Researchers have discovered two new forms of “salty ice” that form when water and regular salt are combined in cold and high-pressure conditions. These strange structures — which were created in a lab by compressing salt water between two diamonds — may be found at the surface and depths of icy, ocean-bearing moons around Jupiter and Saturn. Alongside being important for planetary science, the findings could also have implications for the study of physical chemistry and even energy research — in which these chemical lattices, known as hydrates, are used for energy storage.
Paper author and planetary scientist Professor Baptiste Journaux of the University of Washington: “It’s rare nowadays to have fundamental discoveries in science.
“Salt and water are very well-known at Earth conditions. But beyond that, we’re totally in the dark.
“And now we have these planetary objects that probably have compounds very familiar to us, but at very exotic conditions.
“We have to redo all the fundamental mineralogical science that people did in the 1800s, but at high pressure and low temperature. It is an exciting time.”
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As Prof. Journaux explains, at cold temperatures, water and salts combine to form a rigid, salted, icy lattice — known as a hydrate — that is held together by hydrogen bonds.
Until now, sodium chloride (table salt) was only known to form one hydrate, a relatively simple structure which sports one salt molecule for every two molecules of water.
However, the two new hydrates discovered by the researchers at moderate pressures and low temperatures are quite different.
One has two salt molecules for every 17 of water, and the other one sodium chloride molecule per 13 water molecules.
In their study, Prof. Journaux and colleagues placed a tiny drop of salty between a pair of diamonds that they used to compress the liquid up to a whopping 25,000 times atmospheric pressure.
Prof. Journaux said: “We were trying to measure how adding salt would change the amount of ice we could get, since salt acts as an antifreeze.
“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.”
Moreover, he noted, “it has the structure that planetary scientists have been waiting for.”
Chemical signatures from the surface of Jupiter’s moon appear more “watery” than expected — a fact that might be explained by different water-to-ice ratios found in the higher pressure hydrates discovered by Prof. Journaux and his team.
In fact, according to the researchers, the kinds of cold, high-pressure conditions that they replicated in their diamond press would be common on the Jovian moons, where scientists think that three–six-mile-thick crusts of ice hide oceans up to several hundred miles deep.
It is even possible that, at the bottom of these seas, even denser forms of ice exist.
Prof. Journaux explained: “Pressure gets the molecules closer together, so their interaction changes — that is the main engine for diversity in the crystal structures we found.”
The team found that one of the two hydrates even remained stable after the pressure applied to the diamond press was released.
Prof. Journaux said: “We determined that it remains stable at standard pressure up to about -50C [-58F].
“So, if you have a very briny lake — for example in Antarctica — that could be exposed to these temperatures, this newly discovered hydrate could be present there.”
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With their initial study complete, the researchers are now looking to either produce — or collect — larger samples of the new hydrates.
This would allow them to analyse the chemical structures in more detail, and determine whether their signatures match those that have been detected from Jupiter’s icy moons.
Prof. Journaux said: “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.
“They are, in my opinion, the best place in our solar system to discover extraterrestrial life.
“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.”
Two upcoming missions are planning to explore the Jovian moons — the European Space Agency’s Jupiter Icy Moons Explorer, which launches this April, and NASA’s Europa Clipper, which is scheduled to blast off in October next year.
Alongside these, NASA is planning another mission — Dragonfly — which will be launching to Saturn’s moon Titan at sometime in 2026.
According to the researchers, a better understanding of what chemicals these missions will encounter may help to refine their approach for searching out signatures of alien life.
The full findings of the study were published in the Proceedings of the National Academy of Sciences.
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