retrieved 2009-11-20.
- "Ice deposits found at Moon's pole", BBC News, 2 March 2010
- "NASA Radar Finds Ice Deposits at Moon's North Pole". NASA. March 2010. Retrieved 2012-03-26.
- Elston, D.P. (1968) "Character and Geologic Habitat of Potential Deposits of Water, Carbon and Rare Gases on the Moon", Geological Problems in Lunar and Planetary Research, Proceedings of AAS/IAP Symposium, AAS Science and Technology Series, Supplement to Advances in the Astronautical Sciences., p.441
- NASA - Lunar Prospector
- "Character and Spatial Distribution of OH/H2O on the Surface of the Moon Seen by M3 on [[Chandrayaan-1]]". Science Mag. 15 September 2009. Retrieved 26 September 2009. Wikilink embedded in URL title (help)
- "Welcome To ISRO:: Press Release:: 29 August 2009". 101004 isro.org
- "It's not lunacy, probes find water in Moon dirt". USA Today. 23 September 2009. Retrieved 26 September 2009.
- "Water discovered on Moon?: "A lot of it actually"". The Hindu. 23 September 2009. Retrieved 26 September 2009.
- Watson, K., B. C. Murray, and H. Brown (1961), The Behavior of Volatiles on the Lunar Surface, J. Geophys. Res., 66(9), 3033–3045.
- "It's Official: Water Found on the Moon", Space.com, 23 September 2009
- "Water on The Moon, I. Historical Overview". Astronomical Review 6 (8): 4–20. October 2011. Bibcode:2011AstRv...6h...4C. Retrieved 2012-10-3.
- Moon Once Harbored Water, Lunar Lava Beads Show, Scientific American, July 9, 2008
- Freeman, J.W., Jr.,H.K. Hills., R.A. Lindeman, and R.R. Vondrak, Observations of Water Vapor at the Lunar Surface, The Moon, 8, 115-128, 1973
- Akhmanova, M; Dement’ev, B; Markov, M (February 1978). "Water in the regolith of Mare Crisium (Luna-24)?". Geokhimiia (in russian) (285).
- Akhmanova, M; Dement’ev, B; Markov, M (1978). "Possible Water in Luna 24 Regolith from the Sea of Crises". Geochemistry International 15 (166).
- Crotts, Arlin (2012). "Water on The Moon, I. Historical Overview". arXiv:1205.5597v1 [astro-ph.EP].
- The Clementine bistatic radar experiment — Science
- Clementine Probe
- Simpson, Richard A.; Tyler, G. Leonard (1999). "Reanalysis of Clementine bistatic radar data from the lunar South Pole". Journal of Geophysical Research 104: 3845. Bibcode:1999JGR...104.3845S. doi:10.1029/1998JE900038.
- Campbell, Donald B.; Campbell, Bruce A.; Carter, Lynn M.; Margot, Jean-Luc; Stacy, Nicholas J. S. (2006). "No evidence for thick deposits of ice at the lunar south pole" (PDF). Nature 443 (7113): 835–7. Bibcode:2006Natur.443..835C. doi:10.1038/nature05167. PMID 17051213.
- Linda, Martel (June 4, 2003). "The Moon's Dark, Icy Poles".
- "Eureka! Ice found at lunar poles". August 31, 2001.
- Lunar Prospector Science Results NASA
- Prospecting for Lunar Water, NASA
- Neutron spectrometer results
- Kemm, Kelvin (October 9, 2009). "Evidence of water on the Moon, Mars alters planning for manned bases". Engineering News. Retrieved 2009-10-09.
- No water ice detected from Lunar Prospector, NASA website
- Paul Spudis (2006). "Ice on the Moon".
- Kaguya Gamma Ray Spectrometer, JAXA
- Japan's now-finished lunar mission found no water ice
- Japanese probe crashes into Moon
- "Who's Orbiting the Moon?", NASA, February 20, 2008
- "Chandrayaan team over the Moon". The Hindu. 2008-11-15.
- "Spacecraft see 'damp' Moon soils", BBC, 24 September 2009
- "NASA confirms water on Moon". 2009-11-13. Retrieved 2009-11-18.
- "Moon crash will create six-mile plume of dust as Nasa searches for water", The Times, October 3, 2009
- Discovery of water on Moon boosts prospects for permanent lunar base, The Guardian, 24 September 2009
- "The nature of lunar volatiles as revealed by Mini-RF observations of the LCROSS impact site". Journal of Geophysical Research - Planets 116 (E01005): 8. 13 January 2011. Bibcode:2011JGRE..11601005N. doi:10.1029/2010JE003647. Retrieved 2012-03-26. "the Mini-RF instruments on ISRO's Chandrayaan-1 and NASA's Lunar Reconnaissance Orbiter (LRO) obtained S band (12.6 cm) synthetic aperture radar images of the impact site at 150 and 30 m resolution, respectively. These observations show that the floor of Cabeus has a circular polarization ratio (CPR) comparable to or less than the average of nearby terrain in the southern lunar highlands. Furthermore, <2% of the pixels in Cabeus crater have CPR values greater than unity. This observation is not consistent with the presence of thick deposits of nearly pure water ice within a few meters of the lunar surface, but it does not rule out the presence of small (<∼10 cm), discrete pieces of ice mixed in with the regolith."
- LCROSS mission overview, NASA
- Moon River: What Water in the Heavens Means for Life on Earth, by Randall Amster, The Huffington Post, November 30, 2009.
- Colaprete, A.; Schultz, P.; Heldmann, J.; Wooden, D.; Shirley, M.; Ennico, K.; Hermalyn, B.; Marshall, W; Ricco, A.; Elphic, R. C.; Goldstein, D.; Summy, D.; Bart, G. D.; Asphaug, E.; Korycansky, D.; Landis, D.; Sollitt, L. (22 October 2010). "Detection of Water in the LCROSS Ejecta Plume". Science 330 (6003): 463–468. Bibcode:2010Sci...330..463C. doi:10.1126/science.1186986. PMID 20966242.
- Nozette, Stewart; Spudis, Paul; Bussey, Ben; Jensen, Robert; Raney, Keith; et al. (January 2010). "The Lunar Reconnaissance Orbiter Miniature Radio Frequency (Mini-RF) Technology Demonstration". Space Science Reviews 150: 285–302. Bibcode:2010SSRv..150..285N. doi:10.1007/s11214-009-9607-5.
- C. D. Neish, D. B. J. Bussey, P. Spudis, W. Marshall, B. J. Thomson, et al. (January, 2011). "The nature of lunar volatiles as revealed by Mini-RF observations of the LCROSS impact site". Journal of Geophysical Research.
- Researchers Estimate Ice Content of Crater at Moon's South Pole (NASA)
- [Erik]; Thomas Weinreich, Alberto E. Saal, Malcolm C. Rutherford, James A. Van Orman (26 May 2011). "High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions". Science Express (AAAS) 10 (1126). Bibcode:2011Sci...333..213H. doi:10.1126/science.1204626. ISSN 1095-9203. Retrieved 31 May 2011.
- "Lunar Hydrogen Distribution after KAGUYA(SELENE)" (PDF). 2009 Annual Meeting of LEAG (2009). Retrieved 2009-11-18.
- Ice on the Moon, NASA
- Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies ("Outer Space Treaty"), UN Office for Outer Space Affairs
- "Moon Water: A Trickle of Data and a Flood of Questions", space.com, March 6, 2006
- Agreement Governing the Activities of States on the Moon and Other Celestial Bodies ("Moon Treaty"), UN Office for Outer Space Affairs
- Ice on the Moon — NASA Goddard Space Flight Center
- Fluxes of fast and epithermal neutrons from Lunar Prospector: Evidence for water ice at the lunar poles — Science
- Moon has a litre of water for every tonne of soil — Times Online
- Unambiguous evidence of water on the Moon — Slashdot Science Story