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We think the moon is a dry and dusty place, but over the years, scientists have learned that the Earth's satellite is rather loaded with water – in volcanic deposits, in its cold poles, in the permanent shadows of its craters.
But it seems that the water has also been hidden elsewhere, in considerable abundance: the lunar subsoil, a few centimeters from the surface of the Moon.
We had hints that it was there. Cassini detected traces during its flight over 1999. The NASA Deep Impact probe detected it during a 2009 flyby, similar to the Indian lunar spacecraft Chandrayaan-1 . But the source of this water was unknown – scientists thought it was deposited there, perhaps by the solar wind or by meteoroids.
We did not really think that the surface water could have been there from the beginning. But new research has shown that it was probably the case.
In 2013 and 2014, NASA's lunar atmosphere and dust environment explorer (LADEE) circled the moon, using its Neutral Mass Spectrometer (NMS) to take readings from the very fine atmosphere of the moon. He made 736 positive detections of water.
Two hundred and fourteen of them constituted 33 unusually high water discharges, 29 of which coincided with known annual meteoroid currents, with many small lunar impacts. Thus, the meteoroid strikes Moon; in a way, water appears. But do not meteorites contain water?
Well yes. But not enough to create the observed abundances in the lunar atmosphere.
"To take into account the relatively high intensities of the events detected, our analysis reveals that the impacts must provide a quantity of water from 5.4 to 12.8 times the mass of the impactor." A few tenths to a few percent of its total mass, "the researchers wrote in their article.
"Instead, it most likely comes from adsorbed water on the grains of regolith that is desorbed by the shock wave that is spreading to the site of the impact."
So what about these four water releases that did not coincide with known meteor currents? According to the researchers, they could indicate meteor currents unknown to us.
In analyzing the amount of water released, the research team concluded that the first eight centimeters (three inches) of the lunar regolith are dry as mummy bone.
However, go deeper than this – and at least up to a depth of three meters -, the water is evenly distributed, at concentrations up to 0.05%. It is the lower end of the permanent wilting point here on Earth, so still quite dry. But no completely dry, as previously thought.
The team also calculated the amount of water lost each year due to lunar impacts and reached 200 tons. This would seem to indicate that the moon has always, or almost always, had a whole lot of water.
"The moon – and especially the permanently shaded areas – is very likely to have a net water loss regime," the researchers wrote. "To maintain this rate of losses over geological time, the water must have been delivered long ago, otherwise it was present at lunar formation."
Will it help us to establish a lunar base? Perhaps. It may not be practical to extract this water, for example, but it may lead to a better understanding of the origin of water on the moon and how it is occurring. Is gradually exhausting.
The search was published in Nature Geoscience.
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