Mechanism responsible for the low abundance of Helium (He) Second lightest and second most abundant element (after Hydrogen) in the universe. The most abundant isotope is 4He (99.9998%), 3He is very rare. Helium comprises ~8% of the atoms (25% of the mass) of all directly observed matter in the universe. Helium is produced by hydrogen burning inside of Saturn’s atmosphere. Helium condenses in the upper layers to form a mist, which “rains” down toward Saturn’s interior, just as water vapor forms into rain in the atmosphere of Earth. Also called “helium rain.”
Some or all content above used with permission from J. H. Wittke.