Ice in Space Isn't the Same as Ice on Earth
Next time you're drinking a frosty iced beverage, think about the structure of the frozen chunks chilling it down. Here on Earth, we generally see ice in many forms: cubes, sleet, snow, icicles, slabs covering lakes and rivers, and glaciers. Water ice does this thanks to its hexagonal crystal lattice. That makes it less dense than nonfrozen water, which allows it to float in a drink, in a lake, and on the ocean.
Water ice exists across the Solar System beyond Earth, and it’s abundant in the larger Universe. For example, it shows up in dense molecular clouds. These are star- and planet-forming crèches laced with water ice throughout, as well as in the resulting cometary nuclei. That material is called "low-density amorphous ice (LDA)" and it doesn’t have the same rigid structure as Earth ice does.
We all know that water is the basis for life on this planet. Despite how common it may appear across the Universe, scientists still don’t fully understand it. Studying amorphous ice may help explain its still-to-be-solved mysteries. Here in the Solar System, large amounts of LDA exist in the realm of the ice and gas giants, throughout the Kuiper Belt, and the Oort Cloud. A team of scientists at University College London investigated the form of this ice using computer simulations. They found that the simulations matched the makeup of ice that isn’t completely amorphous and has tiny crystals embedded within.
Jupiter's moon Ganymede is covered with water ice. It likely has a deep, subsurface ocean. Other moons in the Solar System, such as Enceladus, also show evidence of water ice and scientists are interested in the structure of that material.
Courtesy NASA/JPL-Caltech/SwRI/MSSS/Kalleheikki Kannisto
What the Difference Means
Scientists long assumed that "space ice" would be "disordered" without the structure we see in ice on Earth. Why does the structure of ice matter? According to researcher Michael Davies, who led the research team, water ice plays a crucial role in materials and structures across the cosmos. “This is important as ice is involved in many cosmological processes,” he said, “for instance in how planets form, how galaxies evolve, and how matter moves around the Universe.” In addition, understanding the structure of this ice in comparison to ice that formed on Earth has implications for understanding other similar "ultrastable glass" substances that form similar to the way ice does.
Low-density water ice was first discovered in the 1930s and a high-density version was discovered in the 1980s. Davies and his team discovered medium-density amorphous ice in 2023. This is a form of water ice that has the same density as liquid water. Unlike the ice cubes in our theoretical drink, such water ice would neither sink nor float in water, which seems strange to us.
Davies’s team’s work also has interesting implications for a speculative theory called Panspermia. It looks at how life on Earth began and suggests that the building blocks of life came to the infant planet as part of a barrage of icy comets. LDA ice could have essentially been the carrier for material such as simple amino acids. However, according to Davies, that "flavor" of ice isn't likely the transporter of choice. “Our findings suggest this ice would be a less good transport material for these origin of life molecules,” he said. “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.”
Testing the Water Ice
According to Davies, water ice is an important material not just for life, but for other uses. “Ice is potentially a high-performance material in space,” he said. “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.”
The research team used two computer models of water and froze these virtual “boxes” of water molecules by cooling to -120 °C at different rates. Those different rates had different results, creating varying amounts of crystalline and amorphous ice. The team also created larger boxes of water ice containing many small, closely packed ice crystals. Then, they heated the resulting ice so it could form crystals. Eventually, differences in the resulting crystals showed up, based on their original formation.
The result was an LDA ice with about a quarter of its mass in crystalline form. This was indirect evidence, they said, that low-density amorphous ice contained crystals. If it was fully disordered, the ice would not retain any memory of its earlier forms. The tests raise a lot of questions about the nature of amorphous ices and the role they play in processes such as planet formation. Davies’s co-author Professor Christoph Salzmann, of UCL Chemistry, described the difference between the very structured ice on Earth (and implications for its formation) and the amorphous ice in space. “Ice on Earth is a cosmological curiosity due to our warm temperatures,” he said. “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.”
Implications
The result of the team’s simulations shows that the theory of liquid water going straight to a blob of amorphous ice isn’t completely true. Salzmann also suggests that the lab work they did could have important implications for other similar substances. “Our results also raise questions about amorphous materials in general,” he said. “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.”
In layperson’s terms, these substances beyond water ice are part and parcel of such technologies as OLEDs and fiber optics. In the future, an amorphous silicon, for example, could be studied in the same way and lead to major improvements in technologies that depend on the resulting ultrastable glasses.
