Experimental Pathways for Detecting Double Superionicity in Planetary Ices

TL;DR

This paper proposes experimental pathways using dynamic compression and X-ray diffraction to detect double superionic states in planetary ices, advancing understanding of planetary interior properties.

Contribution

It introduces two novel experimental methods for generating and detecting double superionic states in planetary ices under extreme conditions.

Findings

Double superionic states can be reached with shock and ramp compression.

X-ray diffraction can detect the melting of heavy nuclei sublattice.

Conditions of ~3500 K and >200 GPa are necessary for H3NO4.

Abstract

The ice giant planets Uranus and Neptune are assumed to contain large amounts of planetary ices such as water, methane, and ammonia. The properties of mixtures of such ices at the extreme pressures and temperatures of planetary interiors are not yet well understood. Ab initio computer simulations predicted that a number of ices exhibit a hydrogen superionic state and a doubly superionic state [DOI: 10.1038/s41467-023-42958-0]. Since the latter state has not yet been generated with experiments, we outline here two possible pathways for reaching and detecting such a state with dynamic compression experiments. We suggest X-ray diffraction as the principal tool for detecting when the material becomes doubly superionic and the sublattice of one of the heavy nuclei melts. That would require a temperature of $\sim$3500 K and pressures greater than $\sim$200 GPa for H$_3$NO$_4$, which we use as an example material here. Such conditions can be reached with experiments that employ an initial shock that is followed by a ramp compression wave. Alternatively, one may use triple-shock compression because a single shock does not yield sufficiently high densities.