Plastic deformation of superionic water ices

F. Matusalem, J. Santos Rego, M. de Koning, PNAS, 119 (45), e2203397119 (2022). DOI:

Superionic (SI) ices are high-pressure, high-temperature phases of water in which oxygen ions occupy a rigid crystalline lattice, whereas the protons diffuse in a liquid-like manner. They are believed to be abundant in the universe, in particular in the interiors of Neptune and Uranus, in which they are conjectured to constitute a thick solid mantle. Here, we investigate the mechanical deformation properties of these phases using state-of-the-art computational techniques. The results indicate that SI face-centered cubic ice is very malleable, suggesting that the rheology of the icy internal layers of these planets may be orders of magnitude faster than previously thought, possibly having important implications for the interior dynamics of Neptune and Uranus.

Due to their potential role in the peculiar geophysical properties of the ice giants Neptune and Uranus, there has been a growing interest in superionic (SI) phases of water ice. So far, however, little attention has been given to their mechanical properties, even though plastic deformation processes in the interiors of planets are known to affect long-term processes, such as plate tectonics and mantle convection. Here, using density functional theory calculations and machine learning techniques, we assess the mechanical response of high-pressure/temperature solid phases of water in terms of their ideal shear strength (ISS) and dislocation behavior. The ISS results are well described by the renormalized Frenkel model of ideal strength and indicate that the SI ices are expected to be highly ductile. This is further supported by deep neural network molecular dynamics simulations for the behavior of lattice dislocations for the SI face-centered cubic (fcc) phase. Dislocation velocity data indicate effective shear viscosities that are orders of magnitude smaller than that of Earth's lower mantle, suggesting that the plastic flow of the internal icy layers in Neptune and Uranus may be significantly faster than previously foreseen.

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