Helium rain is plausible inside Saturn but unlikely in Jupiter's warmer interior, according to research published in Nature Communications. Researchers mapped the planet's hydrogen-helium immiscibility boundary by computing composition-dependent chemical potentials from large-scale molecular dynamics driven by machine learning potentials.
Across a pressure range of 100 to 1,000 GPa, demixing temperatures were typically around 2,000 K lower than those reported in earlier ab initio simulations that used small system sizes. The authors reported that this discrepancy is larger than the estimated errors from density functional approximations, machine learning potentials, statistical uncertainties, and the neglect of nuclear quantum effects.
The team trained machine learning potentials on three density functional approximations: PBE, vdW-DF, and the hybrid functional HSE. To capture the thermodynamic driving force for phase separation, the researchers fitted the hydrogen-helium mixing free energy to a Redlich-Kister regular solution model, producing a predictive representation of the boundary.
The location of this boundary governs whether helium demixes from hydrogen in gas giants. By comparing their calculations to current planetary interior profiles, the authors found demixing occurs in Saturn but is avoided in Jupiter. The findings provide updated inputs for planetary models that couple demixing, heat transport, and composition gradients.
Xiaoyu Wang, S. Hamel, and Bingqing Cheng conducted the study across UC Berkeley, Lawrence Livermore National Laboratory, and the Institute of Science and Technology Austria. High-performance computing facilities were provided by ISTA, Berkeley Research Computing at UC Berkeley, and LLNL. Author Bingqing Cheng declared an equity stake in AIMATX Inc.
