Water (\(\text{H}_2\text{O}\)) is common, but its solid form, ice, is far more complex than the cubes found in a freezer. The “coldest ice” refers not to a single temperature, but to the different physical structures water molecules adopt under extreme thermal and pressure conditions. As temperature drops, the structure of ice changes profoundly, moving from familiar crystalline arrangements to exotic, structureless states. Studying these unique forms provides scientists with deep insight into water’s fundamental properties under conditions rarely found on Earth.
Defining “Coldest”: The Physical Limits of Temperature
The ultimate limit of coldness is Absolute Zero, 0 Kelvin (K) or \(-273.15^\circ\text{C}\). At this theoretical point, all classical molecular motion ceases, and a system possesses its minimum internal energy. The Kelvin scale is the absolute temperature scale, where \(0^\circ\text{C}\) is \(273.15\ \text{K}\).
While Absolute Zero is impossible to reach practically, scientists have achieved temperatures in the picokelvin range in laboratory experiments. These experiments set the theoretical boundary for the lowest temperatures at which any form of ice could exist. However, the physical state of ice is determined by a combination of both temperature and pressure.
Crystalline Ice Polymorphs at Extreme Cold
The ice found on Earth, known as Ice \(\text{I}_{\text{h}}\) (hexagonal ice), is only one of at least 22 known crystalline structures, or polymorphs. These distinct forms are created when water molecules arrange themselves into different lattice structures under varying combinations of low temperature and high pressure. The familiar hexagonal ice remains stable down to temperatures as low as \(5\ \text{K}\) under standard atmospheric pressure.
Other polymorphs require high pressure to maintain their structure at extremely low temperatures. For instance, Ice IX is a proton-ordered form of ice stable below \(140\ \text{K}\) (\(-133^\circ\text{C}\)) at pressures between \(0.2\) and \(0.4\) GigaPascals (GPa). The difference between many of these crystalline phases often relates to the arrangement of hydrogen atoms. These arrangements can be either disordered or highly ordered, with the ordered forms generally being stable at the lowest temperatures.
Amorphous Ice: The Structureless State
Amorphous ice lacks the long-range crystalline order of its polymorph counterparts, resembling a glass instead of a crystal. This structureless state is formed by rapidly cooling liquid water so fast that the molecules do not have enough time to organize into a crystal lattice. The glass transition temperature for water is approximately \(136\ \text{K}\) (\(-137^\circ\text{C}\)).
Amorphous ice is broadly categorized into density types, primarily Low-Density Amorphous (LDA) ice and High-Density Amorphous (HDA) ice. LDA ice has a density of about \(0.94\ \text{g}/\text{cm}^3\), similar to regular ice, and is often created by depositing water vapor onto a very cold surface below \(120\ \text{K}\). HDA ice is denser, at about \(1.17\ \text{g}/\text{cm}^3\), and is usually formed by compressing crystalline ice at low temperatures.
Recent studies have also revealed a Medium-Density Amorphous (MDA) ice, with a density near \(1.06\ \text{g}/\text{cm}^3\). The existence of these distinct amorphous forms suggests that liquid water itself might have two different liquid states at supercooled temperatures. These glassy ices are thought to be the most prevalent form of water ice throughout the universe.
Where the Coldest Ice Exists
The coldest naturally occurring ice ever measured was detected by the James Webb Space Telescope (JWST) in the Chameleon I molecular cloud, a stellar nursery about 500 light-years from Earth. This interstellar ice was found at approximately \(11\ \text{K}\) (\(-263^\circ\text{C}\)). The ice coats dust grains and is primarily amorphous, a structure common in the extreme vacuum and cold of space where crystallization is hindered.
Amorphous ice is also the dominant form on distant, cold bodies in our solar system, such as objects in the Kuiper Belt and the surfaces of icy moons like Europa and Enceladus. These extraterrestrial forms often contain trapped molecules like methanol and ammonia, providing crucial building blocks for planetary formation.
Crystalline ice polymorphs, while requiring high pressure for stability, are relevant to the internal structure of large icy bodies. Examples include the cores of ice giant planets or the deep layers of large icy moons. Laboratory cryostats and high-pressure devices recreate these extreme conditions to study the phase transitions between the different ice polymorphs. Understanding where these different ice forms are found helps scientists model the chemical evolution of planetary systems and the distribution of water across the cosmos.