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water ice, hydroxyl, water molecules. over the past thirty years, as one probe and lander after another has passed over the moon, data and raw materials have been continuously transmitted back. scientists have worked tirelessly, and the "dry and waterless" moon has gradually shown us its other side.

when did things turn around? was it the glimpse of clementine over the lunar south pole, the improvement in the accuracy and sensitivity of modern ion probe technology, the precise positioning of water molecules at high latitudes by sofia's wider spectrum spectrometer, or the return of fresher samples from chang'e 5?

written by lin honglei, associate researcher, institute of geology and geophysics, chinese academy of sciences

yao zhonghua associate professor, university of hong kong

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the ups and downs of the search for water on the moon

in 1994, the united states' clementine spacecraft headed for the moon. this was the first dedicated lunar exploration mission by the united states in 21 years after the apollo moon landing era.

unlike previous apollo sample return missions, this one will orbit the moon to conduct a detailed global survey. it carries a radar observation system to search for evidence of water at the lunar poles by coordinating with the deep space network receivers on earth.

in 1996, the u.s. department of defense announced that data from the clementine spacecraft showed that there was water ice deposited at the bottom of a crater in a permanently shadowed area at the south pole of the moon (nozette, et al., 1996, science). the volume of the deposit was about 60,000 to 120,000 cubic meters, equivalent to a small lake with an area of ​​four football fields and a depth of 5 meters. this discovery brought the study of lunar water back into people's attention.

why is it called "renewed"? in fact, humans have always been full of infinite reverie about whether there is water on the moon. since galileo invented the telescope in 1609, which can observe the moon more clearly than the human eye, astronomers have discovered that the moon's surface mainly has two distinct areas: white and black. they believe that the black areas on the moon may be covered by liquid water, so they use words and phrases such as "sea", "ocean", "creek" and "bay" to describe the morphology of water systems to name the black areas on the lunar surface (wei yong et al., 2024, journal of the chinese academy of sciences). by the middle of the 20th century, scientists used numerical simulations to show that water vapor can be trapped in permanent shadows on a geological time scale. from 1969 to 1976, the us "apollo" and soviet "lunar" missions collected a large number of samples from the moon and returned them to earth, finally giving people the opportunity to directly measure the water content in lunar samples. unfortunately, the analysis results showed that the lunar soil was very dry, and the trace amounts of water measured could not exclude contamination from the earth's atmosphere. at the same time, the instruments left by astronauts on the lunar surface to detect the atmosphere could not detect water, which seemed to make "the moon is dry" a fact. this also gave rise to the theory of the lunar giant impact.

the radar detection results of the clementine spacecraft undoubtedly released a positive signal! however, the observations of the arecibo ground-based radio telescope further showed that even in areas without permanent shadows (where water ice cannot be preserved), there are similar radar signals, which may be caused by other factors such as surface roughness (stacy, et al., 1997, science). the question of whether there is water ice in the lunar poles has once again fallen into the fog.

water is composed of hydrogen and oxygen. measuring the hydrogen content of the moon may be a way to detect water. therefore, in 1998, the lunar prospector carried a neutron spectrometer to measure the distribution of hydrogen on the moon, and a large amount of hydrogen enrichment was found at the poles (feldman et al., 1998, science). is there water on the moon again? the debate on whether there is water on the moon continued until 2008.

2008 was an extremely unusual year for the search for water on the moon, with two major events. the first was the indian chandrayaan-1 spacecraft carrying the american lunar mineralogy mapper into lunar orbit, the first spectrometer that could directly measure lunar hydroxyl/water molecules.

soon, the team of professor carle pieters of brown university, who was in charge of the spectrometer, found obvious signals of hydroxyl/water molecules in the data, especially in the middle and high latitudes of the moon. this made professor carle pieters' team very excited, but also very surprised: how could there be such a strong water signal? was there something wrong with the calibration of the instrument? they spent several months checking the data and finally believed that these signals were real and valid (pieters et al., 2009, science).

to further ensure that water was indeed detected, the team requested the deep impact probe, which was heading to the comet at the time, to return and conduct spectral measurements of the moon (sunshine et al., 2009, science). they also re-analyzed the spectral observation data from the cassini spacecraft, which was heading to saturn, when it flew over the moon in 1999 (clark, 2009, science), cross-confirming that the signal of lunar water was real.

at the same time, there were new major breakthroughs on the ground, which is the second thing.

professor alberto saal's team, also from brown university, spent three years using ion microprobe technology (sims) to reanalyze the volcanic glass (a product of volcanic eruptions) in the apollo lunar soil and found up to 50 ppm of water (saal et al., 2008, nature), challenging the traditional view that "there is no water on the moon" with conclusive evidence. ion microprobe technology has been around since the 1950s, and it uses high-energy ion beams to bombard the sample surface to release hydrogen ions, which are then measured.

this technique was also used in the analysis of early lunar soil, but the spatial resolution and sensitivity were relatively low. with the improvement of ion source and detector technology, modern sims technology can detect water content as low as 5 ppm (parts per million), and technological advances have made it possible to accurately measure water in lunar soil.alberto saal mainly studies the internal composition of the earth. after coming to brown university, he began to expand his research to the moon and planets. when he proposed to study the topic of "whether there is water inside the moon", his colleagues warned him that it was unlikely to find new information from existing lunar samples. but he did not give up. with the blessing of modern advanced technology, he finally achieved great success. he attributed this success to his lack of experience in the field of planetary science and his lack of fixed thinking.

in 2009, the united states' lunar crater observation and sensing satellite mission (lcross) conducted an impact experiment in the lunar polar region. the observation and analysis of the impact splashes confirmed the presence of water (colaprete et al., 2010, science).since then, it has become a general consensus that there is water on the moon.

laboratory sample analysis and remote sensing observations provide evidence that there is water on the moon, but there is still a missing link in the chain of evidence, that is, field measurements on the lunar surface.china's chang'e-5 probe successfully landed in the mid-latitude region of the moon on december 1, 2020. this is mankind's first sample return mission since 1976. it carried a lunar mineral spectrometer and acquired the spectrum of the lunar surface during the sampling process. it was like we had a "field trip" on the moon. for the first time, we detected the signal of water at such a close distance and high resolution on the lunar surface (lin, li, xu et al., 2022, science advances; liu et al., 2022, nature communications), providing another solid evidence that water is widely distributed on the moon.

after the chang'e-5 samples returned to earth, they aroused great research enthusiasm among researchers, especially for the study of water in lunar soil. because compared with previous lunar sampling missions (i.e., the u.s. "apollo" and the soviet union's "lunar"), the chang'e-5 lunar soil comes from a higher latitude and is younger, which is completely different from the existing lunar soil samples of mankind and is very precious. so far, the national space administration has issued 7 batches of 85.48 grams of scientific research samples to 131 research teams in china. on average, each team only received a few hundred milligrams of samples. using such a small amount of samples, researchers used nano-ion probe technology and infrared spectroscopy technology to discover a high content of water in the lunar soil (xu, tian et al., 2022, pnas; zhou et al., 2022, nature communications).

where is the water mainly stored? after ruling out many possibilities, the researchers turned their attention to the impact glass beads in the lunar soil of chang'e 5.this is a type of sample that has rarely received attention before, because this type of sample has a "normal origin", and is the product of lunar soil being melted by meteorite impact. this process is accompanied by the mixing and transformation of the original rock, which makes it unable to reflect the information of the interior of the moon like "volcanic glass", so few people were interested in it in the early days.however, the amount of impact glass in the lunar soil is quite abundant. thanks to the ultra-high spatial resolution of the nano-ion probe analysis technology, researchers obtained the accurate water content of the chang'e-5 lunar soil impact glass beads, confirming that the impact glass beads are an important reservoir of lunar water and have the ability and potential to maintain the lunar surface water cycle (he et al., 2023, nature geoscience). with the deepening of the understanding of lunar water, the study of the lunar water cycle has gradually become a focus.

what is this lunar water we are talking about?

when it comes to water, the first thing people think of is: can we drink it? when we say there is water on the moon, we are not referring to oceans or lakes, but to water molecules and hydroxyl groups (oh) in the structure of lunar soil particles, or water ice in the permanently shadowed polar regions. water ice refers to a solid substance composed of many water molecules, usually in a hexagonal crystal form, while hydroxyl groups/water molecules on the moon are usually combined with minerals and exist in the mineral structure.

the laboratory ion probe technology measures the hydrogen content, which is then converted into water content, without considering whether the water exists in the form of hydroxyl, water molecules or hydrogen. the spectrometers of chandrayaan-1 and chang'e-5 are unable to distinguish between hydroxyl and water molecules due to their narrow wavelength range.

in 2011, researchers combined ion probes and fourier infrared spectroscopy to directly measure hydroxyl water in the apollo lunar soil (liu et al., 2011, nature geoscience). in 2020, the stratospheric observatory for infrared astronomy (sofia) in the united states used a wider spectrum spectrometer to detect relatively stable molecular water outside the permanent shadow area, confirming for the first time the existence of molecular water on the lunar surface (honniball et al., 2021, nature astronomy). this blockbuster news ignited people's hopes for the utilization of lunar water resources, because molecular water is easier to use than hydroxyl. however, no clear evidence of the existence of molecular water has been found in lunar soil samples. studies on chang'e 5 samples found that impact glass contains a higher water content (he et al., 2023, nature geoscience). is the probability of molecular water in impact glass greater? with this question in mind, researchers conducted detailed measurements of the 12 impact glass beads of the chang'e 5 sample, detected the signal of molecular water, and found that 20%-35% of the water in the impact glass was molecular water (zhou, mo et al., 2024, science advances).

where does the water on the moon come from? this question is mainly answered by hydrogen isotopes.hydrogen mainly has protium (h) and deuterium (d) stable isotopes, with increasing masses. by measuring the deuterium/protium (d/h) ratio in the sample and then comparing it with possible target sources, it is possible to determine where the water came from.if the d/h value is low, similar to that of the sun, it means that the water was brought to the moon by the solar wind; if the d/h value is high, similar to that of a comet, it means that the water may have been brought to the moon by a comet impact; if the d/h value is close to that of the earth, excluding the influence of the earth, it is the water on the moon itself; if the d/h value is between the earth and a comet, it may have been brought to the moon by asteroids containing water.of course, sometimes the measured d/h ratio is the result of a mixture of multiple sources. at this time, it is necessary to combine other isotopes, such as carbon and nitrogen, to further determine the source of water. through the measurement of hydrogen isotopes of lunar soil particles, evidence has been found that water comes from multiple sources such as the lunar interior, solar wind injection, and asteroid/comet impacts (saal et al., 2008, nature; greenwood et al., 2011, nature geoscience; liu et al., 2011, nature geoscience; barnes et al., 2016, nature communications). these sources were even found simultaneously in the impact glass of the chang'e 5 sample (he et al., 2023, nature geoscience; zhou, mo et al., 2024, science advances), which increases the complexity of the lunar water source problem.

in order to determine the main source of water with more statistical significance, researchers conducted a particle-size spectral analysis of a large number of samples from chang'e-5, providing evidence that solar wind is the main source of lunar soil water (lin et al., science bulletin, 2024). the changing pattern of lunar surface water content obtained based on remote sensing spectroscopy also indicates different sources of water. the lunar surface water content has daily variations, with high levels in the morning and evening and low levels at noon, indicating the solar wind source of water, because only solar wind can quickly replenish the water lost at noon due to high temperatures (li & milliken, 2017, science advances; wöhler et al., 2017, science advances); some areas have abnormally high water content, combined with topographic features, confirming the existence of water in the lunar interior (klima et al., 2013, nature geoscience; milliken & li, 2017, nature geoscience). polar water ice may also be formed by the diffusion and migration of solar wind-generated water, which eventually condenses and forms in the permanent shadow areas at the lunar poles (pieters et al., 2009, science). analysis of lcross elemental data indicates that comet impacts are also a reasonable source (mandt et al., 2022, nature communications).

of course, since the moon spends nearly a third of each month in the earth's magnetosphere, the contribution of earth's wind to lunar water cannot be ruled out (wei et al., 2020, geophysical research letters; wang et al., 2021, astrophysical journal letters; li et al., 2023, nature astronomy).the question of the origin of lunar water is extremely complex and requires samples from more areas, more observations and deeper analysis to gain further understanding.it is worth pointing out that the solar wind and the earth's wind are not only the source of lunar water, but also the space radiation environment that we need to directly face when conducting manned and unmanned exploration missions to the moon. research on their radiation characteristics and changing mechanisms is one of the important guarantees for lunar missions, and therefore it is also worthy of special attention.

how much water there is and whether it can be used are issues that humans are more concerned about in the future when building a lunar base and staying there for a long time. this issue should be discussed from two aspects based on the existence of water on the moon. first, the polar regions, because of the low temperature, theoretically store a higher content of water than other regions, and exist in the form of dirty ice (mixed with lunar soil) in the permanent shadow area. the lcross mission impacted the permanent shadow area of ​​the lunar south pole, analyzed the plume produced by the impact, and detected that the impact point contained about 5.6 wt.% of water ice (colaprete et al., 2010, science). infrared spectrum detection also obtained similar results, and found that in some areas with relatively pure water ice signals, the water ice content can even reach 30 wt.% (li et al., 2018, pnas). water ice in the polar regions is also the most promising lunar resource at present, and it is also the focus of exploration by countries around the world. researchers have proposed ideas such as heated mining and heated drilling to extract water ice.however, due to the complexity of the environment, such as low temperature, low gravity, and lack of light, there is still a long way to go to achieve this goal. after mining, issues such as separation, purification, and storage must also be considered.

on the other hand, there is water in the lunar soil. this water exists in the mineral structure, and the water content is only tens to hundreds of ppm (there are tens to hundreds of grams of water in one ton of lunar soil), which is drier than the desert. taking the chang'e 5 sampling area as an example, there are a maximum of 120 grams of water in one ton of lunar soil (lin, li, xu et al., 2022, science advances), but this is also related to the latitude. at higher latitudes, it can even reach 500-750 grams (li & milliken, 2017, science advances). the water in the lunar soil mainly comes from solar wind injection. since fine-grained lunar soil has a larger specific surface area, the water content will be higher than that of coarse particles.particle size screening may be a way to utilize lunar soil water at low and mid-latitudes.108 kg of water can be extracted from 1,000 cubic meters of fine-grained lunar soil (less than 45 microns). if it can be screened finer, 840 kg of water can be extracted from 1,000 cubic meters of lunar soil less than 10 microns (lin et al., science bulletin, 2024). in addition, the volcanic debris area of ​​the moon preserves a large amount of water, which can reach 500 ppm (milliken & li, 2017, nature geoscience). however, extracting this water from the lunar soil requires a higher heating temperature, and how to efficiently obtain the maximum input-output ratio is a challenge.

although there are many difficulties in utilizing lunar water, relevant prototypes have already begun to be developed. with the advancement of technology, i believe that astronauts will be able to drink lunar water in the near future, and even rocket fuel can be made from lunar water.

where to go from here

water is the source of life, and its importance is self-evident. in the evolution of the moon, water also played a very critical role. the new round of lunar exploration has shifted from pure scientific exploration to a focus on both scientific research and application, and has proposed future plans such as "moon village" and "moon research station". water as an important resource has also received more attention. lunar water will play an important role in future space exploration and lay an important foundation for human civilization to move into space.there is still controversy over the spatial and temporal distribution of water on the moon. obtaining high-precision and high-resolution data on the water content and its distribution on the moon will be a key focus of future exploration.

my country's chang'e-7 and chang'e-8 will both conduct detailed explorations at the south pole of the moon, especially explorations of water resources in the polar region, and will build the basic framework of an international lunar research station around 2030. manned lunar exploration has also been put on the agenda. the united states' artemis program will once again realize manned lunar landings after the apollo mission, conduct investigations, experiments and collect samples at the south pole of the moon, and build a base to support long-term human activities on the lunar surface. the united states also plans to launch a small exploration mission, the lunar pioneer, to specifically study the distribution of water on the moon through orbiting exploration. in addition, europe, japan, india, south korea and other countries also regard the exploration of lunar water as an important content. in order to make use of lunar water, we first need to have a clearer understanding of lunar water, which requires more detailed observation data. the moon has the advantage of having no atmosphere and an orbit that is convenient for close and continuous operation. based on this,researchers have proposed a "near-lunar orbit constellation" scheme to achieve continuous high-resolution measurements in the near-lunar orbit (below 30 kilometers), which will help us gain unprecedentedly accurate information about lunar water (wei yong et al., 2024, bulletin of the chinese academy of sciences).