Take water. Add sodium chloride. Chill and squeeze into salty ices.
Scientists have discovered two new forms of salty ice that probably do not exist naturally on Earth but might be found on icy moons farther out in the solar system.
“These structures are nothing like anything that has been described before,” said Baptiste Journaux, an acting assistant professor of earth and space sciences at the University of Washington.
Writing in the Feb. 20 issue of The Proceedings of the National Academy of Sciences, Journaux and his colleagues describe two new solid, icy combinations of two of the most common substances found on Earth: water and sodium chloride, better known as table salt.
The newly discovered crystals formed when salty water was chilled to low temperatures and squeezed to high pressures.
Saltwater is plentiful on Earth and chemists have long known how it behaves under Earth conditions. Ice on this planet is rarely salty.
Indeed, sodium chloride (each molecule consists of one sodium atom and one chlorine atom) is often thought of first as an antifreeze, lowering the freezing temperature of water. That’s why it is spread on roads during snowstorms. When salty water does freeze, the ice crystals that form are made of pure water with the sodium and chloride ions pushed out into the remaining liquid.
At cold enough temperatures, the residual supersalty water begins to solidify, forming hydrohalite, a rigid, water- containing crystal or hydrate. Hydrohalite consists of two water molecules for each sodium chloride.
In recent decades, planetary scientists have discovered a slew of worlds in the outer solar system that possess liquid water oceans under their icy crusts. Those include Europa and Ganymede, two moons of Jupiter, and Titan and Enceladus, two moons of Saturn. Journaux wanted to study the role that salt might play in keeping the oceans on these worlds from freezing.
To reproduce those conditions, a smidgen of salty water was chilled to temperatures as low as minus 190 degrees Fahrenheit and squeezed between two pieces of diamond to pressures up to 25,000 times the usual 14.7 pounds per square inch that air presses against us at the Earth’s surface.
“Initially we did these experiments because we wanted to study the antifreeze effects of sodium chloride, of salt, because it is predicted as probably the most common solute in extraterrestrial oceans as it is in Earth’s oceans,” Journaux said. “We were expecting to see something somewhat similar to what we see on Earth, which is the salts would be rejected from the ice as it grows.” Instead, the antifreeze froze.
“We had a new crystal that came out of nowhere that we were not at all expecting,” Journaux said. “So that was very serendipitous.”
The crystals were tiny, roughly the width of a human hair. Bouncing X-rays off the crystals showed the scientists that they had created two new hydrates. One had a crystal structure of two sodium chloride molecules for every 17 water molecules. That one formed at a temperature of about minus 100 degrees Fahrenheit and a pressure of 5,000 times the usual atmospheric pressure. At higher pressures, yet another, less salty hydrate formed, one with 13 water molecules for every sodium chloride molecule.
The scientists also saw signs of a third form, but the needlelike crystals were too thin to study the crystal structure. “It’s very pretty,” Journaux said, “but it’s so thin, it’s hard to get the data.”
The new hydrates could help explain a mystery on Europa. Observations in 2019 using the Hubble Space Telescope identified sodium chloride in yellowish streaks on the moon’s surface.
It is highly unlikely to be in the form of grains of pure salt, but other observations — colors of infrared light absorbed by the surface, which serve as identifying fingerprints of specific compounds — offered
no convincing signs of hydrohalite, the known salt hydrate.
The scientists showed that the new hydrate that formed at 5,000 times atmospheric pressure remained stable after the pressure was removed and at perhaps temperatures as warm as minus 40 degrees Fahrenheit. That suggests this hydrate could have formed in Europa’s subsurface and would remain in that form if it were pushed to the surface.
“We have long known that some sort of material is mixed in with the water ice,” said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology who was one of the scientists who made the Hubble observations identifying sodium chloride on Europa. “And we have long suspected that it might actually just be salt derived from the interior ocean, but we’ve never quite been able to get a good fit. Maybe it is this new form of salt.”
Sodium chloride “is one of the simplest and most understood things in the world,” Brown said. “And yet Journaux just discovered a new form of it that had never been seen before.”
The stability of the hydrate also suggests that there may be a way to create it without the high pressures, perhaps enabling the growth of larger crystals. That could lead to experiments that would measure the absorption of infrared light and then directly compare that to the Europa measurements.
Journaux said the hydrate might even exist naturally on Earth. Some parts of Antarctica get cold enough, and the hydrate could solidify in briny lakes.
The other hydrate, with 13 water molecules for every sodium chloride, might be found at the bottom of the oceans of the icy worlds, Journaux said.