Helium abundance in giant planets and the local interstellar medium
  The sun and giant planets are generally thought to have the same helium
abundance as that in the solar nebula from which they were formed 4.6 billion
years ago. In contrast, the interstellar medium reflects current galactic
conditions. The departure of current abundances from the primordial and
protosolar values may help trace the processes that drive the nucleosynthesis
evolution of the galaxy and planetary interior formation and evolution. The
Galileo probe measured the He abundance in situ the atmosphere of Jupiter,
showing that He is only slightly depleted compared to the solar value. For
Saturn, contradictory estimates from past Voyager observations make its He
abundance very uncertain. Here, we use He 58.4 nm dayglow measured from the
outer planets by the Voyager ultraviolet spectrometers to derive the He
abundance in the atmosphere of Jupiter and Saturn. We also use the solar He
58.4 nm line measured by the Solar Heliospheric Observatory to derive the He
abundance inside the focusing cone. Finally, we compare He abundances derived
here with primordial and protosolar values, stressing the unique opportunity
offered by inner heliosphere observations and future Voyager in situ local
interstellar medium measurements to derive the He abundance in the very
interstellar cloud in which we reside.
