Take Water. Add Sodium Chloride. Chill and Squeeze Into Salty Ices.

Scientists have discovered two new forms of salty ice that probably don’t exist naturally on Earth, but could be found on icy moons farther out in the solar system.

“These structures are nothing like what has been described before,” said Baptiste Journaux, interim assistant professor of earth and space sciences at the University of Washington.

Written in the February 20 issue of the Proceedings of the National Academy of SciencesDr. Journaux and his colleagues describe two new solid and 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, unexpectedly, when salty water was cooled to low temperatures and squeezed out at high pressures.

Salt water is abundant on Earth, after all, it fills the oceans, and chemists have long known how it behaves under Earth conditions. The ice on this planet is rarely salty.

In fact, sodium chloride (each molecule consists of one sodium atom and one chlorine atom) is often thought of first as an antifreeze, which lowers the freezing temperature of water. That is why it spreads on the roads during snow storms. When salt water freezes, the ice crystals that form are made of pure water with the sodium and chloride ions expelled into the remaining liquid.

At cold enough temperatures, residual supersalt water begins to solidify, forming hydrohalite, a rigid water-bearing crystal or hydrate. The hydrohalite consists of two water molecules for each sodium chloride.

In recent decades, planetary scientists have discovered a large number of worlds in the outer solar system that have oceans of liquid water beneath their icy crusts. These include Europa and Ganymede, two moons of Jupiter, and Titan and Enceladus, two moons of Saturn. Dr. Journaux wanted to study the role that salt might play in keeping the oceans of these worlds from freezing.

To reproduce those conditions, a pinch of salt water was cooled to temperatures as low as -190 degrees Fahrenheit and squeezed between two pieces of diamond at pressures up to 25,000 times the usual 14.7 pounds per square inch that air presses against. us on the surface of the Earth. .

“We initially did these experiments because we wanted to study the antifreeze effects of sodium chloride, of salt, because it is predicted to be probably the most common solute in extraterrestrial oceans as it is in Earth’s oceans,” said Dr Journaux. . “We expected to see something similar to what we see on Earth, which is that salts would be pushed out of the ice as it grows.”

Instead, the antifreeze froze.

“We had a new crystal that came out of nowhere that we weren’t expecting at all,” Dr. Journaux said. “So that was very fortuitous.”

The crystals were tiny, at most 1/250th of an inch across, or about the width of a human hair.

X-rays bouncing 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 normal atmospheric pressure. At higher pressures, another, less salty hydrate formed, one with 13 molecules of water for every molecule of sodium chloride.

The scientists also saw signs of a third form, but the needle-like crystals were too thin to study the crystal structure. “It’s very pretty,” Dr. Journaux said, “but it’s so thin that it’s hard to get the data.”

The new hydrates could help explain a mystery in Europe. Observations in 2019 using the Hubble Space Telescope unequivocally identified sodium chloride in yellowish streaks on the moon’s surface. It is highly unlikely that it is in the form of pure salt grains, but other observations (colors of infrared light absorbed by the surface, which serve as identifying fingerprints of specific compounds) did not offer convincing signs of hydrohalite, the well-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 perhaps at temperatures as warm as minus 40 degrees Fahrenheit. That suggests that this hydrate could have formed underground on Europa and would remain in that form if pushed to the surface.

“We’ve known for a long time that some type of material is mixed with the water ice,” said Michael E. Brown, professor of planetary astronomy at the California Institute of Technology, who was one of the scientists who made the Hubble observations for identify sodium chloride. in Europe. “And we’ve long suspected that it might actually just be salt derived from the inland ocean, but we’ve never been able to get a good fit. Maybe it’s this new form of salt.”

Sodium chloride “is one of the simplest and best understood things in the world,” said Dr. Brown. “And yet Journaux has just discovered a new shape that he has never seen before.”

The stability of the hydrate also suggests that there may be a way to create it without the high pressures, perhaps allowing for the growth of larger crystals. That, in turn, could lead to experiments that would measure the absorption of infrared light, and then compare it directly with measurements from Europa.

Dr. Journaux spoke with Christoph Salzmann, professor of chemistry at University College London in England, who was one of the scientists this month who reported a new form of glass-like water ice that is created when normal ice melts. shake with steel balls.

Starting with salt water, that same technique could perhaps also create the new hydrate. “We will definitely give it a try,” Dr. Salzmann said. “If the new hydrate is stable at low temperatures, perhaps the mixture provided by the ball mill is all that is needed for it to form.”

Dr. Journaux said that the hydrate could even exist naturally on Earth. Some parts of Antarctica get cold enough that the hydrate could solidify into salty lakes.

The other hydrate, with 13 water molecules for every sodium chloride, could be found at the bottom of the oceans of icy worlds, Dr. Journaux said.

Several robotic spacecraft will head to the outer solar system in the coming years to study these intriguing icy worlds, which many scientists say are the most promising places in the solar system to search for extraterrestrial life. The European Space Agency’s Juice mission, a reduction of the Jupiter Icy Moons Explorer, is scheduled to launch in April. NASA plans to launch its Europa Clipper spacecraft in October 2024 to study Europa and Dragonfly in 2026 to head towards Saturn’s largest moon Titan.

The hydrates could even become a way to store power generated by solar panels and wind turbines for use when the sun isn’t shining or the winds are still. “So this could have some real-life implications as well,” Dr. Journaux said.