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The sun and giant planets are generally thought to have the same heliumabundance as that in the solar nebula from which they were formed 4.6 billionyears ago. In contrast, the interstellar medium reflects current galacticconditions. The departure of current abundances from the primordial andprotosolar values may help trace the processes that drive the nucleosynthesisevolution of the galaxy and planetary interior formation and evolution. TheGalileo probe measured the He abundance in situ the atmosphere of Jupiter,showing that He is only slightly depleted compared to the solar value. ForSaturn, contradictory estimates from past Voyager observations make its Heabundance very uncertain. Here, we use He 58.4 nm dayglow measured from theouter planets by the Voyager ultraviolet spectrometers to derive the Heabundance in the atmosphere of Jupiter and Saturn. We also use the solar He58.4 nm line measured by the Solar Heliospheric Observatory to derive the Heabundance inside the focusing cone. Finally, we compare He abundances derivedhere with primordial and protosolar values, stressing the unique opportunityoffered by inner heliosphere observations and future Voyager in situ localinterstellar medium measurements to derive the He abundance in the veryinterstellar cloud in which we reside.

Helium abundance in giant planets and the local interstellar medium