Helium abundance in giant planets and the local interstellar medium

TL;DR

This study measures helium abundance in giant planets and the local interstellar medium using ultraviolet observations, providing insights into planetary formation, galactic evolution, and the current state of the interstellar environment.

Contribution

It introduces a novel method of deriving helium abundance from Voyager ultraviolet data and compares planetary and interstellar helium levels with primordial values.

Findings

Jupiter's helium abundance is slightly depleted compared to the solar value.

Saturn's helium abundance remains uncertain due to past measurement discrepancies.

Inner heliosphere observations offer a unique opportunity to assess interstellar helium abundance.

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. 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.