Water ice molecules are among the most common in the cosmos and influence the interior and exterior of many planetary bodies in our solar system. Glaciers shape parts of Earth’s surface, and dwarf planet Pluto, along with moons such as Europa, Ganymede, Titan, and Enceladus, have whole landscapes made up of ice alone, including boulders, mountains, and even volcanoes.
Under high-pressure or very low temperature conditions, ice forms different crystal structures than those that occur naturally on Earth. Identifying and measuring those structures on worlds such as Ganymede would provide unique data on the interiors of these celestial bodies, in the same way studying mantle rocks pushed to the surface on Earth reveals our planet’s deep geology.
In the lab, researchers can bombard ice with X-rays or neutrons to understand its structure. But such instruments aren’t practical to fly on spacecraft.
“The ices that we prepare in the lab only occur naturally in space.”
Now, new experiments conducted by Christina Tonauer and her colleagues at Universität Innsbruck in Austria show how to distinguish between ice structures using infrared spectroscopy. The analyses, published in Physical Review Letters earlier this summer, can be done using observations from NASA’s James Webb Space Telescope (JWST) or the European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission currently en route to Jupiter.
“The ices that we prepare in the lab only occur naturally in space,” said Tonauer, whose work combines her field of physical chemistry with her love for planets. “I’m also really interested in astronomy, and this is what hooked me to water ice.”
During Tonauer’s Ph.D. work in the early 2020s, JWST was still to be launched, but it was clear the infrared observatory would open avenues for studying the ice-covered moons of the outer solar system. When she and her collaborators delved into the literature, they realized that a lot of spectroscopic work on ice—research that largely predated the leaps in understanding gained from the Voyager and Cassini missions—considered infrared (IR) wavelengths longer than those JWST could measure.
It seemed fruitful to Tonauer and her colleagues to study the shorter-wavelength IR spectrum (near-IR) emitted by ice on these distant worlds.
Ice Maker, Ice Maker, Make Me Some Ice
As of 2025, 21 different phases of ice have been identified in laboratory experiments, although only one form exists under normal conditions on Earth. That form is called ice Ih (pronounced “ice one aitch”), where “h” refers to the hexagonal pattern the molecule’s oxygen atoms take when viewed from one direction.
The conditions that allow researchers to study other ice phases in the lab exist naturally on other planets and moons, however, and scientists have concluded the phases might exist there.
Ganymede and other worlds in the outer solar system likely have something akin to mantle dynamics, for example, but with ice instead of silicate minerals.
Ganymede’s mantle could be 800 kilometers thick and consist of several forms of ice that are known only from laboratory experiments on Earth. Tonauer and her collaborators selected ice V and ice XIII for their study, because they form under the high pressures and low temperatures present inside Ganymede and other moons. These phases have the same arrangement of oxygen atoms, but different orientations of hydrogen atoms: In ice V, hydrogen is jumbled around, whereas hydrogen in ice XIII is structured.
Making these types of ice in the lab requires cooling liquid water with liquid nitrogen under about 5,000 atmospheres (500 megapascals) of pressure. As long as the samples are kept cold after forming, Tonauer noted, they don’t require high pressure to remain stable because the atoms move so slowly.
However, that slow motion still stretches the bonds between molecules, a vibration that produces IR signals. Using spectroscopy to interpret the emissions, Tonauer and her colleagues discovered that these signals are different for ice V and ice XIII. That difference provided the first experimental demonstration of using IR to distinguish hydrogen configurations within different phases of ice. It also highlighted a way to identify them remotely.
The researchers used a JWST simulator to show that a few hours of observation would be enough to distinguish between these ice phases on Ganymede.
A Peek at Deep Ice
The stability of these ice phases is key to understanding their potential presence on the surface of Ganymede: The phases require high pressure to form, but if brought to the lower-pressure surface, they can maintain their exotic crystal structure indefinitely. In that way, the presence of ice V or XIII would provide details about the icy mantle that would otherwise be inaccessible.
Past and present missions to the Jovian system have clearly indicated that Ganymede’s interior contains a liquid water ocean sandwiched between ice layers, but the ices’ crystalline structures, as well as how the layers move and evolve, have not been verified by empirical data. According to models of icy moon interiors, the high-pressure environment should produce ice V, which phenomena such as the tidal force from Jupiter might bring to the surface.
“We can now potentially detect subtle structural differences on icy moons without needing a lander or sample return.”
These new infrared spectroscopy analyses show how to distinguish between ice Ih, ice V, and ice XIII—not to mention amorphous ice, which lacks a clear crystal structure—without having to return samples to Earth for laboratory analysis (a prohibitively expensive proposition). The method could provide an observational way to verify or refute models of interior ice dynamics, sharpen our picture of Ganymede’s internal structure, and help us understand how different flavors of ice behave and interact with each other in a natural environment.
“We can now potentially detect subtle structural differences on icy moons without needing a lander or sample return,” said Danna Qasim, a laboratory astrophysicist at the Southwest Research Institute in Texas who was not involved with the new study.
Qasim pointed out that if the grains of these ices are small and jumbled together, it might be difficult to extract their IR signature. As other recent research has shown, amorphous ice in space likely contains chunks of crystalline ice joined together at odd angles, which also might make identification more difficult.
However, the new method seems promising and could well answer vital questions about the internal structure of icy moons.
“We invest billions of dollars in these spectacular space missions,” Qasim said. “If we want to truly understand what the data is telling us about these enigmatic beautiful worlds, it is absolutely necessary to have laboratory experiments like the ones performed here.”
—Matthew R. Francis (@BowlerHatScience.org), Science Writer