The helium abundance of Uranus from Voyager measurements

Voyager radio occultation and infrared spectroscopy measurements are combined to infer a helium mole fraction in the upper troposphere of Uranus of 0.152 ± 0.033; the corresponding mass fraction is Y = 0.262 ± 0.048. This value is in agreement with recent estimates of the solar helium abundance, suggesting that helium differentiation has not occurred on Uranus. Comparisons with values previously obtained for Jupiter and Saturn imply that migration of helium toward the core began long ago on Saturn and may also have recently begun on Jupiter. The protosolar helium abundance inferred from the Uranus measurements and from recent solar evolutionary models is used along with an assumed primordial helium mass fraction of 0.23–0.24 to estimate a 3–4% enrichment of helium in the interstellar medium between the big bang and the origin of the solar system. The result is in agreement with galactic chemical evolution models which include a substantial decrease in deuterium during the evolutionary process.