Atmospheric Helium Abundances in the Giant Planets

TL;DR

This study models atmospheric helium depletion in giant planets using phase diagrams and equations of state, revealing different internal structures and the importance of in-situ measurements for understanding their composition.

Contribution

It applies H/He phase diagrams and EOS models to explain helium depletion, offering new insights into the internal structures of Uranus, Neptune, and Saturn.

Findings

Strong helium depletion in ice giants if adiabatic.

Weak depletion in Uranus with a thermal boundary layer.

Possible dissimilar internal structures between Uranus and Neptune.

Abstract

Noble gases are accreted to the giant planets as part of the gas component of the planet-forming disk. While heavier noble gases can separate from the evolution of the hydrogen-rich gas, helium is thought to remain at the protosolar H/He ratio Yproto~0.27-0.28. However, spacecraft observations revealed a depletion in helium in the atmospheres of Jupiter, Saturn, and Uranus. For the gas giants, this is commonly seen as indication of H/He phase separation at greater depths. Here, we apply predictions of the H/He phase diagram and three H/He-EOS to compute the atmospheric helium mass abundance Yatm as a result of H/He phase separation. We obtain a strong depletion Yatm<0.1 for the ice giants if they are adiabatic. Introducing a thermal boundary layer at the Z-poor/Z-rich compositional transition with a temperature increase of up to a few 1000 K, we obtain a weak depletion in Uranus as observed. Our results suggest dissimilar internal structures between Uranus and Neptune. An accurate in-situ determination of their atmospheric He/H ratio would help to constrain their internal structures. This is even more true for Saturn, where we find that any considered H/He phase diagram and H/He-EOS would be consistent with any observed value. However, some H/He-EOS and phase diagram combinations applied to both Jupiter and Saturn require an outer stably-stratified layer at least in one of them.