Plastic deformation of superionic water ices

Area of Science:

• Planetary Science
• Materials Science
• Geophysics

Background:

• Superionic (SI) water ice phases are crucial for understanding the geophysical properties of ice giants like Neptune and Uranus.
• The mechanical properties and plastic deformation of these deep planetary interiors remain under-explored.
• Deformation processes significantly influence long-term planetary evolution, including mantle convection.

Purpose of the Study:

• To investigate the mechanical response of high-pressure/temperature solid water phases, specifically superionic ices.
• To determine the ideal shear strength (ISS) and dislocation behavior of these exotic ice phases.
• To assess the implications for plastic flow within the interiors of ice giant planets.

Main Methods:

• Density functional theory (DFT) calculations were employed to determine mechanical properties.
• Machine learning techniques, including deep neural network molecular dynamics, were utilized.
• Analysis focused on ideal shear strength and lattice dislocation behavior in SI water ice.

Main Results:

• Superionic ices exhibit high ductility, as predicted by the renormalized Frenkel model for ideal strength.
• Deep neural network simulations revealed insights into dislocation dynamics in the SI face-centered cubic (fcc) phase.
• Calculated effective shear viscosities are significantly lower than Earth's lower mantle viscosity.

Conclusions:

• Superionic water ices are mechanically ductile and prone to plastic deformation.
• The plastic flow in the interiors of Neptune and Uranus may occur at rates substantially faster than previously estimated.
• These findings necessitate revisions to models of ice giant internal dynamics and evolution.