Laser-driven shock experiments on precompressed water: Implications for “icy” giant planets

Abstract

Laser-driven shock compression of samples precompressed to 1 GPa produces high-pressure-temperature conditions inducing two significant changes in the optical properties of water: the onset of opacity followed by enhanced reflectivity in the initially transparent water. The onset of reflectivity at infrared wavelengths can be interpreted as a semiconductor↔ electronic conductor transition in water, and is found at pressures above ϳ130 GPa for single-shocked samples precompressed to 1 GPa. Our results indicate that conductivity in the deep interior of "icy" giant planets is greater than realized previously because of an additional contribution from electrons.

Mixtures of planetary ices at extreme conditions

“…Intermolecular models used so far to determine the equations of state of methane-water fluid mixtures are based on rigid molecules 7 and may be unable to describe mixing above a few GPa. At more extreme conditions, we find that the mixture becomes electronically conducting at pressures below 120 GPa, where pure water is reported to be still an opaque semiconductor 17 . Because electronic conductivity generally dominates over the ionic contribution, the source of the planetary magnetic fields observed at Uranus and Neptune may be shallower than currently inferred from conductivity data on pure substances, in agreement with revised planetary models 24 .…”

“…The transient formation of H 2 molecules is also observed. The conductivity of pure water at similar conditions is known to be far from metallic at these conditions 16,17 . However, the density of electronic states extracted from our simulations indicates an almost vanishing excitation energy already at 50 GPa, and full gap closure at 120 GPa (Fig.…”

“…According to conductivity measurements on pure H 2 O (ref. 17), the second and third conditions correspond to a semiconducting behaviour, whereas the fourth condition corresponds to an electronic conductor.…”

“…For example, diamond formation in icy planets is inferred from studies based on hydrocarbon dissociation in the absence of water [19][20][21]23 . Similarly, estimates of the electrical conductivity in the planetary cores are essentially based on studies on pure water 2,[13][14][15][16][17] , with the only exception of a study carried out on the mixture up to 77 GPa (ref. 13), which is at the lower end of the region of pressure where water is dissociated.…”

Implications of Shock Wave Experiments With Precompressed Materials for Giant Planet Interiors

“…This impasse can now be addressed with a new experimental technique [8] that combines static and dynamic compression. By first compressing the sample statically in a diamond anvil cell, the starting density can be sufficiently increased so that a subsequent shock experiment yields equation of state data along a different Hugoniot curve at higher density.…”