Helium in Jupiter's atmosphere: Results from the Galileo probe Helium Interferometer Experiment

On December 7, 1995, the NASA Galileo probe provided the first in situ measurements of the helium abundance in the atmosphere of Jupiter. Our Jamin interferometer measured precisely the refractive index of the Jovian atmosphere in the pressure region from 2 to 12 bars. From these measurements, we derive the atmospheric helium mole fraction to be 0.1359 +/- 0.0027. The corresponding helium mass fraction matches closely, but accidentally, the current helium abundance of the atmosphere of the Sun. However, both the Jovian and the solar value fall somewhat below the protosolar value. This suggests that in both Jupiter and the Sun processes are active which separate helium from hydrogen.