Though ice is a widespread form of water, not all water ice is the same. The inside molecules’ arrangement varies considerably, according to pressure and temperature conditions under which it gets formed.
There are approximately 18 distinct ice phases known to science currently, some only seen in the laboratory, controlled environments.
Approximately three years ago, a team of researchers worked on an existing structure, changing it into a form known as ice β-XV. Members of that team figured out its precise crystal structure, answering questions like how it forms and naming it ice XIX.
That discovery may help scientists better understand how ice forms and evolves in alien environments very different from those on our planet.
The ice that forms in a regular refrigerator, or falling from the sky in the form of snowflakes or hailstones, is the most common version of ice on our planet. That is called ice I, and its oxygen atoms form a hexagonal grid. The structure is “geometrically frustrated” due to the hydrogen atoms that are much more disordered.
When ice gets cooled in a particular way, the hydrogen atoms can occasionally become ordered on top of the oxygen atoms. That is how scientists, in laboratory conditions, can form various phases of ice that present significantly more ordered crystalline molecule lattices than their disordered, primary forms.
A team of physical chemists from the University of Innsbruck in Austria worked on something called phase ice VI. That is one of the ice forms that can be found rarely in nature, only under intense pressures 10,000 times greater than atmospheric pressure at sea level (or approximatively one gigapascal), like those pressures available in Earth’s mantle.
Similarly to ice I, ice VI is somewhat disordered. Its hydrogen-ordered form, known as ice-XV, was discovered only ten years ago. It is formed by cooling the ice to under 130 Kelvin (-226 degrees Fahrenheit or -143 degrees Celsius) at pressures of about one gigapascal.
Some years ago, by altering that process, scientists created a new phase of ice. They decreased the cooling speed, took it under 103 Kelvin, and boosted the pressure to 2 gigapascals. That resulted in a second arrangement of hydrogen molecules, different from ice XC, called ice β-XV.
Confirming that the ice was a new phase was a tough job, needing the regular water to be replaced by “heavy water.”
Regular hydrogen presents no neutrons in the nucleus. Heavy water is based on deuterium, a variation of hydrogen that has a neutron in the nucleus.
To uncover atoms’ ordering in a crystal lattice, scientists needed to identify neutrons from nuclei, so regular hydrogen atoms wouldn’t falsely trigger a positive alarm.
Thomas Loerting, a physical chemist from the University of Innsbruck, said:
“Unfortunately, this also changes the time scales for ordering in the ice manufacturing process […] But Ph.D. student Tobias Gasser then had the crucial idea of adding a few percent of normal water to the heavy water – which turned out to speed up the ordering immensely.”
That helped the researchers obtain the neutron information they needed to uncover the crystal structure. As they believed, it was different from ice XV, marking a whole new known phase, called ice XIX.