Ice VII is an extraordinary form of water ice, unlike the common ice we encounter daily. Unlike the ice that floats in our drinks, this unique solid phase of water forms only under immense pressure. It represents one of many known crystalline structures water can adopt when subjected to extreme conditions. This high-pressure ice captivates scientists due to its unusual properties and the environments where it naturally occurs.
How Ice VII Forms
Ice VII forms under conditions of extreme pressure, far exceeding what is found at Earth’s surface. It can crystallize from liquid water at pressures greater than approximately 2 to 3 gigapascals (GPa), which is tens of thousands of times the atmospheric pressure at sea level. Ice VII can also form rapidly through dynamic compression, such as from liquid water subjected to laser-induced shock waves at around 5 GPa.
This high-pressure ice can exist across a significant temperature range. Laboratory experiments have shown its stability from relatively low temperatures up to several hundred degrees Celsius. The combination of high pressure and varying temperatures underscores the unique conditions necessary for its formation, making it a rare sight in everyday environments.
The Unique Structure and Characteristics of Ice VII
The atomic arrangement of Ice VII distinguishes it from other forms of ice. Common ice, known as Ice Ih, has a hexagonal crystal structure. In contrast, Ice VII possesses a compact cubic crystal structure. This cubic arrangement means its water molecules are packed much more tightly together.
As a result of this dense packing, Ice VII is significantly more compressed than regular ice. It is approximately one and a half times denser than common ice. Despite its extreme formation conditions, Ice VII is a transparent solid.
Where Ice VII is Found
Ice VII is not found on Earth’s surface because the necessary high-pressure conditions do not exist there. However, it naturally occurs deep within Earth’s interior, particularly in the mantle. Scientists have found Ice VII trapped as inclusions within diamonds that formed at depths of up to 400 miles (approximately 640 kilometers). These diamond inclusions provide a unique sample of the deep Earth environment.
The presence of Ice VII in these diamonds indicates water-rich regions in the transition zone between the upper and lower mantle, suggesting that water is transported to great depths by subducting oceanic plates. Beyond Earth, Ice VII is a plausible constituent of large, rocky exoplanets often referred to as “water worlds.” On such planets, the immense pressure at the bottom of vast oceans, which can be hundreds of miles deep, would cause water to solidify into high-pressure ice forms like Ice VII.
Why Ice VII Matters to Science
Studying Ice VII provides valuable insights into the composition and processes occurring within Earth’s deep interior. Its discovery in diamonds offers direct evidence of aqueous fluid within the Earth’s mantle, influencing our understanding of the planet’s global water budget and the movement of heat-generating elements. This helps researchers model the dynamics of plate tectonics and the deep water cycle.
In planetary science, Ice VII is important for understanding the internal structures of “water worlds” and other icy bodies in the universe. Models of these exoplanets suggest that layers of high-pressure ice, including Ice VII, could exist beneath their vast liquid oceans. Understanding how Ice VII forms and behaves under extreme conditions aids in predicting whether these distant planets could support life by influencing the transport of nutrients from rocky cores to liquid oceans. Research into Ice VII also contributes to the broader field of high-pressure physics and materials science, exploring how matter behaves under conditions not found on the surface of Earth.