I'm using Chaisson'/McMillan's "Astronomy, a Beginner's Guide to the Universe"/7th Edition In Chapter 7, it describes the gas giants, and says that Jupiter, Uranus, and Neptune all have "86% Hydrogen, 14% Helium" in their atmospheres, while Saturn has about 92% hydrogen, and 7% Helium, in its atmosphere. As an explanation, it offers that on Saturn, Helium liquifies, and condenses toward the center of the planet. If that's accurate, it would have to mean that Helium must undergo an inelastic collision in the core... What mechanism could cause that? Helium is a noble gas, so it doesn't interact easily with other chemicals. It is also is incredibly stable as an isotope, so it isn't going to interact easily with other nucleons. How does Helium get trapped towards the surface or core of Saturn? Also, Saturn is an outlier as far as densities go. 710 kg/cubic meter for saturn, as opposed to (1300, 1200, and 1700) for Jupiter, Uranus, and Neptune, respectively. How strong is their modeling for Helium rain? Should an alternative hypothesis be explored that the bulk of Saturn might actually be captured from a more ancient solar system? Or has such an alternative hypothesis already been explored and rejected?