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According to the following article, scientists have known the lunar surface contains deposits of helium-3 since the Apollo program. The Chinese Chang'e 5 mission appears to confirm it after returning from the moon with a new mineral containing the isotope. This raises a lot of questions. How is it possible to find any amount of helium-3 on the moons surface? It seemingly contracts what I've learned from science.

First, helium is a noble gas in all its isotopic forms, making it incapable of chemically bonding with other elements. Wouldn't any amount of it would exist as an isolated gas? Second, given the moon is about 1/6th of the earth's gravity due to its small mass. Wouldn't such a light gas as helium quickly dissipate out into outer space as would hydrogen on earth?

It could be argued that trace quantities of helium-3 could be trapped inside crystal or other molecular structures of the moon's regolith. helium-3, however, is a small atom. Wouldn't it easily escape the confines of a molecular lattice, especially when it's chemically unbounded to the structure?

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    $\begingroup$ A helium-3 atom is lighter, not smaller than helium-4 one. Finding He inside rocks with alpha decaying elements is common. $\endgroup$
    – Poutnik
    Commented Nov 23, 2022 at 17:29
  • $\begingroup$ Here is a source (the source, perhaps): thehill.com/opinion/technology/… $\endgroup$
    – Karsten ♦
    Commented Nov 23, 2022 at 19:37
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    $\begingroup$ @Poutnik, According to Karsten's article, the amount of Helium-3 is greater than here on earth, but I don't understand how the moon could have more of the alpha decaying elements than the earth. Being smaller, there's just less alpha decaying elements available. Moreover, it still doesn't explain how Helium can even accumulate in such quantities given the conditions on the moon. $\endgroup$
    – user148298
    Commented Nov 24, 2022 at 2:06
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    $\begingroup$ Note that I said He (implicitly helium-4) , not helium-3. Part of trapped helium-3 may be formed by cosmic ray spallation (forced fission), similarly as most of Earth beryllium and boron, as these rays are more intense on the Moon surface due lack of atmosphere and magnetic field. // but main probably by the solar wind too as Nilay Ghosh says, as fast nuclei are better projectiles than bullets. $\endgroup$
    – Poutnik
    Commented Nov 24, 2022 at 9:08
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    $\begingroup$ It is not helium as free gas to escape. $\endgroup$
    – Poutnik
    Commented Nov 24, 2022 at 18:13

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Unlike Earth, which is protected by its magnetic field, the Moon is always bombarded with large quantities of Helium-3 by the solar wind (Helium-3 is emitted from the Sun and carried throughout the Solar System by solar winds). Since the Moon has a weak magnetic field and the atmosphere is extremely thin, Helium-3 is deposited in significant quantities, getting embedded in the upper layer of regolith (ref.2 on how helium-3 gets trapped in the regolith, thanks to OP). You can find more information in ref.1

References:

  1. Estimated solar wind-implanted helium-3 distribution on the Moon by Jeffrey R. Johnson,Timothy D. Swindle,Paul G. Lucey, Volume 26, Issue 3, 1999 DOI: 10.1029/1998GL900305
  2. Taking advantage of glass: capturing and retaining the helium gas on the moon by Ao Li et al 2022 Mater. Futures 1 035101 DOI: 10.1088/2752-5724/ac74af
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