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Europa's surface
An image from NASA’s Galileo orbiter shows Europa’s icy surface, crisscrossed by reddish-brown streaks of radiation-darkened salt. (Credit: NASA / JPL / Ted Stryk)

A prime target in the search for extraterrestrial life is Europa, a moon of Jupiter that’s covered with a sheet of salty ice. But what kind of salt is there? Researchers say they’ve created a new kind of salt crystal that could fill the bill, and perhaps raise hopes for finding life under the ice.

This salt crystal is both exotic and common: It’s actually table salt — also known as sodium chloride, with the chemical formula NaCl — but bound up with water molecules to form a hydrate that doesn’t exist naturally on Earth.

Earthly sodium chloride hydrates are composed of one salt molecule linked by hydrogen bonds with two water molecules. In contrast, the hydrates created in the lab consist of two NaCl molecules to 17 water molecules, or one NaCl molecule to 13 water molecules. (The structure for a third type of “hyperhydrated hydrate” couldn’t be determined.)

That’s promising news for scientists who study Europa and other ice-covered worlds — including two other Jovian moons, Callisto and Ganymede; and the Saturnian moons Enceladus and Titan. Spectral observations indicate that Europa’s surface ice contains salts, including sodium chloride, but the observed levels of concentration don’t match up well with Earth’s run-of-the-mill NaCl hydrates.

“Other explanations have involved other types of chlorinated salts that had large water/salt ratios,” University of Washington planetary scientist Baptiste Journaux, the lead author of a paper published by the Proceedings of the National Academy of Sciences, told GeekWire in an email. “The issue with those is that we don’t have chemical models to explain why they would form rather than NaCl.”

The trick is that the new forms of NaCl hydrates can form on Earth only under extreme pressures. In the lab, a tiny bit of salty water was compressed between two diamonds that were the size of grains of sand, at pressures up to 25,000 as great as standard atmospheric pressure. As the transparent specks of diamonds squeezed in, the researchers watched the hydrate crystals form through a microscope.

“We definitely were not expecting to find such new structures,” Journaux said.

The cold, high-pressure conditions that created the crystals in the lab could be common on Europa, where a miles-thick layer of ice is thought to press down on a hidden ocean that’s dozens of miles deep.

This photomicrograph 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 in cold, low-pressure conditions. (Credit: Journaux et al. / PNAS)

“The ocean composition directly controls the type of organic chemistry and processes possible for the emergence and sustainability of extraterrestrial life,” Journaux said. “Knowing that sodium chloride is a major ingredient, as it is in Earth’s oceans, will help astrobiologists determine how best to characterize life on another ocean world — and possibly detect it. Life emerging in water-ammonia oceans, for example, might be very different from life emerging from a salt-water ocean.”

Journaux said there’s a chance that the newly discovered types of hydrates could be formed in Antarctica’s ice-covered lakes, if the temperature gets cold enough and the pressure gets high enough. And generally speaking, learning more about how hydrates work could advance fields ranging from battery storage technology to climate science.

But the big question is whether the exotic NaCl hydrates really do exist on worlds beyond Earth. That question could be answered for Europa by the European Space Agency’s Jupiter Icy Moons Explorer mission, set for launch in April; or by NASA’s Europa Clipper mission, which lifts off next year. NASA’s Dragonfly mission, which heads for Titan in 2026, could conceivably sample the ice on Saturn’s smog-shrouded moon. Meanwhile, Journaux and his colleagues plan to produce larger samples of the hydrates and learn more about them.

“Only with the recent development of high-pressure, low-temperature technology are we able to explore the oceans and interiors of water-rich worlds,” he said. “This underlines how little we currently know about the minerals forming on and inside these icy worlds. These are exciting times — what has been done for Earth mineralogy in the 1800s and 1900s has to be done again for icy worlds, now that we are actually going there.”

Journaux’s co-authors on the PNAS paper, titled “On the Identification of Hyperhydrated Sodium Chloride Hydrates, Stable at Icy Moon Conditions,” include J. Michael Brown and Jason Ott of the University of Washington. Co-authors from other institutions: Anna Pakhomova, Ines Collings, Sylvain Petitgirard, Tiziana Boffa Ballaran, Steven Vance, Stella Chariton, Vitali Prakapenka, Dongyang Huang, Konstantin Glazyrin, Gaston Garbarino, Davide Comboni and Michael Hanfland.

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