February 20, 2021: Some Researchers Characterize A New Form Of Ice That May Be Present Beneath The Surface Of Icy Worlds Like Europa Or Titan

A new study entitled "Structural characterization of ice XIX as the second polymorph related to ice VI", proposed by a group of researchers involving Thomas Loerting and published in the journal Nature Communications on February 18, 2021 reveals the discovery or the characterization of a new form of ice taking shape in environments undergoing extremely high pressures. The new form of ice is now known as ice XIX. Until now, scientists have been in a position to identify 19 ice polymorphs or 19 forms of ice. The typical ice that we regularly encounter on Earth unveils an hexagonal shape of oxygen atoms. The common hexagonal form of ice is characterized by hydrogen disorder and geometric frustration. The hydrogen atoms which are connected to the oxygen atoms of the hexagon tend to be disordered or tend to have irregular positions due to a relatively high dynamics. There are numerous configurations of the lattice at relatively high temperatures below the freezing point of water around minus 10 degrees Celsius. The ice we encounter on Earth is in fact a relatively dynamic molecular structure compared to the other forms of ice one can encounter in nature. The behavior of ice can significantly change if the environmental pressure is much higher.

In the Outer Solar System, the environmental conditions are likely to allow the presence of other forms of ice. The high pressures encountered in the atmosphere of Jupiter or Saturn are likely to allow the development of other types of molecules than what we usually encounter on Earth. Most moons of Saturn are extremely rich in water ice and at a certain depth, new types of water ice can potentially be found. Mimas, Enceladus, Tethys, Dione and Rhea are bright moons unveiling an external crust dominated by water ice. A world like Titan appears to be more varied than the other moons of the Ringed Planet Saturn but its external crust may also be rich in water ice. Therefore, those moons may unveil some forms of ice that can take shape in environments dominated by relatively high pressures. Even if most of the icy worlds found in the Outer Solar System are devoid of any significant atmosphere, some forms of ice like ice XIX can potentially be encountered beneath the surface of those worlds. The typical form of water ice may be widespread in the upper layer of icy worlds like Europa or Titan but at a relatively high depth beneath the surface of those moons, some exotic forms of water ice may be found due to relatively high pressures.

On Earth, we are used to ice during the Winter season, in the high latitudes or in the polar regions like Antarctica. For the general public, ice is water ice and water ice is always the same form of ice but in reality, the nature of ice can be remarkably diverse. At lower temperatures, water ice will tend to be more stable since the degree of mobility of hydrogen atoms will decrease. Researchers observe that a form of ice known as ice VI unveils a large range of thermodynamic stability and advance that this polymorph of ice exists in the interior of our planet and in the interior of icy moons. The moons of Saturn are really perfect places to study the structure and the dynamics of water ice. The study of other worlds can tell us a lot regarding the fundamental rules of physics or the fundamental rules of nature. The team of Thomas Loerting had previously produced a research work suggesting that ice Beta-XV was a polymorph resulting from disordered ice VI and from ice XV. In the new study, the researchers characterized the structure of the deuterated polymorph depending on the environmental temperature by mobilizing external calorimetry and high-resolution neutron powder diffraction.

Ice Beta-XV or ice XIX turns out to be partially antiferroelectrically ordered and appears to crystallize in a particular supercell. The powder data of the group collected at subambient pressure are more in line with the structural model in space group P4. The synthesis or the generation of deuterated ice XIX was made possible thanks to the use of a DCl-doped D2O/H2O mixture in which the limited fraction of H2O increases ice XIX nucleation kinetics. The team of scientists also observed the transition from ice XIX to the relatively close ice, ice XV, during a heating process. The process occurs via a transition state (ice VI) incorporating a disordered H-sublattice. The researchers of the research work advance that the process may be the first order-order transition known in the field of ice physics. At the relatively high environmental temperature found on Earth, the degree of mobility of the atoms in the relatively simple molecules of water ice remains relatively high so that there is a certain disorder. However, if the level of energy in the environment is lower, the dynamics of the molecule will be more limited and the order in the structure of the molecule will be closer to the perfect order.

The zero-entropy form of ice is supposed to be reached when the level of energy is at its lowest point or when the environmental temperature is at the level of 0 Kelvin. The hydrogen atoms which are connected to the oxygen atoms of the hexagonal structure tend to move if the environmental temperature is relatively high beneath the freezing point of water. The thermodynamically most stable configuration of water molecules represents the configuration in which water molecules are aligned and in which hydrogen atoms are in a perfect order. A similar level of stability in the configuration of hexagonal ice can be obtained if the environmental temperature is below -200 degrees Celsius. If one lowers the environmental temperature, the ice can keep some disorder because the level of mobility rapidly decreases so that the atoms tend to stop moving. It will take time for the atoms to move to their stable or final position. The researchers need to add catalysts in order to speed up the dynamics of the hydrogen bond network. The ordered form in the hexagonal structure of water ice represents the ice XI polymorph today. Thus, extremely low environmental temperatures are necessary for that type of ice to take shape.

The study of the team focused on ice VI which represents one of the numerous high-pressure ice polymorphs and which represents a geometrically frustrated ice. The blankets of ice encountered on Earth are not deep enough to allow the presence of that type of ice. However, that polymorph of water ice can represent a mineral in the mantle of our planet at depths of several hundred kilometers and can even be found in the form of inclusion inside diamonds at the level of the surface. Ice VI can also be found in the interior of the icy worlds or moons located in the Outer Solar System. There is a certain disorder in ice VI. Relatively recently, researchers managed to produce the ordered counterpart to ice VI by cooling ice VI around 1 GPa to below -140 degrees Celsius and by mobilizing HCl as dopant and catalyst. Ice XV appears to be partially H-ordered and is characterized by a structure in which water molecules are aligned in a way engendering antiferroelectricity. The configuration of ice XV is only one configuration among many configurations of ice. Scientists have been in a position to determine that there is a second ordered ice related to ice VI relatively recently as well.

The researchers observed that ice VI evolves toward a form of ice that is different from ice XV when the cooling process is very slow around 2 GPa instead of 1 GPa. The new polymorph was Beta-XV. The new challenge appeared to determine the exact crystal structure of the polymorph. The new study proposed by the team of Thomas Loerting reveals that the nucleation and growth kinetics for deuterated ice Beta-XV are significantly boosted when a small fraction of H2O is incorporated into the D2O sample. A neutron diffraction investigation was undertaken by the team of researchers on the basis of the High-Resolution Powder Diffraction (HRPD) instrument at the Rutherford Appleton Laboratory. It turns out that more than one thousand different types of configuration related to hydrogen atoms can be envisaged. Therefore, the correct structure of the polymorph is difficult to determine. Ice Beta-XV which is now called ice XIX appears to be a new form of ice in which the network of hydrogen atoms is structured in a partly ordered antiferroelectric network. It is clearly remarkable to obtain an ordered polymorph of water ice in an experimental work.

Researchers must simulate extreme environments in order to obtain the exotic types of water ice. The atmosphere of the four Gas Giants of the Solar System is very deep so that the atmospheric pressure can become extremely high in the interior of the world. The type of molecule or the type of structure of atoms or molecules will be closely linked to the nature of the atoms found in the local environment as well as to the combination of environmental temperature and environmental pressure. Environmental temperatures can become extremely high at a high depth in the interior of Jupiter but atmospheric pressures are so high that new types of substance or molecule can take shape. An ocean of metallic hydrogen may exist inside Jupiter for instance. The experiments performed by the team of researchers clearly show that ice XIX can exist beneath the external crust of worlds like Europa, Titan or Pluto. The environmental temperatures at the level of the surface of a world like Pluto are generally well below -200 degrees Celsius. Beneath the external crust of the Dwarf Planet, stable forms of ice we never or almost never encounter on Earth can potentially be found. The level of order of the ice polymorph will tend to increase if the environmental temperature drops and it will tend to decrease if the environmental temperature goes up.

The image above reveals Saturn's largest moon Titan and Rhea, another major moon of the Ringed Planet, at scale. The original image of Titan whose file name is N00205583.jpg represents a raw view obtained from the Cassini orbiter on April 13, 2013 on the basis of the CL1 filter and of the UV3 filter. The original image of Rhea was acquired in visible light with the Wide-Angle Camera of the Cassini spacecraft on November 21, 2009. Titan and Rhea may unveil various types of water ice. Credit for the original view of Titan: NASA/JPL-Caltech/Space Science Institute. Credit for the original view of Rhea: NASA/JPL/Space Science Institute. Montage credit: Marc Lafferre, 2021.

- To get further information on that news, go to: https://chemistrycommunity.nature.com/posts/order-and-disorder-a-story-about-ice and https://www.nature.com/articles/s41467-021-21161-z .



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