Water can change its structure when squeezed hard enough, according to new research from the University of Nevada, Las Vegas (UNLV), and the results could help scientists understand what lies inside distant planets such as Uranus and Neptune.
The study, published Oct. 15 in the journal Communications Chemistry, found that under extreme pressure, solid water becomes less orderly in its atomic arrangement, not more. That is the opposite of what many scientists expected.
Squeezing water like never before
Physicist Ashkan Salamat, who leads UNLV’s Nevada Extreme Conditions Lab, and his team put water under pressure equal to about 35 million pounds per square inch (PSI). A car tire is about 30 PSI, so that is more than a million times higher. The conditions are closer to those inside Neptune or Uranus.
“Water beyond Earth-like conditions has more complex behavior than we ever previously imagined,” Salamat said. “It’s amazing how water can behave under pressure.”
A surprise in the atoms
Normally, squeezing a material pushes its atoms closer together, and scientists expect them to settle into neater, more symmetrical patterns. Water did the opposite. A very high-pressure form of ice known as ice-X, in which atoms line up in a tidy cube-like pattern, began to lose that pattern at about 222 gigapascals, or roughly 32 million PSI. The structure became a lower-symmetry version of itself.
The team is not yet sure of the exact new arrangement. Computer models favor one candidate structure, but the data could fit a second one, the paper says. If the pressure were released, the water would likely return to a familiar form, like the ice found on Earth, UNLV said.
The researchers squeezed tiny water samples between two diamonds, the hardest known material, and aimed X-rays at them to see how the atoms were arranged. The X-ray work was done at Argonne National Laboratory’s Advanced Photon Source and at the Advanced Light Source.
Why it matters
Uranus and Neptune are “ice giants,” made of hot, dense mixes of water, ammonia and methane, UNLV said. Scientists can’t visit their interiors, but they have data such as Voyager measurements, so labs like Salamat’s recreate the conditions on Earth.
The paper adds that planets between Earth and Neptune in size, the most common type in the galaxy, likely hold huge reservoirs of water too. How water’s atoms are arranged affects how dense it is, how well it conducts electricity and heat, and how planets generate magnetic fields.
What it doesn’t show yet
The tests were done at room temperature, while planet interiors are far hotter, so the new structure may not form inside real planets, the authors note. They also say the data can’t yet pin down which of the two candidate arrangements is correct, and that more experiments are needed. Whether the change affects how water reacts with other materials is still an open question.
“When we eventually get to these worlds, what are we going to see?” Salamat asked. Lab experiments like this one, he said, are an early attempt at answers.