"Space ice" contains tiny crystals and is not, as previously assumed, a completely disordered material like liquid water, according to a new study by scientists at UCL (University College London) and the University of Cambridge.
Ice in space is different to the crystalline (highly ordered) form of ice on Earth. For decades, scientists have assumed it is amorphous (without a structure), with colder temperatures meaning it does not have enough energy to form crystals when it freezes.
In the new study, published in Physical Review B, researchers investigated the most common form of ice in the Universe, low-density amorphous ice, which exists as the bulk material in comets, on icy moons and in clouds of dust where stars and planets form.
They found that computer simulations of this ice best matched measurements from previous experiments if the ice was not fully amorphous but contained tiny crystals (about three nanometres wide, slightly wider than a single strand of DNA) embedded within its disordered structures.
In experimental work, they also re-crystallised (i.e. warmed up) real samples of amorphous ice that had formed in different ways. They found that the final crystal structure varied depending on how the amorphous ice had originated. If the ice had been fully amorphous (fully disordered), the researchers concluded, it would not retain any imprint of its earlier form.
Lead author Dr Michael B. Davies, who did the work as part of his PhD at UCL Physics & Astronomy and the University of Cambridge, said: "We now have a good idea of what the most common form of ice in the Universe looks like at an atomic level.
"This is important as ice is involved in many cosmological processes, for instance in how planets form, how galaxies evolve, and how matter moves around the Universe."
The findings also have implications for one speculative theory about how life on Earth began. According to this theory, known as Panspermia, the building blocks of life were carried here on an ice comet, with low-density amorphous ice the space shuttle material in which ingredients such as simple amino acids were transported.
Dr Davies said: "Our findings suggest this ice would be a less good transport material for these origin of life molecules. That is because a partly crystalline structure has less space in which these ingredients could become embedded.
"The theory could still hold true, though, as there are amorphous regions in the ice where life's building blocks could be trapped and stored."
Co-author Professor Christoph Salzmann, of UCL Chemistry, said: "Ice on Earth is a cosmological curiosity due to our warm temperatures. You can see its ordered nature in the symmetry of a snowflake.
"Ice in the rest of the Universe has long been considered a snapshot of liquid water – that is, a disordered arrangement fixed in place. Our findings show this is not entirely true.
"Our results also raise questions about amorphous materials in general. These materials have important uses in much advanced technology. For instance, glass fibers that transport data long distances need to be amorphous, or disordered, for their function. If they do contain tiny crystals and we can remove them, this will improve their performance."
For the study, the researchers used two computer models of water. They froze these virtual "boxes" of water molecules by cooling to -120 degrees Centigrade at different rates. The different rates of cooling led to varying proportions of crystalline and amorphous ice.
They found that ice that was up to 20% crystalline (and 80% amorphous) appeared to closely match the structure of low-density amorphous ice as found in X-ray diffraction studies (that is, where researchers fire X-rays at the ice and analyse how these rays are deflected).
Using another approach, they created large "boxes" with many small ice crystals closely squeezed together. The simulation then disordered the regions between the ice crystals reaching very similar structures compared to the first approach with 25% crystalline ice.
In additional experimental work, the research team created real samples of low-density amorphous ice in a range of ways, from depositing water vapour on to an extremely cold surface (how ice forms on dust grains in interstellar clouds) to warming up what is known as high-density amorphous ice (ice that has been crushed at extremely cold temperatures).
The team then gently heated these amorphous ices so they had the energy to form crystals. They noticed differences in the ices' structure depending on their origin - specifically, there was variation in the proportion of molecules stacked in a six-fold (hexagonal) arrangement.
This was indirect evidence, they said, that low-density amorphous ice contained crystals. If it was fully disordered, they concluded, the ice would not retain any memory of its earlier forms.
The research team said their findings raised many additional questions about the nature of amorphous ices – for instance, whether the size of crystals varied depending on how the amorphous ice formed, and whether a truly amorphous ice was possible.
Amorphous ice was first discovered in its low-density form in the 1930s when scientists condensed water vapour on a metal surface cooled to -110 degrees Centigrade. Its high-density state was discovered in the 1980s when ordinary ice was compressed at nearly -200 degrees Centigrade.
The research team behind the latest paper, based both at UCL and the University of Cambridge, discovered medium-density amorphous ice in 2023. This ice was found to have the same density as liquid water (and would therefore neither sink nor float in water).
Co-author Professor Angelos Michaelides, from the University of Cambridge, said: "Water is the foundation of life but we still do not fully understand it. Amorphous ices may hold the key to explaining some of water's many anomalies."
Dr Davies said: "Ice is potentially a high-performance material in space. It could shield spacecraft from radiation or provide fuel in the form of hydrogen and oxygen. So we need to know about its various forms and properties."
