Helium Abundance in the Atmosphere of Jupiter and Saturn

The Sun and giant planets are generally thought to have the same He abundance as that in the solar nebula from which they were formed. During the Voyagers' deep space cruises, helium abundance derived for the outer planets suggested that processes in the interior of Jupiter and Saturn may have substantially modified the original ratio as observed for the Sun. Galileo probe measured the helium abundance in situ the atmosphere of Jupiter, showing that He is only slightly depleted compared to the solar value, in contradiction with initial Voyager data analysis. For Saturn, contradictory estimates thus far reported from past Voyager observations make its He abundance very uncertain. Here, we use the He first resonance line (584Å ) dayglow measured from the outer planets by the Voyager ultraviolet spectrometers (UVS) to derive the helium abundance in the atmosphere of Jupiter and Saturn. The He 584Å dayglow corresponds to solar He emission line back-scattered by the upper atmosphere of planets. Our finding is that the [He/H2] mixing ratio is ∼ 0.16±0.03 and ∼ 0.15±0.025 respectively for Jupiter and Saturn. These abundances correspond to a helium mass fraction relative to the total helium and hydrogen Y ∼ 0.24±0.04 for Jupiter and Y ∼0.23± 0.03 for Saturn.The He abundance derived here for Jupiter and Saturn are consistent with Galileo in situ measurement, suggesting little change in the primordial composition of the giants' interior since their formation from the solar nebula.