Abstract
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. With the He abundance reported here, credible models become possible for the interior of Jupiter, Saturn, and extrasolar planets. 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.
Highlights
According to the standard Big Bang Nuclueosynthesis (BBN) model, the universe was dense and hot at the beginning of its expansion phase
4 Conclusions We reviewed the helium abundance at different sites of the universe starting with the Big Bang era, considered the protosolar nebula and giant planet composition, and examined the current local interstellar medium
From the studies far conducted on different astrophysical sites [1, 2, 3], the first lesson learned is that an accurate determination of the He abundance strongly constrains the key processes that control the formation of the astrophysical objects from the primordial universe down to the current sun and giant planets
Summary
According to the standard Big Bang Nuclueosynthesis (BBN) model, the universe was dense and hot at the beginning of its expansion phase. Because non-primordial He and heavy metals could only be produced in stars in contrast to D that is consumed, regions of poor metallicity could reflect the He condition not far from that which prevailed at the end of the expansion phase This simple idea fueled the study of a collection of metal-poor extragalactic H II targets to obtain the He abundance versus the gas metallicity, usually expressed as the O I/H I ratio [2]. To derive the He abundance, we adjust the atmospheric He mixing ratio fHe until our radiation transfer model (RT) of the He 58.4nm sun-reflected emission line of the planet matches the corresponding observed brightness. Saturn (1.5-2.5) 107 a Integrated solar flux at the time of the V1 observation
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