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

Nondipolar magnetic fields exhibited at Uranus and Neptune may be derived from a unique geometry of their icy mantle with a thin convective layer on top of a stratified nonconvective layer. The presence of superionic H2O and NH3 has been thought as an explanation to stabilize such nonconvective regions. However, a lack of experimental data on the physical properties of those superionic phases has prevented the clarification of this matter. Here, our Brillouin measurements for NH3 show a two-stage reduction in longitudinal wave velocity (V-p) by similar to 9% and similar to 20% relative to the molecular solid in the temperature range of 1,500 K and 2,000 K above 47 GPa. While the first V-p reduction observed at the boundary to the superionic alpha phase was most likely due to the onset of the hydrogen diffusion, the further one was likely attributed to the transition to another superionic phase, denoted gamma phase, exhibiting the higher diffusivity. The reduction rate of V-p in the superionic gamma phase, comparable to that of the liquid, implies that this phase elastically behaves almost like a liquid. Our measurements show that superionic NH3 becomes convective and cannot contribute to the internal stratification.