Newly discovered type of salt could explain the mystery of Europa's ice cracks - Space.com

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Scientists on Earth have discovered two new types of solid crystals that form when table salt and water mix in cold temperatures and at low pressures. The down-to-Earth discovery could have out-of-this-world implications, as these salts could be found in deep cracks and crevices across the surface of moons around the solar system.

In particular, the discovery could explain the strange red streaks that crisscross the surface of Jupiter's moon Europa. These scratch-like lines have a chemical signature that doesn't match anything found on Earth, but scientists think it could be a frozen mix of salts and water. The new discovery could finally provide an explanation.

The new substances, a mix of two of Earth's most common molecules, were created by a team of scientists led by the University of Washington, who believe that these salts could form naturally at the bottom of deep oceans on moons like Europa.

"It's rare nowadays to have fundamental discoveries in science," lead author and University of Washington acting assistant professor of Earth and space sciences, Baptiste Journaux, said in a statement (opens in new tab). "Salt and water are very well known in Earth conditions. But beyond that, we're totally in the dark. And now we have these planetary objects that probably have compounds that are very familiar to us but in very exotic conditions."

When water and salts combine at frigid temperatures they form a rigid lattice called a "hydrate" which is held together by hydrogen bonds. Sodium chloride, table salt, a simple structure with one salt molecule for every two water molecules, was previously the only known hydrate.

These two newly discovered compounds are also hydrates but are also very different from sodium chloride. One of the compounds has two sodium chloride molecules for every 17 molecules of water. The other has one sodium chloride molecule for every 13 water molecules.

If these high-water hydrates are found across Europa, it could explain why this Jovian moon's surface and that of others have been found to be more watery than expected. "It has the structure that planetary scientists have been waiting for," Journaux said.

This image shows the newly discovered hydrate that has two sodium chloride molecules for every 17 water molecules. This crystal formed at high pressure but remains stable at cold, low-pressure conditions. — A crystal of a completely new type of salty ice that was previously unknown on Earth but could exist on icy moons in the solar system. (Image credit: Journaux et al./PNAS)

The team discovered the new hydrates while compressing a small sample of salt water between tiny diamonds no bigger than a grain of sand. This meant the liquid experienced pressure as much as 25,000 times the usual atmospheric pressure of Earth.

Because the diamonds were transparent, the researchers could track the progress of the experiment through a microscope. The team found that when the high pressure was released one of the hydrate structures remained stable.

"We were trying to measure how adding salt would change the amount of ice we could get since salt acts as an antifreeze," Journaux said. "Surprisingly, when we put the pressure on, what we saw is that these crystals that we were not expecting started growing. It was a very serendipitous discovery."

Even though such frigid, high-pressure conditions that the team created in the lab aren't common on Earth, they do naturally occur on the moons of Jupiter. On the Jovian moons, conditions are sufficient to create ice sheets 3 to 6 miles (5 to 10 kilometers) thick over oceans that are up to a hundred miles deep. Even denser ice could form at the bottom of these oceans.

"Pressure just gets the molecules closer together, so their interaction changes — that is the main engine for diversity in the crystal structures we found," Journaux said.

The team will now attempt to create a larger sample of their new hydrates to analyze them more thoroughly in an attempt to discover if their chemical structures match signatures from icy moons. The investigation will have assistance from two upcoming missions to explore the moons of Jupiter and one that will explore Saturn's largest moon Titan.

The European Space Agency's Jupiter Icy Moons Explorer mission (JUICE), launching in April 2023, and NASA's Europa Clipper mission, set to launch in October 2024, will both head to the Jovian moon system, while in 2027 NASA's Dragonfly mission will head to Titan. One of the main aims of the missions will be to determine if these moons have the chemistry needed to support life.