The Secret of the Black Ice
We are familiar with the physical states of water – solid, liquid and gas. However, in its solid phase, as ice, water can form more than a dozen different structures whose properties vary. The ordinary ice on the puddles that soon awaits us again in the northern hemisphere is just the tip of the iceberg. Scientists have now succeeded in producing an ice phase in the laboratory that is much darker than normal ice – superionic ice. This type of ice forms at extremely high temperatures and pressures, such as those found deep inside planets like Neptune and Uranus. Until now, researchers have only ever managed to create such ice for brief moments by sending a shock wave through a drop of water. Now, in the new study published in Nature Physics, they found a way to reliably create, preserve and study the ice.
“It was a surprise – everyone thought this phase would only occur at much higher pressures than we found,” said study co-author Vitali Prakapenka, research professor at the University of Chicago. “But we were able to map the properties of this new ice, which represents a new phase of matter, very precisely thanks to several powerful instruments.”
Currently, we still don’t know exactly what’s deep inside Earth or its sibling planets. Even on home turf, we have only dug down less than 15 kilometers before equipment began to melt due to extreme heat and pressure. Under these conditions, rock behaves more like plastic, and even the structures of basic molecules such as water begin to change.
Prakapenka and his colleagues use the APS, a massive accelerator that accelerates electrons to extremely high speeds near the speed of light, to produce X-rays. They squeeze their samples between two pieces of diamond to create high pressure, then shoot lasers through the diamonds to heat the sample. Finally, they send an X-ray beam through the sample to measure the arrangement of atoms inside based on the scattering of the X-rays. When the researchers first performed the experiments, Prakapenka saw readings that were quite different than expected. The researcher assumed that something had gone wrong – such as an unwanted chemical reaction, which is common in such experiments with water. “But when I turned off the laser and returned the sample to room temperature, the ice returned to its original state,” Prakapenka says. “This means that it is a reversible, structural change and not a chemical reaction.”
The researchers then succeeded in precisely mapping the structure and properties of this new ice phase. “Imagine a cube, a lattice with oxygen atoms at the corners connected by hydrogen,” Prakapenka says. “When it turns into this new superionic phase, the lattice expands, allowing the hydrogen atoms to migrate while the oxygen atoms stay in their positions. It’s like a solid oxygen lattice sitting in an ocean of floating hydrogen atoms. This affects the behavior of the ice: it becomes less dense, but much darker, because it interacts with light differently. However, the full range of chemical and physical properties of superionic ice has yet to be explored. It is a new state of matter, which means it basically behaves like a new material.”
The results were also surprising because, while theoretical scientists had predicted this phase, most models assumed that it would not occur until the water was compressed to a pressure greater than 50 gigapascals (roughly equivalent to the conditions found in rocket fuel when it explodes for launch). In these experiments, however, the pressure was only 20 gigapascals. The new dark ice phase could prove important to our ideas about how planets form. Scientists believe that similar conditions exist inside Neptune and Uranus and other cold, rocky planets in other parts of the universe. Are you also thinking of “Amphitrite – the black planet”? Superionic ice is particularly important because it plays a role in a planet’s magnetic fields, which in turn have a major impact on its ability to harbor life.