Fluid-like elastic response of superionic NH3 in Uranus and Neptune

Abstract

Significance H 2 O and NH 3 are representative materials comprising the mantle of ice giant planets. While H 2 O probably exists as the superionic state, in which hydrogen rapidly diffuses through oxygen sublattices, in the deep mantle of Uranus and Neptune, the stability field and physical properties of the superionic phase of NH 3 are poorly understood. Our Brillouin study for NH 3 suggested a new superionic phase exhibiting a longitudinal wave velocity comparable to that of the liquid, which is stable in the mantle of Uranus and Neptune at pressures higher than ∼50 GPa. We suggest that the superionic NH 3 is nonviscous and thus cannot contribute to the internal stratification responsible for generation of the nondipolar magnetic fields observed at the icy planets.