Excuse me while I point and laugh. 4Helium is 28 ppm of lunar regolith, which I admit is better than the numbers for lunar 3He. It's about 500 ppm in Earth's atmosphere, iirc. Which makes more sense as a source, the 28 ppm in the baked regolith of a hostile alien world on the other side of a delta vee gulf, or the 500…