Amorphous ice

Amorphous ice

While most forms of ice are crystalline, several amorphous (or "vitreous") forms of ice also exist. Such ice is an amorphous solid form of water, which lacks long-range order in its molecular arrangement. Amorphous ice is produced either by rapid cooling of liquid water to its glass transition temperature (about 136 K or −137 °C) in milliseconds (so the molecules do not have enough time to form a crystal lattice), or by compressing ordinary ice at low temperatures. The most common form on Earth, low-density ice, is usually formed in the laboratory by a slow accumulation of water vapor molecules (physical vapor deposition) onto a very smooth metal crystal surface under 120 K. In outer space it is expected to be formed in a similar manner on a variety of cold substrates, such as dust particles. By contrast, hyperquenched glassy water is formed by spraying a fine mist of water droplets into a liquid such as propane around 80 K, or by hyperquenching fine micrometer-sized droplets on a sample-holder kept at liquid nitrogen temperature, 77 K, in a vacuum. Cooling rates above 10 K/s are required to prevent crystallization of the droplets. At liquid nitrogen temperature, 77 K, hyperquenched glassy water is kinetically stable and can be stored for many years.