In a new paper led by OU researcher Dr Rachael Hamp, the team shows that when salty water is rapidly frozen to extremely low temperatures, in a process often called 'flash' freezing, it can produce a type of salt crystal that is totally new to science.

The new type of salt is made from simple ingredients: sodium chloride (or table salt) and water.

This is only the third new type of sodium chloride to be discovered in over 200 years, and the first discovered in that time that doesn't require extremely high pressures to form.

Sodium chloride is the main salt in Earth's oceans and has all kinds of uses ranging from food production to medicine and the chemical industry.

Dr Hamp, lead author of the new study, says: “The discovery of this new salt phase is important because it hints at the variety of minerals yet to be discovered that could be vital for our understanding of the icy worlds like Europa and Enceladus".

Image, right: Jupiter's icy moon Europa, which contains a salty subsurface ocean and where sodium chloride has been detected. Credit: NASA / Jet Propulsion Lab-Caltech / SETI Institute.

Very little is known about how sodium chloride behaves at the extremely low temperatures expected at the surfaces of icy worlds like Europa or Enceladus. At these low temperatures, salts can combine with water to form cold-stable crystals that have water molecules locked into their solid crystal structures. For over 200 years, only one such type of sodium chloride crystal was known. In 2023, two more were discovered, forming only under very high pressures. This new crystal described by Dr Hamp and co-authors can be produced easily by flash freezing at Earth surface pressure.

Neutrons were key both to the discovery and study of the new material, allowing the team to investigate its crystal structure and understand how it relates to other crystalline forms of sodium chloride. Neutrons are particularly sensitive to the position of hydrogen (or, its heavier form, deuterium) in the crystal lattice, and because the new salt contains a lot of water molecules, using a tool that allowed the team to observe structure in those water molecules was very important. Both NIMROD and Polaris at ISIS were used, alongside the I-11 beamline at Diamond.

We wouldn't expect to find this new material appearing naturally at the surface of the Earth, because it begins to break down when warmed above around -80 Celsius. But it could form in various industrial processes that use salts alongside low temperature. It should also be stable at the surfaces of icy worlds, where temperatures can be as low as -190 Celsius. If we were to detect it on the surface of an icy world, it would act as a signpost for a location where salty water from deep below the ice has frozen rapidly.

Such locations are of great interest to upcoming space missions like ESA's JUpiter ICy moons Explorer (JUICE) and NASA's Europa Clipper, because they could contain chemical traces of the oceans far below.

Dr Hamp added: “It's incredibly exciting that there are currently two missions en route to the icy moons of Jupiter which could search for evidence of this new salt phase!".

The full paper can be found at DOI: 10.1021/acs.jpclett.4c02752​

A version of this article originally appeared on The Open University website.