Ammonia and hydrogen may separate inside Uranus and Neptune

Simulations show that ammonia and hydrogen can separate into distinct layers under some planetary conditions. The boundary between mixing and separation crosses expected temperature profiles inside Uranus and Neptune, suggesting ammonia may be raining out there. In warmer sub-Neptune TOI-270 d, the two substances are likely to remain mixed.

Star system: TOI-270 | Distance from the Sun: 73.3 light years

Ammonia and hydrogen can separate into distinct, hydrogen-rich and ammonia-rich phases under conditions expected inside some planets. Simulations indicate that the boundary between mixing and separation crosses published temperature profiles for Uranus and Neptune. This suggests ammonia may be raining out inside both planets today, moving from their hydrogen-rich envelopes into ammonia-rich regions.

The simulations covered temperatures from 500 to 4,000 kelvins and pressures from 0.1 to 100 gigapascals. A gigapascal is a billion pascals, a unit of pressure. The calculations used machine-learning models of how atoms interact, trained on quantum-mechanical calculations, and simulated two phases in contact. This approach allowed the researchers to map the conditions where ammonia and hydrogen mix completely or separate.

The temperature marking the transition between mixing and separation rises with pressure, from 500 kelvins at 0.1 gigapascals to about 2,000 kelvins near 30 gigapascals. Above roughly 30 gigapascals and 2,000 kelvins, the boundary becomes nearly flat as molecules begin to break down into atoms. The simulations found no separation at 3,000 or 4,000 kelvins, even at pressures up to 100 gigapascals. For a warm sub-Neptune such as TOI-270 d, ammonia and hydrogen are likely to mix in all proportions. The results also support a sequence in which silicates, water and then ammonia separate from hydrogen as planets cool, affecting their composition, heat flow and internal structure.

Key findings
Ammonia and hydrogen can form separate phases across parts of the simulated range of 500 to 4,000 kelvins and 0.1 to 100 gigapascals.
The transition temperature increases from 500 kelvins at 0.1 gigapascals to about 2,000 kelvins near 30 gigapascals.
No phase separation occurs at 3,000 or 4,000 kelvins in the simulations, even up to 100 gigapascals.
The phase boundary crosses expected temperature profiles inside Uranus and Neptune, suggesting ammonia may be raining out there.
Ammonia and hydrogen are likely to mix in all proportions in warm sub-Neptune TOI-270 d.

Scientific study

Ammonia-Hydrogen Phase Equilibria in Planetary Envelopes from First-Principles-Based Machine-Learning Molecular Dynamics (external link)