Water is a unique substance on Earth, commonly known to transition from liquid to solid at \(0^{\circ}\text{C}\) or \(32^{\circ}\text{F}\) at standard atmospheric pressure. However, this definition applies only to pure, fresh water. Natural environments often contain dissolved substances and experience intense pressures that fundamentally change this behavior. In the deep trenches and polar regions, liquid water can be found at temperatures that would instantly freeze a glass of tap water. This investigation requires exploring the deepest ocean basins and the most frigid polar environments, where physics and chemistry combine to depress the freezing point far below the familiar mark.
Identifying the World’s Coldest Water
The coldest naturally occurring liquid water on Earth is found in the deep Southern Ocean surrounding Antarctica. This frigid water mass is known as Antarctic Bottom Water (AABW), which forms the deepest and densest layer of the global ocean circulation system. While the bulk of AABW ranges from \(-0.8^{\circ}\text{C}\) to \(2^{\circ}\text{C}\), the lowest recorded temperature for liquid seawater was found beneath an Antarctic glacier, reaching approximately \(-2.6^{\circ}\text{C}\). This ultra-cold water is primarily formed in specific coastal areas, such as the Weddell Sea and the Ross Sea, through a process called brine rejection. As sea ice forms on the ocean surface, the salt is expelled into the surrounding water, creating a layer of highly saline water. This remaining liquid water becomes extremely cold and dense, causing it to sink rapidly to the ocean floor where it flows northward, spreading across all three major ocean basins. Water flowing out from beneath the massive Antarctic ice shelves can also be supercooled, meaning it is below its freezing point but remains liquid because it lacks a surface or nucleus upon which to form ice crystals.
The Science of Extreme Cold Water
The existence of liquid water at temperatures like \(-1.9^{\circ}\text{C}\) or even lower is explained by two primary physical mechanisms that alter its freezing point. The most significant of these factors is salinity, the concentration of dissolved salts in the water. This phenomenon is known as freezing point depression, where the presence of dissolved ions interferes with the ability of water molecules to arrange themselves into the rigid, crystalline lattice structure of ice. Average ocean water, with a salinity of about 3.5\%, has a freezing point of approximately \(-1.8^{\circ}\text{C}\). Because the water forming the Antarctic Bottom Water is made denser by the rejection of brine from forming sea ice, it can achieve salinities slightly higher than average, thus depressing the freezing point even further. The immense hydrostatic pressure in the deep ocean environment provides the second contribution to the low temperature. Water is unusual because it expands when it freezes. High pressure acts to oppose this expansion, which in turn slightly lowers the temperature at which the liquid-to-solid phase change can occur. Therefore, the combination of high salinity and the crushing weight of several kilometers of ocean above allows the AABW to remain liquid just a fraction of a degree above its freezing point.
Life in Sub-Zero Environments
Despite these hostile conditions, the near-freezing waters of the Southern Ocean support a specialized and diverse biological community. Organisms that thrive in these extremely cold environments are often referred to as psychrophiles. The challenge for marine life here is that their internal body fluids are typically less salty than the surrounding seawater, meaning their blood would freeze at a warmer temperature, around \(-1.0^{\circ}\text{C}\). To survive, many Antarctic fish, particularly the notothenioids, have evolved remarkable biological adaptations, most notably the production of antifreeze proteins (AFPs) or glycoproteins (AFGPs). These specialized proteins circulate in the fish’s blood and other bodily fluids. They do not lower the freezing point by altering the overall concentration, but rather by physically binding to the surface of any tiny ice crystals that may form. By coating these nascent ice crystals, the AFPs prevent them from growing large enough to cause cellular damage, effectively inhibiting the freezing process. This mechanism allows the fish to maintain their body fluids in a liquid state even when the external water temperature is below their expected freezing point. Some invertebrates, such as sea stars and sea spiders, utilize a different strategy, maintaining the salt concentration of their internal fluids to match the surrounding seawater, which naturally keeps their freezing point low.