The hydrogen to helium ratio in Jupiter and Saturn

The high gravity and low external temperatures of Jupiter and Saturn preclude the escape of light elements, even hydrogen. For this reason, it is logical to think that these planets possess the same chemical composition as the primitive nebula from which they were formed. Measurements of the H2/He ratio in the outer atmospheres of these planets should therefore provide the relative abundance of helium in the primitive nebula, the solar helium abundance, and, finally, an upper limit to the primordial abundance according to considerations of the nucleosynthesis theory. According to this theory, the present helium content of the universe is composed of a primordial component produced in the big bang and a small amount of helium subsequently produced in stars and brought to the interstellar medium by supernova explosions.