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The hydrogen-helium (H/He) immiscibility boundary, which dictates helium rain in giant planets, was mapped using large-scale molecular dynamics simulations with machine learning potentials trained on three density functional approximations (PBE, vdW-DF, and HSE). These simulations, which overcame system size limitations of previous ab initio methods, revealed demixing temperatures approximately 2000 K lower than prior estimates across 100-1000 GPa. Analysis suggests helium rain is plausible in Saturn but unlikely in Jupiter, refining planetary interior models.
Helium rain in gas giants may be less frequent than we thought, thanks to new simulations that significantly lower the estimated hydrogen-helium demixing temperatures.
The location of the hydrogen-helium (H/He) immiscibility boundary controls whether and where helium rain occurs in giant planets, yet it remains uncertain because high-pressure experiments are challenging and ab initio simulations are limited in system size and simulation time. We map this boundary by computing composition-dependent chemical potentials from large-scale molecular dynamics driven by machine learning potentials trained on three density functional approximations (PBE, vdW-DF, and the hybrid HSE). The three functionals yield consistent immiscibility boundaries, and the demixing temperatures are typically ~2000 K lower than previous ab initio simulations using small system sizes across the pressure range of 100-1000 GPa. Fitting the H/He mixing free energy to a Redlich-Kister regular solution model rationalizes the thermodynamic driving force for phase separation and provides a predictive representation of the boundary. Comparison with current planetary interior profiles indicates that helium rain is plausible in Saturn but unlikely in the warmer interior of Jupiter. Our results narrow the uncertainty in the H/He immiscibility boundary and provide inputs for planetary models that couple demixing, heat transport, and composition gradients in gas giants.
BAIR