# Why exactly would “permanently shadowed polar lunar craters… have substantially higher levels of ³He than sunlit lunar surfaces?”

Researching this answer led to ³He in permanently shadowed lunar polar surfaces published in Icarus. The abstract is tantalizing but terse:

Abstract

Because of their cryogenic temperatures, analysis indicates that permanently shadowed polar lunar craters may have substantially higher levels of ³He than sunlit lunar surfaces and are conservatively estimated to contain as much as 50 ppb or more.

Wikipedia's Helium-3; Solar nebula (primordial) abundance says:

One early estimate of the primordial ratio of ³He to ⁴He in the solar nebula has been the measurement of their ratio in the atmosphere of Jupiter, measured by the mass spectrometer of the Galileo atmospheric entry probe. This ratio is about 1:10,000, or 100 parts of ³He per million parts of ⁴He. This is roughly the same ratio of the isotopes as in lunar regolith, which contains 28 ppm helium-4 and 2.8 ppb helium-3 (which is at the lower end of actual sample measurements, which vary from about 1.4 to 15 ppb).

Question: Why exactly would "permanently shadowed polar lunar craters... have substantially higher levels of ³He than sunlit lunar surfaces?" What is it exactly about permanently shadowing surfaces from the Sun that is thought to allow them to accumulate up to 50 ppb of helium-3 compared to a lunar average of only 2.8 ppb?

Is it the far lower temperature, or the shielding from the solar wind, or something else?

Possibly relevant factoid, the boiling point of helium-3 is only about 3.2 Kelvin, much lower than the 4.2 Kelvin of helium-4.

• Parts per billion are actually very low concentrations. – user8269 Aug 28 at 3:52
• @PhilipNgai compared to what? – uhoh Aug 28 at 3:52
• Compared to any real source of energy. – user8269 Aug 28 at 3:54
• @PhilipNgai I'm just interested in the dynamics of particles from the Sun interacting with lunar regolith. – uhoh Aug 28 at 3:57
• @uhoh I stand corrected; got a little too fast and loose with my terminology there. – Roger Sep 26 at 15:51

• -1 for unsourced, unsupported commentary. Helium-3 storage in regolith is a very complex process, it's not just stuck to the surface, it can be embedded inside the particles as well. So far you haven't convinced me if it's strictly thermal or if there are sputtering effects that come into play as well. Shielding from sunlight also means shielding from solar wind. Just repeating points in my question is not an answer to my question. Can you have a second look at what I've written after Question: and see if you can address that in more detail? Thanks! – uhoh Aug 27 at 14:50