In situ detection by China’s Chang’e-5 lunar probe and subsequent analysis of its returned samples revealed at least two sources of water on the moon, one brought in by the solar wind and the other by indigenous sources.
The results were published Wednesday in the journal Nature Communications..
Chang’e-5, consisting of an orbiter, lander, ascender and returner, was launched in November 2020 and returned to Earth in December. It recovered a total of 1,731 grams of lunar samples, mostly rocks and soil from the moon’s surface.
One of the scientific objectives of the probe was to study the presence of lunar water, key to the formation and evolution of the moon. It also provides important information about the evolution of the solar system. The presence of water should provide support for future in situ human lunar resources.
However, sources of lunar water are still controversial. Lunar water is traditionally assumed to have been implanted by the solar wind or the result of a meteorite or comet. Additionally, minerals like native water-bearing apatite would be present in small amounts and unevenly distributed over the lunar surface.
A group of scientists from the Chinese Academy of Sciences have found new evidence of water on the lunar surface based on in-situ spectral analysis of Chang’e-5 and laboratory results from returned samples.
Their findings revealed that the apatite found in the area of the young mare where the Chang’e-5 landed contained native water in the form of hydroxyl.
According to the study.
Equipped with the Lunar Mineralogical Spectrometer (LMS), the Chang’e-5 lander acquired spectral data of a rock and different lunar soil targets, finding that the hydroxyl content of the rock is much higher than that of the soil lunar.
The researchers said there should be little to no foreign hydroxyl associated with the impact crater or ejecta since Chang’e-5 saw mostly local mare material at its landing site.
Subsequent laboratory analysis on Earth showed that the returned rock fragments, with a considerable amount of apatite grains, have a higher proportion of mineral content with much less glass content compared to the Apollo samples.
The glass content was considered a measure of the solar wind’s contribution to lunar water, and it is quite low in the Chang’e-5 sample, only about a third of that in the Apollo sample. 11, according to the study.
Then, they confirmed the presence of hydroxyl in the apatite grains.
Moreover, the overall hydroxyl content of lunar soil samples detected in situ is relatively low. But one of them labeled 0012 stands out.
Excluding the native water in the mineral, the water content of soil samples from Chang’e-5 should also be about one-third that of the Apollo 11 sample, according to the study. But sample 0012 shows higher hydroxyl content than it should be.
It turns out that the researchers detected hydroxyapatite in the soil sample, strongly suggesting that hydroxyl-containing apatite is likely to be a significant source for the excess hydroxyl observed.
Despite the exception of 0012, the overall hydroxyl content of lunar soils sampled at the Chang’e-5 landing site averages 28.5 parts per million (ppm), which is at the weaker end of the lunar hydration characteristics.
Researchers have proposed three causes for the drier soil samples. First, the probe obtained data when lunar surface temperatures are near a maximum at the same latitude, with most of the molecular water evaporated.
Second, when the probe collected in situ spectra on the lunar surface, the moon coincidentally lies within the Earth’s magnetosphere which shielded it from the solar wind, thereby reducing the contribution of solar wind hydration.
Third, its landing zone is filled with late-stage lunar basalts, leading to relatively low hydroxyl contents, according to the study.
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