Two newly discovered forms of frozen salt water may help scientists solve a mystery about of the solar system icy moons.
When exposed to higher pressures and lower temperatures than those found naturally on Earth, the atoms in hydrated sodium chloride – better known as salt water ice – arranged themselves into never-before-identified structures that have a much higher proportion of water molecules than salt.
This could explain the strange chemical signature of a substance on the surface of Jupiter’s moon Europa, which appears more watery than scientists expect.
“It’s rare today to have fundamental discoveries in science,” says Earth and space scientist Baptiste Journaux at the University of Washington.
“Salt and water are very well known under Earth conditions. But beyond that, we’re completely in the dark. And now we have these planetary objects that probably have compounds that are very familiar to us, but under very exotic conditions. We’re dealing with all the basic mineralogical science that people did in the 19th century, but at high pressure and low temperature. It’s an exciting time.”
Salt and water – also known as sodium chloride and dihydrogen oxide – are both abundant in our home world. When combined, salt molecules dissolve in water to make a solution. The presence of the salt lowers the freezing point of the solution compared to unsalted water, but as the temperature continues to drop under typical Earth atmospheric conditions, it will eventually freeze.
When it does, the molecules arrange themselves into a rigid lattice structure known as a hydrate. On Earth (outside the laboratory) this structure has only one configuration: one salt molecule for every two water molecules.
On moons such as Europa and Ganymede, which orbit Jupiter, and Saturn’s moon Enceladus, scientists have also found evidence of salt and water, only the conditions in which both are found are quite different from those on Earth.
Exposed to the near vacuum of space, far from the Sun, the surfaces of these distant worlds can become extremely cold. Beneath their ice sheets lay oceans that in some cases can be more than 100 times thicker than the deepest water on Earth, producing quite extreme pressures and temperatures.
Journaux and his colleagues attempted to investigate the effect of salt on the production of ice. They compressed a small blob of salt water in a diamond anvil cell under extremely cold conditions, generating pressures up to 25,000 times Earth’s atmospheric pressure while lowering the temperature to -123 degrees Celsius (-190 degrees Fahrenheit).
They didn’t expect what happened next.
“We were trying to measure how adding salt would change the amount of ice we could get, since salt acts as an antifreeze,” explains Journaux. “Surprisingly, when we applied pressure, what we saw was that these crystals that we hadn’t expected started to grow. It was a very serendipitous discovery.”
Under the experiment’s conditions, the researchers saw two new arrangements of salt and water molecules emerge. One contained two salt molecules for every 17 water molecules; the other had 13 water molecules for one salt molecule. Both are very different from the one salt, two water seen naturally on Earth—and consistent with the watery chemical signatures observed on the icy moons.
“It has the structure that planetary scientists have been waiting for,” adds Journaux.
The main factor, the researchers say, is pressure, which squeezes the molecules together and forces them to find new ways to coexist. But even when the pressure was released, one of the newly identified hydrates – the one with 17 water molecules – remained stable up to temperatures of -50 degrees Celsius. This suggests that it can be found here on Earth as well, possibly under the Antarctic ice.
Future research will need to be done to determine if this discovery can solve the icy moon mystery.
“(The hydrate’s) infrared spectra remain to be determined in future studies,” the researchers write, “but its hyperhydrated structure may solve the long-standing mystery of the unidentified hydrate phase on the surface of Europa and Ganymede.”
The research is published in Proceedings of the National Academy of Sciences.