They solve the mystery of cubic ice

Wednesday, March 29, 2023, 9:40 p.m.

Ice, the solid state of water, is one of the most common substances on Earth and in the solar system. Depending on the conditions in which it is created, it gives rise to different forms. Currently, there are 20 different forms of ice known. The most common is called Ih ice (‘Hexagonal ice’, for its acronym in English) and has a hexagonal structure (like snowflakes). However, under certain conditions, ice can form other structures. One of them is ice Ic (‘Cubic ice’), which has a cubic shape, with oxygen atoms and water molecules arranged in a three-dimensional crystalline lattice. This type of ice is produced at extremely low temperatures, below -70 degrees Celsius, and at very high pressures, such as those found at the bottom of the oceans and in deep glaciers.

In the scientific community there is some controversy regarding ice Ic, since it has been difficult to perfectly detect this type of pure phase ice (sample containing only ice Ic, with no other impurities or forms of ice present) in a large number of experiments. On the one hand, there are those who defend that it really exists in nature; while others consider that its existence is the result of measurement errors. This Wednesday, scientists from the Institute of Physics of the Chinese Academy of Sciences (CAS) have made a breakthrough by confirming the formation of pure phase cubic ice under very low temperature conditions. Their results have been published in the journal ‘Nature’.

For their experiment, the researchers used in situ cryogenic transmission electron microscopy (Cryo-TEM), a high-resolution imaging technique used to study biological samples and macromolecular materials in their native state, that is, without the need for dye or fix them, avoiding altering their structure or composition. What is done is to quickly freeze the sample at very low temperatures (generally below -180°C) and then place it in an electron microscope that is in a cryogenic environment (about -153°C).

Ice on other planets

This allowed the authors to follow the formation of ice crystallites in real time at molecular resolution. They thus discovered that the ice that had formed was mostly of the crystalline form Ic, with a small amount of ice Ih and demonstrated that the formation of this type of ice depends strongly on the conditions in which it is produced. Furthermore, they suggest that ice Ic could have been difficult to identify using conventional methods (mainly, X-ray diffraction methods), due to the presence of other types of ice (beyond Ic) in the samples analyzed in other studies, which made the interpretation of the data obtained difficult.

Ice Ic is of scientific interest because it is believed to be an indicator of the presence of water on other planets and moons in the solar system, such as Europa (on Jupiter) and Enceladus (on Saturn), which are thought to have underground oceans of liquid water. In addition, it can help understand the formation of ice crystals in clouds and be used in various technological applications, such as in the manufacture of advanced materials for engineering or electronics, among others. For example, phase-pure ice Ic (created under laboratory conditions) has been used in the manufacture of superconducting components and in research into new materials for energy storage.

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