Using advanced photon sources, scientists recreated the structure of the ice that formed at the centers of planets such as Neptune and Uranus. Everyone knows the three states of water: ice, liquid, and water vapor —but, depending on the conditions, water can actually form a dozen different structures. Scientists have now added a new stage to the list: superionized ice.
This type of ice forms at extremely high temperatures and pressures, such as in the depths of planets such as Neptune and Uranus. Previously, superionic ice was only glimpsed for a brief moment when scientists sent shock waves through water droplets, but in a new study published in Nature Physics, scientists have found a reliable way to create, maintain, and examine this ice.
Study co-author Vitali Prakapenka said: "It's a surprise — everyone thinks this phase won't come up until you're under a lot more pressure than where we first discovered it," said University of Chicago research professor and beamline scientist at Advanced Photon Sources (APS), a user facility at the U.S. Department of Energy's (DOE) Office of Science at DOE Argonne National Laboratory. "But we were able to map the properties of this new ice very accurately, which constitutes a new phase of matter, thanks to several powerful tools."
Even though humans have snooped into the beginnings of the universe — and into the smallest particles that make up all matter — we still don't understand what lies deep inside Earth, let alone our fraternal planets in our solar system. The scientists only dug about 7.5 miles below the Earth's surface before the equipment began to melt due to extreme heat and pressure. Under these conditions, rocks behave more like plastics, and even the structure of basic molecules like water begins to shift.
Since we can't actually get to these places, scientists have to turn to laboratories to reproduce extreme heat and stress conditions. Prakapenka and his colleagues used APS, a giant accelerator that drives electrons to extremely high speeds close to the speed of light to produce brilliant X-ray beams. They squeezed the sample between two diamonds, the hardest substance on Earth, to simulate a strong pressure, and then shot through the diamond with a laser to heat the sample. Finally, they sent a beam of X-rays through the sample and pieced together the arrangement of the internal atoms based on how the X-rays were scattered across the sample.
When they first conducted the experiment, Prakapenka saw that the readings of the structure were very different from what he had hoped. He thought something was wrong, there was an unnecessary chemical reaction, which often happened in water in such experiments. "But when I turned off the laser and the sample went back to room temperature, the ice was back to its original state." "This means that this is a reversible, structural change, not a chemical reaction."
Observing the structure of the ice, the team realized it had a new phase in hand. They are able to precisely map their structure and properties.
Prakapenka said: "Imagine a cube, a lattice of aerobic atoms in four corners, connected by hydrogen. When it transforms into this new superion phase, the lattice expands, allowing hydrogen atoms to migrate around while oxygen atoms remain in stable positions. It's a bit like a solid oxygen lattice sitting in a floating ocean of hydrogen atoms. "
This has an impact on the way ice behaves. It is less dense, but noticeably darker because it interacts differently with light. But the full chemical and physical properties of superionic ice remain to be explored. It's a new state of matter, so it's basically as a new material, and it might be different from what we think.
The findings are also a surprise, because while theoretical scientists have predicted this phase, most models believe it won't appear until the water is compressed to a pressure of more than 50 gigapascals (about the same conditions as when rocket fuel is detonated into the air). But these experiments were only carried out at a pressure of 20 gigapascals.
Mapping the exact conditions of the different stages of ice is important for understanding the formation of planets, and even where to look for life on other planets. Scientists believe that similar conditions exist in the interiors of Neptune and Uranus, and that similar cold rocky planets exist elsewhere in the universe. The properties of these ices play a role in a planet's magnetic field, which has a huge impact on its ability to carry life. Earth's powerful magnetic field protects us from harmful incident radiation and cosmic rays, while the barren surfaces of Mars and Mercury are exposed. Understanding the conditions that affect magnetic field formation could guide scientists in searching for potentially life-carrying stars and planets in other solar systems.
There are also many angles to explore, such as conductivity and viscosity, chemical stability, and what changes when water mixes with salt or other minerals, as it often happens deep below the Earth's surface.