Newly discovered type of salt could explain the mystery of Europa's ice cracks

A color view of the Jovian moon Europa captured by NASA's Galileo spacecraft in the late 1990s.
The chemical composition of material that makes up the red streak on the surface of Jupiter's moon Europa is different from anything known to exist on Earth. (Image credit: NASA/JPL-Caltech/SETI Institute)

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. 

Related: Behold! Our closest view of Jupiter's ocean moon Europa in 22 years

"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. "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. 

"These are the only planetary bodies, other than Earth, where liquid water is stable at geological timescales, which is crucial for the emergence and development of life," Journaux said. "They are, in my opinion, the best place in our solar system to discover extraterrestrial life, so we need to study their exotic oceans and interiors to better understand how they formed, evolved, and can retain liquid water in cold regions of the solar system, so far away from the sun."

The discovery has implications beyond planetary science, too. Energy research could also benefit from these new salty ices as hydrates can be used for energy storage. 

"We have to redo all the fundamental mineralogical science that people did in the 1800s but at high pressure and low temperature," concluded Journaux. "It is an exciting time."

The team's research is published in the Feb. 20 edition of the Proceedings of the National Academy of Sciences. 

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.