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The search for the presence of lunar water has attracted considerable attention and motivated several recent lunar missions, largely because of water usefulness in making long-term lunar habitation feasible.
The Moon is believed to be generally anhydrous after analysis of Apollo mission soil samples. It is understood that any water vapor on the surface would generally be decomposed by sunlight, leaving hydrogen and oxygen lost to outer space. However, subsequent robotic probes found evidence of water, especially of water ice in some permanently shadowed craters on the Moon; and in 2018 water ice was confirmed in multiple locations. Water and Ices on the Moon", science.nasa.gov, fetched 11 June 2024 Water on the Moon: Direct evidence from Chandrayaan-1's Moon Impact Probe. Published on 2010/04/07. This water ice is not in the form of sheets of ice on the surface nor just under the surface, but there may be small (less than about ) chunks of ice mixed into the regolith, and some water is chemically bonded with minerals. Other experiments have detected water molecules in the negligible lunar atmosphere, and even some in low concentrations at the Moon's sunlit surface.
On the Moon, water (H2O) and Hydroxy group group (-OH) are not present as free water but are chemically bonded within minerals as hydrates and hydroxides, existing in low concentrations across the lunar surface. Adsorption water is estimated to be traceable at levels of 10 to 1000 ppm. The presence of water may be attributed to two primary sources: delivery over geological timescales via impacts and in situ production through interactions of solar wind hydrogen ions with oxygen-bearing minerals.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 Confirmed hydroxyl-bearing materials include glasses, apatite Ca5(PO4)3(F,, and novograblenovite (NH4)MgCl3·6H2O.
NASA Ice-Mining Experiment-1 (launched on the PRIME-1 mission on 27 February 2025) is intended to answer whether or not water ice is present in usable quantities in the southern polar region.
In 1834–1836, Wilhelm Beer and Johann Heinrich Mädler published their four-volume Mappa Selenographica and the book Der Mond in 1837, which established the conclusion that the Moon has no bodies of water on the surface nor any appreciable atmosphere.
Earth-based radar measurements were used to identify the areas that are in permanent shadow and hence have the potential to harbour lunar ice: Estimates of the total extent of shadowed areas poleward of 87.5 degrees latitude are for the north and south poles, respectively. Subsequent computer simulations encompassing additional terrain suggested that an area up to might be in permanent shadow.
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.
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, Clementine Probe and their significance has been questioned.
India's ISRO spacecraft Chandrayaan-1 released the Moon Impact Probe (MIP) that impacted Shackleton Crater, of the lunar south pole, at 20:31 on 14 November 2008 releasing subsurface debris that was analysed for presence of water ice. During its 25-minute descent, the impact probe's Chandra's Altitudinal Composition Explorer (CHACE) recorded evidence of water in 650 mass spectra gathered in the thin atmosphere above the Moon's surface and hydroxyl absorption lines in reflected sunlight.
On September 25, 2009, NASA declared that data sent from its M3 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. "Spacecraft see 'damp' Moon soils", BBC, 24 September 2009 This supports earlier evidence from spectrometers aboard the Deep Impact and Cassini probes. "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 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.
Although M3 results are consistent with recent findings of other NASA instruments onboard Chandrayaan-1, the discovered water molecules in the Moon's polar regions 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 (<~), discrete pieces of ice mixed in with the regolith. Additional analysis with M3 published in 2018 had provided more direct evidence of water ice near the surface within 20° latitude of both poles. In addition to observing reflected light from the surface, scientists used M3's near-infrared absorption capabilities in the permanently shadowed areas of the polar regions to find absorption spectra consistent with ice. At the north pole region, the water ice is scattered in patches, while it is more concentrated in a single body around the south pole. Because these polar regions do not experience the high temperatures (greater than 373 Kelvin), it was postulated that the poles act as where vaporized water is collected on the Moon.
In March 2010, it was reported that the Mini-RF on board Chandrayaan-1 had discovered more than 40 permanently darkened craters near the Moon's north pole that 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.
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 NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) spacecraft that flew through the ejecta plume. LCROSS mission overview , NASA
LCROSS 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" mixed in the regolith. "Ice deposits found at Moon's pole". BBC News, 2 March 2010. Moon River: What Water in the Heavens Means for Life on Earth, by Randall Amster, The Huffington Post, November 30, 2009. What was actually detected was the chemical group hydroxyl (OH), which is suspected to be from water, but could also be , 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 board the Lunar Reconnaissance Orbiter (LRO) observed the plume of debris from the impact of the LCROSS orbiter, and it was concluded that the water ice must be in the form of small (< ~10 cm), discrete pieces of ice distributed throughout the regolith, or as thin coating on ice grains. This, coupled with monostatic radar observations, suggest that the water ice present in the permanently shadowed regions of lunar polar craters is unlikely to be present in the form of thick, pure ice deposits.
The data acquired by the Lunar Exploration Neutron Detector (LEND) instrument onboard LRO show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. Further analysis of LEND data suggests that water content in the polar regions is not directly determined by the illumination conditions of the surface, as illuminated and shadowed regions do not manifest any significant difference in the estimated water content. According to the observations by this instrument alone, "the permanent low surface temperature of the cold traps is not a necessary and sufficient condition for enhancement of water content in the regolith."
LRO 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. Researchers Estimate Ice Content of Crater at Moon's South Pole (NASA)
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.
October 26, 2020: In a paper published in Nature Astronomy, a team of scientists used SOFIA, an infrared telescope mounted inside a 747 jumbo jet, to make observations that showed unambiguous evidence of water on parts of the Moon where the sun shines. "This discovery reveals that water might be distributed across the lunar surface and not limited to the cold shadowed places near the lunar poles," Paul Hertz, the director of NASA's astrophysics division, said.
Chang'e-5 probe
Chang'e 5 measurements on 117 individual spherical glass beads reveal 0–1,909 μg/g of H20, equivalent to 2.7E14 kg of trapped water on the lunar surface pointing to a new mechanism for storing water on the lunar surface. The findings could be useful for future lunar missions by identifying glasses as a potential resources that could be converted to drinking water or rocket fuel. Another unusual hdyroxl-bearing mineral was discovered in the Chang'e 5 regolith novograblenovite (NH4)MgCl3·6H2O. On Earth, this mineral has been identified around fumaroles and hydrothermal activity.
Upon analyzing China’s Chang'
The hydroxyl surface groups (X–OH) formed by the reaction of protons (H+) with oxygen atoms accessible at oxide surface (X=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:
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.
While the ice deposits may be thick, they are most likely mixed with the regolith, possibly in a layered formation. The Moon and Mercury May Have Thick Ice Deposits. Bill Steigerwald and Nancy Jones, NASA. 2 August 2019.
Impactite could store and release water, possibly storing as much as 270 billion tonnes of water.
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 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.
Simulations of lunar thermal conditions show that diurnal temperature variations could drive centimeter-scale water migration and accumulation in the Moon's subsurface.
LADEE data shows that the shock waves from impact events cause water beneath the surface to evaporate.
Warm and pressurized regions in the Moon's interior might still contain liquid water. Underground lakes of liquid water on the Moon require a reservoir of underground water, a source of heat, and a barrier sufficient to stop the water from being lost to space. Subsurface ice layers may block the diffusion of deeper liquid water, so subterranean "lakes" could be present underneath a region with surface or subsurface ice.
The Moon Treaty specifically stipulates that exploitation of lunar resources is to be governed by an "international regime", but that treaty has only been ratified by a few nations, and primarily those with no independent spaceflight capabilities. Agreement Governing the Activities of States on the Moon and Other Celestial Bodies ("Moon Treaty") , UN Office for Outer Space Affairs
Luxembourg and the US have granted their citizens the right to mine and own space resources, including the resources of the Moon. US President Donald Trump expressly stated that in his executive order of 6 April 2020.
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.
In October 2020, astronomers reported detecting Water on the sunlit surface of the Moon by several independent scientific teams, including the Stratospheric Observatory for Infrared Astronomy (SOFIA). The estimated abundance is about 100 to 400 ppm, with a distribution over a small latitude range, likely a result of local geology and not a global phenomenon. It was suggested that the detected water is stored within glasses or in voids between grains sheltered from the harsh lunar environment, thus allowing the water to remain on the lunar surface. Using data from the Lunar Reconnaissance Orbiter, it was shown that besides the large, permanently shadowed regions in the Moon's polar regions, there are many unmapped cold traps, substantially augmenting the areas where ice may accumulate. Approximately 10–20% of the permanent cold-trap area for water is found to be contained in "micro cold traps" found in shadows on scales from 1 km to 1 cm, for a total area of ~40,000 km2, about 60% of which is in the South, and a majority of cold traps for water ice are found at latitudes >80° due to permanent shadows.
Lunar IceCube is a 6U (six unit) CubeSat that was to estimate amount and composition of lunar ice, using an infrared imaging spectrometer developed by NASAs Goddard Space Flight Center. The spacecraft separated from Artemis 1 successfully on November 17, 2022, but failed to communicate shortly thereafter and is presumed lost.
A dedicated on-site experiment by NASA dubbed PRIME-1 is slated to land on the Moon no earlier than November, 2023 near Shackleton Crater at the Lunar South Pole. The mission will drill for water ice.
Slated to launch as a ride-along mission in 2025, the Lunar Trailblazer satellite is part of NASA's Small Innovative Missions for Planetary Exploration (SIMPLEx) program. The satellite carries two instruments—a high-resolution spectrometer, which will detect and map different forms of water, and a thermal mapper. The mission's primary objectives are to characterize the form of lunar water, how much is present and where; determine how lunar volatiles change and move over time; measure how much and what form of water exists in permanently shadowed regions of the Moon; and to assess how differences in the reflectivity and temperature of lunar surfaces affect the concentration of lunar water.
Possible water cycle
Composition and Volatile-Bearing Materials
The composition of lunar water is not yet fully understood and is primarily inferred through remote sensing techniques. The lunar surface, is significantly shaped by meteoritic impacts and likely contains a range of minerals that could harbor hydroxyls. These include hydrated and sulfur-bearing minerals such as epsomite, blödite, gypsum/bassanite, and jarosite. Lunar water may not be pure; instead, it could potentially be a brine, water with dissolved salts and other volatiles. These brines could form from or coexist with minerals delivered by carbonaceous chondrites and CI/CM chondrites, which bring hydrous minerals and potentially soluble compounds to the Moon. Micrometeorites and interplanetary dust particles contribute additional volatile compounds such as H2O, CO, and possibly CO2 and impact the lunar surface with a flux of 1/1m² per day. Furthermore, the potential presence of subsurface brines on larger celestial bodies like Ceres highlights the possibility of similar ices on the Moon.
Data from the Lunar Crater Observation and Sensing Satellite (LCROSS) confirmed a variety of volatiles in the lunar regolith including water (H2O), hydrogen (H2), carbon monoxide (CO), hydrogen sulfide (H2S), ammonia (NH3), sulfur dioxide (SO2), ethylene (C2H4), carbon dioxide (CO2), methanol (CH3OH), mercury (Hg), and methane (CH4).
Confirmed hydroxyl-bearing lunar materials include glasses, apatite, and novograblenovite (NH4)MgCl3·6H2O.
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Production
Lunar water has several potential origins: the Earth, water-bearing (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 (, , 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 mineral surface, must not be confused with water ice.)
or,
where "X" represents the oxide surface.
Trapping
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 (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 Kelvin (about −170 ° Celsius), Ice on the Moon, NASA 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.
Transport
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.
Liquid water
4–3.5 billion years ago, the Moon could have had sufficient atmosphere and liquid water on its surface. Isotope analysis of water in lunar samples suggests that some lunar water originates from Earth, possibly due to the Giant Impact event.
Uses
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.
Ownership
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 moon resources. 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 However most legal experts agree that the ultimate test of the question will arise through precedents of national or private activity.
See also
External links
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