Every human being has an all-encompassing relationship with water that spans their entire lifetime. Our bodies are 50% water. We drink water, bathe in it, and swim in it. We skate on it, snowboard on it, and have snowball fights with it. We can take its familiarity for granted.
On Earth, water is the simplest compound to understand. It's either liquid, solid, or gas, and by high school, most of us know it can be separated into its constituent atoms. But science reveals the strange truth about water ice. The Solar System contains some strange worlds with strange environments. For water to be water, it always has to have two hydrogen atoms and one oxygen atom. But new research shows that under extreme pressure, water ice becomes a bewildering shape-shifter that leaves its molecular identity behind.
The research is titled "Evidence for symmetry-reduction in solid H2O above 222 GPa," and is published in Communications Chemistry. The lead author is Ashkan Salamat, a physicist at the University of Nevada Las Vegas, and director of the university's Nevada Extreme Conditions Lab.
“It’s amazing how water can behave under pressure.” - Ashkan Salamat, UNLV.
There are more than 20 kinds of ice, and each one forms at a different pressure and temperature. They're not governed strictly by temperature, like water is here on the surface of the Earth. Their naming convention is confusing for non-specialists, because they're named in order of discovery, not by ascending pressure and temperature, as one might expect. Among them, ice-X holds a special place.
Typically, a material under pressure has its atoms forced closer together, and as they pressure rices, they form arrangements with higher symmetry. That's what happens with ice all the way up to ice-X. Ice-X forms under extreme pressure that doesn't exist anywhere on Earth, even in Earth's mantle.
A logarithmic-linear pressure-temperature phase diagram of water. Water remains molecular up to ice-X. The phases are named in order of discovery. Image Credit: By Cmglee - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14939155
"Water (H2O), one of the most consequential compounds in the Solar System, governs the structure and dynamics of icy planets, the terrestrial water cycle, and through the pressure-induced transition from hydrogen to ionic bonding, it exhibits marked changes over the pressure range of planetary interiors," the authors write. Ice-X forms under immense pressure, the kind of pressure exerted in Uranus and Neptune, the Solar System's ice giants.
Under these pressures, the oxygen atoms shift. They're forced to sit exactly half-way between two hydrogen atoms, the hallmark of ice-X. This forms a cubic, symmetrical crystal lattice, as expected. This is an important transition, where water ice stops being molecular. But at the same time, it conforms to the rule that under increasing pressure, ice retains symmetry.
This illustration shows ice X, ice that forms under extremely high pressures. It's symmetrical, with each oxygen atom forced to sit exactly half-way between two hydrogen atoms. Image Credit: Militzer and Wilson, 2010. APS.
Though ice-X forms under the extreme pressures in Uranus and Neptune, the researchers wanted to see what happens under even greater pressure, the type of even more extreme pressure that exerts itself deep inside the ice giants. Would it exhibit the expected symmetry under these extreme pressures? What strange behaviour might it exhibit on these worlds?
“Water beyond Earth-like conditions has more complex behavior than we ever previously imagined,” lead author Salamat said in a press release. “It’s amazing how water can behave under pressure.”
In this work, lead author Salamat and his co-researchers placed water in a diamond anvil cell, an experimental device that puts samples under extreme pressures.
“We can’t enter the crust or atmospheres of these planets because of distance, but we have data from things like the Voyager program,” said Salamat. “We have an idea of their compositions, and interior pressure and temperatures. And in labs like mine, we can mimic the chemistry and experiment with what could be occurring inside these planets.”
They subjected the water to 35 million PSI, more pressure than it takes to form ice-X. This extremely high pressure is present in Uranus and Neptune, and the researchers wanted to see what happens.
The researchers used spectroscopy and x-rays to observe the structural arrangement of the hydrogen and oxygen atoms subjected to extreme pressure in the diamond anvil cell. The water did not reach higher symmetry as expected. Instead, it exhibited lower symmetry. "Here we report experimental evidence that water ice undergoes a transformation from ice-X to a loss of cubic symmetry above 222(12) GPa," the researchers write.
222 (+/- 12) gigapascals is an awesome amount of pressure. It's more than 2 million times greater than Earth's atmospheric pressure. "This transition marks the onset of a new regime of structural complexity in high-pressure H2O, redefining the upper stability limit of ice-X," the authors write.
Though the ice loses symmetry at these pressures, it still agrees with theoretical predictions, which takes some of the strangeness away. "The measured equation of state for ice-X and the evolution of the cell parameters of the newly observed phase align well with theoretical models," the authors write, adding that "These experiments highlight the complexities of compressing H2O under high pressures..."
In recent years, we've discovered that some chemicals on other planets may have other pathways of generation than here on Earth. Methane on Mars is one example, and phosphine on Venus may be another. Those aren't rock solid examples, but they're hints that remind us how different planetary environments can be. The lesson is that other planets don't necessarily conform to what we think is allowed based on our Earthly understanding.
This work shows that the same may be true of water ice. We can't assume that things like simple water will behave in predictable ways in the extreme environments elsewhere in the Solar System. Uranus, for example, has an icy-fluid mantle that may reach a whopping 700 gigapascals (GPa) of pressure.
“Understanding the mechanisms of our solar system and our observable universe is critical for the endeavor of human knowledge,” Salamat said. “When we eventually get to these worlds, what are we going to see? We’re really the initial pioneers in trying to answer these questions in the laboratory.”