Amorphous ice is an amorphous solid form of water, meaning it consists of water molecules that are randomly arranged like the atoms of common glass. Everyday ice is a crystalline material where the atoms are regularly arranged in a lattice whereas amorphous ice is distinguished by a lack of long-range order in its atomic arrangement. Amorphous ice is produced either by rapid cooling of liquid water (so the molecules do not have enough time to form a crystal lattice) or by compressing ordinary ice at low temperatures.
Although almost all water ice on Earth is the familiar crystalline Ice Ih, amorphous ice dominates in the depths of interstellar medium, making this likely the most common structure for H2O in the universe at large.
Just as there are many different crystalline forms of ice (currently fifteen known), there are also different forms of amorphous ice, distinguished principally by their densities.
The key to producing amorphous ice is the rate of cooling. The liquid water must be cooled to its glass transition temperature (about 136 K or −137 °C) in a matter of milliseconds to prevent the spontaneous nucleation of crystals.
This is analogous to the production of ice cream, which must also be frozen quickly to prevent the growth of crystals in the mixture. The difference is that pure water forms crystals much more readily than the heterogeneous mixture of ingredients in ice cream, so amorphous ice is more difficult to produce, requiring a physics lab rather than an ice cream churn.
Pressure is another important factor in the formation of amorphous ice, and changes in pressure may cause one form to convert into another.
Chemicals known as cryoprotectants can be added to water, to lower its freezing point (like an antifreeze) and increase viscosity, which inhibits formation of crystals. Vitrification without addition of cryoprotectants can be achieved by very rapid cooling. These techniques are used in biology for cryopreservation of cells and tissues.
Low-density amorphous ice
Low-density amorphous ice, also called LDA, vapor-deposited amorphous water ice, amorphous solid water (ASW) or hyperquenched glassy water (HGW), 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. It is expected to be common in the subsurface of exterior planets and comets. It may also form in the coldest region of the Earth's atmosphere, the summer polar mesosphere, where noctilucent clouds exist.
Melting past its glass transition temperature (Tg) between 120 and 140 K, LDA is more viscous than normal water. Recent studies have shown the viscous liquid stays in this alternative form of liquid water up to somewhere between 140 and 210 K, a temperature range that is also inhabited by ice Ic. LDA has a density of 0.94 g/cm3, less dense than the densest water (1.00 g/cm3 at 277 K), but denser than ordinary ice (ice Ih).
Hyperquenched glassy water (HGW) 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 104 K/s are required to prevent crystallization of the droplets. At liquid nitrogen temperature, 77 K, HGW is kinetically stable and can be stored for many years.
High-density amorphous ice
High-density amorphous ice (HDA) can be formed by compressing ice Ih at temperatures below ~140 K. At 77 K, HDA forms from ordinary natural ice at around 1.6 GPa and from LDA at around 0.5 GPa (approximately 5,000 atm). At this temperature, it can be recovered back to ambient pressure and kept indefinitely. At these conditions (ambient pressure and 77 K), HDA has a density of 1.17 g/cm3.
Peter Jenniskens and David F. Blake demonstrated in 1994 that a form of high-density amorphous ice is also created during vapor deposition of water on low-temperature (< 30 K) surfaces such as interstellar grains. The water molecules do not fully align to create the open cage structure of low-density amorphous ice. Many water molecules end up at interstitial positions. When warmed above 30 K, the structure re-aligns and transforms into the low-density form.
Very-high-density amorphous ice
Very-high-density amorphous ice (VHDA) was discovered in 1996 by Mishima who observed that HDA became denser if warmed to 160 K at pressures between 1 and 2 GPa and has a density of 1.26 g/cm3 at ambient pressure and temperature of 77 K. More recently it was suggested that this denser amorphous ice was a third amorphous form of water, distinct from HDA, and called it VHDA.