Superionic ammonia in the lab sheds light on magnetic fields of Uranus and Neptune

Exploring Uranus and Neptune in the lab

Current models assume that the mantle of these icy giants is made up of water. However, their unusual magnetic fields suggest the presence of conducting fluids. Some scientists believe that under these planets' hydrogen and helium-filled atmosphere, there is a mantle made up of water, methane, and ammonia.

The water and ammonia are hypothesized to be in a superionic state under the extremely high pressures and temperatures inside the planets. Replicating these superionic states has been attempted earlier.

Researchers at the Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) used a static compression approach where they compressed ammonia between two anvils made from diamond or sapphire but could reach the superionic state.

Scientists at LULI used dynamic compression and created a shock wave using a laser, which was also insufficient to achieve the superionic state.

Static and dynamic together

The teams at the two centers then got together to use both approaches. The anvils at IMPMC were used to precompress ammonia into liquid or even solid state. Then an intense laser beam sent a shockwave to send ammonia into a superionic ice phase.

In this phase, ammonia displays properties that are common to solids as well as liquids. The nitrogen atoms in the molecule organize like a solid, while hydrogen atoms move around more chaotically.

The researchers also found a pattern of temperature versus pressure where the temperature increased very little even as pressure continued to rise. Once the pressure rose above a certain level, the temperature rose again. The team believes this pattern is consistent with phase transformation, where energy supplied is used to change state rather than increase temperature.

Putting together the data from these experiments with their previous work, the researchers are confident that above pressures of 100 gigapascals (1 gigapascal = 10,000 atmospheric pressures), superionic ammonia ice melts at temperatures lower than superionic water ice.

This can explain the melting of ammonia-rich regions inside the mantle of the two planets and the unusual magnetic activity that a water-only mantle cannot. The team also found that superionic ammonia has higher electrical conductivity than water inside these planets.

The research findings were published in the journal Nature Physics.

Abstract

Under high pressures and temperatures, molecular systems with substantial polarization charges, such as ammonia and water, are predicted to form superionic phases and dense fluid states with dissociating molecules and high electrical conductivity. This behaviour potentially plays a role in explaining the origin of the multipolar magnetic fields of Uranus and Neptune, whose mantles are thought to result from a mixture of H2O, NH3 and CH4 ices. Determining the stability domain, melting curve and electrical conductivity of these superionic phases is therefore crucial for modelling planetary interiors and dynamos. Here we report the melting curve of superionic ammonia up to 300 GPa from laser-driven shock compression of pre-compressed samples and atomistic calculations. We show that ammonia melts at lower temperatures than water above 100 GPa and that fluid ammonia’s electrical conductivity exceeds that of water at conditions predicted by hot, super-adiabatic models for Uranus and Neptune, and enhances the conductivity in their fluid water-rich dynamo layers.