The evolution of Jupiter and Saturn with helium rain

Studying the interiors of giant planets is important for understanding their origin. While accurate measurements of gravity field and seismology from Juno and Cassini teach us about the present-day internal structures of Jupiter and Saturn, further insights can be gained from simulating their evolution. Understanding the long-term thermal evolution of the giant planets is also critical to connect the planetary formation and the current-state structure of planets.

In Jupiter and Saturn, a phase separation between hydrogen and helium occurs, resulting in the formation of helium droplets which settle towards the deep interior. This phenomenon, also known as helium rain, can strongly affect their thermal evolution. Therefore, studying the evolution of Jupiter and Saturn, and giant planets in general, requires a good comprehension of the phase diagram of hydrogen and helium. However, the latter remains uncertain given the discrepancy between various theoretical calculations and experimental data.

I will present results of evolution models of Jupiter and Saturn with helium rain, using different phase diagrams. We find that a consistency between Jupiter's evolution and the Galileo measurement of its atmospheric helium abundance is achieved only if a shift in temperature in the existing phase diagrams is applied. We next use the shifted phase diagrams to model Saturn's evolution and find consistent solutions for both planets. We confirm that demixing in Jupiter is modest while in Saturn, the process of helium rain is significant. I will discuss the inferred structure of both planets, the inferred atmospheric helium content in Saturn and show the importance of equations of state and atmospheric models on evolution models. This analysis reveals key information on the interiors of giant planets and can also be used to constrain the phase diagram of hydrogen and helium.