Scientists have recently discovered a new form of “salty” ice that could exist on the surface of some extraterrestrial moons in our solar system. This new type of ice is a combination of water and a type of salt called magnesium sulfate, which is also known as Epsom salt.
An international team of researchers discovered two new crystal structures for salty ice, also known as solid hydrate made of water and sodium chloride. The properties of the newly discovered material match those of the substance seen on the surface of icy moons like Europa and Ganymede, and may provide clues to their icy oceans. The red streaks that crisscross Europa, one of Jupiter’s moons, are striking. Scientists believe it is a frozen mixture of water and salts, but its chemical signature is enigmatic because it corresponds to no known substance on Earth.
The discovery of a new type of solid crystal that forms when water and table salt combine in cold and high-pressure conditions by a team led by the University of Washington may have solved the puzzle. The new substance created in a lab on Earth, according to researchers, could form at the surface and bottom of these worlds’ deep oceans.
The study, published in the Proceedings of the National Academy of Sciences, announces a new combination of two of the most common substances on Earth: water and sodium chloride, also known as table salt.
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 that are very familiar to us, but in at very exotic conditions.
Baptiste Journaux
“It’s rare nowadays to have fundamental discoveries in science,” said lead author Baptiste Journaux, a UW acting assistant professor of Earth and space sciences. “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 that are very familiar to us, but in 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.”
At low temperatures, water and salts combine to form a rigid salted icy lattice called a hydrate, which is held together by hydrogen bonds. The only sodium chloride hydrate previously known had a simple structure with one salt molecule for every two water molecules.
But the two new hydrates, found at moderate pressures and low temperatures, are strikingly different. The first contains two sodium chlorides for every 17 water molecules, while the second contains one sodium chloride for every 13 water molecules. This would explain why the signatures from Jupiter’s moons’ surfaces are more “watery” than expected.
“It has the structure that planetary scientists have been waiting for,” Journaux said.
The discovery of new types of salty ice is significant not only for planetary science, but also for physical chemistry and energy research, which uses hydrates for energy storage, according to Journaux. The experiment involved compressing a small amount of salty water between two diamonds the size of a grain of sand and squeezing the liquid up to 25,000 times the standard atmospheric pressure. The transparent diamonds enabled the team to observe the process under a microscope.
“We were trying to see how adding salt affected the amount of ice we could get, because salt acts as an antifreeze,” Baptiste explained. “Surprisingly, when we applied pressure, we noticed that these unexpected crystals began to grow. It was a very fortunate discovery.”
Such cold, high-pressure conditions created in the lab would be common on Jupiter’s moons, where scientists believe 5 to 10 kilometers of ice would cover oceans hundreds of kilometers thick, with even denser ice possible at the bottom.
“Pressure simply brings the molecules closer together, changing their interaction — that is the main engine for diversity in the crystal structures we discovered,” Journaux explained. After the newly discovered hydrates formed, one of the two structures remained stable after the pressure was released.
“We determined that it remains stable at standard pressure up to about minus 50 C. 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,” Journaux said.
The team hopes to create or collect a larger sample to allow for more thorough analysis and to confirm whether the signatures from the newly discovered hydrates match the signatures from icy moons.
The European Space Agency’s Jupiter Icy Moons Explorer mission, which will launch in April, and NASA’s Europa Clipper mission, which will launch in October 2024, will both explore Jupiter’s icy moons. In 2026, NASA’s Dragonfly mission will launch to Saturn’s moon Titan. Knowing what chemicals these missions will come into contact with will allow them to better target their search for signs of life.
“These are the only planetary bodies, other than Earth, where liquid water is stable over geological timescales, which is critical for the emergence and development of life,” said Journaux. “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, far-away regions of the solar system.”
