Composite image of the Moon's south polar region, captured by NASA's Clementine probe over two lunar days. Permanently shadowed areas could harbour water ice.

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.^[1] In fact, adsorbed water is calculated to exist at trace concentrations of 10 to 1000 parts per million.^[2]

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, India's Chandrayaan-1 detected water on the Moon^[3][4] 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;^[5] this may be attributed to water-bearing materials^[6] – what appears to be "near pure crystalline water-ice".^[7] In March 2010, NASA reported that the Mini-SAR radar aboard Chandrayaan-1 detected what appear to be more than 40 small craters hypothesized to contain up to 1.3 trillion pounds (600 million metric tons) of water ice.^[8]

Water may have been delivered to the Moon over geological timescales by the regular bombardment of water-bearing comets, asteroids and meteoroids ^[9] or continuously produced in situ by the hydrogen ions (protons) of the solar wind impacting oxygen-bearing minerals.^[10]

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

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.^[11] 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.^[12] However, a 2008 study of lunar rock samples revealed evidence of water molecules trapped in volcanic glass beads.^[13]

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.^[14]

The first 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.^[15] 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,^[16] and their significance has been questioned.^[17][18] 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.^[19]

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.^[20] 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,^[21] 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.^[22][23]

More suspicions about the existence of water on the Moon were generated by inconclusive data produced by Cassini–Huygens mission,^[24] 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.^[25]

  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.^[26] In June 2009, NASA's Deep Impact spacecraft, now redesignated EPOXI, made further confirmatory bound hydrogen measurements during another lunar flyby.^[12]

Kaguya

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.^[27] 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,^[28] and it ended its mission by crashing into the lunar surface in order to study the ejecta plume content.^[29]

Chang'e 1

The People's Republic of China's Chang'e 1 orbiter, launched in October 2007, took the first detailed photographs of some polar areas where ice water is likely to be found.^[30]

  Image of the Moon taken by the Moon Mineralogy Mapper. Blue shows the spectral signature of hydroxide, green shows the brightness of the surface as measured by reflected infrared radiation from the Sun and red shows a mineral called pyroxene.

  On Friday 9 October 2009, as the spent upper stage of NASA's Lunar Reconnaissance Orbiter launcher smashed into the Cabeus lunar crater, the LCROSS probe attempted to detect water in the ejecta plume before it also crashed itself onto the lunar surface.

Chandrayaan-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.^[31]

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,^[24] albeit in low concentrations and in the form of hydroxyl group ( · OH) chemically bound to soil.^[6][32][33] This supports earlier evidence from spectrometers aboard the Deep Impact and Cassini probes.^[12][34][35]

On March 2010, it was reported that the Mini-SAR on board the 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.^[7][36] 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. ^[36] The estimated amount of water ice potentially present is comparable to the quantity estimated from the previous mission of Lunar Prospector’s neutron data.^[36]

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^[36]) 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.^[37]

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.^[38] 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.^[5][39] However, what was actually detected was the chemical group hydroxyl ( · OH), which is suspected to be from water,^[1] 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;^[5] 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.^[7] A later definitive analysis found the concentration of water to be "5.6 ± 2.9% by mass".^[40] 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.^[41][42]

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.^[43]

Melt inclusions in Apollo 17 samples

In May 2011, Erik Hauri et al. reported^[44] 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

  Production

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.^[45] (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

or,

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.

  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 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),^[46] 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.^[13][16]

  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.^[12]

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.

  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 in-situ resources.^[47][48] 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.^[49]

  See also

Moon portal

• 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
• Water on Mars

  References

  External links

• 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

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