China has discovered water inside impact glass beads retrieved by the Chang'e-5 lander from lunar soils. This marks the discovery of a new water reservoir on the Moon.
Chang'e-5 is China's first lunar sample return mission that was launched on November 23, 2020, landed on the Moon on December 1 the same year, collected a total of 1,731 grams of lunar samples, and returned to Earth on December 16 that year. The lunar samples primarily consisted of rocks and soil from the Moon.
The study describing the findings was published March 27 in the journal Nature Geosciences. A team of researchers led by Professor Hu Sen from the Institute of Geology and Geophysics of the Chinese Academy of Sciences conducted detailed studies on the glass beads, and concluded that they are likely a new water reservoir on the Moon.
Since lunar surface water has the potential for in-situ resource utilisation by future lunar exploration missions and other space missions, water on the Moon is intriguing to all.
Why is a water reservoir necessary on the Moon?
Since several lunar missions have confirmed the presence of structural water or water ice on the Moon, there is no doubt that most of the Moon's surface harbours water. However, the amount of water on the lunar surface is much less than that on Earth.
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Water on the lunar surface exhibits diurnal cycles, which means that it undergoes periodic processes every 24 hours. The water on the Moon is also lost to space. This indicates the fact that there should be a hydrated layer or reservoir at depth in the lunar soils to sustain the retention, release and replenishment of water on the lunar surface, or a lunar surface water cycle.
Previous studies could not explain the lunar surface water cycle
Previously, studies were conducted on the water inventory of fine mineral grains in lunar soils, agglutinates (masses formed by things sticking together) produced by impact, volcanic rocks, and pyroclastic glass beads, or glass beads formed as a result of volcanic action.
However, these studies had been unable to explain the retention, release and replenishment of the water on the lunar surface. This is what made researchers think that there must be a yet-unidentified water reservoir in the lunar soils with the capacity to buffer the lunar surface water cycle.
How the study was conducted
HE Huicin, a doctoral student under the guidance of Professor Sen, proposed that impact glass beads were a potential candidate for investigation of the unidentified hydrated layer or water reservoir in lunar soils.
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In order to identify and characterise the missing water reservoir on the lunar surface, Huicin systematically characterised the petrography (study of the appearance of thin, transparent sections of rocks), water abundance, major element composition and hydrogen isotope composition of the impact glass beads that were returned to Earth by the Chang'e-5 mission.
What was found in the glass beads?
The study found that the Chang'e-5 impact glass beads have homogeneous chemical compositions and smooth exposed surfaces, and are characterised by the abundance of water of up to about 2,000 micrograms per gram of glass beads.
How did researchers conclude that water in the glass beads came from solar winds?
The glass beads had extreme deuterium-depleted characteristics, which means that they had features indicating the loss of deuterium, an isotope of hydrogen. This implies that there is a negative correlation between water abundance and hydrogen isotope composition, indicating that the water in the impact glass beads came from solar winds.
Solar winds brought with them deuterium to the Moon, which formed water, explaining why the beads were depleted of the hydrogen isotope.
Impact glass beads served as a sponge for buffering the lunar water cycle
The team analysed water abundance along six transects (straight lines along a natural surface) in five glass beads, and obtained the hydrogen profiles of solar wind-derived water.
A subsequent degassing event occurred in some glass beads. As a result, the impact glass beads served as a sponge for buffering the lunar surface water cycle.
How much water do glass beads contribute to the Moon?
According to the study, the amount of water contributed by impact glass beads to lunar soils varies from 3 × 10¹¹ kilograms to 2.7 × 10⁴ kilograms. This means that there could be a minimum of 270 trillion kilograms of water on the Moon. However, this is less than the water in Earth's oceans.
The lunar surface water cycle: explained
The glass beads record the dynamic ingress and egress of solar wind-derived water, and act as a buffer for the lunar surface water cycle. Impact glass beads are a ubiquitous component in lunar soils with an amorphous nature.
The first stage of the lunar surface water cycle is the formation of impact glass beads by asteroid impacts.
In the second stage, hydrogen ions from solar winds are implanted on the glass beads. This is called solar-wind derived water implantation.
The third stage is the inward and outward diffusion of solar wind-derived water.
Significance of the study
In a statement released by the Chinese Academy of Sciences, Professor Sen said the findings indicate that the impact glasses on the surface of the Moon and other airless bodies of the solar system such as Mercury are capable of storing solar-wind derived water and releasing it into space.
Hu also said that the water trapped in impact glass beads is relatively easy to extract and could represent a potential water resource for future lunar exploration.
According to the study, the distribution of water within individual beads showed that water can rapidly accumulate in glass beads by diffusion over only a few years and be rapidly released. This presents an efficient recharge mechanism for the lunar surface water cycle.