An in situ detection by China’s Chang’e-5 lunar probe and subsequent analysis of the returned samples revealed at least two sources of water on the moon, one from solar wind and the other from indigenous sources.
The findings were published Wednesday in the journal Nature Communications†
Chang’e-5, consisting of an orbiter, a lander, an ascender and a returner, launched in November 2020 and returned to Earth in December. It retrieved a total of 1,731 grams of lunar samples, mostly rocks and soil from the lunar surface.
One of the probe’s scientific objectives was to investigate the presence of lunar water, which is a 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 is expected to support future in-situ human resources on the moon.
However, the sources of lunar water are still controversial. It is traditionally believed that lunar water was implanted by solar wind or the result of a meteorite or comet. Also, minerals such as apatite that contain native water are believed to be present in small amounts and unevenly distributed on the lunar surface.
A group of scientists from the Chinese Academy of Sciences found new evidence of water on the lunar surface based on Chang’e-5’s in situ spectral analysis and lab results of returned samples.
Their findings revealed that the apatite found in the young mare area where the Chang’e-5 landed contained native water in the form of hydroxyl.
The Chang’e-5 landing site is in one of the youngest extrusive lava flows on the moon, making the apatite, the latest state in the formation of that basalt, an ideal target to look for evidence of native water, according to the study.
Equipped with the lunar mineralogical spectrometer (LMS), the Chang’e-5 lander collected spectral data from a rock and several lunar soil targets, and found that the rock’s hydroxyl levels are much higher than those of lunar soil.
The researchers said there should be little or no external hydroxyl associated with impact crater or ejecta, as Chang’e-5 saw mostly local mare material at its landing site.
Subsequent laboratory analysis back on Earth showed that the returned rock fragments, containing a significant amount of apatite grains, have a higher mineral content with much less glass content compared to Apollo samples.
The glass content has been considered a measure of the solar wind’s contribution to the lunar water, and it’s quite low in the Chang’e-5 sample, only about a third of that in the Apollo 11 sample, according to the study. .
They then confirmed the presence of hydroxyl in the apatite grains.
Also, the total hydroxyl content of in-situ detected lunar soil samples is relatively low. But one of them labeled 0012 stands out.
Excluding the native water in the mineral, the water content in Chang’e-5 soil samples should also be about a third of that in Apollo 11 sample, according to the study. But the sample 0012 shows a higher hydroxyl content than it should.
It turns out that the researchers detected the hydroxyapatite in the soil sample, strongly suggesting that the hydroxyl-containing apatite is likely a major source of the observed excess hydroxyl.
Despite the 0012 exception, the total hydroxyl content of the lunar soils sampled at the Chang’e-5 landing site is at a mean value of 28.5 parts per million (ppm), which is on the weaker side of the lunar hydration characteristics.
The researchers suggested three causes of the drier soil samples. First, the probe obtained data when the moon’s surface temperatures are close to a maximum at the same latitude at which most of the molecular water evaporates.
Second, when the probe collected in situ spectra on the lunar surface, the moon happened to lie in Earth’s magnetosphere shielding it from the solar wind, reducing the solar wind’s contribution of hydration.
Third, the landing area is filled with basalt in the moon’s last phase, leading to the relatively low hydroxyl levels, according to the study.
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