GEOGRAPHY

Scientists discover lunar “mini-reservoir”


Over the past two decades, scientists have found many traces of the presence of water on the lunar surface. Some scientists speculate that there is an aquifer deep in the lunar soil, which drives the entire moon’s water cycle. So, where does the water on the moon come from? Where is it stored?

Now, Hu Sen, a researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences (hereinafter referred to as the Institute of Geology and Geophysics), Hui Hejiu, a professor at the School of Earth Sciences and Engineering of Nanjing University, and other scientists at home and abroad have cooperated to find that the impact glass beads may be the moon’s “mini reservoir” – a ton of glass beads can contain an average of one pound of water. The study was published March 28 in Nature Earth Sciences.

“Our study demonstrates that impacting glass beads may be a new lunar water storage material, supporting the hypothesis of the lunar surface water cycle and explaining the potential source of water on asteroids like the moon that have no atmosphere.” Hu Sen, the corresponding author of the paper, told China Science News.

With the increasing advancement of lunar exploration, researchers believe that the future of extracting water from a large number of glass beads is promising.

An unexpected discovery

One day in the winter of 2021, when He Huicun, a doctoral student at the Key Laboratory of Earth and Planetary Physics of the Institute of Geology and Earth, used a nanoion probe to detect the impact glass beads in the Chang’e-5 lunar soil sample, he found that the water abundance in the beads gradually decreased from the outside to the inside, forming a “ring band feature”.

She presented the results to her mentor, Hu Sen. Hussen’s first reaction was “can’t believe it.”

Soon, Hu Sen called Hui Hejiu, another co-corresponding author of the paper. Despite being familiar with lunar samples, the partner’s reaction was the same as his: “Could it be that the experiment was wrong?” ”

From the moon to Mars and beyond, the search for clues related to “water” has always been a starting point for scientists to study the history of planets and the habitability of life.

At present, scientists have visited the moon the most times of any planet, and studies have shown that although the moon has water, the content is low and dynamic.

For example, the lunar surface water content is only 10~1000ppm (that is, one ton of lunar soil contains 0.01kg~1kg of water). At the same time, the water content in the high-latitude region is significantly higher than that in the low-latitude region, and there may be water ice in the polar region; The lunar surface water content also showed changes related to the lunar day and night, with the water content at dusk at the same location being significantly higher than at noon.

Although the Moon has almost no atmosphere, there may also be a water cycle on the surface of the Moon.

NASA’s Lunar Atmosphere and Dust Environment Exploration Mission has detected that the escape time of water molecules in the lunar space environment is related to meteor shower events. However, after calculating that the input amount of meteor showers is significantly less than the amount of water escaping in lunar space, NASA scientists speculate that when meteor showers hit the moon, part of the water stored in lunar soil will be evaporated into lunar space, resulting in escape, and proposed the lunar surface water cycle conjecture. The hypothesis suggests that there may be undiscovered aquifers in the 10 cm to 3 m area of the lunar surface.

However, in recent years, many researchers at home and abroad, including Hussen, have not supported this hypothesis by studying different minerals or components of the moon. “There is very little water in these minerals.” Hu Sen told China Science News that he and his collaborators had previously found that the total rock water content in the Chang’e-5 lunar basalt was only 62~133 ppm.

So, does the lunar surface water cycle conjecture hold? Are there undiscovered water storage materials in the lunar soil?

Hussen and his collaborators wanted to try to answer this question by hitting the glass bead. Impact glass beads are formed after meteorites and asteroids in space melt the soil and rock on the lunar surface after hitting the moon, and the melt sputters to form droplets after cooling. Some of them have a single internal composition, while others contain different components such as bubbles, metals, and sulfides. The lunar soil returned from the Chang’e-5 and Apollo missions contains rocks, mineral debris, volcanic glass beads, impact glass beads and other different components, and the abundance of water in impact glass beads has not been studied in detail before.

The preliminary results completely exceeded their expectations: the “ring signature” of the abundance of water in the glass beads suggested that it most likely came from the solar wind.

In order to verify this result, they repeatedly experimented, successively selected 32 glass beads with smooth surfaces and chemical composition consistent with lunar basalt from more than 110 lunar soil glass beads, and tested them through scanning electron microscopy, ion probes and Raman spectroscopy, and found that the results were the same as at first.

They found that the water abundance of glass beads is higher than that of lunar soil debris, reaching up to nearly 2,000 ppm. “This is equivalent to about 2 kilograms of water in a ton of sample.” Hu Sen explained to China Science News.

“Open up” the lunar water circulation chain

Scientists have many conjectures about the source of lunar surface water. For example, hydrogen ions carried by the solar wind are injected into lunar surface minerals; The Moon was aqueous when it formed early and remains to this day; Comets and asteroids are carried when they hit the Moon.

In fact, both magmatic activity and impact on celestial bodies (which carry water sources) may form impact glass beads through high-temperature melting. How can researchers determine that the “water” in the impact glass beads comes from the sun by virtue of the “ring band feature”?

In this regard, Hui Hejiu explained to China Science News: “Usually, when glass beads are formed by high temperature melting, ‘water will run out’, so the water abundance in the glass beads is more likely to be ‘high inside and low outside’. ”

At the same time, as the lightest hydrogen in the periodic table, scientists can identify its source by identifying its isotope (argon, deuterium, tritium) signature. By monitoring the hydrogen isotopes in the glass beads, the researchers found that they were consistent with the hydrogen isotope composition of the solar wind and were extremely deuterium-poor.

These phenomena led them to speculate that hitting the glass beads may test the lunar surface water cycle conjecture: over the past few billion years, after meteorite fire asteroid impacts formed glass beads on the surface of the moon, hydrogen ions in the solar wind continuously arrived at the moon and injected into the surface glass beads, forming a source of lunar water and maintaining the water cycle on the lunar surface.

“The main component of the impact glass beads is silicate, like a sponge, into which hydrogen injection may combine with oxygen to form water, or it may be released under other conditions.” Hussen explained that they also found that some glass beads even experienced some degree of impact or heating event at a later stage, resulting in a water content profile superimposed on a degassing process.

The “ploughing effect” brought about by the impact of space celestial bodies for billions of years has made impact glass beads (content about 3~5vol) prevalent in lunar soil (content of about 3~5vol%). The researchers estimate that the amount of water carried by hitting glass beads in lunar soil can reach 2.7×1014 kilograms. “This is far lower than the water reserves of the Earth’s four oceans (~1×1021 kilograms), but relatively speaking, it is quite considerable.” Hussen said.

Based on these results, they believe that these impact glass beads (about 30~150 microns) with a diameter equivalent to the thickness of a human hair are the main reservoirs in the soil on the lunar surface, but do not include the ice at the lunar poles.

Photo courtesy of Hu Sen et al

The prospect of glass beads taking water is promising

With the gradual advancement of lunar water research, if astronauts are stationed on the moon, is it possible to directly “take water in situ”?

Hui Hejiu said that extracting water from a large number of glass beads is promising, but this water is different from the water commonly seen on Earth, and the water in the glass beads mainly exists in the form of hydroxyl (-OH), and how to develop and utilize it still needs further research.

International reviewers found the study interesting and presented important new findings. “In 1972, a study of Apollo hitting glass beads reported the inward diffusion of volatile elements, but the discovery of water in the impact glass beads has not been reported. The authors conducted a comprehensive set of analyses that strongly demonstrated evidence for the presence of hydroxyl groups. One reviewer wrote.

Another reviewer noted that the strong deuterium-loss characteristics of glass beads in the Chang’e-5 return sample suggest that the water in it comes from the solar wind, not from comets or gases released from the moon’s interior, solving a decades-old problem about the origin of trace amounts of hydrogen found on the lunar surface in sunlight.

In addition, with the improvement of detection capabilities, scientists have discovered that more and more atmosphereless objects may have water on the surface, such as Vesta, Mercury, etc. The amount of water detected on the surface of these celestial bodies is far inferior to that of Earth, but it is still “attracting countless fans”. The researchers say the study also has important implications for understanding the source and origin of water at the planet’s low latitudes.

“Further in-depth and detailed research, such as the source, preservation, and migration mechanism of surface water of non-atmospheric objects, has important reference value for the scheme design and extraction strategy of in-situ water resources utilization for future deep space exploration missions.” Hussen said. (Source: China Science News Feng Lifei)

Related paper information:https://doi.org/10.1038/s41561-023-01159-6



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