Fluid-like elastic response of superionic NH ₃ in Uranus and Neptune

Abstract

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 (Vp) by ∼9% and ∼20% relative to the molecular solid in the temperature range of 1,500 K and 2,000 K above 47 GPa. While the first Vp reduction observed at the boundary to the superionic α 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 γ phase, exhibiting the higher diffusivity. The reduction rate of Vp in the superionic γ 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.

In Situ Measurement Techniques Using Diamond Anvil Cell at High Pressure–Temperature Conditions: A Review

“…Brillouin spectroscopy facilitates the measurement of acoustic vibration frequencies [78] in a sample through nonelastic light scattering, thereby facilitating the direct determination of parameters such as adiabatic sound velocity, elastic modulus, and pressure-induced phase transition. [79] Moreover, when combined with the LH-DAC technology, Brillouin measurements offer microscopic diagnostics [80,81] of structural transformations under high pressures and temperatures. Further, the refractive index of the sample can be obtained from the Brillouin data.…”

Anomalous density, sound velocity, and structure of pressure-induced amorphous quartz

“…An Ar laser with a wavelength of 532 nm was used as the probe beam focused to a spot size of about 10 μm. The scattered light was analyzed through a Sandercock-type 6-pass tandem Fabry-Perot interferometer as described elsewhere [51]. The collection geometry for the Brillouin experiments was set at a scattering angle of 50°and carefully measured using a standard reference BK7 glass with known sound-wave velocities at ambient conditions [52].…”

Plastic deformation of superionic water ices

“…Even so, such crystal plasticity effects are known to affect long-term geophysical processes, such as plate tectonics and mantle convection on Earth ( 30 , 31 ), and are also expected to be involved in the evolution of the internal structure of Neptune and Uranus. Indeed, the ease/difficulty of plastic flow under the influence of external stresses should directly affect the dynamics of their solid mantles, playing a potentially important role in such questions as to whether or not these ice masses may contribute to the formation of stratified, nonconvective layers ( 32 ), envisioned to be involved in the magnetic anomalies of these ice giants ( 25 , 26 ).…”