There is a longstanding argument about water on the moon. Does it exist? Hence, if the Chinese say that their lander has found water there (and they do), that’s a big deal.

If this is tl:dr skip down to the ***

In fact there is a longstanding argument about exactly where the water on earth came from, and the discussion is relevant to the moon question. Was water present in the original material that somehow coalesced into the planets? Or was water a later arrival from cometary collisions, for example. Door #2 was preferred for years. Door #1 was opened recently. Either answer could account for water in moon rocks. But water is not in the rocks.

How do you account for the incredible dryness of the moon? Because it is really dry. What does it mean to say that the moon is dry? For one thing a dry moon has never had running water. Harold Urey believed in the sixties that there had been running water, and argued with John O’Keefe about it. A dry moon is one whose minerals and rocks are dry. Inclusions or bubbles in those rocks are dry. The rocks that came back from the moon are nearly all dryer than dry. (And tektites are dry in this interior way.)

[[This is a reference, with a quote, from Harold Urey on how obvious it is that rills on the moon were caused by water. https://www.nature.com/articles/2161094a0 (1967) ]]

The point in general is that at some very early moment all the water was wrung out of the moon somehow, possibly along with other material that would vanish into space if given a chance.

But … recently … water has been reported on or near the surface in two different ways.

First, water ice has been reported on the moon in Permanently Shadowed Regions known as PSRs. (Water ice as opposed to chlorine ice or ammonia ice or …) If sunlight never reaches an area in space, then that place is beyond cold. On earth: a) the tilt ensures that sunlight reaches almost everywhere and b) the atmosphere helps to distribute heat in various ways and c) there aren’t any visible craters at the north or south poles since the poles are already covered with ice anyway, so PSRs do not exist in the same way here.

There are places on Mercury that have ice in deep craters, PSRs, even though Mercury in general is four times closer to the Sun. Venus does not have this ice, most likely because its entire surface was melted some 500,000,000 (five hundred million) years ago and it is still overall incredibly hot. As well, its atmosphere, with two hundred mile per hour winds, probably helps to keep heat distributed.

Where did the ice on the moon come from? Well this does seem to be from impact sources. Comets, which are described as dirty snowballs, crashing on the moon could leave the amount of water believed to be frozen in the PSRs, over the billions of years the moon has existed. The moon has very little tilt so deep craters near its poles will never receive sunlight.

Next …

The Chinese lander Chang’e 5 is reported to have found water on the moon. https://www.nature.com/articles/s41467-022-30807-5

There is an elaborate explanation for this. The rocks on the moon have a lot of oxygen. The solar wind has a lot of hydrogen. As the solar wind passes Mars, some of the hydrogen and oxygen combine to create water. This is a peculiar deal (I’m not saying it’s wrong), because the solar wind is also used to account for the stripping of the atmosphere of Mars.

In other words, on Mars, the solar wind strips off all the light gases like oxygen and hydrogen but on the Moon the solar wind provides hydrogen so that in combination with oxygen we get water. Okay? I will also mention that the earth’s upper atmosphere is considered to lose oxygen, and I think water, through the solar wind process.

On the moon, the proposition is that there is a cycle where sometimes the water is formed, and sometimes it is stripped away again. When I went looking for more information I found an article on an Indian moon probe from 2008. It found the same “beads” the Chinese seem to be discussing but it has a different water producing mechanism. (Update: different from the comets)

https://www.nasa.gov/press-release/nasa-s-sofia-discovers-water-on-sunlit-surface-of-moon

*** This article from the Nasa.gov website discusses finding water in sunlit parts of the moon. The water is trapped in beads, the same mechanism mentioned in respect to the Chinese lander. The mechanism suggested is that the water comes in with the micrometeorites; the beads are formed by the high temperatures caused by the impact of the micrometeorite. As the beads cool, they trap the water that they brought. But not all of it. And over time the beads degrade in the harsh environment of the moon and the water goes back into space. The water/ice, found by the Indian moon probe launched in 2008, is discussed here as being confirmed by the mineralogy mapper aboard that probe.

https://www.nasa.gov/feature/ames/ice-confirmed-at-the-moon-s-poles (2018)

This is closer to the original date of finding. (First discussion of finding of water.)

https://www.nasa.gov/multimedia/imagegallery/image_feature_1478.html (2009)

The Luna 24 Russian mission, that returned soil samples to earth, also reported water. This report was ignored at the time at least partly because of the tiny amounts we are talking about, and partly because there was a question of contamination by the rockets used to land the missions.

https://airandspace.si.edu/stories/editorial/revisiting-soviet-lunar-sample-return-missions

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I also found a discussion of the formation of hematite on the moon. Hematite is similar to rust and requires water and oxygen to form. These conditions are rare on the lunar surface so hematite is a surprise. This article argues that, very occasionally, the conditions are met and some hematite forms. Generally, the article is also arguing that the same micrometerites under discussion for making “wet” beads, are responsible bringing in the the water. (The authors didn’t make this particular connection; I did.) Once formed hematite/rust is reasonably stable. Unlike those beads. Certainly, the earthly experience of rust is that it’s easier to acquire it than to get rid of it…

https://www.jpl.nasa.gov/news/the-moon-is-rusting-and-researchers-want-to-know-why

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One absolutely fascinating part of all of this is the idea that the water can be trapped at the moment of impact. That is totally contrary to what people say about tektites. Tektites are incredibly dry. How did all the water, present everywhere on earth, get driven out? This is one of the fundamental problems in tektite formation, one of those “interesting” questions that people ignore.

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Lengthy discussion of water on the moon found in ** Astronomical Review, Volume 6, Issue 7, Arlin Crotts 2011. I didn’t finish reading it and the URL kept blowing up my draft. Look at your own peril.

UPDATE

A couple of paragraphs from the above paper with footnote [12] included.

Urey suffered sharp criticism for his openness to ideas of water on the Moon. It wore on him, so much that he confessed (in a letter to the prestigious journal Nature) that some thought him under the influ- ence of more intoxicating liquids. [10] The most substantial attack came from NASA planetary scientist John O’Keefe (who suffered his own controversy regarding the lunar origin of glassy globules called tektites, now thought to ejected from large terrestrial impacts). O’Keefe showed how an ice layer under the lunar surface would distort and flow like a glacier if it exceeded a kilometer thickness, in contradic- tion to craters two kilometers deep absent signs of their walls flowing onto their floors. [11] The criti- cism is not definitive: a thinner layer would not deform but might conceivably produce effects Urey claimed in Ranger photos. Urey grew haggard defending himself and authors of similar ideas. [12]

[12] Urey retorted to O’Keefe: “All right, attack if you wish to. This is, so far as I recall, my suggestion, not that of my good friend, T. Gold. Possibly Lingenfelder et al. considered some modification of this idea. I am not at all convinced that Gold’s mechanism may not contribute to the problem to some extent.” (“Water on the Moon” by Harold C. Urey, 1969, Science, 164, 1088.) Lingenfelter et al. postulated that a 1-kilometer thick ice layer on the Moon, shielded from sublimation into the vacuum by a 100-meter overburden of regolith, could melt when impacted by a meteorite and flow underground to form rilles as in Figure 2.

This changed with the first samples returned from the Moon, with almost no signs of hydrated minerals. The only layered lunar rocks result from layered lava. Hadley Rille, visited by Apollo 15, was obviously made by flowing lava, not water. Urey relented, abandoning any idea of lunar water. [13] Eventually, Urey and O’Keefe published scientific papers together but none about lunar hydration. Urey died in 1981, long before minds began to change regarding water on the Moon.