Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets

Deep beneath the icy shells of moons like Europa and Enceladus, or inside the vast, water-rich worlds orbiting distant stars, lies a layer of frozen water so thick that the pressure at its base is millions of times greater than the air pressure at sea level. In these crushing depths, water does not behave like the ice we know from a winter lake. Instead, it transforms into a dense, crystalline solid known as ice VII, a form of water that remains solid even at temperatures far above the boiling point of water at the surface. For decades, planetary scientists have operated under a simple assumption about what happens when salt dissolves in these deep oceans: as the water freezes into this high-pressure ice, the salt is pushed out, rejected by the crystal structure and left behind in the liquid ocean above. This idea has shaped our understanding of how these worlds are built, how their interiors move, and whether their hidden oceans could support life. If salt cannot enter the deep ice, then the oceans remain salty while the ice shells remain pure, creating a specific kind of density layering that drives the planet's internal currents.

However, a new study challenges this long-held view by revealing that under the right conditions, salt does not have to be left behind. Researchers have discovered that when a salty solution is frozen rapidly and then squeezed under immense pressure, the resulting ice can trap a significant amount of salt inside its crystal structure. This finding suggests that the deep interiors of water-rich planets may be far more chemically complex than previously thought, with salty ice acting as a hidden reservoir that could alter how heat and material move through these worlds. The study focuses on sodium chloride, the same common table salt found in Earth's oceans, and demonstrates that it can become an integral part of the ice itself, rather than just an impurity sitting between the crystals.

The team, led by researchers from institutions across Europe, set out to test what happens to salt when it is subjected to the extreme conditions found deep inside icy moons. They began with a concentrated solution of salt and water, similar to the brine found in Earth's oceans but much stronger. Instead of simply freezing this mixture slowly, which would naturally push the salt out, they used a technique called "quench-and-compress." First, they cooled the liquid so quickly that it turned into a glass-like, disordered solid without forming any crystals. Then, while keeping the sample frozen, they placed it inside a device capable of generating pressures far higher than anything found on Earth's surface. As they squeezed this amorphous, glassy salt-water mixture, they warmed it back to room temperature. This specific path—freezing fast, then squeezing—allowed the salt ions to get trapped inside the forming ice structure before the crystal had a chance to reject them.

When the researchers examined the resulting material using powerful X-ray beams and neutron scattering, they found something unexpected. The ice that formed was not the standard, pure ice VII. Instead, it was a new, expanded version of the crystal that contained no separate pockets of salt. The salt ions, sodium and chlorine, had taken the place of water molecules within the ice lattice, creating a solid solution where the salt was woven directly into the fabric of the ice. This new form of ice had a unit cell, the basic repeating building block of the crystal, that was about eight and a half percent larger than pure ice VII. The structure was highly disordered, with the salt ions causing the surrounding water molecules to twist and reorient themselves to accommodate the foreign atoms. Crucially, there was no sign of separate salt crystals or salt hydrates; the salt was fully integrated into the ice.

The researchers then tested how stable this salty ice really was. They slowly reduced the pressure on the sample while keeping the temperature constant. At a pressure of roughly 5.1 gigapascals, a dramatic change occurred. The salty, expanded ice suddenly expelled the salt, which crystallized into separate, solid salt grains. The ice returned to its pure form, and the salt was left behind as a distinct mineral. When they tried to squeeze the sample again to recreate the salty ice, it did not happen. The salt remained separate, refusing to re-enter the ice structure. This revealed that the formation of this salty ice is not a simple, reversible process that happens automatically at high pressure. Instead, it depends entirely on the history of the material: it only forms if the salt is trapped in a disordered state before the ice crystallizes. Once the salt is expelled, it cannot easily be forced back in.

This discovery has profound implications for how we understand the interiors of water-rich planets. Because the salty ice is less dense than a mixture of pure ice and separate salt crystals, it can float more easily within the deep mantle of a planet. This means that salty ice could be carried upward by the slow, churning motion of the planet's interior, transporting salt from the deep rock interface toward the surface ocean. However, as this salty ice rises and the pressure drops, it will eventually reach the critical threshold of 5.1 gigapascals. At this point, the ice will release its salt, which will then sink back down because it is denser than the surrounding ice. This process acts as a filter, allowing some salt to travel upward but trapping a significant portion of it at a specific depth.

The study suggests that the chemical composition of subsurface oceans on moons and exoplanets might be controlled by this pressure-induced filter. If the deep ice can hold onto salt, the oceans above might be less salty than models predict. Conversely, if the ice releases salt at a specific depth, it could create a layer of dense, salty material that inhibits further mixing. The researchers note that while their experiments used very high salt concentrations to make the effect clear, the principle likely applies to the more dilute brines found in nature, though the exact depth of this filtering effect would depend on the specific temperature and salt levels of each world. By showing that ice VII can act as a host for salt under certain conditions, the study forces a rethinking of how chemical elements move through the deep interiors of the solar system's most mysterious worlds. The deep ice is not just a passive, salt-excluding barrier; it is a dynamic participant in the chemical evolution of these planets, capable of trapping, transporting, and releasing salt in ways that were previously unimagined.